The stability of shallow tunnels and underground openings in cohesive material

Abstract

A basic engineering decision to be made in designing a tunnel in soft clay is whether or not the tunnel can be excavated without internal support. The safety of constructing a shallow heading in soft clay can be assessed in terms of the fluid support pressure which may be required to maintain stability. This can be estimated by means of the lower and upper bound theorems of plasticity. The Paper considers three different shapes of shallow underground opening relevant to tunnelling and upper and lower bound stability solutions are derived for collapse under undrained conditions. Solutions are also derived for assessment of the risk of blow-out failure caused by excessively high fluid pressures. Conditions are considered under which local collapse can occur, independent of the cover above the tunnel. © 1980, Thomas Telford Ltd. All rights reserved.