The Influence of Fresh and Weathered Rock Foliation on the Stability of the Monte Seco Tunnel

The Monte Seco tunnel (MST) is one of several old unlined tunnels constructed for Brazilian railways in the 1950s, and the MST still receives intense train traffic (primarily for iron ore transportation). The MST was constructed in a fractured gneissic rock mass, which has potential instability problems associated with gravity-driven block falls (at low stress confinements) defined by three fracture sets and the metamorphic foliation. Previous geological and geotechnical surveys at the MST site have included basic geomechanical classifications from outcrops and drill core specimens, detailed terrestrial laser scanning (TLS) mapping of foliation and fracture planes, geoelectrical imaging and laboratory tests. These surveys discovered two sections of the tunnel (referred to herein as MS1 and MS2) with major potential for instability problems (i.e., poor rock mass qualities). In this paper, the 3D discrete element method (DEM) was used to assess the potential stability problems in these sections with particular interest on the role of the rock foliation strength on the current stability state of these sections. Three instability indicators were developed and monitored during and after the DEM simulations: the total failed block volume (FBV), the average displacement of the tunnel face (AD), and the percentage of slipping contacts near the tunnel face (SC). The weathering effects were considered in two different scenarios: the occurrence of isolated weathered foliation planes (WFPs) and the general weathering advance through potentially weathered regions. In the latter scenario, the percentage of weathered contacts (WC) increased progressively. The overall results indicated that fresh foliation planes ensured the current tunnel stability due to the moderate mica content (M-a) in such geological structures. Thus, underestimating the strength of foliation planes causes unrealistic instability conditions in the tunnel. However, the weathering advance on fractures and foliation planes can trigger important instability problems over time.