Numerical Investigation on the Interaction between a U-Shaped Pile Supporting Structure and an Adjacent Gravity Retaining Wall in River Dredging

Abstract

There is significant interaction between the new supporting structure and the existing adjacent retaining wall in the dredging and excavation project of urban rivers. In addition, the three factors, the spatial location, the stiffness of the structures, and the soil conditions of the two sides of the interaction will exert effects on the bearing properties of the two structures. Combined with an actual dredging project, FLAC^3D software was applied to analyze the influencing rule of U-shaped concrete sheet pile (USCSP) section size, pile length, retaining wall height, and pile–wall spacing on the supporting structure and the bearing properties of the existing gravity retaining wall during dredging excavation. The results are that when the length of the sheet pile increases, the horizontal displacement of the pile gradually decreases, the horizontal displacement of the existing retaining wall declines, and the earth pressure at the wall’s back rises. With the increase in the section size of the sheet pile, its bending resistance enhances gradually, and the horizontal displacement of the existing retaining wall reduces, while the earth pressure slightly increases. When the pile–wall spacing grows, the interaction between the supporting structure and the retaining wall is gradually weakened under the process of excavation, the horizontal displacements of the sheet pile and the retaining wall decrease continuously, and the earth pressure at the retaining wall’s bottom continues to strengthen. Moreover, with the retaining wall growing, the passive resistance from the soil in front of the wall is greater for keeping the stability of the retaining wall, and the horizontal displacement and the stress of the sheet pile increase significantly after excavation. The above results indicate that the characteristics of the pile–wall interaction should be deeply considered in designing and constructing such projects in order to determine the overall stability of the retaining pile and the existing retaining wall. In this study, FLAC^3D software was used to analyze the influence of various factors on the structure in order to provide reference for ensuring the safety of the whole structure.

1. Introduction

River dredging is an important measure to improve urban flood control systems and enhance flood resilience. When the river channel is adjacent to existing structures, the unloading effect caused by dredging and excavation will have a significant impact on the structural buildings near the river channel. The interaction between excavation and retailing structures is very complex, and excavation unloading will lead to the destruction of the original ground stress equilibrium so that the soil’s stress state is constantly changed until a new equilibrium is reached [1,2,3,4]. The reduced solum strength caused by the process of re-equilibrium will give rise to the change in soil pressure on the supporting structure, so the development rule of supporting structure deformation needs to be simulated and predicted [5,6,7] to ensure the stability of the supporting structure. Moreover, the foundation of the adjacent existing buildings will generate additional stress and lateral displacement because of soil displacement after excavation [8,9,10,11,12,13], and the deformation of the adjacent buildings will be most significant when the foundation pit is dug into the bottom area [14]. Biao et al. [15] found that soil erosion between supporting piles is the main element resulting in the uneven settlement of adjacent shallow foundation buildings. Li et al. [16] found by means of numerical calculation that the excavation of the foundation not only causes additional horizontal displacement of the adjacent existing wall but also makes the retaining wall be lifted unevenly which is caused by the excavation rebound effect. At present, in terms of the influence of soil excavation on adjacent existing structures, studies have been conducted at home and abroad in the field and are described below: the horizontal displacement of the pile foundation of a building will gradually change with time and exert adverse effects on the building [17,18], the settlement of the roadbed will increase rapidly during the construction period and affect the safety of traffic [19], the tunnel will produce uneven deformation thus induce various kinds of problems [20,21,22,23], and the disturbance of the surrounding strata will produce additional internal forces and deformation in station and then affect its normal operation [24], damage the underground pipeline, and threaten its safety [25,26]. However, there are few reports that cover the interaction between the supporting structure and the adjacent existing gravity retaining wall during river dredging excavation today.

During dredging excavation support, compared with traditional pile types, U-shaped concrete sheet piles are widely applied because of their great bending resistance, large retaining width, and easy and fast construction [27]. The cross-section of the USCSP is similar to the I-beam one, and its bearing capacity can be calculated by an equivalent method [28]. This paper uses a large number of numerical simulations to study the interaction effect of the USCSP and an adjacent existing gravity retaining wall during dredging excavation, explores the influence of sheet pile cross-sectional size, pile length, gravity retaining wall height, and pile wall spacing on the bearing performance of the supporting structure and the existing retaining wall, and aims to provide theoretical guidance for both the design and construction of similar projects.

2. Project Profile

The project background is taken from the comprehensive improvement project of the Fengmen River system in Lucheng District, Wenzhou City. The river’s total length is 8.99 km. One side of the river is a municipal road, and the slope is collected by a gravity retaining wall. The stratum of the project area is mainly Holocene alluvium or Holocene marine silt, silty soil, clay, mud-bearing powder-fine sand, and a powder-fine sand layer. The design U-shaped sheet pile is 13 m-long where the loaded section is 5 m and the entrenched section is 8 m. The horizontal distance (pile–wall spacing) between the support pile and the edge of the existing retaining wall base is 1–4 m, the existing retaining wall height is 3–6 m, the wall’s top width is 0.5 m, the slope rate of the breast slope is 1:0.11, and the slope rate of the back slope in Figure 1 is 1:0.3.

3. Finite Element Modeling

3.1. Model Geometry Boundary Conditions

FLAC^3D software (This study uses FLAC^3D 6.0 version, developed by Itasca, IL, USA, and launched in 2017.) was used for simulation analysis. The model is 35 m long, 7.5 m wide, and 23.5 m high, as presented in Figure 2. The computational model of the soil layer is the Moore–Coulomb model, and the physical and mechanical parameters of the soil layer are displayed in

Table 1. Mechanical parameters of the model’s soil layers.

Soil Layer Number	Designation	Thickness (m)	Poisson’s Ratio	Young’s Modulus (MPa)	Density (kg/m3)	Friction Angle (°)	Cohesive Force (kPa)
1	Silt	2	0.35	5.1	1630	13	8
2	Powdery clay	8.3	0.33	12.6	1860	20	10
3	Powdered fine sand	15	0.33	20	1890	20	18
4	Asphalt concrete	0.15	0.3	1200	2400

.

Taking a USCSP with a 1200 mm-high section, a 200 mm-thick slab, and a 1500 mm-wide section as an example, its initial model is shown in Figure 3. The bottom surface of the model is fully constrained, the upper boundary is free, and the rest of the outer boundary is normally constrained.

3.2. Model Analysis Steps

After the initial model was built, ground stress equilibrium was first performed then the construction process was simulated. The simulation process includes three stages: in the first stage, the USCSP is used to support the river channel, in the second stage, the standard axle load vehicle load is applied on the road surface, which is simplified to a rectangular area of 0.34 m × 0.23 m with a uniform load intensity of 0.7 MPa, and in the third stage, the dredging excavation process is simulated with an excavation depth of 2 m in two steps, with each step being 1 m. The step flowchart of the research method is as follows (Figure 4).

4. Results and Discussion

Finite element simulations with different USCSP section sizes, pile lengths, gravity-type retaining wall heights, and pile wall spacing were carried out to analyze the interaction effects between the USCSP and the adjacent existing gravity-type retaining walls during excavation, and the other factors were not changed when analyzing the effects of a certain factor.

4.1. The Influence of Pile Length

4.1.1. The Analysis of the Pile Length’s Effects on the Bearing Characteristics of the USCSP

(1) Horizontal displacement of the USCSP

The pile lengths of 8 m, 9 m, 10 m, 11 m, 12 m, and 13 m were chosen, and the length of the loaded section was guaranteed to be unchanged during the calculation. The changes rule of pile displacement with α for different pile lengths during excavation is displayed in Figure 5. The figure shows that the horizontal displacement of the pile decreases gradually with the growth of α. With the pile length expanding, the horizontal displacement of the pile top decreases continuously. At 1 m excavation, the pile length increases from 8 m to 13 m, and the maximum displacement of the pile top reduces from 11.9 mm to 5.96 mm, decreasing by 49.9%; at 2 m excavation, the pile length increases from 8 m to 13 m, and the maximum displacement of the pile top reduces from 25.9 mm to 11.8 mm, decreasing by 54.4%.

(2) Internal force of the USCSP

Figure 6 shows the variation in pile section stresses for different pile lengths. With α increasing, the pile stress shows a trend of improving first and then decreasing, and the pile stress gradually increases with the increase in the pile length, and when the pile length increases to 12 m, the increasing trend of the pile stress starts to be weakened. When the pile length increases from 8 m to 13 m at 1 m excavation, the maximum pile tensile stress increase from 242.3 kPa to 441.1 kPa, increasing by 82.2% and when the maximum pile compressive stress increased from 414.5 kPa to 635.6 kPa, increasing by 53.3%. When the pile length increases from 8 m to 13 m at 2 m excavation, the maximum tensile stress in the pile increased from 304.1 kPa to 852.1 kPa, increasing by 180.2% and the maximum compressive stress in the pile increased from 600.8 kPa to 1131.4 kPa, increasing by 88.3%. In addition, as the pile expanded, the relative position of the maximum stress point corresponding to the pile shifted upward; for example, the α values corresponding to the maximum stress point of the pile were 0.68, 0.58, and 0.53 when the pile was at 8 m, 10 m, and 13 m, respectively.

4.1.2. The Influence of Pile Length on the Bearing Characteristics of the Retaining Walls

(1) Horizontal displacement of the retaining wall

Figure 7 shows the change rule of horizontal displacement of the existing retaining wall corresponding to different pile lengths after excavation, and the retaining wall shows the displacement pattern of horizontal movement and clockwise rotation along the bottom of the wall. With the increase in pile length, the horizontal displacement of the retaining wall keeps decreasing. The maximum displacement of the retaining wall occurs at the top of the wall; when the pile length increases from 8 m to 13 m at 1 m excavation, the maximum displacement of the top of the wall decreases from 4.28 mm to 3.49 mm, decreasing by 20.3%, and when the pile expands from 8 m to 13 m at 2 m excavation, the maximum displacement of the wall’s top decreases from 8.77 mm to 4.59 mm, decreasing by 47.7%. This is due to the fact that when the pile expands, the overall bending resistance of the support pile is enhanced, corresponding to the reduction in the horizontal displacement of the top of the supporting pile, and the horizontal displacement of the retaining wall is gradually reduced.

(2) Distribution of soil pressure

Figure 8 shows the distribution of the soil pressure at the back of the retaining wall for various pile lengths, and with the excavation deepening, the displacement of the retaining wall gradually expands and the soil pressure at the wall’s back gradually weakens; the maximum soil pressure on the retaining wall is at the wall’s bottom. At 1 m excavation, the pile length increases from 8 m to 13 m, and the soil pressure at the bottom of the wall increases from 24.8 kPa to 38.2 kPa, increasing by 54.0%; at 2 m excavation, the pile length increases from 8 m to 13 m, and the soil pressure at the bottom of the wall increases from 21.3 kPa to 32.2 kPa, increasing by 51.2%. At the same time, with the increase in the pile length, the earth pressure in the wall’s back gradually increases; this is because excavation unloading gives rise to the displacement of the retaining wall from the fill, and the retaining wall tends toward the active limit equilibrium state. The larger displacement of the retaining wall leads to lower earth pressure.

4.2. Effect of U-Shaped Sheet Pile Section Size

Three types of U-shaped sheet piles were selected for analysis, and their cross-sectional dimensions are shown in

Table 2. Cross-sectional dimensions of different types of U-plates.

Cross-Section Type	Type I Plate	Type II Plate	Type III Plate
Plate thickness (mm)	120	160	200
Cross-sectional width (mm)	1000	1200	1500
Cross-sectional height (mm)	500	900	1200

. When building the model, the support pile was 13 m and the pile–wall spacing was 2 m.

4.2.1. Effect of Cross-Sectional Dimensions on the Bearing Characteristics of the U-Shaped Piles

(1) Horizontal displacement of the USCSP

Figure 9 indicates the variation curves of the horizontal displacement of piles under different cross-sectional dimensions of the U-slab pile. Compared with type II and type III, type I has the smallest cross-sectional size, the least flexural modulus, and the largest horizontal displacement at the top of the pile. When the excavation deepens from 1 m to 2 m, the horizontal displacement of the type I sheet pile increases from 10.3 mm to 18.9 mm, the horizontal displacement of the type II sheet pile increases from 6.6 mm to 12.6 mm, and the horizontal displacement of the type III sheet pile increases from 6.4 mm to 11.3 mm.

(2) Internal force of the USCSP

Figure 10 shows the pile stress distribution curves corresponding to different USCSP cross-section sizes. With the increase in the pile section size, the flexural section modulus increases and the maximum tensile and compressive stresses in the pile gradually decrease. When excavating to 1 m, the maximum tensile stress in the corresponding section of Ⅰ plate is 1566.4 kPa, the maximum tensile stress in the corresponding section of the Ⅱ plate is 457.4 kPa, and the maximum tensile stress in the corresponding section of the Ⅲ plate is 419.8 kPa; when excavating to 2 m, the maximum tensile stress in the corresponding section of the type I plate is 2853.6 kPa, the maximum tensile stress in the corresponding section of type Ⅱ plate is 891.1 kPa, and the maximum tensile stress in the corresponding section of the type Ⅲ plate is 793.1 kPa. It can be seen that when the excavation is at 2 m, the maximum tensile stress of the corresponding section of the type Ⅰ plate has exceeded its tensile strength, so it is not suitable to be used in this project.

4.2.2. Influence of Cross-Sectional Dimensions on the Bearing Characteristics of the Retaining Walls

(1) Horizontal displacement of the retaining wall

Figure 11 indicates the change rule of horizontal displacement of the retaining wall at different excavation depths. The retaining wall shows a displacement pattern of horizontal movement and clockwise rotation along the wall’s top, and the horizontal displacement of the retaining wall gradually decreases with the expansion of the cross-sectional area of the USCSP, and the trend of rotation gradually slows down. When the excavation deepens from 1 m to 2 m, the horizontal displacement of the top of the retaining wall increases from 5.5 mm to 9.6 mm for the type I sheet pile, from 4.2 mm to 6.0 mm for the type II sheet pile, and from 3.2 mm to 5.2 mm for type III sheet pile. This indicates that a lower bending resistance of the support pile will lead to more significant interaction between the supporting structure and the retaining wall as well as a more obvious effect between the supporting structure and the retaining wall.

(2) Distribution of soil pressure

Figure 12 indicates the distribution rule of soil pressure at the back of the retaining wall at different excavation depths. The larger the cross-sectional area of the USCSP, the smaller the horizontal displacement of the retaining wall, and the larger the soil pressure at the back of the wall; with the increase in the excavation depth, the soil pressure at the back of the wall gradually weakens. When the excavation depth increases from 1 m to 2 m, the soil pressure at the retaining wall’s bottom corresponding to the type I sheet pile decreases from 38.7 kPa to 34 kPa, the soil pressure at the retaining wall’s bottom corresponding to the type II sheet pile decreases from 40.7 kPa to 39.7 kPa, and the soil pressure at the retaining wall’s bottom corresponding to the type III sheet pile decreases from 43.9 kPa to 41.1 kPa.

4.3. Pile–Wall Spacing Factor Analysis

The pile wall spacing was selected as 1.0 m, 1.5 m, 2.0 m, 2.5 m, 3.0 m, and 4.0 m for analysis and calculation, and the retaining wall’s height was 3 m and the length of the U-plate piles was 13 m during the calculation.

4.3.1. Influence of Pile–Wall Spacing on the Bearing Characteristics of the USCSP

(1) Horizontal displacement of the USCSP

Figure 13 shows the comparative analysis of pile displacement for different pile–wall spacing, and the pile displacement gradually decreases with the increase in pile–wall spacing. This is because the increase in pile–wall spacing leads to a weakened interaction effect between the supporting structure and the retaining wall caused by excavation unloading. At 1 m excavation, when the pile–wall gap increases from 1.0 m to 4.0 m, the pile top displacement reduces from 7.2 mm to 6.2 mm, decreasing by 13.9%; at 2 m excavation, when the pile–wall gap increases from 1.0 m to 4.0 m, the pile top displacement reduces from 14.4 mm to 11.7 mm, decreasing by 18.6%.

(2) Internal force of the USCSP

Figure 14 shows the variation rule of pile section stress at different pile–wall spacing values, and the pile stress gradually decreases as the pile–wall spacing increases, and the interaction influence between the supporting structure and the retaining wall gradually weakens as the pile–wall gap expanding. The maximum stress of the pile section appears at about 7.5 m from the pile’s top. When the pile–wall gap grows from 1.0 m to 4.0 m at 1 m excavation, the maximum tensile stress reduces from 526.1 kPa to 400.9 kPa, decreasing by 23.8% and the maximum compressive stress reduces from 743.3 kPa to 569.0 kPa, decreasing by 23.4%; when the pile–wall gap expands from 1.0 m to 4.0 m at 2 m excavation, the maximum tensile stress decreases from 998.3 kPa to 784.5 kPa, decreasing by 21.4%, and the maximum compressive stress decreases from 1323.5 kPa to 1067.2 kPa, decreasing by 19.4%.

4.3.2. The Influence of the Pile–Wall Gap on the Bearing Characteristics of Retaining Walls

(1) Horizontal displacement of the retaining wall

Figure 15 shows the change rule of horizontal displacement of the retaining wall at different pile–wall spacing, and the horizontal displacement of the retaining wall decreases gradually with the increase in the pile–wall gap, and the interaction between the pile–wall weakens. When excavating at 1 m, the pile–wall gap expands from 1 m to 4 m, and the top displacement of the wall decreases from 4.78 mm to 2.12 mm, decreasing by 55.6%; when excavating at 2 m, the pile–wall gap increases from 1 m to 4 m, and the top displacement of the wall decreases from 9.95 mm to 4.28 mm, decreasing by 56.9%.

(2) Distribution of soil pressure

Figure 16 indicates the distribution rule of soil pressure at the back of the wall with different pile–wall gap values, and a larger pile–wall gap leads to smaller horizontal displacement of the retaining wall when excavating and unloading and greater soil pressure at the wall’s back. With the increase in the excavation depth, the soil pressure of the retaining wall weakens, and the maximum soil pressure of the retaining wall is located at the wall’s bottom. When excavating at 1 m, the pile–wall gap increases from 1 m to 4 m, and the maximum earth pressure at the wall’s bottom increases from 33.1 kPa to 39.7 kPa; increasing by 16.6%; when excavating at 2 m, the pile–wall gap increases from 1 m to 4 m, and the maximum earth pressure at the wall’s bottom increases from 32.5 kPa to 37.7 kPa, increasing by 16.0%.

4.4. Effect of the Height of the Retaining Wall

When analyzing the influence of retaining wall height on the bearing properties of the supporting structure, the retaining wall heights of 2.5 m, 3.0 m, 3.5 m, 4.0 m, 5.0 m, and 6.0 m were selected for analysis and calculation, and the base elevation of the retaining wall was unchanged. The length of the support pile is 13 m, and the pile–wall spacing is 2 m.

(1) Horizontal displacement of the USCSP

Figure 17 shows the change curve of horizontal displacement of the USCSP under different retaining wall heights. The figure shows that the horizontal displacement of the pile gradually increases after excavation with the rise in the retaining wall’s height; this is because a higher retaining wall leads to stronger soil pressure on the wall’s back and higher passive resistance of the soil in front of the wall when the retaining wall remains stable. In addition, the passive resistance will react on the loaded section of the supporting pile. When excavating at 1 m, the wall height rises from 2.5 m to 6.0 m, and the pile top displacement increases from 5.8 mm to 11.0 mm, increasing by 89.7%; when excavating at 2 m, the wall height rises from 2.5 m to 6.0 m, and the pile top displacement increases from 10.9 mm to 29.4 mm, increasing by 169.7%.

(2) Internal force of the USCSP

Figure 18 shows the stress distribution curves of the piles corresponding to different retaining wall heights. With the retaining wall’s height growing, the load on the loaded section of the USCSP increases, resulting in a gradual increase in pile stress; at 1 m excavation, the wall height rises from 2.5 m to 6.0 m, and the maximum tensile stress in the pile increases from 331.2 kPa to 948.7 kPa, increasing by 186.4%, and the maximum compressive stress in the pile enhances from 514.3 kPa to 1243.3 kPa, increasing by 141.7%. At 2 m excavation, the wall height rises from 2.5 m to 6 m, the maximum tensile stress of the pile increases from 565.1 kPa to 2220.3 kPa, increasing by 292.9%, and the maximum compressive stress of the pile increases from 852.3 kPa to 2640.9 kPa, increasing by 209.9%.

5. Conclusions

FLAC^3D software was used to carry out a numerical simulation analysis of the interaction between the USCSP supporting structure and the adjacent existing gravity type retaining wall during dredging excavation, and the following conclusions were obtained:

➢

It is necessary to comprehensively consider the overall structural safety in line with the internal force and deformation of supporting piles and retaining walls. The increase in sheet piles is able to significantly reduce the horizontal displacement of the pile body and the existing retaining wall after excavation but increases the pile body stress and the retaining wall back soil pressure at the same time. Therefore, the pile length design should be a comprehensive process to combine the pile body material strength with existing retaining wall stability.

➢

When the cross-sectional size of the USCSP increases, the bending resistance of its cross-section is improved, and the horizontal displacement at the pile’s top and the stress in the pile is reduced. In addition, the horizontal displacement of the retaining wall gradually decreases, and the tendency of rotation gradually slows down. The type I plate is not suitable for application in this project and cannot meet the tensile strength requirements of C60 concrete.

➢

With the pile–wall spacing expanding, the interaction effect between the supporting structure and the retaining wall caused by excavation unloading is gradually weakened, the horizontal displacement of the pile body, pile stress, and horizontal displacement of the retaining wall are gradually reduced, and the soil pressure of the retaining wall’s back is increased.

➢

As the retaining wall grows, the passive resistance of the soil in front of the wall grows when the retaining wall retains its stability, which results in a constant increase in the horizontal displacement of the pile and pile stress after excavation. In this study, a new type of support structure, a U-shaped concrete sheet pile, was used for numerical simulation to provide guidance for the future application of U-shaped concrete sheet piles in various support projects. Due to the limitations of the numerical simulation software, it is impossible to fully consider the various influencing factors in real engineering. Thus, this study focuses on analyzing the effects of overall structural safety.