Experimental Study for the Embedded Depth of Support Structure Foundation Pit in Granite Residual Soil Area

Abstract

In order to deeply understand the appropriate embedded depth of the foundation pit diaphragm wall in granite residual soil area, a physical model of the diaphragm wall with inner support for foundation excavation was constructed according to the actual project in the proportion of 1 : 30. The distribution of Earth pressure, the horizontal displacement of the wall, and the settlement behind the wall were obtained by physical experiments. The numerical simulation was then performed to authenticate the results from physical modeling. It was observed that the embedded depth of the diaphragm wall had the most obvious influence on the horizontal displacement of the wall. Moreover, the final soil settlement and its influence were significantly increased with the decrease in embedded depth. The analysis results also suggested that the control value for the embedded depth of the wall should not be less than 0.36 H (H is the excavation depth of the foundation pit).

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1. Introduction

More excavation support problems related to underground space engineering are highlighted with the increasing contradiction between the rapid development of cities and the lack of land resources in China. The granite residual soil is widely distributed in the south of China, and the diaphragm wall has been used in this area due to its high efficiency and safety under the condition that the foundation pit is developing superlarge and ultradeep. The diaphragm wall plays a critical role in the support structure which accounts for 70%–80% of the total support structure costs [1]. Therefore, appropriate control of the embedded depth of the diaphragm wall is of great significance for engineering construction.

In the past 40 years, predecessors carried out further studies about the foundation pit in the granite residual soil area. The development of foundation pit support technology in Shenzhen can be divided into four stages: unconscious application, initial application of various technologies, soil-nail wall era, and rational application of various technologies [2]. After approximately 40 years of development, the foundation pit project in Shenzhen has moved toward the fifth stage, which focuses on deformation control [3]. Nowadays, the diaphragm wall has been widely used in Shenzhen as a kind of excavation engineering technology due to its safety, quality, impermeability, and minimized environmental impact [4, 5]. Wang and Liu [6] collected the deformation data of 13 granite residual soil foundation pits in the south of China, combined with the existing statistical results of foundation pit deformation in other areas of China, and made a comparative analysis of the deformation characteristics of deep foundation pits in this area. Bai et al. [7] took a foundation pit in Guangzhou as an example to study the effect of embedded depth on the deformation of the diaphragm wall and found that the horizontal displacement of the diaphragm wall gradually decreases as the embedded depth increases. At present, it can be widely recognized that if the embedded depth is shallower, the horizontal displacement of the diaphragm wall is too large, and the foundation pit can be easily destabilized; if the embedded depth is very large, it is very likely to construct the embedded section into the rock stratum with low weathering, which will cause great difficulties for the construction, and the engineering cost will significantly increase. At this time, the effect of increasing the embedded depth on deformation control of the foundation pit is not obvious.

In order to deeply understand the influence of the diaphragm wall embedded depth of the foundation pit in the granite residual soil area, this study made a physical test model according to a similar theory with two similar soil materials: granite residual soil and fully weathered granite. The whole process of the deep foundation pit excavation with the inner support system was simulated, and numerical simulation was used to compare the changes in the physical model experiment.

2. Scale Similarity and Physical Model

2.1. Scale Similarity

The main requirements for the similarity of the physical model experiment are as follows: the boundary conditions of the model, geometry, density, strength, and stress changes of similar materials should follow certain similar laws. The ratio of physical quantities with the same dimensions between the prototype and the model is called a similar scale and is represented by C. The geometric similarity coefficient generally determines first when conducting a similar model experiment. The geometric similarity coefficient is C_i = n, and the density similarity coefficient is C_y = 1. According to the dimensional analysis method, it should be known that the physical quantities of the same dimension are similar to scale C as follows:$$\begin{array}{c}\text{(1)}& C_y=C_{\phi }=C_{\mu }=C_g=1,C_o=C_l=C_E=C_{\sigma }=n,\end{array}$$where $C_y$ is the similarity scale of density, $C_{\phi }$ is the similarity scale of internal friction angle, $C_{\alpha }$ is the similar scale of Poisson’s ratio, $C_g$ is the similarity scale of strain, $C_o$ is the similarity scale of cohesion, $C_E$ is the similarity scale of elastic modulus, and $C_{\sigma }$ is the similar scale of stress.

2.2. Engineering Prototype

2.2.1. Project Overview

The proposed site is located at No.1, Tairan 7th Road, Chigongmiao Industrial Zone, Futian District, Shenzhen City. It is adjacent to Tairan 8th Road on the north side and Binhe Avenue on the south side. Shenzhen Metro Line 9 passes near the site and the nearest station is about 41 m from the study site. The west side is Tairan 9th Road, and the east side is another construction project. The overall terrain of the proposed site is relatively flat, with a construction land area of 5,775.05 m². The site is planned for the business center, with 4 basement floors. The foundation pit site and research scope are shown in Figures 1 and 2, respectively.

[figure omitted; refer to PDF]

The physical experiments and numerical simulation of the soil settlement are shown in Figure 10, both of which are “groove-shaped,” with the maximum settlement values of 7.7 mm and 6.6 mm, respectively, and the maximum values appeared at 0.15 H and 0.40 H from the pit edge. From the abscissa at 22 m in Figure 10, it could be found that the settlement value of the numerical simulation was larger at the edge of the pit. This was caused by a certain time effect in the physical experiment process, but it did not affect the deformation law of the settlement curve.

[figure omitted; refer to PDF]

It can be seen from Figures 11 and 12 that the Earth pressure of the physical experiments is greater than the numerical simulation. As the physical experiment soil sample was reshaped, it caused undergoing a series of steps as drying, stirring, and compaction during the experiments. Then, the structure of the undisturbed soil had been lost, resulting in a certain difference between the results of physical experiments and numerical simulations. In general, it can be seen from Figures 9–12 that the results and changing trends of the physical model experiments and the numerical simulation are basically the same, which can represent the accuracy of the physical model experiments.

[figure omitted; refer to PDF][figure omitted; refer to PDF]

6.2. Comparison between Physical Experiment and Monitoring Results

A comprehensive dynamic monitoring of the foundation pit supporting structure and the surrounding environment was carried out during the construction of the foundation pit, including the lateral displacement (XC) of the diaphragm wall and the soil settlement (W) behind the wall as shown in Figure 2. The layout of the measuring points is also shown in Figure 2. The results of the physical experiments were enlarged by 30 times and compared with the monitoring results. The results of the settlement and displacement are very close as shown in Figure 13. The two monitoring points of XC1 and W2 were 10.2 mm and 7.4 mm, and the difference from the physical experiment result was 2.3 mm and 1.6 mm, respectively (accounting for 22.5% and 21.6% of the actual measured value, respectively). The difference between the monitoring and the experiment results is not very large, which shows the accuracy of the physical model experiment.

[figure omitted; refer to PDF]

6.3. Minimum Embedded Depth

The wall displacement, soil settlement, and Earth pressure were obtained through the excavation physical experiments of foundation pit with different embedded depths of diaphragm walls (L/H = 0.36, L/H = 0.3).

6.3.1. Wall Horizontal Displacement

It can be seen in Figures 14 and 15 that the lateral displacement of the diaphragm wall is as “large belly”. The horizontal displacement of the diaphragm wall increases continuously with the increase of excavation depth; especially after stage 4, the increase of displacement is particularly obvious. The smaller the embedded depth is, the greater the final horizontal displacement of the wall with the change in embedded depth is. The maximum wall displacement value reaches 0.14% H, which is very close to the average value of the maximum horizontal displacement of the foundation pit wall in the granite residual soil area, as 0.13% H [6]. It was found from experiments that the displacement at the top of the wall is generally small, which shows that the crown beam plays a significant role.

[figure omitted; refer to PDF][figure omitted; refer to PDF]

6.3.2. Soil Settlement Behind the Walls

It can be seen in Figures 16 and 17 that with the increase in distance from the diaphragm wall, the soil settlement will first increase and then decrease significantly, forming a groove-shape. The maximum soil settlement appears at about 0.23 H from the pit. The maximum soil settlement gradually increases with the continuous decrease in embedded depth, and the maximum settlement value under the two embedded depths was 0.25 mm, δ_vm/H = 0.04%, and 0.45 mm, δ_vm/H = 0.07%, respectively. The influence range of the settlement was also expanded significantly with the decrease in embedded depth, and hence the influence range of the last settlement exceeded the monitoring range of 70 cm.

[figure omitted; refer to PDF][figure omitted; refer to PDF][figure omitted; refer to PDF]

6.3.3. Earth Pressure

The interior of the foundation pit belongs to the passive Earth pressure area, while the exterior of the foundation pit belongs to the active Earth pressure area according to the displacement law of the diaphragm wall. The Earth pressure in front of the wall decreased continuously with the increasing depth of excavation. The excavation depth is shallow, so the Earth pressure does not change significantly during the first stage; when the excavation depth was deepened, the Earth pressure significantly changed. The deformation of the wall to the interior of the foundation pit gradually increased with the increase in excavation depth, resulting in a decrease in the Earth pressure behind the wall (Figures 18–21).

[figure omitted; refer to PDF][figure omitted; refer to PDF][figure omitted; refer to PDF]

7. Conclusions

(1) The lateral displacement of the wall during the whole process of foundation pit excavation shows the “big belly” shape: the middle part of the wall has the largest displacement, while the lower and upper parts have small displacement. The horizontal displacement of the wall increased, and the position of the maximum displacement value moves down with the increase of the excavation depth. By decreasing the embedded depth, the greater final horizontal displacement of the wall occurred with the maximum value of 0.9 mm, 0.14 H%.

(2) The soil settlement behind the diaphragm wall was gradually increased with the progress of the excavation. The maximum settlement position appears approximately 0.23 H, with a maximum value of 0.45 mm, δ_vm/H = 0.07%. The final settlement increased significantly with the decrease in embedded depth, and the influence range of settlement also increased significantly, but it did not change the groove-shaped rule of soil settlement.

(3) The passive Earth pressure in front of the wall increases linearly with the depth. The passive Earth pressure in the bottom soil of the pit decreases with the increase of the excavation depth. The excavation depth of stage 1 is relatively shallow, and the decrease of the Earth pressure is not obvious. The Earth pressure decreased obviously with the increase in excavation depth under the latest stages, because the displacement of the wall was very small during the experiment, the Earth pressure in front of and behind the wall is distributed in a triangle, which was more in line with the distribution law of the static Earth pressure.

(4) According to the analysis results of horizontal displacement and settlement, the wall displacement reached the critical value of 30 mm in the “Technical Code for retaining and protection of excavation in Shenzhen city” at the embedded depth of 0.36 H [14]. Therefore, it is recommended that the embedded depth should not be less than 24 cm (0.36 H) for the diaphragm wall with inner supports structure.

Acknowledgments

This research was funded by the National Natural Science Foundation of China (nos. 41572257 and 41972267).

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