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An Investigation into Bolt Anchoring Performance during Tunnel Construction in Bedded Rock Mass

An Investigation into Bolt Anchoring Performance during Tunnel Construction in Bedded Rock Mass applied sciences Article An Investigation into Bolt Anchoring Performance during Tunnel Construction in Bedded Rock Mass 1 , 2 1 , 2 3 1 , 2 , 1 , 2 Zhiqiang Zhang , Yin Liu , Junyang Teng , Heng Zhang * and Xin Chen School of Civil Engineering, Southwest Jiaotong University, Chengdu 610031, China; clark@swjtu.edu.cn (Z.Z.); liuyin901209@163.com (Y.L.); chenxin8090@outlook.com (X.C.) Key Laboratory of Transportation Tunnel Engineering, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China; jteng89@hotmail.com * Correspondence: tunnelzh@home.swjtu.edu.cn; Tel.: +86-028-8763-4386 Received: 18 January 2020; Accepted: 22 March 2020; Published: 28 March 2020 Abstract: The anchor bolt is a key point of tunnel design in bedded rock mass. The previous theory of anchorage support falls does not fulfil engineering requirements, and the stability of bedded rock must be addressed by empirical methods. To investigate the bolt anchoring performance for bedded rock mass under di erent anchoring methods, the rock failure mode under shear and tensile stresses in bedded rock was examined in this paper. The results showed that bolt anchoring for rock is achieved mainly through the bonded restoration of surrounding rock near the drill holes by means of an anchoring agent and the supporting resistance provided by the bolt body. It was observed that the strength parameters of bedded rock were increased under the anchoring e ect. Full anchoring bolts were especially e ective. In addition, it was observed that, in the absence of bolts, the failure form changed from shear to split. In the case of bolting, the failure plane occurred parallel to the bolt’s axis. The shearing began along the interface between the hard and soft rock bedding. Compared to end bolt anchoring, full-length bolt anchoring was more capable of o ering an anchoring e ect. The latter o ered a greater increase in the strength and greater shear-bearing capacity of the rock, which ultimately enabled the rock to bear more load. Keywords: tunnel engineering; anchoring performance; bedded rock mass; laboratory experiment; field application; failure mode; support resistance 1. Introduction In the past decade, the tunnels and underground projects built in China under complex geological and environmental conditions have made great progress [1–6]. Meanwhile, it also faces a series of construction diculties and challenges [7–10]. The tunnel composite support system is divided into a primary support and secondary lining (usually concrete lining). The primary support is vital to the tunnel stability and mainly consists of a bolt, sprayed concrete, and reinforcing mesh, and it is supplemented by joist steel or a lattice girder in accordance with the surrounding rock. Moreover, the bolt is the most important support structure of the primary support on account of its high eciency, economic advantages, and reduced space occupation. Shale forms many bedded and fractured structural planes during the diagenetic process of compaction and cementation, which seriously a ects the tunnel stability. The bolt anchoring characteristics for jointed rock mass have been the focus of intense research in China and abroad. The tunnel bolt is mainly designed for full-length anchoring. During bolting at the construction site, a hole is drilled in the tunnel’s surrounding rock. Then, after grouting or resin application, the Appl. Sci. 2020, 10, 2329; doi:10.3390/app10072329 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 2329 2 of 15 body (bolt) is inserted and screwed. In theory, the bolt and surrounding rock mass are bonded in full length via the anchoring agent; however, as the location and angle of the hole di er, bolt construction diculties vary, especially for the tunnel arch part. Furthermore, it is dicult to ensure the grouting quality of the anchoring agent, which causes the anchorage length of the bolt to vary. Therefore, it is necessary to investigate the impact of anchoring and the stability e ect of the jointed rock mass. The bolt reinforcement e ect on jointed rock mass mainly enhances the shear-bearing capacity of the rock mass joint plane. In addition, it prevents the rock mass from developing an intercalated dislocation along the joint plane [11,12]. Many factors a ect the reinforcing e ect of the bolt on the joint plane, such as the bolt size, hole diameter, pretension force, grouting, and anchored rock mass strength [13–21]. Many studies have been conducted on the above aspects. In 1974, Bjurstrom [11] conducted systematic research on the shear property of granite under full-length anchoring and showed that the tangential shear-bearing capacity of the bolt can significantly enhance the stability of the jointed rock mass. Spang et al. [22] and Haas et al. [23] researched the impact of the bolt on the joint shear-bearing capacity of di erent rocks. Using anchorage tests of jointed rock mass, Yoshinaka et al. [24], Ferrero [25], and Kim et al. [26] analyzed the impact of factors such as the quantity of bolts, elasticity modulus of the bolt body, bolt material, and roughness of the rock joint on the joint shear-bearing capacity. Moreover, Pellet et al. [27] theoretically analyzed the bolt shear-bearing capacity and evaluated the impact of the anchoring angle on the anchoring e ect. Grasselli [28] and Jalalifar et al. [29] conducted laboratory shear tests of anchored and jointed rock mass and employed numerical simulation methods to analyze the shear resistance e ect of the bolt. They respectively showed that the bolt can form plastic hinges on the joint plane, and that bolt failures mainly occur among the plastic hinges soon thereafter. Meanwhile, Zhang et al. [30] conducted research on the deformation property of a pre-tensioned bolt during a shear test. They determined that the bolt shear-bearing e ect occurred after the shear dislocation of the joint plane on account of the bolt “dowelling function”. Teng et al. [31] compared failure modes of end anchoring, full-length anchoring, and non-anchoring by experiment. The result showed that the failure modes of anchored specimens are a ected by anchoring type, and they are further divided into shearing extension and shearing o set. Wang et al. [32] simulated the failure mechanism of tunnel segmental lining joints and confirmed that the deformation of the circumferential joints consisted of opening and dislocation, but dislocation was dominant. In addition, to explore the impact of various factors, Ge et al. [33] carried out shear testing of di erent bolt sizes, materials, installation angles, specimen strengths, and others. Based on their findings, Chen et al. [34] established a computational formula of structural-plane-anchored specimen shearing strength, which they verified through simulation tests. Furthermore, Liu et al. [35] employed a physical simulation method to assess the impact of the bolt pre-stress force on the shear-bearing capacity of rock mass. Zhang et al. [36] studied the mechanical properties of fractured rock mass under anchoring conditions and uniaxial compression. They further verified the bolt “dowelling function” in terms of the fractured rock mass. Chen et al. [37,38] used a method of anchoring the origin rock specimens and performed respective tensile, uniaxial compression, and pressure–shear tests on rocks. They detailed the rules of crack initiation, extension, and so on of the anchored specimens and verified the relationships between the anchorage and enhancement of the specimen’s mechanical strengths (tensile, compressive, and shear). In addition, they analyzed the anchoring performance. Based on the classical beam theory and the variational principle of minimum complementary energy, Yang et al. [39] analyzed and determined the resisting mechanical behavior of anchor bolts for di erent rock mass strengths and bolt diameters. Zhang et al. [40] conducted conventional static and dynamic drawing load tests on bonding bolts with end anchorage. The experimental results showed that the distribution of axial stress of a bonded anchor bolt is triangular under static loading. Zhu et al. [41] designed an artificial material and loading system to study the influence of bedding cohesion and anchoring behavior of bedded rock mass. The results showed that the axial stress–strain curve of Appl. Sci. 2020, 10, 2329 3 of 15 bedded rock mass under the reinforcement of bolts presents the features of strain softening and secondary strengthening. As shown above, considerable studies and research results have helped elucidate the jointed rock mass anchoring performance. Obviously, however, many other factors a ect the bolt shearing property for jointed rock mass, and the complicated rock strengthening of the jointed rock mass by bolting remains not fully understood. Moreover, the existing theory of anchorage support falls does not fulfil engineering requirements, and designers have to adopt empirical methods to address the stability of rock in most cases. The intention of this paper is to explore the performance of bolts for bedded and jointed rock masses and to figure out the mechanical properties and failure mode of the rock by di erent anchoring methods. This study is organized as follows. Section 1 reviews the previous studies made in the field of bolt reinforcement e ects. In Sections 2 and 3, a laboratory test program is designed to analyze the mechanical properties (under uniaxial and shear force) from the viewpoint of mechanical e ects and the failure modes of jointed rock with di erent anchoring methods. Section 4 presents verification of the bolt anchoring e ect on a bedded shale tunnel by means of a site test. Finally, Section 5 concludes the current study. 2. Experimental Procedure C15 specimen material was prepared by mixing cement of river sand and quick lime at a ratio of 1.3:1.5 (river sand to quick lime). It was cured for 28 days at room temperature. The specimens for the uniaxial compression test were prepared with bedding, where mica was selected as the bedding structure. Sheets of mica (100 mesh fineness) were evenly laid between two layers, and the distance between two consecutive layers was 15 mm. The cores were drilled perpendicular to the bedding and were prepared according to the testing standards, i.e., 100 mm in height and 50 mm in diameter. Precast concrete was cut into a cube with a size of 50 mm  100 mm  100 mm for the shear test [42]. To bolster the bolt, #45 steel was processed into the screw, and the bolt diameter was 5 mm. The anchoring agent was properly weakened by using chemical grout mixed with ethyl alcohol. The mechanical properties of the screw and bolt are shown in Table 1. Table 1. Mechanical parameters of the bolt and screw. Material Size/mm Tensile Strength/MPa Shear Strength/MPa Anchoring Force/MPa Normal bolt F16~22 200~600 260~600 50 Selected screw F5 800 400 30~40 The uniaxial compression test and shear test were both performed on an MTS815.03 Electro-hydraulic Servo-controlled Rock Mechanics Testing System (MTS815) rock mechanical experiment system. Both adopted displacement control for loading at a displacement rate of 0.1 mm/min. 2.1. Uniaxial Compression Testing According to the size ratios of the bolt and screw, the geometric similarity ratio of the bolt for the uniaxial compression test was determined to be 4:1. With consideration of the specimen’s geometric size, the geometric similarity ratio of its anchoring parameters was designed to be 13.3:1. Three di erent anchoring schemes were selected: no anchoring, end anchoring, and full-length anchoring. For each kind of anchoring method, three specimens were fabricated to reduce the discreteness. For the anchoring, a torque wrench was used to impose a pre-tightening force of 10 kN. The bedded and anchored specimens are shown in Figure 1. 2.2. Shear Testing The bolt used for the shear testing was bonded with a highly sensitive strain gauge to measure the axial strain value of the bolt during the test. For each selected scheme (mentioned above), three Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 15 Appl. Sci. 2020, 10, 2329 4 of 15 50 mm samples were tested to rule out any error. After the installation, anchored specimens were maintained at normal temperature for seven days, thereby enabling the anchoring agent to completely coagulate. Next, laboratory shear testing was performed. A schematic diagram of the strain gauge arrangement, cable anchored specimen, and test process is shown in Figure 2. Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 15 50 mm bedding cable (a) (b) Figure 1. Uniaxial compression test: (a) Dimensions of an anchored specimen; (b) Test process. bedding 2.2. Shear testing The bolt used for the shear testing was bonded with a highly sensitive strain gauge to measure the axial strain value of the bolt during the test. For each selected scheme (mentioned above), three samples were tested to rule out any error. After the installation, anchored specimens were maintained (a) (b) at normal temperature for seven days, thereby enabling the anchoring agent to completely coagulate. Figure 1. Uniaxial compression test: (a) Dimensions of an anchored specimen; (b) Test process. Figure 1. Uniaxial compression test: (a) Dimensions of an anchored specimen; (b) Test process. 2.2. Shear testing P The bolt used for the shear testing was bonded with a highly sensitive strain gauge to measure bedding the axial strain value of the bolt during the test. For each selected scheme (mentioned above), three samples were tested to rule out any error. After the installation, anchored specimens were maintained at normal temperature for seven days, thereby enabling the anchoring agent to completely coagulate. Next, laboratory shear testing was performed. A schematic diagram of the strain gauge arrangement, anchored specimen, and test process is shown in Figure 2. cable bedding strain gauge (a) (b) cable strain gauge (a) (b) (c) Figure 2. Schematic diagram of the strain gauge arrangement and an anchored specimen for shear Figure 2. Schematic diagram of the strain gauge arrangement and an anchored specimen for shear testing: (a) Arrangement of the strain gauge; (b) Anchored specimen; (c) Test process. testing: (a) Arrangement of the strain gauge; (b) Anchored specimen; (c) Test process. 3. Experimental Results In the figures of this section, each name in the legend is based on the following convention: test method + anchoring method + number of test group. Uniaxial compression testing and shear testing (c) are represented by “C” and “S”, respectively, for the test method. “N”, “E”, and “FL” denote no Figure 2. Schematic diagram of the strain gauge arrangement and an anchored specimen for shear anchoring, end anchoring, and full-length anchoring, respectively. testing: (a) Arrangement of the strain gauge; (b) Anchored specimen; (c) Test process. 20 mm 60 mm 20 mm 20 mm 60 mm 20 mm Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 15 3. Experimental Results In the figures of this section, each name in the legend is based on the following convention: test method + anchoring method + number of test group. Uniaxial compression testing and shear testing Appl. Sci. 2020, 10, 2329 5 of 15 are represented by “C” and “S”, respectively, for the test method. “N”, “E”, and “FL” denote no anchoring, end anchoring, and full-length anchoring, respectively. 3.1. Uniaxial Compression Testing 3.1. Uniaxial compression testing The axial displacement was directly measured using an MTS815 rock mechanical experiment system; curves of the specimens’ axial strain for the earlier mentioned anchoring methods (no anchoring, The axial displacement was directly measured using an MTS815 rock mechanical experiment end anchoring, and full-length anchoring) are shown in Figure 3. C-N-1 C-E-1 C-N-2 C-E-2 5 (d) (e) C-E-3 C-N-3 (c) (b) (a) 02 468 10 12 02 46 8 10 12 -3 -3 Axial strain (10 ) Axial strain (10 ) (a) (b) C-FL-1 C-FL-2 8 C-FL-3 02 468 10 12 -3 Axial strain (10 ) (c) Figure 3. Stress–strain curve: (a) No anchoring; (b) End anchoring; (c) Full-length anchoring. Figure 3. Stress–strain curve: (a) No anchoring; (b) End anchoring; (c) Full-length anchoring. A close examination of the figure reveals the following key observations. A close examination of the figure reveals the following key observations. Five phases of the stress–strain curve in every case are obvious. Taking curve C-N-3 in Figure 3a Five phases of the stress–strain curve in every case are obvious. Taking curve C-N-3 in Figure as an example, we see the (a) initial compression phase, (b) elastic deformation phase,(c) phase of 3(a) as an example, we see the a) initial compression phase, b) elastic deformation phase, c) phase of microfracture, (c) stable development phase, (d) phase of unstable fracture and development, and (e) microfracture, c) stable development phase, d) phase of unstable fracture and development, and e) post-fracture phase. In the first phase, the initial compression phase, the duration of the end anchoring post-fracture phase. In the first phase, the initial compression phase, the duration of the end specimen is longer than that with no anchoring; moreover, it is shortest for the full-length anchoring anchoring specimen is longer than that with no anchoring; moreover, it is shortest for the full-length specimen. The cause of this is analyzed below. anchoring specimen. The cause of this is analyzed below. The initial rock compression closure mainly refers to the closure of the rock’s internal structural The initial rock compression closure mainly refers to the closure of the rock’s internal structural plane and primary microfracture by compression. It is assumed that the secondary cracks produced plane and primary microfracture by compression. It is assumed that the secondary cracks produced during sample preparation (drilling, grouting, etc.) may have an adverse e ect on the bolt because the during sample preparation (drilling, grouting, etc.) may have an adverse effect on the bolt because anchoring range of the end anchoring bolt is relatively small. Its anchoring end is tightly bonded with the rock. Some space exists between the bolt body of the non-anchor segment and the rock. In the full-length anchoring specimen, the anchoring bolt is tightly bonded with the rock by the anchoring agent. Moreover, the anchoring agent has the e ect of bonded amalgam restoration on specimen damage. It forms a reinforcing area within a certain range around the bolt body. Therefore, σ (MPa) σ (MPa) σ (MPa) Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 15 the anchoring range of the end anchoring bolt is relatively small. Its anchoring end is tightly bonded with the rock. Some space exists between the bolt body of the non-anchor segment and the rock. Appl. Sci. 2020, 10, 2329 6 of 15 In the full-length anchoring specimen, the anchoring bolt is tightly bonded with the rock by the anchoring agent. Moreover, the anchoring agent has the effect of bonded amalgam restoration on at the implementation of the uniaxial compression test, for the case of no anchoring or horizontal specimen damage. It forms a reinforcing area within a certain range around the bolt body. Therefore, bedding specimens, the first phase is mainly the closure of the bedding plane. For the end anchoring at the implementation of the uniaxial compression test, for the case of no anchoring or horizontal specimens, it is not only bedding plane closure, but also the compression process of the space between bedding specimens, the first phase is mainly the closure of the bedding plane. For the end anchoring the bolt and rock. For the full-length anchoring specimens, it is mainly the compression of bedding in specimens, it is not only bedding plane closure, but also the compression process of the space between the non-anchor area. the bolt and rock. For the full-length anchoring specimens, it is mainly the compression of bedding After the bearing capacity of the no-anchoring specimen has reached the strength peak, the in the non-anchor area. specimen fails rapidly, and the section of the stress–strain curve after the peak is relatively steep. When After the bearing capacity of the no-anchoring specimen has reached the strength peak, the the bearing capacity of the anchored specimen has reached the strength peak, the load and resulting specimen fails rapidly, and the section of the stress–strain curve after the peak is relatively steep. plastic deformation of the specimen continue to increase. Thus, this section can be referred to as the When the bearing capacity of the anchored specimen has reached the strength peak, the load and “plasticity strengthening section”. resulting plastic deformation of the specimen continue to increase. Thus, this section can be referred After the anchoring of specimens, both the average uniaxial compressive strength and elasticity to as the “plasticity strengthening section”. modulus are increased [42]. As the anchoring methods are di erent, the increased range of the After the anchoring of specimens, both the average uniaxial compressive strength and elasticity anchored specimen di ers. In comparison with the no-anchoring specimens, as shown in Figure 4, the modulus are increased [42]. As the anchoring methods are different, the increased range of the uniaxial compressive strength of the end anchoring specimen is increased by 12.73%, while the uniaxial anchored specimen differs. In comparison with the no-anchoring specimens, as shown in Figure 4, compressive strength of the full-length anchoring specimen is increased by 62.71%. Similarly, the the uniaxial compressive strength of the end anchoring specimen is increased by 12.73%, while the elasticity modulus of the end anchoring specimen is increased by 6.31%, while the elasticity modulus uniaxial compressive strength of the full-length anchoring specimen is increased by of the full-length anchoring specimen is increased by 58.73%. 62.71%. 10 1300 Scattered point Scattered point Mean value Mean value End Full-length End Full-length No bolt No bolt anchoring anchoring anchoring anchoring (a) (b) Figure 4. Figure 4. Rock Rock strength parameters by strength parameters by di er dent ifferent anc anchoring horing method methods: (a) Uniaxial s: (a) Uniax compr ial essi cove mp str ressive ength; streng (b) Elasticity th; (b) Elasticity modulus.modulus. The failure patterns of specimens under di erent anchoring modes are shown in Figure 5. The failure patterns of specimens under different anchoring modes are shown in Figure 5. When there is no anchor, the failure mode of the specimen is mainly shear failure along the When there is no anchor, the failure mode of the specimen is mainly shear failure along the bedding and axial splitting failure perpendicular to the bedding. For end anchoring, the bonding bedding and axial splitting failure perpendicular to the bedding. For end anchoring, the bonding force of the anchoring agent at the end of the bolt limits the surrounding rock’s shear failure along force of the anchoring agent at the end of the bolt limits the surrounding rock’s shear failure along the bedding, and the "pin e ect" of bolts makes it dicult for the specimen to split along the axial the bedding, and the "pin effect" of bolts makes it difficult for the specimen to split along the axial direction. The specimen finally shows axial shear tensile failure, as shown in Figure 6a. direction. The specimen finally shows axial shear tensile failure, as shown in Figure 6(a). Under full-length anchorage, shear failure occurs along the middle or end of the specimen. Under full-length anchorage, shear failure occurs along the middle or end of the specimen. The The reason for this is that under the action of full-length bonding and bolt “pin action”, the strength of reason for this is that under the action of full-length bonding and bolt "pin action", the strength of the the specimen near the two ends of the bolt is increased, and the interior is relatively soft, so there is a specimen near the two ends of the bolt is increased, and the interior is relatively soft, so there is a “soft–hard interface” in the specimen. Shear failure occurs along the “soft–hard interface” under load, "soft–hard interface" in the specimen. Shear failure occurs along the "soft–hard interface" under load, as shown in Figure 6b. as shown in Figure 6(b). σ (MPa) E (MPa) Appl. Sci. 2020, 10, 2329 7 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 15 (a) (a) (b) (b) (c) (c) Figure 5. Failure forms: (a) No anchoring; (b) End anchoring; (c) Full-length anchoring. Figure 5. Failure forms: (a) No anchoring; (b) End anchoring; (c) Full-length anchoring. (a) (b) (a) (b) Figure 6. Failure planes of anchored specimens: (a) End anchoring; (b) Full-length anchoring. Figure Figure 6. 6. Failur Failure planes o e planes off an anchor chored spe ed specimens: cimens: ( (a a) ) End an End anchoring; choring; ( (b b) ) F Full-length ull-length ancho anchoring. ring. On the basis of this, the reinforcement mechanism of the bolt can be summarized as follows. On the basis of this, the reinforcement mechanism of the bolt can be summarized as follows. (1 On the basis ) The ancho of this, the r ring agent ha einforcement s a bonding e mechanis ffect and dam m of the bolt c age repaa ir n ing be summ effect on roc arized as k mass follows. around boreholes (1) The ancho . Joints, firing ssures, agent and ha ot s a bondin her structg e ural ffect plane and dam s often aexist ge rep in a t iring he rock effect for on roc ming process k mass ,aro whic unh d reduce boreholes the . mechanic Joints, fissu al pro res, and perties of roc other struct kur , large al pl- ane scale s o struct ften exist ural planes in the rock , and for ev ming en the c process ontrollin , whicg h factors o reduce the f surroundin mechanicg roc al pro k failure. The anchorin perties of rock, large- g sc ag ale ent enters the structur structural planes, and al plane und even the c er gr ontrollin outing g factors of surrounding rock failure. The anchoring agent enters the structural plane under grouting Appl. Sci. 2020, 10, 2329 8 of 15 (1) The anchoring agent has a bonding e ect and damage repairing e ect on rock mass around boreholes. Joints, fissures, and other structural planes often exist in the rock forming process, which reduce the mechanical properties of rock, large-scale structural planes, and even the controlling factors of surrounding rock failure. The anchoring agent enters the structural plane under grouting pressure and plays a role in bonding and strengthening the surrounding rock near the structural plane, thus forming a “reinforcement area” within a certain range. The size of the “reinforcement area” is related to the grouting pressure, the material properties of the anchoring agent, the pore distribution of the surrounding rock, and so on. (2) The axial tensile and tangential shear capacity of the bolts can improve the stress state of the specimen. Under load, the specimen is a ected by both the shear action along the bedding plane and the tension action along the axis (Figure 5a). When the specimen is deformed, the tension and shear action are applied to the bolt, and the bolt body provides support resistance, which limits the deformation of the specimen. Based on the above findings, we inferred that the anchoring increases the specimen strength and changes the failure plane direction by improving the tensile capacity and tangential shear-bearing capacity. Under axial compression, the specimen simultaneously bears the shear e ect along the bedding and the tensile e ect along the axial direction (as shown by Figure 5a), while the bolt body provides support resistance to restrain the specimen deformation. Based on the analysis of the bolt reinforcing performance, we determined that a di erence existed between the reinforcing e ect of the bolt for full-length anchoring and that for end anchoring. The gap between the bolt body for full-length anchoring and the surrounding rock was filled by the anchoring agent, which could bond and reinforce the surrounding rock with greater scope. In the case of the rock developing shear deformation, the bolt with full-length anchoring could immediately restrain further deformation. Meanwhile, a gap existed between the end anchoring bolt and surrounding rock. The bolt’s tangential anchoring force only played a role when the surrounding rock developed a certain tangential deformation. Therefore, the bolt for full-length anchoring could form an “anchoring area” of greater range and provide greater support resistance than the end anchoring bolt. 3.2. Shear Testing The shear–displacement curves of di erent anchoring methods, which were measured by strain gages, are shown in Figure 7. The average maximum shear force values of the no-anchoring specimen, end anchoring specimen, and full-length anchoring specimen were, respectively, 3.55 kN, 15.34 kN, and 17.35 kN, as shown in Figure 8. It is evident that the maximum shear force of the end anchoring specimen was increased by 332.11% while the maximum shear force of the full-length anchoring specimen was increased by 13.10% compared with that of the end anchoring specimen. The shear–displacement curve of the no-anchoring specimen mainly shows the shear deformation process of the joint plane. Figure 7a shows that the specimen develops buckling failures as it is loaded to its ultimate load, resulting in a loss in the bearing capacity and representing a brittle feature. Moreover, the end anchoring specimen develops a “turning point” of a sudden drop and then a rise in shear force prior to its full yield to failure, as shown by Figure 7b. This occurrence was not observed in the other two cases. This point can be regarded as the decision point in terms of whether the bolt of the end anchoring specimen engages its shear resistant e ect. Before this point, the joint plane mainly bears the shear e ect. The extent of its shear sti ness mainly depends on the friction force of the plane and the pre-stressing force of the bolt. At failure, the joint plane su ered shear failures, and the specimen at both sides developed relative sliding, thus mobilizing the shear strength of the bolt itself. Therefore, this point can also be referred to as the “yield point” of the joint plane and the “mobilization point” of the bolt’s shear strength. After this point, the shear strength of the bolt body enhanced the comprehensive shear-bearing performance of the joint plane. Meanwhile, the shear force increased slowly with increasing shear displacement. These are Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 15 of the full-length anchoring specimen and end anchoring specimen's shear–displacement curves varied somewhat (Figure 7(c)). The bolt of the full-length anchoring specimen was in tight contact with the concrete via the anchoring agent. It could resist the shear resistance effect immediately under tAppl. he loa Sci. d ef 2020 fec , 10 t unt , 2329 il the specimen developed buckling failures. The bolt of the full-length anchoring 9 of 15 specimen and its joint plane jointly bore the shear load. No “turning point” developed. The same occurrence was observed in the end anchoring case. Compared to the no-anchoring specimen, the called the plastic phase and plastic strength phase [34,43]. Others refer to them as the slowly increasing end anchoring specimen and full-length anchoring specimen represented a larger ultimate load, resistance phase [24]. S-N-1 S-E-1 S-N-2 S-E-2 S-N-3 S-E-3 02 46 8 10 12 14 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Shear displacement (mm) Shear displacement (mm) (a) (b) S-FL-1 S-FL-2 S-FL-3 0 2 4 6 8 101214 1618 Shear displacement (mm) (c) Appl. Sci. Figure 7. 2020, 10, x FO Shear–displa R PEER Rcem EVIEW ent cu rves: (a) No bolt; (b) End anchoring; (c) Full-length anchoring. 10 of 15 Figure 7. Shear–displacement curves: (a) No bolt; (b) End anchoring; (c) Full-length anchoring. Scattered point Mean value End Full-length No bolt anchoring anchoring Figure 8. Shear force of the specimens by di erent anchoring methods. Figure 8. Shear force of the specimens by different anchoring methods. The typical strain distribution curves of the end anchoring specimen and full-length anchoring specimen segments obtained by the test are shown in Figure 9. The following can be observed from the strain distribution curves. • The maximum value of the bolt axial force is near the joint plane and is anti-symmetrically distributed at both sides of the joint plane. • The axial force of the bolt for full-length anchoring is mainly concentrated near the joint plane. It decreases rapidly with increasing distance from the joint plane, and its distribution is relatively uniform. • Plastic hinges are produced near the joint plane, which can effectively stop the further spread of the stress force. One end of the plastic hinge bears the tensile stress, while the other end bears the pressure stress. Joint S-FL-1 S-E-2 Cable Strain gauge -500 -1000 -1500 -2000 0 20 40 60 80 100 120 140 160 Anchor length (mm) Figure 9. Strain distribution curves of the bolts. 4. Field Application 4.1. Project Description The test field was the Mazhui Tunnel of DaoZhen Highway from Nanchuan of Chongqing to Guizhou, China. It was designed to be a bidirectional four-lane highway, with a full length of 3711 m and a maximum burial depth of 441 m. The lithological character of the test section is Silurian Longmaxi Formation shale with thin bedding, fracture development, and abundant underground water. The tunnel has a composite lining and is primary supported by bolts, sprayed concrete, and Shear force (kN) Anchor strain (με) Shear force (kN) Shear force (kN) Shear force (kN) Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 15 Appl. Sci. 2020, 10, 2329 10 of 15 Scattered point Mean value In accordance with the characteristics of the bolt resistance increase, the bolt developed plastic hinges at both sides of the joint plane. When the strength of the bolt or concrete reached its limit strength, the anchored specimen developed buckling failures. It was also observed that the patterns of the full-length anchoring specimen and end anchoring specimen’s shear–displacement curves varied somewhat (Figure 7c). The bolt of the full-length anchoring specimen was in tight contact with the concrete via the anchoring agent. It could resist the shear resistance e ect immediately under the load e ect until the specimen developed buckling failures. The bolt of the full-length anchoring specimen and its joint plane jointly bore the shear load. No “turning point” developed. The same occurrence was observed in the end anchoring case. Compared to the no-anchoring specimen, the end anchoring specimen and full-length anchoring specimen represented a larger ultimate load, a more rapid and End Full-length greater increase of bolt resistance, a longer plastic strength phase, and a stronger residual shear-bearing No bolt anchoring anchoring capacity after buckling. The typical strain distribution curves of the end anchoring specimen and full-length anchoring Figure 8. Shear force of the specimens by different anchoring methods. specimen segments obtained by the test are shown in Figure 9. The following can be observed from the strain distribution curves. The typical strain distribution curves of the end anchoring specimen and full-length anchoring specimen segments obtained by the test are shown in Figure 9. The following can be observed from The maximum value of the bolt axial force is near the joint plane and is anti-symmetrically the strain distribution curves. distributed at both sides of the joint plane. • The maximum value of the bolt axial force is near the joint plane and is anti-symmetrically The axial force of the bolt for full-length anchoring is mainly concentrated near the joint distributed at both sides of the joint plane. plane. It decreases rapidly with increasing distance from the joint plane, and its distribution is • The axial force of the bolt for full-length anchoring is mainly concentrated near the joint plane. relatively uniform. It decreases rapidly with increasing distance from the joint plane, and its distribution is relatively Plastic hinges are produced near the joint plane, which can e ectively stop the further spread of uniform. the stress force. One end of the plastic hinge bears the tensile stress, while the other end bears the • Plastic hinges are produced near the joint plane, which can effectively stop the further spread of pressure stress. the stress force. One end of the plastic hinge bears the tensile stress, while the other end bears the pressure stress. Joint S-FL-1 S-E-2 Cable Strain gauge -500 -1000 -1500 -2000 0 20 40 60 80 100 120 140 160 Anchor length (mm) Figure 9. Strain distribution curves of the bolts. Figure 9. Strain distribution curves of the bolts. 4. Field Application 4. Field Application 4.1. Project Description 4.1. Project Description The test field was the Mazhui Tunnel of DaoZhen Highway from Nanchuan of Chongqing to The test field was the Mazhui Tunnel of DaoZhen Highway from Nanchuan of Chongqing to Guizhou, China. It was designed to be a bidirectional four-lane highway, with a full length of 3711 Guizhou, China. It was designed to be a bidirectional four-lane highway, with a full length of 3711 m m and a maximum burial depth of 441 m. The lithological character of the test section is Silurian and a maximum burial depth of 441 m. The lithological character of the test section is Silurian Longmaxi Formation shale with thin bedding, fracture development, and abundant underground Longmaxi Formation shale with thin bedding, fracture development, and abundant underground water. The tunnel has a composite lining and is primary supported by bolts, sprayed concrete, and water. The tunnel has a composite lining and is primary supported by bolts, sprayed concrete, and reinforcing mesh. The parametric description is C20 sprayed concrete of a thickness of 18 cm, F22 Anchor strain (με) Shear force (kN) Appl. Sci. 2020, 10, 2329 11 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 15 cartridge bolts with inter-row spacing of 100  120 cm and 2.5 m in length, and 18# joist steel with reinforcing mesh. The parametric description is C20 sprayed concrete of a thickness of 18 cm, Ф22 longitudinal spacing of 1 m. The secondary lining was molded concrete with a thickness of 40 cm of reinfo cartridge rcing m bolts esh. The pa with inter-row rametric descr spacing ipof tion is C2 100 × 120 0 spray cm ed conc and 2.5 ret m e of a t in length, hickness o and 18# f 18 c joist m, Ф steel 22 C25 sprayed concrete. The support parameters of the tunnel section and geological conditions of the cartridge bolts with inter-row spacing of 100 × 120 cm and 2.5 m in length, and 18# joist steel with testing face surrounding rock are shown in Figure 10. with (a) (b) (a) (b) Figure 10. Field test: (a) Support parameters of the tunnel section; (b) Surrounding rock of the testing Figure 10. Field test: (a) Support parameters of the tunnel section; (b) Surrounding rock of the Figure 10. face. Field test: (a) Support parameters of the tunnel section; (b) Surrounding rock of the testing testing face. face. 4.2. Bolt Arrangement at the Construction Site 4.2. Bolt arrangement at the construction site 4.2. Bolt arrangement at the construction site Bolts with lengths of 3.5 m with diameters of 22 mm were used in the field. The measuring bolts Bolts with lengths of 3.5 m with diameters of 22 mm were used in the field. The measuring bolts and vibration wire steel stress gauges were installed to measure the axial force of the bolts, as shown and v Bol ibrat ts wi ion th wire lengths of steel st 3r .5 e m ss ga wi uge th diameters of s were installed 22 mm to meas were used in ure the ax tia he fie l force o ld. The f thmeas e bolturin s, as g bolt show s n in Figure 11. Each measuring bolt was installed with three vibrating wire steel stress gauges with and vibration wire steel stress gauges were installed to measure the axial force of the bolts, as shown in Figure 11. Each measuring bolt was installed with three vibrating wire steel stress gauges with corresponding measuring lines. At the measuring station, the plug of each measuring line was inserted in correspondin Figure 11. Each me g measur asing uring bolt was lines. At th installed w e measurinig th three vibrating wire station, the plug of esteel stress ach measurin gauge g lin s wit e was h into the frequency recorder to record the value. Each vibrating wire steel stress gauge was calibrated corresponding measuring lines. At the measuring station, the plug of each measuring line was inserted into the frequency recorder to record the value. Each vibrating wire steel stress gauge was indoors to obtain its reference frequency and calibration coecient prior to its installation and use. inserted calibrated into ind the fre oors tq ouenc obtay in recorder its referenc to re ecord the frequency an value. E d caa libr ch v ati ibr on ating w coefficient ire ste prel stre ior to it ss g s inst aug ae wa llatios n The frequencies obtained from the field test were computed on the basis of a calibration formula to calibrated and use. The indoors frequenc to ies obtain obtained its reference from the fie frequency ld test were comp and calibration uted on t coefficient he basis of a prior calib to ratits ion obtain the axial force of the bolt segments. installation formula to obt and use. The frequencies obtained from the field test ain the axial force of the bolt segments. Figure 11. A measuring bolt. Figure 11. A measuring bolt. Figure 11. A measuring bolt. th Two site sections in the field ZK17+360 to ZK17+365 (“ZK17” means the 17 kilometer of left line th Two si Two site te sections sections i innthe the fi field eld ZK17 ZK17+360 +360 to ZK17 to ZK17+365 +365 (“ (“ZK17” ZK17” means mean the s the 17 17th kilometer kilometer of left of left line line of DaoZhen Highway, “360” and “365” mean detailed distance(meters) in ZK17.) were selected to test the of DaoZhen H ofbolt DaoZhen axial fo Highway ighway, “360” rce vari,at “360” ionand “365” s. Thes and “365” e s mean detailed dist ites were mean detailed the tu ance(meters) nnel distance(meters) evacuin ZK17. ation fac in ) were sel e ZK17.) and second wer ected to test the e selected ary lining to , bolt test axi the bolt al fo axial rce var force iation variations. s. These s These ites were sites wer the t e the unnel tunnel evac evacuation uation fac face e an and d second secondary ary llining, ining, respectively. In each section, seven bolts were installed. To compare the impact of the anchoring respectively. respectively methods on the anchorage effect, the Z . In each section, seven bolts were in In each section, seven boltsK wer 17+360 se e installed. stalled. To comp ction adopt To compar ed f are the imp u ellthe -lengt impact h an act of the anc choring, wh of the anchoring hil oring e the methods on the anchorage effect, the Z methods on the anchorage e ect, the ZK17 K17+360 se +360 section ction adopt adopted ed f full-length ull-length an anchoring, choring, wh while ile t the he ZK17+365 section adopted the end anchoring scheme. The distribution method of the measuring bolts ZK1 ZK17 and 7+3 + si365 te const 65 secti section r oucti n adopted t adopted on are shown i the he end a end n anchoring n Fi chori gure 12 ng scheme. scheme. . The test The distri The fiedistribution ld sim bution method of ultaneously m method of easured the measuri the measuring the per ng bol iphery bolts ts a and nd site site const constr ructi uction on a ar re shown i e shown in n Fi Figur gure 12 e 12.. The The test test field field simultaneously simultaneously m measur easured ed the per the periphery iphery convergence and arch crown settlement of the two sections to analyze the support effect of the bolt. convergence conver The peripher gence and and y converg arch cro arch crenc own wn settlemen e was me settlement asured by t of the two sections to an of the two a JSS3 sections 0A converge to analyze alyze the support effect o nce g the auge, w support hile e the ect arch of f the bolt. the crown bolt. The periphery convergence was measured by a JSS30A convergence gauge, while the arch crown settlement was measured by a DSZ2 automatic compensated level. settlement was measured by a DSZ2 automatic compensated level. Appl. Sci. 2020, 10, 2329 12 of 15 The periphery convergence was measured by a JSS30A convergence gauge, while the arch crown settlement was measured by a DSZ2 automatic compensated level. Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 15 (a) (b) Figure Figure 12. 12. S Section ection ddistribution istribution and and sisite te ininstallation stallation of the of the me measuring asuring bolts: ( bolts: a) Distributi (a) Distribution on of the of the (a) (b) measuring measuring bolts; ( bolts; (b b)) Installation Installation of the mea of the measuring suring bolts. bolts. Figure 12. Section distribution and site installation of the measuring bolts: (a) Distribution of the The ultimate axial force distribution of the measuring bolts at each section is shown in Figure 13. The u measu lti rmat ing bolts; ( e axia bl ) f Instal orce lati di on of the mea stribution of sur ing bolts. the mea suring bolts at each section is shown in Figure The positive sign represents compressive force; the negative sign represents tensile force. The figure 13. The positive sign represents compressive force; the negative sign represents tensile force. The The ultimate axial force distribution of the measuring bolts at each section is shown in Figure shows that, at section ZK17+360, all bolts mainly bear tensile stress. As a whole, the stress borne by figure shows that, at section ZK17+360, all bolts mainly bear tensile stress. As a whole, the stress 13. The positive sign represents compressive force; the negative sign represents tensile force. The bolts in the ZK17+360 section is greater than that borne by bolts in the ZK17+365 section. That is, borne by bolts in the ZK17+360 section is greater than that borne by bolts in the ZK17+365 section. figure shows that, at section ZK17+360, all bolts mainly bear tensile stress. As a whole, the stress under full-length anchoring, the bolts can provide greater support resistance and hence obtain a That is, under full-length anchoring, the bolts can provide greater support resistance and hence borne by bolts in the ZK17+360 section is greater than that borne by bolts in the ZK17+365 section. better anchorage e ect. The axial forces of bolts in di erent parts of the test section also have certain obtain a better anchorage effect. The axial forces of bolts in different parts of the test section also have That is, under full-length anchoring, the bolts can provide greater support resistance and hence regularities, as outlined below. certain regularities, as outlined below. ( (a a)) ((b) b) Figure 13. Axial force (KN) distribution of the test section bolts: (a) ZK17+360 section; (b) ZK17+365 Figure Figure 13. 13. Ax Axial ial force for (KN) ce (KN) distribu distribution tion of the of test the section test section bolts: (abolts: ) ZK17+36 (a) 0 se ZK17 ction; ( +360 b)section; ZK17+365 (b) section. ZK17 sectio+ n. 365 section. The #2 bolt on the right arch shoulder bears the maximum stress., while the maximum tensile The #2 bolt on the right arch shoulder bears the maximum stress., while the maximum tensile The #2 bolt on the right arch shoulder bears the maximum stress., while the maximum tensile stresses are distributed near the middle of the bolt. These are 55.2 kN (C2 element for full-length stresses are distributed near the middle of the bolt. These are 55.2 kN (C2 element for full-length stresses are distributed near the middle of the bolt. These are 55.2 kN (C2 element for full-length anchoring) and 27.7 kN (C2 element for end anchoring). This is consistent with the deformation anchoring) and 27.7 kN (C2 element for end anchoring). This is consistent with the deformation anchoring) and 27.7 kN (C2 element for end anchoring). This is consistent with the deformation distribution of the tunnel’s surrounding rock, as shown in Figure 14. Moreover, the maximum distribution of the tunnel’s surrounding rock, as shown in Figure 14. Moreover, the maximum distribution of the tunnel’s surrounding rock, as shown in Figure 14. Moreover, the maximum deformation point of the tunnel arch crown is located at the right spandrel. At the construction site, deformation point of the tunnel arch crown is located at the right spandrel. At the construction site, deformation point of the tunnel arch crown is located at the right spandrel. At the construction site, this location frequently developed cracking and chipping, and it even extruded and deformed the this location frequently developed cracking and chipping, and it even extruded and deformed the this loc lattice gi ation rder. freq uently developed cracking and chipping, and it even extruded and deformed the lattice girder. lattice girder. The axial force of the bolt on the wall at each side of the test section was relatively small (5 to 10 kN), especially on bolts #6 and #7 on the wall at each side. These were installed after the evacuation of the lower bench, while the deformation of the tunnel’s surrounding rock tended to be stable after Appl. Sci. 2020, 10, 2329 13 of 15 evacuation of the lower bench. Bolts in this part did not engage their anchoring e ects; rather, they became a kind of safety stock. Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 15 10 Left point Middle point Right point 0 5 10 15 20 25 30 Time (d) Figure 14. Arch crown settlement curve of the ZK17+365 section Figure 14. Arch crown settlement curve of the ZK17+365 section 5. Discussion The axial force of the bolt on the wall at each side of the test section was relatively small (5 to 10 kN), especially on bolts #6 and #7 on the wall at each side. These were installed after the evacuation In laboratory experiments, the specimen needs to be drilled when installing the bolt. In the process of the lower bench, while the deformation of the tunnel’s surrounding rock tended to be stable after of drilling, the drill stem may disturb the specimen and cause damage to the rock around the drill hole. evacuation of the lower bench. Bolts in this part did not engage their anchoring effects; rather, they Although the rock morphology near the borehole is examined in the test process (to check that there became a kind of safety stock. are no macroscopic damage fractures), the degree of damage to the specimen when drilling and its 5 Discussion influence on the test are open to question. In engineering, carrying out on-site bolt construction will also cause damage to the surrounding rock in a certain area near the borehole. How to quantitatively In laboratory experiments, the specimen needs to be drilled when installing the bolt. In the evaluate and compare the relationship between indoor testing and on-site construction is worthy of process of drilling, the drill stem may disturb the specimen and cause damage to the rock around the drill hole. Although the rock morphology near the borehole is examined in the test process (to check further research and discussion. that there are no macroscopic damage fractures), the degree of damage to the specimen when drilling 6. Conclusions and its influence on the test are open to question. In engineering, carrying out on-site bolt construction will also cause damage to the surrounding rock in a certain area near the borehole. How In this paper, we investigated the bolt anchoring performance for bedded rock mass under to quantitatively evaluate and compare the relationship between indoor testing and on-site di erent anchoring methods, as well as the failure mode under shear and tensile stresses in bedded construction is worthy of further research and discussion. rock. Then, based on field tests, we analyzed the force and support e ects of the di erent anchoring methods. 6 Conc The lusiconclusions on are as follows: In this paper, we investigated the bolt anchoring performance for bedded rock mass under (1) With respect to the anchored specimen strength and strain, the reinforcing mechanism of the different anchoring methods, as well as the failure mode under shear and tensile stresses in bedded bolt for the rock was divided into two aspects: the anchoring agent bonding and restoring rock. Then, based on field tests, we analyzed the force and support effects of the different anchoring the surrounding rock near the reinforcing zone, and the bolt body resistance supporting and methods. The conclusions are as follows: improving the stress state of the rock. The combined action of those two aspects increased the (1) With respect to the anchored specimen strength and strain, the reinforcing mechanism of the strength parameters of the anchored specimen. bolt for the rock was divided into two aspects: the anchoring agent bonding and restoring the (2) Failur surro e ofundin the anchor g rock near the rein ed specimen changed forcing zone, from shear and tsplitting he bolt body resista failure withnno ce anchoring supporting into andshear improving the stress state of the rock. The combined action of those two aspects increased the failure where the failure plane slid parallel to the axial direction of the bolt or shear dislocation strength parameters of the anchored specimen. failure along the soft–hard interface. (2) Failure of the anchored specimen changed from shear splitting failure with no anchoring into (3) Via anchoring of the joint rock mass, the bolt could significantly enhance the shear-bearing shear failure where the failure plane slid parallel to the axial direction of the bolt or shear capacity of the rock mass and increase the stability of the surrounding rock. Compared to the dislocation failure along the soft–hard interface. end anchoring bolt, the bolt for full-length anchoring can form an “anchoring area” of a greater (3) Via anchoring of the joint rock mass, the bolt could significantly enhance the shear-bearing range and provide greater support resistance than the end anchoring bolt; therefore, it had a capacity of the rock mass and increase the stability of the surrounding rock. Compared to the better coupling e ect on the surrounding rock with a greater resistance increase. This occurrence end anchoring bolt, the bolt for full-length anchoring can form an “anchoring area” of a greater enabled range the and surr prov ounding ide greartock er su to pp bear ort re mor sistance e load. than the end anchoring bolt; therefore, it had a better coupling effect on the surrounding rock with a greater resistance increase. This occurrence (4) Full-length anchoring can provide more support resistance and have a better anchoring e ect on enabled the surrounding rock to bear more load. the surrounding rock of a bedded rock tunnel. However, the grouting quality is often dicult to Deformation (mm) Appl. Sci. 2020, 10, 2329 14 of 15 guarantee due to the impact of the bolt insertion angle, so it is necessary to pay attention to the filling quality of the anchoring agent, especially at the vault of the tunnel. Author Contributions: Z.Z. analyzed the calculation results. Y.L. carried out the laboratory test and wrote the article. 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Analytical models for rock bolts. Int. J. Rock Mech. Mining Sci. 1999, 36, 1013–1029. [CrossRef] © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

An Investigation into Bolt Anchoring Performance during Tunnel Construction in Bedded Rock Mass

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Abstract

applied sciences Article An Investigation into Bolt Anchoring Performance during Tunnel Construction in Bedded Rock Mass 1 , 2 1 , 2 3 1 , 2 , 1 , 2 Zhiqiang Zhang , Yin Liu , Junyang Teng , Heng Zhang * and Xin Chen School of Civil Engineering, Southwest Jiaotong University, Chengdu 610031, China; clark@swjtu.edu.cn (Z.Z.); liuyin901209@163.com (Y.L.); chenxin8090@outlook.com (X.C.) Key Laboratory of Transportation Tunnel Engineering, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China; jteng89@hotmail.com * Correspondence: tunnelzh@home.swjtu.edu.cn; Tel.: +86-028-8763-4386 Received: 18 January 2020; Accepted: 22 March 2020; Published: 28 March 2020 Abstract: The anchor bolt is a key point of tunnel design in bedded rock mass. The previous theory of anchorage support falls does not fulfil engineering requirements, and the stability of bedded rock must be addressed by empirical methods. To investigate the bolt anchoring performance for bedded rock mass under di erent anchoring methods, the rock failure mode under shear and tensile stresses in bedded rock was examined in this paper. The results showed that bolt anchoring for rock is achieved mainly through the bonded restoration of surrounding rock near the drill holes by means of an anchoring agent and the supporting resistance provided by the bolt body. It was observed that the strength parameters of bedded rock were increased under the anchoring e ect. Full anchoring bolts were especially e ective. In addition, it was observed that, in the absence of bolts, the failure form changed from shear to split. In the case of bolting, the failure plane occurred parallel to the bolt’s axis. The shearing began along the interface between the hard and soft rock bedding. Compared to end bolt anchoring, full-length bolt anchoring was more capable of o ering an anchoring e ect. The latter o ered a greater increase in the strength and greater shear-bearing capacity of the rock, which ultimately enabled the rock to bear more load. Keywords: tunnel engineering; anchoring performance; bedded rock mass; laboratory experiment; field application; failure mode; support resistance 1. Introduction In the past decade, the tunnels and underground projects built in China under complex geological and environmental conditions have made great progress [1–6]. Meanwhile, it also faces a series of construction diculties and challenges [7–10]. The tunnel composite support system is divided into a primary support and secondary lining (usually concrete lining). The primary support is vital to the tunnel stability and mainly consists of a bolt, sprayed concrete, and reinforcing mesh, and it is supplemented by joist steel or a lattice girder in accordance with the surrounding rock. Moreover, the bolt is the most important support structure of the primary support on account of its high eciency, economic advantages, and reduced space occupation. Shale forms many bedded and fractured structural planes during the diagenetic process of compaction and cementation, which seriously a ects the tunnel stability. The bolt anchoring characteristics for jointed rock mass have been the focus of intense research in China and abroad. The tunnel bolt is mainly designed for full-length anchoring. During bolting at the construction site, a hole is drilled in the tunnel’s surrounding rock. Then, after grouting or resin application, the Appl. Sci. 2020, 10, 2329; doi:10.3390/app10072329 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 2329 2 of 15 body (bolt) is inserted and screwed. In theory, the bolt and surrounding rock mass are bonded in full length via the anchoring agent; however, as the location and angle of the hole di er, bolt construction diculties vary, especially for the tunnel arch part. Furthermore, it is dicult to ensure the grouting quality of the anchoring agent, which causes the anchorage length of the bolt to vary. Therefore, it is necessary to investigate the impact of anchoring and the stability e ect of the jointed rock mass. The bolt reinforcement e ect on jointed rock mass mainly enhances the shear-bearing capacity of the rock mass joint plane. In addition, it prevents the rock mass from developing an intercalated dislocation along the joint plane [11,12]. Many factors a ect the reinforcing e ect of the bolt on the joint plane, such as the bolt size, hole diameter, pretension force, grouting, and anchored rock mass strength [13–21]. Many studies have been conducted on the above aspects. In 1974, Bjurstrom [11] conducted systematic research on the shear property of granite under full-length anchoring and showed that the tangential shear-bearing capacity of the bolt can significantly enhance the stability of the jointed rock mass. Spang et al. [22] and Haas et al. [23] researched the impact of the bolt on the joint shear-bearing capacity of di erent rocks. Using anchorage tests of jointed rock mass, Yoshinaka et al. [24], Ferrero [25], and Kim et al. [26] analyzed the impact of factors such as the quantity of bolts, elasticity modulus of the bolt body, bolt material, and roughness of the rock joint on the joint shear-bearing capacity. Moreover, Pellet et al. [27] theoretically analyzed the bolt shear-bearing capacity and evaluated the impact of the anchoring angle on the anchoring e ect. Grasselli [28] and Jalalifar et al. [29] conducted laboratory shear tests of anchored and jointed rock mass and employed numerical simulation methods to analyze the shear resistance e ect of the bolt. They respectively showed that the bolt can form plastic hinges on the joint plane, and that bolt failures mainly occur among the plastic hinges soon thereafter. Meanwhile, Zhang et al. [30] conducted research on the deformation property of a pre-tensioned bolt during a shear test. They determined that the bolt shear-bearing e ect occurred after the shear dislocation of the joint plane on account of the bolt “dowelling function”. Teng et al. [31] compared failure modes of end anchoring, full-length anchoring, and non-anchoring by experiment. The result showed that the failure modes of anchored specimens are a ected by anchoring type, and they are further divided into shearing extension and shearing o set. Wang et al. [32] simulated the failure mechanism of tunnel segmental lining joints and confirmed that the deformation of the circumferential joints consisted of opening and dislocation, but dislocation was dominant. In addition, to explore the impact of various factors, Ge et al. [33] carried out shear testing of di erent bolt sizes, materials, installation angles, specimen strengths, and others. Based on their findings, Chen et al. [34] established a computational formula of structural-plane-anchored specimen shearing strength, which they verified through simulation tests. Furthermore, Liu et al. [35] employed a physical simulation method to assess the impact of the bolt pre-stress force on the shear-bearing capacity of rock mass. Zhang et al. [36] studied the mechanical properties of fractured rock mass under anchoring conditions and uniaxial compression. They further verified the bolt “dowelling function” in terms of the fractured rock mass. Chen et al. [37,38] used a method of anchoring the origin rock specimens and performed respective tensile, uniaxial compression, and pressure–shear tests on rocks. They detailed the rules of crack initiation, extension, and so on of the anchored specimens and verified the relationships between the anchorage and enhancement of the specimen’s mechanical strengths (tensile, compressive, and shear). In addition, they analyzed the anchoring performance. Based on the classical beam theory and the variational principle of minimum complementary energy, Yang et al. [39] analyzed and determined the resisting mechanical behavior of anchor bolts for di erent rock mass strengths and bolt diameters. Zhang et al. [40] conducted conventional static and dynamic drawing load tests on bonding bolts with end anchorage. The experimental results showed that the distribution of axial stress of a bonded anchor bolt is triangular under static loading. Zhu et al. [41] designed an artificial material and loading system to study the influence of bedding cohesion and anchoring behavior of bedded rock mass. The results showed that the axial stress–strain curve of Appl. Sci. 2020, 10, 2329 3 of 15 bedded rock mass under the reinforcement of bolts presents the features of strain softening and secondary strengthening. As shown above, considerable studies and research results have helped elucidate the jointed rock mass anchoring performance. Obviously, however, many other factors a ect the bolt shearing property for jointed rock mass, and the complicated rock strengthening of the jointed rock mass by bolting remains not fully understood. Moreover, the existing theory of anchorage support falls does not fulfil engineering requirements, and designers have to adopt empirical methods to address the stability of rock in most cases. The intention of this paper is to explore the performance of bolts for bedded and jointed rock masses and to figure out the mechanical properties and failure mode of the rock by di erent anchoring methods. This study is organized as follows. Section 1 reviews the previous studies made in the field of bolt reinforcement e ects. In Sections 2 and 3, a laboratory test program is designed to analyze the mechanical properties (under uniaxial and shear force) from the viewpoint of mechanical e ects and the failure modes of jointed rock with di erent anchoring methods. Section 4 presents verification of the bolt anchoring e ect on a bedded shale tunnel by means of a site test. Finally, Section 5 concludes the current study. 2. Experimental Procedure C15 specimen material was prepared by mixing cement of river sand and quick lime at a ratio of 1.3:1.5 (river sand to quick lime). It was cured for 28 days at room temperature. The specimens for the uniaxial compression test were prepared with bedding, where mica was selected as the bedding structure. Sheets of mica (100 mesh fineness) were evenly laid between two layers, and the distance between two consecutive layers was 15 mm. The cores were drilled perpendicular to the bedding and were prepared according to the testing standards, i.e., 100 mm in height and 50 mm in diameter. Precast concrete was cut into a cube with a size of 50 mm  100 mm  100 mm for the shear test [42]. To bolster the bolt, #45 steel was processed into the screw, and the bolt diameter was 5 mm. The anchoring agent was properly weakened by using chemical grout mixed with ethyl alcohol. The mechanical properties of the screw and bolt are shown in Table 1. Table 1. Mechanical parameters of the bolt and screw. Material Size/mm Tensile Strength/MPa Shear Strength/MPa Anchoring Force/MPa Normal bolt F16~22 200~600 260~600 50 Selected screw F5 800 400 30~40 The uniaxial compression test and shear test were both performed on an MTS815.03 Electro-hydraulic Servo-controlled Rock Mechanics Testing System (MTS815) rock mechanical experiment system. Both adopted displacement control for loading at a displacement rate of 0.1 mm/min. 2.1. Uniaxial Compression Testing According to the size ratios of the bolt and screw, the geometric similarity ratio of the bolt for the uniaxial compression test was determined to be 4:1. With consideration of the specimen’s geometric size, the geometric similarity ratio of its anchoring parameters was designed to be 13.3:1. Three di erent anchoring schemes were selected: no anchoring, end anchoring, and full-length anchoring. For each kind of anchoring method, three specimens were fabricated to reduce the discreteness. For the anchoring, a torque wrench was used to impose a pre-tightening force of 10 kN. The bedded and anchored specimens are shown in Figure 1. 2.2. Shear Testing The bolt used for the shear testing was bonded with a highly sensitive strain gauge to measure the axial strain value of the bolt during the test. For each selected scheme (mentioned above), three Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 15 Appl. Sci. 2020, 10, 2329 4 of 15 50 mm samples were tested to rule out any error. After the installation, anchored specimens were maintained at normal temperature for seven days, thereby enabling the anchoring agent to completely coagulate. Next, laboratory shear testing was performed. A schematic diagram of the strain gauge arrangement, cable anchored specimen, and test process is shown in Figure 2. Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 15 50 mm bedding cable (a) (b) Figure 1. Uniaxial compression test: (a) Dimensions of an anchored specimen; (b) Test process. bedding 2.2. Shear testing The bolt used for the shear testing was bonded with a highly sensitive strain gauge to measure the axial strain value of the bolt during the test. For each selected scheme (mentioned above), three samples were tested to rule out any error. After the installation, anchored specimens were maintained (a) (b) at normal temperature for seven days, thereby enabling the anchoring agent to completely coagulate. Figure 1. Uniaxial compression test: (a) Dimensions of an anchored specimen; (b) Test process. Figure 1. Uniaxial compression test: (a) Dimensions of an anchored specimen; (b) Test process. 2.2. Shear testing P The bolt used for the shear testing was bonded with a highly sensitive strain gauge to measure bedding the axial strain value of the bolt during the test. For each selected scheme (mentioned above), three samples were tested to rule out any error. After the installation, anchored specimens were maintained at normal temperature for seven days, thereby enabling the anchoring agent to completely coagulate. Next, laboratory shear testing was performed. A schematic diagram of the strain gauge arrangement, anchored specimen, and test process is shown in Figure 2. cable bedding strain gauge (a) (b) cable strain gauge (a) (b) (c) Figure 2. Schematic diagram of the strain gauge arrangement and an anchored specimen for shear Figure 2. Schematic diagram of the strain gauge arrangement and an anchored specimen for shear testing: (a) Arrangement of the strain gauge; (b) Anchored specimen; (c) Test process. testing: (a) Arrangement of the strain gauge; (b) Anchored specimen; (c) Test process. 3. Experimental Results In the figures of this section, each name in the legend is based on the following convention: test method + anchoring method + number of test group. Uniaxial compression testing and shear testing (c) are represented by “C” and “S”, respectively, for the test method. “N”, “E”, and “FL” denote no Figure 2. Schematic diagram of the strain gauge arrangement and an anchored specimen for shear anchoring, end anchoring, and full-length anchoring, respectively. testing: (a) Arrangement of the strain gauge; (b) Anchored specimen; (c) Test process. 20 mm 60 mm 20 mm 20 mm 60 mm 20 mm Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 15 3. Experimental Results In the figures of this section, each name in the legend is based on the following convention: test method + anchoring method + number of test group. Uniaxial compression testing and shear testing Appl. Sci. 2020, 10, 2329 5 of 15 are represented by “C” and “S”, respectively, for the test method. “N”, “E”, and “FL” denote no anchoring, end anchoring, and full-length anchoring, respectively. 3.1. Uniaxial Compression Testing 3.1. Uniaxial compression testing The axial displacement was directly measured using an MTS815 rock mechanical experiment system; curves of the specimens’ axial strain for the earlier mentioned anchoring methods (no anchoring, The axial displacement was directly measured using an MTS815 rock mechanical experiment end anchoring, and full-length anchoring) are shown in Figure 3. C-N-1 C-E-1 C-N-2 C-E-2 5 (d) (e) C-E-3 C-N-3 (c) (b) (a) 02 468 10 12 02 46 8 10 12 -3 -3 Axial strain (10 ) Axial strain (10 ) (a) (b) C-FL-1 C-FL-2 8 C-FL-3 02 468 10 12 -3 Axial strain (10 ) (c) Figure 3. Stress–strain curve: (a) No anchoring; (b) End anchoring; (c) Full-length anchoring. Figure 3. Stress–strain curve: (a) No anchoring; (b) End anchoring; (c) Full-length anchoring. A close examination of the figure reveals the following key observations. A close examination of the figure reveals the following key observations. Five phases of the stress–strain curve in every case are obvious. Taking curve C-N-3 in Figure 3a Five phases of the stress–strain curve in every case are obvious. Taking curve C-N-3 in Figure as an example, we see the (a) initial compression phase, (b) elastic deformation phase,(c) phase of 3(a) as an example, we see the a) initial compression phase, b) elastic deformation phase, c) phase of microfracture, (c) stable development phase, (d) phase of unstable fracture and development, and (e) microfracture, c) stable development phase, d) phase of unstable fracture and development, and e) post-fracture phase. In the first phase, the initial compression phase, the duration of the end anchoring post-fracture phase. In the first phase, the initial compression phase, the duration of the end specimen is longer than that with no anchoring; moreover, it is shortest for the full-length anchoring anchoring specimen is longer than that with no anchoring; moreover, it is shortest for the full-length specimen. The cause of this is analyzed below. anchoring specimen. The cause of this is analyzed below. The initial rock compression closure mainly refers to the closure of the rock’s internal structural The initial rock compression closure mainly refers to the closure of the rock’s internal structural plane and primary microfracture by compression. It is assumed that the secondary cracks produced plane and primary microfracture by compression. It is assumed that the secondary cracks produced during sample preparation (drilling, grouting, etc.) may have an adverse e ect on the bolt because the during sample preparation (drilling, grouting, etc.) may have an adverse effect on the bolt because anchoring range of the end anchoring bolt is relatively small. Its anchoring end is tightly bonded with the rock. Some space exists between the bolt body of the non-anchor segment and the rock. In the full-length anchoring specimen, the anchoring bolt is tightly bonded with the rock by the anchoring agent. Moreover, the anchoring agent has the e ect of bonded amalgam restoration on specimen damage. It forms a reinforcing area within a certain range around the bolt body. Therefore, σ (MPa) σ (MPa) σ (MPa) Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 15 the anchoring range of the end anchoring bolt is relatively small. Its anchoring end is tightly bonded with the rock. Some space exists between the bolt body of the non-anchor segment and the rock. Appl. Sci. 2020, 10, 2329 6 of 15 In the full-length anchoring specimen, the anchoring bolt is tightly bonded with the rock by the anchoring agent. Moreover, the anchoring agent has the effect of bonded amalgam restoration on at the implementation of the uniaxial compression test, for the case of no anchoring or horizontal specimen damage. It forms a reinforcing area within a certain range around the bolt body. Therefore, bedding specimens, the first phase is mainly the closure of the bedding plane. For the end anchoring at the implementation of the uniaxial compression test, for the case of no anchoring or horizontal specimens, it is not only bedding plane closure, but also the compression process of the space between bedding specimens, the first phase is mainly the closure of the bedding plane. For the end anchoring the bolt and rock. For the full-length anchoring specimens, it is mainly the compression of bedding in specimens, it is not only bedding plane closure, but also the compression process of the space between the non-anchor area. the bolt and rock. For the full-length anchoring specimens, it is mainly the compression of bedding After the bearing capacity of the no-anchoring specimen has reached the strength peak, the in the non-anchor area. specimen fails rapidly, and the section of the stress–strain curve after the peak is relatively steep. When After the bearing capacity of the no-anchoring specimen has reached the strength peak, the the bearing capacity of the anchored specimen has reached the strength peak, the load and resulting specimen fails rapidly, and the section of the stress–strain curve after the peak is relatively steep. plastic deformation of the specimen continue to increase. Thus, this section can be referred to as the When the bearing capacity of the anchored specimen has reached the strength peak, the load and “plasticity strengthening section”. resulting plastic deformation of the specimen continue to increase. Thus, this section can be referred After the anchoring of specimens, both the average uniaxial compressive strength and elasticity to as the “plasticity strengthening section”. modulus are increased [42]. As the anchoring methods are di erent, the increased range of the After the anchoring of specimens, both the average uniaxial compressive strength and elasticity anchored specimen di ers. In comparison with the no-anchoring specimens, as shown in Figure 4, the modulus are increased [42]. As the anchoring methods are different, the increased range of the uniaxial compressive strength of the end anchoring specimen is increased by 12.73%, while the uniaxial anchored specimen differs. In comparison with the no-anchoring specimens, as shown in Figure 4, compressive strength of the full-length anchoring specimen is increased by 62.71%. Similarly, the the uniaxial compressive strength of the end anchoring specimen is increased by 12.73%, while the elasticity modulus of the end anchoring specimen is increased by 6.31%, while the elasticity modulus uniaxial compressive strength of the full-length anchoring specimen is increased by of the full-length anchoring specimen is increased by 58.73%. 62.71%. 10 1300 Scattered point Scattered point Mean value Mean value End Full-length End Full-length No bolt No bolt anchoring anchoring anchoring anchoring (a) (b) Figure 4. Figure 4. Rock Rock strength parameters by strength parameters by di er dent ifferent anc anchoring horing method methods: (a) Uniaxial s: (a) Uniax compr ial essi cove mp str ressive ength; streng (b) Elasticity th; (b) Elasticity modulus.modulus. The failure patterns of specimens under di erent anchoring modes are shown in Figure 5. The failure patterns of specimens under different anchoring modes are shown in Figure 5. When there is no anchor, the failure mode of the specimen is mainly shear failure along the When there is no anchor, the failure mode of the specimen is mainly shear failure along the bedding and axial splitting failure perpendicular to the bedding. For end anchoring, the bonding bedding and axial splitting failure perpendicular to the bedding. For end anchoring, the bonding force of the anchoring agent at the end of the bolt limits the surrounding rock’s shear failure along force of the anchoring agent at the end of the bolt limits the surrounding rock’s shear failure along the bedding, and the "pin e ect" of bolts makes it dicult for the specimen to split along the axial the bedding, and the "pin effect" of bolts makes it difficult for the specimen to split along the axial direction. The specimen finally shows axial shear tensile failure, as shown in Figure 6a. direction. The specimen finally shows axial shear tensile failure, as shown in Figure 6(a). Under full-length anchorage, shear failure occurs along the middle or end of the specimen. Under full-length anchorage, shear failure occurs along the middle or end of the specimen. The The reason for this is that under the action of full-length bonding and bolt “pin action”, the strength of reason for this is that under the action of full-length bonding and bolt "pin action", the strength of the the specimen near the two ends of the bolt is increased, and the interior is relatively soft, so there is a specimen near the two ends of the bolt is increased, and the interior is relatively soft, so there is a “soft–hard interface” in the specimen. Shear failure occurs along the “soft–hard interface” under load, "soft–hard interface" in the specimen. Shear failure occurs along the "soft–hard interface" under load, as shown in Figure 6b. as shown in Figure 6(b). σ (MPa) E (MPa) Appl. Sci. 2020, 10, 2329 7 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 15 (a) (a) (b) (b) (c) (c) Figure 5. Failure forms: (a) No anchoring; (b) End anchoring; (c) Full-length anchoring. Figure 5. Failure forms: (a) No anchoring; (b) End anchoring; (c) Full-length anchoring. (a) (b) (a) (b) Figure 6. Failure planes of anchored specimens: (a) End anchoring; (b) Full-length anchoring. Figure Figure 6. 6. Failur Failure planes o e planes off an anchor chored spe ed specimens: cimens: ( (a a) ) End an End anchoring; choring; ( (b b) ) F Full-length ull-length ancho anchoring. ring. On the basis of this, the reinforcement mechanism of the bolt can be summarized as follows. On the basis of this, the reinforcement mechanism of the bolt can be summarized as follows. (1 On the basis ) The ancho of this, the r ring agent ha einforcement s a bonding e mechanis ffect and dam m of the bolt c age repaa ir n ing be summ effect on roc arized as k mass follows. around boreholes (1) The ancho . Joints, firing ssures, agent and ha ot s a bondin her structg e ural ffect plane and dam s often aexist ge rep in a t iring he rock effect for on roc ming process k mass ,aro whic unh d reduce boreholes the . mechanic Joints, fissu al pro res, and perties of roc other struct kur , large al pl- ane scale s o struct ften exist ural planes in the rock , and for ev ming en the c process ontrollin , whicg h factors o reduce the f surroundin mechanicg roc al pro k failure. The anchorin perties of rock, large- g sc ag ale ent enters the structur structural planes, and al plane und even the c er gr ontrollin outing g factors of surrounding rock failure. The anchoring agent enters the structural plane under grouting Appl. Sci. 2020, 10, 2329 8 of 15 (1) The anchoring agent has a bonding e ect and damage repairing e ect on rock mass around boreholes. Joints, fissures, and other structural planes often exist in the rock forming process, which reduce the mechanical properties of rock, large-scale structural planes, and even the controlling factors of surrounding rock failure. The anchoring agent enters the structural plane under grouting pressure and plays a role in bonding and strengthening the surrounding rock near the structural plane, thus forming a “reinforcement area” within a certain range. The size of the “reinforcement area” is related to the grouting pressure, the material properties of the anchoring agent, the pore distribution of the surrounding rock, and so on. (2) The axial tensile and tangential shear capacity of the bolts can improve the stress state of the specimen. Under load, the specimen is a ected by both the shear action along the bedding plane and the tension action along the axis (Figure 5a). When the specimen is deformed, the tension and shear action are applied to the bolt, and the bolt body provides support resistance, which limits the deformation of the specimen. Based on the above findings, we inferred that the anchoring increases the specimen strength and changes the failure plane direction by improving the tensile capacity and tangential shear-bearing capacity. Under axial compression, the specimen simultaneously bears the shear e ect along the bedding and the tensile e ect along the axial direction (as shown by Figure 5a), while the bolt body provides support resistance to restrain the specimen deformation. Based on the analysis of the bolt reinforcing performance, we determined that a di erence existed between the reinforcing e ect of the bolt for full-length anchoring and that for end anchoring. The gap between the bolt body for full-length anchoring and the surrounding rock was filled by the anchoring agent, which could bond and reinforce the surrounding rock with greater scope. In the case of the rock developing shear deformation, the bolt with full-length anchoring could immediately restrain further deformation. Meanwhile, a gap existed between the end anchoring bolt and surrounding rock. The bolt’s tangential anchoring force only played a role when the surrounding rock developed a certain tangential deformation. Therefore, the bolt for full-length anchoring could form an “anchoring area” of greater range and provide greater support resistance than the end anchoring bolt. 3.2. Shear Testing The shear–displacement curves of di erent anchoring methods, which were measured by strain gages, are shown in Figure 7. The average maximum shear force values of the no-anchoring specimen, end anchoring specimen, and full-length anchoring specimen were, respectively, 3.55 kN, 15.34 kN, and 17.35 kN, as shown in Figure 8. It is evident that the maximum shear force of the end anchoring specimen was increased by 332.11% while the maximum shear force of the full-length anchoring specimen was increased by 13.10% compared with that of the end anchoring specimen. The shear–displacement curve of the no-anchoring specimen mainly shows the shear deformation process of the joint plane. Figure 7a shows that the specimen develops buckling failures as it is loaded to its ultimate load, resulting in a loss in the bearing capacity and representing a brittle feature. Moreover, the end anchoring specimen develops a “turning point” of a sudden drop and then a rise in shear force prior to its full yield to failure, as shown by Figure 7b. This occurrence was not observed in the other two cases. This point can be regarded as the decision point in terms of whether the bolt of the end anchoring specimen engages its shear resistant e ect. Before this point, the joint plane mainly bears the shear e ect. The extent of its shear sti ness mainly depends on the friction force of the plane and the pre-stressing force of the bolt. At failure, the joint plane su ered shear failures, and the specimen at both sides developed relative sliding, thus mobilizing the shear strength of the bolt itself. Therefore, this point can also be referred to as the “yield point” of the joint plane and the “mobilization point” of the bolt’s shear strength. After this point, the shear strength of the bolt body enhanced the comprehensive shear-bearing performance of the joint plane. Meanwhile, the shear force increased slowly with increasing shear displacement. These are Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 15 of the full-length anchoring specimen and end anchoring specimen's shear–displacement curves varied somewhat (Figure 7(c)). The bolt of the full-length anchoring specimen was in tight contact with the concrete via the anchoring agent. It could resist the shear resistance effect immediately under tAppl. he loa Sci. d ef 2020 fec , 10 t unt , 2329 il the specimen developed buckling failures. The bolt of the full-length anchoring 9 of 15 specimen and its joint plane jointly bore the shear load. No “turning point” developed. The same occurrence was observed in the end anchoring case. Compared to the no-anchoring specimen, the called the plastic phase and plastic strength phase [34,43]. Others refer to them as the slowly increasing end anchoring specimen and full-length anchoring specimen represented a larger ultimate load, resistance phase [24]. S-N-1 S-E-1 S-N-2 S-E-2 S-N-3 S-E-3 02 46 8 10 12 14 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Shear displacement (mm) Shear displacement (mm) (a) (b) S-FL-1 S-FL-2 S-FL-3 0 2 4 6 8 101214 1618 Shear displacement (mm) (c) Appl. Sci. Figure 7. 2020, 10, x FO Shear–displa R PEER Rcem EVIEW ent cu rves: (a) No bolt; (b) End anchoring; (c) Full-length anchoring. 10 of 15 Figure 7. Shear–displacement curves: (a) No bolt; (b) End anchoring; (c) Full-length anchoring. Scattered point Mean value End Full-length No bolt anchoring anchoring Figure 8. Shear force of the specimens by di erent anchoring methods. Figure 8. Shear force of the specimens by different anchoring methods. The typical strain distribution curves of the end anchoring specimen and full-length anchoring specimen segments obtained by the test are shown in Figure 9. The following can be observed from the strain distribution curves. • The maximum value of the bolt axial force is near the joint plane and is anti-symmetrically distributed at both sides of the joint plane. • The axial force of the bolt for full-length anchoring is mainly concentrated near the joint plane. It decreases rapidly with increasing distance from the joint plane, and its distribution is relatively uniform. • Plastic hinges are produced near the joint plane, which can effectively stop the further spread of the stress force. One end of the plastic hinge bears the tensile stress, while the other end bears the pressure stress. Joint S-FL-1 S-E-2 Cable Strain gauge -500 -1000 -1500 -2000 0 20 40 60 80 100 120 140 160 Anchor length (mm) Figure 9. Strain distribution curves of the bolts. 4. Field Application 4.1. Project Description The test field was the Mazhui Tunnel of DaoZhen Highway from Nanchuan of Chongqing to Guizhou, China. It was designed to be a bidirectional four-lane highway, with a full length of 3711 m and a maximum burial depth of 441 m. The lithological character of the test section is Silurian Longmaxi Formation shale with thin bedding, fracture development, and abundant underground water. The tunnel has a composite lining and is primary supported by bolts, sprayed concrete, and Shear force (kN) Anchor strain (με) Shear force (kN) Shear force (kN) Shear force (kN) Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 15 Appl. Sci. 2020, 10, 2329 10 of 15 Scattered point Mean value In accordance with the characteristics of the bolt resistance increase, the bolt developed plastic hinges at both sides of the joint plane. When the strength of the bolt or concrete reached its limit strength, the anchored specimen developed buckling failures. It was also observed that the patterns of the full-length anchoring specimen and end anchoring specimen’s shear–displacement curves varied somewhat (Figure 7c). The bolt of the full-length anchoring specimen was in tight contact with the concrete via the anchoring agent. It could resist the shear resistance e ect immediately under the load e ect until the specimen developed buckling failures. The bolt of the full-length anchoring specimen and its joint plane jointly bore the shear load. No “turning point” developed. The same occurrence was observed in the end anchoring case. Compared to the no-anchoring specimen, the end anchoring specimen and full-length anchoring specimen represented a larger ultimate load, a more rapid and End Full-length greater increase of bolt resistance, a longer plastic strength phase, and a stronger residual shear-bearing No bolt anchoring anchoring capacity after buckling. The typical strain distribution curves of the end anchoring specimen and full-length anchoring Figure 8. Shear force of the specimens by different anchoring methods. specimen segments obtained by the test are shown in Figure 9. The following can be observed from the strain distribution curves. The typical strain distribution curves of the end anchoring specimen and full-length anchoring specimen segments obtained by the test are shown in Figure 9. The following can be observed from The maximum value of the bolt axial force is near the joint plane and is anti-symmetrically the strain distribution curves. distributed at both sides of the joint plane. • The maximum value of the bolt axial force is near the joint plane and is anti-symmetrically The axial force of the bolt for full-length anchoring is mainly concentrated near the joint distributed at both sides of the joint plane. plane. It decreases rapidly with increasing distance from the joint plane, and its distribution is • The axial force of the bolt for full-length anchoring is mainly concentrated near the joint plane. relatively uniform. It decreases rapidly with increasing distance from the joint plane, and its distribution is relatively Plastic hinges are produced near the joint plane, which can e ectively stop the further spread of uniform. the stress force. One end of the plastic hinge bears the tensile stress, while the other end bears the • Plastic hinges are produced near the joint plane, which can effectively stop the further spread of pressure stress. the stress force. One end of the plastic hinge bears the tensile stress, while the other end bears the pressure stress. Joint S-FL-1 S-E-2 Cable Strain gauge -500 -1000 -1500 -2000 0 20 40 60 80 100 120 140 160 Anchor length (mm) Figure 9. Strain distribution curves of the bolts. Figure 9. Strain distribution curves of the bolts. 4. Field Application 4. Field Application 4.1. Project Description 4.1. Project Description The test field was the Mazhui Tunnel of DaoZhen Highway from Nanchuan of Chongqing to The test field was the Mazhui Tunnel of DaoZhen Highway from Nanchuan of Chongqing to Guizhou, China. It was designed to be a bidirectional four-lane highway, with a full length of 3711 Guizhou, China. It was designed to be a bidirectional four-lane highway, with a full length of 3711 m m and a maximum burial depth of 441 m. The lithological character of the test section is Silurian and a maximum burial depth of 441 m. The lithological character of the test section is Silurian Longmaxi Formation shale with thin bedding, fracture development, and abundant underground Longmaxi Formation shale with thin bedding, fracture development, and abundant underground water. The tunnel has a composite lining and is primary supported by bolts, sprayed concrete, and water. The tunnel has a composite lining and is primary supported by bolts, sprayed concrete, and reinforcing mesh. The parametric description is C20 sprayed concrete of a thickness of 18 cm, F22 Anchor strain (με) Shear force (kN) Appl. Sci. 2020, 10, 2329 11 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 15 cartridge bolts with inter-row spacing of 100  120 cm and 2.5 m in length, and 18# joist steel with reinforcing mesh. The parametric description is C20 sprayed concrete of a thickness of 18 cm, Ф22 longitudinal spacing of 1 m. The secondary lining was molded concrete with a thickness of 40 cm of reinfo cartridge rcing m bolts esh. The pa with inter-row rametric descr spacing ipof tion is C2 100 × 120 0 spray cm ed conc and 2.5 ret m e of a t in length, hickness o and 18# f 18 c joist m, Ф steel 22 C25 sprayed concrete. The support parameters of the tunnel section and geological conditions of the cartridge bolts with inter-row spacing of 100 × 120 cm and 2.5 m in length, and 18# joist steel with testing face surrounding rock are shown in Figure 10. with (a) (b) (a) (b) Figure 10. Field test: (a) Support parameters of the tunnel section; (b) Surrounding rock of the testing Figure 10. Field test: (a) Support parameters of the tunnel section; (b) Surrounding rock of the Figure 10. face. Field test: (a) Support parameters of the tunnel section; (b) Surrounding rock of the testing testing face. face. 4.2. Bolt Arrangement at the Construction Site 4.2. Bolt arrangement at the construction site 4.2. Bolt arrangement at the construction site Bolts with lengths of 3.5 m with diameters of 22 mm were used in the field. The measuring bolts Bolts with lengths of 3.5 m with diameters of 22 mm were used in the field. The measuring bolts and vibration wire steel stress gauges were installed to measure the axial force of the bolts, as shown and v Bol ibrat ts wi ion th wire lengths of steel st 3r .5 e m ss ga wi uge th diameters of s were installed 22 mm to meas were used in ure the ax tia he fie l force o ld. The f thmeas e bolturin s, as g bolt show s n in Figure 11. Each measuring bolt was installed with three vibrating wire steel stress gauges with and vibration wire steel stress gauges were installed to measure the axial force of the bolts, as shown in Figure 11. Each measuring bolt was installed with three vibrating wire steel stress gauges with corresponding measuring lines. At the measuring station, the plug of each measuring line was inserted in correspondin Figure 11. Each me g measur asing uring bolt was lines. At th installed w e measurinig th three vibrating wire station, the plug of esteel stress ach measurin gauge g lin s wit e was h into the frequency recorder to record the value. Each vibrating wire steel stress gauge was calibrated corresponding measuring lines. At the measuring station, the plug of each measuring line was inserted into the frequency recorder to record the value. Each vibrating wire steel stress gauge was indoors to obtain its reference frequency and calibration coecient prior to its installation and use. inserted calibrated into ind the fre oors tq ouenc obtay in recorder its referenc to re ecord the frequency an value. E d caa libr ch v ati ibr on ating w coefficient ire ste prel stre ior to it ss g s inst aug ae wa llatios n The frequencies obtained from the field test were computed on the basis of a calibration formula to calibrated and use. The indoors frequenc to ies obtain obtained its reference from the fie frequency ld test were comp and calibration uted on t coefficient he basis of a prior calib to ratits ion obtain the axial force of the bolt segments. installation formula to obt and use. The frequencies obtained from the field test ain the axial force of the bolt segments. Figure 11. A measuring bolt. Figure 11. A measuring bolt. Figure 11. A measuring bolt. th Two site sections in the field ZK17+360 to ZK17+365 (“ZK17” means the 17 kilometer of left line th Two si Two site te sections sections i innthe the fi field eld ZK17 ZK17+360 +360 to ZK17 to ZK17+365 +365 (“ (“ZK17” ZK17” means mean the s the 17 17th kilometer kilometer of left of left line line of DaoZhen Highway, “360” and “365” mean detailed distance(meters) in ZK17.) were selected to test the of DaoZhen H ofbolt DaoZhen axial fo Highway ighway, “360” rce vari,at “360” ionand “365” s. Thes and “365” e s mean detailed dist ites were mean detailed the tu ance(meters) nnel distance(meters) evacuin ZK17. ation fac in ) were sel e ZK17.) and second wer ected to test the e selected ary lining to , bolt test axi the bolt al fo axial rce var force iation variations. s. These s These ites were sites wer the t e the unnel tunnel evac evacuation uation fac face e an and d second secondary ary llining, ining, respectively. In each section, seven bolts were installed. To compare the impact of the anchoring respectively. respectively methods on the anchorage effect, the Z . In each section, seven bolts were in In each section, seven boltsK wer 17+360 se e installed. stalled. To comp ction adopt To compar ed f are the imp u ellthe -lengt impact h an act of the anc choring, wh of the anchoring hil oring e the methods on the anchorage effect, the Z methods on the anchorage e ect, the ZK17 K17+360 se +360 section ction adopt adopted ed f full-length ull-length an anchoring, choring, wh while ile t the he ZK17+365 section adopted the end anchoring scheme. The distribution method of the measuring bolts ZK1 ZK17 and 7+3 + si365 te const 65 secti section r oucti n adopted t adopted on are shown i the he end a end n anchoring n Fi chori gure 12 ng scheme. scheme. . The test The distri The fiedistribution ld sim bution method of ultaneously m method of easured the measuri the measuring the per ng bol iphery bolts ts a and nd site site const constr ructi uction on a ar re shown i e shown in n Fi Figur gure 12 e 12.. The The test test field field simultaneously simultaneously m measur easured ed the per the periphery iphery convergence and arch crown settlement of the two sections to analyze the support effect of the bolt. convergence conver The peripher gence and and y converg arch cro arch crenc own wn settlemen e was me settlement asured by t of the two sections to an of the two a JSS3 sections 0A converge to analyze alyze the support effect o nce g the auge, w support hile e the ect arch of f the bolt. the crown bolt. The periphery convergence was measured by a JSS30A convergence gauge, while the arch crown settlement was measured by a DSZ2 automatic compensated level. settlement was measured by a DSZ2 automatic compensated level. Appl. Sci. 2020, 10, 2329 12 of 15 The periphery convergence was measured by a JSS30A convergence gauge, while the arch crown settlement was measured by a DSZ2 automatic compensated level. Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 15 (a) (b) Figure Figure 12. 12. S Section ection ddistribution istribution and and sisite te ininstallation stallation of the of the me measuring asuring bolts: ( bolts: a) Distributi (a) Distribution on of the of the (a) (b) measuring measuring bolts; ( bolts; (b b)) Installation Installation of the mea of the measuring suring bolts. bolts. Figure 12. Section distribution and site installation of the measuring bolts: (a) Distribution of the The ultimate axial force distribution of the measuring bolts at each section is shown in Figure 13. The u measu lti rmat ing bolts; ( e axia bl ) f Instal orce lati di on of the mea stribution of sur ing bolts. the mea suring bolts at each section is shown in Figure The positive sign represents compressive force; the negative sign represents tensile force. The figure 13. The positive sign represents compressive force; the negative sign represents tensile force. The The ultimate axial force distribution of the measuring bolts at each section is shown in Figure shows that, at section ZK17+360, all bolts mainly bear tensile stress. As a whole, the stress borne by figure shows that, at section ZK17+360, all bolts mainly bear tensile stress. As a whole, the stress 13. The positive sign represents compressive force; the negative sign represents tensile force. The bolts in the ZK17+360 section is greater than that borne by bolts in the ZK17+365 section. That is, borne by bolts in the ZK17+360 section is greater than that borne by bolts in the ZK17+365 section. figure shows that, at section ZK17+360, all bolts mainly bear tensile stress. As a whole, the stress under full-length anchoring, the bolts can provide greater support resistance and hence obtain a That is, under full-length anchoring, the bolts can provide greater support resistance and hence borne by bolts in the ZK17+360 section is greater than that borne by bolts in the ZK17+365 section. better anchorage e ect. The axial forces of bolts in di erent parts of the test section also have certain obtain a better anchorage effect. The axial forces of bolts in different parts of the test section also have That is, under full-length anchoring, the bolts can provide greater support resistance and hence regularities, as outlined below. certain regularities, as outlined below. ( (a a)) ((b) b) Figure 13. Axial force (KN) distribution of the test section bolts: (a) ZK17+360 section; (b) ZK17+365 Figure Figure 13. 13. Ax Axial ial force for (KN) ce (KN) distribu distribution tion of the of test the section test section bolts: (abolts: ) ZK17+36 (a) 0 se ZK17 ction; ( +360 b)section; ZK17+365 (b) section. ZK17 sectio+ n. 365 section. The #2 bolt on the right arch shoulder bears the maximum stress., while the maximum tensile The #2 bolt on the right arch shoulder bears the maximum stress., while the maximum tensile The #2 bolt on the right arch shoulder bears the maximum stress., while the maximum tensile stresses are distributed near the middle of the bolt. These are 55.2 kN (C2 element for full-length stresses are distributed near the middle of the bolt. These are 55.2 kN (C2 element for full-length stresses are distributed near the middle of the bolt. These are 55.2 kN (C2 element for full-length anchoring) and 27.7 kN (C2 element for end anchoring). This is consistent with the deformation anchoring) and 27.7 kN (C2 element for end anchoring). This is consistent with the deformation anchoring) and 27.7 kN (C2 element for end anchoring). This is consistent with the deformation distribution of the tunnel’s surrounding rock, as shown in Figure 14. Moreover, the maximum distribution of the tunnel’s surrounding rock, as shown in Figure 14. Moreover, the maximum distribution of the tunnel’s surrounding rock, as shown in Figure 14. Moreover, the maximum deformation point of the tunnel arch crown is located at the right spandrel. At the construction site, deformation point of the tunnel arch crown is located at the right spandrel. At the construction site, deformation point of the tunnel arch crown is located at the right spandrel. At the construction site, this location frequently developed cracking and chipping, and it even extruded and deformed the this location frequently developed cracking and chipping, and it even extruded and deformed the this loc lattice gi ation rder. freq uently developed cracking and chipping, and it even extruded and deformed the lattice girder. lattice girder. The axial force of the bolt on the wall at each side of the test section was relatively small (5 to 10 kN), especially on bolts #6 and #7 on the wall at each side. These were installed after the evacuation of the lower bench, while the deformation of the tunnel’s surrounding rock tended to be stable after Appl. Sci. 2020, 10, 2329 13 of 15 evacuation of the lower bench. Bolts in this part did not engage their anchoring e ects; rather, they became a kind of safety stock. Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 15 10 Left point Middle point Right point 0 5 10 15 20 25 30 Time (d) Figure 14. Arch crown settlement curve of the ZK17+365 section Figure 14. Arch crown settlement curve of the ZK17+365 section 5. Discussion The axial force of the bolt on the wall at each side of the test section was relatively small (5 to 10 kN), especially on bolts #6 and #7 on the wall at each side. These were installed after the evacuation In laboratory experiments, the specimen needs to be drilled when installing the bolt. In the process of the lower bench, while the deformation of the tunnel’s surrounding rock tended to be stable after of drilling, the drill stem may disturb the specimen and cause damage to the rock around the drill hole. evacuation of the lower bench. Bolts in this part did not engage their anchoring effects; rather, they Although the rock morphology near the borehole is examined in the test process (to check that there became a kind of safety stock. are no macroscopic damage fractures), the degree of damage to the specimen when drilling and its 5 Discussion influence on the test are open to question. In engineering, carrying out on-site bolt construction will also cause damage to the surrounding rock in a certain area near the borehole. How to quantitatively In laboratory experiments, the specimen needs to be drilled when installing the bolt. In the evaluate and compare the relationship between indoor testing and on-site construction is worthy of process of drilling, the drill stem may disturb the specimen and cause damage to the rock around the drill hole. Although the rock morphology near the borehole is examined in the test process (to check further research and discussion. that there are no macroscopic damage fractures), the degree of damage to the specimen when drilling 6. Conclusions and its influence on the test are open to question. In engineering, carrying out on-site bolt construction will also cause damage to the surrounding rock in a certain area near the borehole. How In this paper, we investigated the bolt anchoring performance for bedded rock mass under to quantitatively evaluate and compare the relationship between indoor testing and on-site di erent anchoring methods, as well as the failure mode under shear and tensile stresses in bedded construction is worthy of further research and discussion. rock. Then, based on field tests, we analyzed the force and support e ects of the di erent anchoring methods. 6 Conc The lusiconclusions on are as follows: In this paper, we investigated the bolt anchoring performance for bedded rock mass under (1) With respect to the anchored specimen strength and strain, the reinforcing mechanism of the different anchoring methods, as well as the failure mode under shear and tensile stresses in bedded bolt for the rock was divided into two aspects: the anchoring agent bonding and restoring rock. Then, based on field tests, we analyzed the force and support effects of the different anchoring the surrounding rock near the reinforcing zone, and the bolt body resistance supporting and methods. The conclusions are as follows: improving the stress state of the rock. The combined action of those two aspects increased the (1) With respect to the anchored specimen strength and strain, the reinforcing mechanism of the strength parameters of the anchored specimen. bolt for the rock was divided into two aspects: the anchoring agent bonding and restoring the (2) Failur surro e ofundin the anchor g rock near the rein ed specimen changed forcing zone, from shear and tsplitting he bolt body resista failure withnno ce anchoring supporting into andshear improving the stress state of the rock. The combined action of those two aspects increased the failure where the failure plane slid parallel to the axial direction of the bolt or shear dislocation strength parameters of the anchored specimen. failure along the soft–hard interface. (2) Failure of the anchored specimen changed from shear splitting failure with no anchoring into (3) Via anchoring of the joint rock mass, the bolt could significantly enhance the shear-bearing shear failure where the failure plane slid parallel to the axial direction of the bolt or shear capacity of the rock mass and increase the stability of the surrounding rock. Compared to the dislocation failure along the soft–hard interface. end anchoring bolt, the bolt for full-length anchoring can form an “anchoring area” of a greater (3) Via anchoring of the joint rock mass, the bolt could significantly enhance the shear-bearing range and provide greater support resistance than the end anchoring bolt; therefore, it had a capacity of the rock mass and increase the stability of the surrounding rock. Compared to the better coupling e ect on the surrounding rock with a greater resistance increase. This occurrence end anchoring bolt, the bolt for full-length anchoring can form an “anchoring area” of a greater enabled range the and surr prov ounding ide greartock er su to pp bear ort re mor sistance e load. than the end anchoring bolt; therefore, it had a better coupling effect on the surrounding rock with a greater resistance increase. This occurrence (4) Full-length anchoring can provide more support resistance and have a better anchoring e ect on enabled the surrounding rock to bear more load. the surrounding rock of a bedded rock tunnel. However, the grouting quality is often dicult to Deformation (mm) Appl. Sci. 2020, 10, 2329 14 of 15 guarantee due to the impact of the bolt insertion angle, so it is necessary to pay attention to the filling quality of the anchoring agent, especially at the vault of the tunnel. Author Contributions: Z.Z. analyzed the calculation results. Y.L. carried out the laboratory test and wrote the article. 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Journal

Applied Sciences – Multidisciplinary Digital Publishing Institute

Published: Mar 28, 2020

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