Tunnel Heading Stability in Drained Ground

When considering ground conditions for tunnelling one might distinguish between ground with and without a macro structure due to stratification, schistosity and jointing. In this study attention is focused on soils and very soft rock without a significant macro structure. These materials tend to be encountered for shallow tunnelling in urban areas, whereas macro-structured ground is dominant in deep tunnelling. For soils and very soft rock, as considered in this study, stability is governed by shear strength parameters that can be measured in laboratory tests. For soft soils with little (effective) cohesion, it is necessary to drive the tunnel using a shield. For soils or rock with greater cohesion, it is possible to use an open face tunnelling method such as the NATM. Here, if necessary the stability of the tunnel face can be improved by inclining the face or by reducing the cross section of the excavation. In this study we will consider face stability both for closed-face shield tunnelling and openface NATM tunnelling. When discussing previous research on tunnel heading stability, one has to distinguish between drained and undrained conditions. For undrained conditions, as dominant in clays, practical design curves have been derived on the basis