Abstract: The stability of the retaining structure system of deep foundation pits for shield shafts is crucial to ensuring the safety of shield construction. Taking a deep foundation pit project of a shield shaft on Beijing Metro Line 10 as an example, numerical simulation is used to analyze the interaction mechanism between support axial force and pile horizontal displacement, and the results are compared with field monitoring data. The results show that the axial forces of steel bracing in the foundation pit all reach their maximum values shortly after installation, then decrease and gradually stabilize, with the maximum occurring at corner braces. Steel bracing has a certain limiting effect on the development of pile horizontal displacement, and the position of maximum horizontal displacement gradually moves downward with excavation.
Keywords: deep foundation pit; retaining structure; field monitoring; numerical simulation
0 Introduction
The rapid development of underground engineering has made the construction environment of urban deep foundation pits increasingly harsh, causing difficulties in design and construction. In addition, due to the complexity of geological conditions, construction methods, load levels and other factors, theoretical analysis becomes difficult. Therefore, field monitoring and numerical simulation of deep foundation pit support structure systems are of great significance for safe construction. Using the elastic foundation beam method and finite difference method, the deformation and internal forces of pile-anchor support structures during deep foundation pit excavation were calculated and compared with measured data; the results show that there are differences between theoretical and measured values, and it is difficult to establish a theoretical model. Through field tests, the changes in soil mechanical properties at different excavation stages were studied. The results show that prestressed anchor cable support structures can effectively control deep foundation pit deformation and surface settlement, thereby ensuring the safety of the deep foundation pit itself. At present, most analyses are based on laboratory tests or field tests of a single indicator, and the interaction mechanism among various indicators of the support structure system has not been comprehensively studied.
This paper takes a deep foundation pit project of a shield shaft on Beijing Metro as the background, uses FLAC3D for numerical simulation, analyzes the interaction mechanism between support internal forces and pile horizontal displacement, and compares with field monitoring results. The research results can guide engineering construction and ensure the safety and stability of the deep foundation pit project of the shield shaft.
1 Project Overview
The shield shaft foundation pit of the Bagou section of Phase 2 of Beijing Metro Line 10 is 21.00 m long, 18.00 m wide, rectangular in shape, with an excavation depth of 18.66 m. The retaining structure of the shield shaft adopts bored cast-in-place piles with steel bracing, with a pile length of 21.50 m and a total of 3 levels of steel bracing. The first level of steel bracing is set at the crown beam, and the second and third levels are supported on the bored piles through horizontal steel walings; the spacing between the first and second levels is 9.05 m; the spacing between the second and third levels is 5.30 m. The strata from top to bottom are roughly divided into: artificial fill layer, silty soil layer, silty fine sand layer, pebble gravel layer, fine-medium sand layer and pebble layer. The geotechnical engineering characteristics of each soil layer are described in Table 1. Geological survey found that the bottom of the pebble gravel layer contains phreatic water with a small amount, and no groundwater was found in other strata.
2 Calculation Model and Result Analysis
2.1 Establishment of Geometric Model
The numerical simulation uses FLAC3D finite difference software. According to the geometric dimensions of the shield shaft and the influence range of deep foundation pit excavation, the length, width and depth of the model are 105.00 m × 90.00 m × 58.75 m, respectively. The model dimensions consider the influence of boundary conditions on calculation results. The geometric element mesh is shown in Figure 1.
2.2 Boundary Conditions
During the simulation, the boundary constraint conditions of the model are set as follows: the side and bottom of the model are displacement boundaries; the displacement boundary condition of the model side is to constrain horizontal movement; the displacement boundary at the bottom of the model is a fixed boundary, constraining both horizontal and vertical movement. The upper boundary of the model is the ground surface, which is a free boundary.
2.3 Model Selection and Structural Elements
Combined with the soil characteristics of the Beijing area, the Mohr-Coulomb yield criterion is used to describe the failure of rock and soil in the model. The built-in "pile" element of the software is used to simulate concrete bored piles. The diameter of concrete bored piles is 0.80 m, the pile spacing is 1.40 m, the elastic modulus is 30 GPa, and the Poisson's ratio is 0.2. Considering that it is mainly under compression, the "beam" element is used to simulate steel bracing, with a diameter of 0.63 m, a thickness of 12 mm, an elastic modulus of 250 GPa, and a Poisson's ratio of 0.3. The "shell" element is used to simulate the effect of shotcrete, with an elastic modulus of 14.2 GPa, a Poisson's ratio of 0.20, and a thickness of 0.10 m. In order to consider the interaction between the pile structure and the soil, an "interface" contact element is set between them.
2.4 Analysis of Calculation Results
Based on the actual engineering situation, the horizontal displacement diagrams of piles and the axial force diagrams of steel bracing at different excavation depths were obtained.
2.4.1 Horizontal Displacement of Piles
After the foundation pit was excavated to -9.50 m and the second level of steel bracing was installed, the position of maximum horizontal displacement of the soil behind the pile moved downward, occurring between the first and second levels of bracing. The maximum horizontal displacement of the soil behind the pile is about 5.60 mm, which is different from a cantilever pile; the maximum displacement of a cantilever pile structure occurs at the top, but due to the lateral confinement of the steel bracing, the position of maximum horizontal displacement moves downward, as shown in Figure 2a. After the foundation pit was excavated to the design depth, the maximum horizontal displacement of the soil behind the pile reached 9.50 mm, located between the second and third levels of steel bracing, as shown in Figure 2b. 2.4.2 Axial Force of Steel Bracing
Figures 3a to 3d show the calculation results of axial forces when 4 levels of steel bracing are installed. The calculation results show that after the first level of steel bracing is installed, the maximum axial force of this level occurs at the corner brace, with a maximum value of 131.4 kN/m. After the second level of steel bracing is installed, as the excavation proceeds, the internal force of the second level reaches its maximum value of 205.8 kN/m, still at the corner brace; the internal force of the first level decreases rapidly after the second level is installed and is much smaller than the design value of 205 kN/m. With the excavation and the installation of the third level of steel bracing, the three levels share the soil pressure; the axial force of the first level continues to decrease, while the increasing trend of the second level gradually slows down. As the excavation approaches the design depth, the axial force of the third level also reaches its maximum value of 135.3 kN/m. After the simulation of excavation and base slab casting is completed, the axial forces of the first, second and third levels all decrease. During excavation, the calculated axial force of corner braces is significantly greater than that of straight braces; the axial forces of all braces reach their maximum shortly after installation, then decrease and gradually stabilize.
2.4.3 Comparative Analysis
The calculation results show that with the installation of each level of bracing, the deformation rate significantly decreases, and the position of maximum displacement is always away from the bracing.
3 Field Monitoring
3.1 Monitoring Plan
The monitoring plan is formulated to reflect the deformation of the entire foundation pit during construction, so as to achieve mutual verification of various test results and action relationships. The specific layout of monitoring points is shown in Figure 4.
3.2 Monitoring Results of Pile Horizontal Displacement
Figure 5 shows the horizontal displacement curves of the pile at key construction stages. It can be seen from Figure 5 that during excavation, the position of maximum horizontal displacement gradually moves downward; after the roof slab is completed, the maximum horizontal displacement of the pile is 10.10 mm, located at -8 m from the ground surface. When the first level of steel bracing was installed, the horizontal displacement was relatively small due to the shallow excavation depth. After the second level was installed, the position of maximum horizontal displacement gradually moved downward between the first and second levels, which is basically consistent with the numerical calculation results. After the third level was installed, the horizontal displacement continued to move downward, but did not move between the second and third levels, which is somewhat different from the numerical results. The main reason is that the installation of steel bracing in construction was slightly later than planned, so the pile horizontal displacement had already developed before the bracing was completed; at that time, the pile horizontal displacement caused by excavation was basically completed, so after the bracing was installed, the pile horizontal displacement changed little, resulting in no obvious downward movement of the maximum displacement position. This difference also indicates that when the steel bracing reaches the design load, it can significantly limit the pile horizontal displacement, and timely installation of steel bracing is crucial to the overall stability of the foundation pit. The comparison between numerical simulation and field measured data shows that the two results are basically consistent, indicating that the actual variation of pile horizontal displacement conforms to the theoretical research laws. Based on the comprehensive analysis of the above calculation results and field monitoring values, during excavation, the interaction between steel bracing and pile horizontal displacement can be roughly described in several aspects: (1) Before the first level of steel bracing is installed, the foundation pit is exposed, and the retaining pile exhibits a forward-leaning curve deforming into the pit, with the maximum horizontal displacement at the pile top; after the steel bracing is applied, the horizontal displacement curve of the retaining pile changes from forward-leaning to bow-shaped. (2) After the second level of steel bracing is installed, the soil pressure behind the pile gradually transfers from being borne solely by the pile to being shared by the pile and the steel bracing. At this time, the axial force of the first level decreases somewhat, and due to the lateral confinement of the second level, the position of maximum horizontal displacement of the pile moves downward between the first and second levels, as shown in Figure 3b. (3) The installation of the third level increases the proportion of soil pressure borne by the steel bracing. Throughout the excavation, the axial force of the second level is always the largest, so the second level should be the focus of monitoring. The interaction between bracing and pile horizontal displacement indicates that the timing of steel bracing installation and the magnitude of applied axial force have a great influence on pile horizontal displacement and are crucial to the overall stability of the foundation pit.
4 Conclusions
(1) The axial forces of steel bracing in the foundation pit all reach their maximum values shortly after installation, then decrease and gradually stabilize, with the maximum occurring at corner braces. Throughout the excavation, the axial force of the second level is always the largest.
(2) Steel bracing has a certain limiting effect on the development of pile horizontal displacement, and the position of maximum horizontal displacement gradually moves downward with excavation. The timing of steel bracing installation and the magnitude of applied axial force have a great influence on pile horizontal displacement.
(3) The numerical simulation results are basically consistent with the field measured data, indicating that the variation of pile displacement conforms to the theoretical research laws, which has certain reference significance for the design and construction of retaining structures for deep foundation pits of shield shafts.