GTS Information

The Crystal Caves at Grimsel

After Prof. Hans Anton Stalder

large crystals on display within the Crystal Caves Discovery and exposure
The forward section of the crystal cave was discovered on 4th October 1974 during excavation of the access tunnel to the Grimsel II power station, which is operated by the company Kraftwerke Oberhasli AG (KWO).

After careful recovery of the first crystals, it soon became apparent that the cave was considerably more extensive than originally assumed. This was seen as a unique opportunity to place it under state protection as a natural geological monument and a governmental resolution to this effect came into force on 11th December 1974.

Work on exposing the cave began once the KWO tunnel construction had been completed. The first step was to remove the huge slab of rock which was obscuring most of the cave. This slab of crystal, which weighs 875 kilograms, is now located in the administration building of KWO in Innertkirchen. After removal, it became clear that, beyond this, there was yet another cavity filled with crystals. During the winter of 1985/86, the observation gallery was constructed to provide access to this rear section.

The minerals
The crystal cave contains mainly rock crystal (= quartz, silica). Many of the crystals, which are up to 20 centimetres long, are remarkable for their complete transparency. In addition to the normal hexagonal forms, so-called "Gwindel" (rotated quartzes) can also be found.

Chlorite (a magnesium-/iron-aluminosilicate) is the second most commonly occurring mineral. This dark-green scaly mineral completely filled a large part of the rear section of the cave and more than a cubic metre was removed to provide a better view of the underlying rock crystals. Calcite (calcium carbonate) occurs mainly in the rear section, the green chlorite mass being covered more or less everywhere by thin white tabular calcite crystals. In the rear section of the cave there is also a large rhombohedral calcite crystal. Fluorite (calcium fluoride) is loosely scattered over the entire cave. The pale pink octahedral crystals are generally located on top of rock crystals and are overgrown in some locations with white calcite. In a side-fracture, brassy yellow pyrite (iron sulphide) and galena (lead sulphide) are exposed in addition to chlorite and calcite.

A total of 12 different types of mineral have been identified, including adularia (potassium feldspar) epidote (calcium silicate), titanite (titanium silicate), apatite (calcium phosphate), biotite (dark mica) and milarite (a rare beryllium silicate). The latter six are, however, very restricted in their occurrence and are mainly obscured in the altered rock surrounding the cave.

large band of crystals on display within the Crystal CavesThe origin of the crystal cave
As is the case for all the alpine crystal caves of the Aar Massif, this feature was formed around 16 million years ago, towards the close of the alpine orogeny.

At that time, the following conditions prevailed: both small and large fissures (fractures) formed in the rock and were immediately filled with a hot aqueous salt solution. With a temperature in excess of 400°C, the solution dissolved various minerals (mainly quartz and biotite) out of the surrounding rock. Traces of this process can be seen in the form of bleaching halos around the fissures. These processes occurred at least 10 kilometres below what was then ground surface.

The solution later cooled slowly and the minerals precipitated out in the cave in the form of the beautiful crystals we see today. Since then, the entire rock formation has gradually been uplifted, a process which has been balanced by constant erosion. The crystal cave thus arrived at the location where we can admire it today.

Appraisal of the cave
Every alpine mineral cave is unique and represents a special case. This is particularly true of the cave at Gerstenegg. It is large - but not so large that the crystals have been loosened from the rock by earth tremors. Its location also means that it is not subject to the effects of surface weathering. It has a relatively large aperture - most mineral caves are much narrower. Its location in the KWO tunnel means that it is in a controlled environment which can be sealed off. The fact that the cave is protected in this way, and that anyone is free to visit it, is a unique situation.

To arrange a visit to the Crystal Caves see the Visit the GTS page.


30 Years of History at the Gimsel Test Site (GTS)

full face tunnel boring machine
Full-face tunnel boring machine

Between May and November 1983, the Grimsel Test Site was excavated using a full-face tunnel boring machine with a diameter of 3.5 metres (total tunnel length approx. 1.1 km; broken-out volume 14,800 cubic metres).

Caverns were also excavated by blasting. Extensions to the tunnel system were made in 1995, 1997 and 2000.

The conditions for tests performed at the Grimsel Test Site are particularly favourable because it contains areas of relatively undisturbed homogeneous rock as well as heavily fractured areas with water-bearing zones (shear zones, fractures, lamprophyre and aplitic dykes).

The division of the GTS into individual test caverns and drifts was made on the basis of the prevailing rock features at these locations and ensures optimum conditions for the performance of specific experiments.

Operating phases

In the 25 years of operation at the Test Site, a wide range of investigations have been carried out in many fields, including geology, geophysics, hydrogeology, rock mechanics and nuclide transport. The scientific work performed has been already shown to over 50,000 visitors and has been presented in many papers presented at conferences or published in scientific journals and the extensive list of technical reports given in the publications section.

Phases I and II (1983 - 1990)

In Phases I and II, a comprehensive investigation programme was carried out which included 16 major experiments . In addition to providing detailed information on the geological-hydrological situation, which is required for planning, performing and interpreting later tests, Phases I and II improved the understanding of the interaction between modelling exercises, laboratory experiments and in-situ studies. Progress was also made in developing the methodology for performing scientific investigation programmes under field conditions. Some of the research projects, for example the development of geophysical techniques (underground radar and seismics), the mechanical test of the excavation damaged zone, the heater test and rock stress measurements, were successfully completed during Phase II, others continued during Phase III.

Phase III (1990 - 1993)

Drawing on the experience gained in Phases I and II, the concept developed for Phase III focused on investigating hydraulic and geochemical/physical transport processes in the rock. The experiments during this phase included the fracture system flow test, the ventilation test and the migration experiment. In this phase, the role of associated modelling studies became increasingly important.

Models initially used to interpret field observations were used to predict the results of later experiments and such predictions compared with measured output. This aspect of model testing is particularly important as, in many cases, the manner in which the simulation is carried out can be very objective and, if the "answer" is known, can be biased either consciously or subconsciously. The difference between blind testing of model predictions and testing if a model can simulate particular observations is fundamental, although not evident in the literature.

Phase IV (1994 - 1996)

More than in the previous project phases, specific safety analysis questions guided set up of the investigation programme for Phase IV. The criteria used as a basis for determining the Phase IV programme included the applicability of the results to potential repository sites, an assessment of the chances of success, the suitability of Grimsel as a research site and possible overlap with other national research programmes. Choice of experiments to be included and details of individual research projects were established with input from a range of other national partners. The resultant programme includes testing technology for borehole sealing, further development of seismic tomography, development of methods for characterising the area close to the tunnel and in situ experiments designed to provide a better understanding of transport mechanisms of radionuclides through the geosphere. This experiment culminated in the excavation of the test area and analyses of the distribution of the radionuclides along the test area.

Phase V (1996-2004)

More DETAILS in the Phase V/VI Overview section of this site.

Grimsel Phase V ran from 1997 to 2004. The focus was on investigating geological barrier effectiveness, demonstration of disposal concepts and site characterisation investigations. All the projects were also designed to contribute to the further development of and assessment of modelling capabilities.

Phase V saw the construction of new caverns at the GTS and experience gained in the use of radionuclide tracers was built on during the investigations of the geological barrier.

GTS phase V can be grouped into three main areas of interest.

  • The Engineered Barrier System: FEBEX, GMT and FOM
  • Processes in the Geological Barrier: HPF, CRR and GAM
  • Site Characterisation and Modelling: EFP and CTN projects

Phase VI (2003 » )

More DETAILS in the Phase V/VI Overview section of this site.
Grimsel Phase VI began on the 1st January 2003 and represents a major step forward in the research carried out at the GTS. The focus of the new research will be the examination of waste disposal concepts on more repository-relevant timescales and conditions.

Grimsel Test Site (GTS) - Milestones
1979 Geological mapping
1980 Horizontal exploration boreholes
February 1982 Decisions to construct the GTS
June 1982 Contact with Kraftwerke Oberhasli AG (KWO)
November 1982 Federal operation licence
Sept 1983 Arrival of full-face tunnel boring machine
November 1983 First experiment (Excavation effects)
20th June 1984 Inauguration of GTS
1983-1993 Phases I-III
1994-1996 Phase IV
1997-2004 Phase V
2003-2013 Phase VI


Main experiments at the Test Site

Phase I and II (1983-1990)

 - Exploratory boreholes and geological mapping

AU Excavation effects
BK Fracture flow test (BGR)
EM Electromagnetic high frequency measurements (BGR)
FRI Fracture zone investigation (Nagra/USDoE)
GS Rock stress measurements (BGR)
HPA Hydraulic potential (Nagra)
MI Migration experiment (Nagra/JNC)
MOD Hydrodynamic modelling (Nagra)
NFH Near-field hydraulics (Nagra)
NM Tiltmeters (GSF)
SVP Prediction ahead of the tunnel face (Nagra)
US Underground seismic test (Nagra)
UR Underground radar (Nagra)
VE Ventilation test (GSF)
WT Heater test (GSF)

Phase III (1990-1993)

BK Fracture flow test (BGR/Nagra)
MI Migration test (JNC/Nagra)
MOD Hydrodynamic modelling (Nagra)
ZU Unsaturated zone (Nagra)
VE Ventilation test (GSF/Nagra)
- Large diameter borehole (Andra)

Phase IV (1994-1996)

BOS Borehole sealing (Nagra)
EDZ Excavation disturbed zone (Nagra)
EP Excavation of the MI shear zone (JNC/Nagra)
TOM Further development of seismic tomography (Nagra)
TPF Two phase flow (Nagra)
CP Connected porosities (Nagra/JNC)
ZPK Two phase flow in fracture network of the tunnel near-field (BGR)
ZPM Two phase flow in the matrix of crystalline rocks (GSF)

Phase V (1996-2004)

CRR Colloid and Radionuclide Retardation Experiment (Andra, Enresa, FZK, JNC, Sandia, Nagra)
Effective Field Parameters (BGR)
Full-scale High Level Waste Engineered Barriers Experiment (Project lead by Enresa)
Fiber Optic Monitoring (DBE EEIG Nagra)
Gas migration in shear zones (Andra Enresa CSIC UPC Sandia ETH)
Gas Migration in EBS and Geosphere (RWMC Nagra/Obayashi)
Hyperalkaline Plume in Fractured Rocks (Andra Enresa SKB JNC Sandia)

Phase VI (2003 »)

PSG Pore Space Geometry (Nagra, HYRL, STUK, HYDRASA)
Colloid Formation and Migration (Nagra, JAEA, Andra, BMWi)
Long Term Diffusion concept (Nagra, AIST, HYRL, NRI, JAEA)
Long term Cement Studies
Tele Handling in-situ
Material Testing Facility
ITC International Training Centre

Geology of the Gimsel Test Site (GTS)

Around 300 million years ago, granitic melts (magma) from the earth's interior solidified at a depth of around 10 - 13 km. The volume of the rock decreased on cooling and deep fracture systems formed. Residual magma rose through these to form dyke rocks (lamprophyres and aplites).

The rocks of the Aar Massif remained virtually undisturbed for more than 200 million years. Extensive deformation of the rock body then began during the course of the alpine orogeny, around 40 million years ago. The Aar Massif subsided and was overlain by the alpine nappes moving towards the north.

At the time of maximum overburden (approximately 12 km), the rock was exposed to high temperatures (around 450°C) and pressures (around 300 MPa). The main schistosity and shear zones were formed during this period.

The crystalline rock in the Grimsel area has long been thought of as a "Massif", a large block of crystalline basement pushed up through the overlying sediments. However, recent observations in the new Lötschberg railway tunnel indicate that it is, in fact, a massive thrust sheet, overlying sediments in some areas.

In the uplift phase - still continuing with a rate of around 0.5 to 0.8 mm per year today - the tension joints with their beautiful crystals (see The Crystal Cave) were formed around 16 million years ago.


The GTS underground facilities are also available to interested 3rd parties for underground testing and research. The GTS offers cost-effective access to a fully developed, well characterised underground research facility with round the year logistical support - please contact Dr. Ingo Blechschmidt, Head of the Grimsel Test Site, for further details.
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