Investigating the dynamic response of a Granitoid rock mass to reservoir draining at Grimsel Test Site, Switzerland, as an analogue for Glacial Retreat

The drainage and refilling of a surface water reservoir beside the Grimsel Test Site (GTS) underground rock laboratory in Switzerland, has provided a unique opportunity to study in-situ rock mechanical, hydraulic and chemical interactions under large-scale stress changes. The reservoir was drained in October/November 2014 to enable dam maintenance and extension of the regional hydropower tunnel system. Reservoir drainage will have caused rapid unloading of the rock mass. The GTS sits ~37m below the top of the reservoir and ~200-600m away laterally within the mountainside on the eastern bank of the reservoir. For reference, previous research at Strathclyde in similar bedrock showed that oscillations in surface reservoir depth of only 3-6m could produce microseismicity of magnitude up to 2 (Pytharouli et al., GRL, 2011, doi: 10.1029/2010GL045875). Gradual refilling of the reservoir via natural snowmelt and runoff commenced in February 2015.
Research at Strathclyde, funded by Radioactive Waste Management Ltd., have been investigating mechanical-chemical-hydraulic coupling within the rock mass as an analogue for glacial unloading and loading of a future Geological Disposal Facility. We hypothesise that reservoir unloading and reloading will cause microseismic events due to slip on fractures within the surrounding rock mass. These events will open new pathways for fluid flow, expose fresh mineral surfaces and may release previously trapped pore waters. We have deployed three 3-component and 6 single-component micro-seismometers within the GTS and surrounding hydropower tunnel network. In parallel, we have implemented a groundwater sampling program, using boreholes within the GTS, for temporal determination of geochemistry and flow rate. Preliminary data analyses show
groundwater anomalies during unloading, as well as the detection of microseismic events.