Thermo-hydro-mechanical behavior of clay rock for deep geological disposal of high-level radioactive waste

Chun-Liang Zhang

Gesellschaft für Anlagen-und Reaktorsicherheit(GRS),Repository Safety Research Division,Theodor-Heuss-Str.4,Braunschweig,38122,Germany

Keywords:Repository Clay rock Deformability Swelling Self-sealing Water permeability Gas migration Thermal impact

A B S T R A C T In the context of deep geological disposal of radioactive waste in clay formations,the thermo-hydromechanical(THM)behavior of the indurated Callovo-Oxfordian and Opalinus clay rocks has been extensively investigated in our laboratory under repository relevant conditions:(1)rock stress covering the range from the lithostatic state to redistributed levels after excavation;(2)variation of the humidity in the openings due to ventilation as well as hydraulic drained and undrained boundary conditions;(3)gas generation from corrosion of metallic components within repositories;and(4)thermal loading from high-level radioactive waste up to the designed maximum temperature of 90°C and even beyond to 150°C.Various important aspects concerning the long-term barrier functions of the clay host rocks have been studied:(1)fundamental concept for effective stress in the porous clay-water system;(2)stressdriven deformation and damage as well as resulting permeability changes;(3)moisture influences on mechanical properties;(4)self-sealing of fractures under mechanical load and swelling/slaking of clay minerals upon water uptake;(5)gas migration in fractured and resealed claystones;and(6)thermal impact on the hydro-mechanical behavior and properties.Major findings from the investigations are summarized in this paper.

1.Introduction

Clay rocks are world-widely investigated for deep geological disposal of radioactive waste due to their favorable properties such as large homogeneous rock mass,stable geological structure,extremely low hydraulic conductivity,self-sealing potential of fractures,and high sorption capacity for retardation of radionuclides.Several countries have proposed disposal concepts on the basis of a multi-barrier-system,which comprises the natural geological formations and engineered barriers.In France and Switzerland,for instance,the potential repositories for the disposal of high-level radioactive waste(HLW)will be constructed in the indurated Callovo-Oxfordian(COX)and Opalinus(OPA)argillaceous formations,respectively(Nagra, 2002,2010,2014;Andra,2005,2015).

In the French concept(Andra,2005,2015),the repository will be constructed in the COX clay formation at a depth of about 500 m below the ground surface.HLW canisters will be disposed in horizontal boreholes of 70 cm diameter and tens of meters in length.The boreholes are steel-lined to support the surrounding rock and to ensure emplacement and potential retrieval of waste packages.Each disposal borehole is sealed with swelling clay.The thermal load from HLW is designed to be limited below 90°C in the rock mass.

In the Swiss concept(Nagra,2002,2010,2014),the repository will be constructed in the OPA formation at a depth of 400-700 m below the ground surface.The facility will include a series of deadend emplacement tunnels for HLW disposal,which will be excavated in diameter of 2.5-3 m and lengths of several hundred meters.HLW canisters will be emplaced in the middle of the tunnel section and the rest space will be back filled with expansive bentonite to retard radionuclide migration.The temperature will increase up to 140°C-160°C in the buffer on the canister surface and 75°C-95°C at the buffer/rock interface.

Construction and operation of a repository will inevitably disturb the rock mass,particularly in the near- field,yielding complex transient thermo-hydro-mechanical(THM)processes and interactions in the natural and engineered barriers for long periods of time of thousands of years.The following disturbances are crucial for the repository safety:

(1)Excavation results in a concentration of the rock stress and redistribution of the pore water pressure,which usually generates an excavation damaged zone(EDZ)around the openings that may act as potential pathways for fluid flow and radionuclide migration into the biosphere.

(2)Support of the openings restricts extension of the EDZ and ensures the operation safety;however,the support efficiency tends to decrease with time due to alternation and damage of the support materials(e.g.steel and concrete)in chemical interactions with pore water solutions.

(3)Ventilation with relatively dry air induces suction and causes release of some pore water out of the rock,which may lead to shrinkage of the pore space and increase the inherent cohesion and friction resistance between particles and hence the strength.

(4)Back filling and sealing of the repository with suitable materials limit access of groundwater to the waste and thus release of radionuclides via drifts and shafts on one hand,and support the EDZ and limit new damages in host rock on the other hand.

(5)Heat transfer from HLW produces high temperatures in the near- field,which affects the rock stress and deformation,pore water pressure and transport,chemical interactions between porewater,solutes and mineral surfaces,and causes some changes of the barrier properties of the host rock.

(6)Gas generation from corrosion of metallic components may cause gas overpressure if the gas generation rate exceeds the gas dissipation rate in the geological and engineered barrier system,which in turn may generate cracks or dilatational pathways for transport of contaminated water and radionuclides to the aquifer and the biosphere.

Assessment of the long-term performance of a clay host rock and thus the safety of the repository needs comprehensive knowledge of its THM properties and responses to the dynamic conditions(Yu et al.,2014;Bernier et al.,2017;Jobmann et al.,2017).During the last two decades,the THM behavior of the clay rocks has been extensively investigated in the underground research laboratories(URLs)in the COX clay rock at Meuse/Haute-Marne in France(MHM-URL)(Armand et al.,2017)and in the OPA at Mont-Terri in Switzerland(MT-URL)(Bossart et al.,2017).A number of large/full-scale experiments have been conducted in the URLs under realistic repository conditions.The in situ experiments are supported by laboratory tests on samples for providing robust database and for understanding the material behavior.On the basis of the test data,constitutive models and computing codes have been developed for prediction of coupled THM processes in the geological and engineered barriers.From the research activities,comprehensive results have been achieved and published mostly in the Proceedings of the International Conferences on Clays in Natural and Engineered Barriers for Radioactive Waste Confinement(Reims 2002;Tours 2005;Lille 2007;Nantes 2010;Montpellier 2012;Brussels 2015;and Davos 2017).Some of the publications dealing with the THM behavior of clay rocks are listed as follows:Noynaert(2000),Zhanget al.(2007,2010a,2013),Bock et al.(2010),Tsang et al.(2012),Li(2013),Menaceur et al.(2015,2016a),Armand et al.(2017),Bossart et al.(2017),Chen et al.(2017)and Marschall et al.(2017).

Within the framework of the German site-independent research and development program,GRS(Gesellschaft für Anlagen-und Reaktorsicherheit)has participated in the international research projects in the URLs and investigated theTHM behavior of both COX and OPA claystones with various important aspects concerning the barrier functions of the clay host rocks,including:

·Conceptual stress model for clay rock;

·Stress-driven deformation,damage,reconsolidation and permeability changes;

·Moisture influences on the mechanical properties and behaviors;

·Self-sealing of fractures;

·Gas migration in fractured and resealed claystones;and

·Thermal effects on the hydro-mechanical behaviors and properties.

This paper presents the most important findings from our investigations.Details can be found in the respective references.

2.A conceptual stress model

The studied clay rocks are a complex material in terms of mineralogical composition,microstructural organization and state of pore water(Yven et al.,2007;Mazurek et al.,2008;NEA Clay Club,2011;Robinet et al.,2015;Menaceur et al.,2016b).Fig.1 illustrates schematically the microstructure and the state of pore water in the COX claystone,which is similar for the OPA claystone.The pore sizes in claystones mainly range from nanoscale(＜2 nm)in between the parallel platelets of the clay particles to micro-and meso-scale(2-50 nm)between solid particles.The claystones have a clayey matrix embedding other mineral particles(quartz,calcite,and others).The clayey matrix consists of clay particles with strongly adsorbed interlayer water within the sheet structures and with strongly to weakly adsorbed water at the external surfaces.Large pores are filled with water that can freely migrate.

Taking into account the microstructure and the state of pore water,effective stress in claystone has been examined by Horseman et al.(1996),Rodwell et al.(1999)and Zhang(2017a).It is recognized that the effective stress in a water-saturated claystone is partly or even fully transferred by the bound pore water within the interlayer pores and narrow interparticle pores between clay particles.A conceptual stress model was derived by Zhang(2017a),which suggested that the effective stress in a dense clay-water system is transferred through both the solid-solid contact between non-clay mineral grains and the bound water in narrow pores between clay particles.In the model,clay particles including interlayer water are taken as microstructural units since the water molecules in the interlayer are strongly adsorbed and immobile under usually encountered pressure gradients.Fig.2 shows schematically the stress components acting in any wavy surface that passes through the contact areas between particles in a water saturated claystone.

The total stress acting on the mediumσtcan be expressed as

where σsis the contact stress between solid particles,σlis the average disjoining(swelling)pressure acting in the bound water between clayparticles,and pwis the pressure acting in freewater in macropores.Thus the interparticle or effective stressσeffconsists of two parts acting at solid-solid contact area and in interparticle bound water,i.e.

Fig.1.Schematic sketch of microstructures of the COX claystone(after Yven et al.,2007).

In clay-rich and less cemented materials,the effect of solid-solid contacts between non-clay particles disappears,i.e.σs→0,so that the effective stress is mostly carried by the bound pore water and is equivalent to the swelling pressure,i.e.σeff⇔σl.Conversely,if a claystone contains a large number of non-clay particles and/or is strongly cemented,the effect of bound water is negligible,i.e.σl→0,and thus the externally applied load will be transferred through the solid-solid grain contacts,i.e. σeff⇔σs.This concept provides a reasonable view to the nature of the effective stress in clay rock and clay soil,forming the fundamental basis for understanding their THM behavior.This stress concept has been already confirmed by experimental evidence(Zhang et al.,2013;Zhang,2017a)(see Section 4.1).

Fig.2.Conceptual stress model for water-saturated claystone.

3.Deformation and damage

3.1.Characterization of samples

Most of the mechanical tests were carried out on COX samples that were extracted from boreholes drilled horizontally parallel to bedding planes at the-490 m main level of MHM-URL.In order to prevent desaturation and damage,the drilled cores were confined in special cells developed by Andra(Conil et al.,2018)and stored in a climate-controlled room at 22°C.Cylindrical samples were carefully prepared from the cores.

The COX claystone at the sampling locations contains 27%-42%clay minerals,28%-38%carbonates,26%-36%quartz and small amounts of other minerals(Andra,2005).The physical properties of the samples were determined:grain densityρs=2.7±0.01 g/cm3,bulk densityρb=2.4±0.02 g/cm3,porosityφ=16.8%±1%,water content w=6.5%±0.8%,and degree of water saturation Sl=90%±6%.In order to minimize the effects of sampling-induced micro fissures and desaturation,the samples before testing were pre-consolidated by hydrostatic compression up to the in situ rock stress and even higher stresses.

3.2.Short-term behavior

Fig.3.Stress-strain-permeability behavior of COX claystone including stress thresholds of yield σY,dilatancy σD,percolation σP,and failure σB.

The short-term mechanical behaviors of the clay rocks were examined on the pre-consolidated samples in triaxial compression tests by measurements of deformation and gas permeability(Zhang and Rothfuchs,2004,2008;Zhang,2015a,2016a).Deviatoric loading was performed by axial compression at a rate of 10-6s-1and at a lateral confining stress of 0-12 MPa.Fig.3 shows the typical strain and permeability responses of the COX claystone to deviatoric loading with some stress relaxation phases for the measurement of gas permeability.

The stress-strain-permeability behavior can be characterized in four sequential stages:

(I)The stress-strain curves start with linear axial compression ε1,radial extension ε3and volumetric compaction εvuntil a deviation appears at a deviatoric stressσY,named yield stress.

(II)This yield point indicates a certain damage of the inner pore structure leading to plastic volumetric compaction.With increasing deviatoric stress,the microcracks are created and grow so that the volumetric compaction changes over to dilatation.The corresponding deviatoric stressσDis referred to as dilatancy or damage threshold.

(III)Exceeding the dilatancy threshold,the microcracks grow

and propagate much faster with further deviatoric loading.When some of the microcracks coalesce to a continuous fracture,the permeability increases spontaneously.The corresponding deviatoric stressσPis called percolation threshold.The rapid development of fractures after the dilatancy threshold leads to failure at a peak stressσB.Fracture percolation and peak failure occur almost simultaneously,i.e.σP≈ σB.

(IV)Beyond this peak point,the permeability k increases by several orders of magnitude accompanied by only little additional crack opening or dilatancy.This implies that the rapid permeability rise is governed by the increasing connectivity of the stress-induced cracks.

The damage and the resulting permeability change are inhibited by increasing the lateral confining stress.As the lateral stress is increased,the inner structure of the claystone becomes more compacted,increasing the strength.Fig.4 shows the critical stresses at yield σY,dilatancyσD,percolation σPand peak failureσB,as function of the lateral stressσ3.

The relation of the peak strengthσBto the minor principal stress σ3can be approached by Hoek-Brown criterion:

Fig.4.Stress boundaries of yield,dilatancy,percolation and peak failure for COX claystone.

whereσcis the uniaxial compressive strength which is determined to beσc=19 MPa,and the other parameters m=4 and s=1 for the COX claystone.It is observed that the dilatancy boundaryσDis about 93%of the failure envelope.As mentioned above,high stresses above the dilatancy boundary drive the development of microcracks to coalescence and thus to rupture.

Based on the permeability evolution observed during loading at different confining stresses(Fig.5),a percolation model was proposed by Zhang(2016a)for the fracturing-induced permeability,which is assumed to be contributed by the connectivity and the conductivity of cracks,and can be expressed by

Fig.5.Prediction of the fracturing-induced permeability in the claystone using the percolation model.

where kfis the ultimate permeability of the fractured rock with fully interconnected cracks and P is the percolation probability defining the fraction of cracks belonging to the conductive part of a network.The ultimate fracture permeability kfdecreases with increasing minor principal stressσ3following an exponential function:kf=koexp（-γσ3）,where kois the permeability atσ3=0 andγis a parameter characterizing the dilatability of the interconnected cracks.The percolation probability P is expressed as an exponential function of volumetric dilatancy:P=1-exp（-ΔεD/εP）,where ΔεD=|εv- εP|is the dilatancy after the percolation threshold at εP.

Using the estimated values of the parameters,ko=3×10-13m2andγ=1.9 MPa-1,the permeability variations observed after exceeding dilatancy(ΔεD≥ 0)under different lateral stresses are predicted by the model as shown in Fig.5.It is evident that the permeability evolution with the spontaneous increase due to the formation of a continuous crack network(ΔεD=0 → 0.05%)and with the subsequent slowdown due to further development of the network(ΔεD>0.05%)can be reasonably revealed by the model.

3.3.Long-term behavior

Fig.6.Creep of water-saturated(a)COX and(b)OPA claystones under low deviatoric stresses of 0.8-3.5 MPa.

Fig.7.Creep of COX claystone under multistep triaxial stresses.

The long-term performance of a repository is strongly dependent on the long-term deformability of the host rock.This important issue was investigated on the COX and OPA claystones with natural water contents in triaxial creep experiments.Fig.6 presents the results obtained on water-saturated COX and OPA samples subjected to the low deviatoric stresses ofΔσ = σ1-σ3=0.8-3.5 MPa parallel to bedding planes.It is obvious that the axial and radial strains locally measured by means of strain gages increase gradually with time at each load step.The strain rates decrease during the first transient phase within two to three months and approach asymptotically quasi-constant values.The quasistationary creep rates are very low,at around 10-12s-1.The extremely low stresses at which creep occurs suggest that there exists practically no stress threshold for onset of creep in the saturated claystones.

Fig.7 shows another creep test on COX claystone under multistep triaxial loads over two years.During the first phase at an axial stressσ1=15 MPa,the lateral stressσ3was stepwise reduced.At Δσ=σ1-σ3＜14.5 MPa,the volumetric strain rate was still positive,i.e.dεv/dt>0,indicating a compaction process and thus no damage.The quasi-stationary creep rate increased almost linearly with increasing stress.AtΔσ =14.5 MPa,the volumetric strain rate became negative,i.e.dεv/dt＜ 0,suggesting onset of dilatancy or damage.Thus,creep was accelerated by further damaging at the subsequently increased stresses that reached up toΔσ=18 MPa.When the deviatoric stress was stepwise decreased by increasing σ3to 3 MPa,the creep with damage evolution was impeded.The difference in creeprateat a stress reached by uploading and unloading indicates hardening effects.

Fig.8.Stationary shear creep rates of water-saturated COX and OPA claystones as function of deviatoric stress.

Mitchell(1976,1992)has derived a constitutive equation for creep of clay soils through thermodynamic analysis of soil flow by application of the absolute reaction-rate theory.This model has been slightly modified and applied by Zhang et al.(2013)and Zhang(2015a)for mathematical description of the stationary creep of clay rocks in terms of the stationary shear creep rate in relation to the deviatoric stress and temperature:

where˙ε is the stationary shear creep rate(s-1),T is the absolute temperature(K),R is the universal gas constant(8.31433×10-3kJ mol-1K-1),Q is the apparent activation energy(kJ mol-1),A is a factor in s-1depending on the inherent properties of the material,andαis a parameter in MPa-1.These parameters were determined from the creep data obtained along uploading path for the COX claystone:A=2.1×10-4s-1,α=0.2 MPa-1and Q=45 kJ mol-1.

Fig.8 shows a reasonable agreement of the model with the measured creep rates for the COX claystone.The relatively high creep rates observed on the OPA claystone are probably due to its higher content of clay minerals(～65%)compared to the COX claystone(～40%).At low stresses below about 10 MPa,the time-dependent deformation of water-saturated claystone is likely controlled by diffusive mass transfer or pressure solution processes in interfaces between grains in accordance to Rutter(1983).At high stresses,the creep rate increases exponentially.This may indicate increasing contributions from slips and ruptures of water- films at interparticle contacts between clay particles and solid-solid contacts between non-clay particles.Accumulation of the micro-ruptures between particles results in development of micro-fractures and ultimately in failure(Zhang et al.,2013;Zhang,2015a).This microstructural hypothesis requests to be validated by microscopic observations.

4.Responses to moisture change

4.1.Stress response to moisture change

Fig.9.(a)Principle of uniaxial swelling test under axially- fixed and laterally unconfined conditions and(b)Stress response to humidity change surrounding a claystone.

Stress response of claystone to moisture change was studied with the so-called uniaxials welling test method specially developed by our laboratory(Zhang and Rothfuchs,2007;Zhang et al.,2010b,2013;Zhang,2017a;b).A sample is axially loaded to a desired level and then fixed without lateral confinement(Fig.9a).Under the axially- fixed and laterally free conditions,the sample is dried and wetted by circulating air with varying humidity around the surface,where response of the axial stress is monitored.The resulting variation of water content is measured on an accompanying sample of the same size in the same air circulating system outside the cell.

Fig.9b shows the typical response of the axial stress to humidity change.It can be seen that drying led to a decrease in the axial stress from the preload of 15 MPa down to nearly zero and then the subsequent wetting caused an increase to a high level of 10.5 MPa where failure occurred due to wetting-induced alteration of the inner structure.This stress value is consistent with the uniaxial strength of the claystone at a high water content of 7.7%reached after a month of wetting in water vapor.Actually,drying causes release of the adsorbed water and conversely wetting lets water molecules enter the pores,being adsorbed on internal and external surfaces of clay particles,forming double layers being compressed in narrow pores and resulting in repulsive forces against the rigid confinement in axial direction.The variation of the stress with drying and wetting implies that the adsorbed water- films are capable of carrying effective stress and thus confirms the stress concept(Eq.(2))discussed earlier in Section 2.Actually,the axial stress recorded is the effective one(σeff),since no water pressure was applied to the sample and thus the pressure in the free water in macropores is zero,i.e.pw=0.The axial effective stress is directly related to the swelling pressure σlin the axial direction,i.e.Δσeff= Δσl.The buildup of the uniaxial swelling pressure without lateral confinement indicates its tensor expression(Horseman et al.,1996;Rodwell et al.,1999),which differs from the isostatic pressure state of free water in macropores.

Obviously,the swelling pressureσlis dependent on saturation degree of bound pore water,as shown in Fig.10 representing theσldata in relation to the degree of total water saturation.The saturation degrees of the different types of porewater(interlayer water,interparticle bound water,and free water)are also illustrated schematically.

It can be seen from Fig.10 that:

(1)The pre-applied axial stress ofσo=15 MPa decreases with desaturation to a minimum value ofσs=0.5 MPa at a residual saturation degree of Sr=15%,below which no stress changes occur.

(2)The resaturation increases the stress toσls=9.5 MPa at full saturation(Sl=100%),and even after that,the stress increase continues with more water uptake to a maximum value of σp=10.5 MPa,so that the total maximum swelling pressure is the sum ofσl= σp-σs=10 MPa.

(3)The stress varies almost linearly with degree of water saturation in a range of Sr≤Sl＜80%along drying path and Sr≤Sl＜90%along wetting path.

Based on the test results and taking the fractions of the different contact areas(solid-solid,bound water- films and free pore water)into account,a relationship of the effective stress to the saturation degree of bound pore water was approximately derived by Zhang et al.(2013)and Zhang(2017a):

where Seis the effective saturation degree of bound pore water,Slis the saturation degree of total pore water,Sois the saturation degree of free pore water(So=0 for the tested claystone),Sris the residual saturation degree of the remaining water in the interlayer and adsorbed on the external surfaces as the clay particles are disconnected,πDis the local net disjoining pressure in the bound pore water before full saturation,πOis the swelling pressure after full saturation upon water uptake Δw,σsis the contact stress between solid particles at the residual degree Srof water saturation,andσlis the swelling pressure acting in the bound water between clay particles and consists of two parts by πDSebefore full saturation and πO(Δw)after full saturation.

Because the water sorption process in the claystonesis notreally clear,this preliminary model has to be modified further and needs more precisely physical interpretations.It seems also possible to establish a relation of the effective stress directly to suction,because the saturation degrees of the different types of pore water and the buildup of swelling pressures are governed by suction for a given clay material.

4.2.Swelling upon water uptake

Fig.10.(a)Effective stress and swelling pressure as a function of degree of water saturation and(b)Schematic of saturation of interlayer,interparticle bound and free water in clay.

The studied claystones possess adsorption potentials,under which an amount of water can be taken up from the humid environment.The water uptake enlarges the distances between clay particles and between the interlayers in clay minerals,causing macroscopic swelling.The water uptake and the resulting swelling depend on the humid environment and confining conditions.

During the observation of stress response to drying and wetting on the axially- fixed COX sample(cf.Fig.9),its radial strain was also monitored(Fig.11a)and in parallel,the variation of water content and deformation in axial and radial directions were measured on the other accompanying sample under unconfined conditions(Fig.11b).The results from the unconfined sample(Fig.11b)show that drying caused evaporation and release of bound pore water,resulting in collapse of the pore structure to macroscopic shrinkage.A larger shrinkage was observed in radial direction normal to bedding compared to the axial shrinkage parallel to bedding.Under the axially- fixed condition,the drying-induced reduction of the axial stress(Fig.9)was accompanied by shrinking in radial direction(Fig.11a).The subsequent wetting increased the water content to the initial value of 7.2%,resulting in a radial expansion of the axially- fixed sample(Fig.11a)and expansion of the unconfined sample in all directions(Fig.11b).However,the free swelling observed on the unconfined sample is anisotropic with larger radial strain normal to bedding compared to the axial strain parallel to bedding.The anisotropic response of the claystones to moisture change was also observed by other research groups(Valès et al.,2004;Wan et al.,2013;Wang et al.,2013;Yang et al.,2013;Minardi et al.,2017).

Under unconfined conditions,the claystones can take up great amounts of water up to 12%-15%in water vapor,much more than that of 7%-8%in the in situ confined conditions.The increase in water content contributes to the volumetric expansion up to 8%-12%,as shown in Fig.12 comparing the swelling capacities of the COX and OPA claystones by wetting with water vapor at relative humidity(RH)of 100%.The OPA claystone with higher clay content of～65%takes more water and expands more compared to the COX with clay content of～40%.

Fig.11.Strain responses of COX claystone samples under(a)axially- fixed and(b)unconfined conditions to humidity change.

4.3.Influence of water content on strength

Fig.13.Influence of water content on the uniaxial stress-strain behavior,dilatancy and failure strength of COX claystone.

Increasing water content results in widening of the distances or thickness of bound water- films between clay particles and in turn degradation of the inherent cohesion and friction resistance.Thus the stiffness and strength are degraded with increasing water content.Conversely,drying causes release of the adsorbed water,leading to more solid-solid contacts between particles,so that the inherent cohesion and friction resistance increase and enhance the stiffness and strength.Fig.13 shows the results of uniaxial compression tests on the COX claystone with different water contents of w=1.4%-7.7%.It is obvious that the elastic stiffness,dilatancy and peak failure strengths increase significantly with decreasing water content.The maximum peak strength of about 50 MPa is reached at w=1.4%,four time shigher than that of 10 MPa at w=7.7%.Similar observations were also made by other research groups on the claystones(Blümling et al.,2007;Zhang et al.,2014;Amann et al.,2017).

5.Sealing of fractures

Fig.14.Fracture closure-normal stress relationship obtained on COX claystone.

Because of the rheological deformability and swelling capability of the claystones presented before,a recovery process of the EDZ can be expected due to combined impact of the progressive deformation of the clay host rock,the increasing resistance of the engineered barriers,and the swelling/slaking of clay minerals into fracture interstices during the post-closure phase of the repository.This important issue has been intensively investigated with artificially-cracked samples under various THM conditions(Davy et al.,2007;Zhang and Rothfuchs,2008;Zhang,2009,2011;2013,2016a;Bock et al.,2010;Zhang et al.,2010a,2013;Auvray et al.,2015).Some typical results from our experiments(Zhang,2009,2011,2013)are presented below.

5.1.Under compression

Axially-cracked samples were compressed in triaxial cells,whereby closure of fractures and gas permeability were measured.Fig.14 illustrates some results of fracture closure(Δb)under normal stresses(σn).All the σn-Δb curves express the nonlinear behavior,involving the decayed fracture closure and the hysteresis cycle by loading/unloading.Fracture closure evolves faster at large apertures in the initial stage and then closure rates decrease with increase in normal stiffness of the fractures at small apertures.The relationship of fracture closure with effective normal stress can be approximated by an exponential equation:

wherebois the initial aperture equivalent to the maximum aperture closure,andαandξare the constants.If the stress tends to infinity(σn→+∞),the fractures will be fully closed,i.e.Δb→0.Fitting the data derives a unique set of the parametersα=0.3 andξ=0.5 for the samples with different initial apertures of bo=0.32-1.7 mm.

Fig.15.Fracture permeability-aperture relationship obtained on COX claystone.

The fracture closure results in a decrease in gas permeability fromthe initial value of 2×10-13-5×10-13m2down to 3×10-16-7×10-16m2for the large initial apertures(bo=1.1-1.7 mm)and from 1×10-15-5×10-15m2down to 6×10-18-2×10-20m2for the small initial apertures(bo=0.32-0.61 mm),as shown in Fig.15.Because the intact claystone matrix is practically impermeable at the applied gas pressures below 1 MPa,the permeability of fractured claystone is determined exclusively by the fracture closure,which may be approximated by the “cubic law”for fluid flow through a set of parallel fractures:

where b is the average fracture aperture,s is the mean fracture spacing,R is the roughness factor of the fracture surfaces,and F=R/s represents an integrated character of the set of fractures.As the fracture aperture b decreases to zero,Kgtends to zero.Fitting the data yields different F-values of 1×10-5m-1up to 2×10-2m-1due to the different characteristics of the fractures in samples.

5.2.Under impact of water flow

As water enters and flows through fractures in claystone,the clay matrix can take up water and expand into the interstices.The water-induced swelling,weakening and slaking of the claystone lead to sealing of the fractures.Consequently,the hydraulic conductivity of the fractured claystone decreases.Fig.16 shows the long-term evolution of water permeability obtained on fractured COX samples under low confining stresses of 2-3.5 MPa over more than 3 years.As soon as the water was supplied,the high initial gas permeability of 3×10-12m2dropped immediately by five to seven orders of magnitude down to 10-17-10-19m2,depending on the fracture intensity of each sample.At each load level,the permeability decreased gradually with time.The final permeability values are very low at 3×10-20-7×10-21m2,being the same order of magnitude as that of the undisturbed clay rock.

The permeability values Kwobtained after stabilization at different effective confining stresses σeffare summarized in Fig.17. The effective stress is defined as σeff=σ-pw/2,whereσis the total confining stress,pwpw is the water upstream pressure,and pw/2 is the mean pore pressure at zero downstream pressure.The logKw-σeffdataset for each sample can be approximated by an exponential equation:

Fig.16.Long-term evolution of water permeability obtained on fractured COX claystone.

Fig.17.Water permeability of fractured claystones as a function of confining stress.

where Kwois the water permeability at zero effective stressσeff=0,andβis a parameter characterizing the closure tendency of the pathways.Kwois determined by the initial intensity of fractures in the material and varies from a sample to another,where as β reflects the effects of swelling and deformation of the claystone matrix near the fracture walls into the fracture voids,and thus can be used as an index for the self-sealing capacity.Different Kwo-values are estimated in the range of 2×10-16-4×10-20m2and a uniqueβ-value of 0.58 MPa-1is obtained for all tested COX and OPA samples.This is clearly illustrated in Fig.18 with the normalized permeability to the initial ratio,Kw/Kwo.The same slope of the log(Kw/Kwo)-σeffcurves indicates the same self-sealing capacity of the COX claystone from the-490 m level of the MHM-URL and the OPA sandy claystone from the MT-URL.The OPA sandy claystone has relative lower contents of clay minerals(20%-40%)but higher contents of quartz(30%-45%)and carbonates(20%-40%).

6.Gas migration

Fig.18.Normalized permeabilities of the COX claystone and OPA sandy claystone as a function of effective confining stress.

Fig.19.Gas breakthrough pressures and permeabilities obtained on highly resealed COX and OPA claystones under different confining stresses:(a)5 MPa and(b)15 MPa.

Gas generation from corrosion of metallic components and its impact on the integrity of the natural and engineered barrier systems is one of the most concerns for the repository safety.This important issue was intensively investigated in the European Commission project-FORGE(Fate Of Repository GasEs)(Shaw,2015;Zhang,2015b).The highly-consolidated and water saturated clay rocks are practically impermeable for advective transport of gas under normally-encountered pressure gradients(Horseman et al.,1996;Rodwell et al.,1999;Cuss et al.,2012;Harrington et al.,2013).However,the EDZ may act as conduits for preferential gas flow,depending on the resealing degree of fractures.The gas migration behavior of damaged claystone was examined on pre-fractured claystone samples that had been fully water-saturated and highly resealed(Zhang,2015b).

Fig.19 presents the measured gas breakthrough pressures of the highly resealed samples withvery low water permeabilities of10-19-10-21m2(cf.Figs.16 and 17).It can be seen that gas entry and penetration into the water-saturated and highly resealed claystones need high gas pressures to overcome certain thresholds,which are determined by the sealing intensity of the fractures under confining stress.

The advective gas flow through resealed claystone is accompanied by micro- fissuring and dilation of the generated fissures.Fig.20 shows an example of the gas pressure impact on a resealed sample under axial stress of 16 MPa and lateral stress of 12.7 MPa.During the first stage with stepwise gas pressure increase from 2 MPa to 4 MPa,compressive strains evolved progressively and no gas out flow was detectable.The subsequent pressure increase to 4.5 MPa inhibited the continuation of the radial compression.Further pressure increase to 5-7 MPa led to a slightly gradual dilatancy in radial direction,indicating a gas penetration into the sample.The pathways are probably attributed to reopening of some weakly resealed fractures under the high gas pressures.As the gas pressure was increased up to 8.3 MPa,a sudden dilatancy in all directions took place and gas breakthrough occurred.

As shown above,the gas breakthrough pressures observed on the highly resealed claystones are still below the minor confining stress and thus below the fracturing criterion for the intact clay rock,as shown in Fig.21.This important finding implies that the EDZ,even when highly resealed,will still bear the capacity for gas migration with moderate pressures and thus contribute to avoiding high pressure buildup,so that the host rock is prevented from gas fracturing.The gas breakthrough pressure of water-saturated and resealed claystone can be expressed as a function of water permeability and minimum confining stress:

Fig.20.Gas pressure induced dilatancy in a highly resealed claystone sample under triaxial compression.

Fig.21.Gas breakthrough pressures of highly resealed claystones in comparison with a conservative fracturing criterion for intact clay rock.

where Pbis the gas breakthrough pressure,Kwois the initial water permeability at zero minor principal stressσmin=0,and the parameters are estimated to be B=2×10-7MPa m2/3,n=3 and γ=0.2 MPa-1.The model prediction provides a reasonable agreement with the data as shown in Fig.21.

Further gas pressure rising can reopen more resealed microfissures as well as dilate the gas-occupied pathways.The growth of the micro- fissure networks allows gas to flow easier,so that the gas permeability increases as shown in Fig.22.The relationship of gas permeability to gas pressure rise can be approximately represented by an exponential function:

Fig.22.Gas breakthrough permeability of resealed claystones as a function of gas pressure.

where Kgois the gas permeability at the initial breakthrough pressure Pbo;andωis a parameter characterizing the dilatability and interconnectivity of the gas pathways,which can be determined by the slope of the lnKg-Pbcurve.Fitting the data obtained by increasing gas pressure leads toω-values in a small range of 1.5-2.5 MPa-1.

7.Thermal effects

The most concern over the disposal of heat-emitting HLW is whether and how the favorable barrier properties of the clay host rock will be altered under transient non-isothermal conditions over several thousands of years.This important issue was extensively investigated in the European Commission project-TIMODAZ(Thermal Impact on the Damaged Zone around a Radioactive Waste Disposal in Clay Host Rocks)(Li,2013).Within this project and the other German national projects,thermal effects on claystones were comprehensively investigated on COX and OPA samples under various repository-relevant conditions(Zhang et al.,2005,2006;2007,2008;2009,2010a;2013;Zhang,2016b).Thermal effects were measured by hydro-mechanical responses of the claystones to thermal loading, including thermally-induced expansion and contraction,pore pressure variation,temperature influences on deformability and strength,on swelling as well as on self-sealing capacities.

7.1.Thermal expansion

Thermal expansion was measured by heating the claystones with the natural water contents under decreased confining stresses of 15 MPa,10 MPa,5 MPa and 1 MPa(Zhang et al.,2013,2017).Fig.23 shows the results of a heating test on COX sample at a confining stress of 5 MPa.The water-saturated sample was heated bystepwise increasing temperature from 23°C to 68°C and then cooled down.The measured strains(Fig.23a)show that(1)each temperature increase generates expansion in all directions;(2)the expansion does not change much with time at constant temperatures below 47°C and then turns over to a gradual contraction at higher temperatures;and(3)cooling down yields contraction.The contraction at the high temperatures above 56°C might be caused by mobilization of the heated pore water into some unsaturated pores and also by possible leakage of the isolating boundary against such high temperatures,which allows some release of pore water and thus leading to consolidation under loading.

Based on the data,thermal expansion coefficient can be obtained as a function of temperature(Fig.23b).The coefficient increases almost linearly with increasing temperature in the test range,which can be expressed by

where αs,αwandαmare the linear expansion coefficient of the solid grains,the volumetric expansion coefficients of the pore water and the water-saturated porous medium,respectively;T and Toare the actual and the reference temperatures(20°C),respectively;and ω is a factor.The first term of the right side of the equation represents the thermal expansion coefficient of the material at Toand the second term for the temperature dependence.The parameter values are obtained by fitting the data for the claystone with a porosity ofφ =16.5%:αs=2 × 10-6°C-1for clay minerals,αw=3.4 ×10-4°C-1for pore water,ω =1 ×10-6°C-1.Because of the bedding structure,the thermal expansion is anisotropic.The thermal expansion in direction perpendicular to bedding is higher than that parallel to bedding(Zhang et al.,2008,2010a;2013;Zhang,2016b).

Fig.23.Thermal expansion and contraction of a COX sample during heating and cooling:(a)Strain response to thermal loading,and(b)Thermal expansion coefficient.

7.2.Temperature influences on deformation and strength

Thermal impact on the deformation of the claystones was examined in triaxial creep tests under various temperatures and stresses.Fig.24a shows a typical creep test on the water saturated COX claystone at different temperatures of 28°C-110°C under a triaxial stress state ofσ1/σ3=15 MPa/0.5 MPa and undrained conditions.The strain-time curves show that:(a)each temperature increase leads to a short-time radial expansion but a slight axial compression;(b)the axial,radial and volumetric strains(ε1, ε3, εv)increases quite linearly with time at each elevated temperature below 90°C,suggesting no or less thermal transient creep;(c)at higher temperatures above 90°C,the creep slows down;and(d)cooling down results in a short-term radial contraction and negligible creep at each lowered temperature.The creep acceleration at elevated temperatures up to 90°C probably results from the reduction of viscosity and friction of bound water- films between solid particles.At the higher temperatures,the undrained conditions of the testing system could not be tightly maintained,so that some thermally-mobilized pore water was released up to 2%observed after testing.As a consequence,the pore structure was consolidated,increasing the friction resistance between particles and hence decelerating the creep.Fig.24b illustrates the steady state creep rate as a function of temperature under different stress conditions.The creep acceleration by temperature up to about 90°C can be reasonably revealed by the creep equation(Eq.(5)).

Fig.24.Temperature influence on the creep of water-saturated COX claystone:(a)Triaxial creep at different temperatures,and(b)Stationary creep rate as a function of temperature.

Thermal impact on the strength of a claystone differs from saturated to unsaturated states and from drained to undrained conditions.In saturated and undrained conditions,the claystone becomes more ductile and weaker with increasing temperature,as shown in Fig.25 depicting the undrained stress-strain behavior of the water-saturated OPA claystone at temperatures between 20°C and 116°C and at lateral stresses around 3 MPa.The weakness is probably caused by thermally-induced pore overpressure and the corresponding decrease in effective mean stress on one hand and by thermally-induced reduction of the cohesion and friction resistance of the bound water- films between solid particles on the other hand.

However,However, the temperature influence on the strength of the water saturated COX claystone is less significant,as shown in Fig.26 comparing the undrained strength values at T=20°C-30°C(black points)and at T=40°C-100°C(collar points).The strength curves at different temperatures are close to each other.In contrast to the undrained thermal loading,heating a claystone in drained condition drives the existing water out of the pores,resulting in consolidation of the pore structure under confining stresses,increasing the inner friction resistance between solid particles against shearing and hence strengthening the rock.For instance,heating and drying the COX claystone at high temperatures of90°C-150°C(upper part in Fig.26)can increase the strength of the claystone significantly.

Fig.25.Stress-strain curves obtained on the water-saturated OPA claystone at different temperatures.

7.3.Thermal impact on self-sealing of fractures

Thermal impact on the self-sealing capacity of damaged claystone is one of the most concerns in the assessment of the longterm safety ofHLW repositories.This was examined by measuring water permeability of fractured claystones in various thermal conditions.Fig.27a shows the evolution of water permeability obtained on cracked COX and OPA samples during heating and cooling between 20°C and 90°C.As soon as the water was supplied,the high initial gas permeability of 3×10-12m2dropped immediately by two to five orders of magnitude down to 10-15-10-17m2,depending on the fracture intensity of each sample.This drastic drop in permeability is mainly attributed to the water induced swelling,slaking and clogging of the fractures.During the first stage at 20°C,the water permeability decreased with time from 1×10-15m2to 3×10-18m2at OPA sample and from 1×10-17m2to 5×10-19m2at COX sample,respectively.The permeability reduction rate was less affected by the temperature increase up to 60°C.Further heating up to 90°C and also cooling down back to 60°C had no or only little effect on the permeability reached before.Further cooling down to 20°C,however,induced a further reduction of the permeability to 3×10-19m2and 1×10-19m2at both the samples.

Fig.26.Comparison of drained and undrained thermal strengths of the COX claystone.

Fig.27.Temperature influence on the water permeability of fractured COX and OPA claystones:(a)Water permeability evolution during heating/cooling,and(b)Water permeability as a function of temperature.

The water permeability values obtained after stabilization of each temperature stage are depicted in Fig.27b as a function of temperature.It is obvious that the permeabilities of the fractured claystones are slightly influenced by heating and cooling.They decrease more or less with increasing temperature during heating and drop further down with cooling.Generally speaking,the selfsealing capacity of fractures in the claystones is not remarkably affected by the applied thermal load.

8.Conclusions

The THM behavior of the COX and OPA clay rocks has been extensively investigated with various kinds of laboratory experiments,covering the important aspects concerning the long-term containment of radioactive waste:(1)stress concept,(2)deformability and damage,(3)moisture effects,(4)self-sealing of fractures,(5)gas migration,and(6)thermal impacts.

The stress analysis suggests that the interparticle or effective stress in a dense clay-water system is transferred through both the adsorbed interparticle pore water in narrow pores and the solidsolid contact between non-clay mineral grains.The experiments confirm that the adsorbed pore water in the claystones is capable of bearing effective stresses until the strength is exceeded.

The claystones deform elasto-plastically under rapid load with an overall volume compaction until the onset of dilatancy at a high stress.Closely as the peak stress is reached,shear fractures are created leading to a spontaneous increase in permeability and to failure.The damage and failure are suppressed under high values of the minor confining stress.After failure,the claystone still possesses a certain residual strength.When the minor principal stress is increased again,the fractures tend to be closed.The long-term deformability of the claystones is characterized by creep under constant load.The creep under relatively low shear stresses is probably dominated by diffusion and slip processes in the bound water- films between solid particles.At high stresses above the dilatancy threshold,the creep is enhanced by microcracking through the interfaces between particles and tends towards rupture.

The claystones are sensitive to moisture change.In dry air conditions or high suctions,the pore water moves out of the pore space,leading to shrinkage or compaction of the desaturated pores under load.It enhances the friction resistance between particles and hence the stiffness and strength.In contrast,the claystones can take up water from a humid environment,resulting in expansion or development of swelling pressure under confinement.The enlargement of distances between particles due to water uptake leads to degradation of the stiffness and strength.

The claystones possess significant self-sealing potentials.Fractures in them are going to close up under mechanical compression.The fracture aperture decreases exponentially with the increasing normal confining stress. The resulting decrease in gas permeability is related to the aperture by a cubic law.As water enters and flows through fractures,the clay matrix takes up water and expands into the interstices.Consequently,the hydraulic conductivity decreases dramatically by several orders of magnitude down to low levels of 10-19-10-21m2even at low confining stresses of 2-4 MPa.The decreased water permeabilities are in the same order of the intact claystones.

Before water saturation,fractures in the claystones act as preferential pathways for gas.After water saturation,gas entry and subsequent penetration into the resealed fractures require a certain gas pressure to overcome the breakthrough threshold,which is controlled by the re-sealing degree of fractures and the confining stress.All the experiments show that the gas breakthrough pressures in the water-saturated and highly resealed claystones are still lower than the confining stresses.It implies that the EDZ,even when highly resealed,will still have the capacity for gas migration at moderate pressures.

Thermal impacts on the properties and responses of the claystones are dependent on their inherent properties such as porosity and water saturation,and also on the external conditions such as confining stress,hydraulic drained and undrained boundaries.In the water-saturated claystones,the thermal expansion is predominantly controlled by the pore water because of its much higher expansion coefficient compared to that of the solid grains.Under undrained and confined conditions,the water expansion causes high pore pressures,thus the effective stress decreases.The thermal mobilization of the bound pore water alters the inherent cohesion and friction resistance between particles,accelerating the deformation but decreasing the strength.In contrast,heating in drained conditions drives the thermally mobilized pore water out of the pore space.Under external load,the release of the bound pore water leads to consolidation of the porous medium.With heating and drying,the contacts between particles become increasingly the solid-to-solid type,so that the friction resistance between particles increases,enhancing the stiffness and strength but hindering the creep.Another key point is that the high sealing potentials of the fractured claystones are almost not affected by the applied thermal loads at 90°C-120°C,so that the favorable barrier properties of the clay host rocks will not be altered during the thermal loading from HLW.

Conflict of interest

The author wishes to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Acknowledgements

This work was funded by the German Federal Ministry for Economic Affairs and Energy(BMWi)under contract number 02E10377.