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Failure analysis and control technology of intersections of large-scale variable cross-section roadways in deep soft rock

Failure analysis and control technology of intersections of large-scale variable cross-section... In deep underground mining, achieving stable support for roadways along with long service life is critical and the complex geological environment at such depths frequently presents a major challenge. Owing to the coupling action of multiple factors such as deep high stress, adjacent faults, cross-layer design, weak lithology, broken surrounding rock, variable cross-sections, wide sections up to 9.9 m, and clusters of nearby chambers, there was severe deformation and breakdown in the No. 10 intersection of the roadway of large-scale variable cross-section at the − 760 m level in a coal mine. As there are insufficient examples in engineering methods pertaining to the geological environment described above, the numerical calculation model was oversimplified and support theory underdeveloped; therefore, it is imperative to develop an effective support system for the stability and sustenance of deep roadways. In this study, a quantitative analysis of the geological environment of the roadway through field observations, borehole-scoping, and ground stress testing is carried out to establish the FLAC 3D variable cross-section crossing roadway model. This model is combined with the strain softening constitutive (surrounding rock) and Mohr–Coulomb constitutive (other deep rock formations) models to construct a compression arch mechanical model for deep soft rock, based on the quadratic parabolic Mohr criterion. An integrated control technology of bolting and grouting that is mainly composed of a high-strength hollow grouting cable bolt equipped with modified cement grouting materials and a high-elongation cable bolt is developed by analyzing the strengthening properties of the surrounding rock before and after bolting, based on the Heok-Brown criterion. As a result of on-site practice, the following conclusions are drawn: (1) The plastic zone of the roof of the cross roadway is approximately 6 m deep in this environment, the tectonic stress is nearly 30 MPa, and the surrounding rock is severely fractured. (2) The deformation of the roadway progressively increases from small to large cross-sections, almost doubling at the largest cross-section. The plastic zone is concentrated at the top plate and shoulder and decreases progressively from the two sides to the bottom corner. The range of stress concentration at the sides of the intersection roadway close to the passageway is wider and higher. (3) The 7 m-thick reinforced compression arch constructed under the strengthening support scheme has a bearing capacity enhanced by 1.8 to 2.3 times and increase in thickness of the bearing structure by 1.76 times as compared to the original scheme. (4) The increase in the mechanical parameters c and φ of the surrounding rock after anchoring causes a significant increase in σ ; the pulling force of the cable bolt beneath the new grouting material is more than twice that of ordinary cement grout, and according to the test, the sup- porting stress field shows that the 7.24 m surrounding rock is compacted and strengthened in addition to providing a strong foundation for the bolt (cable). On-site monitoring shows that the 60-days convergence is less than 30 mm, indicating that the stability control of the roadway is successful. Keywords Deep soft rock · Variable cross-section · Roadway intersection · Bolting-grouting integration · New grouting material 1 Introduction * Dongdong Chen chendongbcg@163.com With the exhaustion of shallow coal resources, coal min- School of Energy and Mining Engineering, China University ing in east-central China has shifted to deeper realms (Cai of Mining and Technology, Beijing, Beijing 100083, China and Brown 2017; Chen et al. 2019). Deep ground stress Mine Safety Technology Branch of China Coal Research is high, mining has a considerable influence, surrounding Institute, Beijing 100013, China Vol.:(0123456789) 1 3 19 Page 2 of 23 S. Xie et al. rock deformation has great mobility, expansion, and impact; deep roadway support. Wang et al. (2020) utilized ABAQUS hence, adequate support of roadways is becoming increas- to create a finite element model under the original support ingly problematic (Liu 2011; Fairhurst 2017; Xie et  al. design, suggested a zoned bolt-grouting reinforcement tech- 2018). A long service life and high stability are essential nology, and numerically tested its support impact. Using requirements in the development of roadways. The environ- self-developed random non-destructive testing methodolo- ment at great depths is unique owing to the complicated gies and equipment, Zhang et al. (2017) suggested an early geological conditions (Wagner 2019; Ranjith et al. 2017; warning system for the integrity of the roadway envelope Xue et al. 2020). The No. 10 intersection examined in this based on anchor axial load detection. study includes a portion of the roadway of width 9.89 m The research above provided a sound theoretical and engi- with loose and fractured surrounding rock that was seriously neering foundation for controlling surrounding rocks in deep damaged by strong tectonic stress at that depth and traverses roadways; however, a majority of the studies focus on a sin- uneven strata. Consequently, traditional anchor support is gle geological or roadway attribute, such as soft rock, frac- inadequate to withstand the significant deformation damage tured surrounding rock, flooded roadways, or large section that occurs in practice (Pan et al. 2017). chambers, rather than examining the efficient sustenance of Many academics have carried out thorough studies on deep roadways under the influence of many varying factors. the control and design of surrounding rocks to address the In addition, numerical simulations using a single intrinsic challenge of providing an appropriate support system for relationship ignore the difference in mechanics between a deep roadways in complicated geological settings. Tian et al. tunnel envelope and undisturbed rock formations. Moreover, (2020) suggested a support system for deep soft rock sub- the existing amount of research on the support of variable- merged roads based on high-strength anchoring, a high-stiff- section roadways is relatively small, and the numerical mod- ness spraying layer to prevent water, and deep and shallow eling of variable-section roadways under inclined coal rock hole grouting to rebuild the damaged surrounding rock. Xie layers is over-simplified, which affects the accuracy of the et al. (2019) suggested a complete control approach for deep results. The lack of a theoretical model of bolt (cable) sup- large-section chambers such as strong bolt (cable) support, port based on soft rock environment in deep roadway results thick-walled reinforced concrete pouring, and full-section in inappropriate selection of support materials. The present pressure-regulating grouting behind the walls. Kang et al. cement slurry-based grouting material has a large number (2014) developed a novel form of an integrated support sys- of flaws; therefore, it is difficult to ensure grouting action in tem and floor monitoring technique to prevent and manage deep roadways. the weak floor of a deep roadway. Huang, Li, and Zhang The intersection of the − 760 m level No. 10 roadway in et al. utilized a novel steel pipe concrete reinforced support a coal mine is the subject of research in this study, and that successfully suppressed serious deformation of deep the coupling impact of many variables such as deep high roadways (Huang et al. 2018; Li et al. 2020; Zhang et al. stress, adjacent faults and interlayer arrangement, weak 2018). Wang et al. (2017) investigated the damage and con- lithology, fractured surrounding rocks, varied cross-sec- trol mechanisms of deep soft rock roadways and proposed tions, large cross-sections up to 9.9 m wide, and clusters the idea of “high-strength, integrity, and pressure-relief”. of neighboring chambers were examined as the reasons for Yang et al. (2017) used a combination technique of “bolt- its deformation and collapse. Field observation, borehole- cable-mesh-shotcrete + shell” to successfully control the scoping, and in-situ stress testing were used to determine deformation of a deep soft rock roadway. Wang et al. (2015) the geomechanical characteristics of the roadway. The strain- presented a dynamic damage intrinsic model to evaluate the softening features of the surrounding rock in the post-peak elastic rebound and shear expansion deformation of the sur- stage were modeled and studied, and the internal friction rounding rock during the unloading process and addressed angle and cohesive force weakening law of the rock were the pre-peak and post-peak phases in their theory of rock deduced. Curve fitting of the triaxial test was performed damage in deep roadways. Huang and Li et al. performed a using FLAC 3D; the inverted parameters were applied to the numerical simulation of deep rock cutting and fracture pat- FLAC 3D variable cross-section roadway model to achieve terns (Huang et al. 2016; Li et al. 2016). Peng et al. (2018) the coupling of the surrounding rock strain softening and in their study of the structural damage process of deep road- Mohr–Coulomb constitutive model, to effectively analyze ways, reported that horizontal stress had a significant impact the force and deformation characteristics of the roadway on the stability of the surrounding rock and developed a intersection. For support design analysis, a thick reinforced multi-stage support system based on the structural features compression arch mechanical model, based on the quadratic of the roadway bearing. Shreedharan and Kulatilake (2015) parabolic Mohr strength criteria, was developed for the sur- employed the 3DEC discrete element technique to assess rounding conditions of deep soft rock, and the strengthening the stability of a deep coal mine roadway under various sec- support scheme with the action path of "deep hole grout- tions and support bodies in their numerical simulation of a ing and anchoring → reinforcement of broken surrounding 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 3 of 23 19 rock → mutual cementation into the arch → realization of mudstone, thin mudstone, and sandy mudstone interbeds, self-supporting surrounding rock" was proposed, i.e., the where the rock is broken. The overall stability is poor and anchor injection integrated the support technology based on a bedding of V-level unstable rock mass (It refers to bro- a hollow grouting anchor cable equipped with a modified ken soft rock with basic quality index BQ ≤ 250) is devel- grouting material. Simultaneously, the impact of changes in oped that leads to frequent roof fall in roadway excavation. the parameters of the surrounding rock mechanics before and After the implementation of the original support scheme, after bolting and grouting on strength was evaluated using the accidents of large deformation, roof fall and slope still the Heok-Brown criteria, and a supporting prestress field occur in the roadway, and the original support scheme can was constructed to simulate and validate the plan. The road- not effectively control the surrounding rock deformation way was monitored for displacement and borehole-scoping (the original support scheme and failure condition will be following on-site construction, and it was observed that the shown in detail below). A comprehensive histogram of the control effect on the stability of the surrounding rock was strata is shown in Fig. 1. good, providing a theoretical direction and engineering ref- erence for roadway support under the arduous circumstances at great depths.2.2 Engineering characteristics The largest cross-section at the No. 10 intersection is 2 Engineering background formed by the intersection of people and vehicle parking chambers (referred to as parking chambers), and pedes- 2.1 Geological profile trian and vehicle parking chamber passages (referred to as chamber passages). In view of the geological background The mine studied is equipped with a fully mechanized top- of deep high stress, practical conditions of the concen- coal caving face that is mainly used in the No. 3 coal seam trated chamber group, and large cross-section at the inter- of the Shanxi Formation in the Qinshui Coalfield, and the section, a comprehensive site observation of the deforma- designed annual output reaches 3.0 Mt/a. The bottom yard tion due to the stress environment, characteristics of the of the air intake shaft is located under the No. 3 coal seam surrounding rock, cross-section of the intersection, and with a buried depth of approximately 760 m. The strike construction technology yielded the following character- of the coal and rock strata is north high and south low by istics of the project: 12°, and west high and east low by 5°. The rock layers tra- versed by the roadway at the bottom of the shaft are sandy Thickness Rock stratum Lithology description /m Gray, medium to thick laminated, feldspar, quartz Intersection Medium sandstone 8.70 dominant, uniformly laminated, well sorted, well rounded No.1 coal seam 0.35 Mudstone Black coal line 760 m Air shaft 11.10 m Mudstone 5.15 Gray-black, medium-thick laminated, brittle, flat fracture Intersection Sandy mudstone 2.75 Light gray-black, medium-thick laminate, flatter fracture Light gray-black, medium-thick laminate, brittle, shell- Mudstone 11.10 like fracture Sandy sudstone 12.95 Light gray-black, medium-thick lamellar, jagged fracture Gray-black, medium-thick laminated, brittle, shell-like Mudstone 1.80 fracture, containing plant fossils The passageway of Black, bright coal mainly, dark coal second, semi-bright No.3 coal seam 5.80 parking chamber coal No.10 roadway Mudstone 2.35 Dark gray-black, lumpy, flat fracture Intersection intersection 10 Light gray-black, medium-thick laminated, flat fracture Sandy mudstone 5.20 Intersection Gray, thickly laminated, quartz dominant, feldspar secondary, with coal dust, mica, with muddy streaks, Intersection Fine sandstone 3.20 8 Sandy mudstone sorted Intersection 12.95 m Dark gray-black, blocky, brittle, flat fracture, containing Intersection 3 Sandy mudstone 10.45 4 plant fossils Intersection Gray, medium-thick laminated, feldspar, quartz dominated, three vertical fissures of 0.4 m in length Medium sandstone 4.20 developed Intersection 1 Dark gray-black, lumpy, brittle, flat fracture Mudstone 1.55 Intersection Black coal line 6 No.5 coal seam 0.75 Intersection Dark gray-black, lumpy, brittle, flat fracture Mudstone 1.30 Fig. 1 Comprehensive histogram of the stratum at the intersection of the roadway 1 3 Parking chamber 19 Page 4 of 23 S. Xie et al. 2.2.1 Intersections with a concentrated arrangement original rock stress (This is the result of the later numeri- resulting in stress concentration cal calculation, which is shown below). This directly causes severe deformation of the surrounding rock at the intersec- As shown in Figs. 2, 11, roadway intersections connect three tion giving rise to serious cracking of the shotcrete layer in transport roadways near the − 760 m shaft bottom yard in the roadway. the Nanfeng work area, that form a centralized chamber group along with pipes, pedestrian parking chambers, and 2.2.2 Poor surrounding rock lithology owing horsehead gates. The roof mudstone of the chamber group to the proximity of faults and placement is not strong and is rich in clay minerals such as a mixed through layers layer of illite/montmorillonite and kaolinite. This makes the surrounding rock soft, strong-swelling, easily attacked by As shown in Figs. 3, 4, the south side of the No. 10 inter- chemicals, and readily weathered (Kang et al. 2015; Yu et al. section is near the normal fault CF47, where the ground 2020). In this environment, densely distributed intersections stress is dominated by tectonic stress, the surrounding lead to overlapping stresses. At the largest cross-section at rock strength is low, and integrity is poor. Moreover, the the No. 10 intersection, the range of stress concentration is depth of the roadway is large and the surrounding rock of substantial and the peak stress reaches more than twice the the nearby roadway is broken to some extent, making the Central track roadway Main transformer station Air shaft Main roof Shaft station at -760 m level 11.10 m Deep shaft ingate No. 1 No. 4 Deep soft rock chamber group No. 5 No. 3 Immediate roof The passageway of parking chamber 12.95 m No. 2 No. 10 large section False roof Crosscut 1.80 m roadway intersection Parking chamber No. 6 No. 8 Shunting roadway Central belt roadway No.3 coal seam No. 7 Immediate bottom Basic bottom Fig. 2 Location map of all roadway intersections N Crossing layer roadway Intersection No. 12 Intersection No. 8 Parking chamber 23 m Horizontal distance North roadway Permanent refuge chamber Waiting room Crossing the No. 3 coal seam No.3 coal seam CF47 Normal fault 60 70 H: 0 13 m Fig. 3 Stratigraphic profile at the No. 10 intersection 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 5 of 23 19 Superimposed plastic failure a = 19 42'12" zone of roadway intersection 3.5 m Plastic zone boundary R = 20000 K = 6878 a =5 43'55" 6.6 m T = 3473 R =20000 K =2000 T =1001 Plastic failure zone 4.0 m 4.2 m Large deformation of roadway intersection ZDK938/5/20 Right 11 42'36" Track center line 5.5 m 9.65 m Roadway center line Track center line Parking chamber Roadway deformation Fig. 4 Schematic of the plastic zone of each section at the No. 10 intersection support inadequate (Zhang et al. 2019). The intersection 2.2.4 Difficult construction and maintenance of large crosses over the junction of the layers of mudstone and cross‑sectional intersections. sandy mudstone with the bottom of the roadway as the datum, and is almost 0.7–3.0 m from the datum. Therefore, The occurrence of rib spalling and roof falling is frequent in the No. 10 intersection through the layer triggers the plas- the process of digging and excavating; the roadway is poorly tic zone in each part to be at a higher level thus increasing shaped, especially the No. 10 intersection that is 9.9 m wide the volume of the plastic zone, which directly causes the and up to 6.5 m high. Moreover, the shotcrete layer of the surrounding rock to be loose and broken. major support section contains cracks and falling blocks; thus, breaking and falling off of the wall takes place to the extent of different degrees during roadway maintenance. 2.2.3 A large cross‑sectional area of the intersection extending the range of disturbance of the surrounding rock 3 Damage deformation analysis The above picture shows the field measurement results of On account of the field working conditions of the No. 10 plastic failure zone in the early stage of the original sup- intersection, large deformation of the roadway, and broken port design, and the No.10 intersection is a large section surrounding rock, the methods of drilling peek, in-situ stress chamber with a tunneling width of 9.89 m. The rock body measurement, and numerical simulation were used to ana- is in a long-term rheological deformation process; the lyze the surrounding rock plastic zone, roof displacement, plastic zone is more developed and creates a wide range and stress conditions. of superimposed plastic zones around the junction due to the vast burial depth, enormous section, and fractured sur- 3.1 Drilling peek detection rounding rock (Tan et al. 2019). Further, the strength and integrity of the surrounding rock at the intersection are Drilling peeking at intersection 10 is shown in Fig. 5: (1) poor; hence, the stress causes it to reach the plastic yield The surrounding rock from 0–0.4 m was relatively broken condition, leading to plastic flow on both sides of the road- with intense fissures. (2) There was loose destruction of the way, as well as shear yield and tensile failure in the region. surrounding rock at a depth of 0.4–4.2 m, the open fractures The actual damage at the comprehensive site intersection, were concentrated in the shallow fracture development area, under the effect of strong disturbances, results in overall and cracks were developed intensively within a depth of 4 m. deformation and instability, and causes chain damage to (3) There were a large number of fine original fractures in the cavity group in severe cases. the slight crack area of 4.2–6.6 m, that reduced from the 1 3 12 10 80 4 19 Page 6 of 23 S. Xie et al. Complete original rock zone Slightly fractured zone Fracture development zone Fracture zone Plastic zone 6446 mm boundary Intersection 10 2 12 12 10 8 6 4 2 0 0 46 8 10 9891 mm Fig. 5 Schematic of borehole-scoping and zoning failure of the surrounding rock shallow to deep region. (4) The deep surrounding rock from was σ > σ > σ . The maximum principal stress (near the H V h 6.6–12 m was complete with dense rock formations and no horizontal direction) of the measuring point was nearly obvious cracks. (5) The degree of breakage was greater in 30 MPa and the average ratio of the maximum principal the roof and shoulder of the surrounding rock than that at the stress to vertical stress was 1.67, which is a state of high side; hence, roof control was the focus of our study. tectonic stress. The roof and floor control of the roadway support is particularly important in the case of the in-situ 3.2 In‑situ stress measurement stress field dominated by horizontal stress. The roof of the site was soft mudstone with many broken rock blocks, As shown in Fig. 6, an in-situ stress measurement tech- hence, support was difficult. Therefore, modification and nique based on CSIRO cell was applied to the field meas- strengthening of the surrounding rock was the key to form- urements at the No. 10 intersection. The average result of ing a support system. the measured point data showed that the ground stress type KX-81 hollow inclusion YHY16 intrinsic safety triaxial strain gauge strain gauge for mine Surrounding No. 10 large section rock roadway intersection Borehole Deep s mall hole rock core Variable diameter drill bit Direction finder Rock core rate tester for strain gauge Fig. 6 Schematic of in-situ stress measurement 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 7 of 23 19 3.3 Analysis of the failure of the original support 3.4 Establishment of FLAC 3D model for the No. 10 scheme intersection In Fig.  7, the original support scheme consisting of an Based on the engineering geological characteristics of the anchor network cable spray + double-steel-bar ladder beam No. 10 intersection, a FLAC 3D model that conformed to is shown. After the construction, there was large deforma- the actual condition of the site and showed the crossing tion of the roadway section, sinking and cracking of the roof, form of the roadway to the greatest extent was constructed, roof and rib falling, and failure and falling of bolts (cables). and is shown in Fig.  8. The constitutive relationship of The original supporting bolts (Here is the bolt anchored at the model adopted the Mohr–Coulomb criterion and the the end of the resin cartridge) were 2.4 m long, all of which rock formation (mudstone and sandy mudstone) was cou- were in the fracture development concentrated area where pled with the strain-softening constitutive relationship at they were unable to function; ordinary cable bolts (ordinary the intersection point to indicate that the strength of the cable bolt refers to common cable bolt with 7-core steel broken rock was mostly its residual strength. According strands) were 7.6 m long with large spacing and low elon- to the experiment on rock mechanics conducted with the gation. The broken surrounding rock did not allow for an rock core taken from the site, the mechanical parameters effective anchoring foundation, thus the performance of the of the coal and rock strata given in the following Table 1 anchor cable was inadequate and a stable supporting struc- were adopted for the numerical simulation. ture was not formed. Hence, the supporting body failed to exert the self-bearing capacity of the surrounding rock. Normal cable bolt Row & line space: 1600 800 mm High-strengthened resin bolt Rib spalling Partial fracture Row & line space: 800 800 mm Plastic failure zone Fracture zone No.10 roadway intersection Cracking Invalid anchor Fig. 7 Schematic of original support scheme and roadway breakdown and deformation Passageway Intersection 10 Parking chamber No.10Intersection Overburden rock load 65 m Variable section roadway 150 m Fine sandstone 60 m Fig. 8 FLAC 3D numerical model of the No. 10 intersection 1 3 19 Page 8 of 23 S. Xie et al. Table 1 Parameters of coal 3 Rock stratum Average E (GPa) K (GPa) c (MPa) σ (MPa) φ (°) γ (kg/m ) and rock mechanics used in thickness numerical simulation (m) Medium sandstone 5.55 16.73 10.72 5.54 3.58 37.00 2731 Mudstone 7.65 9.66 7.00 2.83 1.19 32.00 2463 Medium sandstone 1.70 15.50 9.94 5.34 3.45 37.00 2713 Mudstone 1.30 11.34 8.22 2.62 1.12 33.00 2466 Medium sandstone 8.70 15.21 9.75 5.46 3.53 37.00 2801 Mudstone 5.50 8.69 6.30 2.34 1.05 30.00 2414 Sandy mudstone 2.75 12.94 8.63 4.78 2.81 35.00 2567 Mudstone 11.10 9.03 4.80 2.69 1.09 30.00 2453 Sandy mudstone 12.95 13.65 9.10 4.92 3.12 36.00 2646 Mudstone 1.80 10.21 7.40 2.54 1.07 32.00 2429 No. 3 coal seam 5.80 5.20 4.33 1.25 0.82 25.00 1423 Mudstone 2.35 9.46 6.86 2.75 1.13 31.00 2457 Sandy mudstone 5.20 12.30 8.20 4.62 2.80 35.00 2549 Fine sandstone 3.20 23.40 13.93 5.87 3.79 38.00 2815 Sandy mudstone 10.45 13.50 9.00 4.75 2.76 35.00 2606 2010), an ideal trilinear strain-softening model curve was 3.5 Simulation of strain‑softening mechanical constructed (Fig. 10) (Kawamoto and Ishizuka 1981). As characteristics shown in Fig. 10 of the simplified model, OA and OB are the pre-peak elastic deformation stages of the rock and 3.5.1 Strain softening mechanical model the secant of the peak point was used as an approximate replacement, where l is the unloading path. After the peak The No. 10 intersection is at the junction of 12.95  m point, the rock enters the strain-softening stage, The slope sandy mudstone and 11.10  m mudstone. The surround- of the post-peak stage of each triaxial test curve is fitted and ing rock is quite broken and the softening characteristics calculated, and an oblique line is obtained to represent the of the post-peak are the main factors affecting the defor - post-peak strain softening stage (Alonso et al. 2003; Lee mation and deterioration of weak rocks. Therefore, based and Pietruszczak 2008), then the average post-peak slope of on the results of the triaxial compression test of the two triaxial test curves with different confining pressure is taken, types of rocks shown in Fig. 9 (Huang et al. 2014; Lu et al. (σ −σ )/MPa 1 3 σ = 40 MPa σ = 40 MPa 3 3 σ = 30 MPa σ = 30 MPa σ = 20 MPa σ = 10 MPa σ = 20 MPa σ = 10 MPa -0.05-0.04 -0.03-0.02 -0.010 0.01 0.02 0.03 0.04 0.05 0.06 ε ε (a)Sandy mudstone (b)Mudstone Fig. 9 Full stress–strain curves of two types of rocks under different confining pressures in triaxial compression tests 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 9 of 23 19 � � � � � � 2 2 2 p p p p p p ps (1) = 2 −  +  +  + 2 − 1 3 1 3 3 1 p p where   and  are the principal plastic strain components. 1 3 Therefore, the subsequent yield surface of the rock after the peak is expressed as ps f  ,  ,  ,  = 0 (2) 1 2 3 where σ is the first principal stress, σ and σ are the sec- 1 2 3 ond and third principal stresses, respectively, and σ = σ . 2 3 It was assumed that the stress state at a point in the post- ps peak strain-softening stage at different ε is in the critical Fig. 10 Post-peak rock strain-softening plastic strain–stress relation- state of strength failure, i.e., the Mohr–Coulomb criterion ship is satisfied. and a unified post-peak slope is obtained (Yao et al. 2018; 1 + sin  1 + sin f =  −  + 2c Zhang et al. 2008). Assuming that the unloading process is (3) 1 3 � � 1 − sin  1 − sin linearly elastic, there is l//OA, and the same plastic deforma- tion is produced along the unloading path l under different where the internal friction angle φ at the critical state and the confining pressures. ps cohesive force c under different ε are calculated inversely, ps and the law of φ and c changing with ε of the two types of 3.5.2 Yield surface of the strain‑softening stage of the rock rocks under study can be obtained simultaneously. Owing to the accumulation of the history of plastic defor- 3.5.3 Internal friction angle and weakening law mation of the rock and randomness of the instantaneous of cohesion stress state, the subsequent yield surface is different from the elastic stage. To record the history of plastic loading of In the strain-softening stage of the rock, the value of the the rock material, the rock was considered to be anisotropic. principal stress difference in the rock gradually decreases ps The strain-softening parameter ε was used as the plastic as the plastic strain increases, and this decreasing trend state variable (Lu et al. 2010; Zhang et al. 2008) given by is reflected in the change of the rock c and φ values. Fig- the following formula: ure 11 is based on the type (2) (3) to obtain the different σ = 10 MPa ϕ′/° c′/MPa ϕ′/° c′/MPa σ = 10 MPa 40 50 18 18 σ = 20 MPa σ = 20 MPa 3 3 σ = 30 MPa σ = 30 MPa σ = 40 MPa σ = 40 MPa 40 14 35 12 30 10 25 8 6 15 4 0 0 05 10 15 20 25 0123 45 678 ps -3 ps -3 ε /10 ε /10 (a)Sandy mudstone (b) Mudstone ps Fig. 11 Curves of the relationship between c′, φ′, and ε under different confining pressures 1 3 19 Page 10 of 23 S. Xie et al. circumferential pressures c', φ' relationship curve with the Ap Apply ply di diffe ffer re en nt t co conf nfinin ining g pr pres essu sure res s ps ε . By calculating the values of c and φ of the sandy mud- ps stone and the mudstone for 8 groups of different ε , the rela- tionship curve was obtained, shown in Fig. 11. Under differ - St Stan anda dard rd tr tria iaxi xial al ent confining pressures, the values of the cohesive force c of the two types of rocks gradually decreased with an increase ps in the strain-softening parameter ε , whereas the value of φ remained unchanged during the strain-softening process. The linear equation was fitted by the downward trend of the Ho Hoop op st stre ress ss po poin inti ting ng to towa ward rds s 50 50 mm mm th the e ce cent nter er of of th the e ci circ rcle le two types of rocks c and the average gradient of the decrease was obtained (Table 2). Fig. 12 Model of FLAC 3D triaxial test 3.5.4 FLAC 3D triaxial test simulation right sides of the roadway. The front and rear attenuation range was approximately 40 m, and that at the right side was The modified strain-softening model was embedded into the FLAC program to verify the accuracy of the softening model approximately 10 m. In engineering practice, roof falling disasters occur frequently at the intersection of roadways described above. The standard triaxial test model shown was established in FLAC 3D (Fig.  12). By applying different during tunneling, therefore, it is difficult to control large cross-sections of roadways. confining pressures, the stress–strain curves of sandy mud- stone and mudstone based on the above softening model 3.6.2 Analysis of plastic zone were obtained, and the simulation and experimental curves were compared (Fig. 13). The two types of curves were quite The FLAC 3D model of the circumscribed circle of a cross- consistent proving that the softening model can describe the post-peak mechanical properties of the two types of rocks. section of the semi-circular arched roadway at the No. 10 intersection was established. According to the results of 3.6 Analysis of the deformation and force the in-situ stress test, a vertical stress of 18 MPa and hori- zontal stress with the lateral pressure coefficient of 1.67 were 3.6.1 Analysis of the displacement applied to it. As shown in Fig. 15, the plastic zone of the No. 10 cross roadway was 4.20–6.61 m deep into the surround- The displacement of the roof at the intersection was large ing rock. The range of the plastic zone between the roof and shoulder was wide and reduced gradually from the two as compared to that at both the sides of the roadway, espe- cially at the maximum cross-section where it was nearly sides to the bottom corner. The overall shape of the plastic zone was asymmetric because the roadway passed through double; this indicates that the surrounding rock deforma- tion was acute (Fig. 14). The range of displacement is clear the interbedded mudstone and sandy mudstone. The average depth of the plastic zone at the right roof was greater than from the three-dimensional cloud map where the maximum displacement at the roof gradually attenuates from the maxi- that at the left roof. The worst case must be considered in devising a support system; therefore, the effective anchorage mum cross-section of the intersection to the front, rear, and Table 2 Parameters of cohesion reduction trend fitting curve Lithology Confining pressure Straight line equation Correlation coef- Average gradient Average correla- (MPa) ficient R tion coefficient Sandy mudstone 10 y = − 0.7304x + 7.4885 0.8731 − 1.0615 0.8569 20 y = − 1.1686x + 14.358 0.8871 30 y = − 1.2472x + 20.927 0.8368 40 y = − 1.0996x + 32.983 0.8307 Mudstone 10 y = − 0.2513x + 9.606 0.9886 − 0.2658 0.9418 20 y = − 0.2892x + 11.444 0.9709 30 y = − 0.3085x + 15.008 0.9377 40 y = − 0.214x + 15.393 0.87 1 3 100 100 mm mm Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 11 of 23 19 130 Numerical simulation curve 80 Numerical simulation curve 120 Triaxial test curve Triaxial test curve σ =40ΜPa σ =40ΜPa σ =30ΜPa σ =30ΜPa 3 50 σ =20ΜPa σ =20ΜPa 60 3 σ =10ΜPa σ =10ΜPa 0.01 0.02 0.03 0.04 0.05 0.06 0.01 0.02 0.03 0.04 0.05 0.06 Strain Strain (a)Sandy mudstone (b)Mudstone Fig. 13 Fitting graph of numerical simulation and experimental data Displacement/m Displacement/cm 0.0000E+00 -5.0000E-03 -1.0000E-02 Displacement peak zone -1.5000E-02 Large deformation zone -2.0000E-02 -2.5000E-02 -3.0000E-02 -3.5000E-02 -4.0000E-02 -4.5000E-02 -5.0000E-02 -5.5000E-02 No.10 Intersection -6.0000E-02 Maximum cross-section -6.5000E-02 -7.0000E-02 The amount of displacement -7.5000E-02 -8.0000E-02 gradually decreases -8.3912E-02 Fig. 14 Vertical displacement cloud of roadway intersection length of the strengthening support must be greater than the concentration are higher on the side of the passageway near maximum depth of the plastic zone (6.61 m) and mainly, the the roadway intersection. roof and shoulders must be controlled. 4 Research on the integration of bolting 3.6.3 Analysis of the stress and grouting support It is evident from the three-dimensional stress equipotential 4.1 Construction and analysis of thick reinforced surface shown in Fig. 16 that the value of the stress, con- compression arch structure centration coefficient, and range of the roadway intersection are considerably increased. Here, the maximum stress was In view of the characteristics of the large deformation of 42 MPa, which was 2.3 times the original rock stress of the surrounding rock and sizeable range of plastic zones 18 MPa. The stress concentration coefficient at the largest at the No. 10 intersection, and the rapid failure of ordinary section was greater than 2, indicating a strong degree of support schemes, the design of the strengthening support stress concentration. The stress slice of the entire section plan must meet the mechanical properties that can effec- of the crossing roadway shows that the stress concentration tively deep anchor and reinforce the plastic zone. A strength- area ranges from 5.65–6.85 m; the range and degree of stress ening support scheme with hollow grouting anchor cables 1 3 Stress/MPa Stress/MPa 19 Page 12 of 23 S. Xie et al. γΗ Original stress zone Plastic failure zone Asymmetric failure zone of the crossing layer roadway 6.39 m Maximum cross-section 6.57 m 6.61 m λγΗ λγΗ Circumscribed circle No.10 Intersection γΗ Fig. 15 Cloud map of the plastic zone at the No. 10 intersection Peak stress zone Gradually increasing Peak stress zone cross-section No.10 Intersection Stress concentration zone Stress concentration zone Gradually increasing stress concentration zone Horizontal slice Variable section roadway Fig. 16 Cloud map of the stress at the No. 10 intersection combined with high-elongation anchor cables was designed, hollow grouting cable bolt, respectively, and P is the resultant after an analysis of existing support methods, and is shown force. The relationship between them is given by in Fig. 17a. Deep-hole grouting with grouting anchor cables was used to fill the cracks and consolidate the broken rock P = ⎪ L ⋅ W c c mass, thus changing its mechanical properties and improv- (4) ing its integrity (Kang et al. 2014; Li et al. 2006); the closed P = L ⋅ W cracks and pores that could not be filled were compressed h h under the action of pressure. This correspondingly increased where Q and Q are the drawing forces of the high elon- c h the deformability of rock mass. The rock mass played a sig- gation cable bolt and hollow grouting cable bolt, respec- nificant role in compaction as it provided a reliable founda- tively, and L , W , and L , W are the row and line spaces, c c h h tion for the anchor cables and built a thick-layered reinforced respectively. compression arch with a high bearing capacity. The strength According to the mechanical properties of the weaker sur- and stability of the thick-layer reinforced compression arch rounding rock, the supporting rock mass follows the quad- bearing structure are analyzed below: ratic parabolic Mohr criterion (Li et al. 2006): The structural mechanics model of the thick-layered rein- forced compression arch is shown in Fig. 17b. P and P are c h = n  + (5) the restraining resistances of the high elongation cable bolt and 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 13 of 23 19 19 steel strands Thick reinforced compression arch structure Anchorage device Bearing plate Hole packer P Steel strand PP Grouting z Full-length anchorage port Stopping plug End resin anchorage dc dθ Grout outlet F F 0 0 (a) Schematic of hollow grouting cable bolt and high-elongationcable bolt (b) Mechanical model Fig. 17 Construction of thick reinforced compression arch structure where τ is the shear strength of the supporting rock mass, σ m dc = R + d (10) is the tensile strength, and n is an undetermined coefficient. 2 Under the uniaxial compression test, n can be obtained where dc is the differential length unit of the outer arc of the by using the following formula (Li et al. 2006): compression arch, R is the radius of the crossing roadway, m is the thickness of the backside compression arch, and dθ is n =  + 2 ± 2   +  (6) c t t c t the angle differential unit of the compression arch along the center of the roadway. where σ is the compressive strength of the supporting rock From Eqs. (9) and (10), the resultant compression arch mass. bearing force F is obtained as follows: The principal stress of the quadratic parabolic envelope is expressed as: 2 2 F = P + n + 2 P +  n b + k m (11) −  = 2n  +  + 4n − n (7) 1 3 1 3 t where k is the increasing slope of radial stress. The stress on the inner wall of the arch structure is gen- As anchoring is performed in fractured rock, the follow- erally equal to the restraining force of the anchor, i.e., ing relationship exists (Yu et al. 2010): = P 3 (8) k = 0 From Eqs. (7) and (8), the relation between the prin- L tan  − l (12) m = cipal stress in the limit state and support resistance is tan obtained as where L is the average length of the bolt (cable), θ is the = P + n + 2 P +  n (9) 1 t control angle of the cable bolt in the supporting rock mass, In the curve operation of Fig. 18, the general value of θ is To calculate the resultant compressive arch bearing force 45° (Yu et al. 2010), and l is the row and line space between F per unit length along the axial direction of the roadway, the supporting bodies. Thus, the expression for F is as the calculation principle diagram shown in the figure above follows: was established, and the following differential equation was obtained as 1 3 19 Page 14 of 23 S. Xie et al. q (KN/m) strength by 1.4 times, making the load-bearing capacity 1.8–2.3 times that of the original support, and the thick- ness of the load-bearing structure formed was increased by qP =+ 0.856 1.712 52.1P ++ 143.275 44.6 1.76 times. Therefore, the construction of a thick-layered reinforced compression arch was achieved theoretically. 4.2 Mechanism of bolting‑grouting integrated Bearing capacity of surrounding rock under stability control strengthening support scheme Anchor-grouting integrated stability control support tech- qP =+ 0.564 1.128 52.1P ++ 143.275 29.392 nology refers to the collaborative implementation of the P (KN) 350400 450500 550600 (1) bolt with the function of “supporting” and “pressure relief”, (2) cable bolt with the function of “control” and “restriction”, and (3) hollow grouting cable bolt with the Fig. 18 Comparison of bearing capacity of compression arch under original and reinforced support schemes function of “strengthening” and “compacting,” in addition to the “filling” and “consolidation” role of the shotcrete support to achieve stability control over the large cross- section roadway at the intersection. The principle of its L tan  − l F = P + n + 2 P +  n (13) action is shown in Fig. 19a. tan In Fig. 19b, the hollow grouting cable bolt technology is shown where the end anchors are changed to full-length The circular thick-layered compression arch built on the anchors to improve the rigidity and shear resistance of roadway is affected by the uniformly distributed load q of the supporting system. Through grouting, the fractured the deep surrounding rock. Under the action of the total sup- surrounding rock was provided with high-stress radial port resistance P, the hoop axial force F produced by the restraint, so that the fractured rock mass could exert its compression arch is expressed as follows: stress-strengthening characteristics and provide a reli- able foundation for the anchor cable. The formula for the surrounding rock reinforcement theory was derived as 2F − q sin  ⋅ dc = 0 (14) follows: Uniaxial compressive strength of broken surrounding where θ is the angle between the differential element and the rock σ is given by coordinate axis, solving the equation, we get 2c ⋅ cos (17) F = R + q c (15) 1 − sin After the process of grouting in the broken surrounding For the compression arch strength, the total bearing force rock, there was an improvement in the cohesion c, inter- F must be greater than the hoop axial force F (i.e., F ≥ F ) 0 0 nal friction angle φ, and elastic modulus E. The uniaxial to ensure stability of the structural load. When F = F , the compressive strength σ is given by compression arch is in the limit of the equilibrium state and the solution is given by � 2(c +Δc) ⋅ cos ( +Δ) (18) 1 − sin ( +Δ) 2 L tan  − l P + n + 2 P +  n 0 t (16) where Δc and Δφ are the respective increments in the cohe- q = tan  (2R + m) sion and internal friction angle, respectively. According to the Heok-Brown guidelines (Eberhardt 2012), Without considering the change in the mechanical parameters of the grouting reinforcement surrounding =  + m  + s (19) 1 3 c 3 rock, the measured surrounding rock parameters were sub- stituted in Eq. (16) to obtain a comparison of the bearing where, m and s are constants for evaluating the rock proper- capacity of the compression arch formed by the original ties and integrity. and strengthened support schemes shown in Fig. 18. It can When σ = 0, the uniaxial tensile strength σ of the rock 1 t be seen that the thick-layered reinforced compression arch mass is obtained as formed by the reinforced support increased the support 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 15 of 23 19 High pressure deep borehole Thick reinforced Grout outlet grouting reinforcement compression arch structure Cement mortar Grouting cable bolt Broken rock Hole packer Cable bolt Anchorage device Bolt Penetrative cranny Hollow grouting cable bolt Bearing plate Shallow surrounding rock Semicircular arched reinforced by bolts Metal mesh and roadway section shotcrete layer (a) Schematic of anchor-grouting integrated stability control (b)Schematic of deep hole grouting. Fig. 19 Principle of the role of anchor-grouting integrated stable control support � � 1 As shown by the strength curve in Table 3, Before the =  m − m + 4s (20) t c implementation of the project, the strength of rock mass specimens before and after grouting was tested in the With the increase in the uniaxial compressive strength laboratory, and the average result was that the c of rock of the broken surrounding rock after grouting, the tensile mass increased by 2.28 MPa, an increase of 85%, and φ strength σ of the strengthened surrounding rock is obtained increased by 11°, an increase of 37%. The parameters of as the mechanical properties of the surrounding rock were � � improved after grouting, and the values of c, φ, and E of the 2 � m − m + 4s (c +Δc) ⋅ cos ( +Δ) fissure surrounding rock increased by 66%–225%, 4°–22°, (21) 1 − sin ( +Δ) and 14%–61%, respectively (Wang et al. 2019). The analy- sis showed that the changes in the mechanical parameters c where m′ and s′ are the rock evaluation constants after and φ after grouting increased the values of σ and σ sig- c t anchoring. nificantly. From Eq. (21) it is seen that the change in the Table 3 Change in mechanical parameters of surrounding rock after grouting Strength curve of surrounding rock before and after grouting Mechanical parameters Original Parameter The increasing rate parameter increase, Δ of tensile strength values (%) Cohesion c′ 2.69 MPa 1.0 37 1.4 52 2.0 74 2.7 100 Internal friction angle φ′ 30° 5 11 10 24 15 39 30 115 1 3 19 Page 16 of 23 S. Xie et al. values of m and s after anchoring did not produce an obvi- 4.3.1 New high‑strength hollow grouting cable bolts ous increase in σ ; however, when the values of c and φ increase in the same proportion, the rate of increase of σ is A new type of hollow grouting cable bolt made of high- different. As shown in the above table, when the increase strength spiral rib prestressed steel wire was selected for ratio is small, the influence of Δc on σ is greater, and when the design. Its structure and advantages of performance are the increase ratio gradually increases, the influence of Δφ shown in Fig. 21. According to previous tests, it was found becomes dominant. that the anchoring strength increased by 15%–20%, anchor- ing ductility increased by approximately 25%, and the high- 4.3 Design of strengthening support scheme pressure grouting pressure could reach 8 MPa as compared with ordinary grouting anchor cables. In practice, the actual Based on the above field test and theoretical analysis, the anchoring force increased by two to three times to achieve design strengthening support scheme of anchor network high-strength anchoring. cable spray + hollow grouting cable bolt support method was adopted (Fig. 20) to ensure long-term stability of the sur- 4.3.2 New modified cement grouting materials rounding rock of the large-section chamber in the deep well. The specific support content was as follows: (1) The large As shown in Fig.  22 there are many disadvantages of deformed roadway under the original support was expanded cement paste in practical application, we can see that and cleaned on the whole to meet the design requirements cement paste has many disadvantages when it is origi- of the original Sect. (2) In accordance with the character- nally used to strengthen surrounding rock grouting, and istics of a large cross-section and gradual cross-section at the intersection of the roadway, the design used high elon- Newhigh-strength hollow 8spiralrib steelwires grouting cablebolt gation cable bolts and hollow grouting cable bolts on the semicircular section of the roadway to be laid alternately at a • 2.47 times higher anchoring • New hollowstructure with its force compared to smooth own a grouting core tube 800 mm interval of the original design plan. This is because steelstrand • Reverse grouting to ensure the original supporting borehole damaged the integrity of • Equal cross-section of steel slurry fillsthe borehole wire with essentially no loss the surrounding rock and cracks around the borehole were of cross-sectionalarea • Immediateload-bearing after installation,suitable for support developed, which was a key area for grouting strengthen- • Significantly lower prestress of poorly stable surrounding transfer length,suitable for rock ing. (3) High-strength bolts were used at the bottom corners applying high prestress • Threaded locking method, of both sides of the roadway to strengthen the surrounding • Low relaxation values, 1.2 reliable lockingmechanism, to 1.8%loss/KH at 70% suitable for wetand dripping stress rock at the bottom corner. (4) In Fig. 20, multiple support underground environment High-pressure grouting core tube methods combine to construct a thick-layered reinforced • Steel wire specimens retain • The cable structuremeets the a yield ratio of over 83% requirements of high pressure after fatigue compression arch of over 7 m that effectively achieves roof grouting control and reinforcement on both sides of the roadway. Fig. 21 New hollow grouting cable bolt made of spiral rib steel wire High elongation cable bolt: SKP22-1 19/1860-8300 Row & line space: 1600 800 mm Form a thick reinforced compression arch structure over 7 m Hollow grouting cable bolt: SKZ29-1/1770-9300 Metal mesh : Φ 6.5 mm steel bar External dimensions: 1000 2000 mm Row & line space: 1600 800 mm Mesh Size: 100 100 mm Bearing plates are connected by Φ14 double reinforcement ladder beams 6446 mm Roadway centerline High-strenthened resin bolt: Thickness of shotcrete support: 120 mm MSGLW500/22 2400 10 9891 mm Concrete grade: C20 Row & line space: 800 800 mm Type I water ditch Fig. 20 Schematic of strengthening support scheme for the No. 10 intersection 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 17 of 23 19 New modified cement Ordinary cement paste ACZ-I cement grouting additive grouting material • Water-cement ratio is • Early-strength high- too high so that the strength the compressive strength is reduced strength of the stone body is Superplasticizer increased by 5 and 2.5 times • Poor fluidity and high at 3 and 28 d respectively Micro expansive grouting resistance agent • High fluidity performance 8% • Cement has a large the slurry can fill the entire particle size, making it gap between the cable body difficult for the grout to and the surrounding rock be injected into small cracks • Micro expansive agent can Early strength agent compensate for shrinkage • The process of cement hardened generates and offset the tensile stress micro-cracks caused by cement hardening • Difficulty in controlling • Adjustable cement setting 525 ordinary portland cement time slurry setting time Disadvantages Advantages The water-cement ratio is 0.5: 1 Fig. 22 Schematic of the advantages of modified grouting material then using it combined with hollow grouting cable bolt 4.4 Simulation analysis of supporting prestress will undoubtedly magnify these shortcomings. Therefore, field a new type of grouting material which can overcome its defects should be considered to match the use of hollow 4.4.1 Support model and surrounding rock compressive grouting cable bolt. Figure 22 shows a new type of modi- stress field fied cement grouting material containing ACZ-I additives that contributes significantly to reducing water, plasti- According to the strengthened support plan, the bolt-and- cizing, strengthening, and micro-expansion of cement cable support system was simulated in FLAC 3D, as shown materials when grouting and strengthening surrounding in Fig. 23, to form the surrounding rock compressive stress rocks, thereby overcoming the current problems of high field (Xie et al. 2018). On examining the stress slices on the water-cement ratio, low strength, hardening shrinkage, compressive stress field at the intersection, it was observed that and large pumping resistance of cement slurries. Accord- after installation of the end of the anchor cable, a pressure of ing to preliminary field testing, the pulling force of the 0.02–3.00 MPa was applied to the surrounding rock of the free hollow grouting cable bolt utilizing the modified cement section and the thickness of the compressed surrounding rock slurry was more than twice that of ordinary paste, as was 7.24 m. From the stress cloud diagram, it is seen that the shown in Table 4. stresses of each anchor cable were cemented with each other to form a complete stress arch that improved the integrity of the surrounding rock to a great extent. 4.3.3 Selection of bolt (cable) parameters for strengthening support plan 4.4.2 Prestressed field model of the integral support The new high-strength hollow grouting cable bolt + high Figure  24 shows the comparison of pre-stress field elongation cable bolt + high-strength resin bolt were used between original support and reinforced support. Accord- in the support design, as indicated in the Table 5 below. ing to the prestress field formed by the original support scheme in the surrounding rock, it can be seen that the stress structure of the compressed arch can not be formed, Table 4 Test table of pull- Grouting material Cement paste New modified grouting material out force of cable bolt under different grouting materials Specimen number 1 2 3 4 5 6 Pull-out force (kN) 52 47 53 108 105 103 1 3 19 Page 18 of 23 S. Xie et al. Table 5 Parameters of anchor bolt (cable) for strengthening support Parameter Hollow grouting cable bolt High elongation cable bolt High-strengthened resin bolt Model SKZ29-1/1770–9300 SKP22-1 × 19/1860–8300 MSGLW500/22 × 2400 Diameter (mm) 29 22 22 Length (m) 9.3 8.3 2.4 Tensile strength (MPa) 1770 1860 500 Breaking force (kN) 600 582 255 Pre-tightening force (kN) 150 200 120 Elongation 4.5% 7.0% 20.0% Row & line space (mm) 1600 × 800 1600 × 800 800 × 800 Bearing plate (mm) δ 20 × 300 × 300 δ 20 × 300 × 300 δ 10 × 150 × 150 Resin anchorage agent MSK2850 × 1, MSZ2850 × 3 MSK2335 × 1, MSZ2360 × 2 MSK2335 × 1, MSZ2360 × 1 End anchorage length (m) 2.0 1.55 0.95 Others Grouting pressure: ≥ 5.0 MPa Hollow grouting cable bolt Strengthening support scheme End resin anchorage Cable axial force/Pa in the early stage Stress/Pa High elongation cable bolt Stress field boundary Maximum cross-section Intersection 10 Maximum cross-section Variable section roadway Compressive stress field High-strengthened of surrounding rock resin bolt Fig. 23 Roadway supporting structure and surrounding rock compressive stress field and there is a discontinuity between the stresses, thus 5 Evaluation of effect of strengthening becoming discontinuous. In the cloud picture of the support strengthening support scheme, the stress is continuous and complete and forms a thick compressive stress arch 5.1 On‑site implementation effect larger than 0.02 MPa, which can act on the surrounding rock mass at a depth of more than 10 m, making the sur- Figure 26 shows an illustration of the field effect of the rounding rock give full play to its self-bearing capacity, bolt-grouting support for the No. 10 crossing roadway at which is enough to realize the effectiveness of support the bottom of the air intake shaft depot in the Nanfeng and structural stability. working area. After completion of the construction of the support, its functioning was under observation and the deformation was monitored for 60 days. It was found that 4.5 Construction process of strengthening support the surrounding rock of the roadway was grouted tightly with good integrity, and there was no surface fragmenta- As shown in Fig. 25 of the construction technology, the tion, cracking of shotcrete layer, or slag peeling. hollow grouting anchor cable construction involved more steps of installing stop plugs and grouting. When the hole or roof is broken during the drilling construction, the 5.2 Deformation monitoring analysis broken body must be put down and the drilling position cleared to facilitate the sealing and control of the drilling The convergence deformation of the surrounding rock at depth. Drilling an anchor hole must be carried out in the the No. 10 intersection was measured and the measure- middle of the roof first and then the two sides from top ment curve is shown in Fig. 27. As shown in the following to bottom. 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 19 of 23 19 120KN Discontinuity Stress/Pa Pre-stress field boundary Pressurized area 100 KN Previous support plan 200 KN 150 KN Pre-stress field Unable to form a complete structure 100 KN Strengthening support plan Surrounding rock Stress/MPa The thick reinforced Stress/Pa anchored by anchor cables 10.2 m compression pre-stress arch Maximum cross-section Affected Pre-stress field 12.5 m surrounding rock Three-dimensional model of supporting pre-stress field Minimum Variable section cross-section roadway High-strengthened Pre-stress field boundary resin bolt Fig. 24 Comparison of pre-stress field between original support and reinforced support figure, the maximum convergence of the roadway is 0.4 m after carrying out the support scheme, there were cracks in within 3 months after the implementation of the original the surrounding rock (0–0.4 m) in the hole. The 0.4–7.4 m support scheme, and the roof subsidence rate is fast, which rock layer was grouted and modified with good integrity, and has affected the normal service of the roadway. After the width of the longitudinal crack was 7.4–8.3 m. It became strengthening the support, the convergence of the sur- smaller and there were no obvious transverse cracks. The rounding rock surface of the roadway is obviously small, 8.3–13.4 m rock layer was fairly complete, which showed the deformation degree of the two sides is the same as that the designed anchor-grouting integrated stability control that of the floor, and the roof is the smallest. As compared support technology could (1) effectively control the conver - with later monitoring data, the convergence deformation gence deformation of large soft rock roadway intersection of the surrounding rock was faster and the rate of conver- in deep complex geological conditions, (2) notably reduce gence was quite high in the first 10 days; however, it was the roadway deformation, (3) greatly extend the period of not more than 1.5 mm/day. The rate of convergence then roadway stability, (4) ensure adequate maintenance effect, decreased rapidly and was approximately 0.05 mm/day for and (5) generate immense economic and social benefits. 40 days. This shows that the support form and parameters controlled the convergent deformation of the chamber effectively and maintained a relatively stable state.6 Conclusions 5.3 Evaluation of overall effect In this study, the No. 10 intersection roadway was the sub- ject under investigation to determine the efficacy of support As shown in Fig.  28, the surrounding rock at the No. 10 schemes of large-section intersection roadways surrounded intersection was assessed by drilling to evaluate the effect by broken rock in deep formations. After on-site monitoring, of the support scheme as a whole. The results showed that 1 3 19 Page 20 of 23 S. Xie et al. Construction Construction plan of anchor-grouting integrated support technology process High-strengthened resin bolt High elongation cable bolt Hollow grouting cable bolt Drilling MSGW-500/22×2400 SKP22-1×19/1860 SKZ29-1/1770-9300 Row & line space:800×800 mm Row & line space:1600×800 mm Row & line space:1600×800 mm Anchor installation Bearing plate δ:10×150×150 mm Bearing plate δ:20×300×300 mm Bearing plate δ:20 ×300×300 mm Applying Pre-tightening force 120 KN Pre-tightening force 200 KN Pre-tightening force 150 KN preload The grouting slurry is cement slurry with a water-cement ratio of 0.5:1, and ACZ-1 grouting additive with Grouting 8% cement weight is added to the slurry. Mounting the Φ14 double reinforced ladder beams are used to connect the bearing plates horizontally and vertically, the ladder beam, ladder beam has two sizes of 1.0 m and 1.8 m. The lap length of the metal mesh grid is not less than 100 mm, metal mesh spaced not more than 100 mm, and 16 gauge lead wire is used to tie the metal mesh. Sprayed The thickness of the shotcrete support is 120 mm, and the concrete grade is C20 concrete Displacement Convergence meter and roof abscission layer instrumentare used to measure roadway deformationand the monitoring subsidence of the roof of the roadway Fig. 25 Flow chart of construction of strengthening support High elongationcable bolt Hollow groutingcable bolt Sandymudstone Mudstone 2.75 m 11.10 m Reinforcementsupport Mudstone ZDK938/5/20track switch Largecross -section 5.15 m Sandymudstone intersection roadway 12.95 m No.10 roadway Maximumcross -section intersection Track intersection Mudstone 1.80 m Parkingchamber Passageway No.3 coal seam 5.80 m Passageway Type Iwater ditch High-strengthenedresin bolt Fig. 26 Effect of strengthening support on site theoretical analysis, numerical simulation, and construction plastic failure zone of more than 6 m, serious destruc- tests, the following conclusions were drawn: tion of roof, and high tectonic stress of nearly 30 MPa. (2) According to the simulation of the strain-softening (1) With the linked reaction of variable cross-sections characteristics of the surrounding rock by FLAC 3D, having a maximum width of 9.9 m, many surround- the deflection of the intersection roadway increased ing chambers, layout under a condition of deep high with an increase in the size of the cross-section and stress, soft surrounding rock, adjacent faults, and cross- the roof displacement nearly doubled in the frontal area stratification, the intersection roadway exhibited four as compared to that in the normal area. The plastic zone features, namely loose and broken surrounding rock, was deep with asymmetric distribution and had a maxi- 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 21 of 23 19 Fig. 27 Comparison of deformation convergence between original support and reinforced support roadway 0.4 m7.4 m8.3 m 13.4 m Good integrity Fissure development Complete rock structure Microcracks Grouting material Shotcrete Fig. 28 Sketch drawing of borehole detection of surrounding rock after strengthening support mum depth up to 6.61 m. It was mainly noticeable in proportion of increase was low; when the proportion the top and shoulder areas and disappeared gradually of increase rose gradually, the influence of Δ φ became in the sidewall and connection between the sidewall the leading cause of increase in σ . and bottom. The stress concentration factor and range (5) By using a modified cement grouting material and increased as compared to other areas of the roadway; high-strength hollow grouting cable bolt, the drawing they peaked at the frontal area, and the range of stress force increased to more than twice that of the cement concentration at the side of the intersection roadway paste; this enhanced the extent of filling of cracks and close to the passageway was wider and higher. mechanical properties of the surrounding rock signifi- (3) In accordance with the compression arch mechanical cantly. Based on the supporting stress field, the sur - model based on the parabolic Mohr strength theory, rounding rock (7.24 m) was compacted and strength- hollow grouting cable bolt and high elongation grout- ened, providing a solid foundation for the attachment ing cable bolt were utilized to build a thick strength- of the bolt (cable) support. According to the results of ened compression arch more than 7 m high having a field monitoring, the convergence of the gallery was bearing capacity greater by 1.8 to 2.3 times and thick- less than 30 mm in 60 days, indicating that stability ness of the bearing structure increased by 1.76 times as control of the gallery was achieved effectively. compared to that of the original support scheme. Thus, the construction of a large-depth, high-intensity bear- Acknowledgements This work was financially supported by the ing structure of the surrounding rock was accomplished National Natural Science Foundation of China (Grant Nos. 52074296, successfully. 52004286), the China Postdoctoral Science Foundation (Grant Nos. (4) According to laboratory tests, the c and φ of rock mass 2020T130701, 2019M650895). increased by 85% and 37% respectively after grouting. It can be calculated by the formula, an increase in the same proportion of caused a different increasing rate of σ , and Δc influenced σ to a greater extent when the t t 1 3 19 Page 22 of 23 S. Xie et al. Li CG, Ge XR, Zheng H, Wang SL (2006) Two-parameter parabolic Declarations Mohr strength criterion and its damage regularity. Key Eng Mater 306–308:327–332 Ethical approval The experiments comply with the current laws of Li XF, Wang SB, Malekian R, Hao SQ, Li ZX (2016) Numerical simu- China. lation of rock breakage modes under confining pressures in deep mining: an experimental investigation. IEEE Access 4:5710–5720 Conflict of interest The authors declare that they do not have any com- Li G, Ma FS, Guo J, Zhao HJ, Liu G (2020) Study on deformation fail- mercial or associative interest that represents a conflict of interest in ure mechanism and support technology of deep soft rock roadway. connection with the work submitted. Eng Geol 264:105262 Liu CR (2011) Distribution laws of in-situ stress in deep underground coal mines. Procedia Eng 26:909–917 Open Access This article is licensed under a Creative Commons Attri- Lu YL, Wang LG, Yang F, Li YJ, Chen HM (2010) Post-peak strain bution 4.0 International License, which permits use, sharing, adapta- softening mechanical properties of weak rock. Chin J Rock Mech tion, distribution and reproduction in any medium or format, as long Eng 29(3):640–648 ((in Chinese)) as you give appropriate credit to the original author(s) and the source, Pan R, Wang Q, Jiang B, Li SC, Sun HB, Qin Q, Yu HC, Lu W (2017) provide a link to the Creative Commons licence, and indicate if changes Failure of bolt support and experimental study on the parameters were made. The images or other third party material in this article are of bolt-grouting for supporting the roadways in deep coal seam. included in the article's Creative Commons licence, unless indicated Eng Fail Anal 80:218–233 otherwise in a credit line to the material. If material is not included in Peng R, Meng X, Zhao G, Li Y, Zhu J (2018) Experimental research the article's Creative Commons licence and your intended use is not on the structural instability mechanism and the effect of multi- permitted by statutory regulation or exceeds the permitted use, you will echelon support of deep roadways in a kilometre-deep well. PLoS need to obtain permission directly from the copyright holder. To view a One 13(2):e0192470 copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . Ranjith PG, Zhao J, Ju MH, De Silva Radhika VS, Rathnaweera TD, Bandara AKMS (2017) Opportunities and challenges in deep min- ing: a brief review. Engineering 3(4):546–551 Shreedharan S, Kulatilake PHSW (2015) Discontinuum–equivalent continuum analysis of the stability of tunnels in a deep coal References mine using the distinct element method. Rock Mech Rock Eng 49(5):1903–1922 Alonso E, Alejano LR, Varas F, Fdez-Manin G, Carranza-Torres Tan YL, Fan DY, Liu XS, Song SL, Li XF, Wang HL (2019) Numeri- C (2003) Ground response curves for rock masses exhibiting cal investigation of failure evolution for the surrounding rock of strain-softening behaviour. Int J Numer Anal Methods Geomech a super-large section chamber group in a deep coal mine. Energy 27(13):1153–1185 Sci Eng 7(6):3124–3146 Cai MF, Brown ET (2017) Challenges in the mining and utilization of Tian ZC, Tang CA, Liu YJ, Tang YB (2020) Zonal disintegration test deep mineral resources. Engineering 3(4):432–433 of deep tunnel under plane strain conditions. Int J Coal Sci Tech- Chen XJ, Li LY, Wang L, Qi LL (2019) The current situation and pre- nol 7(2):337–349. https:// doi. org/ 10. 1007/ s40789- 020- 00319-y vention and control countermeasures for typical dynamic disasters Wagner H (2019) Deep mining: a rock engineering challenge. Eng in kilometer-deep mines in China. Saf Sci 115:229–236 Mech Rock Eng 52(5):1417–1446 Eberhardt E (2012) The Hoek-Brown failure criterion. Rock Mech Wang MY, Zhang N, Li J, Ma LJ, Fan PX (2015) Computational Rock Eng 45(6):981–988 method of large deformation and its application in deep mining Fairhurst C (2017) Some challenges of deep mining. Engineering tunnel. Tunn Undergr Space Technol 50:47–53 3(4):527–537 Wang Q, Pan R, Jiang B, Li SC, He MC, Sun HB, Wang L, Qin Q, Yu Huang X, Liu QS, Liu B, Liu KD, Huang SB (2014) Experimental HC, Luan YC (2017) Study on failure mechanism of roadway with research on the mechanical behavior of deep soft surrounding soft rock in deep coal mine and confined concrete support system. rock tunneling by TBM. J China Coal Soc 39(10):1977–1986 ((in Eng Fail Anal 81:155–177 Chinese)) Wang Q, Qin Q, Jiang B, Yu HC, Pan R, Li SC (2019) Study and engi- Huang J, Zhang YM, Zhu LS, Wang T (2016) Numerical simulation neering application on the bolt-grouting reinforcement effect in of rock cutting in deep mining conditions. Int J Rock Mech Min underground engineering with fractured surrounding rock. Tunn Sci 84:80–86 Undergr Space Technol 84:237–247 Huang WP, Yuan Q, Tan YL, Wang J, Liu GL, Qu GL, Li C (2018) An Wang H, Jiang C, Zheng PQ, Li N, Zhan YB (2020) Deformation and innovative support technology employing a concrete-filled steel failure mechanism of surrounding rocks in crossed-roadway and tubular structure for a 1000-m-deep roadway in a high in situ its support strategy. Eng Fail Anal 116:104734 stress field. Tunn Undergr Space Technol 73:26–36 Xie HP, Gao MZ, Zhang R, Peng GY, Wang WY, Li AQ (2018) Kang YS, Liu QS, Gong GQ, Wang HC (2014) Application of a com- Study on the mechanical properties and mechanical response bined support system to the weak floor reinforcement in deep of coal mining at 1000m or deeper. Rock Mech Rock Eng underground coal mine. Int J Min Sci Technol 71:143–150 52(5):1475–1490 Kang HP, Lin J, Fan MJ (2015) Investigation on support pattern of a Xie S, Mingming G, Dongdong C, Yanding,S, Hao P, Hai S, Shizhong coal mine roadway within soft rocks — a case study. Int J Coal L (2018) Stability influence factors analysis and construction of Geol 140:31–40 a deep beam anchorage structure in roadway roof. Int J Min Sci Kawamoto T, Ishizuka Y (1981) An analysis of excavation instrain- Technol 28(3):445-451. https:// doi. org/ 10. 1016/j. ijmst. 2017. 11. softening rock mass. Proc Japan Soc Civ Eng 148:107–118 Lee YK, Pietruszczak S (2008) A new numerical procedure for elasto- Xie SR, Pan H, Zeng JC, Wang E, Chen DD, Zhang T, Peng XJ, Yang plastic analysis of a circular opening excavated in a strain-sof- JH, Chen F, Qiao SX (2019) A case study on control technology of tening rock mass. Tunn Undergr Space Technol 23(5):588–599 surrounding rock of a large section chamber under a 1200-m deep goaf in Xingdong coal mine, China. Eng Fail Anal 104:112–125 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 23 of 23 19 Xue D, Zhou J, Liu Y, Gao L (2020) On the excavation-induced stress Zhang F, Sheng Q, Zhu ZQ, Zhang YH (2008) Study on post-peak drop in damaged coal considering a coupled yield and failure cri- mechanical beha viour and strain-softening model of three gorges terion. Int J Coal Sci Technol 7(1):58–67 granite. Chin J Rock Mech Eng 27(S1):2651–2655 ((in Chinese)) Yang SQ, Chen M, Jing HW, Chen KF, Meng B (2017) A case study on Zhang HQ, Miao XX, Zhang GM, Wu Y, Chen YL (2017) Non- large deformation failure mechanism of deep soft rock roadway in destructive testing and pre-warning analysis on the quality of Xin’An coal mine, China. Eng Geol 217:89–101 bolt support in deep roadways of mining districts. Int J Min Sci Yao YM, Neithalath N, Mobasher B (2018) Analysis and design pro- Technol 27(6):989–998 cedures for strain hardening flexural beam and panel. RILEM Zhang JP, Liu LM, Cao JZ, Yan X, Zhang FT (2018) Mechanism and Bookseries 15:518–526 application of concrete-filled steel tubular support in deep and Yu WJ, Gao Q, Zhu CQ (2010) Study of strength theory and applica- high stress roadway. Constr Build Mater 186:233–246 tion of overlap arch bearing body for deep soft surrounding rock. Zhang CY, Pu CZ, Cao RH, Jiang TT, Huang G (2019) The stability Chinese J Rock Mech Eng 29(10):2134–2142 ((in Chinese)) and roof-support optimization of roadways passing through unfa- Yu KP, Ren FY, Puscasu R, Lin P, Meng QG (2020) Optimization vorable geological bodies using advanced detection and monitor- of combined support in soft-rock roadway. Tunn Undergr Space ing methods, among others, in the Sanmenxia Bauxite Mine in Technol 103:103502 China’s Henan Province. Bull Eng Geol Environ 78(7):5087–5099 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png International Journal of Coal Science & Technology Springer Journals

Failure analysis and control technology of intersections of large-scale variable cross-section roadways in deep soft rock

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Springer Journals
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10.1007/s40789-022-00479-z
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Abstract

In deep underground mining, achieving stable support for roadways along with long service life is critical and the complex geological environment at such depths frequently presents a major challenge. Owing to the coupling action of multiple factors such as deep high stress, adjacent faults, cross-layer design, weak lithology, broken surrounding rock, variable cross-sections, wide sections up to 9.9 m, and clusters of nearby chambers, there was severe deformation and breakdown in the No. 10 intersection of the roadway of large-scale variable cross-section at the − 760 m level in a coal mine. As there are insufficient examples in engineering methods pertaining to the geological environment described above, the numerical calculation model was oversimplified and support theory underdeveloped; therefore, it is imperative to develop an effective support system for the stability and sustenance of deep roadways. In this study, a quantitative analysis of the geological environment of the roadway through field observations, borehole-scoping, and ground stress testing is carried out to establish the FLAC 3D variable cross-section crossing roadway model. This model is combined with the strain softening constitutive (surrounding rock) and Mohr–Coulomb constitutive (other deep rock formations) models to construct a compression arch mechanical model for deep soft rock, based on the quadratic parabolic Mohr criterion. An integrated control technology of bolting and grouting that is mainly composed of a high-strength hollow grouting cable bolt equipped with modified cement grouting materials and a high-elongation cable bolt is developed by analyzing the strengthening properties of the surrounding rock before and after bolting, based on the Heok-Brown criterion. As a result of on-site practice, the following conclusions are drawn: (1) The plastic zone of the roof of the cross roadway is approximately 6 m deep in this environment, the tectonic stress is nearly 30 MPa, and the surrounding rock is severely fractured. (2) The deformation of the roadway progressively increases from small to large cross-sections, almost doubling at the largest cross-section. The plastic zone is concentrated at the top plate and shoulder and decreases progressively from the two sides to the bottom corner. The range of stress concentration at the sides of the intersection roadway close to the passageway is wider and higher. (3) The 7 m-thick reinforced compression arch constructed under the strengthening support scheme has a bearing capacity enhanced by 1.8 to 2.3 times and increase in thickness of the bearing structure by 1.76 times as compared to the original scheme. (4) The increase in the mechanical parameters c and φ of the surrounding rock after anchoring causes a significant increase in σ ; the pulling force of the cable bolt beneath the new grouting material is more than twice that of ordinary cement grout, and according to the test, the sup- porting stress field shows that the 7.24 m surrounding rock is compacted and strengthened in addition to providing a strong foundation for the bolt (cable). On-site monitoring shows that the 60-days convergence is less than 30 mm, indicating that the stability control of the roadway is successful. Keywords Deep soft rock · Variable cross-section · Roadway intersection · Bolting-grouting integration · New grouting material 1 Introduction * Dongdong Chen chendongbcg@163.com With the exhaustion of shallow coal resources, coal min- School of Energy and Mining Engineering, China University ing in east-central China has shifted to deeper realms (Cai of Mining and Technology, Beijing, Beijing 100083, China and Brown 2017; Chen et al. 2019). Deep ground stress Mine Safety Technology Branch of China Coal Research is high, mining has a considerable influence, surrounding Institute, Beijing 100013, China Vol.:(0123456789) 1 3 19 Page 2 of 23 S. Xie et al. rock deformation has great mobility, expansion, and impact; deep roadway support. Wang et al. (2020) utilized ABAQUS hence, adequate support of roadways is becoming increas- to create a finite element model under the original support ingly problematic (Liu 2011; Fairhurst 2017; Xie et  al. design, suggested a zoned bolt-grouting reinforcement tech- 2018). A long service life and high stability are essential nology, and numerically tested its support impact. Using requirements in the development of roadways. The environ- self-developed random non-destructive testing methodolo- ment at great depths is unique owing to the complicated gies and equipment, Zhang et al. (2017) suggested an early geological conditions (Wagner 2019; Ranjith et al. 2017; warning system for the integrity of the roadway envelope Xue et al. 2020). The No. 10 intersection examined in this based on anchor axial load detection. study includes a portion of the roadway of width 9.89 m The research above provided a sound theoretical and engi- with loose and fractured surrounding rock that was seriously neering foundation for controlling surrounding rocks in deep damaged by strong tectonic stress at that depth and traverses roadways; however, a majority of the studies focus on a sin- uneven strata. Consequently, traditional anchor support is gle geological or roadway attribute, such as soft rock, frac- inadequate to withstand the significant deformation damage tured surrounding rock, flooded roadways, or large section that occurs in practice (Pan et al. 2017). chambers, rather than examining the efficient sustenance of Many academics have carried out thorough studies on deep roadways under the influence of many varying factors. the control and design of surrounding rocks to address the In addition, numerical simulations using a single intrinsic challenge of providing an appropriate support system for relationship ignore the difference in mechanics between a deep roadways in complicated geological settings. Tian et al. tunnel envelope and undisturbed rock formations. Moreover, (2020) suggested a support system for deep soft rock sub- the existing amount of research on the support of variable- merged roads based on high-strength anchoring, a high-stiff- section roadways is relatively small, and the numerical mod- ness spraying layer to prevent water, and deep and shallow eling of variable-section roadways under inclined coal rock hole grouting to rebuild the damaged surrounding rock. Xie layers is over-simplified, which affects the accuracy of the et al. (2019) suggested a complete control approach for deep results. The lack of a theoretical model of bolt (cable) sup- large-section chambers such as strong bolt (cable) support, port based on soft rock environment in deep roadway results thick-walled reinforced concrete pouring, and full-section in inappropriate selection of support materials. The present pressure-regulating grouting behind the walls. Kang et al. cement slurry-based grouting material has a large number (2014) developed a novel form of an integrated support sys- of flaws; therefore, it is difficult to ensure grouting action in tem and floor monitoring technique to prevent and manage deep roadways. the weak floor of a deep roadway. Huang, Li, and Zhang The intersection of the − 760 m level No. 10 roadway in et al. utilized a novel steel pipe concrete reinforced support a coal mine is the subject of research in this study, and that successfully suppressed serious deformation of deep the coupling impact of many variables such as deep high roadways (Huang et al. 2018; Li et al. 2020; Zhang et al. stress, adjacent faults and interlayer arrangement, weak 2018). Wang et al. (2017) investigated the damage and con- lithology, fractured surrounding rocks, varied cross-sec- trol mechanisms of deep soft rock roadways and proposed tions, large cross-sections up to 9.9 m wide, and clusters the idea of “high-strength, integrity, and pressure-relief”. of neighboring chambers were examined as the reasons for Yang et al. (2017) used a combination technique of “bolt- its deformation and collapse. Field observation, borehole- cable-mesh-shotcrete + shell” to successfully control the scoping, and in-situ stress testing were used to determine deformation of a deep soft rock roadway. Wang et al. (2015) the geomechanical characteristics of the roadway. The strain- presented a dynamic damage intrinsic model to evaluate the softening features of the surrounding rock in the post-peak elastic rebound and shear expansion deformation of the sur- stage were modeled and studied, and the internal friction rounding rock during the unloading process and addressed angle and cohesive force weakening law of the rock were the pre-peak and post-peak phases in their theory of rock deduced. Curve fitting of the triaxial test was performed damage in deep roadways. Huang and Li et al. performed a using FLAC 3D; the inverted parameters were applied to the numerical simulation of deep rock cutting and fracture pat- FLAC 3D variable cross-section roadway model to achieve terns (Huang et al. 2016; Li et al. 2016). Peng et al. (2018) the coupling of the surrounding rock strain softening and in their study of the structural damage process of deep road- Mohr–Coulomb constitutive model, to effectively analyze ways, reported that horizontal stress had a significant impact the force and deformation characteristics of the roadway on the stability of the surrounding rock and developed a intersection. For support design analysis, a thick reinforced multi-stage support system based on the structural features compression arch mechanical model, based on the quadratic of the roadway bearing. Shreedharan and Kulatilake (2015) parabolic Mohr strength criteria, was developed for the sur- employed the 3DEC discrete element technique to assess rounding conditions of deep soft rock, and the strengthening the stability of a deep coal mine roadway under various sec- support scheme with the action path of "deep hole grout- tions and support bodies in their numerical simulation of a ing and anchoring → reinforcement of broken surrounding 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 3 of 23 19 rock → mutual cementation into the arch → realization of mudstone, thin mudstone, and sandy mudstone interbeds, self-supporting surrounding rock" was proposed, i.e., the where the rock is broken. The overall stability is poor and anchor injection integrated the support technology based on a bedding of V-level unstable rock mass (It refers to bro- a hollow grouting anchor cable equipped with a modified ken soft rock with basic quality index BQ ≤ 250) is devel- grouting material. Simultaneously, the impact of changes in oped that leads to frequent roof fall in roadway excavation. the parameters of the surrounding rock mechanics before and After the implementation of the original support scheme, after bolting and grouting on strength was evaluated using the accidents of large deformation, roof fall and slope still the Heok-Brown criteria, and a supporting prestress field occur in the roadway, and the original support scheme can was constructed to simulate and validate the plan. The road- not effectively control the surrounding rock deformation way was monitored for displacement and borehole-scoping (the original support scheme and failure condition will be following on-site construction, and it was observed that the shown in detail below). A comprehensive histogram of the control effect on the stability of the surrounding rock was strata is shown in Fig. 1. good, providing a theoretical direction and engineering ref- erence for roadway support under the arduous circumstances at great depths.2.2 Engineering characteristics The largest cross-section at the No. 10 intersection is 2 Engineering background formed by the intersection of people and vehicle parking chambers (referred to as parking chambers), and pedes- 2.1 Geological profile trian and vehicle parking chamber passages (referred to as chamber passages). In view of the geological background The mine studied is equipped with a fully mechanized top- of deep high stress, practical conditions of the concen- coal caving face that is mainly used in the No. 3 coal seam trated chamber group, and large cross-section at the inter- of the Shanxi Formation in the Qinshui Coalfield, and the section, a comprehensive site observation of the deforma- designed annual output reaches 3.0 Mt/a. The bottom yard tion due to the stress environment, characteristics of the of the air intake shaft is located under the No. 3 coal seam surrounding rock, cross-section of the intersection, and with a buried depth of approximately 760 m. The strike construction technology yielded the following character- of the coal and rock strata is north high and south low by istics of the project: 12°, and west high and east low by 5°. The rock layers tra- versed by the roadway at the bottom of the shaft are sandy Thickness Rock stratum Lithology description /m Gray, medium to thick laminated, feldspar, quartz Intersection Medium sandstone 8.70 dominant, uniformly laminated, well sorted, well rounded No.1 coal seam 0.35 Mudstone Black coal line 760 m Air shaft 11.10 m Mudstone 5.15 Gray-black, medium-thick laminated, brittle, flat fracture Intersection Sandy mudstone 2.75 Light gray-black, medium-thick laminate, flatter fracture Light gray-black, medium-thick laminate, brittle, shell- Mudstone 11.10 like fracture Sandy sudstone 12.95 Light gray-black, medium-thick lamellar, jagged fracture Gray-black, medium-thick laminated, brittle, shell-like Mudstone 1.80 fracture, containing plant fossils The passageway of Black, bright coal mainly, dark coal second, semi-bright No.3 coal seam 5.80 parking chamber coal No.10 roadway Mudstone 2.35 Dark gray-black, lumpy, flat fracture Intersection intersection 10 Light gray-black, medium-thick laminated, flat fracture Sandy mudstone 5.20 Intersection Gray, thickly laminated, quartz dominant, feldspar secondary, with coal dust, mica, with muddy streaks, Intersection Fine sandstone 3.20 8 Sandy mudstone sorted Intersection 12.95 m Dark gray-black, blocky, brittle, flat fracture, containing Intersection 3 Sandy mudstone 10.45 4 plant fossils Intersection Gray, medium-thick laminated, feldspar, quartz dominated, three vertical fissures of 0.4 m in length Medium sandstone 4.20 developed Intersection 1 Dark gray-black, lumpy, brittle, flat fracture Mudstone 1.55 Intersection Black coal line 6 No.5 coal seam 0.75 Intersection Dark gray-black, lumpy, brittle, flat fracture Mudstone 1.30 Fig. 1 Comprehensive histogram of the stratum at the intersection of the roadway 1 3 Parking chamber 19 Page 4 of 23 S. Xie et al. 2.2.1 Intersections with a concentrated arrangement original rock stress (This is the result of the later numeri- resulting in stress concentration cal calculation, which is shown below). This directly causes severe deformation of the surrounding rock at the intersec- As shown in Figs. 2, 11, roadway intersections connect three tion giving rise to serious cracking of the shotcrete layer in transport roadways near the − 760 m shaft bottom yard in the roadway. the Nanfeng work area, that form a centralized chamber group along with pipes, pedestrian parking chambers, and 2.2.2 Poor surrounding rock lithology owing horsehead gates. The roof mudstone of the chamber group to the proximity of faults and placement is not strong and is rich in clay minerals such as a mixed through layers layer of illite/montmorillonite and kaolinite. This makes the surrounding rock soft, strong-swelling, easily attacked by As shown in Figs. 3, 4, the south side of the No. 10 inter- chemicals, and readily weathered (Kang et al. 2015; Yu et al. section is near the normal fault CF47, where the ground 2020). In this environment, densely distributed intersections stress is dominated by tectonic stress, the surrounding lead to overlapping stresses. At the largest cross-section at rock strength is low, and integrity is poor. Moreover, the the No. 10 intersection, the range of stress concentration is depth of the roadway is large and the surrounding rock of substantial and the peak stress reaches more than twice the the nearby roadway is broken to some extent, making the Central track roadway Main transformer station Air shaft Main roof Shaft station at -760 m level 11.10 m Deep shaft ingate No. 1 No. 4 Deep soft rock chamber group No. 5 No. 3 Immediate roof The passageway of parking chamber 12.95 m No. 2 No. 10 large section False roof Crosscut 1.80 m roadway intersection Parking chamber No. 6 No. 8 Shunting roadway Central belt roadway No.3 coal seam No. 7 Immediate bottom Basic bottom Fig. 2 Location map of all roadway intersections N Crossing layer roadway Intersection No. 12 Intersection No. 8 Parking chamber 23 m Horizontal distance North roadway Permanent refuge chamber Waiting room Crossing the No. 3 coal seam No.3 coal seam CF47 Normal fault 60 70 H: 0 13 m Fig. 3 Stratigraphic profile at the No. 10 intersection 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 5 of 23 19 Superimposed plastic failure a = 19 42'12" zone of roadway intersection 3.5 m Plastic zone boundary R = 20000 K = 6878 a =5 43'55" 6.6 m T = 3473 R =20000 K =2000 T =1001 Plastic failure zone 4.0 m 4.2 m Large deformation of roadway intersection ZDK938/5/20 Right 11 42'36" Track center line 5.5 m 9.65 m Roadway center line Track center line Parking chamber Roadway deformation Fig. 4 Schematic of the plastic zone of each section at the No. 10 intersection support inadequate (Zhang et al. 2019). The intersection 2.2.4 Difficult construction and maintenance of large crosses over the junction of the layers of mudstone and cross‑sectional intersections. sandy mudstone with the bottom of the roadway as the datum, and is almost 0.7–3.0 m from the datum. Therefore, The occurrence of rib spalling and roof falling is frequent in the No. 10 intersection through the layer triggers the plas- the process of digging and excavating; the roadway is poorly tic zone in each part to be at a higher level thus increasing shaped, especially the No. 10 intersection that is 9.9 m wide the volume of the plastic zone, which directly causes the and up to 6.5 m high. Moreover, the shotcrete layer of the surrounding rock to be loose and broken. major support section contains cracks and falling blocks; thus, breaking and falling off of the wall takes place to the extent of different degrees during roadway maintenance. 2.2.3 A large cross‑sectional area of the intersection extending the range of disturbance of the surrounding rock 3 Damage deformation analysis The above picture shows the field measurement results of On account of the field working conditions of the No. 10 plastic failure zone in the early stage of the original sup- intersection, large deformation of the roadway, and broken port design, and the No.10 intersection is a large section surrounding rock, the methods of drilling peek, in-situ stress chamber with a tunneling width of 9.89 m. The rock body measurement, and numerical simulation were used to ana- is in a long-term rheological deformation process; the lyze the surrounding rock plastic zone, roof displacement, plastic zone is more developed and creates a wide range and stress conditions. of superimposed plastic zones around the junction due to the vast burial depth, enormous section, and fractured sur- 3.1 Drilling peek detection rounding rock (Tan et al. 2019). Further, the strength and integrity of the surrounding rock at the intersection are Drilling peeking at intersection 10 is shown in Fig. 5: (1) poor; hence, the stress causes it to reach the plastic yield The surrounding rock from 0–0.4 m was relatively broken condition, leading to plastic flow on both sides of the road- with intense fissures. (2) There was loose destruction of the way, as well as shear yield and tensile failure in the region. surrounding rock at a depth of 0.4–4.2 m, the open fractures The actual damage at the comprehensive site intersection, were concentrated in the shallow fracture development area, under the effect of strong disturbances, results in overall and cracks were developed intensively within a depth of 4 m. deformation and instability, and causes chain damage to (3) There were a large number of fine original fractures in the cavity group in severe cases. the slight crack area of 4.2–6.6 m, that reduced from the 1 3 12 10 80 4 19 Page 6 of 23 S. Xie et al. Complete original rock zone Slightly fractured zone Fracture development zone Fracture zone Plastic zone 6446 mm boundary Intersection 10 2 12 12 10 8 6 4 2 0 0 46 8 10 9891 mm Fig. 5 Schematic of borehole-scoping and zoning failure of the surrounding rock shallow to deep region. (4) The deep surrounding rock from was σ > σ > σ . The maximum principal stress (near the H V h 6.6–12 m was complete with dense rock formations and no horizontal direction) of the measuring point was nearly obvious cracks. (5) The degree of breakage was greater in 30 MPa and the average ratio of the maximum principal the roof and shoulder of the surrounding rock than that at the stress to vertical stress was 1.67, which is a state of high side; hence, roof control was the focus of our study. tectonic stress. The roof and floor control of the roadway support is particularly important in the case of the in-situ 3.2 In‑situ stress measurement stress field dominated by horizontal stress. The roof of the site was soft mudstone with many broken rock blocks, As shown in Fig. 6, an in-situ stress measurement tech- hence, support was difficult. Therefore, modification and nique based on CSIRO cell was applied to the field meas- strengthening of the surrounding rock was the key to form- urements at the No. 10 intersection. The average result of ing a support system. the measured point data showed that the ground stress type KX-81 hollow inclusion YHY16 intrinsic safety triaxial strain gauge strain gauge for mine Surrounding No. 10 large section rock roadway intersection Borehole Deep s mall hole rock core Variable diameter drill bit Direction finder Rock core rate tester for strain gauge Fig. 6 Schematic of in-situ stress measurement 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 7 of 23 19 3.3 Analysis of the failure of the original support 3.4 Establishment of FLAC 3D model for the No. 10 scheme intersection In Fig.  7, the original support scheme consisting of an Based on the engineering geological characteristics of the anchor network cable spray + double-steel-bar ladder beam No. 10 intersection, a FLAC 3D model that conformed to is shown. After the construction, there was large deforma- the actual condition of the site and showed the crossing tion of the roadway section, sinking and cracking of the roof, form of the roadway to the greatest extent was constructed, roof and rib falling, and failure and falling of bolts (cables). and is shown in Fig.  8. The constitutive relationship of The original supporting bolts (Here is the bolt anchored at the model adopted the Mohr–Coulomb criterion and the the end of the resin cartridge) were 2.4 m long, all of which rock formation (mudstone and sandy mudstone) was cou- were in the fracture development concentrated area where pled with the strain-softening constitutive relationship at they were unable to function; ordinary cable bolts (ordinary the intersection point to indicate that the strength of the cable bolt refers to common cable bolt with 7-core steel broken rock was mostly its residual strength. According strands) were 7.6 m long with large spacing and low elon- to the experiment on rock mechanics conducted with the gation. The broken surrounding rock did not allow for an rock core taken from the site, the mechanical parameters effective anchoring foundation, thus the performance of the of the coal and rock strata given in the following Table 1 anchor cable was inadequate and a stable supporting struc- were adopted for the numerical simulation. ture was not formed. Hence, the supporting body failed to exert the self-bearing capacity of the surrounding rock. Normal cable bolt Row & line space: 1600 800 mm High-strengthened resin bolt Rib spalling Partial fracture Row & line space: 800 800 mm Plastic failure zone Fracture zone No.10 roadway intersection Cracking Invalid anchor Fig. 7 Schematic of original support scheme and roadway breakdown and deformation Passageway Intersection 10 Parking chamber No.10Intersection Overburden rock load 65 m Variable section roadway 150 m Fine sandstone 60 m Fig. 8 FLAC 3D numerical model of the No. 10 intersection 1 3 19 Page 8 of 23 S. Xie et al. Table 1 Parameters of coal 3 Rock stratum Average E (GPa) K (GPa) c (MPa) σ (MPa) φ (°) γ (kg/m ) and rock mechanics used in thickness numerical simulation (m) Medium sandstone 5.55 16.73 10.72 5.54 3.58 37.00 2731 Mudstone 7.65 9.66 7.00 2.83 1.19 32.00 2463 Medium sandstone 1.70 15.50 9.94 5.34 3.45 37.00 2713 Mudstone 1.30 11.34 8.22 2.62 1.12 33.00 2466 Medium sandstone 8.70 15.21 9.75 5.46 3.53 37.00 2801 Mudstone 5.50 8.69 6.30 2.34 1.05 30.00 2414 Sandy mudstone 2.75 12.94 8.63 4.78 2.81 35.00 2567 Mudstone 11.10 9.03 4.80 2.69 1.09 30.00 2453 Sandy mudstone 12.95 13.65 9.10 4.92 3.12 36.00 2646 Mudstone 1.80 10.21 7.40 2.54 1.07 32.00 2429 No. 3 coal seam 5.80 5.20 4.33 1.25 0.82 25.00 1423 Mudstone 2.35 9.46 6.86 2.75 1.13 31.00 2457 Sandy mudstone 5.20 12.30 8.20 4.62 2.80 35.00 2549 Fine sandstone 3.20 23.40 13.93 5.87 3.79 38.00 2815 Sandy mudstone 10.45 13.50 9.00 4.75 2.76 35.00 2606 2010), an ideal trilinear strain-softening model curve was 3.5 Simulation of strain‑softening mechanical constructed (Fig. 10) (Kawamoto and Ishizuka 1981). As characteristics shown in Fig. 10 of the simplified model, OA and OB are the pre-peak elastic deformation stages of the rock and 3.5.1 Strain softening mechanical model the secant of the peak point was used as an approximate replacement, where l is the unloading path. After the peak The No. 10 intersection is at the junction of 12.95  m point, the rock enters the strain-softening stage, The slope sandy mudstone and 11.10  m mudstone. The surround- of the post-peak stage of each triaxial test curve is fitted and ing rock is quite broken and the softening characteristics calculated, and an oblique line is obtained to represent the of the post-peak are the main factors affecting the defor - post-peak strain softening stage (Alonso et al. 2003; Lee mation and deterioration of weak rocks. Therefore, based and Pietruszczak 2008), then the average post-peak slope of on the results of the triaxial compression test of the two triaxial test curves with different confining pressure is taken, types of rocks shown in Fig. 9 (Huang et al. 2014; Lu et al. (σ −σ )/MPa 1 3 σ = 40 MPa σ = 40 MPa 3 3 σ = 30 MPa σ = 30 MPa σ = 20 MPa σ = 10 MPa σ = 20 MPa σ = 10 MPa -0.05-0.04 -0.03-0.02 -0.010 0.01 0.02 0.03 0.04 0.05 0.06 ε ε (a)Sandy mudstone (b)Mudstone Fig. 9 Full stress–strain curves of two types of rocks under different confining pressures in triaxial compression tests 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 9 of 23 19 � � � � � � 2 2 2 p p p p p p ps (1) = 2 −  +  +  + 2 − 1 3 1 3 3 1 p p where   and  are the principal plastic strain components. 1 3 Therefore, the subsequent yield surface of the rock after the peak is expressed as ps f  ,  ,  ,  = 0 (2) 1 2 3 where σ is the first principal stress, σ and σ are the sec- 1 2 3 ond and third principal stresses, respectively, and σ = σ . 2 3 It was assumed that the stress state at a point in the post- ps peak strain-softening stage at different ε is in the critical Fig. 10 Post-peak rock strain-softening plastic strain–stress relation- state of strength failure, i.e., the Mohr–Coulomb criterion ship is satisfied. and a unified post-peak slope is obtained (Yao et al. 2018; 1 + sin  1 + sin f =  −  + 2c Zhang et al. 2008). Assuming that the unloading process is (3) 1 3 � � 1 − sin  1 − sin linearly elastic, there is l//OA, and the same plastic deforma- tion is produced along the unloading path l under different where the internal friction angle φ at the critical state and the confining pressures. ps cohesive force c under different ε are calculated inversely, ps and the law of φ and c changing with ε of the two types of 3.5.2 Yield surface of the strain‑softening stage of the rock rocks under study can be obtained simultaneously. Owing to the accumulation of the history of plastic defor- 3.5.3 Internal friction angle and weakening law mation of the rock and randomness of the instantaneous of cohesion stress state, the subsequent yield surface is different from the elastic stage. To record the history of plastic loading of In the strain-softening stage of the rock, the value of the the rock material, the rock was considered to be anisotropic. principal stress difference in the rock gradually decreases ps The strain-softening parameter ε was used as the plastic as the plastic strain increases, and this decreasing trend state variable (Lu et al. 2010; Zhang et al. 2008) given by is reflected in the change of the rock c and φ values. Fig- the following formula: ure 11 is based on the type (2) (3) to obtain the different σ = 10 MPa ϕ′/° c′/MPa ϕ′/° c′/MPa σ = 10 MPa 40 50 18 18 σ = 20 MPa σ = 20 MPa 3 3 σ = 30 MPa σ = 30 MPa σ = 40 MPa σ = 40 MPa 40 14 35 12 30 10 25 8 6 15 4 0 0 05 10 15 20 25 0123 45 678 ps -3 ps -3 ε /10 ε /10 (a)Sandy mudstone (b) Mudstone ps Fig. 11 Curves of the relationship between c′, φ′, and ε under different confining pressures 1 3 19 Page 10 of 23 S. Xie et al. circumferential pressures c', φ' relationship curve with the Ap Apply ply di diffe ffer re en nt t co conf nfinin ining g pr pres essu sure res s ps ε . By calculating the values of c and φ of the sandy mud- ps stone and the mudstone for 8 groups of different ε , the rela- tionship curve was obtained, shown in Fig. 11. Under differ - St Stan anda dard rd tr tria iaxi xial al ent confining pressures, the values of the cohesive force c of the two types of rocks gradually decreased with an increase ps in the strain-softening parameter ε , whereas the value of φ remained unchanged during the strain-softening process. The linear equation was fitted by the downward trend of the Ho Hoop op st stre ress ss po poin inti ting ng to towa ward rds s 50 50 mm mm th the e ce cent nter er of of th the e ci circ rcle le two types of rocks c and the average gradient of the decrease was obtained (Table 2). Fig. 12 Model of FLAC 3D triaxial test 3.5.4 FLAC 3D triaxial test simulation right sides of the roadway. The front and rear attenuation range was approximately 40 m, and that at the right side was The modified strain-softening model was embedded into the FLAC program to verify the accuracy of the softening model approximately 10 m. In engineering practice, roof falling disasters occur frequently at the intersection of roadways described above. The standard triaxial test model shown was established in FLAC 3D (Fig.  12). By applying different during tunneling, therefore, it is difficult to control large cross-sections of roadways. confining pressures, the stress–strain curves of sandy mud- stone and mudstone based on the above softening model 3.6.2 Analysis of plastic zone were obtained, and the simulation and experimental curves were compared (Fig. 13). The two types of curves were quite The FLAC 3D model of the circumscribed circle of a cross- consistent proving that the softening model can describe the post-peak mechanical properties of the two types of rocks. section of the semi-circular arched roadway at the No. 10 intersection was established. According to the results of 3.6 Analysis of the deformation and force the in-situ stress test, a vertical stress of 18 MPa and hori- zontal stress with the lateral pressure coefficient of 1.67 were 3.6.1 Analysis of the displacement applied to it. As shown in Fig. 15, the plastic zone of the No. 10 cross roadway was 4.20–6.61 m deep into the surround- The displacement of the roof at the intersection was large ing rock. The range of the plastic zone between the roof and shoulder was wide and reduced gradually from the two as compared to that at both the sides of the roadway, espe- cially at the maximum cross-section where it was nearly sides to the bottom corner. The overall shape of the plastic zone was asymmetric because the roadway passed through double; this indicates that the surrounding rock deforma- tion was acute (Fig. 14). The range of displacement is clear the interbedded mudstone and sandy mudstone. The average depth of the plastic zone at the right roof was greater than from the three-dimensional cloud map where the maximum displacement at the roof gradually attenuates from the maxi- that at the left roof. The worst case must be considered in devising a support system; therefore, the effective anchorage mum cross-section of the intersection to the front, rear, and Table 2 Parameters of cohesion reduction trend fitting curve Lithology Confining pressure Straight line equation Correlation coef- Average gradient Average correla- (MPa) ficient R tion coefficient Sandy mudstone 10 y = − 0.7304x + 7.4885 0.8731 − 1.0615 0.8569 20 y = − 1.1686x + 14.358 0.8871 30 y = − 1.2472x + 20.927 0.8368 40 y = − 1.0996x + 32.983 0.8307 Mudstone 10 y = − 0.2513x + 9.606 0.9886 − 0.2658 0.9418 20 y = − 0.2892x + 11.444 0.9709 30 y = − 0.3085x + 15.008 0.9377 40 y = − 0.214x + 15.393 0.87 1 3 100 100 mm mm Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 11 of 23 19 130 Numerical simulation curve 80 Numerical simulation curve 120 Triaxial test curve Triaxial test curve σ =40ΜPa σ =40ΜPa σ =30ΜPa σ =30ΜPa 3 50 σ =20ΜPa σ =20ΜPa 60 3 σ =10ΜPa σ =10ΜPa 0.01 0.02 0.03 0.04 0.05 0.06 0.01 0.02 0.03 0.04 0.05 0.06 Strain Strain (a)Sandy mudstone (b)Mudstone Fig. 13 Fitting graph of numerical simulation and experimental data Displacement/m Displacement/cm 0.0000E+00 -5.0000E-03 -1.0000E-02 Displacement peak zone -1.5000E-02 Large deformation zone -2.0000E-02 -2.5000E-02 -3.0000E-02 -3.5000E-02 -4.0000E-02 -4.5000E-02 -5.0000E-02 -5.5000E-02 No.10 Intersection -6.0000E-02 Maximum cross-section -6.5000E-02 -7.0000E-02 The amount of displacement -7.5000E-02 -8.0000E-02 gradually decreases -8.3912E-02 Fig. 14 Vertical displacement cloud of roadway intersection length of the strengthening support must be greater than the concentration are higher on the side of the passageway near maximum depth of the plastic zone (6.61 m) and mainly, the the roadway intersection. roof and shoulders must be controlled. 4 Research on the integration of bolting 3.6.3 Analysis of the stress and grouting support It is evident from the three-dimensional stress equipotential 4.1 Construction and analysis of thick reinforced surface shown in Fig. 16 that the value of the stress, con- compression arch structure centration coefficient, and range of the roadway intersection are considerably increased. Here, the maximum stress was In view of the characteristics of the large deformation of 42 MPa, which was 2.3 times the original rock stress of the surrounding rock and sizeable range of plastic zones 18 MPa. The stress concentration coefficient at the largest at the No. 10 intersection, and the rapid failure of ordinary section was greater than 2, indicating a strong degree of support schemes, the design of the strengthening support stress concentration. The stress slice of the entire section plan must meet the mechanical properties that can effec- of the crossing roadway shows that the stress concentration tively deep anchor and reinforce the plastic zone. A strength- area ranges from 5.65–6.85 m; the range and degree of stress ening support scheme with hollow grouting anchor cables 1 3 Stress/MPa Stress/MPa 19 Page 12 of 23 S. Xie et al. γΗ Original stress zone Plastic failure zone Asymmetric failure zone of the crossing layer roadway 6.39 m Maximum cross-section 6.57 m 6.61 m λγΗ λγΗ Circumscribed circle No.10 Intersection γΗ Fig. 15 Cloud map of the plastic zone at the No. 10 intersection Peak stress zone Gradually increasing Peak stress zone cross-section No.10 Intersection Stress concentration zone Stress concentration zone Gradually increasing stress concentration zone Horizontal slice Variable section roadway Fig. 16 Cloud map of the stress at the No. 10 intersection combined with high-elongation anchor cables was designed, hollow grouting cable bolt, respectively, and P is the resultant after an analysis of existing support methods, and is shown force. The relationship between them is given by in Fig. 17a. Deep-hole grouting with grouting anchor cables was used to fill the cracks and consolidate the broken rock P = ⎪ L ⋅ W c c mass, thus changing its mechanical properties and improv- (4) ing its integrity (Kang et al. 2014; Li et al. 2006); the closed P = L ⋅ W cracks and pores that could not be filled were compressed h h under the action of pressure. This correspondingly increased where Q and Q are the drawing forces of the high elon- c h the deformability of rock mass. The rock mass played a sig- gation cable bolt and hollow grouting cable bolt, respec- nificant role in compaction as it provided a reliable founda- tively, and L , W , and L , W are the row and line spaces, c c h h tion for the anchor cables and built a thick-layered reinforced respectively. compression arch with a high bearing capacity. The strength According to the mechanical properties of the weaker sur- and stability of the thick-layer reinforced compression arch rounding rock, the supporting rock mass follows the quad- bearing structure are analyzed below: ratic parabolic Mohr criterion (Li et al. 2006): The structural mechanics model of the thick-layered rein- forced compression arch is shown in Fig. 17b. P and P are c h = n  + (5) the restraining resistances of the high elongation cable bolt and 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 13 of 23 19 19 steel strands Thick reinforced compression arch structure Anchorage device Bearing plate Hole packer P Steel strand PP Grouting z Full-length anchorage port Stopping plug End resin anchorage dc dθ Grout outlet F F 0 0 (a) Schematic of hollow grouting cable bolt and high-elongationcable bolt (b) Mechanical model Fig. 17 Construction of thick reinforced compression arch structure where τ is the shear strength of the supporting rock mass, σ m dc = R + d (10) is the tensile strength, and n is an undetermined coefficient. 2 Under the uniaxial compression test, n can be obtained where dc is the differential length unit of the outer arc of the by using the following formula (Li et al. 2006): compression arch, R is the radius of the crossing roadway, m is the thickness of the backside compression arch, and dθ is n =  + 2 ± 2   +  (6) c t t c t the angle differential unit of the compression arch along the center of the roadway. where σ is the compressive strength of the supporting rock From Eqs. (9) and (10), the resultant compression arch mass. bearing force F is obtained as follows: The principal stress of the quadratic parabolic envelope is expressed as: 2 2 F = P + n + 2 P +  n b + k m (11) −  = 2n  +  + 4n − n (7) 1 3 1 3 t where k is the increasing slope of radial stress. The stress on the inner wall of the arch structure is gen- As anchoring is performed in fractured rock, the follow- erally equal to the restraining force of the anchor, i.e., ing relationship exists (Yu et al. 2010): = P 3 (8) k = 0 From Eqs. (7) and (8), the relation between the prin- L tan  − l (12) m = cipal stress in the limit state and support resistance is tan obtained as where L is the average length of the bolt (cable), θ is the = P + n + 2 P +  n (9) 1 t control angle of the cable bolt in the supporting rock mass, In the curve operation of Fig. 18, the general value of θ is To calculate the resultant compressive arch bearing force 45° (Yu et al. 2010), and l is the row and line space between F per unit length along the axial direction of the roadway, the supporting bodies. Thus, the expression for F is as the calculation principle diagram shown in the figure above follows: was established, and the following differential equation was obtained as 1 3 19 Page 14 of 23 S. Xie et al. q (KN/m) strength by 1.4 times, making the load-bearing capacity 1.8–2.3 times that of the original support, and the thick- ness of the load-bearing structure formed was increased by qP =+ 0.856 1.712 52.1P ++ 143.275 44.6 1.76 times. Therefore, the construction of a thick-layered reinforced compression arch was achieved theoretically. 4.2 Mechanism of bolting‑grouting integrated Bearing capacity of surrounding rock under stability control strengthening support scheme Anchor-grouting integrated stability control support tech- qP =+ 0.564 1.128 52.1P ++ 143.275 29.392 nology refers to the collaborative implementation of the P (KN) 350400 450500 550600 (1) bolt with the function of “supporting” and “pressure relief”, (2) cable bolt with the function of “control” and “restriction”, and (3) hollow grouting cable bolt with the Fig. 18 Comparison of bearing capacity of compression arch under original and reinforced support schemes function of “strengthening” and “compacting,” in addition to the “filling” and “consolidation” role of the shotcrete support to achieve stability control over the large cross- section roadway at the intersection. The principle of its L tan  − l F = P + n + 2 P +  n (13) action is shown in Fig. 19a. tan In Fig. 19b, the hollow grouting cable bolt technology is shown where the end anchors are changed to full-length The circular thick-layered compression arch built on the anchors to improve the rigidity and shear resistance of roadway is affected by the uniformly distributed load q of the supporting system. Through grouting, the fractured the deep surrounding rock. Under the action of the total sup- surrounding rock was provided with high-stress radial port resistance P, the hoop axial force F produced by the restraint, so that the fractured rock mass could exert its compression arch is expressed as follows: stress-strengthening characteristics and provide a reli- able foundation for the anchor cable. The formula for the surrounding rock reinforcement theory was derived as 2F − q sin  ⋅ dc = 0 (14) follows: Uniaxial compressive strength of broken surrounding where θ is the angle between the differential element and the rock σ is given by coordinate axis, solving the equation, we get 2c ⋅ cos (17) F = R + q c (15) 1 − sin After the process of grouting in the broken surrounding For the compression arch strength, the total bearing force rock, there was an improvement in the cohesion c, inter- F must be greater than the hoop axial force F (i.e., F ≥ F ) 0 0 nal friction angle φ, and elastic modulus E. The uniaxial to ensure stability of the structural load. When F = F , the compressive strength σ is given by compression arch is in the limit of the equilibrium state and the solution is given by � 2(c +Δc) ⋅ cos ( +Δ) (18) 1 − sin ( +Δ) 2 L tan  − l P + n + 2 P +  n 0 t (16) where Δc and Δφ are the respective increments in the cohe- q = tan  (2R + m) sion and internal friction angle, respectively. According to the Heok-Brown guidelines (Eberhardt 2012), Without considering the change in the mechanical parameters of the grouting reinforcement surrounding =  + m  + s (19) 1 3 c 3 rock, the measured surrounding rock parameters were sub- stituted in Eq. (16) to obtain a comparison of the bearing where, m and s are constants for evaluating the rock proper- capacity of the compression arch formed by the original ties and integrity. and strengthened support schemes shown in Fig. 18. It can When σ = 0, the uniaxial tensile strength σ of the rock 1 t be seen that the thick-layered reinforced compression arch mass is obtained as formed by the reinforced support increased the support 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 15 of 23 19 High pressure deep borehole Thick reinforced Grout outlet grouting reinforcement compression arch structure Cement mortar Grouting cable bolt Broken rock Hole packer Cable bolt Anchorage device Bolt Penetrative cranny Hollow grouting cable bolt Bearing plate Shallow surrounding rock Semicircular arched reinforced by bolts Metal mesh and roadway section shotcrete layer (a) Schematic of anchor-grouting integrated stability control (b)Schematic of deep hole grouting. Fig. 19 Principle of the role of anchor-grouting integrated stable control support � � 1 As shown by the strength curve in Table 3, Before the =  m − m + 4s (20) t c implementation of the project, the strength of rock mass specimens before and after grouting was tested in the With the increase in the uniaxial compressive strength laboratory, and the average result was that the c of rock of the broken surrounding rock after grouting, the tensile mass increased by 2.28 MPa, an increase of 85%, and φ strength σ of the strengthened surrounding rock is obtained increased by 11°, an increase of 37%. The parameters of as the mechanical properties of the surrounding rock were � � improved after grouting, and the values of c, φ, and E of the 2 � m − m + 4s (c +Δc) ⋅ cos ( +Δ) fissure surrounding rock increased by 66%–225%, 4°–22°, (21) 1 − sin ( +Δ) and 14%–61%, respectively (Wang et al. 2019). The analy- sis showed that the changes in the mechanical parameters c where m′ and s′ are the rock evaluation constants after and φ after grouting increased the values of σ and σ sig- c t anchoring. nificantly. From Eq. (21) it is seen that the change in the Table 3 Change in mechanical parameters of surrounding rock after grouting Strength curve of surrounding rock before and after grouting Mechanical parameters Original Parameter The increasing rate parameter increase, Δ of tensile strength values (%) Cohesion c′ 2.69 MPa 1.0 37 1.4 52 2.0 74 2.7 100 Internal friction angle φ′ 30° 5 11 10 24 15 39 30 115 1 3 19 Page 16 of 23 S. Xie et al. values of m and s after anchoring did not produce an obvi- 4.3.1 New high‑strength hollow grouting cable bolts ous increase in σ ; however, when the values of c and φ increase in the same proportion, the rate of increase of σ is A new type of hollow grouting cable bolt made of high- different. As shown in the above table, when the increase strength spiral rib prestressed steel wire was selected for ratio is small, the influence of Δc on σ is greater, and when the design. Its structure and advantages of performance are the increase ratio gradually increases, the influence of Δφ shown in Fig. 21. According to previous tests, it was found becomes dominant. that the anchoring strength increased by 15%–20%, anchor- ing ductility increased by approximately 25%, and the high- 4.3 Design of strengthening support scheme pressure grouting pressure could reach 8 MPa as compared with ordinary grouting anchor cables. In practice, the actual Based on the above field test and theoretical analysis, the anchoring force increased by two to three times to achieve design strengthening support scheme of anchor network high-strength anchoring. cable spray + hollow grouting cable bolt support method was adopted (Fig. 20) to ensure long-term stability of the sur- 4.3.2 New modified cement grouting materials rounding rock of the large-section chamber in the deep well. The specific support content was as follows: (1) The large As shown in Fig.  22 there are many disadvantages of deformed roadway under the original support was expanded cement paste in practical application, we can see that and cleaned on the whole to meet the design requirements cement paste has many disadvantages when it is origi- of the original Sect. (2) In accordance with the character- nally used to strengthen surrounding rock grouting, and istics of a large cross-section and gradual cross-section at the intersection of the roadway, the design used high elon- Newhigh-strength hollow 8spiralrib steelwires grouting cablebolt gation cable bolts and hollow grouting cable bolts on the semicircular section of the roadway to be laid alternately at a • 2.47 times higher anchoring • New hollowstructure with its force compared to smooth own a grouting core tube 800 mm interval of the original design plan. This is because steelstrand • Reverse grouting to ensure the original supporting borehole damaged the integrity of • Equal cross-section of steel slurry fillsthe borehole wire with essentially no loss the surrounding rock and cracks around the borehole were of cross-sectionalarea • Immediateload-bearing after installation,suitable for support developed, which was a key area for grouting strengthen- • Significantly lower prestress of poorly stable surrounding transfer length,suitable for rock ing. (3) High-strength bolts were used at the bottom corners applying high prestress • Threaded locking method, of both sides of the roadway to strengthen the surrounding • Low relaxation values, 1.2 reliable lockingmechanism, to 1.8%loss/KH at 70% suitable for wetand dripping stress rock at the bottom corner. (4) In Fig. 20, multiple support underground environment High-pressure grouting core tube methods combine to construct a thick-layered reinforced • Steel wire specimens retain • The cable structuremeets the a yield ratio of over 83% requirements of high pressure after fatigue compression arch of over 7 m that effectively achieves roof grouting control and reinforcement on both sides of the roadway. Fig. 21 New hollow grouting cable bolt made of spiral rib steel wire High elongation cable bolt: SKP22-1 19/1860-8300 Row & line space: 1600 800 mm Form a thick reinforced compression arch structure over 7 m Hollow grouting cable bolt: SKZ29-1/1770-9300 Metal mesh : Φ 6.5 mm steel bar External dimensions: 1000 2000 mm Row & line space: 1600 800 mm Mesh Size: 100 100 mm Bearing plates are connected by Φ14 double reinforcement ladder beams 6446 mm Roadway centerline High-strenthened resin bolt: Thickness of shotcrete support: 120 mm MSGLW500/22 2400 10 9891 mm Concrete grade: C20 Row & line space: 800 800 mm Type I water ditch Fig. 20 Schematic of strengthening support scheme for the No. 10 intersection 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 17 of 23 19 New modified cement Ordinary cement paste ACZ-I cement grouting additive grouting material • Water-cement ratio is • Early-strength high- too high so that the strength the compressive strength is reduced strength of the stone body is Superplasticizer increased by 5 and 2.5 times • Poor fluidity and high at 3 and 28 d respectively Micro expansive grouting resistance agent • High fluidity performance 8% • Cement has a large the slurry can fill the entire particle size, making it gap between the cable body difficult for the grout to and the surrounding rock be injected into small cracks • Micro expansive agent can Early strength agent compensate for shrinkage • The process of cement hardened generates and offset the tensile stress micro-cracks caused by cement hardening • Difficulty in controlling • Adjustable cement setting 525 ordinary portland cement time slurry setting time Disadvantages Advantages The water-cement ratio is 0.5: 1 Fig. 22 Schematic of the advantages of modified grouting material then using it combined with hollow grouting cable bolt 4.4 Simulation analysis of supporting prestress will undoubtedly magnify these shortcomings. Therefore, field a new type of grouting material which can overcome its defects should be considered to match the use of hollow 4.4.1 Support model and surrounding rock compressive grouting cable bolt. Figure 22 shows a new type of modi- stress field fied cement grouting material containing ACZ-I additives that contributes significantly to reducing water, plasti- According to the strengthened support plan, the bolt-and- cizing, strengthening, and micro-expansion of cement cable support system was simulated in FLAC 3D, as shown materials when grouting and strengthening surrounding in Fig. 23, to form the surrounding rock compressive stress rocks, thereby overcoming the current problems of high field (Xie et al. 2018). On examining the stress slices on the water-cement ratio, low strength, hardening shrinkage, compressive stress field at the intersection, it was observed that and large pumping resistance of cement slurries. Accord- after installation of the end of the anchor cable, a pressure of ing to preliminary field testing, the pulling force of the 0.02–3.00 MPa was applied to the surrounding rock of the free hollow grouting cable bolt utilizing the modified cement section and the thickness of the compressed surrounding rock slurry was more than twice that of ordinary paste, as was 7.24 m. From the stress cloud diagram, it is seen that the shown in Table 4. stresses of each anchor cable were cemented with each other to form a complete stress arch that improved the integrity of the surrounding rock to a great extent. 4.3.3 Selection of bolt (cable) parameters for strengthening support plan 4.4.2 Prestressed field model of the integral support The new high-strength hollow grouting cable bolt + high Figure  24 shows the comparison of pre-stress field elongation cable bolt + high-strength resin bolt were used between original support and reinforced support. Accord- in the support design, as indicated in the Table 5 below. ing to the prestress field formed by the original support scheme in the surrounding rock, it can be seen that the stress structure of the compressed arch can not be formed, Table 4 Test table of pull- Grouting material Cement paste New modified grouting material out force of cable bolt under different grouting materials Specimen number 1 2 3 4 5 6 Pull-out force (kN) 52 47 53 108 105 103 1 3 19 Page 18 of 23 S. Xie et al. Table 5 Parameters of anchor bolt (cable) for strengthening support Parameter Hollow grouting cable bolt High elongation cable bolt High-strengthened resin bolt Model SKZ29-1/1770–9300 SKP22-1 × 19/1860–8300 MSGLW500/22 × 2400 Diameter (mm) 29 22 22 Length (m) 9.3 8.3 2.4 Tensile strength (MPa) 1770 1860 500 Breaking force (kN) 600 582 255 Pre-tightening force (kN) 150 200 120 Elongation 4.5% 7.0% 20.0% Row & line space (mm) 1600 × 800 1600 × 800 800 × 800 Bearing plate (mm) δ 20 × 300 × 300 δ 20 × 300 × 300 δ 10 × 150 × 150 Resin anchorage agent MSK2850 × 1, MSZ2850 × 3 MSK2335 × 1, MSZ2360 × 2 MSK2335 × 1, MSZ2360 × 1 End anchorage length (m) 2.0 1.55 0.95 Others Grouting pressure: ≥ 5.0 MPa Hollow grouting cable bolt Strengthening support scheme End resin anchorage Cable axial force/Pa in the early stage Stress/Pa High elongation cable bolt Stress field boundary Maximum cross-section Intersection 10 Maximum cross-section Variable section roadway Compressive stress field High-strengthened of surrounding rock resin bolt Fig. 23 Roadway supporting structure and surrounding rock compressive stress field and there is a discontinuity between the stresses, thus 5 Evaluation of effect of strengthening becoming discontinuous. In the cloud picture of the support strengthening support scheme, the stress is continuous and complete and forms a thick compressive stress arch 5.1 On‑site implementation effect larger than 0.02 MPa, which can act on the surrounding rock mass at a depth of more than 10 m, making the sur- Figure 26 shows an illustration of the field effect of the rounding rock give full play to its self-bearing capacity, bolt-grouting support for the No. 10 crossing roadway at which is enough to realize the effectiveness of support the bottom of the air intake shaft depot in the Nanfeng and structural stability. working area. After completion of the construction of the support, its functioning was under observation and the deformation was monitored for 60 days. It was found that 4.5 Construction process of strengthening support the surrounding rock of the roadway was grouted tightly with good integrity, and there was no surface fragmenta- As shown in Fig. 25 of the construction technology, the tion, cracking of shotcrete layer, or slag peeling. hollow grouting anchor cable construction involved more steps of installing stop plugs and grouting. When the hole or roof is broken during the drilling construction, the 5.2 Deformation monitoring analysis broken body must be put down and the drilling position cleared to facilitate the sealing and control of the drilling The convergence deformation of the surrounding rock at depth. Drilling an anchor hole must be carried out in the the No. 10 intersection was measured and the measure- middle of the roof first and then the two sides from top ment curve is shown in Fig. 27. As shown in the following to bottom. 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 19 of 23 19 120KN Discontinuity Stress/Pa Pre-stress field boundary Pressurized area 100 KN Previous support plan 200 KN 150 KN Pre-stress field Unable to form a complete structure 100 KN Strengthening support plan Surrounding rock Stress/MPa The thick reinforced Stress/Pa anchored by anchor cables 10.2 m compression pre-stress arch Maximum cross-section Affected Pre-stress field 12.5 m surrounding rock Three-dimensional model of supporting pre-stress field Minimum Variable section cross-section roadway High-strengthened Pre-stress field boundary resin bolt Fig. 24 Comparison of pre-stress field between original support and reinforced support figure, the maximum convergence of the roadway is 0.4 m after carrying out the support scheme, there were cracks in within 3 months after the implementation of the original the surrounding rock (0–0.4 m) in the hole. The 0.4–7.4 m support scheme, and the roof subsidence rate is fast, which rock layer was grouted and modified with good integrity, and has affected the normal service of the roadway. After the width of the longitudinal crack was 7.4–8.3 m. It became strengthening the support, the convergence of the sur- smaller and there were no obvious transverse cracks. The rounding rock surface of the roadway is obviously small, 8.3–13.4 m rock layer was fairly complete, which showed the deformation degree of the two sides is the same as that the designed anchor-grouting integrated stability control that of the floor, and the roof is the smallest. As compared support technology could (1) effectively control the conver - with later monitoring data, the convergence deformation gence deformation of large soft rock roadway intersection of the surrounding rock was faster and the rate of conver- in deep complex geological conditions, (2) notably reduce gence was quite high in the first 10 days; however, it was the roadway deformation, (3) greatly extend the period of not more than 1.5 mm/day. The rate of convergence then roadway stability, (4) ensure adequate maintenance effect, decreased rapidly and was approximately 0.05 mm/day for and (5) generate immense economic and social benefits. 40 days. This shows that the support form and parameters controlled the convergent deformation of the chamber effectively and maintained a relatively stable state.6 Conclusions 5.3 Evaluation of overall effect In this study, the No. 10 intersection roadway was the sub- ject under investigation to determine the efficacy of support As shown in Fig.  28, the surrounding rock at the No. 10 schemes of large-section intersection roadways surrounded intersection was assessed by drilling to evaluate the effect by broken rock in deep formations. After on-site monitoring, of the support scheme as a whole. The results showed that 1 3 19 Page 20 of 23 S. Xie et al. Construction Construction plan of anchor-grouting integrated support technology process High-strengthened resin bolt High elongation cable bolt Hollow grouting cable bolt Drilling MSGW-500/22×2400 SKP22-1×19/1860 SKZ29-1/1770-9300 Row & line space:800×800 mm Row & line space:1600×800 mm Row & line space:1600×800 mm Anchor installation Bearing plate δ:10×150×150 mm Bearing plate δ:20×300×300 mm Bearing plate δ:20 ×300×300 mm Applying Pre-tightening force 120 KN Pre-tightening force 200 KN Pre-tightening force 150 KN preload The grouting slurry is cement slurry with a water-cement ratio of 0.5:1, and ACZ-1 grouting additive with Grouting 8% cement weight is added to the slurry. Mounting the Φ14 double reinforced ladder beams are used to connect the bearing plates horizontally and vertically, the ladder beam, ladder beam has two sizes of 1.0 m and 1.8 m. The lap length of the metal mesh grid is not less than 100 mm, metal mesh spaced not more than 100 mm, and 16 gauge lead wire is used to tie the metal mesh. Sprayed The thickness of the shotcrete support is 120 mm, and the concrete grade is C20 concrete Displacement Convergence meter and roof abscission layer instrumentare used to measure roadway deformationand the monitoring subsidence of the roof of the roadway Fig. 25 Flow chart of construction of strengthening support High elongationcable bolt Hollow groutingcable bolt Sandymudstone Mudstone 2.75 m 11.10 m Reinforcementsupport Mudstone ZDK938/5/20track switch Largecross -section 5.15 m Sandymudstone intersection roadway 12.95 m No.10 roadway Maximumcross -section intersection Track intersection Mudstone 1.80 m Parkingchamber Passageway No.3 coal seam 5.80 m Passageway Type Iwater ditch High-strengthenedresin bolt Fig. 26 Effect of strengthening support on site theoretical analysis, numerical simulation, and construction plastic failure zone of more than 6 m, serious destruc- tests, the following conclusions were drawn: tion of roof, and high tectonic stress of nearly 30 MPa. (2) According to the simulation of the strain-softening (1) With the linked reaction of variable cross-sections characteristics of the surrounding rock by FLAC 3D, having a maximum width of 9.9 m, many surround- the deflection of the intersection roadway increased ing chambers, layout under a condition of deep high with an increase in the size of the cross-section and stress, soft surrounding rock, adjacent faults, and cross- the roof displacement nearly doubled in the frontal area stratification, the intersection roadway exhibited four as compared to that in the normal area. The plastic zone features, namely loose and broken surrounding rock, was deep with asymmetric distribution and had a maxi- 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 21 of 23 19 Fig. 27 Comparison of deformation convergence between original support and reinforced support roadway 0.4 m7.4 m8.3 m 13.4 m Good integrity Fissure development Complete rock structure Microcracks Grouting material Shotcrete Fig. 28 Sketch drawing of borehole detection of surrounding rock after strengthening support mum depth up to 6.61 m. It was mainly noticeable in proportion of increase was low; when the proportion the top and shoulder areas and disappeared gradually of increase rose gradually, the influence of Δ φ became in the sidewall and connection between the sidewall the leading cause of increase in σ . and bottom. The stress concentration factor and range (5) By using a modified cement grouting material and increased as compared to other areas of the roadway; high-strength hollow grouting cable bolt, the drawing they peaked at the frontal area, and the range of stress force increased to more than twice that of the cement concentration at the side of the intersection roadway paste; this enhanced the extent of filling of cracks and close to the passageway was wider and higher. mechanical properties of the surrounding rock signifi- (3) In accordance with the compression arch mechanical cantly. Based on the supporting stress field, the sur - model based on the parabolic Mohr strength theory, rounding rock (7.24 m) was compacted and strength- hollow grouting cable bolt and high elongation grout- ened, providing a solid foundation for the attachment ing cable bolt were utilized to build a thick strength- of the bolt (cable) support. According to the results of ened compression arch more than 7 m high having a field monitoring, the convergence of the gallery was bearing capacity greater by 1.8 to 2.3 times and thick- less than 30 mm in 60 days, indicating that stability ness of the bearing structure increased by 1.76 times as control of the gallery was achieved effectively. compared to that of the original support scheme. Thus, the construction of a large-depth, high-intensity bear- Acknowledgements This work was financially supported by the ing structure of the surrounding rock was accomplished National Natural Science Foundation of China (Grant Nos. 52074296, successfully. 52004286), the China Postdoctoral Science Foundation (Grant Nos. (4) According to laboratory tests, the c and φ of rock mass 2020T130701, 2019M650895). increased by 85% and 37% respectively after grouting. It can be calculated by the formula, an increase in the same proportion of caused a different increasing rate of σ , and Δc influenced σ to a greater extent when the t t 1 3 19 Page 22 of 23 S. Xie et al. Li CG, Ge XR, Zheng H, Wang SL (2006) Two-parameter parabolic Declarations Mohr strength criterion and its damage regularity. Key Eng Mater 306–308:327–332 Ethical approval The experiments comply with the current laws of Li XF, Wang SB, Malekian R, Hao SQ, Li ZX (2016) Numerical simu- China. lation of rock breakage modes under confining pressures in deep mining: an experimental investigation. IEEE Access 4:5710–5720 Conflict of interest The authors declare that they do not have any com- Li G, Ma FS, Guo J, Zhao HJ, Liu G (2020) Study on deformation fail- mercial or associative interest that represents a conflict of interest in ure mechanism and support technology of deep soft rock roadway. connection with the work submitted. Eng Geol 264:105262 Liu CR (2011) Distribution laws of in-situ stress in deep underground coal mines. Procedia Eng 26:909–917 Open Access This article is licensed under a Creative Commons Attri- Lu YL, Wang LG, Yang F, Li YJ, Chen HM (2010) Post-peak strain bution 4.0 International License, which permits use, sharing, adapta- softening mechanical properties of weak rock. Chin J Rock Mech tion, distribution and reproduction in any medium or format, as long Eng 29(3):640–648 ((in Chinese)) as you give appropriate credit to the original author(s) and the source, Pan R, Wang Q, Jiang B, Li SC, Sun HB, Qin Q, Yu HC, Lu W (2017) provide a link to the Creative Commons licence, and indicate if changes Failure of bolt support and experimental study on the parameters were made. The images or other third party material in this article are of bolt-grouting for supporting the roadways in deep coal seam. included in the article's Creative Commons licence, unless indicated Eng Fail Anal 80:218–233 otherwise in a credit line to the material. If material is not included in Peng R, Meng X, Zhao G, Li Y, Zhu J (2018) Experimental research the article's Creative Commons licence and your intended use is not on the structural instability mechanism and the effect of multi- permitted by statutory regulation or exceeds the permitted use, you will echelon support of deep roadways in a kilometre-deep well. PLoS need to obtain permission directly from the copyright holder. To view a One 13(2):e0192470 copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . Ranjith PG, Zhao J, Ju MH, De Silva Radhika VS, Rathnaweera TD, Bandara AKMS (2017) Opportunities and challenges in deep min- ing: a brief review. Engineering 3(4):546–551 Shreedharan S, Kulatilake PHSW (2015) Discontinuum–equivalent continuum analysis of the stability of tunnels in a deep coal References mine using the distinct element method. Rock Mech Rock Eng 49(5):1903–1922 Alonso E, Alejano LR, Varas F, Fdez-Manin G, Carranza-Torres Tan YL, Fan DY, Liu XS, Song SL, Li XF, Wang HL (2019) Numeri- C (2003) Ground response curves for rock masses exhibiting cal investigation of failure evolution for the surrounding rock of strain-softening behaviour. Int J Numer Anal Methods Geomech a super-large section chamber group in a deep coal mine. Energy 27(13):1153–1185 Sci Eng 7(6):3124–3146 Cai MF, Brown ET (2017) Challenges in the mining and utilization of Tian ZC, Tang CA, Liu YJ, Tang YB (2020) Zonal disintegration test deep mineral resources. Engineering 3(4):432–433 of deep tunnel under plane strain conditions. Int J Coal Sci Tech- Chen XJ, Li LY, Wang L, Qi LL (2019) The current situation and pre- nol 7(2):337–349. https:// doi. org/ 10. 1007/ s40789- 020- 00319-y vention and control countermeasures for typical dynamic disasters Wagner H (2019) Deep mining: a rock engineering challenge. Eng in kilometer-deep mines in China. Saf Sci 115:229–236 Mech Rock Eng 52(5):1417–1446 Eberhardt E (2012) The Hoek-Brown failure criterion. Rock Mech Wang MY, Zhang N, Li J, Ma LJ, Fan PX (2015) Computational Rock Eng 45(6):981–988 method of large deformation and its application in deep mining Fairhurst C (2017) Some challenges of deep mining. Engineering tunnel. Tunn Undergr Space Technol 50:47–53 3(4):527–537 Wang Q, Pan R, Jiang B, Li SC, He MC, Sun HB, Wang L, Qin Q, Yu Huang X, Liu QS, Liu B, Liu KD, Huang SB (2014) Experimental HC, Luan YC (2017) Study on failure mechanism of roadway with research on the mechanical behavior of deep soft surrounding soft rock in deep coal mine and confined concrete support system. rock tunneling by TBM. J China Coal Soc 39(10):1977–1986 ((in Eng Fail Anal 81:155–177 Chinese)) Wang Q, Qin Q, Jiang B, Yu HC, Pan R, Li SC (2019) Study and engi- Huang J, Zhang YM, Zhu LS, Wang T (2016) Numerical simulation neering application on the bolt-grouting reinforcement effect in of rock cutting in deep mining conditions. Int J Rock Mech Min underground engineering with fractured surrounding rock. Tunn Sci 84:80–86 Undergr Space Technol 84:237–247 Huang WP, Yuan Q, Tan YL, Wang J, Liu GL, Qu GL, Li C (2018) An Wang H, Jiang C, Zheng PQ, Li N, Zhan YB (2020) Deformation and innovative support technology employing a concrete-filled steel failure mechanism of surrounding rocks in crossed-roadway and tubular structure for a 1000-m-deep roadway in a high in situ its support strategy. Eng Fail Anal 116:104734 stress field. Tunn Undergr Space Technol 73:26–36 Xie HP, Gao MZ, Zhang R, Peng GY, Wang WY, Li AQ (2018) Kang YS, Liu QS, Gong GQ, Wang HC (2014) Application of a com- Study on the mechanical properties and mechanical response bined support system to the weak floor reinforcement in deep of coal mining at 1000m or deeper. Rock Mech Rock Eng underground coal mine. Int J Min Sci Technol 71:143–150 52(5):1475–1490 Kang HP, Lin J, Fan MJ (2015) Investigation on support pattern of a Xie S, Mingming G, Dongdong C, Yanding,S, Hao P, Hai S, Shizhong coal mine roadway within soft rocks — a case study. Int J Coal L (2018) Stability influence factors analysis and construction of Geol 140:31–40 a deep beam anchorage structure in roadway roof. Int J Min Sci Kawamoto T, Ishizuka Y (1981) An analysis of excavation instrain- Technol 28(3):445-451. https:// doi. org/ 10. 1016/j. ijmst. 2017. 11. softening rock mass. Proc Japan Soc Civ Eng 148:107–118 Lee YK, Pietruszczak S (2008) A new numerical procedure for elasto- Xie SR, Pan H, Zeng JC, Wang E, Chen DD, Zhang T, Peng XJ, Yang plastic analysis of a circular opening excavated in a strain-sof- JH, Chen F, Qiao SX (2019) A case study on control technology of tening rock mass. Tunn Undergr Space Technol 23(5):588–599 surrounding rock of a large section chamber under a 1200-m deep goaf in Xingdong coal mine, China. Eng Fail Anal 104:112–125 1 3 Failure analysis and control technology of intersections of large‑scale variable cross‑section… Page 23 of 23 19 Xue D, Zhou J, Liu Y, Gao L (2020) On the excavation-induced stress Zhang F, Sheng Q, Zhu ZQ, Zhang YH (2008) Study on post-peak drop in damaged coal considering a coupled yield and failure cri- mechanical beha viour and strain-softening model of three gorges terion. Int J Coal Sci Technol 7(1):58–67 granite. Chin J Rock Mech Eng 27(S1):2651–2655 ((in Chinese)) Yang SQ, Chen M, Jing HW, Chen KF, Meng B (2017) A case study on Zhang HQ, Miao XX, Zhang GM, Wu Y, Chen YL (2017) Non- large deformation failure mechanism of deep soft rock roadway in destructive testing and pre-warning analysis on the quality of Xin’An coal mine, China. Eng Geol 217:89–101 bolt support in deep roadways of mining districts. Int J Min Sci Yao YM, Neithalath N, Mobasher B (2018) Analysis and design pro- Technol 27(6):989–998 cedures for strain hardening flexural beam and panel. RILEM Zhang JP, Liu LM, Cao JZ, Yan X, Zhang FT (2018) Mechanism and Bookseries 15:518–526 application of concrete-filled steel tubular support in deep and Yu WJ, Gao Q, Zhu CQ (2010) Study of strength theory and applica- high stress roadway. Constr Build Mater 186:233–246 tion of overlap arch bearing body for deep soft surrounding rock. Zhang CY, Pu CZ, Cao RH, Jiang TT, Huang G (2019) The stability Chinese J Rock Mech Eng 29(10):2134–2142 ((in Chinese)) and roof-support optimization of roadways passing through unfa- Yu KP, Ren FY, Puscasu R, Lin P, Meng QG (2020) Optimization vorable geological bodies using advanced detection and monitor- of combined support in soft-rock roadway. Tunn Undergr Space ing methods, among others, in the Sanmenxia Bauxite Mine in Technol 103:103502 China’s Henan Province. Bull Eng Geol Environ 78(7):5087–5099 1 3

Journal

International Journal of Coal Science & Technology – Springer Journals

Published: Dec 1, 2022

Keywords: Deep soft rock; Variable cross-section; Roadway intersection; Bolting-grouting integration; New grouting material

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