Research on Deformation Characteristics and Design Optimization of Super-Large Cofferdam Enclosure Structure

Abstract

With the rapid development of urban transportation facilities, their construction inevitably encounters unfavorable conditions, such as rivers and lakes. When using the weir construction method to build transportation facilities in lakes, the design scheme for the pit slope excavation inside the cofferdam is related to the success or failure of the construction. In this study, the design scheme for pit excavations in lakes is discussed and analyzed using numerical simulation and on-site monitoring against the background of Jinji Lake Tunnel. The findings are as follows. (1) The greater the distance between the pit and the cofferdam, the smaller the impact of pit excavation on the cofferdam is; when the excavation depth is 10 m and the distance is more than 47 m, the deformation of the whole pile is less than 15 mm. (2) Under the condition that the platform width is kept the same, the smaller the height and width ratio is, the greater the safety factor will be, and under the condition that the height and width ratio is kept the same, the longer the platform width is, the greater the safety factor will be. (3) When the pit and cofferdam are far away from each other, different slope release design schemes have different effects on cofferdam deformation and stability. The results of this research can provide references and guidance for the design and construction of similar projects.

1. Introduction

With the development of urban transportation facilities in sea conditions, such as undersea tunnels and cross-sea bridges, there are more and more projects crossing rivers and lakes. At present, the commonly used construction methods for crossing rivers and lakes are the shield method, cofferdam method, and immersed pipe method [1,2,3,4,5,6]. Compared with the other construction methods, the cofferdam method is characterized by low cost, high efficiency, and low construction difficulty and it is suitable for areas with relatively slow water flow and shallow water depth [7,8]. However, there are fewer engineering cases related to the excavation of oversized, deep foundation pits inside cofferdams, and the form of the cofferdam structure and the spacing between cofferdams and foundation pits are related to the success or failure of engineering construction. Therefore, it is necessary to research the deformation characteristics and design optimization of cofferdam and pit enclosure structures to ensure construction safety, improve construction efficiency, and provide references for similar projects.

Extensive research has been conducted on the structural forms of cofferdams and construction plans [9,10,11,12,13,14] to study the support strength of squares, timber pile islands, and tipping cofferdams. Sulovska et al. [15] used Plaxis finite element software to analyze the stability of a cofferdam backfilled with stone around the outer sheet pile wall, and the horizontal displacement of the sheet pile wall was reduced by 30%. Jiang et al. [16] used the same software to analyze the deformation characteristics and stability of a cofferdam during the super-historic flood period, and the safety factor of the cofferdam was correlated with the cofferdam slope ratio. Wang et al. [17] numerically simulated the excavation process for a steep can-reinforced riverbank slope using FLAC3D finite element software and established that setting single or multiple rows of steel pipe piles could improve the cofferdam’s stability. Cofferdam stability is primarily improved by changing the structural form, backfilling the stone on the outside, and adding steel pipe piles. However, there are few studies on the effect of pit excavation on cofferdams.

During the excavation of the foundation pit, the impact on the surrounding environment can be reduced by improving the support system and changing the excavation method. Wu et al. [18] proposed a method for the Taihu Lake Tunnel; in this method, a cofferdam of double-row steel sheet piles (DSSPs) was designed to divide the overlying excavation into several closed zones. This construction method ensured minimal disturbance to the cofferdam. A bipartition wall (BPW) construction method was introduced by Zhang et al. [19], which was developed to reduce the excavation-induced adverse impact on the supporting structures and surrounding environment for oversized, deep foundation pits. Li et al. [20] divided large, deep foundation pits into small independent areas for excavation and proposed a construction method in separate phases to reduce their impact on the surrounding environment. Ye et al. [21] investigated the effect of pit excavation in Hefei on the deformation of an adjacent subway tunnel and found that the closer the excavation was to the tunnel, the more sensitive the deformation was. Zhang et al. [22] studied the effects of pits with different support and excavation methods on adjacent pipelines and found that slope-release excavation can significantly reduce pipeline deformation. Zhao et al. [23] excavated foundation pits using rotary spray piles and a four-layer support system. According to the monitoring results, this foundation pit support system could limit the settlement around the road, the horizontal displacement of the ring beam, and the deep soil displacement of the foundation pit. These methods reduce the impact on the surrounding environment by dividing the large foundation pit into several areas, changing the support system, and using other conventional methods. However, it is not possible to directly determine the optimal distance or the effect of the excavation method on the surrounding structures.

The foundation excavation process affects the deformation of the surrounding structures, which is usually determined through on-site monitoring; however, monitoring cannot predict future development. Many scholars have used numerical simulations [24,25,26,27,28,29,30] to predict the deformation of surrounding structures and the environment under different construction scenarios to provide a reference for the construction of actual projects.

In this study, we investigated the effects of the lake pit excavation process on cofferdam deformation characteristics, employing an analysis of different distances from the impact of pit excavation, a stability analysis of the use of different slopes in the excavation method, and a comparative analysis of pit excavation distance combined with the excavation method. Looking at the Jinji Lake Tunnel Project, the effect of cofferdam design optimization was verified through on-site monitoring, and the mechanism of action was analyzed.

2. Engineering Background

2.1. Project Overview

This study considered as background a large tunnel project in an inner-city lake. This is the first tunnel project in China that crosses a 5A-level lake scenic area and involves the longest open-cut urban lake-bottom public rail tunnel and the longest and widest cofferdam open-cut tunnel in China. The cofferdam is divided into north and south sides; the starting and ending mileage of the south side is S0+000–S3+201.5 and the total length is 3201.5 m, whereas the starting and ending mileage of the north side is N0+000–N2+890.53, with a total length of 2890.53 m. The total length of the lateral joint of the cofferdam is 877.37 m. The total length of the cofferdam is 6969.4 m. The cofferdam is divided into phase I and phase II, and N1+564.11–N1+864.13 of phase I of the cofferdam is the phase I preemptive implementation section. The specific division of the Jinji Lake cofferdam is shown in Figure 1.

According to the engineering geology, the soil layers at the bed of the lake from top to bottom are silt, powdery clay, powdered earth, chalky soil with chalky sand, powdery clay, clay, powdery clay, and, lastly, chalky soil with chalky sand. The profile of and soil distribution in the lake are shown in Figure 2.

2.2. Construction Method

The structures in the middle section of the lake were constructed using a cofferdam as a temporary water-retaining structure, within which dry-cut construction was carried out. The construction sequence mainly included measurement and sampling, installation of guide frames, setting of ties and walings, cofferdam filling, cofferdam pumping, pit excavation, and tunnel construction. The construction sequence is shown in Figure 3.

The construction sequence was as follows:

(1) Measuring and installing a guide frame to ensure accurate positioning of steel pipe piles and verticality in the process of laying, using a manipulator with a crawler crane and vibratory hammer to lay the steel pipe piles;

(2) Setting of tie rods and purlins and transportation of cofferdam filling soil to the operation section by grab ship from above the main tunnel line. Soil was first placed up to the upper layer of tie rods and then left for a week to allow the filler to naturally settle and solidify; then, the remaining soil was added in layers. The top layer compaction measured with the sand filling method was not less than 90% of the design value. It was compacted by a small roller, and after being left to stand for a period of time to stabilize, the construction of the concrete pavement on the top of the weir was carried out;

(3) A pump was installed every 120 m along the cofferdam on the south side, and during the pumping process, the pumping speed was strictly controlled and the cofferdam was observed. Pit excavation and tunnel construction were then initiated.

3. Introduction to Finite Element Analysis Models

3.1. Calculation Model and Parameter Introduction

To study the effect of pit excavation on the cofferdam enclosure structure, the numerical calculation software Midas GTS NX 2022 was used to calculate and analyze the cofferdam enclosure structure. The linear elastic model was used in the model for locking steel pipe piles, Larsen steel sheet piles, steel pipe piles, walings, and tie rods, and an ideal elastic-plastic model with the Mohr–Coulomb criterion was used for soils and inter-pile soils. The total length of the combined-method piles on the waterward side is 23 m; the embedment depth below the lake bottom line is 15 m and the cantilever height above the lake bottom line is 8 m. Steel pipe piles are used on the backwater side with a total length of 18 m; the embedment depth below the lake bottom line is 10 m and the cantilever height above the lake bottom line is 8 m. A double-layered tie-bar buttress is used for connecting the locking steel pipe piles and the steel pipe piles, and the width of the soil between the cofferdam piles is 8 m. The elevation of a 20-year flood would be 5.0 m. At this time, the deformation and internal force are also the largest, so the model used the water level of 5.0 m for the calculation. Due to the symmetry of the foundation pit, half of the foundation pit was selected for the modeling. The depth of excavation of the foundation pit is 10 m, as shown in Figure 4, and the physicomechanical indexes of the various layers of the soil are shown in

Table 1. Soil layer parameters.

Name of Soil Layer	Volumetric Weight (kN/m3)	Elastic Modulus (kPa)	Poisson’s Ratio	The Angle of Internal Friction (°)
①2 Silt	16.0	15,880	0.49	3.0
③2 Powdery clay	19.3	22,960	0.49	12.7
③3 Powdered earth	19.2	24,720	0.48	28.7
④2 Chalky soil with chalky sand	19.6	23,290	0.45	30.5
⑤1 Powdery clay	19.1	30,560	0.39	12.4
⑥1 Clay	20.4	25,920	0.38	12.1
⑥2 Powdery clay	19.9	27,280	0.42	13.6
⑦2 Chalky soil with chalky sand	19.5	38,720	0.36	29.2

The circled numbers represent different soil layers and are mainly used to differentiate between the different soil layers, which correspond to the contents in Figure 2.

.

3.2. Calculation and Analysis of Parameter Settings

(1)

Effects of different distances of pit excavation on the inside of the cofferdam

To clarify the impact of pit excavation at different distances on the inside of the cofferdam, it was necessary to divide the excavation distance on the inside of the cofferdam (as shown in Figure 5), combine it with the deformation of the cofferdam with different locations for the pit excavation, and select the safe distance that minimizes the impact on the cofferdam. A 1:2 aspect ratio secondary slope excavation form was used for the pit excavation, and the depth of each layer was 5 m. To ensure the safety of the pit excavation, 12 different values were used for the horizontal spacing between the inside of the cofferdam and the top of the pit excavation slope: 5 m, 10 m, 15 m, 20 m, 25 m, 30 m, 35 m, 40 m, 45 m, 46 m, 47 m, 48 m. The excavation was carried out in the form of a two-stage slope to ensure the safety of pit excavation, and the depth of each layer was 5 m.

(2)

Different excavation methods for foundation pits

To determine the most appropriate method of slope release and excavation, three groups of comparison models were established, and there were four different working conditions in each group of models. The slope stability was analyzed under 12 different working conditions, as shown in Figure 6, to choose the most appropriate excavation method under different conditions.

(3)

Effects of different excavations in the pit on cofferdams

Based on the comparison of the excavation distance of the pit, as well as the excavation methods, the appropriate program was selected for comparison, and the effects of different excavation methods on the deformation and stability of the cofferdam were compared in terms of the horizontal displacement and bending moment of the double-row steel pipe piles.

4. Analysis of Calculation Results

4.1. Computational Programming

Through the design and analysis of the calculation scheme, the optimal design optimization scheme was determined in terms of three aspects: different distances of pit excavation on the inside of the cofferdam, different excavation methods for the pit, and the optimal combination of pit excavation distances and excavation methods. The calculation scheme is shown in Figure 7.

(1) The deformation and bending moments of the double-row steel pipe piles at different distances were analyzed for pit excavation with different distances on the inside of the cofferdam, comparing the piles from pit excavation one and two and the piles above and below the bottom surface of the lake to determine the pit excavation distances with the smallest impact.

(2) For different excavation methods for foundation pits, we analyzed the slope stability under different working conditions and compared different platform widths with the same aspect ratio and different aspect ratios with the same platform width to choose the appropriate excavation method for the foundation pit.

(3) Based on the research on different distances for the pit excavation and different excavation methods, a suitable combination of working conditions was selected. We analyzed the deformation, bending moment, overall stability, and the comparison of the excavation cross-section area of the double-row steel pipe piles under different combinations of working conditions and finally determined the optimal design optimization scheme.

4.2. Analysis of the Deformation of the Enclosing Piles on the Waterward Side

(1) As shown in Figure 8a,b, due to the closer distance between the pit and the cofferdam, there is a greater impact from pit excavation unloading on the piles below the bottom surface of the lake, and with the increasing distance, the combined piles on the water-facing side of pit excavations one and two undergo clockwise rotational deformation, which has a greater impact on the piles above the bottom surface of the lake. The pile deforms from an “L” shape to a straight line. In pit excavation one, with the bottom surface of the lake as the dividing interface, the deformation of steel pipe piles above the bottom surface of the lake at distances of 5 m and 10 m is greater than that below the bottom surface of the lake, and due to the proximity, the unloading of the pit produces a bulge and a movement in the side slopes of the pit toward the inward direction, which drives the combined piles to the side facing the water. When the distance is 15 m, the deformation of the combined piles is approximated as a straight vertical translation due to the increase in distance and the effect of pit unloading on the piles’ movement towards the backwater side. At distances of 5, 10, and 15 m, the combined pile deformations are rotated clockwise using a position 2 m above the bottom surface of the lake as the axis point.

In pit excavation two, due to the use of a 1:2 aspect ratio slope release excavation form, the distance to the cofferdam is greater than the distance for excavation one, so when the distance is 20 m, due to the increase in excavation depth, the combination piles at a depth −8 m below are mainly subject to inward movement due to the slope of the pit. When the deformation of the pit tilts inward, the combination piles at a depth −8 m above are subject to the unloading of the pit generated by the uplift and the inward movement due to the pit slope of the common pile body seems to be a straight vertical translation. At depths greater than −8 m, the pile body is approximately vertically translated in a straight line due to the joint action of loading and unloading and the inward movement of the foundation slope. The axial point of rotation from a distance of 15 m to a distance of 20 m is 5.6 m above the bottom surface of the lake, and it can be raised under the influence of the augmentation of the unloading from pit excavation one. When the distance is greater than 20 m, it is mainly affected by the inward movement.

(2) As shown in Figure 8c,d, when the distance between the excavation location of the foundation pit and the cofferdam is more than 20 m, the overall deformation of the pile decreases with the increasing distance, and the greater the pile length is, the smaller the deformation. The transformation of the combined piles into a diagonal distribution is mainly due to the inward movement of the slopes of the pit excavation. A plane-rectangular coordinate system was established with the bottom surface of the lake and horizontal displacements of 10 mm and 15 mm as dividing lines. In pit excavation one, the distances of 20 m and 45 m cross two quadrants, and when the distance is greater than 45 m, the deformation of the whole of the combined piles is less than 10 mm. In the range from 20 to 45 m, the combined piles all pass through three quadrants, whereas in the range near 35 m, the combined piles pass through the first and third quadrants. When the distance is greater than 35 m, the combined piles have a displacement of less than 10 mm for the part of the piles above the bottom surface of the lake, and when the distance is less than 35 m, the combined piles have a displacement greater than 10 mm for all of the piles above the bottom surface of the lake. In pit excavation two, the excavation distance is greater and the excavation area smaller compared to excavation one, so the deformation increases to a lesser extent than in excavation one. When the distance is greater than 47 m, the deformation of the whole combined piles is less than 15 mm.

4.3. Analysis of Deformation of Backwater-Side Enclosure Piles

(1) As shown in Figure 9a,b, the rotation and deformation trends for the steel pipe pile on the backwater side are the same as those for the combined piles on the waterward side, but the rotation angle of the steel pipe piles on the backwater side is smaller than that of the combined piles on the waterward side. In pit excavation one, with a distance of 20 m, the deformation of steel pipe piles above the bottom surface of the lake is less than that below the bottom surface of the lake. For foundation pit excavation two, with a distance of 30 m, the combined piles at depths −5 m above, due to the foundation pit unloading generated by the uplift and the foundation pit, slope to the inner side of the movement of the joint role of the pile body and approximate a straight line vertical translation. Depths below −5 m are mainly subject to rumbling from pit unloading and slope towards the waterward side.

In pit excavation one, the point where the shaft rotates from a distance of 5 m to a distance of 10 m is at a location 3 m above the bottom surface of the lake, the point where the shaft rotates from a distance of 10 m to a distance of 15 m is at a location 1 m above the bottom surface of the lake, and the point where the shaft rotates from a distance of 15 m to a distance of 20 m is at a location 4.5 m above the bottom surface of the lake. In pit excavation two, the point where the axis rotates from a distance of 5 m to a distance of 10 m is at a location 5 m above the bottom surface of the lake, the point where the axis rotates from a distance of 10 m to a distance of 15 m is at a location 2 m above the bottom surface of the lake, and the point where the axis rotates from a distance of 15 m to a distance of 20 m is at a location 8 m above the bottom surface of the lake. Due to the effect of the unloading augmentation of pit excavation one, pit excavation two has a higher axis for the point of rotation.

(2) As shown in Figure 9c,d, we established the plane right-angle coordinate system. When the distance between the excavation position of the foundation pit and the cofferdam is more than 20 m, the steel pipe pile gradually tilts to the backwater side while the deformation gradually decreases with the increasing distance, and the deeper the pile length is, the smaller the deformation. The deformation of steel pipe piles above the bottom surface of the lake is approximately straight, and the deformation of steel pipe piles demonstrates approximately vertical translation with the increase in distance. Due to the structural form of the double-row steel pipe pile combination, the water pressure on the water-facing side is mainly transmitted to the combination of steel pipe piles and inter-pile soils, and the inter-pile soils themselves can provide a certain level of horizontal resistance so that the steel pipe piles on the backwater side are less affected. In pit excavation one, when the distance is more than 45 m, the overall deformation of the steel pipe piles is less than 10 mm, and in pit excavation two, when the distance is more than 47 m, the overall deformation of the steel pipe piles is less than 15 mm.

4.4. Bending Moment Analysis of Double-Row Steel Pipe Piles

As shown in Figure 10, with the gradual increase in the distance, the bending moment of the double-row pile gradually decreases, but the deformation law for the double-row pile bending moment does not change. The bending moment curve for the front row of piles has a reverse “S” shape and the bending moment curve for the back row of piles has an “S” shape. Due to the similarity of the double-row steel pipe piles and the end of the cantilever, there is a maximum value for the bending moment near the bottom surface of the lake, and the bending moment of the combined piles on the water-facing side is larger than the bending moment of the steel piles on the backwater side. With the increase in distance, the deformation patterns of the bending moments of the front and rear rows of steel pipe piles above the lake bottom surface do not change, indicating that the front and rear rows of steel pipe piles above the lake bottom surface are mainly affected by the water pressure, and the excavation of the foundation pit mainly affects the steel pipe piles below the lake bottom surface. As the distance increases further, the bending moment of the front row of steel pipe piles below the bottom surface of the lake decreases gradually, while the bending moment of the back row of steel pipe piles is less related to the distance. The maximum value of the positive bending moment of the combined piles on the waterward side appears near the bottom surface of the lake, and the maximum value of the negative bending moment appears nearly 10 m below the bottom surface of the lake. The maximum value of the positive bending moment of the steel pipe piles on the backwater side appears nearly 1 m below the bottom surface of the lake, and the maximum value of the negative bending moment appears nearly 2.5 m above the bottom surface of the lake.

4.5. Stability Analysis of Pit Slopes

To compare the feasibility and safety of excavation with different slopes, a finite element model was established according to the soil parameters of the site, as shown in Figure 11a, and the strength discount method was used for calculation. The safety factor is not only related to the aspect ratio but also to the platform width. As shown in Figure 11b, for the same platform width, the smaller the aspect ratio is, the greater the safety factor. For the same aspect ratio, the greater the platform width is, the greater the safety factor. From the 1:2 aspect ratio to the 1:2.5 aspect ratio, the safety coefficient increases significantly, and from the 1:2.5 aspect ratio to the 1:3 aspect ratio, the safety coefficient increases slowly. Under the condition of a platform width of 3 m, the safety coefficient of a 1:2.5 aspect ratio is 2.474, and the safety coefficient of a 1:3 aspect ratio is 2.476; the safety coefficients of the two are almost equal. By analyzing the finite element results for the platform width of 3 m and different aspect ratios, it was found that the slip fracture surfaces all show circular arcs, and the areas of larger plastic strains are all at the foot of the slope, indicating that the plastic zone of the slope is not penetrated and no slip damage occurs.

4.6. Analysis of Deformation and Stability of Cofferdam with Different Excavations of the Foundation Pit

Through the stability analysis of the sloping excavation, the excavation method with a platform width of 3 m and 1:2, 1:2.5, and 1:3 aspect ratios was selected. As shown in Figure 12, for the excavation of foundation pit one, the deformations of the combined steel pipe piles on the water-facing side and the steel pipe piles on the backwater side with the three different aspect ratios are very close. For pit excavation two, the three different height-to-width ratios have different deformations, and the deformation size relationship is 1:2 > 1:2.5 > 1:3. Above the bottom surface of the lake, the deformation of the combined steel pipe piles on the water-facing side decreases gradually from the top of the piles to the end of the piles, and the maximum horizontal displacement occurs at the top of the piles. This displacement is maximized at the top of the steel pipe piles on the water-facing side under the action of the tie rods. Below the bottom surface of the lake, the horizontal displacement of the steel pipe piles on the backwater side is significantly larger than that of the combined steel pipe piles on the waterward side. There are three main reasons for this: the first is that the waterward side is further away from the excavation of the foundation pit compared to the backwater side; the second is that the embedment depth of the steel pipe piles on the waterward side is greater than that of the steel pipe piles on the backwater side; and the third is that there is an inter-pile between the waterward side and the backwater side, and the gravitational force of the soil between the piles can increase the vertical stress on the soil body below the inter-pile and this vertical stress can increase the horizontal resistance of the soil.

As shown in Figure 13, the effects of the three different height-to-width ratios on the bending moments of the piles are very small, and the bending moments of the steel pipe piles on the backwater side hardly change from the top of the piles to the end of the piles. However, the combined piles on the waterward side change slightly under the combined effect of the water pressure and the excavation of the foundation pit. In the height range of the lake, the bending moment decreases as the height-to-width ratio decreases, and at the depths below 5 m in excavation two, the smaller the height-to-width ratio is, the smaller the bending moment is, which is due to the deep embedment of the combined piles on the water-facing side, which hinders their deformation.

As shown in Figure 14, the intensity discount method was used to analyze the pit excavation with three aspect ratios. In pit excavation one, plastic areas appeared in the slopes and cofferdams with the three aspect ratios, but there was no penetration, which indicated that the cofferdams and the slopes had the same degree of danger, and the safety coefficients for the aspects ratios of 1:2, 1:2.5, and 1:3 were 3.60, 3.80, and 3.87, respectively. In pit excavation two, for the three aspect ratios, only the side slopes showed plastic areas, and all of them showed plastic deformation at the foot of the slope, but no penetration occurred, indicating that the danger for the side slopes at this time was greater than that for the cofferdams, and the coefficients of safety for the aspects ratios of 1:2, 1:2.5, and 1:3 were 1.99, 2.20, and 2.30, respectively. There was not a great difference between the coefficients of safety for the aspects ratios of 1:2.5 and 1:3. A comparison of the excavation cross-sections for both the 1:2.5 and 1:3 aspect ratios revealed that the 1:2.5 excavation cross-section was 5.38% smaller than the 1:3 excavation cross-section, as shown in Figure 15, and that the use of the 1:2.5 aspect ratio could save costs and reduce the duration of the project.

5. Analysis of Monitoring Data

5.1. Monitoring Program

This project is located inside the Jinji Lake and uses a large cofferdam open excavation method to ensure water-free operation in the foundation pit. Monitoring cofferdams is an important task to ensure safe construction, and monitoring tasks were carried out based on the following measures:

(1) The monitoring points for stress and deformation were selected scientifically and reasonably, and the monitoring points were marked on the top of the piles of the cofferdam. The distance between the monitoring points at the top of the piles on the same side was 300 m. The monitoring points are shown in Figure 16, where I-ZD is the horizontal displacement of the locking steel pipe pile and I-WD is the horizontal displacement of the steel pipe pile;

(2) The horizontal displacement of monitoring pile at the top of the cofferdam was measured using a Leica TS09 total station, a real-time grasp, and an analysis of cofferdam deformation to ensure cofferdam safety;

(3) The inspection cycle and frequency for cofferdams and back-pressure soil should be enhanced, especially in the presence of water leakage and broken ties, once abnormalities have been quickly marked and reported.

The cofferdam was used to monitor data to ensure the safety of the construction. The numerical simulation results were used as references to compare the deformations in the field. Monitoring was divided into three stages: slope release excavation stage one, slope release excavation stage two, and the tunnel construction stage. The monitoring phases were divided to comprehensively analyze the deformation in each construction phase and guarantee construction safety.

5.2. Monitoring Analysis

(1) To increase the validity of the data, six monitoring points (I-ZD21–I-ZD26) in the cofferdam were selected for comparative analysis. As shown in Figure 17, the variation pattern of the horizontal displacement remained almost the same for the six monitoring points. There was no monitoring for data failures or corruption. With the increasing depth of the slope release excavation, the horizontal displacement of the pile top increased until it stabilized at the end of the slope release excavation. The change in horizontal displacement at the top of the pile fluctuated significantly during the process of slope release excavation one, and the maximum value of the horizontal displacement reached 8.2 mm, which was smaller than the numerical simulation result of 9.14 mm. The fluctuation in the horizontal displacement of the pile top in the process of slope release excavation two was small, and the horizontal displacement reached 12.6 mm, which was also smaller than the finite simulation result of 13.7 mm.

(2) As shown in Figure 18, the variation law for the horizontal displacement of the top of the pile on the backwater side was the same as that for the horizontal displacement of the top of the pile on the waterward side. During slope release excavation one, the maximum horizontal displacement of the pile top reached 8.5 mm, which was smaller than the finite element simulation result of 9.0 mm. The horizontal displacement during slope release excavation two reached 11.89 mm, which was less than the finite element simulation result of 13.48 mm.

(3) As shown in Figure 19, the pile site monitoring maps were all smaller than the simulated values, and they were all nonlinearly correlated. The deformations of the piles below the lake bottom line were all smaller than for the piles above the lake bottom line. With the appropriate pit excavation distance and method, the deformation of the cofferdam was effectively suppressed, and the on-site monitoring data fully proved the significant effect of the design optimization.

6. Conclusions and Recommendations

In this paper, the combination of numerical simulation and on-site monitoring was used to study the structure of a cofferdam, and some understanding of regularities was obtained. It was found that, when large-scale pit excavation was carried out inside the cofferdam, the appropriate pit excavation distance and excavation method could effectively protect the cofferdam’s safety when the actual conditions of the project permitted it.

(1) The greater the distance between the pit and the cofferdam, the smaller the influence on the cofferdam will be, and the deeper the embedding of the pile, the smaller the deformation of the pile buried in the soil. If the excavation depth is 10 m and the distance between the pit and the cofferdam is greater than 47 m, the deformation of the whole pile will be less than 15 mm.

(2) With the same platform width in the slope excavation, the smaller the aspect ratio is, the greater the safety coefficient will be; with the same aspect ratio, the wider the platform is, the greater the safety coefficient will be. When the aspect ratio and platform width increase to a certain degree, the safety coefficient tends to stabilize, and the selection of the appropriate aspect ratio and platform width is effective in ensuring safety.

(3) When the pit is far away from the cofferdam, the appropriate sloping excavation method can ensure a reduced impact on the cofferdam and overall stability. Field application showed that the appropriate pit excavation distance and method effectively reduced the impact on the cofferdam, providing a reference for the design and construction of cofferdam pit excavations in lakes.

(4) In this paper, wave loads were not considered in the numerical simulation, and the effect of the cofferdam on the pit excavation under dynamic loads should be considered in subsequent work. In the stability analysis of the cofferdam, only the pit excavation was studied in detail, and we did not study the influence of the cofferdam structural form. Subsequent work can study the cofferdam pile spacing, the number of layers of tie rods, and other aspects.