Modeling of Large Deformations
in Tunnels

Recent tunnel modeling work performed by Itasca Consultores S.L. (Oviedo, Spain) has dealt with the case where, when large stress/strength ratios are present, tunnels in weak ground present a squeezing behavior that translates to constant closure rates far from the tunnel face. It is not always possible to reproduce this behavior with conventional elastic-plastic constitutive models (below left). Reducing the stiffness of the ground usually produces abnormally large deformation far away from the tunnel. Reducing the strength of the ground in the model can incorrectly produce tunnel collapse at a low deformation level relative to the real deformations (adjacent figure).

For these cases, often it is necessary to use a creep constitutive model (cpow in this case) that introduces a shear strain rate that depends on the deviatoric stress invariant. With this constitutive model it has been possible to calibrate the evolution of the deformations of the heading phase (below right) and then to simulate the behavior of the tunnel in a future phase (the support of the bench phase, the invert or the lining in a long term).