Numerical analysis of tunnels in strain softening soil

Some aspects are discussed of the finite element analysis of underground excavations in soil and rock masses characterized by strain softening behaviour. First the basic features are recalled of a procedure for detecting the bifurcation condition and of a stress-strain relationship taking into account the loss of the mechanical resistance with increasing shear deformation. Then the results of a small scale test on a 3D tunnel model, aimed at investigating the stability at the tunnel face, are compared with those obtained through 3D and 2D finite element analyses, accounting also for softening material behaviour. Finally some comments are presented on the effectiveness of the adopted numerical models.