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Nuclear Engineering and Design 265 (2013) 330– 340

Contents lists available at ScienceDirect

Nuclear Engineering and Design

j ourna l h om epa ge: www.elsev ier .com/ locate /nucengdes

ydro-mechanical characterization of Barmer 1 bentonite fromajasthan, India

udhakar M. Rao ∗, Ravi K.epartment of Civil Engineering, Indian Institute of Science, Bangalore 560012, India

i g h l i g h t s

Hydro-mechanical characteristics of bentonite buffer from India are presented.Bentonite from Barmer district in Rajasthan state is the identified buffer.Properties of Barmer clay and bentonite buffers from literature are compared.Equations to design hydro-mechanical properties of bentonite buffer are presented.

r t i c l e i n f o

rticle history:eceived 5 June 2012eceived in revised form 2 September 2013ccepted 3 September 2013

a b s t r a c t

The conceptual model for deep geological disposal of high level nuclear waste (HLW) is based on multiplebarrier system consisting of natural and engineered barriers. Buffer/backfill material is regarded as themost important engineered barrier in HLW repositories. Due to large swelling ability, cation adsorptioncapacity, and low permeability bentonite is considered as suitable buffer material in HLW repositories.Japan has identified Kunigel VI bentonite, South Korea – Kyungju bentonite, China – GMZ bentonite,Belgium – FoCa clay, Sweden – MX-80 bentonite, Spain – FEBEX bentonite and Canada – Avonseal ben-tonite as candidate bentonite buffer for deep geological repository program. An earlier study on Indianbentonites by one of the authors suggested that bentonite from Barmer district of Rajasthan (termedBarmer 1 bentonite), India is suited for use as buffer material in deep geological repositories. However,

the hydro-mechanical properties of the Barmer 1 bentonite are unavailable. This paper characterizesBarmer 1 bentonite for hydro-mechanical properties, such as, swell pressure, saturated permeability,soil water characteristic curve (SWCC) and unconfined compression strength at different dry densities.The properties of Barmer 1 bentonite were compared with bentonite buffers reported in literature andequations for designing swell pressure and saturated permeability coefficient of bentonite buffers werearrived at.

. Introduction

Construction of deep geological repositories for safe disposalf high level nuclear waste is a widely accepted concept. Theunction of deep geological repositories is based on the multiplearrier concept and consists of natural barrier and the engineeredarrier system (EBS) which isolates the potentially harmful radioac-ive waste from the biosphere (Dixon et al., 1985; Bucher andonmoos, 1989; Cho et al., 1999; SKB, 2006). The natural barrier

s the host geological stratum (hard crystalline rock/clay/salt) andts surroundings while the EBS consists of the canister buffer andack fill. The system of successive superimposed barriers prevents

∗ Corresponding author. Tel.: +91 8022932812; fax: +91 8023600404.E-mail addresses: msrao@civil.iisc.ernet.in (S.M. Rao),

avik@civil.iisc.ernet.in (R. K.).

029-5493/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.nucengdes.2013.09.012

© 2013 Elsevier B.V. All rights reserved.

circulation of radionuclides to the environment and thereby pro-tects the biosphere from detrimental effects of radiation. Thebuffer is expected to perform two important functions (1) pre-vent advective transport of groundwater to the waste canisterand (2) retain radionuclides escaping from the breached canis-ter (Borgesson et al., 1988; OECD, 2003; Posiva, 2006; Lee et al.,2012).

Densely compacted bentonite/bentonite-sand mix is identifiedas buffer material owing to its favorable physico-chemical andhydro-mechanical properties (Yong et al., 1986; Guven, 1990;Pusch, 1992; Komine and Ogata, 1996; Lloret et al., 2003; Garciaet al., 2006; Garcia, 2007). Review of literature brings out thatJapan has identified Kunigel VI bentonite, South Korea – Kyungju

bentonite, China – GMZ bentonite, Belgium – FoCa clay, Sweden– MX-80 bentonite, Spain – FEBEX bentonite and Canada – Avon-seal bentonite as candidate bentonite buffer for deep geologicalrepository program (Cho et al., 1999; JNC, 2000; Villar and Lloret,

ring an

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S.M. Rao, R. K. / Nuclear Enginee

004; OECD, 2003; Zhijian, 2006). Rao et al. (2008) have recentlyxamined the physico-chemical and engineering properties of ben-onites from Gujarat and Rajasthan states of India as potential buffer

aterials for deep geological repositories and identified bentoniterom Barmer district of Rajasthan (termed Barmer 1 bentonite) asuitable buffer material. This paper presents comprehensive char-cterization of physico-chemical and hydro-mechanical propertiesf Barmer 1 bentonite and compares the properties of Barmer 1entonite with bentonite buffers reported in literature.

. Experimental program

.1. Materials

Bentonite from Barmer district, Rajasthan, India (termed Barmer bentonite) was used in the experimental program. The ben-onite was pulverized in mineral-crushing unit to pass 425 �mieve prior to use in laboratory investigations. The hygroscopicater content of the pulverized Barmer 1 bentonite corresponds

o 12%. Specific gravity, particle size distribution, total surface area,ation exchange capacity, methylene blue capacity and SWCC testsere performed with oven-dried bentonite samples. Oven-drying

f bentonite was performed by drying the specimens at 105 ◦Cor 24 h. Atterberg limits, swell pressure and unconfined compres-ion strength tests were performed without subjecting the sampleso drying procedure. Three broad types of tests are performed inhis study: index property tests (specific gravity, Atterberg lim-ts, grain size distribution), physico-chemical tests (CEC, surfacerea, smectite content) and hydro-mechanical tests (swell pressure,aturated permeability, SWCC, unconfined compressive strength).hree trials were performed for each test categorizing under index,hysico-chemical and hydro-mechanical properties. The averagef three trials with percent variation from average is reported forach test in the manuscript.

.2. Determination of index properties

Specific gravity, particle size distribution and Atterberg limitsf Barmer 1 bentonite were determined as per Indian Standard (IS)rocedures IS 2720 (Part 3, 1980), IS 2720 (Part 4, 1985) and IS 2720Part 5, 1985) respectively. All tests were performed in triplicatend the average of three measurements is reported in the study.pecific gravity of bentonite was determined by testing 5–10 g ofven-dried samples in 50 mL density bottles. Particle size test waserformed with 200 g of oven-dried sample. The bentonite sampleas washed through IS 75 �m sieve until the washings were free of

urbidity. The fraction retained on the 75 �m sieve was oven-driednd weighed to calculate the sand-sized fraction (4.75–0.075 mm)resent in the sample. The bentonite-suspension passing 75 �mieve was oven dried and hand-crushed to break clods. 50 g ofrushed sample was transferred to dispersion cup of mechani-al mixer containing 100 mL of dispersing solution (prepared byissolving 33 g of sodium hexametaphosphate + 7 g of sodium car-onate in 1000 mL of distilled water). An additional 150 mL ofistilled water was added to the dispersion cup and the suspen-ion was stirred for 15 min. The suspension was left undisturbedor 24 h. After 24 h the suspension was transferred from the disper-ion cup to 1000 mL measuring jar. 500 mL of distilled water wassed to wash all traces of suspension adhering to the dispersionup. The suspension was exactly made up to 1000 mL with dis-illed water and subjected to vigorous shaking before proceeding

or sedimentation analysis.

150 g of bentonite sample passing IS 425 �m sieve was used intterberg limits test. The liquid limit of bentonite was determinedsing cone penetration method (mass of cone and plunger = 80 g)

d Design 265 (2013) 330– 340 331

while the plastic limit was determined by surface rolling method(thread diameter = 3 mm). The bentonite sample was mixed withdistilled water to form homogenous paste that was stored for 24 hin a desiccator to attain moisture equilibration. After 24 h, thepaste was subjected to gradual air-drying with thorough hand-mixing to generate 5 points of the flow-curve. The last sample (leastwater content) on the flow-curve was used to determine plasticlimit.

2.3. Determination of physico-chemical properties

The total surface area of bentonite specimen (passing IS 425 �msieve) was determined by the Ethylene Glycol Monoethyl Ether(EGME) adsorption method (Cerato and Lutenegger, 2002). 1 gof oven-dried sample was exactly weighed (accuracy = 0.0001 g)in a stainless steel cup (cup volume = 25 cm3) and covered with3 mL of laboratory reagent grade EGME solution. The suspen-sion was gently stirred until uniform slurry resulted. The cupcontaining the slurry was transferred to vacuum desiccator andevacuated for 12 h. After 12 h, the cup was removed and weighedto obtain the mass of the bentonite–EGME mix. The evacuationstep was repeated at least 4 times till the mass of the bentonite–EGME mix remained constant (variation < 0.001 g). Triplicate mea-surements were performed with 1 g of oven-dried bentonitespecimens and the average of three measurements is reported. Theexternal surface area of oven-dried bentonite specimen (passingIS 425 �m sieve) was measured by the BET nitrogen adsorptionmethod (Brunauer et al., 1938) using Smart Sorb 92/93 supplied byM/s Smart Instruments Co. (P) Ltd.

The cation exchange capacity (CEC) of bentonite was deter-mined by extracting the exchangeable cations with neutralammonium acetate solution (Sumner and Miller, 1996). 5 g of oven-dried bentonite (passing IS 75 �m sieve) was weighed in 100 mLcentrifuge bottle. 25 mL of neutral 1 M ammonium acetate solu-tion was added to the bottle and the suspension was agitated for1 h using mechanical flask shaker. After agitation, the suspensionwas centrifuged at 30,000 rpm and clear supernatant was decantedinto 100 mL volumetric flask. The residue was re-suspended with25 mL of neutral 1 M ammonium acetate solution and the proce-dure repeated. The extraction procedure with the clay residue wasrepeated two more times. The supernatants from the 4 extractioncycles were combined and the volume made up to 100 mL with1 M ammonium acetate solution. The concentrations of displacedexchangeable calcium, magnesium, sodium and potassium ionsin the 100 mL solution were estimated using inductively coupledplasma optical emission spectrometer (ICP-OES, model: Thermo-ICAP 6500). The sum of the exchangeable calcium, magnesium,sodium and potassium ions gave the CEC of the bentonite specimen.

XRD pattern of bentonite powder was obtained after solvatingthe oven-dried sample with ethylene glycol (Greene-Kelly, 1952).JEOL X Ray Diffractometer (Model No. JDX 8030) was used to obtainthe XRD pattern (2� range, 3–60◦) and the minerals present wereanalyzed using Xpert high score software.

The methylene blue capacity of bentonite was determined as perASTM C837-09 (2009) method to estimate the smectite content inthe bentonite specimen (Pusch, 2002). 2 g of oven-dried bentonitesample passing IS 75 �m sieve was placed in 600 mL beaker. 300 mLof distilled water was added to the clay and the suspension wasacidified (pH: 2.5–3.8) using 0.1 N sulfuric acid solution. The acid-ified suspension was titrated against methylene blue solution (1 gof methylene blue dissolved in 250 mL of distilled water) to obtain

the end point by halo method. The smectite content is evaluated bycalculating the methylene blue capacity (Pusch, 2002). Three trialswere performed and the average value was used to estimate thesmectite content in the bentonite specimen.

332 S.M. Rao, R. K. / Nuclear Engineering and Design 265 (2013) 330– 340

Table 1aSpecimen details swell pressure/saturated permeability test specimens.

Dry density (Mg/m3) Bulk mass ofbentonite (g)

Specimenvolume (cm3)

Watercontent (%)

1.4 36 22.62 121.5 38 22.62 12

2

dBa2Tsagp

dsaidttrtnlps

ertdtaasta

Table 1bSpecimen details of SWCC test specimens.

Dry density(Mg/m3)

Watercontent (%)

Specimenvolume (cm3)

Bulk mass ofbentonite (g)

1.5 9

125.6

20515 21621 22726 238

1.75 6

125.6

23310 24214 25118 260

2.0 4 2617 268

1.75 45 22.62 122.0 51 22.62 12

.4. Swell pressure

The swell pressure of compacted bentonite specimens wasetermined by the constant volume method of Komine (2004).atches of bentonite (w = 12%) were hand-mixed to break clodsnd were compacted in oedometer rings (diameter 60 mm, height0 mm) to 8 mm thickness using compaction stress of 35 MPa.he dry densities of specimens were varied by maintaining con-tant compaction stress (35 MPa), sample dimension (22.62 cm3)nd varying the bulk mass. The bulk mass, specimen volume andravimetric water content of swell pressure test specimens areresented in Table 1a.

A 10 kN load cell (sensitivity ± 0.01 kN) and LVDT (maximumisplacement = 20 mm, sensitivity = ±0.01 mm) was used in thewell pressure experiments. A seating pressure of 100 kPa waspplied to the oedometer cell to ensure tight contact between load-ng pad and load cell. The load cell and LVDT measured the loadeveloped and any volume changes experienced by the oedome-er specimen upon inundation with distilled water as function ofime. Measurements were continued until marginal changes wereecorded by the load cell for over 48 h; the steady state was inferredo represent specimen saturation (Pusch, 1980). The LVDT recordedo volume change during the inundation process. The equilibrium

oad divided by the specimen cross sectional area gave the swellressure of the compacted specimen. Fig. 1 presents the schematicketch of the test set up.

Swell pressure measurements were performed in triplicate forach test condition and the average of three measurements iseported in the study. The thickness of each specimen belongingo the triplicate set was measured at 4 locations using sensitiveial gage (least count = 0.01 mm) after compaction in the oedome-er cell and before commencement of the swell pressure test. Theverage of 4 measurements gave the specimen thickness (% vari-tion from average = ±0.1–0.2%). Similar procedure measured the

aturated specimen thickness after completion of swell pressureest. The specimen volumes and dry density values before andfter the swell pressure test (Table 2) illustrate that the specimen

Fig. 1. Schematic representation of test setup for swell pressure measurement.

125.69 27412 281

volumes and dry densities remain constant after the swell pressuremeasurement.

2.5. Saturated permeability

After completion of swell pressure test, determination of thesaturated permeability coefficient commenced using the sameoedometer assembly. The specimen was permeated with distilledusing a stand-pipe under varying head of 650–630 mm. The initialand final heads and the time needed for drop in head were recordedand saturated permeability coefficient was then estimated usingequation (Holtz and Kovacs, 1981):

k = aL

Atln

(h1

h2

)(1)

where, a is the area of the buret, L is the length of the compactedspecimen, A is the cross sectional area of the specimen and t is timerequired for head drop from h1 to h2. Pusch (1980) adopted a similarprocedure to saturate the clay specimens and determine saturatedpermeability coefficients.

2.6. Soil water characteristic curve (SWCC)

Soil water characteristic curves of compacted Barmer 1 ben-tonite specimens were generated at dry densities of 1.5, 1.75 and2 Mg/m3. The total suction of compacted specimens at initial degreeof saturation of 30–90% was estimated from relative humidity(RH) measurements. The total suction of saturated specimens wasmeasured by ASTM non-contact filter paper technique (ASTM D5298-03, 2003). Oven-dried bentonite specimens were used in theRH experiments. Saturated specimens from permeability tests (Sec-tion 2.5) were used in non-contact filter paper method.

2.6.1. RH measurements200 g batches of oven dried bentonite (passing 425 �m sieve)

were mixed with pre-determined volumes of distilled water(8–52 mL) and equilibrated for 24 h in desiccator to facilitate uni-form moisture distribution. The gravimetric moisture contentsof the moist specimens ranged from 4 to 26%. After equilibra-tion, the moist specimens were statically compacted (compactionstress = 35 MPa, specimen thickness = 25 mm) in stainless steelrings (diameter 80 mm, height 30 mm) to desired dry density(1.5/1.75/2 Mg/m3) by varying the bulk mass of the specimens(Table 1b). The Sr values of the compacted specimens ranged from

30 to 90%. A commercial RH probe (Hygroclip CPXXPVC, RotronicAg, Switzerland) was used in the experiments. The probe hasdiameter of 15 mm and measures humidity (range: 0–100%, accu-racy ± 1.5%) and temperature (range: 233–353 K, accuracy ± 0.3 K).

S.M. Rao, R. K. / Nuclear Engineering an

tdthkirrl

vw

2

etsTrtt4

2

tToib

TS

Fig. 2. Photograph of the test setup used for suction measurement.

The bentonite specimen contained in the metallic ring wasightly enclosed at the base and top; a 15 mm (diameter) hole wasrilled at the top of the compacted specimen using hand augero insert the RH probe (Fig. 2); after insertion of the probe, theole was rendered air-tight with sealing tape. The specimens wereept in temperature-controlled room (20 ◦C). The relative humid-ty (and temperature) of the specimens was monitored until theelative humidity attained near constancy in about 10–12 h. Theelative humidity is expressed as total suction ( ) using Henry’saw (Fredlund and Rahardjo, 1993).

= − RT

�woω�ln(RH) (2)

In Eq. (2), R is universal gas constant, T is absolute temperature,wo is the specific volume of water and ωv is the molecular mass ofater vapour.

.6.2. ASTM filter paper methodAfter completion of the falling head tests, the specimens were

jected from the oedometer cells and transferred to air-tight con-ainers; Whatmann No. 42 filter paper (diameter = 60 mm) wasuspended approximately 5-mm above the saturated specimens.he specimen containers were kept in temperature-controlledoom (20 ◦C) for three weeks. After three weeks, the water con-ent of the filter paper was determined and was correlated withotal suction using the standard calibration curve for Whatmann2 filter paper (ASTM D5298-03).

.7. Unconfined compressive strength

Unconfined compressive strengths of compacted Barmer 1 ben-onite specimens were determined as per IS 2720 (Part 10, 1991).

he bulk mass, specimen volume and gravimetric water contentf unconfined compression strength test specimens are presentedn Table 1c. Batches of bentonite (w = 12%) were hand-mixed toreak clods and statically compacted (compaction stress = 35 MPa)

able 1cpecimen details of unconfined compression strength test specimens.

Dry density(Mg/m3)

Bulk mass ofbentonite (g)

Specimenvolume (cm3)

Watercontent (%)

1.4 136 86.19 121.5 145 86.19 121.75 169 86.19 122.0 193 86.19 12

d Design 265 (2013) 330– 340 333

in cylindrical molds to yield specimens (diameter 38 mm and length76 mm) of desired dry densities (Table 1c). Unconfined compres-sive strength of compacted bentonite specimens were measured atstrain rate of 0.12 mm/min in tri-axial compression machine.

3. Results and discussions

3.1. Index properties, physico-chemical and mineralogicalanalysis

The XRD pattern of ethylene glycol solvated bentonite speci-men (Fig. 3) exhibited c-axis spacing at 17.70 A (2� = 6.8◦), typicalof montmorillonite (Jasmund and Mering, 1979). Kaolinite andquartz occurred as accessory minerals in Barmer 1 bentonite (Fig. 3,Table 2).

Table 3 compares the physico-chemical and index properties ofBarmer 1 bentonite with bentonite-buffers reported in literature.The montmorillonite/smectite contents of various bentonite spec-imens ranged from 48 to 92% with average value of 73%. The CECvalues ranged from 58 to 119 mequiv./100 g with an average valueof 80 mequiv./100 g. The total surface area of bentonites rangedfrom 348 to 725 m2/g with an average value of 528 m2/g. The smec-tite content of Barmer 1 bentonite was 7% lower than the averagevalue; the CEC of Barmer 1 bentonite was 0.6% larger than theaverage value, while, its total surface area was 15% smaller thanthe average. The variations from the average of triplicate measure-ments of smectite content, CEC and surface area ranges from ±4 to8%, ±1 to 4% and ±1 to 5% respectively.

The plasticity index (Ip) of various bentonites ranged from 48 to383% with an average value of 232%. The Ip value of Barmer 1 ben-tonite was 58% larger than the average value. The variations fromthe average of triplicate measurements of liquid limit and plasticityindex ranges from ±0.1 to 0.6% and ±0.1 to 1% respectively.

Though some bentonites possessed similar clay contents(80–86%, FoCa and MX-80 bentonites), they exhibited large vari-ations in liquid limits (wl = 112% for FoCa and wl = 400% for MX-80).The variations in liquid limits suggested that besides amount ofclay, other physico-chemical factors could influence the Atterberglimits. To examine the influence of physico-chemical factors, liquidlimit of bentonites were correlated with their exchangeable sodiumpercentage (ESP); ESP is defined as (Mitchell and Soga, 2005):

ESP = [Na+]E

CEC× 100 (3)

where [Na+]E is the concentration of exchangeable sodium ion(mequiv./100 g) and CEC represents the cation exchange capacity(mequiv./100 g). Fig. 4 plots the variation of liquid limit with ESP forvarious bentonites. The liquid limit exponentially increased withESP (R2 = 0.77) and followed the equation:

wl = 89.04e0.02ESP (4)

3.2. Swell pressure

An average swell pressure of 0.6 MPa was developed by theBarmer 1 bentonite specimen on compaction to dry density of1.4 Mg/m3; the average swell pressure increased to 1.3, 2.7 and3.4 MPa with increase in dry densities to 1.5, 1.75 and 2.0 Mg/m3

respectively. The percent variation from average (of triplicate mea-surements) for given compacted specimen ranged between ±1 and7%.

The variation in the average swell pressure (SP) with dry density(�d) for Barmer clays followed the equation:

SP = 8 × ln(�d) − 1.97 (5)

334 S.M. Rao, R. K. / Nuclear Engineering and Design 265 (2013) 330– 340

0

200

400

600

800

1000

1200

1400

1600

1800

706050403020100

Inte

nsi

ty (

Co

un

ts)

2θ (Degree)

MK

M Q

Q

M-K Q

Q-K

Q Q-K

K

M- Montmorillonite

K - Kaol inite

Q - Quar tz

Fig. 3. X-ray diffraction pattern of ethylene glycol treated Barmer 1 bentonite.

Table 2Volume and dry density of triplicate specimens used for swell pressure and saturated permeability measurements.

Compaction series (Mg/m3) Specimen volume (cm3) Dry density (Mg/m3)

Compacted state Saturated state Compacted state Saturated state

1.4 20.74 20.63 1.38 1.3920.57 20.66 1.40 1.3920.57 20.68 1.37 1.38

1.5 20.72 20.89 1.49 1.4820.57 20.66 1.51 1.5220.25 20.36 1.48 1.47

1.75 20.18 20.36 1.77 1.7620.57 20.38 1.75 1.7520.25 20.67 1.76 1.76

2.0 20.14 20.03 1.97 1.9820.57 20.38 1.98 1.9920.25 20.39 1.97 1.97

Table 3Index, physico-chemical and mineralogical data of bentonites.

Bentonite Country Specificgravity

Liquid limit (%) Plasticityindex (%)

Clay (%) Smectite/montmorillonite(%)

CEC (mequiv./100 g) Total surfacearea (m2/g)

ESP (%)

Barmer 1 India 2.69 409 367 86 68 80 450 65.00Kunigel VI

(Komine, 2004)Japan 2.79 474 447 65 48 73 525 55.00

Kyungju (Cho et al.,1999)

Korea 2.74 245 198 40 70 58 348

GMZ (Zhijian,2006; Zhanget al., 2009, 2012)

China 2.70 228 196 74 75 77 570 56.09

FoCa clay (VanGeet et al., 2009;Cuii et al., 2011)

Belgium 2.68 112 62 – 80 71 426 5.07

MX-80a Sweden 2.71–2.76 400–520 330–478 84–85 65–92 65–85 562 82.11FEBEX (Villar and

Lloret, 2008)Spain 2.70 104 48 70 92 119 725 22.69

Avonseal (JNC,2000)

Canada 2.80 257 208 60 79 82 615 56.71

a Pusch (1980, 2001), Mata (2003), Herbert et al. (2008), Espina and Villar (2010), Wang et al. (2012).

S.M. Rao, R. K. / Nuclear Engineering and Design 265 (2013) 330– 340 335

R2 = 0.7 7

ESP-Exchangeable sodiu m percentage

0

100

200

300

400

500

0 20 40 60 80 100

odiu m

Liq

uid

lim

it (

%)

Barmer 1Kunigel VI_Komine (2004)GMZ_Zhijia n (20 06)FoCa_ van Geet et al. (2009)MX-80_Mata (2003)FEBEX_Villar and Lloret (2008)Avonseal_JNC (2000)

ge (ES

d

l

p(t

S

s(staclonp

Exch angeable s

Fig. 4. Variation of liquid limit with exchangeable sodium percenta

Comparatively, swell pressures of FEBEX bentonite varied withry density as (Villar and Lloret, 2004):

n(SP) = 6.77�d − 9.07 (6)

Zhijian (2006) had provided the swell pressure data of com-acted GMZ bentonite specimens at different dry densities1.2–1.8 Mg/m3); the data was analyzed and was observed to followhe equation:

P = 8 × 10−6e7.9�d (7)

Eqs. (5) (for Barmer 1) and (7) (for GMZ bentonite) predictedimilar swell pressures at dry densities of 1.4 and 1.5 Mg/m3; Eq.7) predicted 3.2 times larger swell pressure than Eq. (5) at dry den-ity of 1.75 Mg/m3. Eq. (6) (for FEBEX bentonite) predicted 2.1–6.4imes larger swell pressures than Eq. (5) at dry densities of 1.5nd 1.75 Mg/m3. The variability in swell pressure predictions atommon dry density arose from differences in suction, montmoril-

onite content, type of exchangeable cations, chemical compositionf pore water and compaction stress to name a few. It was henceecessary to identify parameters that could normalize variability inarameters influencing the swell pressures of bentonites sourced

0

2

4

6

8

10

12

1.3 1.4 1.5 1.6 1

Dry densit

Sw

ell

pre

ssu

e (M

Pa

)

Kunig el VI_ Komine an d Og ata (1994)

Kyungju_Lee et al. (2012)

GMZ_Zhijian (2006)

FoCa_Geneste et al. (1990)

MX-80_Karnlan d a nd Birger sson (2006)

FEBEX_Villa r and Llor et (2004 )

Barmer1

Barmer 1_Predict ed by Eq.8

FEB EX_Pre dicted by Eq.8

GMZ_Predi cted by Eq.8

Fig. 5. Variation of swell pressure with dry density: Barmer 1

percentage, ESP (% )

P): Barmer 1 bentonite and bentonite buffer reported in literature.

from different geographical locations. Identification of normalizingparameters is useful to develop predictive equations for designingbentonite buffer with required properties.

Fig. 5 examines the variation in swell pressure with dry densityfor the bentonite specimens. The swelling pressure data of FoCaclay in Fig. 5 is based on the equation provided by Geneste et al.(1990)

ln (SP) = −9.46�d

+ 6.55 (8)

In Eq. (8), SP represents swell pressure (MPa), and �d repre-sents the dry density of water saturated specimens (Mg/m3). FoCaclay is sedimentary clay from the Paris Basin, extracted in the Vexinregion (Fourges-Cahaignes, France) of France (Van Geet et al., 2009)and has been used as a part of engineered barrier system (EBS) byBelgium (OECD, 2003). France is using MX-80 in its buffer exper-iments (Espina and Villar, 2010). Variations of swelling pressures

with dry density of MX-80 reported by Karnland and Birgersson(2006) are included in Fig. 5; swell pressures of 1, 2, 8 and 10 MPaare developed at dry densities of 1.2, 1.45, 1.55 and 1.60 Mg/m3 bythe MX-80 specimens.

.7 1.8 1.9 2.0 2.1

y (Mg/m3)

bentonite and bentonite buffer reported in literature.

336 S.M. Rao, R. K. / Nuclear Engineering and Design 265 (2013) 330– 340

0

2

4

6

8

10

12

14

40 50 60 70 80 90 10 0

Montmoril lon ite content (%)

Sw

ell

pre

ssu

re (

MP

a)

1.6 Mg/m³

1.7 Mg/m³

1.9 Mg/m³

2 Mg/m³

F nite an1 EBEX

litbm1o(dv1isM

S

p

aps

Fb(L

ig. 6. Variation of swell pressure with montmorillonite content: Barmer 1 bento994), Kyungju (Lee et al., 2012), GMZ (Zhijian, 2006), FoCa (Geneste et al., 1990), F

Fig. 6 presents the variation in swell pressure with montmoril-onite/smectite content (denoted as M). The plots in Figs. 5 and 6llustrate that dry density and montmorillonite/smectite con-ent have strong influence on the swell pressure of compactedentonites. Thus to capture the influence of dry density andontmorillonite/smectite content, the swell pressure of Barmer

bentonite and other bentonite buffers is expressed as functionf M/e ratio, where M is the montmorillonite/smectite contentexpressed as decimal) and e is the void ratio. Increasing M andecreasing e is expected to facilitate increase in swell pressure. Theariations of M/e with swell pressure are plotted in Fig. 7 for Barmer, Kunigel VI, Kyungju, GMZ, FoCa and FEBEX bentonites. The data

n Fig. 7 is restricted to dry density values ≤2 Mg/m3. Interestinglywell pressures of different bentonites plot uniquely as function of/e ratio (R2 = 0.71) and follow the equation:

P = 0.4 × e1.3M/e (9)

Eq. (9) describes the best-fit for the experimental data of swellressure (SP) on the y-axis plotted against M/e ratio on the x-axis.

Swell pressures predictions for Barmer 1 (Eq. (5)), FEBEX (Eq. (6))nd GMZ (Eq. (7)) bentonites were compared with swell pressuresredicted by Eq. (8) for the three bentonites at different dry den-ities in Fig. 5. Eq. (8) appears to provide a reasonable estimate of

R2 = 0.71

0

4

8

12

16

20

0.5 1.0 1. 5 2.0 2. 5 3.0M/e

Sw

ell

pre

ssu

re (

MP

a)

Kuni gel VI

Kyungj u

GMZ

Barmer 1

FEBEX

FoC a

ig. 7. Variation of swell pressure with M/e: Barmer 1 bentonite and bentoniteuffer reported in literature (data for Kunigel VI (Komine and Ogata, 1994), KyungjuLee et al., 2012), GMZ (Zhijian, 2006), FoCa (Geneste et al., 1990), FEBEX (Villar andloret, 2004)).

d bentonite buffer reported in literature (data for Kunigel VI (Komine and Ogata,(Villar and Lloret, 2004)).

the experimental swell pressures of the three bentonites, except atdry density of 1.75 Mg/m3 for GMZ specimens.

3.3. Saturated permeability

The average saturated permeability (k) coefficient (from trip-licate measurements) of specimens compacted to 1.4 Mg/m3

corresponded to 1.5 × 10−14 m/s; the average k values reducedto 1.4 × 10−14, 1.3 × 10−14 and 1.0 × 10−14 m/s at dry densities1.5, 1.75 and 2.0 Mg/m3 respectively. The percentage variationfrom average (triplicate measurements) for given specimen rangedbetween ±1 and 10%. The permeability of Barmer 1 bentonite is notvery sensitive to increase in dry density possibly as the physico-chemical factor (diffuse double layer forces) predominate over thephysical factor (dry density) in controlling the k values.

The variations of saturated permeability coefficient (k) with drydensity (�d) for Barmer 1 bentonite specimens (Fig. 8) followed theequation:

k = 4 × 10−14e−0.69�d (10)

Fig. 8 also includes the k versus dry density plots of Kyungju,

GMZ, FoCa and FEBEX bentonite specimens and shows the variationin k values at common dry density. The saturated permeability (k)of Kyungju, GMZ, FoCa and FEBEX bentonites show marked reduc-tion in k with increase in dry density. Comparatively, the saturated

1.0E-15

1.0E-14

1.0E-13

1.0E-12

1.0E-11

1.2 1.4 1.6 1.8 2.0

Dry density (Mg/m3)

Sa

tura

ted

per

mea

bil

ity

(m

/s)

Kyungju_Cho e t al. (1999)GMZ_Zhijian (2006)

FoCa_Genes te et al. (1 990)FEBEX_ENRESA (2000)Barme r1

Fig. 8. Variation of permeability coefficient with dry density: Barmer 1 bentoniteand bentonite buffer reported in literature.

S.M. Rao, R. K. / Nuclear Engineering and Design 265 (2013) 330– 340 337

Table 4Experimental data of SWCC test specimens.

Degree of saturation, Sr (%) Water content (%) Total suction (MPa)

a �d = 1.5 Mg/m3 �d = 1.75 Mg/m3 �d = 2 Mg/m3 �d = 1.5 Mg/m3 �d = 1.75 Mg/m3 �d = 2.0 Mg/m3

30 9 6 4 84 128 22950 15 10 7 52 98 11170 21 14 9 32 54 74

12 20 26 57

k(t(

gft

k

3

mfrm

rwa

S

wptTomawid

Fr(

Table 5Brooks and Corey curve fitting parameters for compacted Barmer 1 bentonitespecimens.

Dry density (Mg/m3) 1.5 1.75 2

Curve fittingparameters

B 1.07 1.02 1.05D 0.02 0.01 0.01

90 26 18

a �d = dry density.

of Barmer 1 bentonite was not sensitive to dry density variationsFig. 8). Such a situation possibly arose as the physico-chemical fac-or (diffuse double layer forces) predominated over physical factordry density) in controlling the saturated k of Barmer 1 bentonite.

In order to normalize the k values of bentonites from differenteographical locations, Fig. 9 plots the variation in k with M/e ratioor Barmer 1, Kyungju, GMZ, FoCa and FEBEX bentonite specimens;he k values plotted uniquely (R2 = 0.8) and followed the equation:

= 3 × 10−11e−4.2M/e (11)

.4. Soil–water characteristic curve (SWCC)

Table 4 presents the experimental data for SWCC measure-ents. Total suction measurements were performed in triplicate

or each specimen and the average of three measurements iseported. The percentage variation from average for a given speci-en ranged between ±2 and 4%.The experimental (total suction)–Sr (degree of saturation,

ange: 30–100%) data of compacted Barmer 1 bentonite specimensas analyzed using the Brooks and Corey equation (Fig. 10) given

s (Sillers et al., 2001):

r = B − D m (12)

here, Sr is the degree of saturation, B and D are the curve fittingarameters related to soil capillarity, m is the curve fitting parame-er related to pore size distribution and represents total suction.he air entry value ( a) of compacted Barmer 1 specimens wasbtained from the Brooks and Corey plot (Fig. 10) by double tangentethod (Fredlund and Xing, 1994). The air entry value (Fredlund

nd Xing, 1994; Lu and Likos, 2004) represents the soil suction athich degree of saturation commences to reduce below 100% dur-

ng drying path. The a values ranged from 20 to 55 MPa at dryensities of 1.5–2 Mg/m3. The curve fitting parameters (B, D and

R2 = 0.8

1E-15

1E-14

1E-13

1E-12

1E-11

0.5 1 1.5 2 2.5M/e

Sa

tura

ted

per

mea

bil

ity

(m

/s)

ig. 9. Variation of saturated k with M/e: Barmer 1 bentonite and bentonite buffereported in literature (data for Kyungju (Cho et al., 1999), GMZ (Zhijian, 2006), FoCaGeneste et al., 1990), FEBEX (ENRESA, 2000)).

m 0.84 1.03 0.77Air entry value ( a, MPa) 20 37 55

m) and the air entry values from Brooks and Corey plot for com-pacted Barmer 1 bentonite are provided in Table 5. The Brooks andCorey equation was used to analyze the experimental data as it isextensively used to model the SWCC behavior of compacted clays(Sillers et al., 2001).

The –w (gravimetric water content) plots of Barmer 1 ben-tonite and other bentonite buffer reported in literature arepresented in Fig. 11. Researchers have used different techniques togenerate the w– relationships. ENRESA (2000) and Van Geet et al.(2009) adopted unconfined wetting of compacted FEBEX bentoniteand FoCa clay specimens to obtain SWCC plots. Chen et al. (2006),Johannesson et al. (1999) and Lee et al. (2011) used vapor equilib-rium/relative humidity methods to obtain SWCC of GMZ, MX-80and Kyungju bentonites respectively. Fig. 11 shows that despitevariations in bentonite type, dry density and measurement tech-nique, the SWCC data of different bentonites plot in single envelope.

3.5. Unconfined compressive strength

The unconfined compressive strengths of Barmer 1 bentonitespecimens increased from 1.7 to 3.2 MPa with increase in dry den-sity 1.4–2.0 Mg/m3. Triplicate measurements were performed witheach compacted specimen and the average of three measurementsis reported. The percentage variation from average for given spec-imen ranged between ±4 and 6%.

The secant modulus at peak stress (Es) of the compacted spec-imen ranged from 510 to 666 MPa and increased with dry density(Table 6). The variations in unconfined compressive strength withdry density of Barmer 1 bentonite and GMZ bentonite is com-pared in Fig. 12. The GMZ bentonite developed larger compressivestrength than Barmer 1 bentonite at similar dry densities. The mix-

ing water contents of the compacted GMZ and Barmer 1 bentonitespecimens were similar (12–13%). Apparently, the GMZ and Barmer1 specimens developed different compression strengths from

Table 6Unconfined compressive strength and secant modulus of compacted Barmer 1 ben-tonite specimens.

Dry density(Mg/m3)

Gravimetricwater content(%)

Unconfinedcompressivestrength (MPa)

Secant elasticmodulus Es

(MPa)

1.4 12 1.72 5101.5 12 2.08 5181.75 12 2.41 5752 12 3.16 666

338 S.M. Rao, R. K. / Nuclear Engineering and Design 265 (2013) 330– 340

0.0

0.2

0.4

0.6

0.8

1.0

0001001011

Total suctio n (M Pa)

Deg

ree

of

satu

rati

on

Brooks and Corey _1.5 Mg/m³Brooks and Corey_1.75 Mg/m³ Brooks and Corey_2 Mg/m³ Expt_1.5 Mg/m³Expt_1.75 Mg/m³ Expt_2 Mg/m³

Fig. 10. Experimental and predicted (Brooks and Corey model) SWCC plots for Barmer 1 bentonite specimens.

0.01

0.1

1

10

100

1000

0 10 20 30 40 50 60

Gravimteric water content (%)

To

tal

suct

ion

(M

Pa

)

Barmer1_1.75 Mg/m³

FEBE X_1 .75 Mg /m³

GMZ_1 .75 Mg/m³

MX-80_a ll densities

Kyungju_1. 5 Mg/m³

Kyungju_1. 6 Mg/m³

FoCa_1 .65 Mg /m³

Fig. 11. SWCC plots of Barmer 1 bentonite and bentonite buffers (Data for Kyungju (Lee et al., 2011), GMZ (Chen et al., 2006), FoCa (Van Geet et al., 2009), MX-80 (Johannessonet al., 1999), FEBEX (ENRESA, 2000)).

0.0

2.0

4.0

6.0

8.0

0.0 0. 5 1.0 1.5 2.0 2.5

Dry density (Mg/m3)

Un

con

fin

ed c

om

pre

ssiv

e st

ren

gth

(M

Pa

)

Barmer 1 (w=1 2 %)

GMZ (w=13 %)

Fig. 12. Variation of unconfined compressive strength with dry density of Barmer 1 and GMZ (Zhijian, 2006) bentonites.

ring an

do

asbTpciw

4

ftmat11siadcrpBtp1cw

tdcswrvr

vatif

A

Dt

R

A

A

B

B

S.M. Rao, R. K. / Nuclear Enginee

ifferences in preparation methods (example, compaction stress)f the specimens.

Eqs. (7) and (9) predict desirable combinations of dry densitynd smectite/montmorillonite content of bentonites that wouldatisfy the swell pressure and permeability requirements of bufferased on controlled laboratory data with homogenous specimens.he equations however do not consider factors of scale effect, sam-le homogeneity and compaction control representative of fieldonditions. Due to these limitations, the equations could serve asndicators for buffer identification that needs to be further validated

ith careful laboratory and field tests.

. Conclusions

The hydro-mechanical characterization of Barmer 1 bentoniterom Rajasthan, India was performed and evaluated in the con-ext of results reported for bentonite buffers in literature. The

ontmorillonite content of Barmer 1 bentonite was 7% lower thanverage value for bentonite buffers (73%); the CEC was 0.6% largerhan the average (80 mequiv./100 g) and total surface area was5% smaller than the average (528 m2/g). The Ip value of Barmer

bentonite was 58% larger than the average value (232%). Thewell pressures of compacted Barmer 1 bentonites were stronglynfluenced by compaction dry density and ranged from 0.6 MPat 1.4 Mg/m3 to 3.4 MPa at 2 Mg/m3. Similar variations in dryensity had marginal influence on saturated permeability coeffi-ient (k) of compacted Barmer 1 bentonite specimens and valuesanged between 1 and 1.5 × 10−14 m/s. The –Sr relations of com-acted Barmer 1 bentonite specimens were analyzed using therooks and Corey equation; the air-entry values ranged from 20o 55 MPa at dry densities of 1.5–2 Mg/m3. The unconfined com-ressive strengths ranged from 1.7 to 3.2 MPa at dry densities of.4–2.0 Mg/m3. The Es (secant modulus at peak stress) values of theompacted specimens ranged from 510 to 666 MPa and increasedith dry density.

The swell pressure equations of Barmer 1, GMZ and FEBEX ben-onites predicted swell pressures ranging from 0.5 to 16 MPa at dryensities of 1.4–1.75 Mg/m3. The ratio of smectite/montmorilloniteontent to void ratio (M/e) was observed to uniquely represent thewell pressure and saturated k values of bentonite buffers. Like-ise the exchangeable sodium percentage (ESP) could uniquely

epresent the liquid limits of various bentonite buffers. Despiteariations in bentonite type, and measurement technique, the –welations of various bentonite buffers plot as a single envelope.

The values predicted by Eqs. (8) and (10) are at-best guidancealues for agencies to gauge experimental ranges as the resultsre based on homogenous specimens tested in laboratory condi-ions and do not include issues of scale effects, non-homogeneityn composition and compaction conditions that may influence per-ormance under field conditions.

cknowledgements

The authors thank the Board of Research in Nuclear Sciences,epartment of Atomic Energy, Government of India for supporting

his study through research grants.

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