Abstract：The subway crossing project through Hongxu Road in Shanghai is the pipe-roof project with the biggest cross-section box culvert up to now in the world. It is constructed in saturated soft soil stratum in the Beihong Road located in the middle ring-road in Shanghai. In order to keep cutting surface stable during pushing the box culvert in the front of which many steel grids are inserted；it is very important to design rational sizes of grids. According to analysis of soil arch effect near the cutting surface of the back end of a grid，rational design approach of cross-section sizes of grids is given on the basis of three important conditions，namely mechanical equilibrium equations，shear intensity principle of soil，and self-stability condition of soil mass. Its concrete calculation procedure is given in detail with the practical project.
