Failure Modes of Tunnels with Improved Soil Surrounds

Abstract

This paper presents the study of possible failure mechanisms and stability of large-diameter tunnels in cement-treated soil surrounds. The failure modes and critical tunnel support pressure of tunnels with varying strength and thickness were observed from centrifuge model tests. The results of the centrifuge model tests were used to develop a numerical model in geotechnical finite-element software. Both centrifuge and numerical studies were conducted for the plane-strain condition. The results show that the improved soil surround may fail before a generalized plastic collapse occurs in the soft soil. Three failure modes were identified using centrifuge and numerical modeling: shear failure in weak and thin tunnels, multiple cracks or rupture in medium-strength tunnels, and tension failure in higher strength and thicker tunnels. Taken together, these failure modes define a transition from shear failure in thin, weak improved soil surrounds to tension failure in thick, strong improved soil surrounds. A failure map and stability chart have been proposed to enable some form of preliminary design for such cement-treated soil surrounds.