Tunnel stability with special reference to the shear strain energy criterion

Abstract

A comparison of the results of the application of the finite element method and of a modified KIRSCH's solution for the calculation of stress around an unlined tunnel in London Clay is undertaken. Some conventional methods and the finite element method for the calculation of stress and displacement around a tunnel are referred to. An estimate of the reduction of the shear strength parameters along fissures is obtained by laboratory tests on fissured and intact samples of London Clay. On the basis of the MOHR-COULOMB criterion of failure and by the use of a computer technique a delimitation of potentially unstable orientation zones on equal area projections is obtained for all the elements around a tunnel in London Clay. The shear strain energy stored around the discontinuities, which fall in the potentially unstable zones, is computed for the case of a non-random discontinuity fabric of London Clay and the values are normalized to the values corresponding to an equivalent isotropically discontinuous fabric. Then the instability of each area around the tunnel is defined in terms of the principal stresses σ(_1), σ(_2), σ(_3), the shear strength characteristics of the material and the discontinuity fabric.