Stability of unlined twin tunnels in undrained clay

This note presents solutions for the problem of the stability of twin tunnels excavation in soft ground. A compatible displacement field has been derived to idealise ground deformations around shallow, unlined, twin tunnels embedded within an undrained clay layer. Within the boundaries of this displacement field, the soil is required to shear compatibly and continuously with no relative sliding at the boundaries. Upper bounds on the loads required to resist collapse are derived and presented in the form of dimensionless stability graphs that account for the variation of shear strength with depth, the embedded depth of tunnels, and spacing between tunnels.