On the stabilizing effect of advance drainage in tunnelling / Über die statische Auswirkung der vorauseilenden Gebirgsdrainagen im Tunnelbau

The paper reviews the favourable effects of advance drainage with respect to three tunnel engineering problems in water-bearing ground: the stability of the tunnel face, the stability of grouting cylinders in geological fault zones and the convergences in squeezing ground. In a high permeability ground, advance drainage improves the stability of the tunnel face because it reduces the pore pressures and their gradients in the ground ahead of the face. If the ground exhibits a low permeability, advance drainage increases its short-term shear resistance and thus the short-term stability and the stand-up time of the face. In fault zones with flowing ground, tunnel advance is often possible only after strengthening and sealing the ground around the tunnel by grouting. As the permeability of the grouted body is very low in relation to that of the surrounding untreated ground, large seepage forces may develop inside the grouted body. Systematic drainage reduces the pore pressure gradient within the grouted body, improving thereby its stability while reducing the risk of inner erosion and uncontrollable water and mud inrush. Squeezing ground usually exhibits low permeability and a pronounced time-dependent behaviour. It is well known that high pore pressures favour the development of squeezing. The pore pressure relief consolidates the ground, increasing as a result its undrained shear strength. This is favourable with respect to the short-term convergences in the vicinity of the tunnel heading. Advance drainage is favourable also with respect to the long-term ground response because it eliminates the high pore pressure gradients associated with the first stages of the consolidation process around the opening.