Deformation Characteristics and Optimization Design for Large-Scale Deep and Circular Foundation Pit Partitioned Excavation in a Complex Environment

Hai Shi; Zhilei Jia; Tao Wang; Zhiqiang Cheng; De Zhang; Mingzhou Bai; Kun Yu

doi:10.3390/buildings12091292

Shi, Hai;Jia, Zhilei;Wang, Tao;Cheng, Zhiqiang;Zhang, De;Bai, Mingzhou;Yu, Kun

2022-08-23 00:00:00

buildings Article Deformation Characteristics and Optimization Design for Large-Scale Deep and Circular Foundation Pit Partitioned Excavation in a Complex Environment 1 , 2 1 1 , 3 , 3 , 4 1 1 Hai Shi , Zhilei Jia , Tao Wang *, Zhiqiang Cheng * , De Zhang , Mingzhou Bai and Kun Yu School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China Key Laboratory of Track Engineering Beijing, Beijing 100044, China Shanghai Road and Bridge Group Co., Ltd., Shanghai 200433, China Shanghai Engineering Research Center of Green Pavement Materials, Shanghai 200433, China * Correspondence: wangtao1@bjtu.edu.cn (T.W.); cr1903@tongji.edu.cn (Z.C.) Abstract: With the rapid development of urban rail transit, the impact of the construction of large- scale deep and circular foundation pits (LDCFPs) on the surrounding environment has become increasingly prominent; however, there is no deﬁnite standard for the design theory and construction method of the retaining structure of LDCFPs; therefore, the such standard needs further research and exploration. The paper is based on the LDCFP engineering of a subway station in Wuhan, China. Two partitioned excavation methods (step-by-step and synchronous excavation) of LDCFP were discussed by numerical simulation; the deformation characteristics of the soil around the foundation pit, the lateral deformation of the retaining structure in each partitioned district, and the distribution of the supporting axial force were analyzed; the deformation law of LDCFP was revealed; and the safe and economic excavation method was determined. The ﬁeld monitoring test was carried out to Citation: Shi, H.; Jia, Z.; Wang, T.; reveal the deformation characteristics and stress evolution behavior for the supporting structure of Cheng, Z.; Zhang, D.; Bai, M.; Yu, K. the LDCFP in the subway station. It was concluded that the construction method of synchronous Deformation Characteristics and partitioned excavation in the same layer is more conducive to controlling the overall deformation, Optimization Design for Large-Scale with the deformation of the circular retaining structure being uniform and the “vault effect” of the Deep and Circular Foundation Pit Partitioned Excavation in a Complex circular retaining structure having a certain constraint on the deformation of the foundation pit. The Environment. Buildings 2022, 12, 1292. research results can provide guidance for the design and construction of supporting engineering and https://doi.org/10.3390/ similar engineering. buildings12091292 Keywords: LDCFPs; deformation characteristics; partitioned excavation; control measures; Academic Editor: Suraparb optimization design Keawsawasvong Received: 21 June 2022 Accepted: 11 August 2022 Published: 23 August 2022 1. Introduction Publisher’s Note: MDPI stays neutral With the continuous progress of urbanization, the development of urban under- with regard to jurisdictional claims in ground space in China is continuously evolving from point-line planes to blocks and published maps and institutional afﬁl- networking [1–3], which shows the characteristics of a larger spatial scale, longer structural iations. span, deeper underground structure, complex foundation pit support, and more excavation steps [4–6]. Foundation pit engineering is an important part of urban underground space in con- struction. It usually relies on temporary or permanent support structures to bear the Copyright: © 2022 by the authors. surrounding soil pressure and ﬁnally form a safe and reliable underground space. In Licensee MDPI, Basel, Switzerland. the process of deep foundation pit construction, engineering accidents frequently occur This article is an open access article for many reasons, such as construction technology and management. The deformation distributed under the terms and behavior, reinforcement, and support measures of deep foundation pits during excavation conditions of the Creative Commons were studied by an increasing number of scholars [7–10]. Houhou [11] and Teparaksa [12] Attribution (CC BY) license (https:// analyzed the whole construction process of deep foundation pits in clay layers using creativecommons.org/licenses/by/ 4.0/). Buildings 2022, 12, 1292. https://doi.org/10.3390/buildings12091292 https://www.mdpi.com/journal/buildings Buildings 2022, 12, 1292 2 of 28 numerical simulations and studied the characteristics of surface settlement and the defor- mation behavior of supporting structures. Yajnheswaran et al. [13] studied the supporting and displacement control effect of deep foundations by pit anchor bolts. Cui et al. [14] analyzed the surface settlement and horizontal displacement caused by deep foundation pit excavation and predicted the location of the maximum displacement. At present, the construction of urban underground space with “multidimensional space, large scale, and complex structure” has become an inevitable development trend [15,16]. The new con- struction concept puts forward higher requirements for the construction of foundation pit engineering, especially the construction of high standard special-shaped deep and large foundation pit engineering, which is particularly challenging [17]. Research on foundation pit engineering mostly focuses on normal shape founda- tion pits, such as rectangular and long strip shapes. During this period, to alleviate the contradiction between urban land supply and demand, protect underground pipelines and surrounding buildings, and make full and rational use of the existing land, various unconventional-shaped deep and large foundation pits will inevitably appear in the con- struction of subway stations [18,19]. At present, there are few studies on the deformation characteristics, support measures, and stress evolution process of various unconventional- shaped deep and large foundation pits, such as LDCFPs. Because the circular foundation pit has special spatial stress characteristics, it can give full play to the characteristics of strong compressive capacity under the action of axial load. It has the advantages of large stiffness, small wall deformation, and convenient mechanized operation of LDCFP [20,21], such as the LDCFP engineering of the Shanghai global ﬁnancial center and the Wuhan Optics Valley Plaza complex project in China. Some results were obtained from research on LDCFP, including ﬁeld monitoring [22,23], numerical simulation [24,25], and theoret- ical solutions [26,27]. For example, Tobar [28], Cheng [29], and other scholars proposed the circumferential stress coefﬁcient, and it is suggested to take the static earth pressure coefﬁcient as the circumferential stress coefﬁcient. Tan et al. [30] studied the performance of medium and large circular foundation pits excavated in clay gravel pebble mixed strata and analyzed the inﬂuence of foundation pit excavation on the surrounding environment. However, the circular foundation pit was mainly subjected to circumferential axial pres- sure; its deformation is mainly composed of circumferential compression deformation, mud compression in the groove section, and asymmetric load; and the LDCFP is very sensitive to inhomogeneous load [31,32]. Therefore, the symmetry of the circular founda- tion pit between earth pressure and horizontal deformation is particularly important in the construction period. There is a lack of sufﬁcient understanding of the deformation characteristics and the impact on the surrounding environment caused by the excavation of circular foundation pits, and it is also the main technical obstacle to realizing the best engineering design and construction of circular foundation pits. At present, the foundation pit engineering of urban subways is usually located in the dense areas of buildings and urban lifeline engineering [33,34], which puts forward higher requirements for the supporting structure and excavation sequence of LDCFPs. Therefore, the paper is based on the LDCFP engineering of a subway station in Wuhan, China. The deformation characteristics of two partitioned excavation methods (step-by- step and synchronous excavation) of LDCFP were discussed by numerical simulation, and the reasonable excavation method was determined. The ﬁeld monitoring test was carried out to reveal the deformation characteristics and stress evolution behavior for the supporting structure of the LDCFP in the subway station, and economical and effective control measures were proposed to provide guidance for the design and construction of the engineering and another similar engineering. 2. Project Overview 2.1. Project Overview and Surrounding Environment The paper is based on the LDCFP engineering of a subway station in Wuhan, China. The project includes rail transit engineering, municipal engineering, and underground Buildings 2022, 06, x FOR PEER REVIEW 3 of 29 2. Project Overview Buildings 2022, 12, 1292 3 of 28 2.1. Project Overview and Surrounding Environment The paper is based on the LDCFP engineering of a subway station in Wuhan, China. The project includes rail transit engineering, municipal engineering, and underground public space, and the surrounding environment of the foundation pit is complex. High- public space, and the surrounding environment of the foundation pit is complex. High-rise rise buildings, pedestrian crossing channels, and urban transportation are distributed buildings, pedestrian crossing channels, and urban transportation are distributed around around the foundation pit (as shown in Figure 1). Therefore, the stability requirements of the foundation pit (as shown in Figure 1). Therefore, the stability requirements of the the foundation pit engineering are higher. foundation pit engineering are higher. Figure 1. Excavation site of LDCFP construction. Figure 1. Excavation site of LDCFP construction. LDCFP engineering mainly includes three layers of underground structures. The first LDCFP engineering mainly includes three layers of underground structures. The underground layer is the subway station hall and underground public space, and the first ﬁrst underground layer is the subway station hall and underground public space, and underground layer contains the mezzanine structure, which is the platform of the subway the ﬁrst underground layer contains the mezzanine structure, which is the platform of transfer station passing through a highway tunnel. The second underground layer is the the subway transfer station passing through a highway tunnel. The second underground subway transfer hall and equipment room, which passes through another highway tun- layer is the subway transfer hall and equipment room, which passes through another nel. The third underground layer is the platform floor of another subway station. During highway tunnel. The third underground layer is the platform ﬂoor of another subway the construction of the foundation pit, the geological environment varies greatly, the sur- station. During the construction of the foundation pit, the geological environment varies rounding environment of the foundation pit is complex, and the traffic flow is large. greatly, the surrounding environment of the foundation pit is complex, and the trafﬁc ﬂow Therefore, ensuring the construction safety of foundation pits, reducing the impact of con- is large. Therefore, ensuring the construction safety of foundation pits, reducing the impact struction on surrounding buildings, and maintaining the continuous flow of road traffic of construction on surrounding buildings, and maintaining the continuous ﬂow of road are difficult tasks for LDCFP engineering construction. trafﬁc are difﬁcult tasks for LDCFP engineering construction. The LDCFP engineering mainly adopts a multi-span frame structure with three un- The LDCFP engineering mainly adopts a multi-span frame structure with three under- derground layers; the buried depth of the baseboard on the first underground layer is 14 ground layers; the buried depth of the baseboard on the ﬁrst underground layer is 14 m, m, the buried depth of the baseboard on the second underground layer is 21 m, the third the buried depth of the baseboard on the second underground layer is 21 m, the third underground layer is typical pit-in-pit engineering, and the maximum depth is 33 m. The underground layer is typical pit-in-pit engineering, and the maximum depth is 33 m. The plane diameter of the LDCFP engineering is 200 m, and the surrounding environment of plane diameter of the LDCFP engineering is 200 m, and the surrounding environment of Buildings 2022, 06, x FOR PEER REVIEW 4 of 29 the LDCFP engineering is shown in Figure 2. the LDCFP engineering is shown in Figure 2. Figure 2. The surrounding environment of foundation pit engineering. Figure 2. The surrounding environment of foundation pit engineering. 2.2. Overview of Engineering Geology According to the engineering exploration of LDCFP engineering, it is divided into five layers from top to bottom based on the stratum lithology and characteristics. The soil parameters are determined according to the engineering investigation and geotechnical test; the soil layer parameters of the foundation pit engineering are shown in Table 1. The stagnant water level in the upper layer of the foundation pit is 1.20–5.30 m below the ground, and the static water level is 0.3–3.0 m below the ground. The reason for the large range of these water levels is that the monitoring time is just at the time of more rainfall. Table 1. The soil layer parameters. Shear Strength Secant Tangent Modulus of Poisson’s Unit Soil The Angle of In- Stiffness Stiffness Elasticity Cohesion Soil Layer Ratio (v) Weight (γ) ternal Friction Thickness (E50) (Eoed) (Eur) (c) (φ) 2 2 2 — kN/m³ kN/m kN/m kN/m (kPa) (°) m Fill soil 0.35 19.5 4500 5400 31,500 5 15 4.5 Clay 0.36 19.0 6075 7290 42,525 22 10 4.5 Residual soil 0.33 19.50 9450 11,340 66,150 35 13 3 Strongly weathered 0.3 22.5 12,600 15,120 88,200 50 20 4 Mudstone Moderately Weath- 0.3 24.3 82,800 99,360 579,600 80 22 15 ered mudstone Slightly weathered 0.28 26.5 135,000 162,000 945,000 100 27 — mudstone 2.3. Design of Foundation Pit Supporting Structure According to the geological conditions and surrounding environment of foundation pit engineering, the supporting structure adopted by the foundation pit includes retaining piles, supports of each layer, column piles, top beams, and wai purlins. 2.3.1. Design of Retaining Piles The foundation pit is supported by bored piles and constructed by the open excava- tion method. The rotary drill is used to complete the pile drilling, and the impact drill is used to form the hole in the limited area. In the foundation pit engineering, retaining piles Buildings 2022, 12, 1292 4 of 28 2.2. Overview of Engineering Geology According to the engineering exploration of LDCFP engineering, it is divided into ﬁve layers from top to bottom based on the stratum lithology and characteristics. The soil parameters are determined according to the engineering investigation and geotechnical test; the soil layer parameters of the foundation pit engineering are shown in Table 1. The stagnant water level in the upper layer of the foundation pit is 1.20–5.30 m below the ground, and the static water level is 0.3–3.0 m below the ground. The reason for the large range of these water levels is that the monitoring time is just at the time of more rainfall. Table 1. The soil layer parameters. Shear Strength Secant Tangent Modulus of Poisson’s Unit Soil The Angle Stiffness Stiffness Elasticity Cohesion Weight ( ) Ratio (v) Thickness Soil Layer of Internal (E ) (E ) (E ) 50 oed ur (c) Friction (') 3 2 2 2 - kN/m kN/m kN/m kN/m (kPa) ( ) m Fill soil 0.35 19.5 4500 5400 31,500 5 15 4.5 Clay 0.36 19.0 6075 7290 42,525 22 10 4.5 Residual soil 0.33 19.50 9450 11,340 66,150 35 13 3 Strongly weathered 0.3 22.5 12,600 15,120 88,200 50 20 4 Mudstone Moderately 0.3 24.3 82,800 99,360 579,600 80 22 15 Weathered mudstone Slightly weathered 0.28 26.5 135,000 162,000 945,000 100 27 - mudstone 2.3. Design of Foundation Pit Supporting Structure According to the geological conditions and surrounding environment of foundation pit engineering, the supporting structure adopted by the foundation pit includes retaining piles, supports of each layer, column piles, top beams, and wai purlins. 2.3.1. Design of Retaining Piles Buildings 2022, 06, x FOR PEER REVIEW 5 of 29 The foundation pit is supported by bored piles and constructed by the open excavation method. The rotary drill is used to complete the pile drilling, and the impact drill is used to form the hole in the limited area. In the foundation pit engineering, retaining piles with a diameter of '1200@1500 mm are used for the supporting structure. There are more than with a diameter of φ1200@1500 mm are used for the supporting structure. There are more 3400 retaining piles in total, and the slurry is used for wall protection. A schematic diagram than 3400 retaining piles in total, and the slurry is used for wall protection. A schematic of the retaining piles is shown in Figure 3. diagram of the retaining piles is shown in Figure 3. Figu Figure re 3. Sc3. heSchematic matic diagdiagram ram of th of e r the etain retaining ing piles piles. . 2.3.2. Supports of Each Layer Design 2.3.2. Supports of Each Layer Design Supports Design in the Central District of LDCFP Supports Design in the Central District of LDCFP The central district of the foundation pit is designed as three layers underground The central district of the foundation pit is designed as three layers underground structure, and the maximum excavation depth is 33 m. The ﬁrst and second layers of structure, and the maximum excavation depth is 33 m. The first and second layers of the the underground structure are deﬁned as the bowknot district, and the third layer is the underground structure are defined as the bowknot district, and the third layer is the long- long-shaped excavation of the foundation pit (that is, pit in pit district). The soil layer is shaped excavation of the foundation pit (that is, pit in pit district). The soil layer is divided divided into six layers according to the altitude of support during excavation. The proﬁle into six layers according to the altitude of support during excavation. The profile of the of the foundation pit support is shown in Figure 4. foundation pit support is shown in Figure 4. Figure 4. Supports design in the central district. Supports Design in the Northern and Southern Districts of LDCFP Both the southern and northern districts of the foundation pit are two layers of un- derground structures. The first layer (the soil layer between the first support and the sec- ond support) is approximately 4.5–5 m deep and is constructed in two steps. The second layer (the soil layer between the second support and the bottom layer) is approximately 9–10 m deep and is constructed in two steps. The profile of the foundation pit support is shown in Figure 5. Buildings 2022, 06, x FOR PEER REVIEW 5 of 29 with a diameter of φ1200@1500 mm are used for the supporting structure. There are more than 3400 retaining piles in total, and the slurry is used for wall protection. A schematic diagram of the retaining piles is shown in Figure 3. Figure 3. Schematic diagram of the retaining piles. 2.3.2. Supports of Each Layer Design Supports Design in the Central District of LDCFP The central district of the foundation pit is designed as three layers underground structure, and the maximum excavation depth is 33 m. The first and second layers of the underground structure are defined as the bowknot district, and the third layer is the long- shaped excavation of the foundation pit (that is, pit in pit district). The soil layer is divided Buildings 2022, 12, 1292 5 of 28 into six layers according to the altitude of support during excavation. The profile of the foundation pit support is shown in Figure 4. Figure Figure 4. 4. Supports Supports d design esign i in n t the he c central entral district. district. Supports Design in the Northern and Southern Districts of LDCFP Supports Design in the Northern and Southern Districts of LDCFP Both the southern and northern districts of the foundation pit are two layers of Both the southern and northern districts of the foundation pit are two layers of un- underground structures. The ﬁrst layer (the soil layer between the ﬁrst support and the derground structures. The first layer (the soil layer between the first support and the sec- second support) is approximately 4.5–5 m deep and is constructed in two steps. The second ond support) is approximately 4.5–5 m deep and is constructed in two steps. The second layer (the soil layer between the second support and the bottom layer) is approximately Buildings 2022, 06, x FOR PEER REVIEW 6 of 29 layer (the soil layer between the second support and the bottom layer) is approximately 9–10 m deep and is constructed in two steps. The proﬁle of the foundation pit support is 9–10 m deep and is constructed in two steps. The profile of the foundation pit support is shown in Figure 5. shown in Figure 5. Figure 5. Supports design in the northern and southern districts. Figure 5. Supports design in the northern and southern districts. 3. Finite Element Simulation Analysis 3. Finite Element Simulation Analysis 3.1. Model Establishment 3.1. Model Establishment In this paper, Midas/gts software was used for numerical simulation, and the model In this paper, Midas/gts software was used for numerical simulation, and the model was established by spatial strain. The geometric model was established according to the was established by spatial strain. The geometric model was established according to the engineering geological data, including the central district, the northern district, and the engineering geological data, including the central district, the northern district, and the southern district of the foundation pit. The foundation model in the central district was southern district of the foundation pit. The foundation model in the central district was established according to the support design, and the excavation was divided into 6 layers, established according to the support design, and the excavation was divided into 6 layers, with depths of 9 m, 5 m, 6 m, 4 m, 5 m, and 4 m. The foundation pit in the northern with depths of 9 m, 5 m, 6 m, 4 m, 5 m, and 4 m. The foundation pit in the northern and and southern districts is 14 m deep and was excavated in two layers, with excavation southern districts is 14 m deep and was excavated in two layers, with excavation depths depths of 4.5 m and 8.5 m, respectively. Since the inﬂuence range of the foundation pit of 4.5 m and 8.5 m, respectively. Since the influence range of the foundation pit construc- construction is approximately 2–3 times the plane size of the foundation pit, the model size tion is approximately 2–3 times the plane size of the foundation pit, the model size is 400 is 400 m 400 m 100 m. m × 400 m × 100 m. The self-weight load of the structure was considered in this model, and the direction of the load was vertically downward. The boundary conditions in the model are automat- ically constrained; that is, the normal horizontal displacement is constrained around the model, the displacement in three directions (x, y, z) is constrained at the bottom of the model, and the earth’s surface is a free surface. At the same time, the vertical torsional restraint is set at the bottom of the column piles. Assuming that the soil load is uniformly distributed, the overburden load of the surrounding main roads was designed to be 15 2 2 kN/m , and the surrounding building load was accumulated according to 18 kN/m per layer. The modified Mohr–Coulomb model was adopted for the constitutive relationship of the soil layer. The tangent modulus Et of stress–strain is [35],  R 1−− sinϕσ σ ( )( ) f 13 EE = 1−⋅ (1)  t 0 2c cosϕσ + 2 sinϕ  3 where Et is the tangent modulus, E0 is the initial modulus of elasticity, Rf is the failure ratio, and its value is between 0.75 and 1.00, c is cohesion, and φ is the angle of internal friction. σ1, σ2, and σ3 are the stresses of the soil. 3.2. Model of Supporting Structure In the model, the retaining pile was transformed into an underground diaphragm wall in the way of equal stiffness [36], and the linear elastic model plate element with a thickness of 965 mm was used for simulation. The transformed formula is shown in Equa- tions (2) and (3), Buildings 2022, 12, 1292 6 of 28 The self-weight load of the structure was considered in this model, and the direction of the load was vertically downward. The boundary conditions in the model are automatically constrained; that is, the normal horizontal displacement is constrained around the model, the displacement in three directions (x, y, z) is constrained at the bottom of the model, and the earth’s surface is a free surface. At the same time, the vertical torsional restraint is set at the bottom of the column piles. Assuming that the soil load is uniformly distributed, the overburden load of the surrounding main roads was designed to be 15 kN/m , and the surrounding building load was accumulated according to 18 kN/m per layer. The modiﬁed Mohr–Coulomb model was adopted for the constitutive relationship of the soil layer. The tangent modulus E of stress–strain is [35], R (1 sin j)(s s ) 1 3 E = 1 E (1) t 0 2c cos j + 2s sin j where E is the tangent modulus, E is the initial modulus of elasticity, R is the failure ratio, 0 f and its value is between 0.75 and 1.00, c is cohesion, and j is the angle of internal friction. s , s , and s are the stresses of the soil. 1 2 3 3.2. Model of Supporting Structure In the model, the retaining pile was transformed into an underground diaphragm wall in the way of equal stiffness [36], and the linear elastic model plate element with a thickness of 965 mm was used for simulation. The transformed formula is shown in Equations (2) and (3), 1 1 3 4 (D + t)h = pD (2) 12 64 h = 0.838D (3) 1 + where D is the diameter of the retaining pile, t is the spacing of the retaining piles, and h is the thickness of the transformed underground diaphragm wall. Six supporting structures were set in the central district of the foundation pit along the depth direction. The ﬁrst to fourth supporting structures were reinforced concrete supports, and the ﬁfth and sixth supporting structures were steel pipe supports. Two concrete supporting structures were set along the depth direction in the northern and southern districts of the foundation pit, and top beams or enclosure beams were set at the ends of the supporting structures. Supports of each layer, column piles, top beams, and wai purlings were simulated by linear elastic beam elements, and the material parameters of the supporting structure are shown in Table 2. The model grid of the supporting structure is shown in Figure 6. Table 2. Material parameters of supporting structure. Material Modulus of Poisson’s Size Properties Elasticity/E Ratio/v Supporting Structure - mm kN/m Retaining piles C35 concrete '1200@1500 3.25 10 0.2 Column pile C35 concrete '1200 3.25 10 0.2 First top beam C35 concrete 1200 2445 3.15 10 0.2 Second and third wai purling C30 concrete 1200 1200 3.00 10 0.2 Fourth top beam C35 concrete 1200 1000 0.2 3.15 10 Fifth and sixth wai purling C30 concrete 1400 1400 0.2 3.00 10 Supports of ﬁrst and fourth layer C40 concrete 800 1000 3.25 10 0.2 Support of second layer C40 concrete 1000 1100 3.25 10 0.2 Support of second layer C40 concrete 1100 1200 3.25 10 0.2 Supports of ﬁfth and sixth layer steels '800@16 steel pipe 7.85 10 0.3 Buildings 2022, 12, 1292 7 of 28 When the model was established, 10-node and tetrahedron elements were used for the soil layer, 3-node line element was used for the beam, and 12-node interface elements were used for the interaction of soil and structure in the process of grid division. The total Buildings 2022, 06, x FOR PEER REVIEW 8 of 29 Buildings 2022, 06, x FOR PEER REVIEW number of model elements was 360,437, and the total number of nodes was 263,819. 8 of The 29 solution type of the model was the construction stage. The model is shown in Figure 7. Figure 6. The model grid of the supporting structure. Figure 6. The model grid of the supporting structure. Figure 6. The model grid of the supporting structure. Figure 7. Three-dimensional finite element model of the LDCFP. FFigure igure 7. 7. TThr hree ee-dimensional -dimensional fiﬁnite nite el element ement m model odel of of th the e LLDCFP DCFP. . During the construction of the foundation pit, the geological environment varies During the construction of the foundation pit, the geological environment varies During the construction of the foundation pit, the geological environment varies greatly, the surrounding environment is complex, and the traffic flow is large. The con- greatly, the surrounding environment is complex, and the trafﬁc ﬂow is large. The con- greatly, the surrounding environment is complex, and the traffic flow is large. The con- struction method of partitioned excavation should be adopted for LDCFP engineering. In struction method of partitioned excavation should be adopted for LDCFP engineering. In struction method of partitioned excavation should be adopted for LDCFP engineering. In order to study the interaction between various districts under different excavation proce- order to study the interaction between various districts under different excavation proce- order to study the interaction between various districts under different excavation proce- dures, different construction organizations of foundation pit excavation were simulated dures, different construction organizations of foundation pit excavation were simulated dures, different construction organizations of foundation pit excavation were simulated ffr ro om m th the e tw two o as aspects pects o of f s step-by-step tep-by-step a and nd ssynchr ynchrono onous us e excavation, xcavation, aand nd tthe he d deformation eformation from the two aspects of step-by-step and synchronous excavation, and the deformation cha characteristics racteristics of of tthe he ffoundation oundation p pit it e excavation xcavation w wer ere e d discussed. iscussed. characteristics of the foundation pit excavation were discussed. 4. Results of Numerical Simulation 4. Results of Numerical Simulation 4. Results of Numerical Simulation 4.1. Deformation Characteristics of Step-By-Step Partitioned Excavation 4.1. Deformation Characteristics of Step-By-Step Partitioned Excavation 4.1. Deformation Characteristics of Step-By-Step Partitioned Excavation 4.1.1. Model Working Conditions Design 4.1.1. Model Working Conditions Design 4.1.1. Model Working Conditions Design Three working conditions of the step-by-step partitioned excavation method were Three working conditions of the step-by-step partitioned excavation method were Three working conditions of the step-by-step partitioned excavation method were simulated. (1) The central district of the foundation pit was excavated ﬁrst, and then the simulated. (1) The central district of the foundation pit was excavated first, and then the simulated. (1) The central district of the foundation pit was excavated first, and then the southern and northern districts were excavated at the same time, which was recorded as southern and northern districts were excavated at the same time, which was recorded as so condition uthern and 1. nor (2) The thern southern districts and were northern excavat districts ed at the of sa the mefoundation time, which pit wwe as rec re excavated orded as condition 1. (2) The southern and northern districts of the foundation pit were excavated condition 1. (2) The southern and northern districts of the foundation pit were excavated at the same time, and then the central district of the foundation pit was excavated, which at the same time, and then the central district of the foundation pit was excavated, which awas t ther s ecor ame ded time as , condition and then th 2.e(3) cen The tralnorthern district of district the fou of nd the ation foundation pit was epit xcawas vateexcavated d, which was recorded as condition 2. (3) The northern district of the foundation pit was excavated w ﬁrst, as rethen corde the d acentral s condit distri ion 2ct . (3 of ) T the he foundation northern dis pit, tricand t of tﬁnally he found the atsouthern ion pit wa district s excavof ate the d first, then the central district of the foundation pit, and finally the southern district of the first, then the central district of the foundation pit, and finally the southern district of the foundation pit, which was recorded as condition 3. The analysis steps of each working foundation pit, which was recorded as condition 3. The analysis steps of each working condition are shown in Figure 8. condition are shown in Figure 8. north area bowk not area pit-in-pit are a south area north-south area bowk not area pit-in-pit are a n orth area b owk no t area pit-in-pit are a south area bowk not area pit-in-pit are a north-south area north-south area b owk no t area pit-in-pit are a b owk not area pit-in-pit are a north-south area Buildings 2022, 12, 1292 8 of 28 Buildings 2022, 06, x FOR PEER REVIEW 9 of 29 foundation pit, which was recorded as condition 3. The analysis steps of each working Buildings 2022, 06, x FOR PEER REVIEW 9 of 29 working Condition 1 working Condition 2 working Condition 3 condition are shown in Figure 8. Initial crustal stress working Condition 1 working Condition 2 working Condition 3 Construction of retainin g-piles and column-pile Initial crustal stress layer 1: top beam Constru ction of retainin g-piles and column-pile layer 1: top beam and support layer 1: top beam and support and support layer 2:wai purli n layer 1: top beam layer 1: top beam layer 2:wai purli n and support and support layer 1: top beam layer 2:wai purli n and support and support and support and support layer 2:wai purli n layer 3:wai purli n layer 2:wai purli n and support layer 2:wai purli n and support and support and support layer 1: top beam layer 3:wai purli n layer 1: top beam and support and support and support layer 4: top beam layer 1: top beam and support layer 2:wai purli n layer 1: top beam and support and support and support layer 2:wai purli n layer 4: top beam and support and support layer 5:wai purli n layer 2:wai purli n layer 2:wai purli n and support layer 3:wai purli n and support and support and support layer 5:wai purli n layer 3:wai purli n layer 3:wai purli n and support and support layer 6:wai purli n and support layer 3:wai purli n and support and support layer 6:wai purli n layer 4: top beam and support layer 4: top beam and support layer 4: top beam and support and support layer 4: top beam layer 1: top beam layer 5:wai purli n and support and support layer 1: top beam and support layer 5:wai purli n layer 5:wai purli n and support and support layer 5:wai purli n and support layer 6:wai purli n layer 2:wai purli n and support layer 6:wai purli n and support layer 2:wai purli n and support layer 6:wai purli n and support and support layer 6:wai purli n and support and support layer 1: top beam layer 1: top beam and support and support layer 2:wai purli n layer 2:wai purli n and support and support Figure 8. The analysis steps of step-by-step partitioned excavation method. Figure 8. The analysis steps of step-by-step Figure 8. Thpartitioned e analysis step excavation s of step-by method. -step partitioned excavation method. 4.1.2. The Deformation Characteristics of the Ground Surface around the Foundation Pit 4.1.2. The Deformation Characteristics of the Ground Surface around the Foundation Pit 4.1.2. The Deformation Characteristics of the Ground Surface around the Foundation Pit In order to study the deformation characteristics of the ground surface around the In order to study the deformation characteristics of the ground surface around the In order to study the deformation characteristics of the ground surface around the foundation pit, modeling and calculation were carried out according to the above working foundation pit, modeling and calculation were carried out according to the above working foundation pit, modeling and calculation were carried out according to the above working conditions, and the measuring point was selected in the eight directions of the foundation conditions, and the measuring point was selected in the eight directions of the foundation conditions, and the measuring point was selected in the eight directions of the foundation pit; it was named P1-P8 clockwise from due north. The surface deformation within 50 m pit; it was named P1-P8 clockwise from due north. The surface deformation within 50 m of of the retaining piles was analyzed p . B it;y i tt aw kia ng s n wame ork C do P nd 1- it P ion 8 c1 lock as aw n i ese xa m frp olm e, tdu he s e u nor rface th. The surface deformation within 50 m the retaining piles was analyzed. By taking work Condition 1 as an example, the surface deformation characteristics around of t the he frou eta nd in ain tion p g pile it s a re was show anal n i yn F zed ig . B ury e t 9a . T kihe ng m wa o xi rk - Condition 1 as an example, the surface deformation characteristics around the foundation pit are shown in Figure 9. The maximum mum deformation of three working conditions is shown in Figure 10. deformation characteristics around the foundation pit are shown in Figure 9. The maxi- deformation of three working conditions is shown in Figure 10. mum deformation of three working conditions is shown in Figure 10. Distance from retaining piles(m） 10 20 30 40 50 Distance from retaining piles(m） 10 20 30 40 50 -2 -2 -4 2 P P2 8 P -4 P P 2 P 3 5 7 P1 -6 P P2 P4 P8 6 P 7 4 P P 8 P 3 5 7 -6 P -8 4 P6 Figure 9. The surface deformation of the ground surface around the foundation pit. -8 Figure 9. The surface deformation of the ground surface around the foundation pit. Figure 9. The surface deformation of the ground surface around the foundation pit. The surface deformation(mm) The surface deformation(mm) Buildings 2022, 06, x FOR PEER REVIEW 10 of 29 Buildings 2022, 12, 1292 9 of 28 Buildings 2022, 06, x FOR PEER REVIEW 10 of 29 Figure 10. The maximum deformation of three working conditions. Figure 10. The maximum deformation of three working conditions. Figure 10. The maximum deformation of three working conditions. Figure 10 shows that the maximum deformation of three working conditions is work- Figure 10 shows that the maximum deformation of three working conditions is work- Figure 10 shows that the maximum deformation of three working conditions is work- ing ing Condition Condition 3 3 > > w working orking CCondition ondition 2 2 > w > or working king Cond Condition ition 1. T 1. hisThis is beis cau because se the sy the m- ing Condition 3 > working Condition 2 > working Condition 1. This is because the sym- symmetrical metrical exca excavation vation met method hod was was adop adopted ted forfor Cond Conditions itions 1 a1 nd and 2, a 2, nd and the the spspacing acing effe efc fect t of metrical excavation method was adopted for Conditions 1 and 2, and the spacing effect of of the foundation pit was used. the foundation pit was used. the foundation pit was used. 4.1.3. Deformation of Supporting Structure 4.1.3. Deformation of Supporting Structure 4.1.3. Deformation of Supporting Structure After the numerical calculation of the model is completed according to the excavation After the numerical calculation of the model is completed according to the excavation After the numerical calculation of the model is completed according to the excavation steps, the deformation nephogram of the supporting structure is shown in Figure 11. steps, the deformation nephogram of the supporting structure is shown in Figure 11. steps, the deformation nephogram of the supporting structure is shown in Figure 11. (a) (b) (a) (b) (c) (c) Figure 11. The deformation nephogram of the supporting structure. (a) working Condition 1. (b) Figure Figure 11. 11. TThe he def deformation ormation nenephog phogram ram of th ofe s the upsupporting porting strustr ctu uctur re. (ae. ) w( o ark ) i working ng CondiCondition tion 1. (b) 1. working Condition 2. (c) working Condition 3. working Condition 2. (c) working Condition 3. (b) working Condition 2. (c) working Condition 3. Buildings 2022, 12, 1292 10 of 28 Buildings 2022, 06, x FOR PEER REVIEW 11 of 29 The deformation nephograms of the supporting structure under three working condi- The deformation nephograms of the supporting structure under three working con- tions were compared, and it can be seen that the deformation difference of the supporting ditions were compared, and it can be seen that the deformation difference of the support- str in uctur g stru ecis tuobvious. re is obvio The us. T dif he fer dience fferen is cemainly is mainr leﬂected y reflected i in the n thshar e sha ed red retaining retaining piles pilesof ofthe central the cen and tral southern–northern and southern–northern districts. districtUnder s. Under condition condition 1,1the , the shar shaed red retaining retaining piles piles by the by so thuth–north e south–nort rows h row wer s we ere alladeformed ll deformed to tothe the inner inner s side ide o of f th the e cen central tral didistrict; strict; unde under r Condition Condition2, 2,part partof of the the s shar hared ed ret retaining aining p piles iles bby y th the e so south–north uth–north ror w ows s wer wer e d eef deformed ormed to to the outside of the central district; under Condition 3, part of the shared retaining piles by the outside of the central district; under Condition 3, part of the shared retaining piles by the north row deformed to the outside of the central district, and part of the shared retain- the north row deformed to the outside of the central district, and part of the shared retain- ing piles by the south row deformed to the inside of the central district. The deformation ing piles by the south row deformed to the inside of the central district. The deformation nephogram shows that the final deformation of the shared retaining pile is related to the nephogram shows that the ﬁnal deformation of the shared retaining pile is related to the excavation sequence of the foundation pit. excavation sequence of the foundation pit. Deformation of the Exterior Retaining Structure Deformation of the Exterior Retaining Structure The deformation of retaining piles in each district under the working condition of the The deformation of retaining piles in each district under the working condition of the step-by-step partitioned excavation method was further analyzed. The deformation of the step-by-step partitioned excavation method was further analyzed. The deformation of the retaining piles in the excavation process of the foundation pit is shown in Figure 12. retaining piles in the excavation process of the foundation pit is shown in Figure 12. The deformation of retaining piles(mm) The deformation of retaining piles(mm) 2 4 6 8 10 12 14 2 4 6 8 10 12 14 -5 -5 -10 -10 -15 -15 1 -20 -20 -25 P -25 -30 -30 (a) (b) The deformation of retaining piles(mm) 2 4 6 8 10 12 14 -5 -10 -15 -20 -25 6 -30 (c) Figure 12. Deformation of the exterior retaining structure. (a) working Condition 1. (b) working Condition 2. (c) working Condition 3. Buried depth(m) Buried depth(m) Buried depth(mm) Buildings 2022, 06, x FOR PEER REVIEW 12 of 29 Buildings 2022, 12, 1292 11 of 28 Figure 12. Deformation of the exterior retaining structure. (a) working Condition 1. (b) working Condition 2. (c) working Condition 3. The deformation law of the exterior retaining pile under each working condition is The deformation law of the exterior retaining pile under each working condition is basically consistent, and the pile shaft was “waist drum shaped”; except for measuring basically consistent, and the pile shaft was “waist drum shaped”; except for measuring point P1, the deformation of all retaining piles was concentrated in the range of 7–11 mm. point P1, the deformation of all retaining piles was concentrated in the range of 7–11 mm. The above analysis showed that the deformation law of the exterior retaining structure is The above analysis showed that the deformation law of the exterior retaining structure is less affected by the excavation sequence in different districts of the foundation pit, and less affected by the excavation sequence in different districts of the foundation pit, and the deformation the deformatof ion the of exterior the exter rietaining or retainstr ing uctur struc etis urr eelatively is relativ uniform. ely uniform. Retaining Structure Deformation in the Pit-in-Pit District Retaining Structure Deformation in the Pit-in-Pit District Three reference points of retaining piles were selected along with the equal distance to Three reference points of retaining piles were selected along with the equal distance the north side of the foundation pit, which denotes K1 (west side pit angle), K2, and K3 to the north side of the foundation pit, which denotes K1 (west side pit angle), K2, and K3 (east side pit angle). Two reference points of the retaining piles were selected along with (east side pit angle). Two reference points of the retaining piles were selected along with the equal distance of the south side of the foundation pit, which denotes K4 (west side pit the equal distance of the south side of the foundation pit, which denotes K4 (west side pit angle) and K5 (short side midpoint). The ﬁnal deformation of the retaining structure in the angle) and K5 (short side midpoint). The final deformation of the retaining structure in pit-in-pit district is shown in Figure 13. the pit-in-pit district is shown in Figure 13. The deformation of the retaining structure(mm) The deformation of the retaining structure(mm) 2 3 4 5 6 7 8 2 4 6 8 -20 -20 -25 -25 -30 -30 K1 K1 K2 K2 K3 -35 -35 K3 K4 K4 K5 K5 -40 -40 (a) (b) The deformation of the retaining structure(mm) 2 3 4 5 6 7 8 -20 -25 -30 K1 K2 K3 -35 K4 K5 -40 (c) Figure 13. Retaining structure deformation in pit-in-pit district. (a) working Condition 1. (b) working Condition 2. (c) working Condition 3. Buried depth(m) Buried depth(m) Buried depth(m) Buildings 2022, 12, 1292 12 of 28 Figure 13 shows that: (1) The K1, K3, and K4 measuring points have almost no difference in the deformation of the pile shaft under different excavation steps. The maximum deformation at the top of the pile is concentrated in the range of 4–5 mm, and the deformation decreases gradually with increasing excavation depth; (2) By comparison, it was found that the deformation of the retaining piles under Condition 2 is small. This is because when the pit-in-pit district is excavated, the soil in the northern and southern districts is excavated. Due to the unloading effect of the soil, the load effect of the upper soil in the pit-in-pit district is weakened, so the deformation is small. The excavation sequence of the foundation pit has a certain inﬂuence on the deformation of the retaining piles in the pit-in-pit district; (3) Deformation of the shared retaining structure The shared retaining structure of LDCFP refers to the retaining piles used to separate the central district and the northern–southern districts. There are three rows of shared retaining piles in this engineering. The north row is located on the north side of the bowknot district (the length is 26 m), the south-1 row is located on the southwest side of the bowknot district (the length is 26 m), and the south-2 row is located in the southeast side of the bowknot district (the length is 39 m). Three reference points were selected along the north row, which denote ZN1 (1/4 side), ZN2 (midpoint), and ZN3 (3/4 side). Five reference points were selected along the south-1 and south-2 rows, which denote ZS1 (1/3 south-1 row), ZS2 (2/3 south-1 row), ZS3 (1/4 south-2 row), ZS4 (south-2 row midpoint), and ZS5 (3/4 south-2 row). The deformation of each reference point in the construction stage of the bowknot district, pit-in-pit district, and northern–southern districts is shown in Figure 14. Figure 14 shows that: (1) In the excavation process of the bowknot district, the deformation of shared re- taining piles shaft is approximately a “waist drum-shaped”. The deformation is mainly concentrated 10 m underground, and the deformation of the pile shaft below 10 m gradually decreases. The deformation of the retaining pile in the center is greater than that of the retaining piles on both sides; (2) When the pit-in-pit district is excavated, the deformation of the bottom of the retaining piles (long piles) in the south-2 row continues to increase, and the maximum deformation position gradually moves down with the excavation; (3) When the foundation pit is constructed in the northern and southern districts, the deformation of the short pile shaft changes from the original “waist drum shaped” to the approximate “vertical shaped”, due to the unloading effect of the soil behind the pile, and the pile shaft is mainly affected by the axial force of the support in the foundation pit; (4) The deformation of the retaining piles mainly occurs in the excavation stage of the northern–southern districts. Buildings 2022, 12, 1292 13 of 28 Buildings 2022, 06, x FOR PEER REVIEW 14 of 29 The deformation(mm) The deformation(mm) 4 8 12 16 20 0 4 8 12 0 0 ZN1 ZN2 ZN3 ZS1 -10 -10 ZS2 ZS3 ZS4 ZS5 -20 -20 ZN1 ZN2 ZN3 ZS1 -30 -30 ZS2 ZS3 ZS4 ZS5 -40 -40 (a) (b) The deformation(mm) 4 8 12 16 20 ZN1 ZN2 ZN3 ZS1 -10 ZS2 ZS3 ZS4 ZS5 -20 -30 -40 (c) Figure 14. Deformation of the shared retaining structure. (a) working Condition 1. (b) working Figure 14. Deformation of the shared retaining structure. (a) working Condition 1. (b) working Condition 2. (c) working Condition 3. Condition 2. (c) working Condition 3. 4.2. Deformation Characteristics of Synchronous Partitioned Excavation 4.2. Deformation Characteristics of Synchronous Partitioned Excavation 4.2.1. Design of Model Working Conditions 4.2.1. Design of Model Working Conditions The synchronous partitioned excavation of the central district and the northern– The synchronous partitioned excavation of the central district and the northern– southern district of the foundation pit is divided into two cases: synchronous excavation southern district of the foundation pit is divided into two cases: synchronous excavation of of the same layer and the layered synchronous excavation. The construction steps for each the same layer and the layered synchronous excavation. The construction steps for each condition are shown in Figure 15. condition are shown in Figure 15. Buried depth(m) Buried depth(m) Buried depth(m) bowknot area north-south area pit-in-pit are a bowknot area north-south area pit-in-pit are a bowknot area north-south area pit-in-pit are a bowknot area north-south area pit-in-pit are a Buildings 2022, 06, x FOR PEER REVIEW 15 of 29 Buildings 2022, 12, 1292 14 of 28 Buildings 2022, 06, x FOR PEER REVIEW 15 of 29 working Condition 4 working Condition 5 Initial cru stal stres s working Condition 4 working Condition 5 Construction of r etaining-piles and column-pile Initial cru stal stres s layer 1:top beam Construction of r etain in g-p iles an d column-pile and support layer 2:wai purli n layer 1:top beam and support layer 1: top beam and support and support ll aa yy erer 2:w 3:w ai pur ai pur li n li n and support layer 1: top beam and support layer 2:wai purli n and support and support layer 3:wai purli n and support + layer 2:wai purli n and support layer 3:wai purli n layer 1: top beam and support layer 3:wai purli n layer 1: top beam and support layer 4:top beam and support layer 4:top beam and support layer 4: top beam layer 5:wai purli n layer 4: top beam layer 5:wai purli n and support and support and support + and support la la yy erer 2:w 2:w ai pur ai pur li n li n layer 5:wla aiy pur er l5:w i n ai purli n and support and support and support and support layer 6:wai purli n layer 6:wai purli n layer 6:wai purli n layer 6:wai purli n and support and support and support and support Figure 15. Simulation calculation of conditions. Figure 15. Simulation calculation of conditions. Figure 15. Simulation calculation of conditions. 4.2.2. The Deformation Characteristics of the Ground Surface around the Foundation Pit 4.2.2. The Deformation Characteristics of the Ground Surface around the Foundation Pit 4.2.2. The Deformation Characteristics of the Ground Surface around the Foundation Pit The modeling and calculation were carried out according to the above working con- The modeling and calculation were carried out according to the above working condi- ditions. After the calculation of the foundation pit model, the reference point was selected tions. The After mod the elcalculation ing and ca of lc the ula foundation tion werepit camodel, rried ou the t r a efer ccor ence ding point to t was he a selected bove working con- at the same position as the previous working condition, which is denoted as M1–M8. The at the same position as the previous working condition, which is denoted as M1–M8. The ditions. After the calculation of the foundation pit model, the reference point was selected deformation characteristics of the ground surface around the foundation pit are shown in deformation characteristics of the ground surface around the foundation pit are shown in at the same position as the previous working condition, which is denoted as M1–M8. The Figure 16. Figure 16. deformation characteristics of the ground surface around the foundation pit are shown in Figure 16. (a) (b) Figure 16. The deformation characteristics of the ground surface. (a) working Condition 4. (b) work- Figure 16. The deformation characteristics of the ground surface. (a) working Condition 4. (b) work- ing Condition 5. ing Condition 5. T The he d deformation eformation cha characteristics racteristics of of the the ssurface urface d deformation eformation of of tthe he g gr ro ound und ssurface urface aar ro ound und tthe he ffoundation oundation p pit it u under nder C Conditions onditions 4 4 aand nd 55 aar re b e basically asically tthe he ssame. ame. B Because ecause oof f th the e (a) (b) symmetrical partitioned excavation method was adopted in both conditions. symmetrical partitioned excavation method was adopted in both conditions. Figure 16. The deformation characteristics of the ground surface. (a) working Condition 4. (b) work- ing Condition 5. The deformation characteristics of the surface deformation of the ground surface around the foundation pit under Conditions 4 and 5 are basically the same. Because of the symmetrical partitioned excavation method was adopted in both conditions. Buildings 2022, 12, 1292 15 of 28 Buildings 2022, 06, x FOR PEER REVIEW 16 of 29 4.2.3. Deformation of Retaining Structure 4.2.3. Deformation of Retaining Structure When the calculation was completed according to the excavation steps of working When the calculation was completed according to the excavation steps of working Conditions 4 and 5, the deformation of the retaining structure was analyzed. Conditions 4 and 5, the deformation of the retaining structure was analyzed. DDeformation eformation of E of xtExterior erior RetaRetaining ining Struct Str uructur e e The deformation of the retaining structure during the synchronous partitioned exca- The deformation of the retaining structure during the synchronous partitioned excava- vation was analyzed, as shown in Figure 17. tion was analyzed, as shown in Figure 17. The deformation of the retaining structure(mm) The deformation of the retaining structure(mm) 4 6 8 10 12 4 6 8 10 12 -5 -5 -10 -10 -15 -15 -20 -20 M -25 -25 -30 -30 (a) (b) Figure 17. Deformation of the exterior retaining structure. (a) working Condition 4. (b) working Figure 17. Deformation of the exterior retaining structure. (a) working Condition 4. (b) working Condition 5. Condition 5. The deformation curve shows that under the condition of synchronous excavation in The deformation curve shows that under the condition of synchronous excavation in different districts, the deformation of the surrounding retaining pile was not affected by different districts, the deformation of the surrounding retaining pile was not affected by the excavation process, and the deformation is relatively uniform. the excavation process, and the deformation is relatively uniform. Deformation of the Retaining Structure in the Pit-in-Pit District Deformation of the Retaining Structure in the Pit-in-Pit District Five reference points were selected with the same location as the step-by-step parti- Five reference points were selected with the same location as the step-by-step parti- tioned excavation, named M1′, M2′, M3′, M4′, and M5′. Under working Conditions 4 and 0 0 0 0 0 tioned excavation, named M1 , M2 , M3 , M4 , and M5 . Under working Conditions 4 and 5, the deformation of retaining piles is shown in Figure 18 after the synchronous excava- Buildings 2022, 06, x FOR PEER REVIEW 17 of 29 5, the deformation of retaining piles is shown in Figure 18 after the synchronous excavation tion in different districts was completed. in different districts was completed. (a) (b) Figure 18. Deformation of retaining structure in pit-in-pit district. (a) working Condition 4. (b) Figure 18. Deformation of retaining structure in pit-in-pit district. (a) working Condition 4. working Condition 5. (b) working Condition 5. Figure 18 shows that the deformation of the retaining pile in the pit-in-pit district is consistent with the final deformation of the step excavation, indicating that the synchro- nous excavation sequence of the foundation pit has little effect on the deformation of the retaining pile in the pit-in-pit district. Figure 18 shows that the deformation of the retaining pile in the pit-in-pit district is consistent with the final deformation of the step excavation, indicating that the synchro- nous excavation sequence of the foundation pit has little effect on the deformation of the retaining pile in the pit-in-pit district. Deformation of the Sharing Retaining Structure Eight reference points were selected with the same location as the step-by-step parti- tioned excavation. The deformation of each retaining pile at different excavation stages was analyzed, as shown in Figure 19. The deformation(mm) The deformation(m) 4 8 12 16 20 0 2 4 6 8 10 -10 -10 -20 ZN1 -20 ZN1 ZN2 ZN2 ZN3 ZS1 ZN3 ZS2 ZS1 -30 -30 ZS3 ZS2 ZS4 ZS3 ZS5 ZS4 ZS5 -40 -40 (a) (b) Buried depth(m) Buried depth(m) Buried depth(m) Buried depth(m) Buildings 2022, 06, x FOR PEER REVIEW 17 of 29 (a) (b) Figure 18. Deformation of retaining structure in pit-in-pit district. (a) working Condition 4. (b) working Condition 5. Figure 18 shows that the deformation of the retaining pile in the pit-in-pit district is consistent with the final deformation of the step excavation, indicating that the synchro- Buildings 2022, 12, 1292 16 of 28 nous excavation sequence of the foundation pit has little effect on the deformation of the retaining pile in the pit-in-pit district. Figure 18 shows that the deformation of the retaining pile in the pit-in-pit district is Figure 18 shows that the deformation of the retaining pile in the pit-in-pit district is consistent with the final deformation of the step excavation, indicating that the synchro- consistent with the ﬁnal deformation of the step excavation, indicating that the synchronous nous excavation sequence of the foundation pit has little effect on the deformation of the excavation sequence of the foundation pit has little effect on the deformation of the retaining retaining pile in the pit-in-pit district. pile in the pit-in-pit district. Deformation of the Sharing Retaining Structure Deformation of the Sharing Retaining Structure Eight reference points were selected with the same location as the step-by-step parti- Eight reference points were selected with the same location as the step-by-step parti- tioned excavation. The deformation of each retaining pile at different excavation stages tioned excavation. The deformation of each retaining pile at different excavation stages was analyzed, as shown in Figure 19. was analyzed, as shown in Figure 19. The deformation(mm) The deformation(m) 4 8 12 16 20 0 2 4 6 8 10 -10 -10 -20 ZN1 -20 ZN1 ZN2 ZN2 ZN3 ZN3 ZS1 ZS2 ZS1 -30 -30 ZS3 ZS2 ZS4 ZS3 ZS5 ZS4 Buildings 2022, 06, x FOR PEER REVIEW ZS5 18 of 29 -40 -40 (a) (b) The deformation(mm) 4 8 12 16 20 -10 ZN1 -20 ZN2 ZN3 ZS1 ZS2 ZS3 -30 ZS4 ZS5 -40 (c) Figure 19. Deformation of the shared retaining structure under working Condition 4. (a) excavate Figure 19. Deformation of the shared retaining structure under working Condition 4. (a) excavate to to the bottom of the first layer. (b) excavate to the bottom of the second layer. (c) excavate to the the bottom of the ﬁrst layer. (b) excavate to the bottom of the second layer. (c) excavate to the bottom bottom of the foundation pit. of the foundation pit. Figure 19 shows that under working Condition 4: Figure 19 shows that under working Condition 4: (1) When the excavation of (1) When the firs the t la excavation yer is comp of lethe ted,ﬁrst the d layer eform isacompleted, tion of the tthe op deformation of the top retaining pile is small un rd etaining er the cons pile tra is int small of the under first athe nd sconstraint econd supp of orthe ts, aﬁrst nd the and masecond x- supports, and the imum deformation of thmaximum e pile shaft deformation is located ap of pr the oxipile mate shaft ly 10is m located underg appr round oximately ; that is, 10 m underground; that between the second supp is, orbetween t (−4 m) ithe n the second northern support –south ( ern 4 m) disin trithe cts a northern–southern nd the bottom (−14 districts and the bottom (14 m) in the northern–southern districts; m) in the northern–southern districts; (2) When the LDCFP is excavated to the first layer in the central district, due to the unloading effect of the foundation pit soil behind the pile, which is approximately linear- shaped, showing the rebound deformation of the retaining pile shaft. There is no obvious deformation of the pile top, which is due to the concrete support erected before excavation hindering the rebound of the retaining pile; (3) When the LDCFP is excavated to the third layer in the central district, the defor- mation of each retaining pile further increases and the deformation of the pile shaft is the most obvious in the range of 10–20 mm; in the excavation of the pit-in-pit district, only the ZS4 and ZS5 retaining piles (long piles) continue to deform downward. Figure 20 shows that under Condition 5: (1) When the foundation pit is excavated in the bowknot district, the shared retaining pile is only subjected to unilateral earthwork unloading, and the deformation trend of the retaining pile in each district is relatively close, indicating that the stress of the retaining pile in each district is consistent; (2) In the early stage of excavation, due to the unloading effect of the soil behind the pile, the overall deformation of the retaining pile decreases when the southern and north- ern districts of the foundation pit are excavated simultaneously. With the advance of ex- cavation steps, the deformation of the retaining structure continues to increase, which is manifested as large deformation of short piles as a whole, and obvious deformation of long piles only occurs at the upper part of the pile shaft; (3) When the foundation pit excavation is completed, the deformation of the long pile is parabolic-shaped, and the short pile still has a certain drum-shaped; that is, the defor- mation of the retaining pile is not fully restored, and the overall deformation of the retain- ing pile is larger, and the top deformation of ZS5 is larger than that of ZS3 and ZS4, which is related to the stiffness of the corner brace. This shows that the internal force changes greatly and is prone to mutation when the layered synchronous excavation is carried out in different districts. Buried depth(m) Buried depth(m) Buried depth(m) Buildings 2022, 12, 1292 17 of 28 (2) When the LDCFP is excavated to the ﬁrst layer in the central district, due to the unloading effect of the foundation pit soil behind the pile, which is approximately linear- shaped, showing the rebound deformation of the retaining pile shaft. There is no obvious deformation of the pile top, which is due to the concrete support erected before excavation hindering the rebound of the retaining pile; (3) When the LDCFP is excavated to the third layer in the central district, the defor- mation of each retaining pile further increases and the deformation of the pile shaft is the most obvious in the range of 10–20 mm; in the excavation of the pit-in-pit district, only the ZS4 and ZS5 retaining piles (long piles) continue to deform downward. Figure 20 shows that under Condition 5: (1) When the foundation pit is excavated in the bowknot district, the shared retaining pile is only subjected to unilateral earthwork unloading, and the deformation trend of the retaining pile in each district is relatively close, indicating that the stress of the retaining pile in each district is consistent; (2) In the early stage of excavation, due to the unloading effect of the soil behind the pile, the overall deformation of the retaining pile decreases when the southern and northern districts of the foundation pit are excavated simultaneously. With the advance of excavation steps, the deformation of the retaining structure continues to increase, which is manifested as large deformation of short piles as a whole, and obvious deformation of long piles only occurs at the upper part of the pile shaft; (3) When the foundation pit excavation is completed, the deformation of the long pile is parabolic-shaped, and the short pile still has a certain drum-shaped; that is, the deformation of the retaining pile is not fully restored, and the overall deformation of the retaining pile is larger, and the top deformation of ZS5 is larger than that of ZS3 and ZS4, which is related to the stiffness of the corner brace. This shows that the internal force changes greatly and is prone to mutation when the layered synchronous excavation is carried out in different districts. 4.3. Comparison of Partitioned Excavation Methods From the above analysis, it can be seen that: (1) In the step-by-step excavation of the foundation pit, the deformation of the ground surface around the foundation pit under different working conditions is characterized by lateral displacement and uplift of the soil at the bottom of the foundation pit. The surface settlement of symmetrical excavation under working Conditions 1 and 2 is small. The ﬁnal deformation of the shared retaining pile tilts toward the excavation district. When the soil on both sides of the pile shaft is unloaded, the deformation of the retaining pile is “linear shaped”; (2) When the foundation pit is adopted synchronous partitioned excavation, the deformation of the ground surface around the foundation pit, the deformation of the exterior retaining pile, and in the pit-in-pit district under various working conditions are consistent with the deformation of step-by-step excavation. The deformation difference of the shared retaining pile is obvious. The deformation of the shared retaining pile is small, and the deformation is consistent when the same layer is excavated synchronously. The deformation of the pile shaft is complex, uneven distribution of internal forces that are prone to mutation under the layered synchronous excavation; (3) When the foundation pit is excavated step-by-step, the symmetrical excavation method should be preferred, and the smaller district should be excavated ﬁrst; the con- struction method of synchronous excavation in the same layer is more beneﬁcial to control the overall deformation of the foundation pit under the fully considering the engineering economy and the feasibility of construction organization. Buildings 2022, 12, 1292 18 of 28 Buildings 2022, 06, x FOR PEER REVIEW 19 of 29 The deformation(mm) The deformation(mm) 0 4 8 12 16 4 8 12 16 20 -10 -10 -20 -20 ZN1 ZN1 ZN2 ZN2 ZN3 ZN3 ZN4 ZS1 ZN5 -30 -30 ZS2 ZN6 ZS3 ZN7 ZS4 ZN8 ZS5 -40 -40 (a) (b) The deformation(mm) 4 8 12 16 20 -10 -20 ZN1 ZN2 ZN3 ZS1 -30 ZS2 ZS3 ZS4 ZS5 -40 (c) Figure 20. Deformation of the shared retaining structure under working Condition 5. (a) excavate Figure 20. Deformation of the shared retaining structure under working Condition 5. (a) excavate to to the bottom of the first layer. (b) excavate to the bottom of the second layer. (c) excavate to the the bottom of the ﬁrst layer. (b) excavate to the bottom of the second layer. (c) excavate to the bottom bottom of the foundation pit. of the foundation pit. In summary, the LDCFP engineering based on the paper adopts the construction 4.3. Comparison of Partitioned Excavation Methods method of step-by-step partitioned excavation using Condition 1. From the above analysis, it can be seen that: (1) In the step-by-step excavation of the foundation pit, the deformation of the 5. Foundation Pit Monitoring Design ground surface around the foundation pit under different working conditions is charac- In order to ensure the stability of the foundation pit during construction, according to terized by lateral displacement and uplift of the soil at the bottom of the foundation pit. the characteristics of step-by-step partitioned excavation in different districts of the LDCFP The surface settlement of symmetrical excavation under working Conditions 1 and 2 is engineering, in-site monitoring of deformation during construction was carried out. The small. The final deformation of the shared retaining pile tilts toward the excavation dis- surface deformation around the foundation pit and retaining structure was analyzed. trict. When the soil on both sides of the pile shaft is unloaded, the deformation of the retaining pile is “linear shaped”; 5.1. Monitoring Plan Design (2) When the foundation pit is adopted synchronous partitioned excavation, the de- Based on the LDCFP engineering, the lateral displacement of the retaining pile, the sur- formation of the ground surface around the foundation pit, the deformation of the exterior face deformation around the foundation pit, and the supporting axial force were monitored. retaining pile, and in the pit-in-pit district under various working conditions are con- The locations of the measuring points are shown in Figure 21. sistent with the deformation of step-by-step excavation. The deformation difference of the shared retaining pile is obvious. The deformation of the shared retaining pile is small, and Buried depth(m) Buried depth(m) Buried depth(m) Buildings 2022, 06, x FOR PEER REVIEW 20 of 29 the deformation is consistent when the same layer is excavated synchronously. The defor- mation of the pile shaft is complex, uneven distribution of internal forces that are prone to mutation under the layered synchronous excavation; (3) When the foundation pit is excavated step-by-step, the symmetrical excavation method should be preferred, and the smaller district should be excavated first; the con- struction method of synchronous excavation in the same layer is more beneficial to control the overall deformation of the foundation pit under the fully considering the engineering economy and the feasibility of construction organization. In summary, the LDCFP engineering based on the paper adopts the construction method of step-by-step partitioned excavation using Condition 1. 5. Foundation Pit Monitoring Design In order to ensure the stability of the foundation pit during construction, according to the characteristics of step-by-step partitioned excavation in different districts of the LDCFP engineering, in-site monitoring of deformation during construction was carried out. The surface deformation around the foundation pit and retaining structure was ana- lyzed. 5.1. Monitoring Plan Design Based on the LDCFP engineering, the lateral displacement of the retaining pile, the Buildings 2022, 12, 1292 19 of 28 surface deformation around the foundation pit, and the supporting axial force were mon- itored. The locations of the measuring points are shown in Figure 21. Figure 21. The location of measuring points. Figure 21. The location of measuring points. (1) Install the inclinometer tube in the retaining structure. The inclinometer tube depth (1) Install the inclinometer tube in the retaining structure. The inclinometer tube is the same as the length of the retaining structure and is laid every 25–30 m along the depth is the same as the length of the retaining structure and is laid every 25–30 m along longitudinal direction of the foundation pit, named CX1–CX61. The settlement measuring the longitudinal direction of the foundation pit, named CX1–CX61. The settlement meas- points are laid into section form along with the foundation pit, and the spacing is 25 m; uring points are laid into section form along with the foundation pit, and the spacing is (2) The support system of this engineering is complex, in which six supports are set 25 m; along the depth direction of the foundation pit in the central district, and 35 axial force (2) The support system of this engineering is complex, in which six supports are set monitoring points are set for each support layer in the bowknot district. Two reinforced along the depth direction of the foundation pit in the central district, and 35 axial force concrete support and one steel support are used in the pit-in-pit district, and 30 axial force monitoring points are set for each support layer in the bowknot district. Two reinforced monitoring points are set in each layer. Two supports are set along the depth direction of concrete support and one steel support are used in the pit-in-pit district, and 30 axial force the foundation pit in the northern and southern districts, and 17 axial force monitoring monitoring points are set in each layer. Two supports are set along the depth direction of points are set in each layer. the foundation pit in the northern and southern districts, and 17 axial force monitoring points are set in each layer. 5.2. Control Standard of Foundation Pit Deformation Due to the large excavation volume, complicated excavation steps, and complex sur- rounding environment, the LDCFP engineering is determined to be a primary underground space foundation pit after engineering risk assessment. According to the relevant provi- sions on the design and monitoring value of foundation pit deformation and the maximum horizontal displacement allowable value of the support structure in the current Chinese speciﬁcation [37], the importance level is determined as the ﬁrst level of the deformation control value, as shown in Table 3. Table 3. Deformation control values of LDCFPs. Monitoring Content Control Value Displacement of pile top <30 mm Surface subsidence 0.002H, <30 mm Retaining structure horizontal displacement 0.0015H, <20 mm Maximum displacement <50 mm Note: 1 H is the depth of excavation. 5.3. Deformation Monitoring Results 5.3.1. Surface Settlement Outside the Foundation Pit In this LDCFP engineering, a total of 27 surface subsidence monitoring points were laid. The surface deformation around the foundation pit is shown in Figure 22. Among them, the upward displacement (uplift) is positive, and the downward displacement (settlement) is negative. Buildings 2022, 06, x FOR PEER REVIEW 21 of 29 5.2. Control Standard of Foundation Pit Deformation Due to the large excavation volume, complicated excavation steps, and complex sur- rounding environment, the LDCFP engineering is determined to be a primary under- ground space foundation pit after engineering risk assessment. According to the relevant provisions on the design and monitoring value of foundation pit deformation and the maximum horizontal displacement allowable value of the support structure in the current Chinese specification [37], the importance level is determined as the first level of the de- formation control value, as shown in Table 3. Table 3. Deformation control values of LDCFPs. Monitoring Content Control Value Displacement of pile top <30 mm Surface subsidence 0.002H, <30 mm Retaining structure horizontal displacement 0.0015H, <20 mm Maximum displacement <50 mm Note: 1 H is the depth of excavation. 5.3. Deformation Monitoring Results 5.3.1. Surface Settlement Outside the Foundation Pit In this LDCFP engineering, a total of 27 surface subsidence monitoring points were laid. The surface deformation around the foundation pit is shown in Figure 22. Among Buildings 2022, 12, 1292 20 of 28 them, the upward displacement (uplift) is positive, and the downward displacement (set- tlement) is negative. D2 D4 D6 D8 D10 D12 D14 D16 D18 D20 D22 D24 D26 -4 -8 Stage 1 -12 Stage 2 Stage 3 Stage 4 Stage 5 -16 Stage 6 Figure Figure 22. 22. Surface Surface d deformation eformationof ofexcavation excavationin inthe the central centraldistrict. district. From Figure 22, the following can be seen: From Figure 22, the following can be seen: (1) Most of the surface deformation values of each monitoring point are negative, and (1) Most of the surface deformation values of each monitoring point are negative, and there is surface uplift at some monitoring points because the pile shaft is ﬂoating at the there is surface uplift at some monitoring points because the pile shaft is floating at the stage of the construction of the retaining pile and the column pile, resulting in individual stage of the construction of the retaining pile and the column pile, resulting in individual surface soil uplift, and the uplift is within 5 mm; surface soil uplift, and the uplift is within 5 mm; (2) In the process of foundation pit excavation, the surface deformation caused by the (2) In the process of foundation pit excavation, the surface deformation caused by the ﬁrst and second steps of excavation is small, while the surface deformation signiﬁcantly first and second steps of excavation is small, while the surface deformation significantly after the completion of the third step of excavation, and the deformation of the northern after the completion of the third step of excavation, and the deformation of the northern side of the bowknot is close to the ﬁnal deformation. Because the pit-in-pit district is far side of the bowknot is close to the final deformation. Because the pit-in-pit district is far from the northern side of the foundation pit, the excavation of the pit-in-pit has little effect from the northern side of the foundation pit, the excavation of the pit-in-pit has little effect on the northern supporting structure; on the northern supporting structure; (3) There are two valley values in the surface deformation curve after the completion (3) There are two valley values in the surface deformation curve after the completion of foundation pit excavation. The maximum deformation of measuring point D17 can reach of foundation pit excavation. The maximum deformation of measuring point D17 can 16.35 mm. It was observed that the two monitoring points are located near the midpoint reach −16.35 mm. It was observed that the two monitoring points are located near the of the “linear pit edge” of the foundation pit. The deformation of measuring points D8-D14 and D23-D27 located at the “arc pit edge” of the foundation pit has little difference in each excavation stage, and the deformation is generally small. This shows that the deformation around the foundation pit in the central district is affected by the geometric shape of the foundation pit; that is, the deformation at the midpoint of the “straight pit edge” is the maximum, and the maximum decreases gradually as the pit angle increases, while the deformation of around the “arc pit edge” is relatively uniform. 5.3.2. Deep Displacement of the Retaining Structure Deep Displacement of Bowknot District Excavation When excavating to the bottom of the bowknot district (the third layer of excavation is completed), the deep displacement of the measuring points (CX1–CX27) in the central district of the foundation pit is shown in Figure 23. The displacement of the retaining pile toward the foundation pit is positive, and the displacement to the pit is negative. Surface settlement (mm) Buildings 2022, 06, x FOR PEER REVIEW 22 of 29 midpoint of the “linear pit edge” of the foundation pit. The deformation of measuring points D8-D14 and D23-D27 located at the “arc pit edge” of the foundation pit has little difference in each excavation stage, and the deformation is generally small. This shows that the deformation around the foundation pit in the central district is affected by the geometric shape of the foundation pit; that is, the deformation at the midpoint of the “straight pit edge” is the maximum, and the maximum decreases gradually as the pit an- gle increases, while the deformation of around the “arc pit edge” is relatively uniform. 5.3.2. Deep Displacement of the Retaining Structure Deep Displacement of Bowknot District Excavation When excavating to the bottom of the bowknot district (the third layer of excavation is completed), the deep displacement of the measuring points (CX1–CX27) in the central Buildings 2022, 12, 1292 21 of 28 district of the foundation pit is shown in Figure 23. The displacement of the retaining pile toward the foundation pit is positive, and the displacement to the pit is negative. The deformation(mm) The deformation(mm) -10 -5 0 5 10 15 -10 -5 0 5 10 15 -5 -5 -10 -10 -15 -15 -20 CX15 CX1 CX16 CX2 -20 CX17 -25 CX3 CX21 CX7 CX22 CX8 -30 CX24 -25 CX9 CX26 CX10 CX27 -35 CX12 -30 Figure 23. Deep displacement of the retaining structure during bowknot district excavation. Figure 23. Deep displacement of the retaining structure during bowknot district excavation. TThe he m monitoring onitoring d data ata in inFi Figur guree 223 3 sshow how tthe he fo following: llowing: (1) Because the first layer of the foundation pit adopts slope excavation, the slope (1) Because the ﬁrst layer of the foundation pit adopts slope excavation, the slope rrange ange is isla lar rge ge, , aand nd the the ddistrict istrict isis co complex; mplex; th thus, us, so some me ret retaining aining ppiles iles hhave ave di dif ffe fer ren ent t deg degr ree ees s of of ne negative gative ddir ireection ction (t (the he d displacement isplacement aar rou ound nd the the fo foundation undation ppit) it) w within ithin the the eexcavation xcavation range of the ﬁrst layer (0–9 m depth); range of the first layer (0–9 m depth); (2) From the depth of the maximum lateral displacement position, the maximum (2) From the depth of the maximum lateral displacement position, the maximum dis- displacement position of the retaining pile is mainly concentrated in the depth range of placement position of the retaining pile is mainly concentrated in the depth range of 10– 10–15 m of the foundation pit. The foundation pit gradually enters the rock layer from 15 m of the foundation pit. The foundation pit gradually enters the rock layer from below below 16 m, and the tangential displacement of the retaining structure is signiﬁcantly 16 m, and the tangential displacement of the retaining structure is significantly weakened. weakened. This shows that the vertical space effect exists in the circular foundation pit. This shows that the vertical space effect exists in the circular foundation pit. When the When the bottom of the foundation pit is a hard rock soil layer, the maximum deformation bottom of the foundation pit is a hard rock soil layer, the maximum deformation position position of the retaining structure appears above the bottom of the foundation pit because of the retaining structure appears above the bottom of the foundation pit because the lat- the lateral constraint of the rock layer on the retaining structure is signiﬁcantly increased. eral constraint of the rock layer on the retaining structure is significantly increased. When When the bottom of the foundation pit is soft soil, the maximum deformation of the the bottom of the foundation pit is soft soil, the maximum deformation of the retaining retaining structure occurs under the bottom of the foundation pit because the support structure occurs under the bottom of the foundation pit because the support system is just system is just too small; too small; (3) From the maximum lateral displacement of the monitoring points, the monitoring (3) From the maximum lateral displacement of the monitoring points, the monitoring values in the central district of the foundation pit edge in groups I and III are signiﬁcantly values in the central district of the foundation pit edge in groups I and III are significantly greater than those at the pit foundation angle, indicating that the corner support and the greater than those at the pit foundation angle, indicating that the corner support and the foundation pit angle effect increase the corner stiffness, reﬂecting the spatial effect of the foundation pit angle effect increase the corner stiffness, reflecting the spatial effect of the foundation pit. However, the monitoring values of groups II and IV are not signiﬁcantly different, indicating that the deformation of retaining piles in the peripheral arc section is not signiﬁcantly different, and the arc section is not affected by the foundation pit space. Deep Displacement in the Pit-in-Pit District After the excavation in the pit-in-pit district is excavated to the bottom (the sixth layer of excavation is completed), the deep displacement of the measuring points on each side of the pit-in-pit district is shown in Figure 24. Among them, CX15, CX16, CX17, and CX27 are the original monitoring points for the construction of the bowknot district, and CX28–CX39 is the new deep displacement of the measuring points for the construction of the pit-in-pit district. Depth(m) Depth(m) Buildings 2022, 06, x FOR PEER REVIEW 23 of 29 foundation pit. However, the monitoring values of groups II and IV are not significantly different, indicating that the deformation of retaining piles in the peripheral arc section is not significantly different, and the arc section is not affected by the foundation pit space. Deep Displacement in the Pit-in-Pit District After the excavation in the pit-in-pit district is excavated to the bottom (the sixth layer of excavation is completed), the deep displacement of the measuring points on each side of the pit-in-pit district is shown in Figure 24. Among them, CX15, CX16, CX17, and CX27 are the original monitoring points for the construction of the bowknot district, and CX28– Buildings 2022, 12, 1292 22 of 28 CX39 is the new deep displacement of the measuring points for the construction of the pit-in-pit district. The deformation(mm) 0 4 8 12 16 -20 -24 -28 CX28 CX29 CX30 CX31 -32 (a) (b) The deformation(mm) 0 5 10 15 20 25 -20 -24 CX32 -28 CX33 CX34 CX35 CX36 CX37 CX38 -32 CX39 (c) FFigure igure 24. 24. D Deep eep d displacement isplacement of of the the r retaining etaining str suctur tructu erduring e durin pit-in-pit g pit-in-pi district t distr excavation. ict excavati(on a) .deep (a) d displacement eep displacem (CX15, ent (CX15, CX16, CCX17 X16, C and X17CX27). and CX27). (b) deep (b) d displacement eep displacem (CX28, ent (C CX29, X28, C CX30 X29, C and X30 an CX31). d C (X31). c) deep (c)displacement deep displacem (CX32, ent (C CX33, X32, C . .X33 . and , ... an CX39). d CX39). The monitoring data in Figure 24 show the following: The monitoring data in Figure 24 show the following: (1) With the downward excavation of the foundation pit, the maximum displacement of CX15, CX16, CX17, and CX27 are increased, and the position of the maximum depth is also gradually moved downward with the excavation and ﬁnally stabilized between 15 mm and 20 mm below the foundation pit, which is still above the bottom of the foundation pit; (2) There is still a spatial effect in the pit-in-pit district; that is, the deformation of the retaining pile near the foundation pit angle is smaller than that near the center of the foundation pit, and the soil at the foundation pit angle and the diagonal part of the angle has a constraint effect; (3) The deformation of the CX32-CX39 is a “waist drum shaped”, and the maximum lateral displacement of the retaining pile is concentrated in the middle, up to more than 15 mm, and the lateral displacement at the corner is relatively small; (4) CX15-CX17 and CX28-CX31 are located on the same foundation pit slope at the depths of 20–33 m (the fourth, the ﬁfth, and the sixth layers of the excavation stage), but the Depth(m) Depth(m) Buildings 2022, 06, x FOR PEER REVIEW 24 of 29 (1) With the downward excavation of the foundation pit, the maximum displacement of CX15, CX16, CX17, and CX27 are increased, and the position of the maximum depth is also gradually moved downward with the excavation and finally stabilized between 15 mm and 20 mm below the foundation pit, which is still above the bottom of the foundation pit; (2) There is still a spatial effect in the pit-in-pit district; that is, the deformation of the retaining pile near the foundation pit angle is smaller than that near the center of the foun- dation pit, and the soil at the foundation pit angle and the diagonal part of the angle has a constraint effect; (3) The deformation of the CX32-CX39 is a “waist drum shaped”, and the maximum lateral displacement of the retaining pile is concentrated in the middle, up to more than 15 mm, and the lateral displacement at the corner is relatively small; (4) CX15-CX17 and CX28-CX31 are located on the same foundation pit slope at the depths of 20–33 m (the fourth, the fifth, and the sixth layers of the excavation stage), but the deformation is obviously different, mainly reflected in the following two aspects: the deformation of the retaining pile at the CX28-CX31 measuring point is a “waist drum Buildings 2022, 12, 1292 23 of 28 shaped”; that is, the maximum deformation is mainly distributed in the middle of the retaining pile. The deformation of the retaining pile below 20 m at the CX15–CX17 meas- uring point is “parabolic shaped”, that is, the deformation decreases with the increasing deformation is obviously different, mainly reﬂected in the following two aspects: the defor- mation of the retaining pile at the CX28-CX31 measuring point is a “waist drum shaped”; excavation depth. The deformation of CX15–CX17 and CX28–CX31 measuring points at that is, the maximum deformation is mainly distributed in the middle of the retaining pile. the same burial depth is quite different, which are generally not more than 10 mm for the The deformation of the retaining pile below 20 m at the CX15–CX17 measuring point is former and nearly 20 mm for the latter. This is because the upper part of the retaining pile “parabolic shaped”, that is, the deformation decreases with the increasing excavation depth. The deformation of CX15–CX17 and CX28–CX31 measuring points at the same burial depth at the CX15–CX17 measuring point is simultaneously supported by the first, second, and is quite different, which are generally not more than 10 mm for the former and nearly third layers of the foundation pit, which plays a constraint role in the deformation of the 20 mm for the latter. This is because the upper part of the retaining pile at the CX15–CX17 lower part of the retaining pile. measuring point is simultaneously supported by the ﬁrst, second, and third layers of the foundation pit, which plays a constraint role in the deformation of the lower part of the retaining pile. The Deep Deformation in the Northern and Southern Districts The Deep Deformation in the Northern and Southern Districts When the excavation in the northern and southern districts is completed, the deep When the excavation in the northern and southern districts is completed, the deep deformation of the measuring points in the central district (CX40–CX61) is shown in Fig- deformation of the measuring points in the central district (CX40–CX61) is shown in ure 25. Figure 25. Figure 25. The deep deformation in the central district(CX40–CX61). Figure 25. The deep deformation in the central district(CX40–CX61). Figure 25 shows that the deformation of the retaining piles in the northern and south- Figure 25 shows that the deformation of the retaining piles in the northern and south- ern districts is relatively consistent, showing a “waist drum shaped”. The retaining pile is weakly affected by the spatial effect of the foundation pit. The deformation of the retaining ern districts is relatively consistent, showing a “waist drum shaped”. The retaining pile is structure in the northern–southern districts is signiﬁcantly smaller than that in the central weakly affected by the spatial effect of the foundation pit. The deformation of the retaining district, which reﬂects the uniform stress characteristics of the circular retaining structure. structure in the northern–southern districts is significantly smaller than that in the central The stress characteristics of the circular retaining structure are conducive to the control of the overall deformation of the foundation pit. 5.3.3. Support Axial Force Monitoring By taking the central district as an example, the axial force variation curve of each layer support in each construction stage was analyzed, as shown in Figure 26. The axial force of the internal support of the foundation pit is shown as the second, third and fourth internal supports when excavating to the bottom of the foundation pit. Buildings 2022, 06, x FOR PEER REVIEW 25 of 29 district, which reflects the uniform stress characteristics of the circular retaining structure. The stress characteristics of the circular retaining structure are conducive to the control of the overall deformation of the foundation pit. 5.3.3. Support Axial Force Monitoring By taking the central district as an example, the axial force variation curve of each layer support in each construction stage was analyzed, as shown in Figure 26. The axial force of the internal support of the foundation pit is shown as the second, third and fourth internal supports when excavating to the bottom of the foundation pit. The axial force is significantly greater than that of the first support, and the axial Buildings 2022, 12, 1292 24 of 28 forces of the second and third supports are relatively close. The axial force of the fourth support is the largest. The axial force of the reinforced concrete support in each channel The axial force is signiﬁcantly greater than that of the ﬁrst support, and the axial forces generally increases with the increasing excavation depth, and the growth rate gradually of the second and third supports are relatively close. The axial force of the fourth support slows with the excavation. After the excavation of the first soil layer, the axial force of the is the largest. The axial force of the reinforced concrete support in each channel generally first support basically remained stable at approximately 1500 kN; the axial force of the increases with the increasing excavation depth, and the growth rate gradually slows with the excavation. After the excavation of the ﬁrst soil layer, the axial force of the ﬁrst support second support obviously changed during the excavation of the second and third layers, basically remained stable at approximately 1500 kN; the axial force of the second support and the axial force tended to be stable during the excavation. The axial force of the third obviously changed during the excavation of the second and third layers, and the axial force support increases slowly with the excavation; the axial force of the fourth concrete support tended to be stable during the excavation. The axial force of the third support increases slowly with the excavation; the axial force of the fourth concrete support is much larger is much larger than that of the fifth and sixth steel supports, which is related to the for- than that of the ﬁfth and sixth steel supports, which is related to the formation conditions mation conditions of the two supports. of the two supports. Figure 26. The axial force of the support in the central district. Figure 26. The axial force of the support in the central district. In summary, the deformation of the surrounding soil and retaining structure does not appearIas n s anuearly mma warning ry, the in d the efwhole ormamonitoring tion of th period, e surr which ound reﬂects ing so the il a constr nd r uction etaining structure does safety of retaining piles in LDCFP. It is indicated that a reasonable support method is the not appear as an early warning in the whole monitoring period, which reflects the con- premise of the foundation pit safety. struction safety of retaining piles in LDCFP. It is indicated that a reasonable support 5.3.4. Comparison of Monitoring Results and Numerical Simulation method is the premise of the foundation pit safety. In order to verify the reliability of the foundation pit excavation design, numerical simulation and monitoring results were compared. After the foundation pit excavation 5.3.4. Comparison of Monitoring Results and Numerical Simulation was completed, the comparison between surface deformation monitoring and numerical simulation In or calculation der to v ater each ify t monitoring he reliab point ilityof of the the foundation founda pit tion p is shown it e in xca Figur vaeti27 on d . esign, numerical Only part of the retaining piles after the foundation pit excavation were selected for simulation and monitoring results were compared. After the foundation pit excavation comparative analysis of the deformation of the retaining structure, and the measuring was completed, the comparison between surface deformation monitoring and numerical points CX10, CX14, CX24, and CX40 were selected as the research objects. After the foundation simulation c pit excavation alculation was at completed, each moni thetor comparison ing point between of the the fomonitoring undation and pit is shown in Figure numerical simulation of the deformation of the retaining piles is shown in Figure 28. 27. Only part of the retaining piles after the foundation pit excavation were selected for It can be seen from Figures 27 and 28 that the monitoring results and the numerical comparative analysis of the deformation of the retaining structure, and the measuring simulation results have the same deformation trend, but there are certain differences in points CX10, CX14, CX24, and CX40 were selected as the research objects. After the foun- the numerical value, which is shown by the large ﬂuctuation of the monitoring value and the concentrated change in the numerical simulation results. The reasons are as follows: dation pit excavation was completed, the comparison between the monitoring and nu- (a) The deformation of retaining structure involves a wide range of soil, but in the numerical merical simulation of the deformation of the retaining piles is shown in Figure 28. simulation, the scope of calculation and simulation is reduced by conditional assumptions; Buildings 2022, 06, x FOR PEER REVIEW 26 of 29 It can be seen from Figures 27 and 28 that the monitoring results and the numerical Buildings 2022, 06, x FOR PEER REVIEW 26 of 29 simulation results have the same deformation trend, but there are certain differences in the numerical value, which is shown by the large fluctuation of the monitoring value and the concentrated change in the numerical simulation results. The reasons are as follows: It can be seen from Figures 27 and 28 that the monitoring results and the numerical (a) The deformation of retaining structure involves a wide range of soil, but in the numer- simulation results have the same deformation trend, but there are certain differences in ical simulation, the scope of calculation and simulation is reduced by conditional assump- the numerical value, which is shown by the large fluctuation of the monitoring value and the concentrated change in the numerical simulation results. The reasons are as follows: tions; (b) The physical and mechanical parameters of the soil are affected by various fac- Buildings 2022, 12, 1292 25 of 28 (a) The deformation of retaining structure involves a wide range of soil, but in the numer- tors such as construction conditions and have randomness. The foundation pit space of ical simulation, the scope of calculation and simulation is reduced by conditional assump- the project is large, and the stratum conditions and groundwater conditions are complex; tions; (b) The physical and mechanical parameters of the soil are affected by various fac- (c) The size of this simulation is too large, and there are many grid elements. The research tors such as construction conditions and have randomness. The foundation pit space of (b) The physical and mechanical parameters of the soil are affected by various factors such has simplified the actual soil layer and has a certain impact on the numerical calculation; the project is large, and the stratum conditions and groundwater conditions are complex; as construction conditions and have randomness. The foundation pit space of the project (d) It is difficult to accurately calculate the disturbance degree load of undisturbed soil (c) The size of this simulation is too large, and there are many grid elements. The research is large, and the stratum conditions and groundwater conditions are complex; (c) The caused by construction. has simplified the actual soil layer and has a certain impact on the numerical calculation; size of this simulation is too large, and there are many grid elements. The research has (d) It is difficult to accurately calculate the disturbance degree load of undisturbed soil simpliﬁed the actual soil layer and has a certain impact on the numerical calculation; (d) It is cadi us fe ﬁd cult by to cons accurately truction. calculate the disturbance degree load of undisturbed soil caused by construction. Monitoring results Numerical simulation -2 Monitoring results Numerical simulation -2 -4 -4 -6 -6 -8 -8 -10 -10 Monitoring point Monitoring point Figure 27. Comparison of surface settlement. Figure Figure 27. 27. Comparison Comparison of of s surface urface s settlement. ettlement. TT he he d ede for form matio atio n(m n(mm m) ) -2 0 2 4 6 8 10 12 14 16 -2 0 2 4 6 8 10 12 14 16 -5 -5 -10 -10 -15 -15 CX10(M) -20 CX13(M) CX24(M) CX10(M) -20 CX40(M) CX13(M) -25 CX10(N) CX24(M) CX13(N) CX40(M) CX24(N) -25 CX10(N) CX40(N) -30 CX13(N) CX24(N) Figure 28. Comparison of the deformation for the retaining structure. CX40(N) -30 6. Conclusions and Discussion Figure Figure 28. 28. Comparison Comparison of of the the deformation deformafor tion the fo rr etaining the reta str in uctur ing s e.tructure. 6. Conclusions and Discussion 6. Conclusions and Discussion Based on the LDCFP engineering of a subway station in Wuhan, China, the paper analyzes the characteristics of various partitioned excavation methods. step-by-step and synchronous excavation. The surface deformation around the LDCFP and the deformation of the retaining structure are analyzed by numerical simulation and ﬁeld monitoring, and the excavation deformation characteristics of the circular foundation pit are revealed. The main conclusions are as follows: SurSu facrefa sc ettle e se me ttle nt( m mm) ent(mm) Depth(m） Depth(m ） D8 D8 D9 D9 D10 D10 D11 D11 D12 D12 D13 D14 D13 D23 D14 D24 D23 D25 D24 D26 D25 D27 D26 D40 D27 D41 D40 D42 D43 D41 D44 D42 D45 D43 D46 D44 D47 D45 D48 D46 D49 D47 D50 D51 D48 D52 D49 D53 D50 D54 D51 D55 D52 D56 D53 D57 D54 D58 D55 D59 D60 D56 D61 D57 D58 D59 D60 D61 Buildings 2022, 12, 1292 26 of 28 (1) When the LDCFP is excavated step by step, the symmetrical excavation method should be preferred, and the smaller district should be excavated ﬁrst; in the synchronous excavation of the foundation pit partition, under the premise of fully considering the engineering economy and the feasibility of construction organization, the construction method of synchronous excavation of the same layer is more conducive to controlling the overall deformation of the foundation pit; (2) Under different excavation processes, the deformation characterized by lateral displacement around the foundation pit, surface deformation, soil uplift at the bottom, the deformation of the retaining piles in the periphery, and the pit-in-pit district is consistent. The deformation difference caused by the process is mainly reﬂected in the deformation of the shared retaining pile, and the ﬁnal deformation of the pile is tilted toward the excavation district of the foundation pit. When both sides of the pile are unloaded, the deformation of the retaining pile rebounds in a linear shape; (3) During the excavation of the foundation pit, the axial force of the support in each layer gradually increases with the excavation. The axial force of the support increases rapidly in the early stage and slows down in the late stage. Under the same constraint conditions, the axial force of the bottom support is greater than that of the upper support; (4) Through the analysis of monitoring data and numerical simulation results, it was found that the deformation of the circular retaining structure is small and relatively uniform, indicating that the “vault effect” of the circular structure has a certain constraint on the deformation of the foundation pit and the retaining structure; (5) The circular foundation pit has special spatial stress characteristics; it can give full play to the characteristics of strong compressive capacity under the action of axial load. It has the advantages of large stiffness, small wall deformation, and convenient mechanized operation of LDCFP. As the trafﬁc environment and geological environment around each LDCFP engineering are different, speciﬁc analysis is required. Author Contributions: Conceptualization, H.S., Z.J. and M.B.; methodology, H.S. and Z.C.; software, H.S.; validation, T.W., Z.C. and K.Y.; formal analysis, D.Z. and K.Y.; investigation, Z.C.; data curation, H.S. and T.W.; writing—original draft preparation, H.S. and M.B.; writing—review and editing, Z.J. and T.W.; visualization, D.Z. and Z.C.; supervision, K.Y.; project administration, H.S. and Z.C.; funding acquisition, H.S., T.W. and Z.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Fundamental Research Funds for the Central Uni- versities (No.2021RC208), the National Key Research and Development Program of China (No. 2018YFC0808701), the Shanghai Science and Technology Committee (No. 20DZ1202100, 20DZ2251900), the Tianjin Transportation Technology Development Plan Project (No. 2019B-25), and the central government guides local science and technology development fund projects (No. 216Z3802G). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conﬂicts of Interest: We declared that we have no conﬂict of interest in this work. We declare that we do not have any commercial or associative interest that represents a conﬂict of interest in connection with the work submitted. References 1. Boone, S.J. Analysis of wall and ground movements due to deep excavations in soft soil based on a new worldwide database. Soils Found. 2005, 45, 165–166. [CrossRef] 2. Greeman, A. Station stops. Tunn. Tunn. Int. 2012, 7, 52–56. 3. Zhou, N.; Vermeer, P.A.; Lou, R.; Tang, Y.; Jiang, S. Numerical simulation of deep foundation pit dewatering and optimization of controlling land subsidence. Eng. Geol. 2010, 114, 251–260. [CrossRef] 4. Hsieh, H.S.; Huang, Y.H.; Hsu, W.T.; Ge, L. On the system stiffness of deep excavation in soft clay. J. GeoEngineering 2017, 12, 21–34. 5. Tan, Y.; Wang, D.L. Characteristics of a large-scale deep foundation pit excavated by the central-island technique in Shanghai soft clay. II: Top-down construction of the peripheral rectangular pit. J. Geotech. Geoenviron. Eng. 2013, 139, 1894–1910. [CrossRef] Buildings 2022, 12, 1292 27 of 28 6. Sharma, J.s.; Hefny, A.M.; Zhao, J.; Chan, C. Effect of large excavation on deformation of adjacent MRT tunnels. Tunn. Undergr. Space Technol. 2001, 16, 93–98. [CrossRef] 7. Zhou, F.; Zhou, P.; Li, J.; Lin, J.; Ge, T.; Deng, S.; Ren, R.; Wang, Z. Deformation characteristics and failure evolution process of the existing metro station under unilateral deep excavation. Eng. Fail. Anal. 2021, 131, 105870. [CrossRef] 8. Finno, R.J.; Blackburn, J.T.; Roboski, J.F. Three-dimensional effects for supported excavations in clay. J. Geotech. Geoenviron. Eng. 2007, 133, 30–36. [CrossRef] 9. Ding, Z.; Jin, J.; Han, T.C. Analysis of the zoning excavation monitoring data of a narrow and deep foundation pit in a soft soil area. 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Lim, A.; Ou, C.Y. Stress paths in deep excavations under undrained conditions and its inﬂuence on deformation analysis. Tunn. Undergr. Space Technol. 2017, 63, 118–132. [CrossRef] 16. Wu, J.; Peng, L.; Li, J.; Zhou, X.; Zhong, J.; Wang, C.; Sun, J. Rapid safety monitoring and analysis of foundation pit construction using unmanned aerial vehicle images. Autom. Constr. 2021, 128, 103706. [CrossRef] 17. Tan, Y.; Lu, Y. Responses of shallowly buried pipelines to adjacent deep excavations in Shanghai soft ground. J. Pipeline Syst. Eng. Pract. 2018, 9, 05018002. [CrossRef] 18. Jamsawang, P.; Voottipruex, P.; Tanseng, P.; Jongpradist, P.; Bergado, D.T. Effectiveness of deep cement mixing walls with top-down construction for deep excavations in soft clay: Case study and 3D simulation. Acta Geotech. 2018, 14, 225–246. [CrossRef] 19. Rouainia, M.; Elia, G.; Panayides, S.; Scott, P. Nonlinear ﬁnite-element prediction of the performance of a deep excavation in Boston Blue Clay. J. Geotech. 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Deformation Characteristics and Optimization Design for Large-Scale Deep and Circular Foundation Pit Partitioned Excavation in a Complex Environment

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ISSN: 2075-5309
DOI: 10.3390/buildings12091292
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Buildings – Multidisciplinary Digital Publishing Institute

Published: Aug 23, 2022

Keywords: LDCFPs; deformation characteristics; partitioned excavation; control measures; optimization design

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Deformation Characteristics and Optimization Design for Large-Scale Deep and Circular Foundation Pit Partitioned Excavation in a Complex Environment

Deformation Characteristics and Optimization Design for Large-Scale Deep and Circular Foundation Pit Partitioned Excavation in a Complex Environment

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Deformation Characteristics and Optimization Design for Large-Scale Deep and Circular Foundation Pit Partitioned Excavation in a Complex Environment

Deformation Characteristics and Optimization Design for Large-Scale Deep and Circular Foundation Pit Partitioned Excavation in a Complex Environment

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