Large Scale Monitoring (LASMO) Results
Rock volume characterisation at GTS
The rock volume surrounding the GTS is mainly composed of Central Aar Granite and Grimsel Granodiorite. Metabasic dykes crosscut the plutonic rocks. Deformation related to the Alpine orogeny resulted in a pervasive foliation and localized high strain zones.
Figure showing a lower hemisphere equal area projection of the normal to the foliation plane
Figure showing the gradual deformation of the rock volume surrounding the GTS with decreasing temperature and a change from ductile to brittle deformation
Figure showing a conceptual fault zone in the granites surrounding the GTS
Geometry of the fault zones in 3D
Based on the orientation of the fault zones mapped in the GTS and at the surface a 3D structural model was created to represent the geometry of the faults in three dimensions. The structural modelling started with a lineament mapping showing linear features identified on an orthophoto or the digital elevation model.
Figure showing the results of the lineament mapping
Based on the lineament mapping and extensive field work, a structural map of the Juchlistock area was drawn.
Figure showing the structural map of the Juchlistock areaThe combination of the surface trace, the orientation of the faults and the digital elevation model a fault 3D model of the GTS area was compiled.
Figure showing the geometry of the faults connecting the topography with the GTS
Movements along the faults
An extensive slip monitoring survey was implemented within the frame of LASMO. Multiple faults in the GTS were equipped with a fully automated 3D extensometer allowing movement detection down to the nanometer-scale.
Figure showing fully automated 3D extensometer
The slip monitoring allowed for the detection of movements induced by nearby drilling activities, however no evidence for neotectonics was found.
Figure showing the response of two dilatometer to nearby drilling activities
In situ stress state
The in situ stress state was estimated in three locations based on three dedicated borehole having a precise spatial layout.
Figure showing the strain gauge used to estimate the stress field and the spatial layout of the dedicated boreholes
The strain monitoring detected strain changes due to the Earth tides, however the in situ stress field could not be successfully inferred. This is probably due to the proximity of the tunnel and thus strong influence on the stress field.
Micro-seismic survey
A micro-seismic survey was designed to monitor rock volume reactions to perturbations such as lake level changes.
Figure showing the increased event frequency detected by the north array during the drainage and the refill of the Lake Rätrichsboden
Most of the picked events are located in the Grimsel area, but not in the direct vicinity of the GTS. Qualitative comparison between the event locations and available structural maps of the surface indicated that the events are located along known faults. Detection of events close-by the sensors would require a higher sampling frequency.
Water-conducting features in the GTS
Water percolation in fractured crystalline rock occur along faults this is a well-known fact. However, water inflow mapping performed in the GTS showed that not all faults are water-conducting. What structural features govern water flow? Water inflow points were compared with fault intersections and slip tendency maps. Slip tendency describes the probability for a given fault to be reactivated under current stress conditions. The comparison showed that fault intersections are the structural feature most prone for water flow. Moreover, the slip tendency correlates positively with a qualitative trend of the hydraulic conductivity of faults.
Figure showing the distribution of fault intersection, high slip-tendency faults and their density on a map of the GTS
Groundwater composition in the GTS
Groundwater from various borehole intervals was sampled in order to determine the baseline composition of the groundwater. Stable water isotopic analysis showed that the groundwater is of meteoric origin. The chemical composition of the groundwater results from water-rock interactions during the downwards percolation of the groundwater. Strikingly, groundwater in the GTS shows different biomarker content indicating various origin (lake water or mountainside water).
Figure showing a conceptual cross-section along the GTS with the major findings from the groundwater baseline survey
