Investigation on Dewatering Scheme Optimization, Water Levels, and Support Layout Influences for Steel Sheet Pile Cofferdams

Abstract

Based on the steel sheet pile cofferdam project for the main bridge piers of a cross-sea bridge, finite element numerical simulations were conducted to analyze the influence of construction sequences in marine environments, as well as the effects of initial water levels and support positions under various construction conditions on the stress and deformation behavior of steel sheet piles. Using a staged construction simulation with a Mohr–Coulomb soil model and stepwise activation of loads/excavation, this study delivers practically relevant trends: staged dewatering halves the sheet pile head displacement (top lateral movement <0.08 m vs. ~0.16 m in the original scheme) and mobilizes the support system earlier, while slightly increasing peak bending demand (~1800 kN·m) at the bracing elevation; the interaction between water head and brace elevation is explored through fitted response curves and summarized in figures/tables, and soil/structural properties are tabulated for reproducibility. The results indicate that a well-designed dewatering process, along with the coordination between water levels and internal support positions, plays a critical role in controlling deformation. The findings offer valuable references for the design and construction of sheet pile cofferdams in marine engineering under varying construction methods and water level conditions.

1. Introduction

Steel sheet piles are often used in the construction of dry docks, offshore engineering piles, and undersea tunnels in offshore engineering. They can separate the internal working surface from the external seawater, forming a highly sealed and water-tight construction working area, and play a temporary support role in the early stage of engineering construction. Steel sheet pile cofferdams are commonly used in the construction process of deep-water foundations for the main bridge piers of large-scale bridges [1]. Tian et al. [2] conducted a study on the critical construction techniques of an interlocking steel pipe pile cofferdam for the main pier of the Shuidu No. 2 bridge located in Danjiangkou City. Figure 1 shows the steel sheet pile cofferdam of the main pier of a certain cross-sea bridge.

In order to reduce the additional bending moment generated by steel sheet piles during the construction process, Crawford et al. [3] proposed a numerical model for predicting the bending stress of Larsen-type piles. Osthoff et al. [4] conducted a numerical simulation analysis of deformational behavior of steel sheet piles during jacking using the coupled Euler–Lagrange (CEL) method. The results show that, due to the blockage of the soil, the stress state between the web and the flange surfaces increases, especially for U-shaped and Z-shaped sections. There are also studies on similar steel sheet pile structure cofferdams in the marine area. Kimura [5] developed a new type of H-joint steel pipe sheet piles (SPSPs) technology, aiming to enhance the performance of the SPSP technology and expand its application scope. Steel sheet piles can also play a role in preventing water overflow in marine engineering. Mitobe et al. [6] proposed a new embankment reinforcement method using steel sheet piles to resist tsunami overflow.

The influence of the construction sequence on the deformation behavior of steel sheet pile cofferdams has been relatively understudied. Lv et al. [7] selected the temporary cofferdam of the Shenzhen–Zhongshan Railway as a case study and developed finite element models for steel sheet pile cofferdams with varying construction sequences. It highlights the significant impact of the layered backfilling process in the steady-state seepage condition on the bending moment and deformation behavior of steel sheet piles. The analysis results demonstrate that the rational investigation of the construction process effectively reduces the bending moment experienced by the steel sheet piles.

As a critical component of the steel sheet pile cofferdam project, the internal support system significantly contributes to the load-bearing capacity and structural stability of the steel sheet pile assembly. Considering the reduced modulus action (RMA) of U-shaped steel sheet piles caused by the incomplete transfer of shear force due to inter-pile sliding, Wang et al. [8] developed a three-dimensional finite element model of the steel sheet pile cofferdam incorporating supporting structures, performed a parametric analysis of RMA, and subsequently proposed an optimized design strategy for the supporting system. Wang et al. [9] deliberated and analyzed on the design scheme for the foundation pit excavation. Moreover, they probed into the influence of the height-to-width ratio of the cofferdam on the stability of the foundation pit. Xue et al. [10] investigated the stability failure mechanism of Pile Wall Frame Structures (PWFSs). Subsequently, through structural stability analysis employing the limit equilibrium method, they proposed a design methodology for PWFSs that accounts for the influence of pile cutting forces under varying row spacing. Furthermore, they assessed the impact of key design parameters—such as frame width, pile spacing, pile length, and pile diameter—on the overall structural stability.

In addition to the aforementioned research, extensive research has been dedicated to evaluating the stability of steel sheet pile cofferdams under critical geotechnical and hydraulic conditions. For instance, foundational investigations on soft soil behavior (e.g., interaction mechanisms of individual pile head displacements, the m-value for backfilled sand compaction, and riprap–soil interface stability) [11], as well as storm surge-induced hydrodynamic loads (including wave impact and transient pore pressure distributions) and their correlation with water level/wave height thresholds [12], have provided essential insights into cofferdam failure mechanisms under complex environmental interactions. In offshore engineering, the influence of water level on the design and construction of sheet piles cannot be ignored. Wang et al. [13] developed a medium- and short-term water level prediction framework in the construction conditions of steel sheet pile cofferdams. However, the position of the internal supports, the structural configuration parameters, and the construction conditions were not comprehensively considered. In offshore engineering, soil liquefaction and constitutive behavior are frequently examined due to the influence of wind and wave loading [14,15]. The Mohr–Coulomb model is extensively utilized in engineering practice owing to its straightforward parameterization and ease of implementation. In this research, the conventional Mohr–Coulomb constitutive model was applied to investigate relevant engineering problems.

Existing research exhibits significant limitations in the following aspects: (1) Wang et al.’s research on support systems did not fully consider the impact of water level fluctuations; (2) most analyses of construction sequences did not involve staged dewatering techniques; and (3) there is a lack of quantitative analysis of the coupling effect between water level and support position. Through parametric analysis, this study innovatively establishes a dynamic response relationship among water level, support position, and construction stage, filling the knowledge gap in existing research regarding dynamic construction control in marine environments. The calculation data were fitted into curves using the least squares method for comparative analysis. This approach further elucidated the variations in deformation control mechanisms under different operational conditions, thereby providing a solid theoretical basis for engineering decision-making.

Given the inherently challenging operational conditions characteristic of offshore engineering projects [16], stringent precision in deformation mitigation and load management for cofferdam retaining structures is imperative [17]. The implementation of rigorous construction process control protocols constitutes a critical determinant for ensuring the successful execution of marine infrastructure development. This study employs a finite element modeling numerical–analytical approach in combination with actual engineering conditions and discusses the influence of construction procedures on the force and deformation of offshore sheet pile cofferdams during the construction process. The deformation and stress of the sheet pile support system under different construction conditions such as dewatering and excavation of silt were considered, and the coupling correlation of the initial water level condition and the design of the internal support position on the deformation control of sheet piles was discussed. The research findings establish a valuable reference framework for structural design and construction methodology assessment of marine sheet pile cofferdams under varying hydrological conditions and construction sequences.

2. Project Overview

2.1. Project Description

Focusing on the large-scale caisson project for the main bridge pier of a cross-sea bridge, this study investigates the structural behavior, bracing effectiveness, and influencing factors of the steel sheet pile cofferdam. The deformation and stress characteristics of the cofferdam in the marine environment were analyzed, specifically considering seawater level, internal bracing positions, and construction dewatering schemes. Through calculation and analysis, an optimal dewatering strategy for construction was identified, along with design recommendations for internal bracing positions under varying water levels.

Offshore marine engineering sites exhibit complex geological conditions distinct from inland areas, requiring consideration of both seawater overburden and unique seabed soil composition. The seabed sediment at this site contains organic matter including fish, shrimp, seashells, and marine animal feces. The upper soil stratum consists of silty soil averaging 8 m in thickness. The main pier foundation employs a pile group with a two-tier platform structure (upper and lower platforms). The construction methodology involves the following:

(1)

Placing a 3 m concrete seal atop the soil layer.

(2)

Installing pile foundations.

(3)

Constructing the platform.

To prevent mud intrusion into the working area during construction, foundation pit dredging is required. Consequently, the numerical model incorporates dredging conditions, simulating excavation of the silty soil at the pit base. Figure 2 presents the cross-section of the offshore steel sheet pile cofferdam.

2.2. Model Establishment

Finite element analysis software was employed to simulate the support system involving steel sheet piles and steel supports. During the simulation, the Mohr–Coulomb soil constitutive model was adopted. The physical and mechanical properties of the soil layers, namely ① mucky soft soil, ② silt clay, and ③ sandstone, are summarized in

Table 1. Physical and mechanical performance parameters of each soil layer.

Soil layer	h (m)	φ (0)	c (kPa)	γ (kN/m3)	γsat (kN/m3)	υ	E (kN/m2)	k (cm/s)
① Mucky soft soil	8	13	8	19.2	20.4	0.30	800	1.6 × 10−7
② Silty clay	14	26	19	20.1	21.3	0.3	2000	1.2 × 10−6
③ Sandstone	-	43	36	21.3	21.6	0.25	1.06 × 105	2.7 × 10−9

.

Table 2. Physical and mechanical parameters table of structural components.

Component	EA (kN/m)	EI (kN/m2/m)	A (m2)	w (kg/m)	υ
Inner support	4.5 × 107	2.4 × 106	0.8	47.9	0.25
Steel sheet pile	8.0 × 107	6 × 108	0.12	52.8	0.3

provides the corresponding physical and mechanical parameters of the structural components. In the table, the soil layer thickness is h (m), the internal friction Angle is φ (°), the cohesion is c (kPa), the soil layer density is γ (kN/m³), the saturation density is γ_sat (kN/m³), Poisson’s ratio is υ, the elastic modulus is E (kN/m²), and the cross-sectional area per square meter is A (m²). The mass is w (kg/m), and the moment of inertia of the cross-section is I (m⁴). The permeability coefficient of the soil is k (cm/s).

To ensure high computational accuracy, a 15-node triangular element is employed in the analysis. The top surface of the soil layer is designated as the ±0 m elevation, with an excavation depth of 8 m for the foundation pit and a retaining pile length of 24 m. Considering the excavation of silty soil at the pit bottom, groundwater control during construction requires the water table to be lowered below the pit base. According to the relevant specifications, the water level must be reduced to at least 0.5 m beneath the pit bottom. In the original scheme, under the initial working condition, the water level is located at +10 m elevation, resulting in a total drawdown depth of 18 m. A single layer of steel support is installed to ensure structural stability.

A two-dimensional soil model was established using the finite element simulation software. The proposed construction site is an ocean site with steel sheet pile cofferdams for support and a single layer of internal supports. Initial working conditions: the original marine site, with a 10 m thick overlying seawater layer. The soil layers are as follows: 8 m thick mucky soft soil, 14 m thick silty clay, and the bottom is sandstone. The numerical simulation model size is 100 m × 60 m. Figure 3 shows the numerical simulation model of the steel sheet pile cofferdam project established.

3. Research on the Impact of Construction Dewatering Schemes on Sheet Piles

3.1. Construction Plan

The construction plan design is categorized into two types: the original construction plan and the staged dewatering construction plan. The cloud map of the total vertical displacement u_x of the foundation pit under the initial oceanic sites working conditions are provided in Figure 4.

The original construction plan involved six construction processes. The detailed working conditions are as follows:

Step 1. Steel sheet pile installation: The steel sheet piles were installed by vibratory driver.

Step 2. Dewatering: Lowering the water level to the top surface of the soil layer (the original seawater level was +10 m, but it has now dropped to ±0 m).

Step 3. Installation of struts: After the dewatering, the internal supports were installed at the desired depth (support position +5 m).

Step 4. Dewatering: Lowering the water level to the position −8 m below the bottom of the pit.

Step 5. Excavation: Excavate the silty soil to the position −8 m below the bottom of the pit.

Step 6. Casting bottom concrete, caps, and piers.

Figure 5 shows the lateral displacement u_x and longitudinal u_y displacement cloud maps of the foundation pit soil from dewatering to the top surface of the soil layer under working condition two of the original scheme. After the precipitation reaches the top surface of the soil layer, the maximum lateral displacement of the soil occurs at the position where the steel sheet piles are inserted on the surface of the soil layer, and the maximum vertical displacement of the soil occurs at the middle part of the base pit bottom.

The multiple-stage dewatering scheme modifies the original plan by inserting an additional dewatering procedure before Phase 2 (dewatering to the soil layer’s top surface). Specifically, the revised workflow first lowers the water table to the installation elevation of internal struts/stiffening beams. After constructing the internal support system, dewatering proceeds to reach the final target level at the soil layer’s top surface (±0 m). This sequential approach enhances excavation stability through staged reinforcement implementation. The initial phase site conditions remain consistent with the aforementioned original scheme, with the seawater level positioned at +10 m above the soil stratum interface.

The multiple precipitation construction plan involved seven construction processes. Changing the construction process to a multiple dewatering construction plan, the detailed working conditions are as follows:

Step 1. Steel sheet pile installation: The steel sheet piles were installed by vibratory driver.

Step 2. Dewatering: Lowering the water level to +5 m (relative to the soil stratum interface).

Step 3. Installation of struts: After the dewatering, the internal supports were installed at the desired depth (support position +5 m).

Step 4. Dewatering: Lowering the water level to the top surface of the soil layer.

Step 5. Dewatering: Lowering the water level to the position −8 m below the bottom of the pit.

Step 6. Excavation: Excavate the silty soil to the position −8 m below the bottom of the pit.

Step 7. Casting bottom concrete, caps, and piers.

3.2. Soil Deformation and Internal Force Analysis

Figure 6 shows the maximum transverse and longitudinal effective stresses (σ′_xx, σ′_yy) and total stress (σ_xx, σ_yy) values of the soil under two construction schemes. Negative values indicate that the soil is under pressure. As illustrated in the figure, the lateral effective stress, vertical effective stress, lateral total stress, and vertical total stress in the soil mass progressively increase with the advancement of construction activities. However, during the excavation phase of the silty clay layer, all four stress components (lateral/vertical effective and total stresses) decreased significantly compared to the previous construction stages. Among them, the changes in the longitudinal effective stress (σ′_yy) and total stress (σ_yy) were more pronounced. This phenomenon arises from the unloading-dominated nature of soil excavation, which inherently induces stress relaxation within the soil mass.

Under both construction schemes, the magnitudes of total stress and effective stress in the soil are numerically similar. The multiple dewatering scheme shows slightly more favorable stress reduction effects, though the numerical differences are insignificant. Construction dewatering affects both lateral and vertical effective stresses and total stresses in the soil, with a more pronounced impact on the lateral components. This can be observed from the slopes of the black and red curves in the figure: after support installation, both lateral effective stress and total stress exhibit sharp increases with continued dewatering.

Figure 7 presents the lateral and longitudinal deformations of the soil under two distinct construction schemes. As illustrated in the figure, the lateral deformation slightly decreases after the installation of the internal support, indicating that the supporting structure contributes to controlling soil deformation. However, the lateral deformation gradually increases during the processes of precipitation and construction. The huge spacing between the black squares and red circles in the figure demonstrates that the staged precipitation construction scheme exerts a more pronounced control over lateral deformation. Particularly under initial construction conditions, staged precipitation achieves better control of lateral deformation, with an average reduction of approximately 0.03 m compared to one-time precipitation. Furthermore, during the subsequent excavation of silt soil after the completion of the precipitation process, the stepwise precipitation construction plan approach continues to exhibit superior control over lateral deformation compared to the original method. Regarding longitudinal deformation, the soil experiences downward settlement as the groundwater level drops, accompanied by an increase in effective stress. After the excavation of the silt soil, a slight rebound occurs, followed by an increase in longitudinal deformation. However, the staged precipitation construction scheme demonstrates a less significant effect on controlling longitudinal deformation compared to its influence on lateral deformation, with only a minor improvement observed.

Based on the analysis of soil stress and deformation at each construction stage under the two construction schemes, the stepwise multiple dewatering method proves more effective in controlling lateral soil stress and deformation. Its influence on longitudinal stress and deformation, however, is relatively minor. In both construction schemes, the longitudinal stress and deformation remain comparatively low, with minimal differences observed between them. The dewatering phase exerts a more significant impact on lateral stress and deformation. As the groundwater level decreases, both lateral deformation and stress increase rapidly. With the progression of construction, the lateral deformation under the stepwise dewatering approach gradually converges toward that of the one-time dewatering method. The excavation of muddy soil has a more pronounced effect on longitudinal stress and deformation. During this phase, the soil experiences rebound due to unloading, resulting in a decrease in longitudinal stress accompanied by an increase in deformation.

3.3. Analysis of Deformation and Stress of Steel Sheet Piles

Figure 8 and Figure 9 present the lateral displacement and bending moment diagrams of the steel sheet piles under the original construction scheme. As shown in the figures, the maximum lateral displacement occurs at the top of the steel sheet pile, reaching approximately 0.16 m. Under the condition of mucky soft soil excavation, the lateral displacement of the steel sheet pile below the +5 m elevation has changed. Specifically, the displacement at the top decreases, while the +5 m position corresponds to the location of internal support reinforcement, indicating that the internal support system effectively controlled the deformation of the steel sheet pile. With the continuation of silt soil excavation, significant changes in lateral displacement occur, with the influence extending from +5 m to −20 m in depth. Within this range, the lateral displacement is notably greater compared to previous construction stages, demonstrating that mucky soft soil excavation exerts a significant influence on the lateral displacement of the steel sheet pile. The presence of internal support also leads to stress redistribution, causing the maximum deformation to shift toward the middle and lower sections of the steel sheet pile. The bending moment diagram further illustrates that the installation of internal support results in an increased bending moment at the +5 m position. Affected by dewatering and soil excavation, the deformation in the middle section of the steel sheet pile increases, indicating that silt soil excavation has a considerable impact on the bending moment in this region.

Figure 10 illustrates the lateral displacement of steel sheet piles under the stepwise dewatering scheme. Compared to the original scheme, the stepwise dewatering approach demonstrates superior control over the displacement at the top of the steel sheet piles. The maximum displacement at the top is less than 0.08 m, representing a reduction of over 50% compared to the original scheme. Furthermore, once internal bracing is installed, it immediately contributes to deformation control, effectively restraining displacement during subsequent dewatering stages. In contrast, the original scheme delays the installation of internal bracing, allowing lateral deformation to accumulate significantly during dewatering before the bracing is activated, thereby limiting its effectiveness due to the shorter timeframe for intervention. Therefore, phased dewatering not only optimizes overall displacement control but also enables earlier utilization of the passive support effect provided by internal bracing.

Figure 11 shows the bending moment diagram of steel sheet piles under the stepwise dewatering scheme. Compared to the original scheme, the maximum bending moment of the steel sheet piles slightly increases to approximately 1800 kN·m under the stepwise approach. Similarly, due to the configuration of internal bracing, the bending moment peaks at the +5 m elevation. The combined effects of dewatering and soil excavation significantly amplify mid-span deformation in the steel sheet piles. For both schemes, excavation of silty clay exerts considerable influence on the bending moment in the middle section of the piles. The location of the maximum bending moment in the mid-section coincides with the position of maximum displacement, both occurring near the −3 m elevation. Compared to the original scheme, the stepwise dewatering method enhances the structural efficiency of the retaining system, achieving superior deformation control and improved mobilization of internal forces.

3.4. Deformation and Force Analysis of Internal Supports

Figure 12 and Figure 13 present the vertical displacement and bending moment diagrams, respectively, of the internal support under the two construction schemes. Plan 1 represents the original scheme, while Plan 2 corresponds to the staged dewatering scheme. It can be observed that the staged dewatering scheme demonstrates a significantly better deformation control effect for the internal support across all construction stages compared to the original scheme. Additionally, the bending moment is greater under Plan 2, which facilitates the structural performance of the internal support. The maximum vertical displacement occurs at the mid-span of the internal support. Both dewatering and mucky soft soil excavation conditions contribute to reduced deformation and increased bending moment, with the excavation condition exhibiting a more pronounced effect. Excavation of mucky soft soil constitutes an unloading process. As indicated by the comparison of bending moment diagrams, the bending moment increases under excavation conditions, while vertical displacement decreases. This suggests that the internal support assumes a more significant supporting role after soil excavation, bearing increased lateral earth pressure from the foundation pit walls, thereby resulting in a higher bending moment.

However, factors such as foundation pit deformation and base heave may reduce vertical displacement, leading to a discrepancy between deformation and internal forces. Throughout the construction process, the maximum vertical displacement of the internal support remains below 0.025 m. Following the excavation of soil, the bending moment increases sharply, ranging from 1200 kN·m to 1700 kN·m. Therefore, when evaluating the deformation and internal forces of the internal support and sheet piles, a comprehensive analysis should be conducted, taking into account construction conditions, groundwater levels, the position of the internal support, and the mechanical behavior of other structural components. Relying solely on deformation data is insufficient to fully assess engineering safety. Close attention should also be given to the internal bending moments of the internal support and sheet piles to prevent structural damage caused by stress concentration during construction.

4. Research on the Influence of Water Level and Internal Support Position

4.1. Analysis of Initial Water Level Influence

To examine the effects of varying water levels and internal support positions on the lateral displacement of steel sheet piles, the internal support locations were set at +1 m, +3 m, and +5 m, while the water levels were maintained at +10 m, +8 m, and +6 m, respectively, in accordance with the staged water drainage construction plan. Under each scenario, both lateral and longitudinal deformations of the steel sheet piles were analyzed. As illustrated in Figure 14, the maximum lateral displacement values of the steel sheet piles under different combinations of water levels and internal support positions are summarized.

The figure illustrates that, as the initial water level decreases from 10 m to 6 m, the lateral displacement of steel sheet piles gradually reduces. When maintaining a constant initial water level and lowering the internal support position from +5 m to +1 m, the lateral deformation of steel sheet piles under the two dewatering plans progressively increases with the descent of internal supports. However, under silty clay excavation conditions, the lateral deformation decreases as the support position lowers, demonstrating effective deformation control. This occurs because lower-positioned internal supports, being closer to the excavated soil mass, provide enhanced restraint during excavation. Nevertheless, such low support positions exhibit inferior deformation control during early construction phases. Consequently, the optimal positioning scheme for internal supports should comprehensively account for water level variations, construction stages, and their respective durations. For prolonged marine or riverine excavation projects requiring extended foundation engineering after silty clay removal, priority should be given to lateral displacement control during excavation. Conversely, if stringent deformation control is mandated during extended dewatering stages, the design should primarily address lateral displacement constraints under dewatering conditions.

4.2. Correlation Analysis of the Influence of Water Level and Support Position on the Deformation of Sheet Piles

Figure 15 demonstrates the variation of steel sheet pile lateral deformation with internal support positions under two dewatering conditions: dewatering to the soil layer surface and dewatering to the −8 m elevation of excavation base. The experimental data were fitted using the formula y = y₀ + A₁e^−x/t₁, and the specific parameters can be found in Appendix A,

Table A1. Formula y = y₀ + A₁e^−x/t₁ fitting parameter index.

Water Level/Operating Condition		Parameters
		y0	A1	t1	k	tau
Water level +10 m	4	0.044	0.041	2.187	0.457	1.516
	5	0.047	0.032	3.820	0.262	2.648
Water level +8 m	4	0.008	0.055	6.206	0.161	4.302
	5	0.040	0.026	2.090	0.478	1.449
Water level +6 m	4	0.011	0.038	3.191	0.313	2.212
	5	0.031	0.027	0.875	1.142	0.607

. The three datasets arranged from bottom to top correspond to initial water levels at +6 m, +8 m, and +10 m, respectively. The results indicate significant water level dependence: higher initial water levels induce greater lateral displacements, though these can be mitigated through internal support repositioning. Elevating supports from +1 m to +5 m under high water levels (+10 m) produces measurable deformation reduction.

All six curves in the figure exhibit concave profiles. Slope analysis indicates that, when internal supports are relocated from +1 m to +3 m, the lateral deformation decreases more rapidly, demonstrating more pronounced control effectiveness compared to support repositioning from +3 m to +5 m. Under the −8 m dewatering condition (red curves), the steel sheet piles exhibit greater lateral deformation at the higher initial water level (+10 m) than under lower water levels. Adjusting supports upward from +1 m to +5 m in this high-water-level configuration accelerates deformation reduction and enhances displacement control effectiveness. Conversely, at the lower initial water level (+6 m), elevating supports from +3 m to +5 m under this condition yields diminished improvement in deformation mitigation. This confirms that initial water level elevation modulates the sensitivity of lateral deformation control to support repositioning in specific operational scenarios. However, under the surface dewatering condition (black curves), the curves exhibit consistent descent rates. Gradual upward adjustment of supports from +1 m to +5 m produces progressive reduction of lateral deformation throughout the relocation range, demonstrating effective transverse displacement control through support position optimization in this operational mode.

Figure 16 illustrates the variation curves of steel sheet pile lateral deformation with initial water levels under different internal support positions across operational conditions, fitted by the formula y′ = y₁ + A₂e^−x′/t₂ (fitting parameters detailed in Appendix A,

Table A2. Formula y′ = y₁ + A₂e^−x′/t₂ fitting parameter index.

Internal Support Position/Operating Condition		Parameters
		y1	A2	t2	k	tau
Support position +5 m	2	−0.849	0.812	−111.640	−0.009	−77.383
	3	−0.073	0.049	−12.228	−0.082	−8.476
	4	−0.041	0.034	−10.249	−0.098	−7.104
	5	−0.005	0.017	−8.013	−0.125	−5.554
	6	−0.027	0.054	−13.214	−0.076	−9.159
Support position +3 m	2	−0.074	0.066	−14.978	−0.067	−10.382
	3	0.279	−0.311	30.989	0.032	21.480
	4	0.115	−0.158	10.442	0.096	7.238
	5	−0.056	0.058	−14.163	−0.071	−9.817
	6	−0.146	0.166	−32.372	−0.031	−22.439
Support position +1 m	2	−7.969	7.962	−1034.333	−0.001	−716.945
	3	−5.968	5.960	−776.000	−0.001	−537.882
	4	−5.967	5.960	−776.000	−0.001	−537.882
	5	−11.988	11.982	−1552.000	−0.001	−1075.764
	6	−0.057	0.077	−18.832	−0.053	−13.053

). As illustrated in figure, the lateral deformation of the steel sheet piles during construction was directly proportional to the rise in the initial water level height. Under pre-excavation conditions (silty clay intact), lowering support positions systematically amplifies deformation magnitudes. However, the deformation pattern inverts after foundation pit excavation. The black curve (originally lowest deformation trace) shifts to the uppermost position post-excavation, indicating maximum deformation at +5 m support elevation—diametrically opposed to pre-excavation behavior where +5 m positioning minimized deformation.

As the construction progresses, the curve undergoes convergence evolution. Progressive water level drawdown drives deformation pattern convergence among the three curves, demonstrating a diminishing influence of the support positioning. Mechanistically, post-dewatering conditions reduce passive earth pressure on the excavation-side sheet piles, with subsequent silty clay removal further diminishing soil resistance. This dual pressure reduction renders lower support positions (+1 m to +3 m) more effective in controlling lateral movement during the active excavation phases. Therefore, when designing the internal support position, it is necessary to comprehensively consider the influence of the dewatering water level on the inner side of the foundation pit and the excavation depth based on the specific working conditions.

5. Conclusions

Through systematic analysis of the mechanical behavior and deformation characteristics of marine steel sheet pile structures under various construction schemes, this study establishes the influence patterns of different construction sequences on pile stress distribution and deformation. The investigation further quantifies the coupled effects of water level variations and internal support positioning on pile performance. By integrating correlation analyses between construction methodologies, initial hydraulic conditions, support configurations, and operational parameters (including dewatering and excavation phases), the following conclusions are derived for engineering reference:

The staged dewatering construction scheme demonstrates a superior effect on controlling lateral deformation of sheet piles compared to the one-time dewatering construction scheme and results in a more reasonable distribution of the forces acting on the structural components. Compared with the one-time dewatering scheme, staged dewatering enables more effective control of pile head displacement, allowing internal supports to function in a timely manner and thereby optimizing the overall internal force distribution of the supporting structure. However, its influence on longitudinal deformation control is indirect and less significant than its effect on lateral deformation. In the case of one-time dewatering, close monitoring of pile head displacement is required during the early stages of construction, which is not conducive to deformation control. Adjusting the dewatering scheme has limited impact on the longitudinal deformation of sheet piles, and the influence of dewatering construction conditions on such deformation is not significant. In contrast, excavation conditions have a notable effect on the longitudinal stress and deformation of both the soil and the sheet piles.

Under certain specific working conditions, the initial water level height will affect the sensitivity of the internal support position change to the control of the lateral deformation of the sheet pile. The initial water level elevation and different dewatering conditions collectively influence the effectiveness of internal support relocation in controlling lateral deformation of steel sheet piles. In general, higher initial water levels lead to greater lateral deformation, while lower operational water levels (i.e., deeper dewatering depths) also exacerbate this deformation. Upward relocation of internal supports from the soil layer surface progressively reduces lateral deformation. Regardless of initial water level elevations, upward adjustment of internal support positions effectively controls lateral deformation under soil surface dewatering conditions. However, in deeper dewatering scenarios, the efficacy of upward support relocation diminishes significantly when combined with low initial water levels. Therefore, internal support design must comprehensively consider the following factors: the initial water level conditions; the dewatering elevation under critical operational conditions, particularly the magnitude of water level drawdown (greater drawdown extents correlate with intensified deformation effects); and the spatial relationship between current operational water levels and support positions.

The initial water level and the position of the internal support significantly influence the deformation behavior of steel sheet piles. Generally, with the internal support position held constant, a lower initial water level corresponds to reduced pile deformation. Under precipitation conditions, lateral deformation decreases as the internal support is positioned higher; in contrast, under soil excavation conditions, lateral deformation increases with a higher internal support position. Therefore, in both the design of supporting structures and foundation pit construction, it is essential to account for specific engineering and construction conditions. For instance, in riverine or marine environments, after soil excavation, a certain period is typically required before subsequent pile foundation construction begins. During this interval, particular attention should be given to lateral displacement of the steel sheet piles under excavation conditions. If the internal support is placed at a higher elevation in such cases, deformation control may become compromised, potentially leading to stress concentration in the surrounding soil and adversely affecting overall structural stability. Consequently, it is necessary to integrate considerations of initial water level, geological conditions, construction schedule, and environmental factors to rationally adjust the positioning of internal supports and the dewatering strategy. In soft soil regions, the long-term effects of soil consolidation settlement on steel sheet pile deformation should also be evaluated to ensure structural safety and construction quality.