IMPORTANT CONSIDERATIONS FOR CHARACTERIZING TECTONIC MELANGE FOR TUNNELLING

Tunneling in a tectonic mélange presents many challenges to both tunnel engineers and engineering geologists. In order to develop reliable predictions of the rock mass conditions and thus the rock mass behavior, principles from structural geology need to be used to help define the probable variations in the rock mass structure. These include using the results from field mapping, kinematic analyses, laboratory investigations and numerical simulations to develop realistic distributions of relevant geologic structures. Early work performed by Lindquist and Medley on the engineering characterization of the Franciscan Mélange demonstrated that identifying the block proportion (BP) was critical for evaluating the global strength of the rock mass. The evaluation of monitoring data from tunnels constructed in a polygenetic tectonic mélange, associated with the Mur-Mürztal fault system in the Semmering region of Eastern Austria, have shown that the spatial characteristics of the blocks relative to the tunnel also have a large impact on the rock mass behavior.

Three basic rock mass behavior types have been identified by considering the BP, matrix dominated, block influenced, and block dominated. In order to evaluate the BP, the geologic conditions observed at the face must be extrapolated to a representative element volume (REV) that is related to the zone influenced by the excavation. The behavior of a matrix dominated region is controlled by the local foliation orientation and the associated strength anisotropy. A block influenced behavior is the most difficult to define. If blocks are evenly distributed (size and spatially) then the response can also be homogeneous, when there is an heterogeneous distribution in either size or location, the location of the blocks relative to the tunnel perimeter is the dominate factor for evaluating the behavior. The behavior of a block dominated region approaches that of a faulted rock mass. The location of the matrix zones relative to the excavation dominate the response. The behavior is usually highly anisotropic and the most critical zone is where the matrix zone is located just outside of the excavation. Potential rapid brittle failure of the blocks poses the greatest hazard in this rock mass type. Groundwater problems also increase with increasing BP.