Euradwaste '08 - EU Bookshop - Europa

)
a)
e)
d)
f)
d)
e)
f)
c)
Figure 3.1: Overcoring of the strike-parallel SELFRAC [9] borehole at Mont Terri; a) conceptual
model; b) modelling result for volumetric strain from Nagra (2002)[3]; c) 300 mm diameter slice
of the overcore. The old borehole position is visible in the centre (the circular element is a section
of the resin injection pipe); d) regularly spaced fractures in the collapsed region that follow the
bedding planes; e) non-continuous cracks that limit collapsed regions cross-cut the bedding
planes; f) cracks along the central region of the collapsed zone also cross-cutting the bedding.
Sigma 1 orientation is sub-vertical.
The self-sealing behaviour which is obvious for plastic clays was also investigated for indurated
clays in laboratory tests. Here, this process is dominated by the combined impact of recompaction
and clay swelling. A self-sealing test on a large fractured Opalinus Clay sample (L/D= 600 mm /
260 mm) from the Mont Terri Rock Laboratory was performed by GRS within the framework of
NF-PRO. As a result of coring and a long storage time without sufficient confinement, the sample
was already highly damaged and desaturated before testing started (Fig. 3.2). Along the bedding
planes, multiple macro-fractures developed through the whole sample more or less parallel to its
axis. This damaged state may represent the situation in-situ near the drift wall. The large-scale sealing
test focused on examining the effect of normal stress and water resaturation on gas permeability
along fractures.
The fractured sample had a high initial permeability of 5·10 -14 m 2 , which was measured with a radial
stress of 3 MPa and an axial stress of 19 MPa. Keeping the axial stress constant, the radial stress
normal to the fractures was increased stepwise to 18 MPa, resulting in a drastic decrease in permeability
down to 10 -19 m 2 , which is around five orders of magnitude lower than the initial value.
When the radial stress was reduced again to its initial level, the measured permeability of 10 -16 m 2
was still two orders of magnitude lower than the initial value.
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