Numerical modelling of soil-infilled rock discontinuities

Numerical modelling often form an essential part in the design of urban tunnels in order to evaluate the interaction of the geo-structure with other structures in the vicinity of the excavation (buildings, public services, driveways etc.), the design of the support and to establish limits of alert during the construction. With the understanding that rock joints play a key role in a rock mass, the approach used for such prediction should be able to describe all important mechanisms characterising a discontinuous medium. In this scenario, rock joints that are filled with fine materials are likely to be the weakest elements in a rock mass, having a dominant influence on its overall shear behaviour because of the low frictional properties of the infill. This paper presents a discussion on some of the relevant factors affecting the shear behaviour of soil-infilled rock discontinuities. To illustrate the effect of some of those factors in a class A prediction, a hypothetical example of a cavern excavation in a jointed medium is presented. The analysis is based on a discontinuum approach and performed using the distinct element program UDEC. A new semi-empirical yield criterion for soil-infilled rock discontinuities under constant normal stiffness conditions has been employed and compared to the conventional Mohr-Coulomb slip model. The new criterion better describes the shear behaviour of the rock joint resulting in a difference of approximately 50% in the induced settlement at the surface.