## 3 Establishment of Numerical Analysis Models
### Validation of Model Correctness  
Based on the comparison of the monitoring results of the deformation and internal force of steel cofferdam and tie rods with the numerical simulation results, the correctness of the established numerical analysis model could be verified.  
For the deformation of the steel cofferdam, measurement points were set every 50 m along the axis of the cofferdam, and measurement points BJ17 to BJ23 and BJ27 to BJ31 (a total of 12 points) were selected. A comparison between the monitoring results settlement and numerical simulation results of steel cofferdam is shown in Figure 6a, and the comparison between the monitoring results and numerical simulation results of steel cofferdam horizontal displacement is shown in Figure 6b. For the internal force of the steel cofferdam tie rod (the tension is positive), three sets of measurement points, MG5, MG6 and MG7, were taken. The comparison between the monitoring results and the numerical simulation results is shown in Figure 6c.  
From Figure 6a,b, it can be seen that after construction, the simulated values of the steel cofferdam settlement and horizontal displacement at each monitoring point were basically consistent with the monitoring value changes, indicating that the numerical analysis model was reliable. The simulated average settlement value at each monitoring point was 31.8% higher than the monitoring result, and the simulated average horizontal displacement value was 21.6% higher than the monitoring result. The possible reasons for this are: (1) the numerical model did not consider the effect of time on water seepage in the lake, resulting in a simulated value for the steel cofferdam settlement that was greater than the monitoring value; (2) during simulation, double-row steel sheet piles and tie rods were arranged in the steel cofferdam at the same time, while in the actual project, double-layer steel sheet piles were first constructed, followed by round steel tie rods, forming a complete steel cofferdam. The different construction sequences of round steel tie rods resulted in the simulated value of steel cofferdam displacement, which was greater than the monitoring value.  
Figure 5: Section sizes of steel sheet piles.  
It can be seen from Figure 6c that the simulated internal force of the tie rod of the steel cofferdam was tension, and the variation law was basically consistent with the monitoring values. Since the steel cofferdam tie rod played the role of limiting the displacement of the double-layer steel sheet pile, the upper end of the double-layer steel sheet pile existed far away from the center of the steel cofferdam, and the internal force of the tie rod took on tension as a whole. The maximum internal force simulation value was in MG7, followed by MG5, and the minimum was in MG6. This is basically consistent with the pattern reflected by the monitoring value, which further verified the reliability of the numerical analysis model. The simulation results of the average internal force values of the three measurement points MG5, MG6, and MG7 were, respectively, greater than the monitoring values by 23.1%, 39.8%, and 25.6%. The reason why numerical simulations significantly predicted a force larger than the observed data was that the double-row steel sheet piles and ties in the steel cofferdam were arranged simultaneously in the simulation, while the actual double-row steel sheet piles and ties were not arranged simultaneously; the stress in the soil was released instantaneously in the simulation while the actual construction process was slow; the physical and mechanical properties of the materials used in the simulation were mostly the results of indoor tests, which could differ from the actual situation.