Barrier Effect of Existing Building Pile on the Responses of Groundwater and Soil During Foundation Pit Dewatering

Abstract

In regions with abundant groundwater resources, pre-excavation dewatering on deep foundation pits often leads to the deformation of the enclosure wall and settlement of the surrounding ground. Based on a series of engineering measurements, we conducted a series of numerical simulations to investigate the behaviors of wall and soil during pre-excavation dewatering with and without the existing pile foundations and under different distances between the existing pile foundations and foundation pits (D). Numerical results indicated that when the foundation pit is adjacent to existing building pile foundations, the soil was restricted by the pile foundations (i.e., soil-blocking effect). When D ≤ 40 m, the soil-blocking effect grows stronger as D gets smaller; while when D > 40 m, the soil-blocking effect is significantly weakened and the water-blocking effect (i.e., the blockage of groundwater seepage by the building pile foundation) gradually appears, which intensifies the ground surface settlement. The maximum settlement position of the soil behind the pile foundation of the existing building is farther away from the foundation pit as the soil-blocking effect becomes stronger. The coupling effect of soil-blocking and water-blocking on the ground deformation should be considered in the design of the foundation pit project to get a more reasonable support and dewatering scheme.

1. Introduction

With the accelerating urbanization in China, numerous deep excavation projects are emerging in densely populated urban areas [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]; foundation pits located adjacent to existing underground building structures have become very common; the interaction between adjacent structures and excavation pits has become a focal point of research [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41].

Currently, soil excavation and dewatering are two major factors causing the deformation of the foundation pit and ground environment during the pit construction process [42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65]. The environmental disturbance caused by excavation has drawn extensive attention from scholars [59,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87]. Zhang et al. [88] conducted numerical analysis and experiments to study the damage to existing buildings caused by an adjacent foundation pit during excavation. Zhu et al. [89] introduced a safety monitoring and warning system used to analyze the influence of excavation pits on adjacent buildings. Zhang et al. [90] studied the time-domain analysis of viscoelastic mechanisms during the excavation of foundation pits on adjacent building deformations; Wei et al. [91] researched the effect of foundation pit excavation and dewatering on adjacent buildings in detail.

Regarding the impact of dewatering from a foundation pit on the surrounding environment, several scholars have conducted relevant studies as follows. Zeng et al. [51] used ABAQUS 6.14-4 to establish a three-dimensional fluid–solid coupling element model to study the enclosure wall deflection induced by pre-excavation dewatering under different construction conditions. Zhang et al. [92] studied effective control of ground settlement during dewatering in a Double-Layered Foundation pit based on the Seepage Control–Recharge Coupling Model.

The research mentioned above focuses on the impact of foundation pit excavation and dewatering on the surrounding environment. However, existing underground buildings around the foundation pit also affect the deformation of the foundation pit [64,93,94]. For example Liao et al. [95], through numerical parametric studies and case studies, have demonstrated that the presence of existing building piles creates a soil-blocking effect (i.e., hindering the soil settlement). This results in different ground deformations outside the foundation pit when there are existing building piles compared to when there are none. However, they did not consider how the foundation pit dewatering process with existing building piles might affect the ground strata.

Therefore, this study establishes a numerical model for deep foundation pit dewatering based on a subway station in Nankai District, Tianjin, China. Using the ABAQUS finite element analysis software, a numerical model of deep foundation pit dewatering was established to study the blocking effects of existing building pile foundations on water and soil and how these influence the deformation characteristics of the enclosure walls and surrounding soil layers during pre-excavation dewatering. The findings aim to provide technical references for related engineering designs.

2. Engineering Background

2.1. Project Description

Figure 1 presents an excavation project for a subway station site located in Nankai District, Tianjin, China. The pit was approximately 153.6 m long, 37.6 m wide, and with an excavation depth of about 16.9 m. The enclosure wall of the foundation pit adopts an underground diaphragm wall that was constructed using reinforced concrete, with a thickness of 0.8 m and a length of 32.5 m (the hydraulic connection between the inside and outside of the foundation pit was not cut off).

The horizontal support of the subway station’s main structure excavation adopts three layers of C30 concrete supports. To minimize construction disturbances to the enclosure wall, the first support was installed in advance. There were 25 pumping wells installed inside the foundation pit to lower the water level. Among these, 22 pumping wells were opened to withdraw groundwater, while the remaining 3 pumping wells were unoperated and functioned as observation wells to monitor the variations in groundwater level within the foundation pit. Simultaneously, several groundwater observation wells were arranged outside the foundation pit to monitor the variations in water levels in different aquifers.

2.2. Geological Conditions

As depicted in Figure 1, the south of the pit site is an open area without buildings, while on the north, there is a row of residential community buildings with foundations supported by pipe piles. The buildings in the community are approximately 36 m away from the foundation pit, with the pile foundation buried at a depth of 17.5 m. Within the 50 m depth range of the ground surface in the construction site, the foundation pit site mainly consisted of silty clays, clayey or sandy silts, and silty sands. There are five aquifers (designated as Aq0, AqI, AqII, AqIII, and AqIV, respectively), separated by aquitards (designated as AdI, AdII, AdIII, and AdIV). Figure 2 shows the distribution of soil layers and the main soil parameters (the data were obtained from site investigation and laboratory soil tests), where γ represents the unit weight; K₀ denotes the coefficient of earth pressure at rest; ω indicates the water content; e represents the initial void ratio; E_S is the constrained modulus; Vs is the shear wave velocity.

2.3. Pumping Test

Figure 2 shows the positions and relationships among various enclosure walls, pumping wells, and observation wells. The enclosure wall does not completely cut off AqII, and the dewatering wells inside the pit are inserted into AqII. Therefore, dewatering inside the foundation pit would also cause groundwater drawdown outside the foundation pit, which further induces ground surface settlement.

To assess the effectiveness of the retaining structure’s water resistance and the pumping wells’ pumping capacity, seven pumping tests were conducted by engineers before soil excavation of the foundation pit, with a total test duration of 21 days. Therein taking a typical group well pumping test as an example, it lasted for 3.2 days. Through observation wells inside the foundation pit and settlement monitoring points outside the pit, it was observed that the water level inside the pit dropped by approximately 15 m. The water levels have dropped by approximately 8 m on the south side and 3 m on the north side outside the foundation pit, respectively. The reason is that the dewatering on the south side occurs in AqII, which is different from the dewatering on the north side that occurs in AqI, and there are building piles on the north side that obstruct the flow of water. Furthermore, the maximum ground surface settlement has reached up to 12 mm [11]. This indicates that the impact of the pumping tests on the surrounding environment of the foundation pit cannot be ignored. In addition, the only difference in the above experimental results is whether or not there are pile-supported buildings between the north and south sides. This implies that the presence of piled foundation structures affects groundwater seepage and stratum deformation.

Therefore, the following section investigates the influence mechanism of the adjacent pile foundation’s seepage barrier effect on groundwater and stratum deformation during the excavation dewatering process. The development mechanism is revealed through a series of numerical simulation studies.

3. Numerical Analysis

3.1. Modeling Scheme

This section utilized ABAQUS to establish two types of models based on the engineering background described in Section 2. The first type of model simulated the pumping test from the previous section (i.e., without a building pile foundation on the north side of the foundation pit, because of the uncertainty and complexity of the real conditions of the pile foundation) and was validated against actual measured data from the engineering. The second type of model is based on the first type, with the premise of incorporating the existing adjacent building pile foundation on the north side outside the foundation pit. It varies the parameters of the distance (D) between the piled foundation and the foundation pit and the depth of pit dewatering (H_d) (where D is set to 5, 10, 15, 20, 40, and 60 m, and H_d is set to 10, 15, 19, and 22 m), to investigate the influence of existing pile foundation seepage barrier effects on strata deformation induced by pit dewatering. Since the processes for establishing the two types of models are similar, the following only shows the modeling process specifically for the second type of model in detail.

3.2. Model Setup

Considering the complexity of modeling for the entire project and the deformations that often occur in the middle section of the foundation pit, this section models the middle section of the elongated foundation pit (as shown in Figure 1), with the three-dimensional model depicted in Figure 3. The dimensions of the enclosure wall and the plan and vertical view of the dewatering wells in the model are roughly like the engineering conditions described in Section 2. The model is constructed based on a common group pile foundation, with a 10 m deep underground building basement and 36 piles, each 31 m long and 0.85 m in diameter (spaced at 3.06 m intervals).

The over-exploitation of groundwater in coastal areas of China and recent restrictions on groundwater extraction, as a result, led to fluctuating groundwater levels in Tianjin, causing the soil to exhibit elastic deformation under typical groundwater drawdown conditions [96,97]. Hence, the soil model utilizes the Mohr–Coulomb (MC) model [49,98,99]. Based on the actual site conditions described in Section 2 of the project, the soil layers in the model are divided into 12 layers, simulated using pore-pressure solid elements (C3D8P). Consideration of fluid–structure interaction is included in the calculations. The Poisson ratio for silty clays and clay is 0.33, while for silt, silty sands, and fine sand, it is 0.30. Detailed model parameters are provided in Table 1. The effective cohesion (c′) and the effective internal friction angle (φ′) of the soil are determined based on on-site investigation and laboratory soil tests. The elastic modulus (E) and the horizontal and vertical permeability coefficient (K_H and K_V) are obtained through inverse calculation, comparing the model computation results with the measurements from pumping tests described in Section 2. For detailed procedures of the inverse calculation, please refer to the article by Wang et al. [100]. The enclosure wall and building piles are simulated using solid incompatible elements C3D8I, the dewatering wells are simulated using shell elements S4, and the first line supports are simulated using linear beam elements B31. According to previous studies and experience from scholars [27], the modeling process neglects the gap between the dewatering well casing and the wellbore. The (μ) between the dewatering wells, building pile foundations, and enclosure wall with the soil is taken as 0.30. Assuming linear elastic behavior for the stress–strain relationship of retaining structures, dewatering wells, and building pile foundations, their respective elastic moduli are 30 MPa, 210 MPa, and 30 MPa.

Due to the hydraulic connection between the inside and outside of the pit, to avoid the settlement of soil affecting areas beyond the model boundary during dewatering inside the pit, the influence range (R) of dewatering, assuming a maximum drawdown of 35.5 m for AqII, using the Sichardt Formula (1) [101], is defined as follows:

(1)$R={10}_{SW}\sqrt{K_h},$

where SW and K_h are the groundwater drawdown and horizontal permeability of the pumped aquifer, and R is influencing a range of dewatering.

The calculation yields R = 615 m (K_h = 3 m/d). To prevent the influence of water level changes during pit dewatering from extending beyond the model boundary, the distance between the model soil boundary and the pit is set to 800 m, which is 40 times the maximum drawdown depth.

Apart from the top surface of the model, horizontal constraints are applied to the entire model boundary. Additionally, vertical constraints are applied to the bottom surface of the soil, and the water level is also set at the constant head hydraulic supply position at the model’s ground surface boundary. To simulate dewatering, a constant head boundary is established on the soil surface in contact with the dewatering wells. For example, for the model where the water level inside the pit drops by 22 m (with a model well bottom depth of 24.5 m), the zero-pore pressure boundary condition can be set within the range of 0 to 22 m, and a fixed head boundary condition based on hydrostatic pressure can be sit in the range of 22 to 24.5 m. Therefore, the simulation of different dewatering depths only requires adjusting the ranges of the zero-pore pressure and fixed head boundary conditions accordingly.

3.3. Model Validation

Following the numerical simulation based on the pumping test process, the variations in water levels inside and outside the pit and ground surface settlement caused by pumping are obtained. By comparing these with the measured values, we found that the simulated values have a good level of consistency with the measured values. However, due to the existence of differences in actual geological conditions, such as uneven soil layer thickness or variations in soil properties, the measured values and simulated values cannot be completely consistent. In summary, this study’s numerical simulation broadly reflects the development of practical water level change and soil deformation caused by pumping. The detailed comparison results can be referred to in the article by Zeng et al. [8]. Therefore, the second type of model established based on this model is considered relatively reliable.

4. Simulation Results and Analysis

4.1. Distribution of Groundwater Drawdown

Figure 4 shows the distribution pattern of groundwater drawdown outside the foundation pit as a function of depth, using a dewatering depth H_d = 22 m as an example. The three main points that can be observed are as follows:

1. Compared to the side without building pile foundations, the side with building pile foundations has a greater depth of groundwater drawdown. This is because groundwater flow toward the foundation pit is hindered by the presence of building piles, causing some groundwater to circumvent the underground structures, thereby elongating its seepage pathway;

2. As D increases, the drawdown of water level outside the foundation pit decreases, approaching the drawdown observed without building piles. This indicates that the farther the pile foundations are from the foundation pit, the weaker their barrier effect on water;

3. In Aq0, the variation in water levels outside the foundation pit is significantly influenced by D on the sides with pile foundations. However, below Aq0, the differences in water level variations between sides with and without pile foundations gradually diminish. This is because within the depth range of the Aq0, there are underground structures such as building basements, which create a strong barrier effect on groundwater. Below this depth range are pile foundations, where the soil between the piles allows groundwater to seep through, thus significantly reducing their barrier effect on water.

Figure 5 shows the horizontal variation curve of drawdown in the water level of the first confined aquifer AqI under the condition of H_d = 22 m. From the figure, it can be observed that along the horizontal distribution, as the distance from the foundation pit increases, the drawdown in water level decreases; as D increases, the barrier effect of the pile foundation on water gradually diminishes. Furthermore, comparing the groundwater drawdown on the side of the foundation pit with and without pile foundations, it is evident that regardless of D, the side with pile foundations consistently shows greater water level variations than the side without pile foundations. This corresponds with the vertical distribution of groundwater drawdown shown in Figure 4.

To assess the water-barrier effect of building pile foundations during pre-watering excavation, the water resistance intensity factor η = S₂/S₁. (S₂ is the maximum groundwater drawdown on the northern side of the foundation pit, while S₁ is the maximum groundwater drawdown on the southern side without a building pile foundation). Figure 6 shows the trend of η as D varies when H_d = 22 m. As shown in Figure 6, regardless of D, the η is always greater than one. This indicates that the existing building pile foundations obstruct the water flow into the foundation pit, resulting in a greater groundwater drawdown compared to the condition without buildings. In addition, the fitted curve of η shows a decreasing trend. This indicates that as D increases, η gradually decreases, which means the obstructive effect of building pile foundations on water becomes weaker gradually.

4.2. Distribution of Enclosure Wall Deflection and Ground Surface Settlement

Figure 7, taking a dewatering depth of H_d = 22 m as an example, demonstrates the pattern of ground surface settlement and enclosure wall deflection outside the foundation pit induced by dewatering, under different distances (D) between building pile foundations and the foundation pit. From the enclosure wall deflection pattern the following is observed:

1. Regardless of whether there are pile foundations outside the foundation pit or not, dewatering inside the foundation pit causes the enclosure wall to undergo an inward ‘bulging’ type of deflection. This is because the initial support provided by the first construction stage before dewatering causes minimal lateral movement at the head on the enclosure wall. At the same time, the deeper soil exerts a constraining effect on the bottom of the enclosure wall, resulting in less lateral movement at deeper positions of the enclosure wall. Therefore, the deformation of the enclosure wall mainly occurs in the near-middle section;

2. When there are foundation piles outside the foundation pit, as D increases, the lateral movement of the enclosure wall caused by dewatering also increases but overall remains smaller than when there are not pile foundations (i.e., at the south side of the foundation pit).

From the ground surface settlement pattern outside the foundation pit the following is observed:

1. Dewatering causes a dual settlement groove pattern in the ground surface on the north side of the foundation pit (i.e., the settlement groove behind the existing pile foundations and the settlement groove between the enclosure wall and the existing pile foundations). While on the south side without pile foundations, the ground settlement outside the foundation pit exhibits a single settlement groove pattern;

2. For the scenarios with ground surface settlement involving existing pile foundations, when D < 40 m (i.e., D = 5 m, 10 m, 15 m, and 20 m), the ground settlement between the enclosure wall and the existing pile foundations is less than without pile foundations (i.e., the ground settlement corresponding to the south side of the pit); when D ≥ 40 (i.e., D = 40 m and 60 m), the ground settlement between the enclosure wall and the existing pile foundations is greater than without pile foundations. This indicates that during the pre-excavation dewatering process, as D increases, both the soil-blocking effect and the water-blocking effect gradually diminish. Moreover, when D < 40 m, the barrier effect of the soil plays a predominant role, thereby hindering the settlement of the soil between the existing pile foundations and the enclosure wall. When D > 40 m, the water-blocking effect plays a predominant role, thereby lengthening the flow path, resulting in a more significant decrease in groundwater level between the existing pile foundations and the enclosure wall, consequently leading to greater settlement.

From the settlement pattern of the existing pile foundations the following is observed:

1. When existing pile foundations hinder the movement of soil, they themselves deform with the movement of the soil, resulting in uneven settlement. This is particularly evident when they are close to the foundation pit, but the settlement of the pile foundations is always less than the settlement of the nearby ground surface;

2. As D increases, the settlement of the building transitions from a greater settlement at the front side compared to the back side to gradually less settlement at the front side compared to the backside. It is important to consider this pattern and its impact on the structural integrity during the project.

4.3. Distribution of Maximum Ground Settlement and Maximum Enclosure Wall Deflection

Figure 8 shows the relationship between the maximum ground settlement (δ_vm) in front and behind existing pile foundations outside the foundation pit, as well as the maximum lateral displacement of the enclosure wall (δ_hm) at different dewatering depths and D. The following can be observed:

1. Overall, the pumping causes both the front δ_vm and δ_hm of the existing pile foundations outside the pit to increase with the increase in D. This trend becomes more pronounced under deeper dewatering depths. This reflects the impact of the continuous weakening effect of existing pile foundations outside the foundation pit on restraining soil movement;

2. As in Figure 8a, in the front of the existing pile foundations, when D ≤ 40 m, the δ_vm on the side with existing pile foundations is significantly smaller than the δ_vm under the same conditions when there are not pile foundations. This is because the barrier effect of the building has a positive influence. Furthermore, it primarily relies on the water-blocking effect. However, when (D > 40 m), the δ_vm on the side with existing pile foundations is slightly larger than the δ_vm under the same conditions without pile foundations. This is because the barrier effect of the building has a negative impact, primarily relying on the soil-blocking effect. It can be considered that under the same dewatering depth, there exists a critical value for D in terms of the influence of dewatering-induced δ_vm. As D changes, the coupled effects of barrier pile foundations on the soil and pile foundations on the water gradually become apparent;

3. As in Figure 8b, At the same dewatering depth, the δ_vm behind the existing pile foundations gradually decreases with increasing D. The δ_vm on the side with/without existing pile foundations is roughly the same. This is because water can permeate between the pile foundations; Thus, the extent of water level decline behind the pile foundations is similar to that on the side without pile foundations (see Figure 5), resulting in similar consolidation settlement of the soil mass;

4. From Figure 8c, it can be observed that with increasing dewatering depth, δ_hm shows a pattern of initially increasing and then decreasing, reaching its maximum at H_d = 15 m. This is because when the dewatering depth is shallow (H_d ≤ 15 m), there is less groundwater replenishment from outside the foundation pit to the inside, resulting in smaller maximum lateral displacement under corresponding conditions. However, when the dewatering depth is greater (H_d > 15 m), the water level inside the foundation pit decreases significantly, while groundwater from outside the foundation pit promptly replenishes into it. This results in a significant decrease in water levels both inside and outside the foundation pit, causing the enclosure wall to exhibit lateral displacement tendencies in both directions—towards the inside and outside of the foundation pit. Due to these two trends counteracting each other, the maximum lateral displacement under these conditions is smaller compared to when the dewatering depth H_d = 15 m. Because the barrier effect plays a positive role in restraining the lateral deformation of the enclosure wall, δ_hm is consistently less than the maximum lateral displacement of the enclosure wall without pile foundations.

Figure 9 shows the relationship between maximum ground surface settlement (δ_vm) and maximum lateral displacement of the enclosing wall (δ_hm) under different dewatering depths (H_d). To facilitate comparison with previous studies, this paper calculates non-dimensionless δvm and δhm by dividing them by H_d, resulting in δ_vm/H_d and δ_hm/H_d. Among them, the horizontal data points in Figure 9, from top to bottom, represent the variation in D from 60 m to 5 m, respectively. The following can be observed:

1. δ_vm mostly lies between δ_vm = 1.88 δ_hm and δ_vm = 5.52 δ_hm, caused by pre-watering excavation, with δ_vm ranging from 0.051% H_d to 0.114% H_d, and δ_hm changes from 0.015 H_d to 0.044 H_d. The ratio relationship between δ_vm and δ_hm in this study is generally within the range of the measurement data from Zhang et al. [102] (i.e., from δ_vm = δ_hm to δ_vm = 10 δ_hm). Zhang et al. and this study both investigate the relationship between the changes in δ_vm and δ_hm caused by foundation pit dewatering when there is hydraulic connectivity between the inside and outside of the foundation pit (i.e., dewatering inside the pit leads to a rapid decline in the water level outside the pit);

2. As D continues to increase, the slope of the relationship between δ_vm and δ_hm also increases correspondingly. This is because, as D increases, the barrier effect of the building gradually weakens, which aggravates surface ground settlement while also restraining the lateral displacement of the enclosure wall. In addition, as the depth of dewatering (H_d) increases, it will also lead to an increase in the slope of the relationship between δ_vm and δ_hm.

4.4. Maximum Settlement Increment and Maximum Lateral Displacement Increment

Figure 10 illustrates the relationship between the maximum settlement increment of the ground surface between existing pile foundations and the foundation pit (Δδ_vm), as well as the maximum lateral displacement increment of the enclosure wall (Δδ_hm), concerning the distance between existing pile foundations and the enclosure wall (D) and the dewatering depth (H_d).

From Figure 10, it can be observed that as D increases, both Δδ_vm and Δδ_hm decrease nonlinearly, when D ranges from 5 to 40 m, the variations in Δδ_vm and Δδ_hm are significant, indicating that Δδ_vm and Δδ_hm are highly sensitive to changes in both D and H_d. To simplify the calculation of maximum surface settlement outside the foundation pit (δ_vm) and maximum lateral displacement of the enclosure wall (δ_hm) caused by dewatering, Origin software (OriginPro 2018C) is used to fit the scatter data points into two types of mathematical functions, respectively, describing the trends of Δδ_vm and Δδ_hm changes. Therefore, based on the deformation observation results of the existing foundation pit, preliminary estimates can be made for the maximum lateral displacement of the enclosure wall and maximum settlement caused by dewatering in the new construction foundation pit under similar geological conditions at different distances (D). This can provide initial references for the dewatering design of the foundation pit.

5. Discussion

In summary, the factors affecting the seepage barrier effect are mainly the differences in the barrier structures and the distance (D) between the building pile foundations and the foundation pit. When the barrier structure is a basement, its water-barrier effect is significantly stronger than that of building pile foundations (like the third conclusion in Section 4.1). Due to the coupling effects of the water-barriers effect and soil-barriers effect from existing nearby buildings, when the distance D between the building pile foundations and the pit is less than 40 m, the soil-barrier effect dominates; when D is greater than 40 m, the water-barrier effect dominates. This paper considers pre-excavation dewatering without building pile foundations and does not discuss dewatering during the excavation process. Due to the current lack of measured data, this will be a focus of future research, comparing it with the actual excavation process.

6. Conclusions

This article investigates the influence of the seepage barrier effect of existing building pile foundations on the deformation of strata induced by foundation pit dewatering using three-dimensional finite element numerical simulation. Based on this study, the following conclusions can be obtained:

1. From the perspective of the depth distribution of water level drawdown outside the pit, the water level drawdown on the side with a building pile foundation is greater than that on the side without any buildings. It is noteworthy that in the aquifer layer Aq0, the changes in water level outside the foundation pit on both sides with and without building pile foundations are significantly influenced by distance (D), while below the aquifer layer Aq0, the differences in water level changes between both sides gradually diminish. From the perspective of the horizontal distribution of water level drawdown outside the foundation pit, the further away from the foundation pit, the smaller the water level drawdown. As the distance (D) increases, the barrier effect of the building pile foundation on the water gradually weakens;

2. From the perspective of enclosure wall deformation patterns, due to the presence of the first support at the top of the diaphragm wall of the foundation pit, the maximum lateral displacement generated by the enclosure wall occurs primarily around a depth of 10 m, regardless of the presence or absence of building pile foundations. Under the same dewatering depth conditions, the lateral displacement of the enclosure wall increases gradually with the distance between existing building pile foundations and the foundation pit. Additionally, the lateral displacement of the enclosure wall on the north side of the foundation pit is consistently smaller than that on the south side at corresponding positions. This indicates that both the barrier effect of the existing building pile foundation soil and the barrier effect of the existing building pile foundation water positively influence the lateral displacement pattern of the enclosure wall;

3. From the perspective of ground surface settlement patterns between the existing building pile foundation and the pit, when there are not building pile foundations, the settlement outside the pit caused by pre-excavation dewatering exhibits a single settlement groove. If there are building pile foundations, it manifests as a double settlement groove pattern. Under the same dewatering conditions, the distance (D) between the building pile foundation and the pit increases, and the settlement of the soil outside the pit also increases. When D < 40 m, the soil-blocking effect gradually diminishes with increasing D, resulting in the settlement of the soil being less than that at the corresponding position on the south side of the pit. However, when D > 40 m, the soil-barrier effect significantly weakens, while the pile foundations enhance their obstruction to groundwater flow, leading to settlement being greater than that at the corresponding position on the south side of the foundation pit. From the perspective of ground surface settlement patterns behind the building pile foundation outside the foundation pit, the settlement of the soil behind the building pile foundation is greater compared to the settlement at corresponding positions without building pile foundations. Moreover, the settlement of the soil behind the existing building pile foundations gradually approaches the settlement at corresponding positions on the south side of the foundation pit as D increases;

4. Regarding the maximum settlement of the soil outside the foundation pit and the maximum lateral movement of the enclosure wall, as well as their increments, δ_vm mostly lies between δ_vm = 1.88 δ_hm and δ_vm = 5.52, with δ_hm caused by pre-watering excavation. It is largely consistent with the measurement data from Zhang et al., ranging from δ_vm = δ_hm to δ_vm = 10 δ_hm. There exists a critical value for the influence of different D values on δ_vm of the soil between the building pile foundation and the foundation pit (i.e., D = 40 m). The settlement of the soil behind the building pile foundation decreases linearly with increasing D. The greater the depth of dewatering, the more pronounced that the influence of the building pile foundation on deformation induced by pumping becomes.

Author Contributions

Conceptualization, C.Z.; Methodology, C.Z.; Software, D.H.; Formal analysis, D.H.; Investigation, H.S.; Resources, Y.Z. and L.H.; Data curation, Y.Z. and L.H.; Writing—original draft, D.H. and X.X.; Writing—review & editing, C.Z. and C.X. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to confidentiality requirement.

Acknowledgments

The authors are grateful for the warm and efficient work by editors and reviewers.

Conflicts of Interest

Author Youwu Zhao was employed by the company China Construction Fifth Engineering Division Corp., Ltd. Author Lei Han was employed by the company China Construction Eighth Engineering Division Corp., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

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Figures and Table

Figure 1. Plans and sections of an excavation and layout of instrumentation.

Figure 2. Strata distribution and the main soil mechanical parameters.

Figure 3. Finite element material diagram of the model (adapted from Reference [8]).

Figure 4. Groundwater drawdown outside the pit along the depth.

Figure 5. Distribution curves of groundwater drawdown outside along level in AqI.

Figure 6. Variation of η = S2/S1 with D.

Figure 7. Settlement of the ground outside the pit and adjacent structures and lateral displacement curve.

Figure 8. Relation between δhm and D under different dewatering depths: (a) relation between pile foundation and foundation pit δvm and D; (b) relation between behind the pile foundation δvm and D; (c) relation between δhm and D.

Figure 8. Relation between δhm and D under different dewatering depths: (a) relation between pile foundation and foundation pit δvm and D; (b) relation between behind the pile foundation δvm and D; (c) relation between δhm and D.

Figure 9. Relationship between δvm/Hd and δhm/Hd [102].

Figure 10. Relation between D and Hd of maximum ground settlement increment and maximum lateral displacement increment of enclosure structure between adjacent structure and foundation pit: (a) Δδvm/ΔD relative to D/Hd; (b) Δδhm/ΔD relative to D/Hd.

Figure 10. Relation between D and Hd of maximum ground settlement increment and maximum lateral displacement increment of enclosure structure between adjacent structure and foundation pit: (a) Δδvm/ΔD relative to D/Hd; (b) Δδhm/ΔD relative to D/Hd.

Table 1

Soil distribution and parameters used in the model (adapted from Reference [8]).

Hydrogeology	Soil Classification	Buried Depth/m	KH(m/d)	KV(m/d)	E(Map)	C′(Kpa)	φ′
Aq0	Silty clays with silt seams	0~10	0.03	0.003	43.5	17	25
AdI	Silty clays	10~15	0.025	0.001	56.3	18	23
AqI	Clayey silts	15~19	0.2	0.1	137.6	10	34
AdII	Silty clays	19~22	0.006	0.001	118.6	19	26
	Sandy silts	22~24.5	2.5	0.5	151.8	8	34
	Sandy silts	24.5~29.5	1	0.2	153.3	8	36
AqII	Silty clays with silt seams	29.5~32.5	1	0.16	128	17	26
	Silty sands	32.5~35.5	3	0.6	178.5	7	37
AdIII	Silty clays	35.5~37	0.02	0.004	152.1	19	26
AqIII	Sandy silts	37~41	3	0.9	214.5	10	34
AdIV	Silty clays	41~47	0.005	0.001	198.4	18	27
AqIV	Silty sands	47~50	3.5	1.5	257	7	38

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