Influence of bedding planes to EDZ-evolution and the ... - Andra

P/EDZ/1
INFLUENCE OF BEDDING PLANES
TO EDZ-EVOLUTION AND THE COUPLED HM
PROPERTIES OF OPALINUS CLAY
INTERNATIONAL MEETING, SEPTEMBER 17...>...18, 2007, LILLE, FRANCE
CLAYS IN NATURAL &ENGINEERED BARRIERS
FOR RADIOACTIVE WASTE CONFINEMENT
Till Popp, Klaus Salzer &Wolfgang Minkley
Institut für Gebirgsmechanik GmbH, 02479 Leipzig, Germany ( till.popp@ifg-leipzig.de)
INTRODUCTION
Besides other host rocks argillaceous clay rock formations are considered for the long term storage of
radioactive waste to exclude athreat toactual and future generations. Since the transport properties of the
clay rock are responsible for the demanded integrity, knowledge about the relationship between the
development of damage (dilatancy) and hydraulical properties is of utmost importance. Determination of
the stress dependent onset of dilatancy, i.e. described by the criteria “dilatancy boundary”, is aprerequisite
for an appraisal ofbarrier properties of solid rocks. But clay rocks are inherently anisotropic, which means
that the sedimentary and tectonically induced bedding can act as preferential flow paths and mechanical
weakness planes. Both overlapping effects are particularly important during rock stress redistribution in the
EDZ. Using Opalinus clay as reference material we will highlight the following topics:
1) Mechanicaland transport properties of the rock matrix and bedding planes
2) Determination of onset of dilatancy and its evolution (healing and damage)
3) Derivation of material lawparameters referring to matrix and bedding plane properties
4) Case studywith numerical modelling of EDZ phenomenafor the MontTerrisite
RESULTS AND INTERPRETATION
Concerning the mechanical properties, it has been confirmed that the strength behaviour is strongly
anisotropic referred to the bedding and increases with increasing minimal stress. However, detection of
initial micro cracking in indurated clay under lab conditions is not asimple task due to the dominating
effects of matrix compaction and depends, therefore, mainly on the sensitivity of the measured physical
parameter (e.g. Popp &Salzer, 2006).
Importantly, the velocity decrease ofradially measured p-waves or s-waves (oscillation direction ⊥ to crack
planes) shows initial crack opening at 50 –60% ofthe failure stress. However, only at ~90% ofthe failure
stress an increase ofvolumetric strain was observed associated with increasing permeability. Consequently,
two pronounced damage-related stress boundaries exist:
σ initial damage ≈ 0.5 -0.6 *speak resp. σ dilatancy ≈ 0.8 -0.9 * σ peak
In conclusion, although the relevancy of the term „dilatancy“ regarding its importance for describing the
evolution of the EDZ in indurated clays needs tobe discussed, the general reliability of the “dilatancy
concept” regarding onset of damage respectively sealing (i.e. decrease ofpermeability) isdemonstrated.
Concerning the hydraulical properties, both, seismic monitoring and permeability measurements clearly
indicate pre-damage of the investigated core samples, and, in addition, only partial saturation due to sample
disturbances during sample recovery and preparation whereby the relation to the bedding is apparent.
Increase of confining stress contributes largely to compaction whereby sealing is most efficient
perpendicular tothe bedding.
In consequence, gas transport in Opalinus clay is affected by 2-phase flow whereby capillary gas threshold
pressure isafunction of permeability. Based on our data asimple coupling between mechanical (e.g.
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P/EDZ/1
Rock matrix
(Visco-)elasto-plastic material model
it describes as afunction of the plastic strain :
Axial stress σ 1 (MPa )
Volumetric strain V/V (%)
60,0
40,0
30,0
20,0
10,0
Stoffansatz :
Minkley, W.; Menzel, W.; Konietzky, H.;
te Kamp, L.(2001)
Figure 1: Our modelling approach to describe the mechanical evolution of the EDZ for aspecific drift
situation at the Mont Terri site.
damage) and hydraulical properties seems unlikely. Gas-transport is controlled by two-phase flow. In
addition, aunique permeability /porosity relationship is not available.
Finally, asshown inFigure 1,anew modelling approach ispresented consisting of two parts, i.e. of a
(visco-)elasto-plastic constitutive model, comprising the hardening/softening behaviour and dilatancy
effects of the rock mass, and aspecific friction model, which described displacement- and velocitydependent
shear strength softening for the bedding planes (Minkley &Mühlbauer, 2007). Using our sitespecific
material parameters, EDZ phenomena for alocal drift situation in the Mont Terri lab could be
nicely simulated.
However, literature results reveal alarge scattering of mechanical strength date ofindurated clay, e.g. from
the Mont Terri Site, which isattributed besides others mainly tothe possible influence ofpore pressures
or de-saturation. From the experimental point of view the knowledge regarding coupling between
mechanical (e.g. damage) and hydraulical properties is not sufficient i.e. the effective Biot-coefficient is
unknown. Preliminary tests with application of pore pressures demonstrate the difficulty ensuring stable
conditions in short term tests due to the low rock permeability. This implies that more experimental work
is necessary to quantify the respective parameters and processes.
References:
Popp, T.&Salzer, K., 2006. Anisotropy of seismic and mechanical properties of Opalinus clay during
triaxial deformation in amulti-anvil apparatus. Proceedings International Symposium: "Using Natural
and Engineered Clay-based Barriers for the Containment of Radioactive Waste ",Tours, 14-18 March
2005. Physics and Chemistry of the Earth, Parts A/B/C, available online 6October 2006.
Minkley, W.&Mühlbauer, J., 2007. Constitutive models todescribe the mechanical behavior of salt rocks
and the imbedded weakness planes. In Proc. 6th Conference onthe Mechanical Behaviour of Salt,
Hannover, 22-25 May 2007, in press. Rotterdam: Balkema.
Page 522
⎛
50,0
σ Max − σ ⎞ D
σ1= σ D + ⎜ 1 +
⎟ σ 3
⎝ σ ϕ + σ
⎟
3 ⎠
⋅
⎛
50,0
σ Max − σ ⎞ D
σ1= σ D + ⎜ 1 +
⎟ σ 3
⎝ σ ϕ + σ
⎟
3 ⎠
⋅
⎛
50,0
σ Max − σ ⎞ D
σ1= σ D + ⎜ 1 +
⎟ σ 3
⎝ σ ϕ + σ
⎟
3 ⎠
⋅
0% plast. Verformung
0,04% plast. Verformung
0,1% plast. Verformung
0,24 plast. Verformung
0,4% plast Verformung
ε p (%) 0,00 0,04 0,10 0,24 0,40
σ D (MPa ) 10,5 8,6 6,0 2,0 0,0
σ φ (MPa (MPa (MPa ) 5,0 5,0 5,0 5,0 5,0
σ Max (MPa ) 55,0 48,0 43,0 38,0 35,0
0,0
0 2 4 6 8 10 12 14 16 18 20
2,5
2,0
1,5
1,0
0,5
Confining stress σ 3 (MPa )
ε p (%) 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 2 4 6 8 10 12 14 16 18 20
Confining pressure σ 3 (MPa )
• hardening
• softening
• dilatancy
⇒ Implementation
of pore pressures ?
(Konietzky, 2001)
Bedding plane properties
Newdeveloped shear model
to describe the behaviour ofweakeness planes
on the basis of:
τ = μ 1 + Δ μ ⋅ σ +
μ K
adhesion
=kinetic friction
Δμ =adhesive friction
c =cohesion
K
( ) c
τ
Shear stress (MPa )
INTERNATIONAL MEETING, SEPTEMBER 17...>...18, 2007, LILLE, FRANCE
CLAYS IN NATURAL &ENGINEERED BARRIERS
FOR RADIOACTIVE WASTE CONFINEMENT
N
σ N
Normal Normal stress
stress
σ σ Ν Ν = = 10 10 MPa
MPa
Shear displacement (mm)
Medium Medium 1
1
Medium Medium 2
2