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a)
b) c)
Figure 2.2: a) Large-scale in-situ heater test (Praclay heater experiment) to be performed in HA-
DES, Mol; b) the heater test is instrumented to monitor the THMC evolution and c) accompanied by
long-term seismic measurements to study the EDZ evolution.
3.1 EDZ in clay rock
In the national programmes in Belgium, France and Switzerland, different clay host rocks have
been evaluated, ranging from more plastic clays such as the Boom Clay to overconsolidated claystone
such as the Opalinus Clay. The latter is strongly influenced by material anisotropy, which has
a significant influence on the failure mechanisms around underground openings. Texture analysis of
the Jurassic Opalinus Clay shows that clay minerals are well aligned with the bedding planes [7].
The same applies for large carbonatic shell fragments which can be shown to be the origin of microcracks
that lead to macroscopic failure in triaxial experiments [8].
In the Mont Terrri Rock Laboratory, bedding plane slip dominates the failure mechanisms from
borehole scale to gallery scale. Figure 3.1 shows the fracture pattern around a recently overcored
101 mm borehole. After removal of the previously installed packer system, the lower part of the
borehole had been injected with dyed resin. An analysis of the collapse structures shows a distinct
sequence of fracture evolution in the borehole EDZ. Bedding plane failure is followed by shear
cracks, limiting the failure to the sides of the borehole. The latter links up to form a continuous zone
of cracks that limits the highly deformed region. The stack of plates is then pushed into the opening
and progressive bending leads to the development of cracks along their centres.
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