Limit face pressure analysis of an EPB shield under suffusion conditions

Abstract

This study investigated the excavation face stability of an Earth-Pressure-Balanced (EPB) shield tunnel under suffusion conditions using a finite difference method. Based on the seepage erosion tests on gap-graded sand, the suffusion process was properly simulated by the gradual variations in porosity, fines content, permeability coefficient, and dry density. The variation in eroded soil strength was based on the coupling effect of fines content and relative particle roundness according to a series of tests on binary mixtures. A prism-wedge equilibrium model was revised to obtain the limit support pressure of the excavation face under suffusion conditions. It was found that the amount of fine particle loss and relative particle roundness were two key factors in determining the limit support pressure, and the decrease in seepage force resulting from the increase in eroded soil permeability contributed to a smaller limit support pressure than those under seepage-only conditions. The limit support pressure under seepage-only conditions is relatively conservative and is adequate to maintain face stability.