ABSTRACT
ABSTRACT: This paper reviews unique case histories from the Central Swiss Alps that illustrate the effects intermediate and deep tunnels in crystalline rocks can have on groundwater flow systems, surface springs and surface deformations. Serious impacts on surface waters occur in weathered and de-stressed high permeability areas close to the tunnel portals, or along tunnels running only a few hundred meters below ground surface. In these areas, water table drawdown can reach the elevation of the tunnel and significantly affect surface springs and wetlands over lateral distances of several kilometers within a few weeks. Similarly, deep tunnels located 500-1500 m below ground surface can have considerable impacts on natural flow systems, although their effects are much less visible; groundwater flow is redirected towards the draining tunnel, re-locating groundwater divides and reversing flow directions over lateral distances larger than one kilometer. In addition, strong pressure reductions created by draining tunnels at depth can result in consolidation of the rock mass and large scale deformations leading to surface settlements in excess of 10 cm.
