An improved 3D wedge-prism model for the face stability analysis of the shield tunnel in cohesionless soils

Sufficient support pressure is required for tunneling by the shield machine to prevent the face collapse. It is found that the failure mechanism and soil aching effect have great influence on the accurate evaluation of the required support pressure by using the limit equilibrium methods, especially in cohesionless soils. In this paper, an improved 3D wedge-prism model which considers the height of the prism and the effect of soil arching was proposed for the analysis of tunnel face stability. In this model, the height of the prism is thought to be min{C, 2L} (where C is the cover depth and L is the width of the prism) by summarizing the results of previous researchers. The effects of soil arching are mainly embodied in the lateral stress ratio between the prism and the adjoining soils. An analytical formula for the lateral stress ratio Ks between the prism and the adjoining soils which reflects soil arching effects was also proposed. Comparing the results (i.e., the limit support pressure) from theoretical models and typical model tests (i.e., centrifuge model test and 1 g model test), the accuracy of the improved wedge-prism model was verified. A design chart was finally proposed for determining the limit support pressure during tunneling in the cohesionless ground.