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Figure 1 depicts stress strain curves in relation to deformation induced changes of the volumetric
strain and permeability from the various strength tests on Opalinus clay with different loading directions.
Despite the limited number of tests the rock-mechanical test results clearly document that
the strength of Opalinus Clay is sensitive to minimal stress and the stress direction related to the
bedding. In this context it has to be mentioned that detection of initial micro crack-opening in indurated
clay under lab conditions depends mainly on the sensitivity of the measured physical parameter
because during triaxial loading an overall matrix compaction of the clay rock occurs.
Under deviatoric conditions local initial crack opening at 50 – 60% of the failure stress is only indicated
by velocity decrease of radially measured p-waves or s-waves (oscillation direction to crack
planes, compare [2]). Primary at around 90% of the failure stress an increase of volumetric strain
was observed associated with a permeability increase. In consequence, two pronounced stress
boundaries have to be considered representing different stages of damage:
initial damage � 0.5 - 0.6 * peak , respectively dilatancy 0.8 - 0.9 * peak
Rock matrix
(Visco-)elasto-plastic model
it describes as a function of elasto/plastic strain :
60
• hardening
⎛ σMax − σ ⎞ D
50
σ1= σD+
⎜
⎜1
+
⎟ σ 3
σ ϕ + σ ⎟
⎝
3 ⎠
• softening / failure
40
• dilatancy
⋅
(Visco-)elasto-plastic model
it describes as a function of elasto/plastic strain :
60
• hardening
⎛ σMax − σ ⎞ D
50
σ1= σD+
⎜
⎜1
+
⎟ σ 3
σ ϕ + σ ⎟
⎝
3 ⎠
• softening / failure
40
• dilatancy
⋅
Axial stress σ 1 ( MPa )
Volumetric strain V/V ( % )
30
0% plast. strain
0,04% plast. strain
0,1% plast. strain
0,24 plast. strain
20
0,4% plast strain
ε p ( % ) 0,00 0,04 0,10 0,24 0,40
10
σ D ( MPa )
σ φ ( MPa MPa )
10,5
5,0
8,6
5,0
6,0
5,0
2,0
5,0
0,0
5,0
0
σ Max ( MPa ) 55,0 48,0 43,0 38,0 35,0
0 5 10 15 20
2,5
2,0
1,5
1,0
0,5
Confining pressure σ 3 ( MPa MPa )
εp εp ( % ) 0,00 0,04 0,10 0,24 0,4
σ ψ ( MPa ) 3,2 3,1 2,9 2,5 2,0
tan β° 0,15 0,3 0,5 1,0 2,0
0% plast. strain
0,04% plast. strain
0,1% plast. strain
0,24 plast. strain
0,4% plast strain
0,0
0 5 10 15 20
Confining pressure σ 3 ( MPa )
Bedding plane properties
Extended Minkley shear model
it describes the behaviour of weakeness planes
on the basis of: μ K = kinetic friction
Δμ = adhesive friction
τ adhesion = μ K ( 1 + Δ μ ) ⋅ σ N + c
c = cohesion
Numerical modeling of clay rocks as
„discrete material“ using UDEC: The
bedding planes are treated as discontinuities
between blocks (matrix)
479
τ
Shear stress ( MPa )
σ N
Normal stress
σ Ν = 10 MPa
Shear displacement ( mm )
Medium 1
Medium 2
Figure 2. The new modelling approach based on matrix and bedding plane properties. (centre) the
reference case: a specific drift situation at the Mont Terri site with the relevant deformation styles,
i.e. extensional failure and bedding plane slip in the roof respectively brittle failure in the wall. (left
side) the MINKLEY-elasto-plastic constitutive model for the rock matrix (parameter curves of the experimental
data); (right side) the new developed MINKLEY-shear model for describing bedding
plane properties. The inset shows the numerical simulation of a shear test. For details of the used
models see [1].
3.2 Modelling of the EDZ around a specific drift situation at Mont Terri
With respect of a prognosis of the EDZ a new modelling approach has been developed based on the
obtained experimental results (Fig. 2). It consists of two parts, i.e. of the MINKLEY-(visco-)elastoplastic
constitutive model comprising the hardening/softening behaviour and dilatancy effects of the
rock mass and a specific shear friction model, which describes displacement- and velocity-