Geological structures

The main tectonic element in the investigation area is the Mont Terri anticline. This is a dome-shaped fold that, due of the pressure of the Alps (Fernschub = distant push hypothesis), was pushed around 1 km over the “Table Jura” low-lying mountains in the Ajoie area.

The rock laboratory lies completely within the Opalinus Clay, in the south limb of the Mont Terri anticline. The rock layers in the laboratory dip roughly 42° to the SSE.

Geological profile
Geologic profile through the Mont Terri anticline with the location of the rock laboratory and the most important deep drillholes BDS-5 and BDB-1.

In the rock laboratory, all structures are documented with the great precision. There are three processes that produce fault-bounded structures found in the Mont Terri rock laboratory:

  • Natural faults caused by tectonic forces
  • Faults caused by tunnel excavation
  • Desiccation and swelling due to shrinkage/expansion of the Opalinus Clay over the years

Geological map of the rock laboratory
Geological map of the rock laboratory with all mapped faults and their orientations. The stereoplot displays three groups with different spatial orientations.

Natural faults

Given the tectonic history of regional and local faults, the Opalinus Clay is pervaded by rock structures indicating past movements. In the rock laboratory, three groups of faults can be observed: faults lying SSE (red), S to SW (light blue) and WNW or ESE (purple). Faults lying SSE and S to SW arise from thrusting and folding due to pressure from the Alps. The flat to steep faults lying WNW or ESE are reactivated older structures.

 

In the laboratory there are two larger fault zones, neither of which is active. These fault zones are interpreted as branch faults from basal shearing and the associated thrusting action.

Lineation can be observed on the fault surfaces, which indicate the direction of movement and, in places, create a sense of movement.

 

Major tectonic element in the rock laboratory
The major tectonic element in the rock laboratory is a SSE-dipping thrust zone, the “Main Fault” (red).

Rock sample
Rock sample with slickensides along the fracture surface. These indicate the direction of shear movement and in some cases the sense of shear.

Faults caused by excavation

Besides faults created by natural tectonic movements, excavation of caverns and tunnels has created cracks. Relief cracks occur roughly parallel to the tunnel wall. Such occurrences are called an “excavation damaged zone” (EDZ). These form an interconnected crack network with pre-existing faults that penetrates about 1 m into the tunnel wall. Only a few isolated EDZ cracks exhibit penetration depths of up to 2 m.

EDZ structure
Exfoliation (pressure release) joints with fine gypsum crystals on the joint surface. Gypsum precipitates when pore-water evaporates on the rock surface.

Plumose structure
EDZ pressure release joint with plumose structure. Clearly visible are the concentric “ripples” that indicate the spreading direction of the joint.

Desiccation and swelling over the years

Shrinkage cracks form by the drying out of the rock in the tunnel during the six months of winter. In the summer and in conditions of 100% air humidity the cracks partially close up again due to swelling of clay minerals. Thus there is an annual cyclic movement of cracks and fissures. Over longer periods we observe that the overall tendency is towards closing up of cracks (the self-healing effect).

Desiccation cracks
In winter the rock dries out and shrinks, causing cracks. These shrinkage cracks spread out along the layering (in the image: from bottom left to top right).