Study on the Influence and Deformation Control of Rich Water Foundation Excavation on Adjacent Buildings

Abstract

Taking the foundation pit of the Suzhou Chunshenhu Road Expressway Reconstruction Project as an example, the excavation process of the foundation pit was numerically simulated using a three-dimensional finite element method. The measured data and simulated data of the lateral deformation of the enclosure structure, surface settlement deformation of the ground outside the pit, and settlement deformation of the building were compared to analyze the impact of foundation pit construction on adjacent buildings. The influence of foundation pit floor and diaphragm wall thickness on wall displacement, building settlement, and foundation pit uplift was also discussed. The results showed the following: (1) Adding a foundation pit floor has a significant effect on reducing the lateral displacement of the diaphragm wall, settlement of the building, and uplift of the foundation pit. Increasing the thickness of the foundation pit floor has a limited effect on reducing the displacement, while increasing the thickness of the diaphragm wall has a small effect. (2) The displacement curve of the underground diaphragm wall increases with depth. It reaches a maximum at the excavation surface and then decreases gradually. (3) The surface settlement increases first and then decreases with distance from the foundation pit, showing a concave shape. As the depth of excavation increases, the settlement value increases. (4) Through analysis of the monitoring data of vertical displacement of buildings, it can be seen that during foundation pit excavation, buildings undergo five stages: initial slow descent, steep descent, mid-term slow descent, late steep descent, and stable deformation. The buildings are dominated by settlement deformation.

1. Introduction

The acceleration of large-scale underground municipal transportation engineering construction has greatly driven the development of deep foundation pit engineering technology. This has led to higher requirements for the design and construction of deep foundation pits [1,2,3]. The construction of a deep foundation pit will lead to large-scale unloading of the soil, which will inevitably cause deformation of the surrounding strata [4,5,6,7]. In particular, the excavation of a foundation pit in a water-rich soft soil area will cause greater deformation [8], adversely affecting the surrounding structures, which greatly increases the difficulty of the project [3,9,10]. In deep foundation pit engineering, support measures are often used to reduce the displacement of the formation and the impact on the surrounding buildings, so as to ensure the safety of the foundation pit construction. It is of great significance to study the influence of foundation pit excavation on the surrounding buildings and the deformation control effect of the supporting structures.

At present, some progress has been made in the research on the impact of foundation pit excavation on tunnels [11,12,13,14]. Shengyuan fan et al. [15] analyzed the deformation of a tunnel and the stress change of the lining structure during the construction of the foundation pit by Midas GTS/NX and discussed the rationality of the engineering protection measures by combining the measured monitoring values of the tunnel uplift deformation and convergence deformation. Shuaihua et al. [12] analyzed the influencing factors of a foundation pit excavation on a subway and found that the deformation was more sensitive when the excavation depth was close to the tunnel, and the disturbance in the foundation pit piling process had the greatest impact on the supporting structure.

The growing development of urban underground spaces has prompted increased research into the influence of foundation pit excavation on underground building structures [16]. Zhijian Jiang et al. [17] used FLAC 3D software to establish a three-dimensional numerical model of a foundation pit, an existing subway station, and its tunnel structure, and combined with the field monitoring data, the deformation characteristics of the supporting structure, an adjacent subway station, and its tunnel at different stages of the deep foundation pit were analyzed.

In addition to underground structures, many scholars have studied the impact of foundation pit excavation on nearby aboveground building structures [17]. Jiangpeng Wu et al. [18] analyzed the influence of different construction stages on the deformation and overturning rate of adjacent viaducts and their variation laws based on measured data and numerical simulations and summarized the influence of the distance between the viaduct and the foundation pit on the viaduct. The above research mainly focuses on the impact of deep foundation pit excavation on underground structures or deep foundation structures. In contrast, shallow foundation buildings are more sensitive to settlement due to their special structure. Previous studies have shown that the deformation effects of soil and buildings caused by a foundation pit excavation in a water-rich area are more obvious [19], in which it easily causes cracking and the failure of shallow foundation buildings.

In order to ensure the stability of foundation pit construction in water-rich areas, scholars have studied the foundation pit supporting structure and soil deformation [20,21]. Yang Jin et al. [22] studied the influence of steel support cross-section size and preload axial force level on the stability of a foundation pit through numerical simulation. Based on model experiments, Chuanzhao Xu [23] analyzed the coordination relationship between the lateral deformation of the diaphragm wall and the axial force of the inner brace. Tao Yang et al. [24] analyzed field monitoring data from soft soil strata in Suzhou, identifying factors influencing diaphragm wall deformation and building settlement under various support schemes. Junding Liu et al. [25] studied a foundation pit in Taizhou, Zhejiang, and found that retaining wall displacements and nearby building settlement increased progressively during excavation. Zheyuan Feng et al. [26], through finite element simulations of a Suzhou foundation pit, demonstrated that maximum ground settlement and horizontal displacement correlate positively with excavation depth.

Currently, extensive research has been conducted on the impact of deep foundation pit excavations on adjacent buildings. However, limited attention has been given to the effects of excavation on shallow foundation buildings in soft soil areas, despite their higher sensitivity to deformation due to a weaker bearing capacity and structural rigidity. Moreover, the influence of support structure dimensions on adjacent building stability remains insufficiently analyzed, resulting in a lack of effective guidance for structural design. Addressing these gaps is crucial to ensuring the safety of buildings surrounding deep foundation pits, particularly in water-rich soft soil areas.

In this paper, the K10 + 800–K11 + 100 section of the rapid reconstruction project of Chunshenhu Road and the dormitory building near it were taken as the research object. This study monitored surface settlement, diaphragm wall deformation, and building settlement during excavation, analyzing deformation patterns to evaluate the impact of foundation floor and diaphragm wall thickness. The findings can provide valuable references for deformation control in similar projects.

2. Project Basics

2.1. Project Overview

The Suzhou Chunshenhu Road Expressway Reconstruction Project (Section 5) is situated east of the Suzhou–Jiaxing–Hangzhou Expressway. This road project uses a tunnel structure, extending eastward from Wangxiang Port North to Linjia Port South, continuing along Hubin Road to Yangcheng West Lake, and ultimately connecting to the Suzhou Industrial Park section.

Figure 1 provides a schematic of the project layout. On the south side of the K10 + 800–K11 + 100 foundation pit section lies the School Dormitory, a nearby building with an elevation of 6.1 m. The building is situated a minimum of 10 m from the foundation pit (Figure 1), with a maximum distance of approximately 17 m. The dormitory building is rectangular, measuring about 100 m in length and 15 m in width. Its main structural elements consist of reinforced concrete columns, beams, and slabs, while the structural form is a brick–concrete system, with expanded shallow foundations as the base.

2.2. Geology and Hydrological Characteristics

The region’s topography is generally flat, with some localized undulations. According to the site survey data, the foundation soil at a depth of 90 m below the surface of the construction site mainly includes Quaternary littarian, Quaternary floodplain, and riverbed facies sediments.

The stratum around the foundation pit area mainly consists of silt and clay, as shown in Figure 2. The hydrogeological conditions of this project are complex, with variable groundwater levels, which are categorized into three types: phreatic, low-pressure confined, and confined aquifers. The groundwater distribution is as follows: Phreatic water is primarily found in shallow fill and clayey soils, with a depth ranging from 0.70 to 4.10 m. Low-pressure confined water exists in silty sand layers, characterized by moderate water abundance and good permeability. Confined water is mainly distributed in the silty sand layers, which exhibit moderate water abundance and are relatively well-sealed.

2.3. Preliminary Support Scheme

The foundation pit’s enclosure structure uses underground diaphragm walls. The northern side (without buildings) has a diaphragm wall thickness of 1000 mm, while the southern side (with buildings) has an initial diaphragm wall thickness of 1000–1400 mm, with a depth of 43.5 m and an average panel width of 6 m. The walls are cast with C35 concrete underwater. As shown in Figure 2, the standard section employs five layers of radial supports, with the first layer composed of reinforced concrete (C30 concrete), spaced horizontally at 9.0 m, with a cross-sectional dimension of 1000 × 1200 mm, and a top ring beam dimension of 1400 × 1200 mm. The third layer of reinforced concrete support is spaced at 9.0 m with a cross-sectional dimension of 1200 × 1400 mm and a ring beam dimension of 1600 × 1400 mm. Steel supports are of two types: Φ609 mm and Φ800 mm steel pipes, each with a wall thickness of 16 mm and a horizontal spacing of 3.0 m. The steel waler uses twin H588 × 300 steel girders for the second, fourth, and fifth layers. Each reinforced concrete support layer is accompanied by two temporary support columns, which are embedded in drilled piles from the tunnel’s main structure. Girders are used for bracing, and the foundation pit floor is preliminarily set at 0–500 mm.

3. Deformation and Stress Analysis Under Different Conditions

3.1. Finite Element Model and Calculation Process

Midas GTS/NX is suitable for nonlinear analysis of foundation pit excavation processes due to its efficient meshing and computational capabilities, which can handle complex 3D geological models and multistage construction conditions. The software supports a variety of soil constitutive models and structural elements, which can accurately simulate the soil–structure interaction and the response of the support system. In addition, its powerful visualization and postprocessing capabilities make displacement, stress distribution, and plastic zone analysis more intuitive. However, the simulation results are highly sensitive to parameter settings and boundary conditions. Idealized assumptions in the model, such as material homogeneity, limit the accurate reflection of the actual operating conditions to some extent.

This software was employed to simulate the effects of excavation and support dewatering processes on the nearby building during foundation pit excavation. Given the length of the tunnel relative to the foundation pit, a three-dimensional model was created based on the pit and adjacent building segment. The model’s dimensions were set to 300 m (L) × 150 m (W) × 60 m (H), as shown in Figure 3.

The Z-axis was oriented vertically upward, with the X- and Y-axes representing horizontal directions aligned with the actual project’s north and east. Soil layers were modeled using 3D hexahedral solid elements and assigned a modified Mohr–Coulomb model; diaphragm walls and building foundation slabs were modeled using 2D surface elements with elastic properties. Horizontal and vertical supports used 1D truss elements with elastic properties. Normal displacement constraints were applied to all vertical boundaries and the base surface of the three-dimensional numerical model. Referring to the hydrogeological profile of the project, the soil was stratified according to the actual situation, and considering the feasibility of the model calculation, the soil layers with similar properties were combined for modeling, so there were nine layers of soil in the model, and the specific material properties are shown in

Table 1. Physical and Mechanical Parameters of Soil Layers.

Soil Layer	Unit Weight (kN/m3)	Elastic Modulus (MPa)	Internal Friction Angle (°)	Cohesion (kPa)	Poisson’s Ratio
Layer 1	19.5	15	15.5	54.2	0.33
Layer 2	18.8	50	28.8	5.2	0.32
Layer 3	18.9	41.6	15.8	22.8	0.35
Layer 4	19.7	64.6	15.4	54.7	0.31
Layer 5	19.5	120	28	6	0.29
Layer 6	19.1	45.4	14	23	0.33
Layer 7	19.1	83.2	23.5	8.9	0.31
Layer 8	19.0	75	14.7	25.8	0.33
Layer 9	19.4	100	15.2	33	0.33

.

During the simulation, the initial groundwater level was set at 4.0 m below the datum based on the monitoring well data. Precipitation was simulated in alignment with the on-site construction plan by adjusting the node water level in real time during each excavation stage, enabling a dynamic representation of the dewatering process. According to the construction plan of the site, the simulation process is shown in

Table 2. Simulated construction procedures.

Number	Working Conditions	Implementation Process	Excavation Thickness (m)
1	Initialize geological and hydrological states	Activate all strata and initial water head Clear the displacement after the in situ stress equilibrium	-
2	Diaphragm wall construction	Activate the diaphragm underground wall	-
3	Construction 1	3.1 Excavation of the first layer of soil 3.2 Construction of the first support	5.5
4	Construction 2	4.1 Foundation pit dewatering 4.2 Excavation of the second layer of soil 4.3 Construction of the second support	3
5	Construction 3	5.1 Foundation pit dewatering 5.2 Excavation of the third layer of soil 5.3 Construction of the third support	5
6	Construction 4	6.1 Foundation pit dewatering 6.2 Excavation of the forth layer of soil 6.3 Construction of the forth support	3.5
7	Construction 5	7.1 Foundation pit dewatering 7.2 Excavation of the fifth layer of soil 7.3 Construction of the fifth support 7.4 Construction of the concrete floor	4.5

.

To assess the effects of varying diaphragm walls and foundation floor thicknesses on the diaphragm wall lateral displacement, pit floor uplift, and building settlement, nine specific working conditions were considered, as listed in

Table 3. Specific working conditions.

Condition	Diaphragm Wall Thickness/m	Foundation Floor Thickness/m
Condition 1	1.2	0.5
Condition 2	1.0	/
Condition 3	1.2	/
Condition 4	1.4	/
Condition 5	1.0	0.3
Condition 6	1.2	0.3
Condition 7	1.4	0.3
Condition 8	1.0	0.5
Condition 9	1.4	0.5

. Condition 1 was set according to the construction plan of the site, and the rest of the working conditions were taken by the control variable method combined with the empirical value.

3.2. Total Ground Movements

In the foundation pit excavation process, key evaluation indicators include lateral displacement of the underground diaphragm wall, settlement of the adjacent buildings, uplift at the pit bottom, and internal forces on the support structure. Figure 4 presents cross-section deformation diagrams under different conditions. Analyzing Conditions 1 through 9, we observed uplift at the pit bottom, inward tilting of the diaphragm wall, and settlement, particularly under buildings near the pit. Pit bottom uplift appeared in three bulges, and settlement under the adjacent buildings formed a concave pattern. Comparisons among different conditions reveal the following patterns:

Conditions 2, 3, and 4 (without foundation floor) show that increasing diaphragm wall thickness has a limited effect on reducing pit bottom uplift and adjacent building settlement.

Conditions 1, 3, and 6 demonstrate that adding a foundation floor effectively curtails pit bottom uplift, building settlement, and surface settlement, with increased floor thickness further enhancing these effects. However, merely increasing diaphragm wall thickness yields minimal benefit for reducing pit uplift, surface settlement, or building settlement (Conditions 2, 3, and 4) (Conditions 1, 5, and 6) (Conditions 7, 8, and 9).

3.3. Lateral Displacement of the Diaphragm Wall

The lateral displacement of the diaphragm wall under different working conditions was analyzed. As shown in Figure 5, the lateral displacement curve of the diaphragm wall generally exhibits a convex shape, with the maximum lateral displacement position near the excavation surface and diminishing displacement below it. Comparison of Conditions 1, 3, and 6 reveals that if the foundation floor is not cast in time, the diaphragm wall’s lateral displacement increases by 20%, with maximum lateral displacement shifting below the excavation surface.

Conditions 2, 3, and 4 show that when the foundation pit bottom plate is not poured, only increasing the thickness of the connecting wall has little effect on reducing the horizontal lateral displacement of the connecting wall.

Casting the foundation floor significantly reduces lateral displacement, though merely increasing the wall thickness has minimal impact on horizontal movement (Conditions 2, 6, and 9).

It can be seen that, when the bottom plate is not constructed, the spatial structure formed by the connecting walls, supports, columns, and soil of the pit resists the soil pressure on both sides. The lower elastic modulus of the soil compared to concrete cause greater displacement, resulting in lateral displacement increase at the excavation surface. After pouring the bottom plate concrete, the bottom plate replaces the soil to participate in the force, which can provide better horizontal force support, thereby reducing the lateral displacement of the connecting wall.

In summary, implementing a foundation floor effectively limits lateral displacement of the diaphragm wall, concentrating maximum lateral displacement near the excavation surface. Increasing only diaphragm wall thickness or floor thickness alone is less effective.

3.4. Building Settlement

The building settlement under different conditions was compared and analyzed. Figure 6 illustrates the settlement distribution, where settlement initially increases and then decreases with distance from the foundation pit, creating a concave shape. The settlement on the side of the building closer to the pit was larger than on the side farther from it, with the maximum settlement occurring approximately 17 m from the pit.

Examining Conditions 1, 3, and 6, we see that when the foundation floor is not cast in time, building settlement increases by 33%, with the maximum differential settlement reaching δ = 3.4 mm. Conditions 2, 3, and 4 show that without a foundation floor, merely increasing the diaphragm wall thickness has a negligible effect on reducing vertical settlement of the building. Comparing Conditions 2, 6, and 9, it is clear that adding a foundation floor is the most effective approach for reducing building settlement, with increased floor thickness as the next best measure.

These findings suggest that, for adjacent buildings, particularly those with mat foundations, timely construction of the foundation floor and increasing its thickness can significantly reduce settlement. However, increasing diaphragm wall thickness alone has a limited impact on minimizing building settlement. Monitoring adjacent building settlement during construction is essential, and if excessive settlement is observed, it is advisable to pause until stabilization is achieved before continuing.

3.5. Pit Bottom Uplift

The uplift values at the foundation pit bottom under different conditions were compared. As shown in Figure 7, under the constraint of the enclosure structure, uplift is minimal at the sides of the pit near the walls. The pit floor was divided by two support columns into three sections, with the maximum uplift observed in the middle areas of each section, while uplift decreased significantly near the columns, creating a three-peak “convex” distribution pattern. This pattern occurred because excavation releases the overburden stress on the soil above the excavation level, causing rebound in the soil below the pit, while the columns provide pull resistance to control uplift.

Simulations for Condition 1, based on actual construction parameters, show a maximum uplift value of Δ = 58 mm. Comparisons among Conditions 1, 3, and 6 indicate that when the foundation floor is not promptly cast after excavation, pit bottom uplift increases by 24%. Without casting the foundation floor, merely increasing the diaphragm wall thickness has a limited effect on reducing uplift (as shown in Conditions 2, 3, and 4). Due to the presence of buildings, the vertical stress of the soil on the right side of the foundation pit is greater, resulting in a larger uplift on the right side of the foundation pit.

Comparing Conditions 2, 6, and 9 reveals that constructing a foundation pit bottom plate has the best effect on reducing building settlement, followed by increasing the thickness of the bottom plate. The effect of increasing the thickness of the diaphragm wall is least significant.

3.6. Internal Forces on the Support Structure

Figure 8 shows the internal force distributions in the support structure under various conditions. Across Conditions 1 through 9, the entire support structure is subject to compressive forces, while the columns below the pit floor experience tensile forces. The axial pressure on the horizontal supports increases from top to bottom, with notable axial forces observed on the right side of the third support layer, the middle and left side of the fourth layer, and the middle and left side of the fifth layer. The overall support structure presents a convex shape, with displacement values larger in the center and smaller at the edges.

In Conditions 2, 3, and 4, compared to Condition 1 (which includes a foundation floor), increasing diaphragm wall thickness without a floor causes greater internal forces on the support structure, with a slight reduction in the tensile force on the columns. When the foundation floor is added, as in Condition 1, the maximum axial pressure on the support structure decreases by 30%, while the maximum axial tensile force on the columns increases by 230%. Additionally, comparing Conditions 1 and 6 shows that increasing foundation floor thickness has a limited effect on reducing axial forces in the support structure.

In summary, considering the factors of pit bottom uplift, surface settlement, building settlement, internal forces on the support structure, and economic costs, it is recommended to use a diaphragm wall thickness of 1.2 m (on the building side) and a foundation floor thickness of 0.5 m.

4. Analysis of On-Site Monitoring Results

4.1. Monitoring Point Arrangement and Monitoring Scheme

The monitoring points were arranged as follows:

(1) Surface settlement monitoring points: These were mainly distributed between the School Dormitory building and the foundation pit, with three transverse monitoring points on the left side of the dormitory and five transverse monitoring points on the left side of the pit. (2) Wall lateral displacement monitoring points: These were placed along the walls surrounding the foundation pit, with a spacing of 20 m and an inclinometer depth of 34.5 m. (3) Building settlement monitoring points: L-shaped settlement observation points were set at the four corners of the building, with three additional settlement points placed evenly along the long side of the building.

The monitoring range covered the main three-lane tunnel for a continuous length of 4469.287 m. The monitoring targets included surrounding roads, pipelines, and structures within three times the excavation depth. Settlement monitoring points were placed along both sides of the pit, with settlement monitoring cross-sections arranged approximately every 20 m along the longitudinal direction of the pit. Each cross-section included five monitoring points to effectively measure settlement around the pit. If obstacles such as buildings blocked access, the number of points was reduced, with the initial point located 2 m from the pit edge and subsequent points spaced at 2, 5, 8, 10, and 15 m.

The School Dormitory, a two-story brick–concrete structure close to the pit, is highly susceptible to deformation due to excavation. Therefore, settlement and lateral displacement of the surrounding ground and diaphragm walls were monitored to analyze the impact on the dormitory. Figure 9 provides the monitoring point layout. For analysis purposes, the foundation pit excavation process was divided into five construction stages based on the on-site conditions:

Stage 1: diaphragm wall construction and first excavation layer (5.5 m) and completion of the first layer of supports; Stage 2: second soil layer excavation (3 m) and completion of the second layer of supports; Stage 3: third soil layer excavation (5 m) and completion of the third layer of supports; Stage 4: fourth soil layer excavation (3.5 m) and completion of the fourth layer of supports; and Stage 5: fifth soil layer excavation (4.5 m) and foundation floor construction completed.

4.2. Analysis of Diaphragm Wall Lateral Displacement

During the foundation pit excavation, the deformation of the pit’s enclosure structure affected the surrounding environment. As shown in Figure 10, the maximum lateral displacement of the diaphragm wall generally increases with excavation depth. The third soil layer excavation shows a slight decrease in maximum lateral displacement. However, the displacement spikes significantly during the fourth and fifth layers. Maximum lateral displacement depth also increases with excavation depth. Under actual construction parameters, Condition 1 simulated a lateral displacement value of δ_m = 42 mm, exceeding the observed value of 35 mm.

As excavation progressed, the diaphragm wall’s lateral displacement towards the pit interior gradually increased, displaying a trend of larger displacement in the middle and smaller at the top and bottom. In the final excavation stage, the largest lateral displacement occurred near the pit bottom, with a maximum lateral displacement of 35 mm, approaching the control limit of 0.18% of the excavation depth he = 36 mm.

The inward deformation at the wall’s top was due to increased lateral pressure on the diaphragm wall as the pit was excavated in layers, coupled with increasing difference of water pressure and soil pressure across the wall. Horizontal supports installed after each excavation layer effectively controlled the diaphragm wall’s lateral displacement, though the fourth and fifth layers saw significant displacement due to the excessive thickness of these layers. It is crucial to install horizontal supports promptly after each layer excavation to prevent excessive diaphragm wall displacement. Once the foundation floor is constructed, the floor, horizontal supports, and diaphragm walls form a stable spatial structure, minimizing further lateral displacement of the diaphragm wall.

To mitigate excessive wall displacement during excavation, excavation depths per layer should be kept moderate, and supports should be installed promptly to prevent excessive lateral displacement caused by insufficient support.

4.3. Analysis of Surface Settlement

As shown in Figure 11, the surface settlement trends outside the pit are generally consistent across monitoring points. With increasing excavation depth, the cumulative settlement at surface monitoring points steadily rises, with settlement on the building side greater than that on the non-building side. At 2 m from the pit on the building side, Point 1 consistently showed a higher settlement value. At 5 m from the pit, Point 2 initially exhibited small settlement but fluctuated significantly as excavation progressed. For monitoring points on the non-building side, settlement initially increased and then decreased with distance from the pit; Points 2 and 3 showed larger settlement, while Points 4 and 5 demonstrated decreasing settlement with increasing distance from the pit.

The primary factor causing cumulative settlement increases at surface monitoring points was the excavation within the pit. The progressive removal of soil inside the pit, combined with active soil pressure outside the pit, causes lateral displacement of the diaphragm wall toward the pit, leading to settlement deformation. Additionally, as groundwater levels outside the pit dropped during excavation, the soil experienced reduced pore water pressure, promoting consolidation and increasing cumulative settlement with continued excavation.

Settlement values at monitoring points along both sides of the pit first increased then decreased with increasing distance. Soil near the diaphragm wall is less affected due to wall friction, while soil further from the wall experiences more significant settlement from the combined forces of active soil pressure and groundwater drawdown. Soil farthest from the pit has smaller settlement values due to lower active soil pressure and minimal groundwater decline. Therefore, surface settlement during excavation exhibits a “concave” settlement pattern.

4.4. Analysis of Building Settlement Deformation

The settlement data from ten monitoring points around the School Dormitory were analyzed. As shown in Figure 12a, the settlement curve for the four corner points of the dormitory indicates a general downward trend as excavation progresses, with the maximum absolute settlement reaching 50.19 mm and the minimum at 23.69 mm when excavation concludes. On the same transverse section, settlement trends are consistent, with corner points showing similar settlement values. Settlement is larger at points closer to the foundation pit and smaller at those farther from it. Initially, settlement is concentrated near the pit, but as excavation advances, maximum settlement shifts towards the building’s center, with the highest settlement eventually located at the central floor slab of the structure.

In the first working condition simulated according to the actual construction conditions, the maximum settlement value was Δ = 31 mm, slightly below the observed values. Field monitoring consistently shows that settlement near the foundation pit is greater than that at points farther away, resulting in the building tilting towards the pit. As excavation depth increases, settlement and its impact area expand outward, necessitating timely control of settlement levels during construction.

During the excavation of the fifth soil layer, all four corner points showed a sudden increase in settlement. As illustrated in Figure 12b, settlement changes along the transverse axis are consistent and nearly identical at each point. Differences in settlement are relatively stable after the third excavation layer, with short side settlements remaining uniform, while differences along the long side increase significantly after the third layer excavation. On the long side near the pit, the maximum differential settlement was calculated as Δ/L = 20.39 mm/15 m = 0.00136, while on the farthest long side, it was Δ/L = 13.17 mm/15 m = 0.00088. Comparisons with regulatory standards indicate that the average settlement and tilt of the building are within acceptable limits.

Analysis of vertical displacement monitoring data for the building indicates that during pit excavation, the building experiences five distinct stages: initial slow descent, steep descent, mid-term slow descent, late steep descent, and stable deformation. The reasons for these stages are shown in

Table 4. Five stages of building settlement.

Number	Stage	Condition
1	Initial Stage	In the absence of concrete supports, the soil surrounding the excavation exhibited inward displacement, resulting in minor subsidence.
2	Post-First Excavation Layer	Lack of immediate installation of the first concrete horizontal support led to a delayed settlement in the building; larger settlement occurred once support was installed.
3	Mid-Term Excavation	With soil removed on the outer side of the diaphragm wall, outward movement of the wall caused soil settlement, which led to further building settlement.
4	Late Excavation	The fourth soil layer was excavated at a greater thickness, causing intense building settlement.
5	Final Stabilization	The diaphragm wall, multiple horizontal supports, and the foundation floor formed an integrated structure that resisted inward pressure, creating a new equilibrium. After approximately 200 days, building settlement stabilized.

.

To safeguard adjacent buildings, measures such as reinforcing supports and timely casting of the foundation floor are essential based on the settlement data analysis, ensuring structural safety during excavation.

5. Discussion

This study, based on foundation pit excavation projects in water-rich soft soil areas of Suzhou, analyzed the impacts on adjacent buildings and the effectiveness of control measures through numerical simulations and on-site monitoring. The limitations and generalizability of the results are discussed as follows.

5.1. Limitations of Applicability

The deformation magnitude and patterns caused by foundation pit excavation can vary significantly depending on excavation sequences, methods, and building structural types. In this study, layered dewatering and excavation within the pit were employed. Variations in excavation methods or dewatering sequences may lead to results differing from those presented here. Moreover, the research focused on shallow brick–concrete structures; other building types may exhibit different deformation behaviors due to differences in foundation bearing mechanisms and overall structural rigidity, thus limiting the applicability of the findings. Furthermore, the study utilized the Mohr–Coulomb constitutive model to analyze short-term deformations caused by dewatering and excavation, without addressing long-term deformation behaviors. As a result, the conclusions are primarily applicable to deformation control during the construction phase under similar geological conditions and are not suited for evaluating settlement during building operation phases.

5.2. Generalizability of Structural Control Measures

The study numerically evaluated the effectiveness of support structures in controlling deformations. These results, being independent of specific geological conditions, have broad applicability. The primary cause of settlement in adjacent buildings during foundation pit excavation is soil plastic flow due to unloading. Timely casting of the concrete base slab can mitigate unloading rebound, form a spatially integrated structure with the support system, reduce structural deformation, and decrease settlement of surrounding buildings.

5.3. Future Research Directions

This study did not consider the impacts of dynamic construction loads, which can lead to greater deformation and compromise overall stability. Additionally, creep behavior of the soil during excavation was not accounted for, potentially underestimating settlement magnitudes. Future research should incorporate characteristics of construction-induced dynamic loads, geotechnical creep behavior, and diverse support structure parameter combinations to further evaluate the effects of foundation pit excavation on surrounding buildings.

6. Conclusions

Based on numerical analysis and on-site monitoring of the Chunshenhu Road Expressway Reconstruction Project foundation pit excavation under water-rich sandy soil conditions, the deformation behavior of adjacent structures was systematically investigated. The following conclusions were drawn.

6.1. Effectiveness of Foundation Floor Installation

(1)

Casting the foundation floor significantly reduces diaphragm wall lateral displacement by 27%, building settlement by 26%, and pit uplift by 30%.

(2)

Increasing the foundation floor thickness by 0.2 m further reduces building settlement by 6% and pit uplift by 8%, whereas increasing the diaphragm wall thickness alone has a negligible effect.

(3)

In practical engineering, increasing the foundation floor thickness is recommended over merely increasing wall thickness. Additionally, casting the foundation floor reduces the maximum axial forces in the support structure by 30%, demonstrating its effectiveness in optimizing structural performance.

6.2. Displacement Characteristics of the Diaphragm Wall

(1)

The lateral displacement of the diaphragm wall follows a depth-dependent trend, with displacement increasing to a maximum near the excavation surface and then gradually decreasing with depth.

(2)

Surface settlement exhibits a “concave” shape, initially increasing and then decreasing with distance from the pit. Greater excavation depths lead to higher settlement values.

6.3. Building Settlement Behavior

(1)

Vertical displacement monitoring revealed five distinct settlement phases during excavation: initial slow descent, steep descent, mid-term slow descent, late steep descent, and stable deformation.

(2)

Settlement is the dominant deformation, closely linked to the excavation process. Prompt casting of the foundation floor after excavation completion is recommended to slow and stabilize building settlement effectively.