Hydraulic conductivity distribution in crystalline rocks, derived from inflows to tunnels and galleries in the Central Alps, Switzerland |
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| Authors: | Olivier Masset Simon Loew |
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| Institution: | 1. Geologisches Institut, NO G 69.1, Sonneggstrasse 5, 8092, Zurich, Switzerland
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| Abstract: | Inflow data from 23 tunnels and galleries, 136 km in length and located in the Aar and Gotthard massifs of the Swiss Alps, have been analyzed with the objective (1) to understand the 3-dimensional spatial distribution of groundwater flow in crystalline basement rocks, (2) to assess the dependency of tunnel inflow rate on depth, tectonic overprint, and lithology, and (3) to derive the distribution of fracture transmissivity and effective hydraulic conductivity at the 100-m scale. Brittle tectonic overprint is shown to be the principal parameter regulating inflow rate and dominates over depth and lithology. The highest early time inflow rate is 1,300 l/s and has been reported from a shallow hydropower gallery intersecting a 200-m wide cataclastic fault zone. The derived lognormal transmissivity distribution is based on 1,361 tunnel intervals with a length of 100 m. Such interval transmissivities range between 10?9 and 10?1 m2/s within the first 200–400 m of depth and between 10?9 and 10?4 m2/s in the depth interval of 400–1,500 m below ground surface. Outside brittle fault zones, a trend of decreasing transmissivity/hydraulic conductivity with increasing depth is observed for some schistous and gneissic geological units, whereas no trend is identified for the granitic units. |
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