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Technischer Bericht NTB 85-29

Hydrogeologische Grundlagen für das Modellgebiet Oberbauenstock

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For the project "Gewähr 1985", the data concerning the hydraulic parameters of the host rock (Valanginian Marls) in the modelling area of Oberbauenstock have been reviewed and processed; the hydraulic conductivities, in particular, have been estimated. The corresponding results are shown in the present report.

According to the field observations, the "compact" rock ("rock matrix") is practically impervious and the deep groundwater flows in a network of interconnected discontinuities. The Valanginian Marls show three joint systems of significance for groundwater flow. The geometry of these systems is defined in the present report: in particular, it is shown that the joints are closely interconnected. The hydraulic conductivity of the discontinuities is estimated to be KF = 3·10-9 m/s, the large-scale host rock conductivity K0 = 10-10 m/s with an anisotropy significantly below 10:1. In the vicinity of the existing or possible underground structures as tunnels and caverns, a decompressed zone is produced, whose significance for the hydraulic parameters of the host rock is shown.

Observations made on the water inflow from the Valanginian Marls into the Seelisberg Tunnel have been processed and the seepage rate found to be very limited. The local manifestations of humidity or of drips are well correlated with certain elements of the geological structure and in particular with the thickness of the Valanginian Marls above the tunnel.

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