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Article

The Field Monitoring and Numerical Simulation of Spatiotemporal Effects During Deep Excavation in Mucky Soft Soil: A Case Study

by
Qiang Wu
1,
Jianxiu Wang
2,3,*,
Yanxia Long
2,*,
Xuezeng Liu
2,4,
Guanhong Long
2,
Shuang Ding
4,
Li Zhou
1,
Huboqiang Li
2 and
Muhammad Akmal Hakim bin Hishammuddin
2
1
Suzhou Metro Line 1 Co., Ltd., Suzhou 215300, China
2
College of Civil Engineering, Tongji University, Shanghai 200092, China
3
Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education, Tongji University, Shanghai 200092, China
4
Shanghai Tongyan Civil Engineering Technology Co., Ltd., Shanghai 200092, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(4), 1992; https://doi.org/10.3390/app15041992
Submission received: 5 January 2025 / Revised: 9 February 2025 / Accepted: 11 February 2025 / Published: 14 February 2025
(This article belongs to the Special Issue Geo-Environmental Problems Caused by Underground Construction, 2nd Edition)
Browse Figures
Figure 1
<p>Soil profile along the standard section of Huaxi Park Station.</p> "> Figure 2
<p>Layout of monitoring points of Huaxi Park Station foundation pit. (CX is the monitoring point number of diaphragm wall horizontal displacement; DB is the monitoring point number of surface subsidence).</p> "> Figure 3
<p>A 3D numerical model of the Huaxi Park Station foundation pit.</p> "> Figure 4
<p>Structure of CVISC model.</p> "> Figure 5
<p>Variation rule of maximum horizontal displacement of enclosure wall: (<b>a</b>) maximum horizontal displacement of west end well wall; (<b>b</b>) maximum horizontal displacement of standard section wall. RW means retaining wall.</p> "> Figure 6
<p>Surface settlement outside the pit: (<b>a</b>) surface settlement outside the west end head pit; (<b>b</b>) surface settlement outside the standard section pit. D is the distance from the pit. Notes−excavation step sequence: 1−arrangement of the first steel support; 2−arrangement of the second steel support; 3−arrangement of the third steel support; 4−arrangement of the fourth steel support; 5−excavation completed.</p> "> Figure 7
<p>The relationship between the surface settlement outside the pit and the maximum horizontal displacement of the retaining wall: (<b>a</b>) the west end well; (<b>b</b>) the standard section.</p> "> Figure 8
<p>Pore water pressure contour with different construction steps: (<b>a</b>) before excavation; (<b>b</b>) excavation of third floors; (<b>c</b>) final excavation.</p> "> Figure 8 Cont.
<p>Pore water pressure contour with different construction steps: (<b>a</b>) before excavation; (<b>b</b>) excavation of third floors; (<b>c</b>) final excavation.</p> "> Figure 9
<p>Horizontal displacement of diaphragm wall of Huaxi Park Station foundation pit.</p> "> Figure 10
<p>Horizontal displacement of diaphragm wall (CX5): (<b>a</b>) horizontal displacement versus depth curve; (<b>b</b>) variation curve of maximum horizontal displacement with construction sequence. (Construction sequence: 1—arrangement of the first steel support; 2—arrangement of the second steel support; 3—arrangement of the third steel support; 4—arrangement of the fourth steel support; 5—arrangement of the fifth steel support; 6—completion of the footing pouring.)</p> "> Figure 11
<p>Vertical displacement after the completion of excavation: (<b>a</b>) surface settlement outside the pit; (<b>b</b>) vertical displacement.</p> "> Figure 12
<p>Surface settlement outside the pit after the completion of excavation: (<b>a</b>) DB4; (<b>b</b>) DB6; (<b>c</b>) DB1.</p> "> Figure 13
<p>Change in the axial force of the first concrete support.</p> "> Figure 14
<p>Change in the axial force of the second to fifth supports.</p> ">
Versions Notes

Abstract

:
The issue of geotechnical hazards induced by excavation in soft soil areas has become increasingly prominent. However, the retaining structure and surface settlement deformation induced by the creep of soft soil and spatial effect of the excavation sequence are not fully considered where only elastic–plastic deformation is used in design. To understand the spatiotemporal effects of excavation-induced deformation in soft soil pits, a case study was performed with the Huaxi Park Station of the Suzhou Metro Line S1, Jiangsu Province, China, as an example. Field monitoring was conducted, and a three-dimensional numerical model was developed, taking into account the creep characteristics of mucky clay and spatiotemporal response of retaining structures induced by excavations. The spatiotemporal effects in retaining structures and ground settlement during excavation processes were analyzed. The results show that as the excavation depth increased, the horizontal displacement of the diaphragm walls increased linearly and tended to exhibit abrupt changes when approaching the bottom of the pit. The maximum horizontal displacement of the wall at the west end well was close to 70 mm, and the maximum displacement of the wall at the standard section reached approximately 80 mm. The ground settlement on both pit sides showed a “trough” distribution pattern, peaking at about 12 m from the pit edge, with a settlement rate of −1.9 mm/m per meter of excavation depth. The excavation process directly led to the lateral deformation of the diaphragm walls, resulting in ground settlement, which prominently reflected the time-dependent deformation characteristics of mucky soft soil during the excavation process. These findings provide critical insights for similar deep excavation projects in mucky soft soil, particularly regarding excavation-induced deformations, by providing guidance on design standards and monitoring strategies for similar geological conditions.

1. Introduction

With the continuous development of urban underground space construction in China, particularly in areas with high-water-content soft soils, there has been an increasing number of engineering cases involving deep foundation pits [1,2,3,4]; e.g., approximately 600 deep excavations were reported in the last 15 years in Shanghai [5]. However, the challenging geotechnical conditions of soft soils pose serious risks to excavation stability. These soft soils, characterized by high water content, high compressibility, and low permeability, exhibit pronounced creep behavior due to their elastic viscoplastic nature. The creep characteristics of soft soil can cause considerable deformation in foundation pit retaining structures and ground settlement, gradually increasing over time [6,7,8,9,10]. If supports are not installed in time or construction is delayed for too long a period, it is highly likely to result in foundation pit engineering accidents [11,12]: for example, the collapses of the Nicoll highway excavation for the metro circle line in Singapore (2004) [13] and the Xianghu excavation for a subway station in Hangzhou (2008) [14], and excavation instability at Jinxiu Four Seasons Garden in Zhuhai (2011) [15]. Deep excavation in soft soils exhibits significant spatiotemporal effects [16,17,18], increasing the risk of geotechnical hazards in the surrounding environment [19,20,21]. Therefore, ensuring the safety and stability of deep excavations in soft soil and preventing excessive deformation that may lead to the settlement and cracking of adjacent environments and facilities have become a pressing and critical research focus in engineering.
Numerous scholars have conducted extensive research on the spatiotemporal evolution of deformation in deep excavation in soft soil. Based on the three-dimensional failure mode of pit walls, Liu [22] and Yang [23] proposed formulas for the length of the influence area considering spatial effects and calculations for earth pressure. Lei et al. [24] introduced the concept of the equivalent internal friction angle, treating the soil layers around the pit as non-cohesive homogeneous bodies, and using the theory of plastic upper bounds along with the associated flow rule, provided formulas for calculating spatial effect coefficient. Wu [25] performed numerical simulations to analyze the horizontal deformation of retaining structures at different positions and directions during pit excavation; Wang et al. [26] and Guo et al. [27] initially obtained parameters for the soft soil creep model through triaxial rheological tests and then used numerical simulations to analyze the impact of soil creep on the deformation of retaining structures and the surrounding environment. Zhuang et al. [28] integrated existing creep test curves for soft soils in the Shanghai area to fit the calculation parameters for the soil creep equation and then used the finite element method to analyze the creep deformation and creep strain of soil slopes during the construction process. Xie [29] performed numerical simulations through the secondary development of the viscoelastic–plastic constitutive model to analyze the time-dependent deformation pattern of deep pit retaining structures and the safety of deep pits during a construction process involving timely support and over-excavation. In research on pit deformation in mucky strata, Qin et al. [30] discussed the deformation characteristics of pits in deep mucky strata and systematically analyzed the influence and patterns of factors such as the reinforcement range within the pit, the embedding depth of support piles, and the stiffness of support piles on pit deformation. Lin et al. [31] studied the spatiotemporal evolution pattern of pit retaining structure deformation, ground settlement, and the internal forces of supports in a pit engineering project of Shenzhen Metro Line 11 under marine mucky stratum conditions. Although these studies provide many bases for revealing the spatiotemporal effects of soft soil pit deformation, they mostly focus solely on the spatial or temporal effects of soft soil pit excavation without considering the interaction between the temporal and spatial effects, and there is seldom research on the spatiotemporal effects of pit engineering deformation in mucky soft soil areas considering seepage conditions.
Mucky clay, a typical type of soft soil, has a higher water content, stronger rheological properties, lower permeability, and obvious time-dependent characteristics. These unique properties lead to complex issues related to the spatiotemporal effects of deep excavations. In the subway stations of Suzhou Metro Line S1, unevenly thick flow plastic mucky clay is distributed. The uneven thickness of the flow plastic mucky soils may cause non-uniform settlement during excavation. The time-dependent nature of creep deformation means that it does not occur instantaneously but rather accumulates over time, potentially intensifying during various construction stages, which continuously threatens the stability of the retaining structures and excavations. Therefore, time-dependent deformation characteristics should be carefully considered during excavation, particularly in deep excavation projects in soft soils and high-water-level areas. Given the time-dependent nature of creep deformation, precise control is crucial, as the deformation caused by these construction stages can pose serious risks to the stability of the excavation and supporting structures [32,33].
In previous studies, the retaining structure and surface settlement deformation induced by the creep of soft soil and spatial effect of the excavation sequence were not fully considered. With the Huaxi Park Station pit project of the Suzhou Metro Line S1 as a background, a three-dimensional numerical model was established where the creep characteristics of mucky layers and the spatiotemporal deformation induced by excavation under seepage conditions were considered. The spatiotemporal effects of mucky soft soil, including retaining structure deformation, surface settlement, and the internal forces of supports, during excavation could be fully considered. The CVISC rheological model was capable of simulating the time-dependent characteristics of mucky soils and seepage effects to help understand and predict the behavior of deep excavations in soft soils. This case study can be used as a reference for similar engineering projects.

2. Materials and Methods

2.1. Project Profile

The Huaxi Park Station of Suzhou Metro Line S1 is located in Huaqiao Town, Kushan City, Suzhou, China, extending from east to west. The standard section width is 28.7 m, with the pit excavation depth ranging from 16.5 to 16.9 m, and the pit dimensions are 151.6 m × 28.7 m. The station structure consists of two underground levels in an island-style layout, constructed using the open cut method. According to regional geological data, the strata within the excavation depth of the station include miscellaneous fill soil, plain fill soil, silty clay, and mucky silty clay. The mucky silty clay layer in the station area is relatively thick, with a thickness of approximately 19 m. The mucky silty clay is soft and exhibits a flow−plastic state. The sensitivity of this soil is about 4, and the characteristic foundation-bearing capacity is approximately 60 kPa. The water content is about 43.9%, the liquid limit index IL has an average value of 1.38, and the average compression coefficient is 0.74. The mucky silty clay layer exhibits characteristics of a high water content, low strength, high sensitivity, and high compressibility. The groundwater-level burial depth generally ranges from 1.00 to 2.10 m.
The excavation depth of the pit was large, the groundwater level was high, and the geological conditions were complex. The diaphragm wall had high stiffness and excellent impermeability characteristics, which can effectively control pit deformation, ground settlement, and cut off groundwater. The diaphragm wall and inner supports were selected as the foundation pit retaining structure and waterproof curtain. The standard excavation depth of the pit was 16.5–16.9 m, and the excavation depth of the end shaft was 18.4 m. A diaphragm wall with a thickness of 800 mm was adopted, whose insertion ratio was 1.16–1.48 and depth was 38.5–41.5 m, in accordance with the requirements of the Technical Specification for Retaining and Protection of Building Foundation Excavations (JGJ120-2012). To limit lateral deformation, five layers of supports were constructed, as described in Figure 1. The first and third supports were reinforced concrete with grade C35, while the others were steel supports. The cross-section of the first reinforced concrete support is 800 mm × 1000 mm, while the cross-section of the third reinforced concrete support is 1000 mm × 1000 mm. The fourth and fifth steel supports are φ800 mm (t = 20 mm) steel pipe supports, while all other supports are φ609 mm (t = 16 mm) steel pipe supports. The foundation employs φ850 @600 triple-axis mixing piles for soil reinforcement.

2.2. Monitoring Arrangement

The deformation monitoring data during the whole process of the excavation of the Huaxi Park Station pit were analyzed to understand the deformation temporal characteristics of mucky soil pit engineering in the construction process, so as to provide theoretical support for verifying the correctness of the 3D numerical model and for analyzing the deformation characteristics of mucky soil.
The plan layout of the monitoring points is shown in Figure 2. Monitoring was conducted with a frequency of once per day during all excavation stages. Six main sections were set up along the longitudinal direction of the foundation pit. The top of the diaphragm wall and the deep horizontal displacement monitoring points were set up, and surface settlement monitoring points were set up in the range of 2 times the depth of the excavation of the foundation pit, and the surface settlement monitoring points were gradually thinned out from the foundation pit to the outside. Typical monitoring sections were selected to analyze the horizontal displacement of the diaphragm wall and the surface settlement outside the pit in the end well on one side and the standard section of the pit, respectively. The specific monitoring instruments include the following: (1) the surface settlement (DB1-DB16) was investigated using the Leica LS15 Laser Scanner; (2) the diaphragm wall’s lateral displacement (CX1-CX16) was monitored using inclinometers; and (3) the support axial force was investigated using a portable frequency reader. All of these instruments were examined during the excavation to reduce measurement errors.
According to the foundation pit design requirements and the Technical Specification for Retaining and Protection of Building Foundation Excavations (JGJ120-2012), for station foundation pits with depths exceeding 15 m, the failure of the support structure or excessive soil deformation can severely impact the surrounding environment and the safety of the main structure construction. Therefore, the Huaxi Park Station foundation pit was classified as Grade I in terms of safety level.
Considering the surrounding environmental conditions, the environmental protection level of the main foundation pit was classified as Grade II. Control measures were implemented based on the following criteria: the maximum surface settlement was 0.2%H, and the maximum allowable horizontal displacement of the retaining wall was 0.3%H, where H was the excavation depth.

2.3. Three-Dimensional Numerical Simulation

2.3.1. Numerical Model

The software FLAC3D 6.0 was used to establish the model based on the engineering information. Considering the influence range of the excavation, the boundaries of the peripheral soil body and the depth boundary were set to no less than five times the depth of the pit excavation [34,35]. The final size of the model was determined to be 333 m × 180 m × 70 m (L × W × H), containing 1,033,950 nodes and 1,001,280 grid cells. The 3D modeling included the soil around the pit, the retaining structures, and the support structures (crown beam, concrete supports, steel supports), as shown in Figure 3. The model was divided vertically into ten layers, and the excavation was completed in seven stages along the longitudinal direction of the pit.
The model mesh was denser inside the pit horizontally, gradually diminishing from the pit outward, and uniformly distributed vertically. Displacement boundary conditions and fluid boundary conditions were set in the model; displacement boundary conditions restricted the horizontal and vertical displacement at the bottom and around the model, while the top of the model was the free deformation boundary. The seepage boundary conditions restricted the bottom of the model so that it was the impermeable boundary. The pore water pressure was set to 0 from the surface of the model to more than 1 m under the surface, and the pore water pressure gradually increased below 1 m of the surface. Also, the diaphragm was fixed as the impermeable model.

2.3.2. Constitutive Model and Material Properties

Two constitutive models were employed for the comparative analysis of the soft soil: (1) the MC model, which follows the Mohr–Coulomb failure criterion and is usually used to simulate an isotropic elasto-plastic material, and (2) the CVISC model, which considers the rheological characteristics of soft soils. The viscoelastic–plastic model in FLAC3D is the CVISC model [36], which describes the relationship between strain and time at each stage and is rather sketchy [37]. Compared with the MC model, the CVISC model was expected to simulate the delayed deformation of the foundation pits in soft soils due to surcharge loading or excavation unloading. This was reasonable because the rheological effect was time-dependent. The results of case studies have shown good agreement between prediction and measurements [38,39].
As layer ②y of Huaxi Park Station exhibited low strength, a high water content, and significant rheological behavior, it was essential to accurately present its time-dependent deformation characteristics during excavation. The CVISC rheological model was adopted to simulate the behavior of layer ②y. The CVISC model was a self-contained viscoelastic–plastic model, as shown in Figure 4. The CVISC model consisted of a Kelvin element, a Maxwell element, and a Mohr–Coulomb friction element arranged in series. The Kelvin element represented the instantaneous elastic response and primary creep stage, while the Maxwell element governed long-term viscous deformation, capturing the stable creep phase. The Mohr–Coulomb yield criterion defined the plastic behavior during the accelerated creep stage, where the material experienced plastic deformation due to stress redistribution following excavation unloading or surcharge loading. The plastic flow law used in the model was an uncorrelated Mohr–Coulomb flow law, where the strain rate depends on stress rather than on time.
The CVISC model parameters included traditional mechanical properties such as cohesion, internal friction angle, and tensile strength (as in the MC model) but also incorporated rheological parameters, namely, Maxwell shear modulus ( E M ), Maxwell viscosity ( η M ), Kelvin shear modulus ( E K ), and Kelvin viscosity ( η K ). The rheological characteristics were correlated negatively with the values of E M , E K , η M , and η K . The parameters of the model were determined by laboratory tests and are shown in Table 1.
The corresponding rheological equation for the CVISC model is as follows:
ε t = σ 0 1 E M + t η M + 1 E K 1 − e − t E K / η K + ε P ,
where, E K and E M , and η K and η M , are the modulus of elasticity and coefficient of viscosity of the Kelvin and Maxwell bodies, respectively.
Since the creep effect of strata other than the ②y layer of silty powdery clay was not obvious in the process of excavation, the Mohr–Coulomb model was used for the layers. According to “Huaxi Park Station of Suzhou Metro Line S1 Detailed Geotechnical Engineering Investigation Report” and the creep test results of the Suzhou ②y layer silty powdery clay, the mechanical parameters of each soil layer were obtained and are shown in Table 1 and Table 2.
In order to simulate the interactions between theretaining structure and soil, the diaphragm wall of the pit was simulated by a solid unit, and the steel support and concrete internal support were simulated by a structural unit. Considering that the concrete support was constructed during the excavation of the foundation pit, its strength had a gradual growth process with the advancement of the excavation of the foundation pit. The structural parameters of the foundation pit are shown in Table 3.

2.3.3. Simulation Method

The explicit central difference method was utilized to solve the rheological process. When rheological calculations were performed, it was necessary to first determine the simulation’s problem time and time step, because the quantities related to time in the rheological calculations represented real time, not parameters that achieve computational iterative stability, thereby simulating the impact of time on the deformation of soft soils. Rheology was controlled by deviatoric stress, and the maximum rheological time step can be taken as the ratio of the viscosity coefficient to the shear modulus.
The main steps for performing rheological calculations on soft soil pit engineering are as follows: activate the rheological analysis mode; select the rheological constitutive model based on the rheological characteristics of the geotechnical body, and assign values to the soil’s rheological constitutive parameters; initiate rheological analysis, set the time step, and perform initial earth pressure calculations on the pit model; during the excavation of the pit, rheological analysis must also be initiated; calibrate the simulation results with the stages of the monitored horizontal displacement of the diaphragm wall under the rheological parameters obtained from laboratory experiments.

3. Results and Discussion

3.1. Monitoring Analysis of Spatiotemporal Effect

3.1.1. Horizontal Displacement of Diaphragm Wall

The maximum horizontal displacement of the enclosure wall monitoring volume during pit excavation is shown in Figure 5. According to the on-site monitoring results, the maximum horizontal displacement of the wall of the west end well and the standard section gradually increased with the increase in the excavation depth and sequence. The maximum horizontal displacement of the wall at the west end well was close to 70 mm, and the maximum displacement of the wall at the standard section reached approximately 80 mm, both of which exceeded the allowable limit of 0.3%H specified by JGJ 120-2012. These exceedances indicated a potential risk to the stability of the foundation pit and may have adverse effects on the surrounding environment, necessitating further assessment and mitigation measures. The displacement of the diaphragm wall in the west end well exhibited significant changes as excavation approached the bottom of the pit, with deformation accelerating during the final excavation layer, indicating an increased risk of structural instability. In contrast, the maximum horizontal displacement of the diaphragm wall in the standard section demonstrated a linear correlation with excavation depth, with a maximum average displacement rate of 4.85 mm/m. After the excavation was completed, the deformation of the diaphragm wall continued to increase, highlighting the time-dependent behavior of mucky soil. These monitoring results confirm that the deformation of mucky soil progressively accumulates over time during excavation, emphasizing the need for continued monitoring and assessment to ensure long-term stability.

3.1.2. Temporal Effectiveness of Surface Settlement

The surface settlement observed at various distances from the pit during the excavation of the pit is shown in Figure 6. According to on-site monitoring results, as the distance from the pit increased, the settlement outside the pit first increased and then decreased. As shown in Figure 6a, the surface settlement at the west end well increased almost linearly with excavation depth under the sequence of excavation stages, and the maximum surface settlement during excavation was 35 mm. The maximum surface settlement occurred at a distance of 12 m outside the pit, with a settlement rate of −1.9 mm/m per meter of excavation depth. After the excavation was completed, the deformation continued to increase until the base slab was poured, at which point the deformation began to converge and gradually stabilized. Similarly, the ground settlement in the standard section of the pit correlated positively with the depth of excavation and the sequence of excavation, with a sudden change in settlement occurring during the construction of the third steel support, followed by a trend of accelerated deformation over time. After the excavation was completed, the surface settlement continued to increase, reaching over 50 mm. The relationship between surface settlement outside the pit and the maximum horizontal displacement of the diaphragm wall, as shown in Figure 7, indicated a linear correlation. There was a significant correlation between the maximum surface settlement and the maximum horizontal displacement of the diaphragm wall, with an R2 value of 0.932 for the end well and 0.977 for the standard section. When the horizontal displacement of the diaphragm wall reached approximately 20–30 mm, the surface settlement outside the pit rapidly increased and even underwent abrupt changes, clearly linking ground settlement to the horizontal displacement of the diaphragm wall caused by excavation. These monitoring results indicated that excavation was the main factor causing surface settlement outside the pit, closely related to and controlled by the timing of the excavation. Besides the immediate deformation caused by excavation, the time-dependent deformation of mucky soil was also notably significant.
In deep excavation, excavation unloading caused the lateral horizontal displacement of the diaphragm walls, leading to surface settlement outside the pit. When there was thick mucky soil within the excavation range of the pit, the deformation continuously increased over time, potentially accelerating or even undergoing abrupt change, exhibiting more pronounced time-dependent deformation characteristics, which can easily lead to pit collapse.

3.2. Numerical Analysis of Spatiotemporal Effect

3.2.1. Pore Water Pressure

Before dewatering, the water levels inside and outside the pit were nearly equal, as shown in Figure 8a, with the diaphragm wall serving as a waterproof curtain. In the numerical model, the pore water pressure on the diaphragm wall was assumed to be 0 kPa, considering the impermeability of the wall and the dissipation of excess pore water pressure near the interface. Outside the diaphragm wall, the soil layer’s pore water pressure was consistent with initial equilibrium. During the phased dewatering and excavation of the pit, as shown in Figure 8b,c, under the influence of the seepage field within the pit, the water level inside gradually decreased, with groundwater around the pit seeping into the pit. Due to the deep insertion of the waterproof curtain and the primary focus on dewatering the shallow mucky soil layers, the vertical flow of groundwater in the mucky soil layers dominated, with more groundwater flowing from beneath the pit into the pit than from the outside to the inside laterally. Overall, dewatering has little impact on the water level outside the pit, but it significantly affected the water level at certain depths below the pit base, reducing it. Once the excavation was completed, the water level inside the pit dropped to about 1 m below the pit bottom, significantly lower than the water level outside, because it was cut off by the diaphragm wall. Dewatering was necessary to lower the water level below the excavation surface, ensuring dry excavation. The impact of dewatering on pit stability was considered, and water pressure was calculated in the design.

3.2.2. Horizontal Displacement of Diaphragm Walls

Based on the lateral displacement of the diaphragm walls, as shown in Figure 9, when excavating to the bottom slab, excavation caused the obvious deformation of the retaining structure. The last excavation had no support at the bottom, and the support stiffness above the excavation face was large, which led to the horizontal deformation developing downward, causing large deformation, and the maximum deformation position was shifted downward [3]. From the point of maximum lateral displacement, the lateral displacement of the diaphragm walls gradually decreased both upward and downward. To analyze the rationality of the parameters used in this simulation, the simulated horizontal displacement of the diaphragm walls was compared with the on-site monitoring data, as shown in Figure 10. Figure 10a shows that the lateral displacement of the diaphragm walls increased and then decreased along the depth, with the maximum lateral displacement occurring about 5 m below the base slab of the pit, with both simulated and monitored lateral displacements exceeding 60 mm. The simulation data fitted well with the monitoring data, with a maximum difference of about 5 mm, accurately reflecting the lateral deformation pattern of the diaphragm walls. Figure 10b indicates that both the simulation and monitoring data reflected the deformation pattern of the diaphragm walls’ lateral horizontal displacement increasing with the progress of excavation and the extension of construction time. This was because when excavation was shallow, the pressure difference between the soil inside and outside the wall was very small, leading to minimal changes in lateral displacement. As the depth of excavation increased, the pressure difference also increased, leading to the greater lateral displacement of the walls. Additionally, the excavation range was permeated with flow plastic mucky soil, causing the deformation values to increase and develop over time, with the numerical simulation also considering the rheological characteristics of soft soil. The simulation data and monitoring data generally matched well. However, during the base slab pouring process, the monitoring data show the continued lateral deformation of the diaphragm walls, whereas the simulation data show the lateral displacement beginning to converge. This discrepancy may be due to the presence of large machinery around the site during the actual base slab pouring, which was not considered in the simulation.

3.2.3. Surface Settlement Around the Pit

Based on the vertical deformation of the soil around the pit (Figure 11), it was evident that after the completion of pit excavation, varying degrees of settlement occurred on the surface outside the pit. The surface settlement on both sides of the pit was symmetrical, increasing with distance from the pit edge before decreasing. Settlement on the shorter sides was less than that on the longer sides. Through the selection of monitoring points along the pit section, the comparison between monitored surface settlement data and simulation results, shown in Figure 12, revealed that the simulated surface settlement followed a similar pattern to the monitoring data, initially increasing and then decreasing from the edge of the pit, forming a “concave”-shaped curve, with the maximum settlement point moving away from the pit edge line. The surface settlement reached its maximum at about 12 m from the pit edge, which differs from the commonly observed pattern, where maximum settlement typically occurs at a distance of approximately 0.2 to 0.3 times the excavation depth [3,5,40,41]. This discrepancy may be attributed to the creep behavior and high compressibility of the soft soil, which allowed for the settlement to extend further from the pit edge, especially when the excavation depth was significant. This aligns with findings in studies by Ge et al. (2025) and Tanoli et al. (2022), where maximum settlement in soft soils was observed farther from the excavation boundary [34,35]. After fluid–solid coupling and rheological calculations of the mucky soil, the overall simulation results for the pit’s standard section aligned closely with the on-site monitoring results, with the maximum difference in deformation at the points of the greatest surface settlement not exceeding 5 mm. In the simulation, only the effects of overload were considered, while live loads such as construction vehicles were not taken into account. Relevant studies have shown that live loads, such as those from construction vehicles, can significantly impact surface settlement [5,35]. The simulation results for the end wells were smaller in scope than the monitoring data, which may be attributed to the frequent movement of heavy construction vehicles and public vehicles near the end wells at the actual construction site, leading to excessive surface settlement. Therefore, to protect the surrounding environment during actual construction, the monitoring of surface settlement at each excavation stage should not be neglected, especially at locations where the maximum values of surface settlement may occur during various stages.
Regarding the surface settlement outside the pit and the lateral deformation of the diaphragm walls, a comparative analysis of simulation results and monitoring data revealed that in the standard section of the pit, the rheological constitutive model and parameters selected for mucky soil accurately reflected the deformation pattern of the mucky soil pit, and can be used for deformation prediction in such pits.

3.2.4. Internal Support Axial Force

From the initiation of the second steel support to the completion of the pit base slab, the axial force changes in the first and third (concrete supports) and second, fourth, and fifth (steel supports) supports are shown in Figure 13 and Figure 14. Regarding the first support within the pit plane, it was under pressure when the second support was installed, and during the installation of the third support to the completion of the base slab, except for the end well supports, which were under pressure, the standard section supports were under tension. The axial force of the end well supports was less than that of the standard section supports, primarily due to the three-dimensional effects of the excavation and additional corner bracing. The spatial effect resulted in stronger lateral constraints at the end wells, reducing horizontal displacement and consequently lowering the axial force. Additionally, extra corner bracing was implemented at the end wells, further restricting deformation and distributing the loads more effectively, leading to reduced axial forces in the end well supports. The support axial forces continuously increased until the fourth support was installed. After the installation of the fifth support, the axial forces decreased until the base slab was completed, where they still showed a reduction.
The second to fifth supports within the pit plane were almost always under compression throughout the entire excavation process. The second support inside the end wells shifted to a tension state after installation but then returned to compression, with axial forces initially increasing and then decreasing. The second support in the standard section shifted from compression to tension after the fourth support was installed, with gradually decreasing axial forces. The third support remained under compression during pit excavation, and its axial force increased with the excavation depth, but it decreased gradually after the fifth support was installed. The fourth support changed from tension to compression, and its axial force decreased after the completion of the base slab. The fifth support remained under compression from installation to the completion of the base slab, and its axial force increased. Among the five supports, the third support experienced the highest axial force. This may be mainly attributed to its position and concrete material. The third support was located at the middle of the excavation, which subjected it to the most significant stress redistribution in the excavation process. The higher stiffness of the third concrete support contributed to its ability to resist displacement more effectively, which resulted in a greater concentration of axial force. The fifth support was present for a short duration during pit excavation. Except for the fifth support, the other supports showed a pattern of initially increasing and then decreasing axial forces. As the excavation depth increased, the axial forces in the upper supports decreased, showing signs of stress relaxation.

4. Conclusions

This study aimed to investigate the spatiotemporal effects of deep excavation in mucky soft soil at the Huaxi Park Station foundation pit of Suzhou Metro Line S1. Through field monitoring and numerical simulations, the deformation characteristics of pit retaining structures, surface settlement, and internal support forces were analyzed. A three-dimensional numerical model, incorporating the CVISC viscoelastic–plastic model to account for the creep characteristics of the mucky strata, was developed. The main conclusions can be drawn as follows:
(1) As the construction sequence progressed and the depth of excavation continuously increased, the horizontal displacement of the diaphragm walls showed an approximately linear increase. When the excavation approached the bottom of the pit, sudden changes in the displacement of the retaining walls were likely to occur. After the completion of excavation, the deformation of the diaphragm walls continued to increase over time, reflecting, overall, the significant time-dependent deformation characteristics of the muck soil pit during excavation;
(2) The surface settlement on both sides of the pit was symmetrical, moving outward from the edge of the pit, and the surface settlement outside the pit first increased and then decreased, with the maximum settlement exceeding 40 mm. Excavation was the main factor causing surface settlement outside the pit, which directly caused the lateral deformation of the diaphragm walls, thereby causing surface settlement. Besides the instantaneous deformation caused by excavation, surface settlement was also controlled by the timing of excavation, with the deformation over time being quite significant;
(3) The surface settlement on both sides of the pit exhibited a “trough” distribution pattern and reached its maximum value at about 12 m from the pit edge. Therefore, it was feasible to consider reinforcing the soil within a certain range of this location (especially the mucky layers), such as through stone dumping compaction or planar grid-shaped cement mixing piles to “lock the silt”;
(4) When soft soil layers were excavated, the entire diaphragm wall displacement rate was relatively fast. Before this construction stage, the mud within the excavation range could be reinforced or temporarily supported by several more struts. During the construction stage, it was advisable to accelerate the speed of the excavation of soft soil layers and increase the frequency of monitoring diaphragm wall displacement;
(5) The retaining structure and surface settlement deformation induced by the creep of soft soil and spatial effect of the excavation sequence were considered in this case, which provided critical insights for similar deep excavation projects in mucky soft soil by providing guidance on design standards and monitoring strategies for similar geological conditions;
(6) The simulation primarily focused on the deformation and support forces during the excavation process but did not further explore the evolution of the deformation parameter under the long-term impact of consolidation settlement on the stability of the foundation pit, which can be further studied in future research;
(7) Future research could explore the use of machine learning and data-driven models to predict soil behavior and optimize excavation plans. By leveraging historical and real-time monitoring data, these models could improve prediction accuracy and enable real-time dynamic adjustments during the excavation process. Meanwhile, incorporating real-time monitoring technologies coupled with machine learning models for prediction can be introduced in a future study.

Author Contributions

Methodology, J.W., Q.W., Y.L. and X.L.; Software, Y.L. and H.L.; Validation, J.W., Y.L., X.L., G.L., S.D., L.Z., H.L. and M.A.H.b.H.; Formal analysis, J.W., Y.L., G.L., S.D., L.Z., H.L. and M.A.H.b.H.; Investigation, J.W., G.L., S.D., L.Z., H.L. and M.A.H.b.H.; Resources, J.W., X.L., S.D. and L.Z.; Data curation, X.L., G.L., S.D. and M.A.H.b.H.; Writing—original draft, J.W. and Y.L.; Writing—review and editing, J.W.; Supervision, J.W.; Project administration, J.W., X.L. and L.Z.; Funding acquisition, J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Suzhou Rail Transit Line 1 Co., Ltd. (SURT01YJ1S10002); Research Project of Shanghai Housing and Urban Rural Development Management Committee (2024-Z02-007); Shanghai Shallow Geothermal Energy Engineering Technology Research Center (DRZX-202302); Shanghai Municipal Science and Technology Project (18DZ1201301; 19DZ1200900); Shanghai Tunnel Engineering Co. Ltd.(2022-SK-02); Guangzhou Metro Design and Research Institute Co. Ltd.; Ministry of Housing and Urban-Rural Development of Research and Development Program of the People’s Republic of China (2022-K-044); Shanghai Institute of Geological Survey (2023(D)-003(F)-02); Key Laboratory of Land Settlement Monitoring and Prevention, Ministry of Natural Resources of the People’s Republic of China (KLLSMP202101; KLLSMP202201); China Railway 15 Bureau Group Co., Ltd. (CR15CG-XLDYH7-2019-GC01).

Institutional Review Board Statement

This study did not require ethical approval.

Informed Consent Statement

This study did not involve humans.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We want to express our thanks to the authors whose publications we cited and the efficient editor.

Conflicts of Interest

Authors Qiang Wu and Li Zhou were employed by the company Suzhou Metro Line 1 Co., Ltd. Authors Xuezeng Liu and Shuang Ding were employed by the company Shanghai Tongyan Civil Engineering Technology Co., 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.

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Applsci 15 01992 g001
Figure 1. Soil profile along the standard section of Huaxi Park Station.
Figure 1. Soil profile along the standard section of Huaxi Park Station.
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Figure 2. Layout of monitoring points of Huaxi Park Station foundation pit. (CX is the monitoring point number of diaphragm wall horizontal displacement; DB is the monitoring point number of surface subsidence).
Figure 2. Layout of monitoring points of Huaxi Park Station foundation pit. (CX is the monitoring point number of diaphragm wall horizontal displacement; DB is the monitoring point number of surface subsidence).
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Figure 3. A 3D numerical model of the Huaxi Park Station foundation pit.
Figure 3. A 3D numerical model of the Huaxi Park Station foundation pit.
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Figure 4. Structure of CVISC model.
Figure 4. Structure of CVISC model.
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Figure 5. Variation rule of maximum horizontal displacement of enclosure wall: (a) maximum horizontal displacement of west end well wall; (b) maximum horizontal displacement of standard section wall. RW means retaining wall.
Figure 5. Variation rule of maximum horizontal displacement of enclosure wall: (a) maximum horizontal displacement of west end well wall; (b) maximum horizontal displacement of standard section wall. RW means retaining wall.
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Figure 6. Surface settlement outside the pit: (a) surface settlement outside the west end head pit; (b) surface settlement outside the standard section pit. D is the distance from the pit. Notes−excavation step sequence: 1−arrangement of the first steel support; 2−arrangement of the second steel support; 3−arrangement of the third steel support; 4−arrangement of the fourth steel support; 5−excavation completed.
Figure 6. Surface settlement outside the pit: (a) surface settlement outside the west end head pit; (b) surface settlement outside the standard section pit. D is the distance from the pit. Notes−excavation step sequence: 1−arrangement of the first steel support; 2−arrangement of the second steel support; 3−arrangement of the third steel support; 4−arrangement of the fourth steel support; 5−excavation completed.
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Figure 7. The relationship between the surface settlement outside the pit and the maximum horizontal displacement of the retaining wall: (a) the west end well; (b) the standard section.
Figure 7. The relationship between the surface settlement outside the pit and the maximum horizontal displacement of the retaining wall: (a) the west end well; (b) the standard section.
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Applsci 15 01992 g008aApplsci 15 01992 g008b
Figure 8. Pore water pressure contour with different construction steps: (a) before excavation; (b) excavation of third floors; (c) final excavation.
Figure 8. Pore water pressure contour with different construction steps: (a) before excavation; (b) excavation of third floors; (c) final excavation.
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Figure 9. Horizontal displacement of diaphragm wall of Huaxi Park Station foundation pit.
Figure 9. Horizontal displacement of diaphragm wall of Huaxi Park Station foundation pit.
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Figure 10. Horizontal displacement of diaphragm wall (CX5): (a) horizontal displacement versus depth curve; (b) variation curve of maximum horizontal displacement with construction sequence. (Construction sequence: 1—arrangement of the first steel support; 2—arrangement of the second steel support; 3—arrangement of the third steel support; 4—arrangement of the fourth steel support; 5—arrangement of the fifth steel support; 6—completion of the footing pouring.)
Figure 10. Horizontal displacement of diaphragm wall (CX5): (a) horizontal displacement versus depth curve; (b) variation curve of maximum horizontal displacement with construction sequence. (Construction sequence: 1—arrangement of the first steel support; 2—arrangement of the second steel support; 3—arrangement of the third steel support; 4—arrangement of the fourth steel support; 5—arrangement of the fifth steel support; 6—completion of the footing pouring.)
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Figure 11. Vertical displacement after the completion of excavation: (a) surface settlement outside the pit; (b) vertical displacement.
Figure 11. Vertical displacement after the completion of excavation: (a) surface settlement outside the pit; (b) vertical displacement.
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Figure 12. Surface settlement outside the pit after the completion of excavation: (a) DB4; (b) DB6; (c) DB1.
Figure 12. Surface settlement outside the pit after the completion of excavation: (a) DB4; (b) DB6; (c) DB1.
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Figure 13. Change in the axial force of the first concrete support.
Figure 13. Change in the axial force of the first concrete support.
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Figure 14. Change in the axial force of the second to fifth supports.
Figure 14. Change in the axial force of the second to fifth supports.
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Table 1. Burgers rheological parameters for mucky soils of Huaxi Park Station.
Table 1. Burgers rheological parameters for mucky soils of Huaxi Park Station.
ParameterEM (kPa)EK (kPa)ŋM (kPa·h)ŋK (kPa·h)
Value20010080,0002000
Table 2. Material physical and mechanical parameters of Huaxi Park Station.
Table 2. Material physical and mechanical parameters of Huaxi Park Station.
Layer No.NameDensity
/ ρ 		Hydraulic Conductivity/KCompressive Modulus
E0.1~0.2		Friction/ϕCohesion/cPore
Ratio/e		Poisson/μ
(N/m3)10−6 cm/sMPa°kPa
①1Filler21007000.015.05.00.350
②1Silty clay19006.04.6313.723.50.940.375
②yMucky clay17907.03.078.912.51.230.444
⑤1Silty clay18306.53.9012.818.31.040.429
⑦1Silty clay18508.04.2612.919.61.010.394
⑦2Silt with silt sand193033009.6718.96.60.7980.324
⑦3Silty clay189010.05.0814.225.00.920.367
(11)Silt sand with silt19806500.011.0323.85.10.740.310
(12)Silty clay19106.05.341535.00.8650.310
(13)Silt sand196010,00010.88274.00.7580.333
Table 3. Retaining structure parameters of Huaxi Park Station.
Table 3. Retaining structure parameters of Huaxi Park Station.
Retaining Structureγ
kN/m3		Modulus
E/kPa		Poisson/ν
Diaphragm wall2540 × 1060.200
Crown beam2532 × 1060.167
Concrete support2532 × 1060.200
Steel support7920 × 1070.300
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Wu, Q.; Wang, J.; Long, Y.; Liu, X.; Long, G.; Ding, S.; Zhou, L.; Li, H.; Hishammuddin, M.A.H.b. The Field Monitoring and Numerical Simulation of Spatiotemporal Effects During Deep Excavation in Mucky Soft Soil: A Case Study. Appl. Sci. 2025, 15, 1992. https://doi.org/10.3390/app15041992

AMA Style

Wu Q, Wang J, Long Y, Liu X, Long G, Ding S, Zhou L, Li H, Hishammuddin MAHb. The Field Monitoring and Numerical Simulation of Spatiotemporal Effects During Deep Excavation in Mucky Soft Soil: A Case Study. Applied Sciences. 2025; 15(4):1992. https://doi.org/10.3390/app15041992

Chicago/Turabian Style

Wu, Qiang, Jianxiu Wang, Yanxia Long, Xuezeng Liu, Guanhong Long, Shuang Ding, Li Zhou, Huboqiang Li, and Muhammad Akmal Hakim bin Hishammuddin. 2025. "The Field Monitoring and Numerical Simulation of Spatiotemporal Effects During Deep Excavation in Mucky Soft Soil: A Case Study" Applied Sciences 15, no. 4: 1992. https://doi.org/10.3390/app15041992

APA Style

Wu, Q., Wang, J., Long, Y., Liu, X., Long, G., Ding, S., Zhou, L., Li, H., & Hishammuddin, M. A. H. b. (2025). The Field Monitoring and Numerical Simulation of Spatiotemporal Effects During Deep Excavation in Mucky Soft Soil: A Case Study. Applied Sciences, 15(4), 1992. https://doi.org/10.3390/app15041992

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