Study on the Stability of Buildings During Excavation in Urban Core Areas

Abstract

Excavations in urban cores, in close proximity to existing structures, can significantly influence the structural loading. This study, based on a specific section of the foundation pit for the Yaoziqiu area road network project, constructs a mechanical analysis model to assess the impact of foundation pit construction on adjacent buildings. The research examines how various factors, including the distance between the building and the pit, pile length, retaining wall thickness, and the depth of the retaining wall’s embedment, affect the deformation response of the structures. The results indicate that when the building is 5 m away from the foundation pit, the maximum pile foundation settlement is 13.45 mm. When the distance is 30 m, the settlement value is 6.91 mm, with a decrease of 6.54 mm. The effect of foundation pit excavation on pile foundation settlement is significantly reduced when the pile length exceeds 15 m. When the thickness of the retaining wall increases from 0.5 m to 0.7 m, the maximum settlement of the building foundation is reduced by 1.34 mm, a decrease of 8.38%. When the thickness increases from 0.9 m to 1.1 m, the maximum settlement of the building foundation is reduced by 0.6 mm, a decrease of 4.17%. When the embedded depth of the retaining structure increases from 10 m to 15 m, the maximum settlement is reduced by only 1.05 mm. When the embedded depth is increased to 25 m, the change in the settlement value of the building foundation is within 0.2 mm. This study offers a detailed quantitative analysis of the factors influencing structural deformation, providing specific guidance for developing risk minimization strategies in planning excavations near existing structures.

1. Introduction

Excavation in the urban core presents challenges to the safety and stability of engineering projects, especially when conducted in close proximity to existing buildings. It may also affect the structural load-bearing behavior of nearby buildings. Therefore, it is crucial to study the impact of excavation on the structural forces of existing buildings. Scholars internationally have conducted extensive research in this area.

Burland and Worth [1] developed an evaluation framework for differential settlement, providing a methodological basis for assessing structural responses to uneven subsidence. Boone [2] and Namazi et al. [3] contributed to the quantification of construction impacts on buildings by developing a first-order damage assessment model and a method for predicting ultimate structural strain, respectively. Li et al. [4] derived a pile deformation model based on the principle of ground loss, incorporating soil damage effects and characterizing deformation patterns through monitored data. Bryson [5] investigated the effects of foundation pit excavation on surrounding structures, proposing a correlation model linking the development of retaining structure cracks to building settlement. Zhang et al. [6] established a base–soil coupling model to elucidate the mechanisms by which foundation pit construction affects building structures.

Li Zilin and Wang Tao [7] used the least squares method to fit the settlement data of buildings surrounding a construction site using the SMW (soil mixed wall) method in Tianjin. The settlement data obtained from the fitted curve equation showed a small error compared to the measured data. Ritter [8] combined excavation pore pressure relations, soil stratification models, and consolidation theory to evaluate the long-term impact of excavation on building deformation. Wang Hongyu [9] studied the shear strength values of the foundation near excavation sites and proposed using the average of the softened strength and residual strength to estimate the foundation bearing capacity.

Xu Jia [10] monitored key parameters such as settlement and surface subsidence of buildings surrounding the excavation, revealing the impact patterns of excavation activities. Liu Nianwu et al. [11] found significant differences between the settlement deformations of five-story buildings and surface soil subsidence. Liu Fengzhou et al. [12] evaluated the impact of diaphragm wall construction sequence on building settlement and proposed strategies for structural protection and control. He Chao and Liu Xiuzhen [13] used numerical simulation to analyze the impact of excavation on the deformation of adjacent buildings, finding that the settlement impact on pile foundation structures was minimal. Uribe-Henao et al. [14] evaluated the response of buildings near excavation sites, while Son [15] studied the potential impact of factors such as window opening on structural crack development.

Qin Huilai et al. [16] applied finite element analysis to study the impact of reinforcement areas within the excavation on deformation. Chen Kang [17] comprehensively considered the design parameters of underground continuous walls in deep excavation projects and assessed their combined effect on surrounding environmental deformation. Huang Zhipeng [18] used numerical simulation software to analyze the impact of building pile foundation factors on pile–anchor support structures. Zheng Gang and Li Zhiwei [19,20] analyzed deformation under different distances and angles between buildings and excavations. Wang Haoran et al. [21] developed a simplified calculation model to predict the maximum surface settlement. Huang Maosong et al. [22] proposed a displacement-controlled finite element analysis method. Li Jiayu and Chen Chen [23] pointed out that the foundation stiffness had little impact on the displacement field of the soil, whereas the foundation burial depth had the opposite effect, influencing both the excavation range and building deformation. Fan Fan et al. [24] proposed a simplified algorithm for neighboring building settlement based on assumptions and the pattern of settlement ratios. Zhang Zhiguo et al. [25] analyzed the interaction between shallow foundation buildings and excavation at different angles. Shi Youzhi et al. [26] simulated that the settlement deformation primarily occurs in the basement, with rapid changes in settlement within 1.5 times the excavation depth, and the main impact area within three times the excavation depth.

This study, based on a specific section of the foundation pit for the Yaoziqiu area road network project, employed a method combining field measurement data with numerical simulation to investigate the influence of pit excavation in the urban core area on the forces acting on the adjacent existing building structures. It explores the relationship between different factors and their effects on buildings. The findings provide theoretical support for evaluating the regularity of structural force changes in adjacent buildings during excavation in urban core areas. The findings are important for assessing the structural safety of buildings and for ensuring the safety and stability of nearby structures during excavation.

2. Engineering Context and Monitoring Protocol

Yaoziqu Subdistrict Road Network Project, Phase I, is located in the northwestern Yaoziqu area of the Jiangbei District, immediately adjacent to Chongqing’s Guanyinqiao commercial core. The project encompasses a 1.665 km ground-level road system comprising six streets: a secondary arterial (1.019 km) and five branch roads (0.646 km in aggregate).

The Yaoziqu-1 viaduct segment K0+396.242–K0+567.480 spans 121.238 m above grade. The associated cut-and-cover excavation reaches 25.446 m in depth and 15 m in width (see Figure 1 for plan view).

2.1. Engineering Geology and Hydrogeological Conditions

(1) Engineering geology

The surficial strata consist of 4–5.5 m of man-made fill overlying the Middle Jurassic Shaximiao Formation (J2s), a sequence of sandy mudstone. Groundwater occurs as pore water within the loose overburden and as fissure water in the underlying bedrock.

(2) Hydrogeology

Piezometric levels in the central sector of the site range from 228.0 to 229.5 m above datum, indicating a deep water table. Located in an urbanized area, the plot is devoid of perennial surface runoff; no fluvial channels or standing water were observed.

2.2. Engineering Monitoring Scheme

Within the zone influenced by the excavation, both sides of the site have been designated as demolition areas. A five-story masonry-concrete structure is located approximately 4.8 m from the edge of the excavation and is aligned parallel to the construction boundary line. The building is supported by a pile foundation, selected for its superior deformation compatibility compared to stiffer foundation types such as raft or box foundations.

Excavation and its influence on the surrounding environment constitute a highly dynamic process. To ensure safe and efficient construction, real-time monitoring is essential for tracking structural and geotechnical responses. Such monitoring enables the timely identification of deformation trends and potential hazards, thereby facilitating the implementation of appropriate mitigation measures.

The monitoring scheme is tailored to the specific engineering context and includes the retaining structures, surrounding soil mass, and adjacent buildings. The design of the monitoring plan considers factors such as the nature and significance of the monitored objects, their proximity to the excavation, and the requirements of both structural design and construction activities. A Topcon total station (Model: GM-52) was used to monitor the deformation of the foundation pit, the surrounding soil, and the adjacent existing buildings. The locations of all monitoring points are shown in Figure 2.

The monitoring frequency was determined with reference to the relevant design drawings and the Technical Code for Monitoring in Urban Rail Transit Engineering (GB 50911-2013) [27]. The detailed monitoring schedule is presented in

Table 1. Monitoring frequency table.

Construction Stage		Monitoring Frequency
Excavation Depth	≤5 m	1 time/3 d
	5~10 m	1 time/2 d
	10~15 m	1 time/2 d
	15~20 m	1 time/1 d
	>20 m	1 time/1 d

. Based on ongoing monitoring data and site conditions, the monitoring frequency can be adjusted dynamically. This adaptive approach allows for timely modifications to the existing construction plan, aiming to minimize unnecessary losses during the actual construction process.

2.3. Settlement Data and Analysis

The measured settlement data of the building were compiled, and the settlement values at each monitoring point were plotted as a settlement profile, as shown in Figure 3.

Analysis of Figure 3 reveals the following:

(1) Building Inclination: From the figure, it is evident that all four monitoring points show negative values, and these values progressively increase as the excavation of the foundation pit continues.

(2) Due to the close proximity of the building foundation to the excavation area and the significant lateral unloading it endures, the settlement of the building on the side adjacent to the foundation pit is relatively large. The two curves with the highest settlement values correspond to Monitoring Points 1 and 2, both located near the excavation site. At the final stage of excavation, the maximum settlement values for these points were 17.08 mm and 15.04 mm, respectively. In contrast, the settlement values at Points 3 and 4, located further from the excavation, are much smaller, with maximum values of 8.65 mm and 7.29 mm, respectively. This indicates that the settlement decreases with increasing distance from the excavation site.

3. Numerical Computation Model

3.1. Constitutive Model and Parameter Selection

With the continuous development of geomechanics theory, a variety of constitutive relationships suitable for soil properties have been established successively. During the numerical simulation process, the appropriate selection of constitutive models is the main factor in ensuring the accuracy of the simulation results. MidasGTS/NX (2022) software integrates a rich library of constitutive models. Therefore, this paper uses MIDAS/GTS (2022) to establish a numerical model for the calculation and analysis of the deformation influence law of tunnel construction on adjacent multi—storey frame buildings. In the model, all soil layers were represented using the Modified Mohr–Coulomb model, and the physical and mechanical parameters of each layer are provided in

Table 2. Values of parameters of each rock and soil layer in numerical modeling.

No.	Soil Layer	$\mathit{\gamma }$ (kN/m3)	$\mathit{c}$/kPa	$\mathit{\phi }$/(°)	$\mathit{E}$/MPa	$\mathit{\nu }$	Layer Thickness/m
1	Miscellaneous Fill Soil	17.5	22	25	35	0.3	5
2	Silt	19.1	4	31	48	0.23	5
3	Clayey Sand	19.1	22	26	35	0.33	4
4	Sandy Mudstone	19.6	44	21	37	0.30	6
5	Sandy Mudstone	19.3	42	22	42	0.35	20

.

The physical parameters of each soil layer were selected based on the Geotechnical Report of the Yaoziqiu Area Road Network Engineering Phase I. The values for some parameters are explained as follows:

The actual elastic modulus of the soil is greater than the compression modulus due to its structural and anisotropic characteristics. Based on empirical data, E = (2~5)·Es, and for this study, E = 5 E_S was adopted. The compression modulus is provided in the geotechnical report’s soil test results.

The relationships between the bulk modulus K, shear modulus G, elastic modulus E, and Poisson’s ratio λ are as follows:

$$K=\frac{E}{3(1-2\lambda )}$$

(1)

$$G=\frac{E}{2(1+\lambda )}$$

(2)

The cohesion, internal friction angle, and other parameters of the soil layers were taken from the relevant values in the Geotechnical Report. The tensile strength T of the soil was assumed to be zero, while the tensile strength of the rock was taken as 1/8 to 1/12 of its compressive strength.

In the simulation, the diaphragm wall was modeled using solid elements to capture its overall rigidity. The horizontal supports and building beams were modeled using beam elements to reflect their bending behavior under load. The neutral column, vertical piles, and building piles were each modeled using pile elements, with an elastic constitutive model applied to all components. The specific parameters for the diaphragm wall and adjacent building calculations are provided in

Table 3. Parameter values of supporting structures and buildings in numerical modeling.

No.	Name	Material	Dimensions/mm	E/GPa
1	Diaphragm Wall	Concrete	1000 (Thickness)	30
2	First Horizontal Support	Concrete	800 × 800	30
3	First Horizontal Support Connecting Beam	Concrete	400 × 800	30
4	Second Horizontal Support	Steel	$\Phi$ 690 × 8	200
5	Second Horizontal Support Connecting Beam	Steel	630 × 910 closed-box steel girder	200
6	Neutral Column	Steel	550 × 550 built-up (latticed) steel column	200
7	Vertical Pile	Concrete	1000 (diameter)	30
8	Floor Slab	Concrete	150 (thickness)	30
9	Building Column	Concrete	300 × 300	30
10	Building Pile	Concrete	800 (diameter)	30
11	Building Beam	Concrete	300 × 300	30

.

3.2. Establishment of the Numerical Model

To investigate the influence of excavation on the settlement of adjacent buildings, a five-story building numerical model was established based on this project.

The model extends from the edge of the excavation to 3–5 times the excavation depth. Given the presence of a building on one side of the excavation, the boundary on the building side was appropriately expanded. The model’s left boundary is 11.0 m from the excavation edge, and the right boundary is 26.0 m away. The excavation width is 15 m, and the total model length in the x-direction is 100.0 m. The sandy mudstone in the area is shallow (approximately 20 m below ground), with high strength and good integrity, so a model depth of 40 m was sufficient for the calculation requirements. The final model dimensions are 100.0 m × 50.0 m × 40.0 m (length × width × height).

The field building is a five-story structure, with a length of 19 m, a width of 10 m, parallel to the construction boundary, and located 11 m from the edge of the excavation. The finalized numerical model is shown in Figure 4. The model boundaries are constrained for normal deformation, and the bottom of the model is constrained in all three directions. No constraints are applied to the top of the model.

3.3. Numerical Computation Process

In the Midas GTS NX software platform, the numerical simulation of construction processes is primarily achieved through the “Activation” (Excavate) and “Deactivation” (install) commands. Therefore, it is necessary to establish the entire numerical model, geotechnical layers, boundary conditions, and self-weight loads in accordance with the construction sequence, followed by conducting a construction step analysis. The numerical calculation process is as follows:

(1)

Import the geotechnical layer model and perform a geostatic stress equilibrium analysis.

(2)

Import the building model and perform the geostatic stress equilibrium analysis again.

(3)

The displacements were reset to zero. Subsequently, the diaphragm wall, neutral columns, and vertical piles were installed in the model.

(4)

Excavate the first layer (0 to −5 m) and install the first horizontal support.

(5)

Excavate the second layer (−5 to −10 m) and install the second horizontal support.

(6)

Excavate to the foundation level (−10 to −25 m).

3.4. Displacement Results of Adjacent Buildings to the Excavation

Settlement of the Building

In this project, considering that the building adopts a frame structural system, it is highly sensitive to uneven settlement. Excessive differential settlement may lead to serious issues such as structural cracking. Therefore, controlling building settlement became a key consideration during both design and construction. Specifically, the relative position of the building to the excavation plays a decisive role in determining the direction and focus of the settlement study. During the numerical simulation process, dynamic monitoring of the settlement at the four corner points of the adjacent building was conducted to assess the structural stability. The specific arrangement of the monitoring points is detailed in Figure 5.

To analyze the variation patterns more clearly and intuitively, the settlement data from the actual monitoring points 1, 2, 3, and 4 of the building were selected. Settlement curves under different excavation conditions were plotted, as shown in Figure 6.

As shown in Figure 6, with the progression of excavation, the maximum settlement of the building occurs near Monitoring Points 1 and 2, which are located close to the excavation side. The maximum settlement value is −16.29 mm, while the settlement displacement at other monitoring points increases progressively with the increasing excavation depth.

Data analysis was performed on the four monitoring points of the studied building. A comparison between the actual accumulated deformation and the simulation results is presented in Figure 7.

From the comparison of the curves, it can be seen that due to variable site conditions, weather, and construction tolerances that disturb the soil, a small deviation exists between the measured and simulated deformation. Nevertheless, the measured data and the simulated model’s predicted values show a high degree of consistency. After the completion of the excavation, the differences between the measured and simulated settlement values at Monitoring Points 1 to 4 are 0.79 mm, 0.98 mm, 0.85 mm, and 1.16 mm, respectively. These results indicate that the simulated data can effectively reflect the actual displacement of the building. Furthermore, this validates the applicability of the model, suggesting that simulation results can be used to assess the potential impact of excavation on building deformation, thereby guiding construction activities.

4. Study on the Factors Affecting the Deformation of Buildings Adjacent to Excavations

To investigate the interaction mechanism between excavation and adjacent frame-structured buildings, and to minimize the impact of excavation on neighboring buildings, key factors influencing foundation deformation are further analyzed based on the previously discussed engineering context. The same modeling approach and material parameters as in the previous chapter were adopted. By analyzing factors such as the distance between the building and the excavation, pile length, thickness of the retaining wall, and the insertion depth of the retaining wall, the settlement deformation behavior of the building foundation was studied.

4.1. Layout Plan of Influencing Factors

To investigate the deformation behavior of adjacent existing buildings during excavation under different influencing factors, the study focuses on the parameters of distance between the building and the excavation, pile length, retaining wall thickness, and the insertion depth of the retaining wall. The deformation behavior of the pile foundation is analyzed based on these factors. The specific parameters for each scenario are provided in

Table 4. Specific parameters of each working condition of the influencing factors.

Influencing Factors	Detailed Parameters for Each Condition
Distance Between the Building and the Excavation	5 m, 10 m, 15 m, 20 m, 25 m, and 30 m
Pile Length	5 m, 10 m, 15 m, 20 m, 25 m, and 30 m
Retaining Wall Thickness	0.5 m, 0.7 m, 0.9 m, and 1.1 m
Insertion Depth of the Retaining Wall	10 m, 15 m, 20 m, and 25 m

.

4.2. Study of the Influence of Structural Parameters on Building Deformation

4.2.1. Distance Between the Building and the Excavation

This section mainly investigates the impact of the distance between the building and the excavation on the settlement of the pile foundation. By considering the excavation depth and the primary and secondary influence zones of excavation on the soil, six distances—5 m, 10 m, 15 m, 20 m, 25 m, and 30 m—are selected to evaluate their specific impact on pile foundation settlement.

Figure 8 shows the settlement curves of the foundation at distances of 5 m, 10 m, 15 m, 20 m, 25 m, and 30 m from the excavation. When the building is located closer to the excavation, its stiffness to resist deformation decreases, resulting in more significant deformation caused by excavation. The settlement of the pile foundation also increases. At a distance of 5 m from the excavation, the maximum settlement of the pile foundation is 13.45 mm. As the distance between the building and excavation increases from 10 m to 20 m, the maximum settlement of the pile foundation decreases by 2.09 mm. From 20 m to 30 m, the decrease in settlement is 3.10 mm. These results clearly indicate that distance is a significant factor affecting the deformation of building foundations during excavation. Therefore, when excavating near existing buildings, it is recommended to maintain a safe distance to minimize the adverse impacts on the building.

4.2.2. Pile Length

This section investigates the impact of excavation on the deformation of the building foundation under different pile lengths. The selected pile lengths are 5 m, 10 m, 15 m, 20 m, 25 m, and 30 m.

Figure 9 shows the impact of excavation on pile settlement for different pile lengths. It can be observed that the pile settlement is generally smaller on the side of the pile foundation farther from the excavation, while the settlement is larger on the side closer to the excavation. When the pile length increases from 5 m to 15 m, the settlement decreases more rapidly. The maximum settlement values for the piles are −13.56 mm, −11.63 mm, and −6.44 mm, respectively. As the pile length increases from 15 m to 20 m, the maximum pile settlement decreases from −6.77 mm to −6.54 mm, a reduction of only 0.23 mm, or 3.40%. With further increase in pile length, the settlement of the pile foundation slightly increases, indicating that there is an optimal pile length for minimizing settlement. Beyond this optimal length, increasing the pile length further has a limited effect on reducing settlement.

4.2.3. Diaphragm Wall Thickness

This section primarily investigates the impact of excavation on building foundation deformation when the thickness of the diaphragm wall is 0.5 m, 0.7 m, 0.9 m, and 1.1 m.

Figure 10 illustrates the impact of diaphragm wall thickness on building foundation settlement. From the figure, it can be seen that changes in diaphragm wall thickness significantly affect the settlement of the building foundation. Increasing the thickness of the diaphragm wall can notably reduce the settlement of the building. When the diaphragm wall thickness increases from 0.5 m to 0.7 m, the maximum settlement of the building foundation decreases by 1.34 mm, a reduction of 8.38%. When the diaphragm wall thickness increases from 0.9 m to 1.1 m, the rate of settlement reduction slows down. The maximum foundation settlement decreases by 0.6 mm, or 4.17%.

4.2.4. Diaphragm Wall Depth

This section primarily investigates the impact of excavation on building foundation deformation when the depth of the diaphragm wall is 10 m, 15 m, 20 m, and 25 m.

Figure 11 illustrates the effect of diaphragm wall depth on building foundation settlement during excavation. When the wall depth increases from 10 m to 15 m, the overall settlement of the foundation decreases, but the reduction is small, with the maximum settlement value only decreasing by 1.05 mm. Further increasing the wall depth to 25 m results in minimal change in foundation settlement, with a variation of only 0.2 mm. Continuously increasing the diaphragm wall depth as a strategy to control building settlement is neither efficient nor economical, as it leads to significant cost increases with minimal benefits.

It can thus be concluded that increasing the embedment depth of the retaining wall has a limited effect on reducing the settlement of adjacent buildings. This is because there exists a critical threshold for the restraining effect of the retaining wall’s embedment depth on excavation lateral displacement: when the embedment depth fails to reach this threshold, increasing the depth can significantly reduce wall toe deformation and pit bottom heave, thereby decreasing the additional stress on adjacent building foundations; however, once the embedment depth exceeds the critical threshold, the overall stiffness and lateral displacement resistance of the wall are already sufficient to balance the earth pressure around the pit. Further increasing the depth only increases the consumption of wall materials, but cannot provide additional restraint on the shear deformation and consolidation settlement of the soil surrounding the pit. At this point, the dominant factor contributing to the settlement of building foundations shifts from “insufficient performance of the retaining wall” to “self-compression of the soil surrounding the pit,” and as a result, the influence graph exhibits the characteristic that “the settlement curve tends to flatten out once the depth increases to a certain extent.”

5. Conclusions and Outlook

5.1. Conclusions

This study primarily analyzes the structural and excavation support parameters affecting the foundation deformation of adjacent existing frame buildings during excavation. Based on this analysis, the construction plan for the background project was optimized to control the foundation deformation of the building. The main conclusions are as follows:

(1) Distance to Excavation: When the building is 5 m from the excavation, the maximum settlement is 13.45 mm. Settlement decreases as this distance increases. The settlement reduces by 2.09 mm as the distance increases from 10 m to 20 m, and by 3.10 mm when the distance increases from 20 m to 30 m. These results clearly indicate that the distance is a significant factor influencing the foundation deformation during excavation. Therefore, when excavating near existing buildings, it is recommended to maintain a safe distance to minimize adverse impacts on the building.

(2) Pile Length: There exists an optimal pile length. When the pile length increases from 5 m to 15 m, the settlement of the pile foundation decreases rapidly, with maximum settlement values of −13.56 mm, −11.63 mm, and −6.44 mm, respectively. Beyond 15 m, the effect of excavation on pile settlement diminishes significantly. When the pile length increases from 15 m to 20 m, the maximum settlement decreases from −6.77 mm to −6.54 mm, a reduction of only 0.23 mm, or 3.40%.

(3) Diaphragm Wall Thickness: When the diaphragm wall thickness increases from 0.5 m to 0.7 m, the maximum foundation settlement decreases by 1.34 mm, or 8.38%. As the diaphragm wall thickness increases from 0.9 m to 1.1 m, the rate of settlement reduction slows down, with the maximum settlement decreasing by 0.6 mm, or 4.17%. The insertion depth of the retaining wall has a minimal effect on settlement. When the insertion depth increases from 10 m to 15 m, the overall settlement decreases, but the reduction is small, with the maximum settlement decreasing by only 1.05 mm. Further increasing the insertion depth to 25 m results in an almost negligible change in foundation settlement, with a variation of only 0.2 mm. The depth of the diaphragm wall has a very weak influence on foundation deformation, and continuing to increase the insertion depth has an insignificant effect on the deformation.

5.2. Outlook

Despite the achievements obtained in the selection of deep foundation pit support schemes and the impact on the deformation of adjacent buildings, there are still some deficiencies in this study. Future research can be further expanded in the following aspects:

(1) This study mainly analyzes the impact of single factors on building deformation. However, in practical engineering, multiple factors often act simultaneously. Future research can be conducted on the impact of deep foundation pit excavation under the coupling of multiple factors, such as the comprehensive impact of groundwater level changes, construction loads, and surrounding traffic loads on the foundation pit and adjacent buildings.

(2) The study is mainly based on the geological conditions in Chongqing. The applicability to other complex geological conditions (such as soft soil, karst, and fill soil) is insufficient. Future research can be carried out under different geological conditions to further improve the selection and optimization methods of foundation pit support schemes.