STABILITY CHARTS FOR UNSUPPORTED CIRCULAR TUNNELS IN COHESIVE SOILS

Abstract

This paper investigates the stability of a plane strain circular tunnel in cohesive soils.  A strength reduction technique and the finite difference program FLAC, are used to determine the factor of safety for unsupported circular tunnels. Results from the finite difference approach are presented alongside rigorous upper and lower bound limits computed using Optum G2. A thorough comparison between these two methods finds very good agreement. As the model is shown to be accurate, design charts are presented for a wide range of practical scenarios using dimensionless ratios. The potential usefulness of this approach is demonstrated using a number of examples. This is considered a simpler way of analysing the stability of an unsupported tunnel that may be as practical as similar methods used in slope stability analysis, such as the commonly used Taylor’s design charts.