Large Scale Monitoring (LASMO)

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.

Rock volume characterisation at GTS

 

 

 

Geometry of the fault zones in 3D

Figure showing the structural map of the Juchlistock area

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

Micro-seismic survey

Water-conducting features in the GTS

Groundwater composition in the GTS

The overall goals of the LASMO project are to test monitoring systems, technologies or methods and to collect monitoring data in order to identify baseline conditions and disturbing events. In detail, the objectives are to:

1. Establish and test a multi-parameter monitoring system for collecting data on groundwater pressures and chemistry, seismic signals, displacements, stress and deformation
2. Update existing or develop new (i) geological/structural, (ii) hydraulic, (iii) hydrochemical and (iv) stress models in 3d of the geosphere around the gts and to the surface
3. Monitor tectonic/seismic activity and develop a stress model
4. Identify the characteristics of baseline data and identify deviations that are induced by known natural or anthropogenic perturbations
5. Use perturbations of known timing, orientation, magnitude and frequency to test measurement techniques and the predictability of models
6. Combine the obtained data in an integrated interpretation and iteratively update the above mentioned geosphere models using the new findings
7. Integrate monitoring data into the gts database or create new databases (e.g., microseismics, hydrogeochemistry) and improve data management processes

 RWM, United Kingdom

SURAO, Czech Republic

Nagra, Switzerland

Current programmes for the long-term management of long-lived radioactive waste are focused on underground disposal. Emplacing the waste in deep geological repositories along with multiple engineered barriers is widely accepted as a safe means of isolating it from the biosphere.

Monitoring systems will be necessary to help evaluate the behaviour of repository components, or the impacts of the repository on the environment (IAEA 2001). A monitoring programme is typically divided into the three phases, the so called (1) baseline monitoring, (2) the monitoring during construction and operational until closure and (3) post-closure monitoring (Fig. 1).

Fig. 1: The three phases of a typical monitoring programme (after NRC 2003)

The objective of the LArge Scale MOnitoring (LASMO) project is to evaluate existing monitoring techniques in the near and far field of a repository-like environment during both the baseline and operational phase until closure of the repository. The LASMO project is implemented at the Grimsel Test Site (GTS) located in the Swiss Alps. Since its establishment in 1984, the GTS has hosted a wide range of underground research under repository-relevant boundary conditions, allowing LASMO to build upon 30 years' knowledge and experience regarding the local (hydro-)geology, data acquisition, development and testing of equipment, conceptualisation and modelling (Blechschmidt et al. 2008, Kickmaier et al. 2005, McCombie et al. 1995, Vomvoris et al. 2013, 2015). In addition, the project takes advantage of ongoing and planned construction works and related lake drainage of the local hydro power plant, operated by Kraftwerke Oberhasli AG (KWO). These measures may affect the hydraulic and/or rock mechanical conditions around the GTS and are used as analogues for perturbations during repository construction and/or operation, or eventually closure. Hence, the LASMO project provides a unique opportunity for developing, testing and surveying monitoring strategies and techniques at different phases of repository implementation (baseline, construction, operation) at a large scale and under realistic boundary conditions.

The LASMO project is conducted in the framework of Phase VI of the research programme of the Grimsel Test Site. GTS Phase VI runs from January 2003 to 2018 and is dedicated to repository-relevant (i.e. small to large-scale, long-term) in-situ experiments (www.grimsel.com). Currently (as per 2015), LASMO is a cooperation project with Nagra, SÚRAO and RWM as project partners. The project started in 2013 and is planned to last until 2018.

Fig. 2 illustrates the experimental concept and the timeline of the LASMO project with milestones. The available instrumentation at the GTS and the pre-existing data set on the surrounding geosphere is augmented within the LASMO project by a comprehensive monitoring programme and data acquisition with a focus on rock mass, groundwater and stress characterisation. The extensive monitoring network allows the development of baseline characteristics, and testing the sensitivity of the different monitoring techniques and/or parameters to perturbations that are either of natural (e.g. earthquakes, meteoric events) or manmade origin (e.g. underground excavation, power plant operation). Furthermore, the existing and the collected monitoring data are used to develop and iteratively update geological/structural, hydraulic, hydrochemical and stress models in 3D of the geosphere around the GTS and to the surface.

Milestone 1 reflects the end of the implementation phase, which was largely completed before the emptying of Lake Raeterichsboden in October 2014 (Fig. 2Fig.). Milestones 2 – 4 are the completion of the first status report and annual reports for the years 2016 and 2017, respectively. Milestone 5 will be accomplished with the finalisation of the synthesis report, which is planned for 2018.

Fig. 2: Experimental concept of the LASMO project.