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Journal of Rock Mechanics and Geotechnical Engineering 5 (2013) 169–178

Journal of Rock Mechanics and GeotechnicalEngineering

Journal of Rock Mechanics and GeotechnicalEngineering

journa l homepage: www.rockgeotech.org

n the chemo-thermo-hydro-mechanical behaviour of geological andngineered barriers

u-Jun Cui ∗, Anh Minh Tangaboratoire Navier/CERMES, Ecole des Ponts ParisTech, 6 et 8, Avenue Blaise Pascal, 77455 Marne-la-Vallée cedex 2, France

r t i c l e i n f o

rticle history:eceived 26 March 2013eceived in revised form 10 April 2013ccepted 25 April 2013

eywords:oom Clay (BC)entonite-based materialsechanical behaviourydraulic conductivityore-water salinityechnological voids

a b s t r a c t

An overview of the recent findings about the chemo-hydro-mechanical behaviour of materials used forboth geological and engineered barriers in nuclear waste disposal is presented, through some examplesabout the natural Boom Clay (BC) and compacted bentonite-based materials. For the natural BC, it wasfound that compression index identified from both oedometer and isotropic compression tests is sim-ilar and the compressibility of BC from the Mol site is higher than that of BC from the Essen site; theshear strength of Mol BC is also higher than that of the Essen BC, suggesting a significant effect of carbo-nates content; the thermal volume change is strongly overconsolidation ratio (OCR) dependent—low OCRvalues promote thermal contraction while high OCR values favour thermal dilation; the volume changebehaviour is also strongly time dependent and this time dependent behaviour is governed by the stresslevel and temperature; the effect of pore-water salinity on the volume change behaviour can be signif-icant when the smectite content is relatively high. For the bentonite-based materials, it was found thatthermal contraction also occurs at low OCR values, but this is suction dependent—suction promotes ther-

mal dilation. Under constant volume conditions, wetting results in a decrease of hydraulic conductivity,followed by an increase. This is found to be related to changes in macro-pores size—wetting induces adecrease of macro-pores size, followed by an increase due to the aggregates fissuring. The presence oftechnological voids can increase the hydraulic conductivity but does not influence the swelling pressure.

© 2013 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by

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mcf

. Introduction

In the high-level radioactive waste geological disposal, multi-arrier concept with geological barrier and engineered barrier issually considered. The geological barrier is the natural host forma-ion such as claystones (France and Switzerland), salts (Germany),ranite (China and Sweden) and stiff clays (Belgium). The mainunctions of geological barrier are its low hydraulic conductivitynd its capacity of self healing. The engineered barrier is made up

∗ Corresponding author at: Laboratoire Navier/CERMES, Ecole des Ponts ParisTech, et 8 avenue Blaise Pascal, Cité Descartes, Champs-sur-Marne, 77455 Marne-la-allée cedex 2, France. Tel.: +33 01 64153550.

E-mail address: yujun.cui@enpc.fr (Y.-J. Cui).eer review under responsibility of Institute of Rock and Soil Mechanics, Chinesecademy of Sciences.

674-7755 © 2013 Institute of Rock and Soil Mechanics, Chinese Academy ofciences. Production and hosting by Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jrmge.2013.05.001

oeso

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Elsevier B.V. All rights reserved.

f compacted bentonite-based materials. It is usually used withranite geological barriers (China and Sweden) and constitutes aarrier before the geological one. Note that these materials arelso used as filling materials for other geological barriers. The mainunctions of the bentonite-based materials are their sealing capac-ty related to their volume change behaviour, swelling property,etention capacity and hydraulic behaviour.

In the situation of nuclear waste disposal, the constitutiveaterials of both barriers are subjected to complex coupled

hemo-thermo-hydro-mechanical (CTHM) loadings stemmingrom interaction between clay minerals and pore-water chemistryr other chemicals resulted from concrete alteration, from heatmission of the waste canisters, from water flow and from fieldtress/materials swelling. This justifies the wide studies conductedn their CTHM behaviours in both laboratory and field conditions.

In field, tests have been performed in different Undergroundesearch Laboratories (URLs). For instance, the French agencyANDRA) has carried out various tests in the Bure URL to investigatehe involved Oxfordian claystone behaviour during excavation,

eating, etc. The Swiss agency (Nagra) has done the same in theont Terri URL. The Belgian agency (Ondraf/Euridice) has investi-

ated the hydraulic behaviour and mechanical behaviour of Boomlay (BC) in the Mol URL. Note also that the French institution IRSN

170 Y.-J. Cui, A.M. Tang / Journal of Rock Mechanics and Geotechnical Engineering 5 (2013) 169–178

Table 1Geotechnical properties of the soil cores studied.

Core Depth (m) Member Gs wL (%) IP e0 Carbonate content (%)

Ess75 218.91–219.91 Putte 2.65 78 45 0.785 0.91Ess83 226.65–227.65 Putte 2.64 70 37 0.730 0.76Ess96 239.62–240.62 Terhagen 2.68 69 36 0.715 0.24

68 39 0.700 4.3662 37 0.755 2.6459–83 0.49–0.67 5.9–8.3

(drsmiiht

b(S2ob1R22Vee2

rtaa

2

b(bett

mmtwfiN2

MvBa

c

Table 2Mineralogical composition of clay fraction (<2 �m).

Core Mineral content (%)

Chlorite Kaolinite Illite Smectite Ill/Smecta

Ess75 5 35 20 10 30Ess83 5 35 20 20 20Ess96 5 35 20 10 30Ess104 5 35 15 30 15Ess112 5 35 10 50 10Mol 5 35 20 10 30

30

40

50

60

70

80

90

100

1 10 100

Particle size (µm)

Per

cent

pas

sing (

%)

Ess75

Ess83

Ess96

Ess104

Ess112

Mol (Coll, 2005)

sfi

ccacn4

out, allowing determination of the compression index Cc. Theobtained values of Cc are shown in Table 3. It appears clearly that

Table 3Compressibility parameters.

Core Oedometer compression Isotropic compression

Ess75 0.378 0.383Ess83 0.345 0.313

Ess104 247.90–248.91 Terhagen 2.68

Ess112 255.92–256.93 Terhagen 2.67Mol 223 Putte 2.67

Institut de Radioprotection et de la Sûreté Nucléaire) has been con-ucting the infiltration tests aiming at identifying the key factorselated to the long-term performance of bentonite-based sealingystems when considering the initial technological void. The ther-al loading effect has been also investigated in different URLs, for

nstance, in the Mol URL, a 10-year heating project—Praclay projects underway, aiming at investigating the effect of heating on theydro-mechanical of BC on one hand, and the sealing capacity ofhe compacted bentonite ring on the other hand.

In the laboratory, several studies on the hydro-mechanicalehaviour of geological formations have been conductedHorseman et al., 1987; Baldi et al., 1988, 1991; Sultan, 1997;ultan et al., 2000, 2002; Coll, 2005; Deng et al., 2011a, 2011b,011c, 2012). A large number of studies have also been performedn the thermo-hydro-mechanical (THM) behaviour of bentonite-ased materials (Pusch, 1979; Dixon et al., 1985, 1987, 1992, 1996,999; Yong et al., 1986; Delage et al., 1998, 2006; Lee et al., 1999;omero et al., 1999, 2011; Börgesson et al., 2001; Cui et al., 2002,008, 2011; Marcial et al., 2002; Marcial, 2003; Montes et al.,003; Villar and Lloret, 2004, 2008; ANDRA, 2005; Agus, 2005;illar, 2005; Lloret and Villar, 2007; Agus and Schanz, 2008; Agust al., 2010; Gatabin et al., 2008; Karnland et al., 2008; Kominet al., 2009; Ye et al., 2009; Komine, 2010; Komine and Watanabe,010; Tang and Cui, 2010a, 2010b; Wang et al., 2012).

In this paper, some advances on the CTHM behaviours of mate-ials used for geological and engineered barriers are presentedhrough some examples about the natural BC from both Mol sitend Essen site in Belgium and compacted bentonite-based materi-ls.

. Volume change behaviour of Boom Clay

In the Belgian programme for nuclear waste disposal, BC haseen investigated for both Mol site (URL location) and Essen siteabout 50 km east from Mol). In order to compare its volume changeehaviour for both sites, five cores (1 m long and 100 mm in diam-ter) were taken from the Essen site and one core was taken fromhe Mol site at a depth of 223 m. The geotechnical properties ofhese cores are shown in Table 1.

For the BC at Essen, two cores were taken from the Putteember (Ess75 and Ess83) and three cores from the Terhagenember (Ess96, Ess104 and Ess112). The geotechnical properties of

hese cores are similar: specific gravity, Gs = 2.64–2.68; liquid limit,L = 62%–78%; plastic index, IP = 36–45. The void ratio (e0) ranges

rom 0.700 to 0.785. The carbonate content of core Ess104 (4.36%)s significantly higher than those of other cores (lower than 1%).ote that the carbonate content for Ess112 is also relatively high:.64%.

After Francois et al. (2009), the main parameters of the BC atol, e.g. Gs, wL and IP are similar to those of the BC at Essen. The

oid ratio ranges from 0.49 to 0.67, significantly lower than that of

C at Essen. These differences suggest that the BC at Mol is densernd more carbonated.

Table 2 depicts the mineralogical compositions of differentores. It is observed that BC at Essen contains more active minerals

Fig. 1. Particle size distribution curves.

uch as smectite, but the total amount of smectite and interstrati-ed illite/smectite is similar.

The particle size distribution curves are shown in Fig. 1. Theurves of Ess83 and Ess96 are close to that of Mol, showing alay content (<2 �m) of 57%–60%. The curves of Ess75 and Ess104re slightly below the curves of Ess83, Ess96 and Mol, showing aontent of clay-size particles of 43%–50%. Core Ess112 presents sig-ificantly larger particles and a lower value of clay content (about0%).

Both oedometer and isotropic compression tests were carried

Ess96 0.375 0.280Ess104 0.327 0.346Ess112 0.302Mol 0.405

Y.-J. Cui, A.M. Tang / Journal of Rock Mechanics and Geotechnical Engineering 5 (2013) 169–178 171

F1

bs

3

(1pctc(ccwca

ca

4

eA1h1

(i(c3a6

v

(

F

(

(

(

tr(mcac

5

ieotttfunction. Note that this observation is made for a quite low stressrange; experimental data for a larger stress range are needed toconfirm this point.

ig. 2. Summary of failure envelopes of all Boom Clay considered Bouazza et al.,996.

oth tests give similar results. Comparison between the two siteshows that BC at Mol is more compressible than BC at Essen.

. Shear strength of Boom Clay

In Fig. 2, the results at failure of BC at Mol by various authorsHorseman et al., 1987; Baldi et al., 1991; Van Impe, 1993; Sultan,997; Coll, 2005) and the results on BC at Essen are gathered in′–q plane (mean effective stress-deviator stress plane) for a directomparison. It appears clearly that the slope of intact BC at Mol inhe range of p′ > 2 MPa and the slope of intact BC at Essen is verylose (0.46 against 0.47), suggesting a similar internal friction angle12◦–13◦). However, the q0 value (intersection) for Mol is signifi-antly larger than that for Essen (1.12 MPa against 0.39 MPa). Thisorresponds to a higher effective cohesion for Mol as comparedith that for Essen: 0.53 MPa against 0.19 MPa. This difference in

ohesion is likely due to the difference in carbonate contents: BCt Mol has a higher content of carbonate (see Table 1).

The reconstituted Essen BC has an effective cohesion′ = 0.01 MPa and an internal friction angle ϕ′ = 20◦. These valuesre close to those of core Ess112.

. Thermal volume changes

The overconsolidation ratio (OCR) is known to have a majorffect on the volume change of soil heated under a constant load.t low OCR values, heating induces thermal contraction (Paaswell,967; Plum and Esrig, 1969). On the contrary, at high OCR values,eating results in thermal dilation (Baldi et al., 1988; Towhata et al.,993).

This aspect was investigated for BC at various OCR valuesOCR = 1, 2, and 12) by carrying out heating tests in a triax-al cell from 22 ◦C to 100 ◦C or tests with temperature cycles22 ◦C—100 ◦C—22 ◦C). The results are shown in Fig. 3. The OCR = 1ondition was achieved under confining stresses � ′

c = 1.2 MPa and.85 MPa; the OCR = 2 condition was achieved under � ′

c = 4 MPand the OCR = 12 condition was achieved under � ′

c = 4.2 MPa and MPa, respectively.

The results of Fig. 3 confirm the four major trends of the thermalolume change behaviour of saturated clays:

1) The thermal contraction of a normally consolidated sample is

independent of the mean effective stress (see tests at OCR = 1under 1.2 MPa and 3.85 MPa, and OCR = 12 under 4.2 MPa and6 MPa). This trend was initially demonstrated by Demars andCharles (1982).

FC

ig. 3. Thermal volumetric changes of Boom Clay samples at different OCR values.

2) The thermal contraction increases when OCR is decreased, lead-ing to pure contraction at OCR = 1, in accordance with theresults of Plum and Esrig (1969), Demars and Charles (1982)and Baldi et al. (1988).

3) The slope of the volumetric strain in the cooling stage is inde-pendent of the applied mean effective stress.

4) The temperature at which the transition between thermalexpansion and contraction occurs decreases with OCR (T = 80 ◦Cat OCR = 12, T = 50 ◦C at OCR = 2), in accordance with Baldi et al.(1988) and Towhata et al. (1993).

It is interesting to note that the slope of cooling is parallel tohe slope of heating dilation. As physically the cooling process cor-esponds to a pure thermal contraction of the constituents of soilsolid and water), it can be considered as elastic. Thereby, the ther-

al contraction corresponds to a plastic process. Based on thisonsideration, Cui et al. (2000) developed a constitutive modelllowing describing the thermo-mechanical behaviour of saturatedlays.

. Time dependent behaviour of Boom Clay

The time dependent behaviour of saturated BC was investigatedn a triaxial cell under different isotropic stresses and at differ-nt temperatures (Cui et al., 2009). Fig. 4 presents the variationf consolidation rate from tests 7, 11, 13 and 15 as a function ofemperature. A clear increase in consolidation rate with increasingemperature is observed. Furthermore, the relationship betweenhe consolidation rate and time can be described by an exponential

ig. 4. Consolidation rate versus temperature for different heating tests on Boomlay.

172 Y.-J. Cui, A.M. Tang / Journal of Rock Mechanics and Geotechnical Engineering 5 (2013) 169–178

Fig. 5. Consolidation rate versus temperature for all tests performed on Boom Clay.

Table 4Pore-water salinity of Boom Clay.

Salt Ess83 (10−3 mol/L) Ess96 (10−3 mol/L) Mol (10−3 mol/L)

NaHCO3 7.16 7.16 13.93Na2SO4 5.23 5.81 0.002KCl 0.61 0.61 0.34CaCl2·2H2O 0.52 0.57MgCl2·6H2O 1.44 1.60 0.11NaCl 54.7 61.6 0.17H3BO3 0.70NaF 0.26CaCO3 0.05

+

aimoi

6

Eeaastwt0lt

EwioieE(

E

wc

lcUcg

tsnn

eresults obtained with different water (synthetic water or distilledwater) and by different methods (the direct method based on thewater volume injected by the controller of pressure/volume (CPV)and the indirect Casagrande’s method) are gathered in a same figure

Na 72.32 80.38 14.104

Total salinity (g/L) 5.037 5.578 1.286

Fig. 5 presents the variation of consolidation rate versus temper-ture for all the tests performed. The significant scatter observeds related to the influence of stress level, void ratio, thermal or

echanical loading rate, loading mode and loading interval. In spitef this, a clear trend showing an increase in consolidation rate withncreasing temperature can be observed.

. Effect of chemistry

Table 4 shows the chemical composition of pore-water of coresss83 and Ess96 (De Craen et al., 2006) and that of BC at Mol (Cuit al., 2009). The results show that the pore-water at Essen has NaCls main salt, while the pore-water taken from Mol has NaHCO3s main salt. Comparison between the values for Ess83 and Ess96hows that there is little difference between the two cores. Theotal salinity of the pore-water at Essen site is 5.037–5.578 g/L,hile that at Mol site is only 1.286 g/L. The results also show

hat the main cation at both sites is Na+, with a concentration of.072–0.080 mol/L at Essen and 0.014 mol/L at Mol, i.e. 5–6 times

ower. This difference in salinity shows the need of investigatinghe effect of salinity on the hydro-mechanical behaviour of BC.

Oedometer tests were carried out for this purpose on coresss83 and Ess96 using two different pore-waters: syntheticater in tests Ess83Odo3 and Ess96Odo04; distilled water

n tests Ess83Odo4 and Ess96Odo05. The compression curvesbtained after saturating the soil samples with the correspond-ng pore-water were used to determine different parameters forach loading/unloading stage (see for instance Fig. 6a for testss83Odo3) such as the compression index (C∗

c ), the swelling index

C∗

s ) and the oedometer modulus EOdo that is defined as

Odo = hi

(d� ′

vdh

)(1)

Fig. 6. Determination of parameters from the test Ess83Odo3.

here hi is the initial height of the soil sample and dh is the heighthange induced by a vertical stress increment d� ′

v.In Fig. 6b, the void ratio change versus time is plotted for a

oading step. This consolidation curve allows determination of theonsolidation coefficient Cv following the Casagrande’s method.sing the expression k = Cv�wg/EOdo, the hydraulic conductivity kan be calculated with �w as the density of water and g as theravitational acceleration.

Moreover, the curve shown in Fig. 6b can be used to determinehe secondary deformation coefficient C˛ that corresponds to thelope of the line after the part of primary deformation. This defi-ition implies that C˛ is positive for loading stages as II→III, andegative for unloading stages as I→II and III→IV.

Figs. 7 and 8 show the hydraulic conductivity k versus void ratio for the tests on Ess83 and Ess96, respectively. For each core, the

Fig. 7. Hydraulic conductivity versus void ratio for Ess83.

Y.-J. Cui, A.M. Tang / Journal of Rock Mechanics and Geotechnical Engineering 5 (2013) 169–178 173

fmtstltatcl

rEEEdbp

FtdwtE

Fig. 10. Oedometer modulus versus void ratio for Ess96.

rtdthetic water, especially for the unloading paths (I→II and III→IV).By contrast, for Ess96, this phenomenon is not obvious and thepoints corresponding to two water chemistries almost overlap.

Fig. 8. Hydraulic conductivity versus void ratio for Ess96.

or comparison. For Ess83, it can be observed that the measure-ents by the two methods gave similar results. On the other hand,

he water chemistry did not affect the hydraulic conductivity sinceimilar values were obtained with two different pore-water salini-ies. On the whole, a linear k–e relationship is observed. For Ess96, aittle difference can be identified between test Ess96Odo4 with syn-hetic water and test Ess96Odo5 with distilled water. In addition,

relatively higher value was also obtained in Ess96Odo5 based onhe CPV measurements. These differences are not significant andan be included in the range of experimental errors. As for Ess96, ainear k–e relationship was also obtained.

Figs. 9 and 10 present oedometer modulus EOdo versus voidatio e during both loading and unloading paths for Ess83 andss96, respectively. For Ess83, it is observed that the points of testss83Odo4 with distilled water lie slightly below the points of testss83Odo3 with synthetic water, suggesting a weakening effect ofistilled water. However, for Ess96, a good agreement is observedetween the points of test Ess96Odo5 with distilled water and theoints of test Ess96Odo4 with synthetic water.

The results of consolidation coefficient Cv are shown inigs. 11 and 12 for Ess83 and Ess96, respectively. As for the oedome-er modulus EOdo, the values of Cv from test Ess83Odo4 withistilled water are slightly lower than that from test Ess83Odo3

ith synthetic water; by contrast, test Ess96Odo5 with dis-

illed water gave quite consistent results when compared to testss96Odo4 with synthetic water.

Fig. 9. Oedometer modulus versus void ratio for Ess83.

Fig. 11. Consolidation coefficient versus void ratio for Ess83.

Changes in the secondary deformation coefficient C˛ with voidatio e are depicted in Figs. 13 and 14 for Ess83 and Ess96, respec-ively. For Ess83, the absolute values of C˛ from test Ess83Odo4 withistilled water are larger than those from test Ess83Odo3 with syn-

Fig. 12. Consolidation coefficient versus void ratio for Ess96.

174 Y.-J. Cui, A.M. Tang / Journal of Rock Mechanics and Geotechnical Engineering 5 (2013) 169–178

Etosst

7

wt2ptettaust

asccc

20

30

40

50

60

70

80

−1.0 −0.5 0.0 0.5 1.0

Volumetric stra in (%)

Tem

per

atu

re (

°C)

T2 (s = 39 MPa)

T3 (s = 9 MPa)

Dil ation Contraction

HeatingCooling

Heati ng

Heating

T1 (s = 110 MPa)

Fig. 15. Volumetric strain during thermal loading under constant pressure at0.1 MPa of tests T1, T2 and T3.

20

30

40

50

60

70

80

90

−1.0 −0.5 0.0 0.5 1.0 1.5

Volumetric stra in (%)

Tem

per

atu

re(°

C)

T5 (s = 39 MPa) T4 (s = 110 MPa)

Dil ati on Contractio n

Heati ng

Coolin g Coolin g

Heati ng

Fo

A(tictw

c

Fig. 13. Secondary consolidation coefficient versus void ratio for Ess83.

Examination of the mineralogical composition of Ess83 andss96 showed that the smectite content is likely the main factoro be considered in investigating the pore-water chemistry effectsn the hydro-mechanical behaviour of BC. As Ess83 contains moremectite (10% more, see Table 2), it appears normal that the Ess83hows more effects of pore-water chemistry. But on the whole,hese effects are quite limited.

. Volume change behaviour of bentonite-based materials

The volume change behaviour of compacted MX80 bentoniteas investigated using an isotropic cell that enables suction and

emperature to be controlled (Tang et al., 2007; Tang and Cui,010a). The results of thermal volume change under a pressure

= 0.1 MPa are presented in Fig. 15. The results from tests T1 (suc-ion s = 110 MPa) and T2 (s = 39 MPa) show that heating induced anxpansion. Considering that this expansion is linear, a coefficient ofhermal expansion ̨ = 2 × 10−4 ◦C−1 can be deduced. On the con-rary, the result from test T3 (s = 9 MPa) shows that heating induced

contraction. In addition, the subsequent cooling–reheating cyclendertaken shows a reversible behaviour. Note that significant datacatter was observed in this test and it is thus difficult to quantifyhe volume change behaviour during the cooling–reheating stage.

Fig. 16 presents the results obtained from tests T4 (s = 110 MPa)nd T5 (s = 39 MPa) at p = 5 MPa. It can be observed that, at highuction, heating from T = 25 ◦C to 80 ◦C induced an expansion and

ooling from T = 80 ◦C to 25 ◦C induced a contraction. The volumehange during this thermal cycle is approximately reversible; aoefficient of thermal expansion ̨ = 2 × 10−4 ◦C−1 can be estimated.

Fig. 14. Secondary consolidation coefficient versus void ratio for Ess96.

mtti

F

ig. 16. Volumetric strain during thermal loading under constant pressure at 5 MPaf tests T4 and T5.

t a suction of 39 MPa (test T5), as opposed to the case in test T2s = 39 MPa, p = 0.1 MPa), heating from T = 25 ◦C to 80 ◦C resulted in ahermal contraction and cooling from T = 80 ◦C to 25 ◦C also resultedn a contraction. The volumetric strain and the temperature duringooling from T = 55 ◦C to 25 ◦C can be correlated with a linear func-ion, defining a coefficient of thermal expansion ̨ = 2 × 10−4 ◦C−1

hich is similar to that deduced from tests T1, T2 and T4.Based on the compression curves in the e–ln p plane, the plastic

ompression coefficient �(s) and elastic coefficient � were deter-

ined. It was observed that these parameters are independent of

emperature but strongly dependent on suction (see Fig. 17 for theests at 25 ◦C). It appears that wetting increases the two compress-bility parameters.

ig. 17. Compressibility parameters (� and �(s)) versus suction for the tests at 25 ◦C.

Y.-J. Cui, A.M. Tang / Journal of Rock Mechanics and Geotechnical Engineering 5 (2013) 169–178 175

0

10

20

30

40

50

60

70

80

90

0.1 1 10 100

Tem

per

ature

(°C

)s = 110 MPa

s = 39 MPa

s = 9 MPa

fp

8

anittbpprubo

ksctaib

0.2 0

0.22

0.24

0.26

0.28

0.3 0

0 20 40 60 80

Mac

rosc

opic

void

rat

io

II I

rm

ttccd

dfSv(ipom1tws

c

Yield pressure (M Pa)

Fig. 18. Yield pressure (p0) versus temperature for different suctions.

The values of yield pressure (p0) determined versus temperatureor different suctions are shown in Fig. 18. It can be observed that0 decreases upon heating and wetting.

. Hydraulic behaviour

The hydraulic conductivity of compacted bentonite Kunigel V1t different initial water contents was determined by the simulta-eous profile method (Cui et al., 2008). The results are presented

n Fig. 19. It can be observed that unsaturated hydraulic conduc-ivity kunsat determined by Loiseau et al. (2002) is decreasing overime. The decrease of kunsat during the tests was equally observedy Haug and Wong (1992) and Hoffmann et al. (2007) on com-acted expansive soils. Lloret and Villar (2007) mentioned that theermeability of compacted bentonite measured under water satu-ated conditions is much lower than that measured by gas undernsaturated or dry conditions. This phenomenon can be explainedy the decrease of the volume of macro-pores during wetting asbserved by Cui et al. (2002).

From the analysis mentioned above, it can be concluded thatini corresponds to the initial microstructure of the compactedand/bentonite mixture. Note that the plot of test T04 in Fig. 19orresponds to the k-s curve of a soil sample having a microstruc-

ure similar to the initial microstructure of a compacted soil samplet e = 0.34. Moreover, the soils tested by Loiseau et al. (2002) andn test T04 have similar initial state (e = 0.34, wi = 8.0 ± 0.3%). It cane observed that the k–s curve determined from test T04 trends to

1E-14

1E-13

1E-12

0 10 20 30 40 50 60

s (MPa)

k(m

/s)

T01

T02

T03

T04 (e = 0.34)

Loiseau et al. (2002 )

k ini

k fin al

Fig. 19. Hydraulic conductivity versus suction.

MtmptsIpes

9

wibubbotu

Suction (MPa)

Fig. 20. Changes of macroscopic void ratio with suction.

each kini when suction is decreased to zero. This confirms that theicrostructure of the soil tested in the two tests is similar.On the other hand, kfinal corresponds to the final microstruc-

ure of the soil after wetting under constant-volume conditions;he macro-pore family was almost eliminated. Interestingly, the k-surves obtained from the three tests performed at constant-volumeonditions (T01, T02 and T03) trend to join kfinal when suction isecreased to zero, whatever the water content is.

Upon wetting, the suction is decreasing and hydraulic con-uctivity kunsat is in general increasing because water retentionorce is reduced (Daniel, 1982; Benson and Gribb, 1997; Chiu andhackelford, 1998). On the other hand, wetting under constant-olume conditions reduces the volume of the macro-pores familyCui et al., 2002), giving rise to a decrease of kunsat. These two oppos-ng mechanisms explain the results on constant-volume conditionsresented in Fig. 19: during wetting, k decreases initially becausef the reduction of the volume of macro-pores. As the volume ofacro-pores is dependent on the initial water content (Delage et al.,

996), the decrease of k with suction is different for different ini-ial water contents. Thereby, test T03 shows a constant decreasehereas tests T01 and T02 show that k started to increase from

= 22 MPa.In order to verify the wetting effect on the macro-pores, mer-

ury intrusion porosimetry (MIP) was conducted on the compactedX80/sand mixture wetted at different suctions. The pores larger

han 2 �m are defined as macro-pores. Fig. 20 shows that theacroscopic void ratio (emacro) defined by the volume of macro-

ores changes with decreasing suction. It can be observed thathe macro-pores quantity is progressively reduced with decreasinguction in zone I and then increased in zone II. The increase in zoneI can be explained by the creation of the 2D (two-dimensional)ores due to aggregating fissuring (Wang, 2012). These changes ofmacro upon wetting explain the decrease of hydraulic conductivityhown in Fig. 19.

. Technological void effect

As engineered barriers are often made up of compacted bricks,hen the bricks are placed around waste canisters or to form seal-

ng buffers, the so-called technological voids either between thericks themselves or between bricks, canisters and the host rock arenavoidable. As an example, 10 mm thick gaps between bentonitelocks and canister and 25 mm thick gaps between the bentonite

locks and the host rock have been considered in the basic designf Finland (Juvankoski, 2010). These technological voids appearedo be equal to 6.6% of the volume of the gallery in the FEBEX mock-p test (Martin et al., 2006). Fractures that appear in the excavation

176 Y.-J. Cui, A.M. Tang / Journal of Rock Mechanics and Geotechnical Engineering 5 (2013) 169–178

1E-15

1E-14

1E-13

1E-12

1E-11

1E-10

1.4 1.6 1.8 2.0

Dry density of mixture (Mg/m )

Hydra

uli

c co

nduct

ivit

y (

m/s

)

With technological void (Casagrande method)

With technological void (Constant head test-inflow)

Constant volume (Constant head test-inflow, Gatabin et al. (2008))

dattrmvo

9

edtctwd61

stosdadtthdtt

9

pt

0.01

0.1

1

10

100

0 1 2 3 4

Ver

tica

l st

ress

(M

Pa)

Mixture 70/30 (Present work)

Mixture 70/30 (Karnland et al., 2008)

Pure bentonite (Karnland et al., 2008)

Pure bentonite (Dixon et al., 1996)

Pure bentonite (Komine et al., 2009)

Pure bentonite (Börgesson et al., 1996)

F

tapv(r(

e

e

e

wcu

e

�

wtw1micmLc

tFpdceD

Constant volume (Constant head test-outflow, Gatabin et al. (2008))

Fig. 21. Hydraulic conductivity versus dry density of mixture.

amaged zone within the host rock in the near field constitutedditional voids. In the French concept, the volume of the ben-onite/rock gaps is estimated at 9% of the volume of the gallery byhe French waste management agency (ANDRA, 2005). This valueeaches 14% in the SEALEX in situ test carried out in the Tourne-ire URL (Barnichon and Deleruyelle, 2009). This technological

oid can significantly influence the hydro-mechanical behaviourf bentonite-based material and needs to be investigated in depth.

.1. Effect of technological void on hydraulic conductivity

Compacted MX80/sand mixture was considered to study theffect of technological void on hydraulic conductivity. With a ringiameter of 38 mm, the annular technological void selected (14% ofhe total cell volume representing 17% of the initial sample volume)orresponds to a sample diameter of 35.13 mm. Four tests withhe same technological void of 14% were conducted on samplesith the same initial water content of 11% and various initial dryensities obtained by changing the compaction pressure (between5 MPa and 85 MPa, giving rise to dry densities comprised between.93 Mg/m3 and 1.98 Mg/m3).

The saturated hydraulic conductivity was determined afteraturation of the samples, by both the constant head test andhe indirect Casagrande’s method. Fig. 21 compares the resultsbtained with that obtained by Gatabin et al. (2008) by con-tant head test on homogeneous samples at similar densities. Theata obtained for the heterogeneous samples with both methodsre in good agreement. On the whole, the hydraulic conductivityecreases with density increase, following a slope comparable tohat obtained by Gatabin et al. (2008). Further examination showshat the samples with initial technological voids exhibit higherydraulic conductivity than that by Gatabin et al. (2008), with aifference of one order of magnitude. This difference is suspectedo be due to a preferential water flow in the looser zone (initialechnological voids) around the samples.

.2. Effect of technological void on swelling pressure

In order to analyse the effect of technological void on swellingressure, various constitutive parameters of the compacted mix-ure are defined. It is supposed that the volume of bentonite (Vb) in

tsdt

Bentonite void ratio

ig. 22. Relationship between vertical stress and bentonite (MX80) void ratio.

he mixture is equal to the difference between the total volume (V)nd the volume of sand (Vs). Vb is equal to the sum of the bentonitearticle volume (Vbs) and the volume of void, namely intra-voidolume (Vi). The bentonite void ratio (eb) consists of two parts (Eq.2)), the intra-bentonite void ratio inside the soil (ebi) and the voidatio corresponding to the technological void (etech). Eqs. (3) and4) define these two voids, respectively.

b = ebi + etech (2)

bi = Vi

Vbs(3)

tech = Vtech

Vbs(4)

here Vtech is the volume of technological voids. The value of ebian be deduced from the initial dry unit mass of the mixture (�dm)sing the following equations:

bi = Gsb�w

�db− 1 (5)

db = (B/100)�mGss�w

Gss�w(1 + wm/100) − �m(1 − B/100)(6)

here �w is the water unit mass; Gsb is the specific gravity of ben-onite; �db is the initial dry unit mass of bentonite in the mixture,hich was calculated using Eq. (6) (Dixon et al., 1985; Lee et al.,

999; Agus and Schanz, 2008; Wang et al., 2012); �m is the unitass of the mixture; B (%) is the bentonite content (in dry mass)

n the mixture; Gss is the specific gravity of sand; wm is the waterontent of the mixture. In this study, the decrease of water unitass (�w) during hydration (e.g. Skipper et al., 1991; Villar and

loret, 2004) was not considered and the value was assumed to beonstant (1.0 Mg/m3), B = 70%, Gss = 2.65.

The values of vertical stress measured at the end of the hydra-ion tests on samples with technological voids are presented inig. 22 with respect to the bentonite void ratio. The data of swellingressure measured in homogeneous samples under the same con-itions of constant volume by other authors are also plotted foromparison (MX80 70/30 bentonite/sand mixture from Karnlandt al. (2008), and pure MX80 bentonite from Börgesson et al. (1996),ixon et al. (1996), Karnland et al. (2008), and Komine et al. (2009)).

All data remarkably agree, giving a unique relationship between

he vertical pressure and the bentonite void ratio, regardless of theample nature (homogeneous or not). The correspondence withata from Karnland et al. (2008) (pure bentonite and 70/30 ben-onite sand mixture) at bentonite void ratio close to 1 is particularly

cs and

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ont

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becg

R

A

A

A

A

B

B

B

B

B

B

B

C

C

C

C

C

C

C

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

F

G

H

H

H

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K

K

Y.-J. Cui, A.M. Tang / Journal of Rock Mechani

ood. This confirms that the stress at equilibrium is not affected byhe heterogeneity of the samples. The swelling pressure dependsnly on the global bentonite void ratio (eb), regardless of the tech-ological void and the presence of sand.

0. Conclusions

An overview of the recent findings about the CTHM behaviourf materials used for both geological and engineered barriers inuclear waste disposal is presented, through some examples abouthe natural BC and the compacted bentonite-based materials.

For the natural BC, it was found that: (1) compression indexdentified from both oedometer and isotropic compression tests isimilar and the compressibility of BC from the Mol site is higherhan that of BC from the Essen site; (2) the shear strength of MolC is also higher than that of the Essen BC, suggesting a signifi-ant effect of carbonates content; (3) the thermal volume changes strongly OCR dependent: low OCR values promote thermal con-raction while high OCR values favour thermal dilation. The volumehange behaviour is also strongly time dependent and this timeependent behaviour is governed by the stress level and tem-erature. The effect of pore-water salinity on the volume changeehaviour can be significant when the smectite content is relativelyigh. However, the pore-water chemistry does not seem to affecthe hydraulic conductivity.

For the bentonite-based materials, it was found that thermalontraction occurs also at low OCR values, but this is suctionependent—suction promotes thermal dilation. Under constantolume conditions, wetting results in a decrease of hydraulic con-uctivity followed by an increase. This is found to be related tohanges in macro-pores size—wetting induces a decrease of macro-ores size followed by an increase due to the aggregates fissuring.

The presence of technological voids can form a loose zone withenonite gel after hydration. This zone corresponds to the pref-rential path for water flow, with a relatively higher hydrauliconductivity. However, the swelling pressure depends only on thelobal bentonite void ratio, regardless of the technological void.

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Yu-Jun Cui is working as a professor at Ecole desPonts ParisTech (ENPC), France. His research interests