evaluation of the influence of boundary

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Evaluation of the influence of boundary

confinement on the behaviour of unsaturated

swelling clay soils

Greg Siemens and James A. Blatz

Abstract: Swelling soils are found in many regions throughout the world. Damage caused to infrastructure by these types

of soils is measured annually in billions of dollars. These excessive damages are, in part, due to the lack of proper design,

resulting from a need for better tools for practitioners to assess the impact of swelling soils in typical design applications.

This paper presents an experimental testing program with interpretations to provide a framework for predicting the behav-

iour of swelling soils under general stress and volume state conditions for practical applications. The experimental testing

adopted a new automated triaxial apparatus that controls boundary stress and strain while applying liquid infiltration condi-

tions at the perimeter or center of triaxial specimens. Results demonstrate the influence of a range of boundary conditions

on the behaviour of swelling soil during liquid infiltration. The range of boundary conditions examined in the experimental

testing include constant mean stress (CMS), where the mean stress applied during the swelling stage is constant; constant

volume (CV), where the volume is held constant during the liquid infiltration; as well as a flexible spring-type boundarycondition (CS) that applies increases in stress as a specified function of the volume increase. These boundary conditions

represent the broad spectrum of experiences in the field. The experimental results show the dominance of boundary condi-

tions on the development of swell pressure and volume expansion to give evidence for a new swell equilibrium limit

(SEL) relationship. The SEL shows promise in providing a framework for swelling soils to predict the final soil state

under wetting conditions for the range of boundary conditions examined. Application of the SEL relationship in practice is

presented as a concept for examining swelling induced pressures and volume expansion in applications of liquid infiltration

of swelling soils.

Key words: swelling soil, unsaturated, experimental, boundary conditions, engineered barriers.

Resume: Les sols gonflants sont presents dans plusieurs parties du monde. Les dommages causes aux infrastructures par

ce type de sol totalisent plusieurs millions de dollars annuellement. Ces dommages excessifs sont dus en partie a une

conception inappropriee, resultant du manque doutils pour evaluer les impacts des sols gonflants dans des applications ty-

piques. Cet article presente un programme experimental et les interpretations qui en decoulent, afin doffrir un cadre pour

la prediction du comportement des sols gonflants soumis a

`des conditions de contraintes et de volumes ge

ne

raux pour desapplications pratiques. Un nouvel appareil triaxial automatise a eteutilise; cet appareil controle les contraintes et deforma-

tions aux frontieres de lechantillon tout en appliquant des conditions dinfiltration de liquide sur le perimetre ou sur le

centre des specimens triaxiaux. Les resultats demontrent linfluence dun eventail de conditions frontieres sur le comporte-

ment du sol gonflant lors dune infiltration. Les conditions frontieres evaluees incluent les essais a contrainte moyenne

constante CMS , durant lesquels la contrainte moyenne reste constante durant le gonflement, les essais a volume cons-

tant CV , durant lesquels le volume reste constant durant linfiltration deau, et les essais a conditions de type ressort

flexible CS , qui appliquent des augmentations de contraintes selon une fonction spe cifique de laugmentation de vo-

lume. Ces conditions frontieres representent la variete de conditions frontieres observees sur le terrain. Les resultats experi-

mentaux demontrent limportance des conditions frontieres sur le developpement de la pression de gonflement et de

lexpansion volumetrique, pour amener une nouvelle relation de limite de gonflement a lequilibre SEL . La SEL

pourra fournir un outil de travail pour les sols gonflants afin de pre dire letat final du sol dans des conditions de mouillage

pour la gamme de conditions frontieres evaluees. Lapplication pratique de la relation SEL est presentee en termes de

concept pour examiner les pressions induites par le gonflement et lexpansion volumetrique lors de linfiltration de liquide.

Mots-cles : sol gonflant, non sature, experimental, conditions frontieres, barrieres techniques.

[Traduit par la Redaction]

Received 3 July 2007. Accepted 18 November 2008. Published on the NRC Research Press Web site at cgj.nrc.ca on 11 March 2009.

G. Siemens.1 GeoEngineering Centre at QueensRMC, Department of Civil Engineering, Royal Military College of Canada, 13 GeneralCrerar Crescent, Sawyer Building, Room 2085, Kingston, ON K7K 7B4, Canada.J.A. Blatz. Department of Civil Engineering, University of Manitoba, 246 Engineering Building, Winnipeg, MB R3T 5V6, Canada.

1Corresponding author (e-mail: [email protected]).

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Can. Geotech. J. 46: 339356 (2009) doi:10.1139/T08-125 Published by NRC Research Press


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Introduction

Soils with shrink and swell susceptibility are found inmany regions around the world. Swelling soils have the po-tential to undergo volume change as a result of changes inwater content under constant stress conditions. Alternatively,swelling soils can exert considerable pressure on structuralelements when exposed to increases in water content whileconfined. Much of our built infrastructure is founded onshallow or deep foundations supported by swelling soils. Inmany regions, subgrades for road construction and engi-neered materials for dam construction also employ swellingsoils. Cumulative annual damage caused by swelling soilshas been reported to be in the order of US$2 billion in theUSA alone (Colorado Geological Survey, geosurvey.state.co.us/). However, in individual cases, damage is unfortu-nately often accepted as typical and simply a nuisance. Thisis particularly true for residential dwellings. Clearly, there isa need to better-design infrastructure in regions where swel-ling soils exist to reduce the negative impacts.

Traditionally, swelling behaviour has been examined with

measurements and predictions of one-dimensional volumechanges and one-dimensional swell pressure with as-sumed perfectly rigid lateral boundaries (Komine and Ogata1994; Dixon et al. 1996; Katti and Shanmugasundaram2001; ASTM 2002b; Sridharan and Gurtug 2004; Tripathyet al. 2004; Thakur and Singh 2005). Some two-dimensionalswell measurements have also been made (Parker et al.1980; Al-Shamrani and Al-Mhaidib 2000; Chen and Ng2005) under free-swell or constant-stress conditions, but notwith constant-volume conditions (to directly measure swellpressure). Measurements of swell pressure have not beenmade in devices other than one-dimensional, oedometer-typeapparatuses.

Previous measurements of vertical swell pressure andswell potential with one-dimensional devices implicitly ap-plied two extreme boundary conditions to the soil that in-cluded constant volume and constant stress. There existinfinite possible conditions between the constant-volumeand constant-stress conditions that are a function of the stiff-ness of the confining element. A boundary condition that isneither constant volume nor constant stress is termed con-stant stiffness in this discussion. The intermediate conditioncan take on any relationship of stiffness as a function of dis-placement, but for simplicity, the tests in this program ex-amine constant-stiffness paths. The three possible boundaryconditions of constant volume, stress, and stiffness are illus-trated in Fig. 1a for the case of a deep geologic repository.

Although the example provided is specific to the under-ground repository conditions, the boundary conditions areconsistent with those observed in typical engineering appli-cations. Figures 1a and 1b illustrate the idealized stressvolume paths for swelling soil subjected to liquid infiltrationconditions while under the imposed boundary conditions in-dicated. Constant-volume (CV) boundary conditions are rep-resentative of the area near the rocksoil interface of thedeep geologic repository where there will be little to nospace available for expansion during water ingress into theswelling backfill soil. The constant mean stress (CMS)boundary conditions allow volume increase at constantstress, representing the swelling soil beneath the container

that has a constant load. These represent the two limitingboundary conditions. The intermediate case is where someexpansion can occur, followed by increasing confinementconditions. In the intermediate case, the swelling soil experi-ences both increasing volume and confining stress.

Swelling soil behaviour is influenced greatly by the boun-dary conditions as the porosity and structure can vary under

wetting conditions depending on the ability to change vol-ume. A confined swelling soil will maintain its porosityunder CV conditions, whereas a swelling soil that expandsunder a constant stress can increase in porosity, which canimpact hydraulic performance. One example of this phenom-enon was identified by Thomas et al. (2003) when modelinga full-scale experiment, known as the isothermal test (ITT).The model examined groundwater flow into a borehole (incrystalline granite) filled with a compacted swelling material(Dixon et al. 2002b). Initially, a hydraulic conductivity rela-tionship was used based on laboratory measurements com-bined with a fitted equation for the unsaturated portion.Final water content predictions using this function were

Fig. 1. (a) Infiltration boundary conditions in a deep underground

waste repository and (b) stressvolume paths applied by laboratory

apparatus.

340 Can. Geotech. J. Vol. 46, 2009

Published by NRC Research Press


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over-estimated, so the function was modified to incorporateincreasing hydraulic conductivity from 0%90% saturationand then hydraulic conductivity decreased towards zero at100% saturation. This produced satisfactory final water con-tent results, but the modified function was deemed uncon-ventional by the authors as it was in contravention of thetraditional understanding of increasing hydraulic conductiv-

ity with increasing degree of saturation. Interpretations re-ported in this paper will show that if the porosity isexamined in light of the ability for the material to expandin volume during wetting, the modelling will show that theinfluence of the boundary conditions on the hydraulic per-formance of the materials will produce the proper hydraulicconductivity function.

Swelling is a multi-dimensional phenomenon and therefore,measurements of one-dimensional swell pressure and swellpotential can only partially capture the generalized behaviourof a swelling soil. In addition, traditional test conditions offree swell or constant height only provide two extreme boun-dary conditions for swelling mechanisms to develop. This pa-per provides details on a laboratory test program that utilizes a

new apparatus as well as the interpretation required to assesschanges to the hydro-mechanical behaviour under generalboundary conditions. Results show the dominance of boun-dary conditions on the hydraulicmechanical behaviour ofswelling soil during liquid infiltration. The results also giveevidence of a swell equilibrium limit (SEL) for swelling soilthat can predict the final state of soil, given the initial watercontent and density as well as the boundary conditions im-posed during wetting. The limit is valid over the range of con-ditions applied in this testing program and provides aframework for examining the performance of swelling soilsin traditional applications. Use of the SEL is illustrated for abasement constructed in swelling soil.

Material and methods

Material properties

The material used in all laboratory testing was a 50:50(by mass) claysand mixture known as bentonitesand buf-fer (BSB), previously referred to in the industry and re-search publications as buffer. The clay component wascomposed of a Nabentonite known as Wyoming bentonite,which was obtained from the Bentonite Corporation ofWyoming under the trade name Standard-Western Bentonite(200 mesh). As received, the Nabentonite was in powderform following processing. It was composed of at least 75%montmorillonite with the remaining being quartz and feld-

spars and its cation exchange capacity was 76 meq/100g.Liquid-limit and plastic-limit testing of the Nabentonitegave results of approximately 555% and 43%, respectively,resulting in a plasticity index of 511 (ASTM 2002a). Theextremely high plasticity index is the result of the high ac-tivity of the montmorillonite component of the bentonite.The silica sand component of BSB is an angular materialmixed to a standard grain-size distribution (Dixon et al.1994). The resulting sand mixture is well graded in termsof grain size distribution with properties Cu = 4, Cc = 0.84,d10 = 0.12 mm, and d50 = 0.38 mm. The mixture was com-bined and compacted into test specimens using proceduresdescribed in Siemens and Blatz (2007a). The cylindrical

specimens (Fig. 2) had an initial target gravimetric watercontent of 19.4% and dry density of 1.67 Mg/m3 to give adegree of saturation of approximately 85%.

Examination of the pore-size distribution evolution pro-vides insight into internal changes that will affect large-scalebehaviour. In swelling soils, this is especially true as thepore-size distribution can change significantly at constant

porosity. Mercury intrusion porosimetry (MIP) results and ascanning electron microscopy (SEM) photograph on an as-compacted specimen are illustrated in Fig. 3. The MIP test-ing was completed at Dalhousie University using a Quan-tachrome PoreMaster 33. The MIP testing was performedup to a maximum pressure of approximately 224 800 kPa.For interpretation of the results, parameters including sur-face tension of 480 erg/cm and a contact angle of 1408were used. The SEM images were collected using a Cam-bridge Stereoscan 120 SEM (Cambridge Instruments, Mon-treal, Que.), equipped with a secondary electron (SE)detector, a four-quadrant semiconductor backscatter electron(BSE) detector, and a Phoenix Genesis 4000 energy disper-sive spectrometer (EDS) (EDAX Inc., Mathwah, N.J.).

Specimens were sputter-coated with a 60% Au 40% Pd al-loy and the SEM was operated with a 20 kV electron beamacceleration. For each test specimen, three SEM and MIPtests were completed to spatially define the change in pore-size distribution. Details of specimen preparation and testingas well as extensive results are provided in Siemens andBlatz (2006) and representative results are shown here.

The MIP and SEM specimens were prepared by dryingthem in a dessicator at room temperature until constantmass conditions were achieved. This preparation procedurewas used to allow for comparison with previous pore-sizetests. During drying, the specimens underwent shrinkage totheir shrinkage limit, which in the as-compacted specimencorresponded to about 10% volume strain (Blatz et al.2007). The mass and volume of each MIP specimen wasmeasured before and after drying, allowing the change in to-tal porosity to be quantified. Although the relative change ofthe pore-size distribution is unknown, this allowed the meas-ured MIP porosity to be corrected. To correct for volumechanges during drying, the cumulative porosity functionwas recalculated so that the porosity measured at end of theinfiltration test was attained.

The bimodal distribution for BSB has peaks at pore sizesof approximately 0.02 and 30 mm levels. Pore sizes are alsoevident on the SEM photograph. In both swelling and non-swelling compacted claysand materials, multi-modal pore-size distributions are observed (Garcia-Bengochea et al.

1979; Juang and Holtz 1986). The bimodal pore-size distri-bution is due to both hydration of the particles and the en-ergy used during compaction. During initial hydration,highly plastic clays form peds or groups of clay particles.Pores within individual peds form the microporosity (alsoknown as intraped pores). Microporosity properties, includ-ing size and particle distribution, are a function of the watercontent during preparation (Wan et al. 1990), which also af-fects its water retention characteristics (Blatz et al. 2002).Macroporosity (also known as interped pores) comprises thespace between peds (aggregation of clay minerals) and itsdistribution is a function of the compaction effort (Wan etal. 1990) and of soil suction (Cuisinier and Masrouri 2005).

Siemens and Blatz 341



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Test apparatus and procedureThe experimental apparatus used for this work has been

described in detail previously (Blatz and Siemens 2004,2005, 2006; Siemens and Blatz 2007a). The apparatus ap-plied radial infiltration under controlled boundary conditionsin the triaxial cell and had a Xeritron sensor compacted atthe center to measure total suction. The Xeritron sensormeasures total suction using a cellulose beam that respondsto changes in relative humidity. Figure 2 shows a photo-graph of a prepared specimen with the Xeritron sensor wireexiting the top of the specimen and Fig. 4 illustrates the ap-paratus in detail. The water pressure applied at the perimeterand the Xeritron sensor compacted at the center provide theupstream and downstream hydraulic boundary conditions. Inbetween, pore pressure (both positive and negative), degreeof saturation, and total suction vary along the flow path.

Following preparation, specimens were installed in the tri-axial cell. The experimental procedure for each test includedthree phases: equilibration, isotropic compression, and infil-tration under controlled boundary conditions. The equilibra-tion phase is provided for the Xeritron sensor to equilibratewith the suction environment inside the specimen. Duringthe isotropic compression phase, mean stress was increasedincrementally to the initial target level, which was variedbetween 250 and 1500 kPa. During liquid infiltration, boun-dary conditions were controlled to simulate field conditions

identified in Fig. 1a, including CMS, CV, and constant stiff-ness (CS). The boundary conditions applied an overall con-trol on the specimen stress and volume states, but internalchanges to the specimen were not anticipated to be uniformas the perimeter had longer access to water compared withthe center. To quantify these internal changes, spatial distri-bution of gravimetric water content and bulk density were

measured to calculate dry density and degree of saturation.The SEM photography and MIP testing were also performedon selected test specimens.

Boundary conditions, which are automatically controlledby the custom data acquisition and control system (Blatz etal. 2003; Siemens 2006; Blatz et al. 2007; Siemens andBlatz 2007a), determine the path followed in mean stress volume strain (p-3v) space (Fig. 1b). Boundary conditions,including CMS, CV, and CS, were imposed on specimensto represent idealized field boundary conditions as deter-mined from an underground waste repository as shown inFig. 1a as well as traditional engineering applications. Dur-ing infiltration, water flowed from the perimeter towards thecenter of the specimens and was taken up by the clay par-

ticles, giving the clay particles increased potential for expan-sion. Two selected tests were completed using a slightlydifferent apparatus that allowed drainage out of the centerof the specimens (Siemens and Blatz 2007b). In these tests,outflow through the center of the specimen was permittedusing a central drainage line and the Xeritron sensor wasnot included in the apparatus.

Test results

Infiltration test results are presented in this section. First,a brief description of an example infiltration test, then asummary of tests completed at the 250 kPa isotropic com-pression level, and finally, the remaining experimental pro-

gram will be presented.

250 kPa constant-volume test

An example test is presented to show general trends ineach measurement throughout the tests as well as to displaythe boundary condition control that the system provides dur-ing liquid infiltration. Test results for the 250 kPa constant-volume test are shown in Fig. 5. Following suction sensorequilibration (1.7 days), cell pressure was increased to250 kPa. The specimen compressed (positive volume strain)and suction decreased as a result (Blatz and Graham 2003).At 2.7 days, the constant-volume infiltration phase was initi-ated. Initially, water infiltrated at a relatively high rate, butthen levelled off at about 6.3 days. After this point, onlylimited infiltration occurred. However, mean stress contin-ued to increase until about 10 days. The suction responsewas smooth throughout the infiltration phase and decreaseduntil the end of the test. The test was completed after21 days. A total of 16.1 mL of water was absorbed, an end-of-test mean stress of 915 kPa was observed, and suctionwas reduced to 1.6 MPa.

Following infiltration testing, the final distribution ofgravimetric water content and bulk density were measuredto calculate dry density and degree of saturation. The aver-age final gravimetric water content distribution from the250 kPa CV test is shown in Fig. 6. Water content was the

Fig. 2. Compacted specimen with internal Xeritron sensor.




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greatest at the perimeter and decreased towards the center ofthe specimen with significant nonlinearity noted. This is

consistent with the radial-flow conditions imposed duringthe test, whereby the perimeter of the specimen had accessto water throughout the infiltration phase. Increased watercontent was associated with increasing degree of saturationas well as decrease in density. Due to the constant-volumeboundary conditions applied during liquid infiltration, theoverall porosity remained constant. As anticipated, the low-est dry density and the highest degree of saturation occurredat the perimeter.

250 kPa infiltration tests

Results from the 250 kPa infiltration tests are shown inFigs. 7a, 8a, 9a, and 10 as specific volume versus mean

stress, gravimetric water content versus mean stress, gravi-metric water content versus specific volume, and the final

water content distribution, respectively. In specific volume mean stress space (Fig. 7a), the 250 kPa CV test is shown asa horizontal line, while the CMS test plots as a vertical linealong with the 250 kPa constant mean stress drained(CMSD) test, and the x= 25 kPa/% and x= 75 kPa/% CStests (CS25 and CS75, respectively) plot along the appliedslopes. The straight lines indicate a high level of stress-pathcontrol for the test with applications of various boundaryconditions. As all the specimens followed a similar methodof preparation and testing until application of liquid infiltra-tion, the influence of boundary conditions was observed di-rectly by the stressvolume paths imposed. During liquidinfiltration, the test specimens experienced expansion under

Fig. 3. (a and b) Mercury intrusion porosimetry (MIP) and (b, inset) scanning electron microscopy (SEM) test results on as-compacted

bentonitesand buffer (BSB) (after Siemens and Blatz 2006).




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constant stress (CMS and CMSD), swelling induced pres-sures at CV or a combination of expansion and increase instresses (CS25 and CS75). As mentioned above, the CVspecimen required an increase of a mean stress up to915 kPa to maintain initial volume. The CS specimens re-quired less stress increase to maintain flexible boundary con-ditions. The CMS and CMSD specimens expanded thegreatest under constant stress boundaries. Expansion andswelling induced stresses continued (depending on the boun-dary conditions) until the swell potential of the clay particlesinside the specimen came into equilibrium with the hydraulicand mechanical boundary conditions imposed at the speci-men periphery.

The gravimetric water content response versus meanstress and specific volume plots (Figs. 8a and 9a, respec-tively) also show the dominance of boundary conditions.The greatest expansion and water content increase occurredin the CMSD and CMS tests. During infiltration, the magni-tude of water content increase was limited by the boundarycondition imposed and the available void space of the speci-men. The clay particles in each specimen had the sameswell potential at the beginning of infiltration. As the clayparticles were given access to water, the peds expanded.

This included expansion of the clay peds into the macroporespace, as well as bulk expansion of the specimens. The ex-tent and magnitude of the expansion depended on the ap-plied boundary condition. The CV boundary conditions didnot allow bulk expansion, while CS allowed expansionagainst an increasing stress boundary. The CMS and CMSDconditions allowed the clay peds to expand to their limit un-

til the applied mean stress counteracted their expansion ca-pabilities. Increasing the mean stress during infiltration,against which the clay particles swell, reduced total waterinfiltration. Due to all specimens having started at an unsa-turated condition (approximately 85%), all took in measura-ble water, regardless of the boundary conditions imposed.

The change in pore-size distribution from the 250 kPaCS25 test with the as-compacted condition is shown inFig. 11. Relative to the initial condition, collapse of themacroporosity from the as-compacted specimen is apparentwith little change observed in the micropores. This specimenunderwent isotropic compression and wetting under a con-fined condition, both of which are anticipated to reduce themacropore space as indicated in the figure. Collapse of the

macropores is also a mechanism for causing decreased hy-draulic conductivity with increasing saturation (Cui et al.2001; Siemens et al. 2007).

Figure 9a, which shows gravimetric water content versusspecific volume, indicates that an apparent upper limit towater content increase is reached. As liquid infiltration con-tinued, specimens reached a point where they could not takein any more water without increasing in volume. The boun-dary conditions imposed during liquid infiltration limited ex-pansion along the specified mean stress specific volumerelationship. Both the CV and CS75 specimens increased inwater content up to the limit and then no further water con-tent increase or volume expansion occurred. The CS25,CMS, and CMSD specimens expanded up to the apparent

limit and then followed the limit until no further infiltrationwas observed.

The post-test gravimetric water content distributions,shown in Fig. 10, illustrate that the boundary conditions con-trol bulk behaviour as well as internal behaviour that cannotbe measured from the periphery of the specimen. Gravimet-ric water content distributions are nonlinear for all specimensand greatest near the perimeter due to longer contact withwater and resulting expansion. Expansion continues until itcomes into balance with the imposed boundary conditions.If the overall specimen volume is held constant and expan-sion occurs at the perimeter, compression at the center ofthe specimen is required for strain compatibility of the speci-

men (zero displacement at the boundary). The CMSD testachieved the greatest water content followed by the CMSand CS25 tests, and then the CS75 CV tests. Expansion ofthe specimen allowed further access for water to flow intothe specimen and raised saturation closer to the center.

The 250 kPa tests give clear insight into the dominance ofthe boundary conditions on the behaviour of swelling soilduring liquid infiltration. The boundary conditions limitedthe overall specimen volume until they reached an apparentspecific volume mean stress gravimetric water contentlimit, after which infiltration was halted or continued alongthe limit. In numerical modeling of swelling soil systems,the boundary conditions are defined to predict behaviour.

Fig. 4. Schematic of triaxial specimen surrounded with geotextile

(from Siemens and Blatz 2007a, with permission from ASCE).

(Web version: This material may be downloaded for personal use

only. Any other use requires permission of the American Society of

Civil Engineers. This material may be found at dx.doi.org/10.1061/

(ASCE)1090-0241(2007)133:6(748).) Typ., typical.




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500, 1000, and 1500 kPa infiltration tests

Results from all infiltration tests under controlled boun-dary conditions are shown in Figs. 7, 8, and 9 as specific

volume versus mean stress, water content versus meanstress, and water content versus specific volume, respec-tively. The 250 kPa tests discussed in the previous sectionare plotted along with the 500, 1000, and 1500 kPa liquidinfiltration tests under CMS, CS, and CV boundary condi-tions. The straight stressvolume paths plotted in Fig. 7 dis-play the high level of control provided by the controlsystem. Behaviour observed at the 250 kPa isotropic com-pression level is similarly observed at the higher mean stressvalues except at a reduced magnitude. That is, expansion aswell as increases in swelling-induced stress continues untilthe swell potential of the clay particles comes into equili-brium with the applied boundary conditions. Increasing the

initial compression pressure level reduced the volume ex-pansion in the higher-pressure tests with CMS boundaryconditions. Less swelling-induced stress increase was also

required in the CV tests at higher initial pressure levels.This was due to the influence of the initial conditions onthe soil response during infiltration. In terms of water uptakeby the specimens, reduced expansion is reflected in less in-filtration into the specimens as shown in Figs. 8 and 9.

The pore-size distribution of compacted clay materials hasbeen previously identified as a key factor in understandingbehaviour (Garcia-Bengochea et al. 1979; Cui et al. 2001).Pore-size distributions and example SEM photographs oftwo infiltration specimens are shown in Fig. 11. The MIPsamples for the infiltration specimens were taken near thecenter of the specimen. The SEM photographs are displayedat consistent magnification (2000) and a scale is shown in

Fig. 5. 250 kPa constant volume test results: mean stress, volume strain, total suction, and water added to specimen versus time.

Fig. 6. 250 kPa constant volume test results: end-of-test distribution of gravimetric water content.




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the bottom left corner of each photo. The photographs weretaken at this magnification to identify the macropore modeof the specimens. In comparison with the as-compactedspecimen (Fig. 3), the infiltration specimens have bothundergone decreases to their macropore space. This is espe-cially true for pore sizes greater than 20 mm. The macroporemode has also shifted to the left for the infiltration speci-mens with the 250 kPa specimen having a consistentlygreater volume of pores throughout. This is a result of thehigher mean stress that was previously applied as well asthe ped expansion during infiltration. The micropore modesshow similar increases in volume from the as-compactedstate, which is consistent with the infiltration conditions ap-plied during the test. The MIP results support the previousdiscussions of material behaviour of a collapse of the macro-pores, causing a decrease in hydraulic conductivity follow-ing infiltration. Comparing the infiltration specimens withthe as-compacted ones shows a decrease in macroporosityfor the two specimens with a corresponding increase in mi-croporosity.

The complete infiltration testing results clearly show theinfluence of boundary conditions on the volume change be-haviours of the BSB soil during liquid infiltration. The final

state of the specimens in terms of mean stress, specific vol-ume, and gravimetric water content is a function of the ini-tial conditions, and the location on the limit is determinedby the boundary conditions imposed during infiltration. Theboundary conditions determine the stressvolume path fol-lowed and the soil response is intimately dependent on theseconditions.

Discussion and interpretation

Swell equilibrium limit (SEL) for swelling soils

A practical analytical tool for designs that employ swel-ling soils is developed in this section. Ideally, a practical de-sign tool provides high quality design information with aminimum of input parameters. The experimental datashowed swelling soil that developed swelling inducedstresses and volume expansion under the imposed boundaryconditions. These swell mechanisms of increased confiningstress and swelling during liquid infiltration continued untilthe swell potential of the clay particles came into equilibra-tion with the imposed boundary condition. The relativeamount of volume expansion and confining stress increasedto apply the imposed boundary condition that decreased with

Fig. 7. Specific volume versus mean stress for infiltration tests: (a) 250 kPa; (b) 500 kPa; (c) 1000 kPa; (d) 1500 kPa.




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confining stress. It is noted that the test specimens with dif-fering initial conditions, but converging stressvolume paths,such as the 250 kPa CV test and the 1000 kPa CMS testspecimens, achieve very similar final stress and volumestates (Figs. 7a, 7c). Figure 12 shows the end-of-test statesand a fitted line for all the infiltration tests completed in theexperimental program. Also shown in Fig. 12 is the isotropiccompression line for this material. Along this line, testspecimens are initially subjected to isotropic compression

followed by liquid infiltration under controlled boundaryconditions. During the liquid infiltration phase, specimensswelled, underwent swelling-induced stresses or both untilthey came into equilibrium with the application of hydraulicand mechanical boundary conditions. The fitted line istermed the SEL as it forms a limit to expansion and confin-ing stress increase for this material. Considering the widelyvarying initial and boundary conditions applied in the exper-imental program, the correlation between the end-of-testdata and the fit gives confidence that the SEL is valid overa wide range of stress and volume conditions. Given that atthe pore scale, similar mechanisms are occurring within thepore volume and clay particles, similar final states would be

expected for two specimens with differing initial conditions,but converging stressvolume paths.

The SEL provides a limit to volume expansion and con-fining stress development under general wetting conditions.Establishing initial conditions that include stress, water con-tent, and dry density (specific volume) as well as boundaryconditions that occur during wetting conditions allows de-signers to predict final states. Initial conditions would nor-mally be determined during a site investigation. Boundary

conditions are a function of confinement conditions estab-lished for a particular design. From the initial soil conditionsand boundary conditions provided by a specific design, crit-ical design cases under wetting conditions are establishedand then designed for.

The main implication for designs with swelling soils is toshow the importance of determining the hydraulic and me-chanical conditions as well as the physical limits of the en-gineered materials. Huang et al. (1998) and Fredlund andPham (2006) showed the intimate relationship between thehydraulic and mechanical behaviour of deformable soils dur-ing measurements of permeability and modeling, respec-tively. This relationship is further complicated in swelling

Fig. 8. Gravimetric water content versus mean stress for infiltration tests: (a) 250 kPa; (b) 500 kPa; (c) 1000 kPa; (d) 1500 kPa.




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Fig. 9. Gravimetric water content versus specific volume for infiltration tests: (a) 250 kPa; (b) 500 kPa; (c) 1000 kPa; (d) 1500 kPa.

Fig. 10. 250 kPa test results: end-of-test distributions of gravimetric water content.




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soils, as changes in pore structure affect behaviour even ifbulk volume is constant (Thomas et al. 2003). In engineer-

ing applications, CMS boundary conditions can allow exces-sive volume expansion that will result in a decrease to thestiffness properties of the soil. CV or high-stiffness boun-dary conditions can result in significant swelling-inducedstresses. Consequently, the design will need to take this intoaccount. Examples of pure CV boundary conditions are notthe norm in traditional geotechnical engineering applica-tions, as some displacements are required to mobilize soilstrength. It is likely that some types of flexible boundaryconditions, represented in this testing program as CS, willhave to be designed. The hydraulic conditions that have tobe taken into account are the initial water contents (suction)as well as the water-pressure boundary conditions.

Swelling-soil design example

To illustrate how the SEL can be used as a practical ana-

lytical tool, a design in swelling soil is presented. An exam-ple of a swelling-soil design, a new basement foundation, isillustrated in Fig. 13. Two aspects of the design are particu-larly interesting: below the basement slab and adjacent tothe basement wall. Wetting conditions are anticipated inthis example as the new basement covers ground that previ-ously had vegetation cover. Roots from the vegetation pro-vide suction to the soil and their removal would result in anincrease in water content around the basement.

For the design, the initial soil conditions and boundaryconditions are determined to predict the final conditions byusing the SEL. Initial conditions, including water contentand density, are determined during the site investigation.

Fig. 11. (aand b) Mercury intrusion porosimetry and (b, inset) scanning electron microscope results of infiltration specimens (after Siemens

and Blatz 2006).




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Boundary conditions are established from the design config-uration (Fig. 13). Swelling under constant-stress boundaryconditions are representative of the area below the floorslab. Stress conditions provided by the floor slab are as-sumed constant throughout. In the area next to the basementwall, confined conditions are observed. In the experimentalprogram, this boundary condition was applied as a constant-stiffness boundary. As the soil next to the foundation under-

goes wetting, it will swell against the in situ soil on one sideand the basement wall on the other side. The stiffness of thebasement wall will play a key role in confining stress devel-opment. During wetting conditions, the soil below and adja-cent to the basement will swell and eventually come intoequilibrium with the mechanical and hydraulic conditions.As illustrated on the inset in the figure, the soil below thefoundation will swell until it reaches the SEL. As such, thevertical movement under the foundation can be predictedand accounted for in the design. Adjacent to the basementwall, the stiffness of the wall and in situ soil will determinethe final stress and volume states. Following the establish-ment of the critical design parameters, the foundation slab

and wall can now be properly designed to carry the pre-dicted loads and displacements.

Re-interpretation of swell pressure measurements

Many previous measurements of swell pressure exist inpublished literature (Dixon et al. 1996; ASTM 2002b; Srid-haran and Gurtug 2004; Thakur and Singh 2005; andothers). The experimental evidence from this testing pro-gram indicates that a unique value of swell pressure doesnot exist, but that swelling behaviour is a function of theboundary conditions. Chandler (2005, personal communica-

tion) proposed the means for direct comparison between tra-ditional swell pressure measurements and the SELsummarized here.

Dixon et al. (2002a) reported swell pressures for variousswelling materials as a function of effective montmorillonitedry density (EMDD). In general, swell pressure is shownto increase with EMDD. Past experience showed that con-ductivity in compacted swelling materials was a function ofthe clay dry density with the remaining dry mass of materialserving as filler (Gray et al. 1984). This was later confinedto just the swelling (montmorillonite) portion of the mate-rial. The EMDD is calculated by taking the mass of mont-morillonite divided by the volume of montmorillonite (Grayet al. 1984) and has been used in the past to correlate me-

chanical and hydraulic parameters. The EMDD is calculatedfrom

1 EMDD fmfcrd

1 1fcrd=Gsrw 1fmfcrd=Gnrw

where fm is the montmorillonite fraction of clay, fc is theclay fraction, rd is the dry density, Gs is the specific gravityof nonclay materials, Gn is the specific gravity of nonswel-ling clay, and rw is the wet density.

Using relevant material properties specific to the BSB,EMDD was shown in Siemens (2006) to be calculated as

Fig. 12. Swell equilibrium limit (SEL) and end-of-test specific volume mean stress states (Vp).

Fig. 13. Design application using the swell equilibrium limit for a

shallow foundation.




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2 EMDDBSB 0:45

V=2:7 0:2037

where EMDDBSB is the EMDD for BSB and Vis the speci-fic volume.

Many reported measurements of swell pressure are per-formed in one-dimensional apparatuses. As reported above,

no reports of constant-volume tests performed in the triaxialapparatus were found. Therefore, it is reasonable to assumethat most, if not all, swell pressures reported by Dixon etal. (2002a) were performed in one-dimensional apparatuses.These swell pressures must be converted to equilibriumor end-of-test mean stresses (pequil, BSB) for comparison withthe SEL. As an assumption, elasticity was used to calculatestresses. Siemens (2006) showed that using the assumptionof elasticity, one-dimensional swell pressures reported byDixon et al. (2002a) using distilled deaired water could con-vert end-of-test mean stresses through

3 pequil; BSB 0:003084 exp 2:109

V=

2:7 0

:2037

where pequil, BSB is the equilibrium (end of test) mean stressfor BSB.

To summarize, previous relationships between EMDD andswell pressure were proposed based on a number of swel-ling materials as presented in Dixon et al. (2002a). To com-pare with laboratory tests completed for this research,conversion of one-dimensional swell pressure to meanstress was required. Also, EMDD was converted to specificvolumes and the results are plotted in Fig. 14 as the dashedline. This indicates that the swell pressure EMDD rela-tionship measured in one-dimensional apparatuses may ac-tually be comparable to the SEL that was developed basedon the full range of boundary conditions. This assumes thatswelling is an elastic process, which may or may not betrue. The concept, however, shows excellent promise andshould be explored further.

Extending this concept, a database of swell tests in regionsaround the world with swelling materials will be a valuableresource in developing a SEL for the particular soil. Forthese regions, it is likely that a large number of swell-pressure and swell-potential tests would have already beencompleted on local materials. The SEL provides a frame-work for understanding swell mechanisms from the develop-ment of the SEL and comparison with the BSB specimens toan existing database of tests normalized to EMDD. The SEL

also provides a valuable tool for re-interpreting previoustests in light of this new understanding. The above exampleof a shallow foundation shows the way that the SEL can beused as a promising design tool. A number of tests using thisnew apparatus will be required to calibrate and validate eachdatabase of swell tests for a particular region.

Comparison with full-scale experiment

A part of the original motivation for the creation of thenew apparatus (Siemens and Blatz 2007a) was to improvethe understanding of previous full-scale underground experi-ments, including the ITT (Dixon et al. 2002b; Thomas et al.

2003). Modeling of the ITT required use of an unconven-tional hydraulic conductivity relationship that decreased assaturation was approached. The ITT configuration is similarto the laboratory apparatus used in this study except that theITT allowed water inflow through the base of the borehole.Therefore, changes in density and water content are antici-pated to be comparable in the full-scale and laboratory tests.

Comparison of density measurements from the decommis-sioning of the ITT and end-of-test measurements is shownin Fig. 15a. Also shown in the figure is the initial densityof BSB in the ITT and the 500, 1000, and 1500 kPa testsfor comparison. From this figure, the ITT appears to be ap-proximately a large-scale 1500 kPa CV infiltration test. InFig. 15a, the initial density of BSB is shown as a solid hori-zontal line. This is highly idealized, as Kjartanson et al.(1992) reported that the initial dry density, formed usingcompaction procedures as in the ITT, was in a small rangethat was approximate to this ideal value. Swelling in theBSB is reported at the boundary and compression occurredat the center in the ITT. This is consistent with post-test

measurements in the laboratory tests. One of the differencesbetween the large-scale and laboratory tests is that in thelaboratory, the entire specimen periphery was surroundedby water, but in the ITT, inflow only occurred through dis-crete fractures in the rock. Therefore, it is not surprising thatwater uptake takes relatively longer compared with a two-week CV test.

In addition, the end-of-test distribution of water contentsfrom the ITT and 1500 kPa CV and CMS tests are includedin Fig. 15b. Approximately one-third from the top of theITT borehole was taken as an approximation of the radialflow conditions applied in the laboratory tests, allowing di-rect comparison of end-of-test spatial water contents.Figure 15b includes gravimetric water content versus nor-malized distance for the 1500 kPa CV and CMS tests andlayer C from the ITT. Water content measurements at theend of the 1500 kPa CV test are comparable with the finalmeasurements from the ITT. Additionally, swelling-inducedpressures that were measured during the ITT also coincidewith the 1500 kPa CV test. A range of swelling-inducedpressures between 1250 and 2000 kPa was reported inDixon et al. (2002b). The end-of-test mean stress observedduring the 1500 kPa CV test was 1900 kPa.

Dry density, water content, and final mean stresses fromthe ITT indicate that it is a large-scale 1500 kPa CV test.The major improvement is that now, these conditions canbe applied in the laboratory test apparatus that includes

measurement and control of stress, volume, and suctionstates and can be completed in a two-week time frame.

In light of the SEL concept, the ITT results may now beinterpreted. The ITT and swell tests reported here incorpo-rate similar materials, as both were 50:50 claysand materi-als with only different clay types. Final water content anddensity measurements coincide and overall boundary condi-tions are similar in the ITT as the 1500 kPa CV test(Fig. 15). Therefore, behaviour in the infiltration tests canbe used to interpret the ITT results. Under groundwater in-filtration and nondisplacement boundary conditions providedby the crystalline granite, soil swelling near the perimeter




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would be accompanied with compression at the center. Thismaintains constant porosity over the entire soil massalthough local changes would occur. Given the mainchanges in porosity that occur in the macropore space fromthe MIP results and that water transfer occurs through themacropores, a decrease in hydraulic conductivity will occurunder these conditions. This represents physically what wasrequired by the Thomas et al. (2003) model in which hy-draulic conductivity decreased with increasing suction. Sim-ilar findings are reported in Siemens et al. (2007), in whichthe infiltration tests are modelled using a deformable capil-lary tube. The capillary tube reduces in cross-sectional areaas water flows along its length, representing a loss of macro-pore sizes during water ingress. Considering behaviour at

the specimen scale as well as at the pore level allows thisnew understanding to be used in numerical models.

Predictions using new limit

The SEL is shown to be valid for the tested soils and con-ditions. Some additional predictions can be made based oncurrent understanding. Also, trends in behaviour can bepostulated if specimen and test parameters are altered.

Altering initial conditions, including water content anddry density of specimens, will change the starting pointsprior to infiltration, but the same limit in water content specific volume space (Fig. 9) will eventually be reachedduring liquid infiltration. This limit appears linear over awide range of specific volumes and water contents. If speci-

Fig. 14. (a) Swell pressure versus effective montmorillonite dry density (EMDD). (b) Comparison between swell equilibrium limit (SEL)

and converted swell pressure measurements. DDW, distilled deaired water.




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mens are dried prior to infiltration, this will only alter theinitial state based on the induced changes to water contentand volume. The boundary conditions applied during liquidinfiltration will still dominate behaviour and the water con-

tent specific volume limit will eventually be reached.Alternatively, drying specimens (increasing suction) priorto infiltration will affect the SEL as shown in Fig. 16. Thelimit in water content specific volume space (Fig. 8) formsan asymptotic line to the initial water content. This indicatesthat an initial mean stress exists above which no (or little)water inflow can occur for this water pressure, regardless ofthe boundary condition imposed during infiltration. Eventu-ally, increases in the mean stress will result in significantcompression and water will be pushed out of the soil in aprocess similar to consolidation. If the initial water contentis decreased (increased suction), the SEL will likely remainnonlinear in shape, but the constant will be the reduced ini-

Fig. 15. Comparison of post-test measurements and de-commissioning of isothermal test (ITT data from Dixon et al. 2002b). (a) post-test

dry density; (b) post-test gravimetric water content.

Fig. 16. Predicted trends in water content mean stress limit for

altered initial water content and water pressure. Typ., typical.




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tial water content as illustrated in Fig. 16. Lower initialwater content will give the montmorillonite particles greaterpotential for expansion, resulting in greater stress required tomaintain initial volume.

Results presented in this program are also specific to thewater pressure applied (200 kPa), which is chosen during theinitial testing and is similar to measurements of final water

pressure near the soilrock interface at the end of the ITT(Dixon et al. 2002b). If the water pressure is increased ordecreased, the limit in gravimetric water content meanstress space will shift upwards, as there will be a greaterdriving potential for water inflow and vice versa, as illus-trated in Fig. 16. Increasing the water pressure will also in-crease the mean stress required to maintain initial watercontent, as indicated in the figure. Altering the water pres-sure is not anticipated to affect the gravimetric water con-tent specific volume limit, as it is likely a function of thematerial and not the imposed conditions.

Conclusions

Boundary conditions have been shown to dominate thebehaviour of swelling soils during liquid infiltration. In thispaper, experimental results and interpretations from a newapparatus are presented. Results indicate that there exists alimit to volume expansion and confining stress that occursduring liquid infiltration under controlled boundary condi-tions. This is termed the swell equilibrium limit (SEL).Boundary conditions can range from constant mean stress(CMS) to constant volume (CV), with the intermediateboundary condition termed constant stiffness (CS). Theseboundary conditions comprise the full range of what is nor-mally anticipated in engineering applications. During infil-tration, the test specimens followed the imposed boundary

conditions until they reached the SEL and then no furtherexpansion or stress increase was observed. The SEL wasshown to be valid over the range of pressures and boundaryconditions that were present. The SEL provides a frameworkfor interpretation of swelling soil behaviour under wettingconditions. Final stress and volume states can be predictedusing the SEL for known initial and boundary conditions.

The SEL is shown to provide a new understanding for thebehaviour of swelling materials under wetting conditions.Instead of viewing swelling behaviour as either swell pres-sure or potential, a unified framework is presented that al-lows prediction under general boundary conditions. Adesign example of a basement constructed in swelling soilwas developed to show the value of the SEL as a powerfulanalytical tool to be used by practitioners. In addition, aframework was laid out in the paper for re-interpretation ofone-dimensional swell tests in light of the SEL concept. TheSEL was developed from multi-dimensional swell tests withcontrolled boundary conditions. The framework givesgreater value to previously completed tests and could allowdevelopment of local databases of swell tests for future de-sign applications. The SEL was used to re-interpret a full-scale experiment known as the isothermal test (ITT). Duringthis test, changes to the pore structure of swelling soil oc-curred under confined wetting conditions that can now beinterpreted using the new framework.

Acknowledgments

The authors acknowledge financial support from the Nat-ural Sciences and Engineering Research Council of Canada,Atomic Energy of Canada Limited, Ontario Power Genera-tion, the Civil Engineering Department at the University ofManitoba, the Faculty of Graduate Studies at the Universityof Manitoba, the friends and colleagues of Neil Burgess, and

the Petro-Canada Graduate Scholarship. Valuable input fromBruce Kjartanson, David Toll, Jamie VanGulck, Doug Ruth,David Dixon, and Jim Graham is also noted.

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List of symbols

Cu uniformity coefficientCc concavity coefficient

d10 sieve diameter through which 10% of particles passd50 sieve diameter through which 50% of particles passEMDD effective montmorillonite dry density

EMDDBSB effective montmorillonite dry density for bentonitesand buffer

e void ratiofc clay fraction

fm montmorillonite fraction of clayGn specific gravity of nonswelling clayGs specific gravity of nonclay materialsp mean stress (= (s1 + 2s2)/3)

pequil, BSB equilibrium mean stress for bentonitesand bufferV specific volume (= e + 1)w gravimetric water content




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3v volumetric strainrd dry densityrw density of waters stress

s1, s2 maximum and minimum principal stressJ total suction




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evaluation of the influence of boundary

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