Groundwater is one major reason for tunnel collapses throughout history. Tectonic fault zones, characterized by heterogeneous rock mass composition and low strength, are particularly prone to such events. The article describes the groundwater conditions in heterogeneous ground at high overburden, revealing that high hydraulic gradients may develop close to the face in such situations. Numerical analyses with full hydraulic-mechanical coupling indicate a poorly confined region, subject to seepage forces, forming ahead of the face when approaching a fault zone. The governing hydraulic failure mechanisms for excavation stability can be distinguished into seepage-force driven mechanical failures (so called plug failure and cracking) and erosion processes. For the former, the article describes a novel solution to assess tunnel face stability subject to seepage forces, based on the method of slices applied to a hemispherical failure body.
Several historical and recent cases describe tunnel collapses or critical incidents related to high water or mud ingress into tunnels, often referred to as ‘flowing’ or ‘swimming ground’. Tectonic fault zones are particularly prone to such events. On the one hand such zones usually exhibit low rock mass strength. On the other hand, strongly varying hydraulic parameters within the fault zone may influence the distribution of hydraulic heads around the tunnel and thus yield more adverse groundwater conditions than in homogeneous ground.
Although the geological - geotechnical knowledge and methods developed rapidly during the last decades, the geomechanical failure modes triggering flowing ground conditions in tunnelling are hardly described. Lacking understanding of the failure mechanisms, practicable methods to assess tunnel stability in water-bearing rock mass under high overburden are currently scarce. This paper shall contribute to understanding the hydraulic conditions and failure mechanisms for deep tunnels excavated in weak and water-bearing rock mass.
For assessing groundwater-related failure modes, knowledge of the hydraulic head field (i.e. the distribution of pore pressures) in vicinity of the tunnel face is essential. In homogeneous rock mass, the hydraulic head field predominately depends on the hydraulic conductivity of the rock mass, the advance rate of the excavation, the size of the excavation and the initial groundwater level. The lower the hydraulic conductivity and the faster the excavation advance, the steeper are the hydraulic gradients occurring ahead of the face and the higher are the resulting seepage forces acting towards the tunnel.
