## 4 Analysis of the Mechanical Properties of Steel Sheet Pile Cofferdam Construction
### Mechanical Characteristics of Steel Sheet Pile Cofferdam during Construction  
(1) The settlement of steel cofferdam  
To study the effect of the tunnel's construction on the deformation of adjacent steel sheet pile cofferdam, the simulated values of the settlement on the steel sheet pile cofferdam from the side of the adjacent lake to the side of the foundation pit were extracted from working conditions 1 to 6, and the relationship between the settlement of the cofferdam top and the horizontal distance was obtained (see Figure 7). It can be seen from Figure 7 that the maximum settlement of the cofferdam increased gradually with the construction. During the construction, the maximum settlement appeared on the side near the foundation pit, at a distance of 0.27\(-\)0.53 m; after the completion of construction condition 6, the maximum settlement was 8.16 mm. (2) Horizontal displacement of steel cofferdam  
The simulation results of the horizontal displacement of the steel sheet piles cofferdam at different positions and depths during tunnel construction are shown in Figure 8.  
From Figure 8a, it can be seen that under the influence of multiple factors such as water pressure and soil pressure, the maximum horizontal displacement of the steel sheet pile cofferdam on the side adjacent to the lake always appeared at a depth of 11.27-12.67 m under different construction conditions. With the progress in construction, the maximum horizontal displacement gradually increased. After the completion of construction condition 6, the maximum horizontal displacement (0.12 mm) appeared at a depth of 11.73 m,  
Figure 7: Settlements on cofferdam top at various horizontal distances. indicating that the backfill of the soil and geotextile on the side adjacent to the lake could effectively restrain the horizontal displacement of the cofferdam on the side adjacent to the lake. From Figure 8b, it can be seen that the lateral steel sheet pile of the foundation pit was mainly affected by soil pressure, and the horizontal displacement of the steel sheet pile gradually increased with depth; the maximum horizontal displacement of the sheet pile under different construction conditions always appeared at the bottom of the steel sheet pile, and the average horizontal displacement of the lateral steel sheet pile of the foundation pit was greater than that of the adjacent lakeside; after the completion of construction condition 6, the maximum horizontal displacement (2.92 mm) appeared at the bottom of the steel sheet pile. (3) Axial force on tie rod in steel cofferdam  
Fifteen round steel tie rods were arranged within a 30 m tunnel length range. The eighth rod, located in the middle position, was selected, and the tie rod axial force under different construction conditions was extracted. The simulation results of the tie rod axial force at different positions on the side of the adjacent lake are shown in Figure 9.  
From Figure 9, it can be seen that maximum tension appeared in the lateral tie rod of the adjacent lake; from the adjacent lakeside to the foundation pit side, the internal force  
Figure 8: Relationship between depth and horizontal displacement of cofferdam. (**a**) Adjacent lakeside. (**b**) Foundation pit side. Figure 9: Monitored axial forces of tie rods adjacent to the lake at various distances. of the tie rod gradually decreased, decreased to a minimum value at a distance 3.87 m from the lake, and then gradually increased to the maximum tension, showing a U-shaped distribution overall. (4) Water seepage in the lake  
The simulation results of the seepage path of lake water in the cofferdam under different construction conditions are shown in Figure 10. From Figure 10, it can be seen that water in the lake could flow around the steel sheet pile; as the construction progressed, the seepage gradually increased, and water could push out at the bottom of the foundation pit.