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

In order to deeply understand the appropriate embedded depth of the foundation pit diaphragmwall 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. +e distribution of Earth pressure, the horizontal displacement of the wall, and the settlement behind the wall were obtained by physical experiments. +e 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. +e analysis results also suggested that the control value for the embedded depth of the wall should not be less than 0.36H (H is the excavation depth of the foundation pit).


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. e 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. e diaphragm wall plays a critical role in the support structure which accounts for 70%-80% of the total support structure costs [1]. erefore, 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.
e 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. e 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.

Scale Similarity.
e 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. e ratio of physical quantities with the same dimensions between the prototype and the model is called a similar scale and is represented by C. e geometric similarity coefficient generally determines first when conducting a similar model experiment.
e 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: where C y is the similarity scale of density, C φ is the similarity scale of internal friction angle, C α 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 σ is the similar scale of stress.  e thickness of the reinforced protective layer of the diaphragm wall is 50 mm. e thickness of the reinforced protective layer in inner support is 35 mm. e profile of the foundation pit is shown in Figure 3.

Physical Model.
e foundation pit support scheme is composed of a diaphragm wall with an inner support structure. During the physical modeling experiments, considering the symmetrical shape of the foundation pit and the supporting structure in the foundation pit, a quarter of the foundation pit is assessed by the physical simulation experiment.
e area under study for this experiment is shown in Figure 2.

Test Scheme.
Previous research shows that the influence of dry density and moisture content of the material on its properties was rarely considered when preparing similar materials [8][9][10]. erefore, on the basis of previous research findings, this study has taken quartz sand and bentonite as raw materials, weight ratio of sand and bentonite, dry density, and moisture content as three influencing factors, set up 4 levels, and designed a total of 16 groups of orthogonal tests to conduct the preparation research of similar materials. According to the orthogonal test results, a large number of tests were carried out under the conditions of changing parameters, and finally, two similar materials of simulated granite residual soil and fully weathered granite used in the physical experiments were obtained. In addition, the soil layer above the gravelly clayey soil was simulated by fully weathered granite residual soil. e particle size range of quartz sand was 1 mm to 2 mm. e deformation modulus E 0 is as follows: Monitoring point:

Study area
Internal support  It comprises the tawny clay and brown maroon sand with the coarse side and hardpan strata. And, it is slightly wet, mostly slightly dense state, locally loose state, or moderately dense.
Holocene (Q 4 mc ) 3.0 It mainly comprises the cinereous clay with a small amount of organic matter. e cut surface is smooth with a slight gloss. And, it contains the gravel with lens shape once in a while.
e primary color is tawny, and the brown maroon color, greyish color, etc., are partly distributed on it. And, it mainly comprises sandy clay, clayey sand gravel, and clayey sand.
e color is greyish and brown. e weathered host rock is mainly medium-coarsegrained granite, the structure of the host rock is clearly visible, gravel sand accounts for about 25%, and the composition is quartz.
e primary color is yellowish-brown and reddish-brown. e rock is weathered violently, and the organizational structure has been basically destroyed, but it is still recognizable. e core is in the shape of a hard soil column, which is easy to disintegrate when exposed to water. Yanshanian (λβ 5 K 1 ) 14.4 e primary color is brown-yellow, gray-brown, the rock is weathered strongly, the organizational structure has been partially destroyed, and it disintegrates in water Advances in Civil Engineering 3 e Earth pressure during the experiment would not exceed 50 kPa. e values of p 2 and p 1 were 25 kPa and 50 kPa, respectively, the variable modulus of Earth pressure within 50 kPa was obtained, which was more in line with the experimental situation. e proportions of similar materials used in the study are shown in Table 2. e physical-mechanical parameters of the prototype and model material are shown in Table 3.
According to the scale of the foundation pit and taking the excavation depth H as the reference, the length, height, and width of the model were taken as 4 H, 2 H, and 2 H, and the similarity coefficient as n � 30. e size of the model box was determined as 2.5 m in length, 1.5 m in width, and 1.5 m in height (Figure 4). e diaphragm wall was simulated by the PPR board in the physical model. According to the similarity ratio, the thickness of the board was 33 mm and the length was 98 cm. e inner support and waist beam were simulated by 33 mm and 40 mm thick steel, respectively, and the anchorage to the diaphragm wall was fixed by self-drilling nails. e column was simulated by iron rods with diameters of 3 cm and 2 cm and connected with the inner support by iron wire tying.

Data Collection.
During the experiment, a joint monitoring method of stress and displacement was adopted. e stress monitoring was used for the Earth pressure cell to test the Earth pressure of the diaphragm wall under all working stages. e equipment was a 20-channel 100 Hz dynamicstatic strain apparatus and the Earth pressure cell. Displacement monitoring was mainly performed to monitor the displacement of the diaphragm wall and vertical deformation of the surrounding soil in real time. e equipment adopts a dial indicator and three-dimensional (3D) deformation monitor (Figure 4). e length of the 3D deformation monitor was kept as 100 cm, in which the displacement of one point can be monitored after every ickness (m) 6  25 cm. In this experiment, two 3D deformation monitors were installed staggered on the diaphragm wall. e diaphragm wall was 98 cm long, so that the displacement of the wall was monitored after every 12.5 cm and the displacement data of 8 points were monitored at the same time. e above sensors were arranged in the center of the diaphragm wall. In Figure 5, "A" represents the position sensor, "B" represents the monitoring point of the Earth pressure cell, and "H" represents the length of the diaphragm wall. e layout of each sensor is shown in Figure 6.

Model
Installation. e first step in the experimental procedure was to install the displacement sensors and Earth pressure cell. One Earth pressure cell was arranged on the lower side of each row of waist beams, and three Earth pressure cells were densely arranged in the embedded section of the diaphragm wall. e second step was to embed the diaphragm wall and the columns. e third step was to install the dial indicator on the surface of the soil behind the diaphragm wall after the soil compaction was completed, which was used to determine the vertical deformation of the soil settlement during the excavation of the foundation pit.

Physical Experiment Stages.
e excavation process of the foundation pit was completely in accordance with the actual foundation pit excavation.  Figure 7 In the aforementioned excavation physical process, the soil settlement, Earth pressure and diaphragm displacement were recorded in real time. So far, the whole process of the physical excavation experiments for the embedded length of

Model Parameter Selection.
e study took a quarter of the foundation pit for 3D modeling calculation analysis same as the physical model experiment. Considering the influence of wall displacement and ding to the analysis results of horizontal displacement and settlement, the settlement, the height of the modeled foundation pit was taken as the influence range of the surrounding soil at twice the excavation depth (H � 19.3 m). e overall size was 78.8 m (length) × 72.8 m (width) × 40 m (height), as shown in Figure 8.
e modified Coulomb constitutive model was adopted for soil mass. e traditional Mohr-Coulomb model was an elastoplastic model, which could not consider the influence of soil unloading modulus changes. e final simulated value had a great difference from the  e modified Mohr-Coulomb constitutive model could be regarded as an improved version of the former. e latter had the same shear yield surface as the former, but it used rounded corners to make the model more convergent. e compression yield surface was elliptical, and its shear yield surface and compression yield surface did not affect each other. e model could consider the change of elastic modulus of soil with the change of stress, so it was more practical than the Mohr-Coulomb model. e soil parameters of the model and the supporting structure parameters are shown in Table 4 and 5, respectively.
According to the excavation and support process of the foundation pit, the simulation calculation process was divided into the following 4 stages: Preparation stage: it consists of the generation of the initial stress field, activation of all soil layers, activation of deadweight and boundary displacement constraint conditions, reset of the displacement, and the derivation of diaphragm wall and column

Comparison between the Physical Modeling and Numerical Simulations.
e results of the physical experiments were expanded 30 times in proportion, and compared with the results of the numerical simulation, the comparison diagram of wall displacement, soil settlement, and Earth pressure were obtained successively. Among e trends of numerical simulation and physical experiments are basically the same as depicted in Figure 9. In stage 1, stage 2, and stage 3, the inner supports were installed after excavation, and the wall displacement did not significantly change. Stage 4 had a large excavation depth and no new inner support was installed, resulting in the largest change in the wall displacement. e maximum deformation values of the wall were very close to 11.3 mm and 15.9 mm. e minimum displacement of the wall appeared below the excavation surface of the foundation pit, indicating that the soil had a restraint effect on the wall and the embedded effect was obvious at the excavation surface. Both the physical experiments and the numerical simulation of the wall displacement curve were "big bellyshaped," and the maximum horizontal displacement position d hm was located at the ratios of 0.33 H and 0.57 H, respectively. e average value of the maximum horizontal displacement position of the granite residual soil foundation pit in Shenzhen is 0.56 H [6], which was very close to the experimental results.
e 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. is was caused by a certain time effect in the physical experiment process, but it did not affect the deformation law of the settlement curve.
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. en, 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.   Advances in Civil Engineering

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. e layout of the measuring points is also shown in Figure 2. e results of the physical experiments were enlarged by 30 times and compared with the monitoring results. e results of the settlement and displacement are very close as shown in Figure 13. e 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). e difference between the monitoring and the experiment results is not very large, which shows the accuracy of the physical model experiment.    Figures 14 and 15 that the lateral displacement of the diaphragm wall is as "large belly". e 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. e smaller the embedded depth is, the greater the final horizontal displacement of the wall with the change in embedded depth is. e 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. 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. e maximum soil settlement appears at about 0.23 H from the pit. e 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. e 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.

Earth Pressure.
e 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.
e Earth pressure in front of the wall decreased continuously with the increasing depth of excavation. e 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. e 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).

Conclusions
(1) e 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. e 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) e soil settlement behind the diaphragm wall was gradually increased with the progress of the excavation. e maximum settlement position appears approximately 0.23 H, with a maximum value of 0.45 mm, δ vm /H � 0.07%. e 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) e passive Earth pressure in front of the wall increases linearly with the depth. e passive Earth pressure in the bottom soil of the pit decreases with the increase of the excavation depth. e excavation depth of stage 1 is relatively shallow, and the decrease of the Earth pressure is not obvious.
e 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]. erefore, 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.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.