The convergence-confinement method applied to weak rock masses

Abstract

ln tunneling, the interaction between the ground and the support may be analysed by the so called convergence-confinement method which takes into account the progress of the excavation face and the ground reological behaviour. To be applied to weak rock masses, it is necessary to define an isotropic or anisotropic equivalent continuous medium; usually an elastoplastic with strain-softening behaviour is assumed. To analyse the time-dependent deformations, the advance rate of excavation and the time-dependant strain must be compared. The most used rheological model is the viscoplastic Bingham model. The cases of stiff support and soft support are to be distinguished. The main drawback of this approach is the difficulty to determine ali the ground parameters. Therefore, the tunneling engineer must check the validity of the model by monitoring the convergences during the excavation works. Precise measurements of convergence may give a valuable information on the rock mass behaviour and its interaction with the support.