Large Scale Monitoring (LASMO) Project Perimeter & GTS Layout
The Grimsel Test Site is located in the Hasli Valley to the north of the Grimsel Pass, 400 to 450 m beneath the Juchlistock at an altitude of ~ 1’730 m asl. Fig. 3 shows both the approximate perimeter of GTS and the large-scale perimeter of the Grimsel area being evaluated as part of the LASMO project. Mapping, surface investigations and modelling take place at a large-scale perimeter, while monitoring of groundwater pressures and chemistry as well as rock deformation, stress and microseismic activity concentrate on the GTS perimeter including KWO access tunnels.
Fig. 3: Grimsel area with the large-scale LASMO project perimeter (view to the west).
From a geological point of view, the GTS is situated in the southern section of the Central Aar Massif with the leucocratic Central Aare Granite and Grimsel Granodiorite as host rocks. The formations belong to a large pluton which was formed during the Variscan Orogeny and intruded (~ 290 0300 Ma; Schaltegger & Corfu, 1992, Schaltegger et al. 1994) into a Palaeozoic framework of metamorphic sediments (Labhart 1977, Abrecht 1994) (Fig. 4). The cooling of the pluton led to a system of regional fracture and fault patterns which became pathways for the subsequent intrusion of granodioritic and aplitic dykes and lamprophyres (metabasic dykes). After being buried beneath several kilometres of sediments, the granitic formations rose to their present position as a result of tectonic compression during the Alpine Orogeny (40 Ma) involving regional shear displacement and weak to intermediate metamorphosis (Keusen et al. 1989, Challandes et al. 2008). The latter caused significant foliation whereas the shear movement gave birth to cataclastic and mylonitic shear zones (Rolland et al. 2009). The combination of brittle and ductile deformation has shaped the present system of fault and fracture zones (Fig. 5). Within the GTS area, up to twelve different shear and fracture sets can be observed in detailed drill core and surface evaluation (Steck 1968, Keusen et al. 1989).
Fig. 4: Regional geology of the Hasli Valley (Baumberger 2015).
Fig. 5: Simplified block diagram of the Juchlistock with the GTS and the region’s shear and fracture sets (after Keusen et al. 1989). Main GTS tunnels shown in red.
The GTS is embedded in a 400 km2 catchment of a hydroelectric power plant system owned and operated by KWO (grimselstrom.ch). The system consists of eight storage lakes, two of which are located in the direct vicinity of the GTS: Lake Grimsel in the south and Lake Raeterichsboden to the east (Fig. 3) with peak lake levels of 1909 and 1767 m asl, respectively. Previous studies have found a clear relation between the water levels in these two lakes and the rock mass deformation at the GTS (e.g. Flach & Noell 1989).
The GTS’ branching tunnel system (Fig. 6) is more than 1 km in total length. It was excavated in 1983 and 1984 using both a tunnel boring machine with a diameter of 3.5 m and drill and blast techniques. Expansions of the site in 1995 and 1998 provided space for two large-scale demonstration tests. The laboratory is reached by an access tunnel to one of KWO’s electrical power plants (Grimsel II). The northern entry of the laboratory is positioned 1’020 m south of the access tunnel portal at Gerstenegg. From there, the main drift extends 521 m southwards, rising with a gradient of 1% from an elevation of 1’727 m asl to 1’732 m asl. The last 85 m are referred to as VE drift. At L 202 m and L 309 m, the main drift branches into the WT and AU drifts with lengths of 140 and 181 m, respectively, both dipping 1% northwards. The latter accommodates a radiation controlled zone of IAEA Level B/C, allowing field experiments to be carried out with radioactive tracers. The access tunnel runs parallel to the GTS and rises southward with a gradient of 2.2%. The main access to the GTS is provided via the central facilities that connect the access tunnel (1’733 m asl) with the main drift at L 267 m (1’730 m asl).
Since 1984, more than two dozen organisations and research institutes from twelve countries together with the European Union have participated in the six phases of the GTS research programme. The projects have contributed substantially to the development and confirmation of safe geological disposal concepts and to the characterisation of potential host rock formations. Each phase has focused on the key issues at the time, attempting to anticipate the next steps in national programmes for the long-term management of radioactive waste. The 15-year Phase VI began in 2003 and is dedicated to integrated projects with:
a) field experiments under repository-relevant boundary conditions, i.e. small to large-scale, long-term experiments with realistic hydrogeological settings; and
b) projects addressing the implementation of a geological repository in terms of engineering feasibility, potential construction impacts on the surrounding rock, operational aspects, closure, and monitoring.
The Phase VI projects are illustrated in Fig. 6 at their respective position within the GTS. In contrast to most projects, LASMO is not related to a specific sub-section of the laboratory, but instead covers the full GTS extent as well as the surrounding geosphere up to the surface, and integrates and extends existing instrumentation and data sets.
Fig. 6: Grimsel Test Site layout and Phase VI experiments.