ABSTRACT
NATM/SEM/SCL construction techniques require the exposed excavation face to remain stable while sprayed concrete is applied and reaches design strength. This is typically straight forward in cohesive soils or above the standing groundwater level but potentially problematic where water bearing granular soils or weak rock are exposed or present near the open face. In appropriate hydrogeological settings the risk of groundwater ingress, associated instability or unacceptable groundwater pressures can be mitigated by pre-drainage dewatering methods. These techniques involve the use of pumped wells to lower groundwater levels in advance of excavation. Conventional surface dewatering techniques can be installed in advance of the face and so avoid both the artesian conditions and logistical constraints inherent in below ground working. Conversely in-tunnel or in-shaft techniques avoid the need to undertake work at the surface and can give direct access to target water bearing horizons. An approach which combines both surface and in-tunnel dewatering systems can sometimes be the most efficient approach particularly where sand/clay interface conditions are present within or close to the face.
