STABILITY CHART FOR UNSUPPORTED SQUARE TUNNELS IN HOMOGENEOUS UNDRAINED CLAY

Abstract

This short technical note investigates the stability of a plane strain square tunnel in homogeneous
undrained clay using shear strength reduction technique. The finite difference program FLAC is used to determine
the factor of safety for unsupported square tunnels where an automatic generation of program script is developed
using FISH programming. This developed procedure enables parametric studies to be conducted in an effective
way with great efficiency. Numerical results, expressed in term of factor of safety, are compared with published
classical upper and lower bound limit solutions. The comparison between these two numerical methods finds a
very good agreement and the confidence level of the current FLAC model has increased significantly. With the
success of model validations, a number of stability design charts for square tunnels using dimensionless ratios are
presented for practical scenarios in a similar way to Taylor’s slope stability charts. Examples are illustrated to
show the potential usefulness of the produced design charts for practicing engineers.