Effect of temperature on soil-water characteristics and hysteresis of compacted Gaomiaozi bentonite

J. Cent. South Univ. Technol. (2009) 16: 0821- -0826包SpringerDOI: 10.1007/s11771-009- 0136-xEffect of temperature on soil-water characteristics andhysteresis of compacted Gaomiaozi bentoniteYE Wei-min(叶为民)", WAN Min(万敏)，CHEN Bao(陈宝),CHEN Yong-gui(陈永贵)，CUI Yu-jun(崔玉军)，WANG Ju(王驹)4(1. Key Laboratory of Geotechnical and Underground Engineering, Ministry of Education,Tongji University, Shanghai 200092, China;2. United Research Center for Urban Environment and Sustainable Development,Ministry of Education, Shanghai 200092, China;3. Ecole des Ponts Paries Tech, Paris, France;4. Bejjing Research Institute of Uranium Geology, Beijing 100029, China)Abstract: Laboratory tests under different constraint conditions were carried out to obtain the soil-water retention curves (SWRCs)of highly-compacted confined/unconfined Gaomiaozi (GMZ) bentonite at 20, 40 and 80 °C, respectively. The effect of temperatureon the soil-water characteristics of the highly-compacted GMZ bentonite was analyzed. The results show that the water retentioncapacity of the highly-compacted GMZ bentonite decreases as the temperature increases under unconfined and confined conditions.high suction, but the water retention capacity of the confined specimen is lower than that of the unconfined specimen at low suction.Under unconfined conditions, the hysteretic behaviour of the compacted bentonite decreases with increasing temperature. At highsuction (>4 MPa), the hysteretic behaviour of the unconfined bentonite tends to increase with the decrease of the suction. Insummary, the hysteretic behaviour of the compacted bentonite is not significant.Key words: Gaomiaoz; bentonite; soil-water characteristic; hysteretic behavior; nuclear waste repository; water retention capacity;temperaturehydro-mechanical property of bentonite. Many studies1 Introductionwere conducted on the effect of temperature OIsoil-water characteristics of the highly-compactedIn the design concept of multi-barrier system ofbentonite [2- -5]. ROMERO et al [6], and VILLAR anddeep geological disposal for high-level radioactive waste, .LLORET [7] experimentallyobtained soil-waterthe highly-compacted bentonite has been widely used forretention curves of the compacted BOOM clay andconstructionof artificial barrier. Because of theFEBEX bentonite by different temperature experiments,respectively. JACINTO et al [8] measured the retentioninfiltration groundwater from the surrounding rock andcurves of different dry densities of MX- 80 bentonitethe mechanical stress induced by the hygroscopicfollowing the wetting path at different temperatures, andexpansion of the working compacted bentonite， themodified the van Genuchten's model by considering thehighly-compacted bentonite experiences coupling ofeffect of temperature on the water retention capacity.thermo-hydro-mechanical process. In order to ensure theBut the above studies only concerned about thesafety and effective operation of high-level radioactiveeffect of temperature on the soil-water characteristicswaste repository, it is important to investigate the effectfollowing single drying/wetting path. In fact, under theof temperature on hydro-mechanical characteristics ofinfluence of gas generation and groundwater infiltrationthe compacted bentonite used as buffer materials [1].of radioactive waste disposal, the compacted bentoniteSoil-water characteristic is one of the key indexes ofhas a soil-water hysteresis caused by the multi-drying/Foundation item: Projects(40772180, 40572161, 40802064) supported by the National Natural Science Foundation of China; Project ([2007]831) supported byComision of Science, Technology and Industry for National Defense of China; ProScience Foundation of China; Projcct(20080430680) supported by China P中国煤化Il2c08R214155_ject08R21415)5supported by Shanghai Postdoctoral Scientific Program of China; Proect(B308)MYHCNMH Galemie DsipieReceived date: 2008- 12- -26; Accepted date: 2009- -03- -24Corresponding author: YE Wei-min, PhD, Professor; Tel: +86- 21- -65985086; E-mail: ye_ tju@tongji.edu.cn.822J. Cent. South Univ. Technol. (2009) 16: 0821-0826wetting circles in a certain range of suction [9- -10]. TheTable 1 Properties of GMZ bentonite .studies on the soil-water hysteresis were focused 0PropertyDescriptiondeveloping thermo-dynamic models of hysteresis forSpecific gravity of soil grainsoil-water characteristic curves, such as the boundingpH8.68-9.86surface model [11-12], and the internal-variable modelLiquid limit%276[13]. Based on the intermal-variable theory of capillaryhysteresis, XU et al [14] developed a mathematicalPlastic limit/%37model to describe the soil-water characteristics o1Total specific surface area/570geotechnicalmediaexperiencingarbitrary(m*.g )drying-wetting paths. The effect of air entrapment wasCation exchange capacity/0.773 0(mmol:g )fully taken into account in XU's model. GaomiaoziMain exchanged cation/Nat (0.433 6), Ca+(0.291 4),(GMZ) bentonite was selected as buffer materials of(mmol:g ) .Mg- (0.123 3), K (0.025 1)Chinese geological repository for high level radioactivemontmorillonite(75.4%),nuclear waste [15]. CHEN et al [16] studied thquartz (11.7%),Main mineralssoil-water characteristic curves of highly-compactedfeldspar (4.3%),bentonite using the osmotic technique and vaporcristobalite (7.3%)equilibrium technique to control suction. YE et al [17]studied the swelling pressure characteristics of the GMZTable 2 Initial state parameters of specimenbentonite by the constant volume test, and found thatParameterValuethere was an exponential relationship between theHeight/mm6swelling pressure and the dry density. QIN et al [18]Diameter/mm2(analyzed the effects of the dry density, the vertical stressWater content/%10.65and the soaking liquid on the swelling characteristics ofthe GMZ bentonite. The objective of the present study isDry density/(g.cm-31.7to investigate the temperature effect on the soil-watercharacteristics and hysteretic behaviors of highly-compacted GMZ bentonite under unconfined andconfined conditions.2 Experimental2.1 MaterialsFig.1 Photo of compaction mouldhe properties of GMZ bentonite used in theexperiment are shown in Table 1, indicating that GMZshown in Fig.2, which were employed for suction controlbentonite has high cation exchange capacity, greattest under confined conditions. The holes were designedadsorption ability, good dispersion and good hydraulicas channels for moisture exchanges between specimen inproperties.the cell and the circulating air around it. For the testThe GMZ bentonite powder was compacted into aunder unconfined conditions, the specimen was placedthin cylindrical specimen. A digital frame was employedinside an aluminous container as shown in Fig.2.for the compaction. The compaction was digitallyThe apparatus using vapour phase techniquecontrolled at a vertical velocity of 0.1 mm/min to ensuresuggested by YE et al [20]， as shown in Fig.3, washe specimen uniformity. Even so, it is difficult to placed in the desiccator, and the salt solution was usedachieve the optimum homogenization [19]The initial state parameters of the specimen arelisted in Table 2.2.2 ApparatusThe compaction devices are shown in Fig.l,A4/including a steel mould and a piston. The bentonite中国煤化工ipowder can be compressed into a thin cylindricalspecimen in the steel mould under the universal press.Fig.2 Photos ofTYHCNMHG>nite: (a) Un-Self-designed cells with small holes in two ends areconfined specimen; (b) Confined specimen in self- designed cell.J. Cent. South Univ. Technol. (2009) 16: 0821-0826_823In order to obtain the temperature effect on theOven(Temperaturesoil-water characteristics of highly-compacted GMZcontrol (士0.1)"C)Salt solutionbentonite,，soil-water retention (SWR) tests of GMZbentonite following drying/wetting cycles were carriedout under both unconfined and confined conditions at 20,40 and 80。C respectively, and soil water retentioncurves (SWRCs) and hysteretic circles were obtainedcorrespondingly.SampleSaltAir pump3 Results and discussionFig.3 Setup of vapour phase technique3.1 SWRCs of highly-compacted GMZ bentonitefor suction control. The air pump was used to keep air inThe SWRCs of the highly-compacted GMZcirculation. The whole system was placed in a digitalbentonitefollowing wettingpath at differentoven for temperature control with the resolution oftemperatures under unconfined and confined conditions(土0.1)。C.are shown in Fig.4. It can be seen from Fig.4 that, thevater retention capacity under both confined and2.3 Methods and test procedureunconfined compacted bentonite decreases with2.3.1 Temperature controlincreasing temperature, and the influence increases withIn order to investigate temperature effects odecreasing suction at a certain suction.soil-water characteristics of the compacted GMZbentonite，20， 40 and 80 °C were selected fo28 ro- 20 "C(Unconfined)temperature control in the tests.40 C(Unconfined)24 t2.3.2 Suction control80 C(Unconfined)▲- 40 "C(Confined)The vapour equilibrium method was employed to20◆- 80 C(Confined)control the suction. Different saturated salt solutionswere used to control the relative humidity of the16circulating air in the whole system. Their correspondingimposed suctions at 20, 40 and 80 °C are shown inTable 3. The high suctions (> 4 MPa) were calibrated by8TANG and CUI [21] and the low suctions were4controlled by unsaturated NaCl solutions suggested byLYTTON [22].10110210Suction/MPaTable 3 Corresponding suction of salt solution at differentFig.4 Comparison of SWRCs in unconfined/confined wettingtemperatures (unit: MPa)paths at different temperatures20 °C40 °C80 °CLiCl2309.0The mechanism for the influence of temperature onMgCl150.0162.4 .219.5the water retention capacity of the highly-compactedbentonite can be analyzed in the aspect of the low113.0122.0167.4suction and the high suction, respectively [7]. At highMg(NO3)282.0103.1suction (> 4 MPa), the change of clay fabric andNaNO257.0intra-aggregate water plays a significant role in waterNaNO339.049.569.0retention capacity of GMZ bentonite. Much intra-NaCl38.040.68.0aggregate water moves into the macropore withincreasing temperature. The excessive water flows out(NH)2SO424.932.2from the macropore, for degree of saturation is invariableKCl21.027.838.4in macropores at a certain suction. Therefore, the waterZnSO,12.6retention caparases as th中国煤化工temperatureapillary effectKNO3.0plays a decisivTYHCNMHGofthewaterK2SO4.25..5retention capacity. The changes of surface tension caused.824J. Cent. South Univ. Technol. (2009) 16: 0821-0826by the increase of temperature result in the increase of20 rthe water content of bentonite with increasing。一Unconfinedtemperature at a certain suction.*- ConfinedThe effect of the constraint conditions on the water15-retention capacity of bentonite can also be analyzed inthe aspect of the high and low suctions. At high suction,the influence of constraint conditions on the soil-water0一retention capacity of bentonite is not significant, whichagrees with the result obtained by CHEN et al [16]. Butat low suction, the water retention capacity of th5confined specimen is clearly lower than that ofunconfined specimen. The difference between watecontent of the compacted bentonite under the confined0l102103and unconfinedconditions becomes greater withSuction/MPadecreasing suction, which suggests that water retentionFig.5 Relationship between k, and suction(t1 is40 °C， andt iscapacity has a close relationship with the changes of the80 °C)microstructure. Under the unconfined conditions, thinter-aggregate pores of bentonite expand constantly, anda)water can flow into these pores easily. Therefore, the16-water retention capacity of the unconfined specimenincreases rapidly as the suction decreases. As clayunder confined conditions, the increasing rate of watercontent in the confined specimen is significantly lowerthan that in the unconfined specimen with the decrease ofthe controlling suction.- 20 C, UnconfinedIn order to analyze the influence of temperature on●- 40 C, Unconfinedthe water retention capacity, a ratio (k), which indicatesthat the water content changes with temperature, is110defined as17(b)k,=W_Wzx 100%(1130where Wr and Wr, are the water contents in the。SWRCs at temperature t and t2 for a given suction,respectively.The relationship between k, and the suction i言70temperature range from 40 to 80。C is shown in Fig.5,50-indicating that the influence of temperature on the water。一20 C, Unconfinedretention capacity has a close relationship with the30-suction. And temperature has a great effect on the water01:retention capacity in the suction range from 40 to 80n%MPa. It can also be seen from Fig.5 that the impact ofFig.6 Hysteretic characteristics of unconfined compacted GMZconstraint conditions on k; tends to increase withbentonite at high suction and different temperatures:decreasing suction.(a) Hysteretic characteristic curves; (b) Relationship betweensuction and hysteretic coefficient3.2 Hysteretic behaviour of SWRCThe hysteretic characteristic curves for unconfinedbehaviour of the compacted bentonite is less obvious asGMZ bentonite following the wetting/drying cycles intemperature inc中国煤化工icient, n7, washigh suction at 20 and 40 °C are shown in Fig.6(a). It isintroduced tchystereticquite obvious that the hysteresis loop of SWRC at20。Cbehavior of benYHCNMHGCes followingis larger than that at 40 °C. That is to say, the hystereticwetting/drying cycles, which can be expressed as.J. Cent. South Univ. Technol. (2009) 16: 0821-0826_825Wd-Wwx 100%4 ConclusionsVw(1) The water retention capacity of the highly-and wetting paths at a given suction, respectively. Eq.(2)compacteGMZ bentonite decreases as the temperatureindicates that the hysteretic behaviour is less significantincreases. At the same time, the impact of temperature onwhen n is small.The hysteretic coefficients of unconfined compactedthe water retention capacity has a close relationship withGMZ bentonite changing with suction at 20 and 40。Cthe suction.(2) At a certain temperature, the influence ofwere calculated using Eq.(2). The results are given inFig.6(b). Fig.6(b) shows that the hysteretic coefficientsconstraint conditions on the water retention capacity ofare all less than 7%, and decrease as the temperaturethe compacted bentonite is not obvious at high suction,increases, which indicates that the hysteresis of highly-but the water retention capacity of confined specimen issignificantly lower than that of unconfined specimen atcompacted bentonite is not so significant.The hysteretic behavior of the unconfinedlow suction.(3) The hysteretic behaviour of the compactedcompacted bentonite in different of ranges suction at40 °C is shown in Fig.7. Fig.7(a) shows that thebentonite under unconfined conditions decreases as thehysteretic cycle becomes larger with the decrease oftemperature increases. At high suction (>4 MPa), thesuction.Fig.7(b)alsoindicates that hysteretichysteretic behaviour of the unconfined bentonite tends tocoefficients are lower than 5% at high suction (> 4 MPa),increase with decreasing suction. 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