Improvement of the Internal Force Calculation Method for Composite Underground Structure Walls Based on the Incremental Method

As a composite structure with both support and load-bearing functions, the composite underground structure wall has been widely applied in engineering. However, in terms of scientific research, a simplified calculation method that can reflect the internal force distribution law and the interaction mechanism between the two walls has not been found. In terms of design applications, the internal force calculated by the traditional total method has a relatively large deviation from the actual situation. This study proposes an internal force calculation method for composite underground structure walls based on the incremental method. The difference between the at-rest earth–water pressure and the active earth–water pressure is taken as the load increment, which is applied step-by-step according to the construction conditions. Based on the Wangsheren Subway Station in Jinan, China, the actual bending moments of the diaphragm wall and inner lining wall are back-analysis using Plaxis 2D V20 with measured horizontal deformation as input. Models of the incremental method and total method are built in Midas GEN 2022. Bending moment distributions under various conditions are compared. The results show the following: (1) The absolute values of the bending moments of the two walls calculated by the incremental method are inversely proportional along the depth direction, which is consistent with the trend of back-analysis, while the absolute values of the bending moments of the two walls calculated by the total method are directly proportional. (2) The incremental method has a higher calculation accuracy for the characteristic points of the bending moment. In terms of calculated values, the bending moment of the diaphragm wall is 0.87–1.90 times that of the back-analysis, and that of the inner lining wall is 1.06–4.93 times, while the deviation of the total method is significantly larger (0.47–3.34 times for the diaphragm wall and 1.49–16.64 times for the inner lining wall). (3) Under complex working conditions, the calculation results of the incremental method are still better than those of the total method. This incremental method can better simulate the interaction mechanism and the internal force redistribution characteristics of the composite underground structure wall. The calculation results are more in line with the engineering reality, which can save materials while ensuring the structural safety and provides a more scientific theoretical method for relevant designs.