An Expansive Soil Index for Predicting Shrink–Swell Potential

An Expansive Soil Index for Predicting Shrink-Swell PotentialP. J. Thomas,* J. C. Baker, and L. W. Zelazny

ABSTRACTSoil properties indicative of shrink-swell potential were studied

for 12 soils encompassing several parent materials in Virginia. Soilsare rated from moderate to very high shrink-swell potential. Themineralogy classes, soil series, and (parent materials) examined in-clude: smectitic—Iredell (hornblende gneiss), lackland and Waxpool(diabase); vermiculitic—Kelly (thermal shale); kaolinitic—Cecil(granite gneiss) and Davidson (diabase); and mixed—Carbo and Fred-erick (limestone), Craven and Peawick (Coastal Plain sediments),and Mayodan and Creedmoor (Triassic sandstones). Soil propertiesmeasured were swell index, coefficient of linear extensibility (COLE),particle-size distribution, cation-exchange capacity (CEC), liquidlimit, plasticity index (PI), and clay mineralogy. Soils with estimatedhigh or very high shrink-swell potential were clayey, with clay contentsexceeding 60%. These expansive soils also exhibited high CEC (>15cmolc kg"1 soil), high liquid limits (>70), and appreciable swelling 2:1mineral content (>15% montmorillonite + 1/2 vermiculite on whole-soil basis). An expansive soil rating system, termed the ExpansiveSoil Index (ESI), was developed using the soil properties most corre-lated with shrink-swell potential. The sum of swelling 2:1 minerals,swell index, liquid limit, and CEC gave ESI ratings for each soil series.The higher the ESI, the greater the shrink-swell potential. Whereless-detailed information is required, such as for initial feasibilitystudies, an ESI consisting of liquid limit and CEC is recommended.Finally, the soils were grouped into risk categories based on parentmaterial to allow for classification of similar soils into the ESI ratingsystem. Soils with restricted drainage formed from mafic rocks, car-bonate parent material, and metamorphic shales are at high risk forexpansive soil behavior.

EXPANSIVE SOILS may occur throughout Virginia, butthey especially pose a problem where rapid urban-

ization and development are occurring. As developmentextends into these areas, identification and quantifica-tion of the soil properties that define shrink-swell poten-tial are essential to evaluate properly the stability of asoil as a foundation material.

Shrink-swell classes for each horizon and for the soilprofile are based on the change in length of an uncon-fined clod as moisture content is decreased from a moistto a dry state. If this change is expressed as a percentage,the value used is linear extensibility percentage (LEP).If it is expressed as a fraction, the value used is COLE(Soil Survey Staff, 1996). The shrink-swell classes are

P.J. Thomas, J.C. Baker, and L.W. Zelazny, Dep. of Crop and SoilEnvironmental Sciences, Virginia Polytechnic Inst. and St. Univ.,Blacksburg, VA 24061-0404. Received 18 June 1998. *Correspondingauthor ([email protected]).

Published in Soil Sci. Soc. Am. J. 64:268-274 (202000).

defined as follows: low (LEP <3%, COLE <0.03); mod-erate (LEP 3-6%, COLE 0.03-0.06); high (6-9%,COLE 0.06-0.09); and very high (LEP >9%, COLE>0.09) (Soil Survey Staff, 1997a). Occasionally shrink-swell classes are estimated from accessory soil character-istics such as field-determined plasticity and stickinessand texture. Thus, accurate quantitative measures oflinear extensibility are not always available.

Presently, no one method of soil analysis estimatesshrink-swell potential accurately for all soils. Soil scien-tists recognize that shrink-swell behavior can best bepredicted by examining a combination of physical,chemical, and mineralogical soil properties. A protocolthat integrates these properties and then establishes ashrink-swell model that can be extrapolated across thesame or similar parent materials is needed.

Most studies examining expansive soils have beenconducted on Vertisols and high base, montmorillonitic(smectitic) soils. In these studies several parametershave been identified as correlated with swelling. Poten-tial volume change of expansive soils in the westernUSA had been linked to clay content and PI (Holtz andGibbs, 1956). The variation in swelling of Hapludalfs inOhio was related to clay content in a study where allother parameters were held constant (McCormack andWilding, 1975). Swell potentials of montmorilloniticsoils in southern Ontario were correlated with clay con-tent and specific surface area (SSA), and SSA explainedmore of the variability in shrink-swell potential thandid clay content (Ross, 1978). In Usterts and Torrertsof arid regions, swell potential, as measured by theCOLE, was related to the fine clay content and ex-changeable Na percentage (Anderson et al., 1973).Schafer and Singer (1976) determined that the percent-age of expandable clays explained most of the variabilityin swelling potential in soils of Yolo County, California.Shrink-swell potential in kaolinitic and mixed mineral-ogy soils and acid montmorillonitic soils are often moredifficult to predict. Franzmeier and Ross (1968) ob-served that soils having equal amounts of kaolinite andmontmorillonite behaved like montmorillonitic soils,whereas soils with appreciable amounts of montmoril-lonite had wide ranges in swelling potentials. They pos-tulated the variability was related to the amount of clay

Abbreviations: CEC, cation-exchange capacity; COLE, coefficient oflinear extensibility; ESI, Expansive Soil Index; LEP, linear extensibil-ity percentage; PI, plasticity index; PVC, potential volume change;SSA, specific surface area.

THOMAS ET AL.: SOIL INDEX FOR PREDICTING SHRINK-SWELL POTENTIAL 269

Table 1. Classification and estimated shrink-swell potential of selected soil series.Physiographicprovince

Valley and Ridge

Piedmont

Triassic Basins

Coastal Plain

Parent material

Limestone

Granite gneissHornblende gneissDiabase (diorite)

Thermal shaleSandstone/shale

Marine sedimentsFluvial sediments

Soil series

FrederickCarboCecilIredellJacklandWaxpoolDavidsonKellyCreedntoorMayodanCravenPeawick

Classification!

Mixed, semiactive, Typic PaleudultMixed, active, Typic HapludalfKaolinitic, Typic KanhapludultSmectitic, Typic HapludalfSmectitic, Aquic HapludalfSmectitic, Aerie EpiaqualfKaolinitic, Rhodic KandiudultVermiculitic, Aquic HapludalfMixed, semiactive, Aquic HapludultMixed, semiactive, Typic HapludultMixed, subactive, Aquic HapludultMixed, active, Aquic Hapludult

Shrink-swellpotential

HighVery highModerateVery highVery highVery highModerateHighHighHighModerateHigh

t All soils are in the fine family particle-size class (350-600 g kg"1 clay), except for Carbo, which is very fine (>60 g kg ' clay). Frederick, Carbo, Jackland,Waxpool, and Kelly are in the mesic soil temperature class; other soil series are in the thermic soil temperature class.

and the soil fabric. Acid montmorillonitic and mixedmineralogy Alfisols and Ultisols in Alabama showedweak correlations between COLE and potential volumechange (PVC) (Karathanasis and Hajek, 1985). HigherAl saturations may contribute to resistance of the clayminerals to dehydration. Acid conditions were found tofavor interlayer formation with Al and Fe in montmoril-lonite (Carstea et al., 1970) and to inhibit swelling (Rich,1968). Iron coatings have also been found to reduceswell potential (Davidson and Page, 1956). In a study ofAlabama soils ranging from kaolinitic Davidson (Kandi-udult) to montmorillonitic Houston soils (Hapludert),CEC was highly correlated with SSA, -1500 kPa mois-ture content, and PI (Gill and Reaves, 1957).

As can be surmised from the discussion above, severalphysical, chemical, and mineralogical soil properties in-fluence shrink-swell behavior, with no one propertyaccurately predicting shrink-swell potential for all soils.Often most expansive soils are clayey with high CECs,high SSAs, and high liquid limits. Smectite typicallycomprises most of the soil clay fraction.

Our study was undertaken with the hypothesis thatno one soil property or expansive soil test can preciselypredict shrink-swell potential for all soils. However, aset of soil properties that estimates shrink-swell behav-ior can usually be determined for clayey soils with ka-olinitic, mixed, or smectitic mineralogy formed from avariety of parent materials. The objectives of our studywere (i) to quantify properties and shrink-swell indicesof 12 expansive soils in four major physiographic prov-inces in Virginia, (ii) to correlate shrink-swell potentialwith soil properties and shrink-swell indices, (iii) todevelop an expansive soil rating system using soil prop-erties correlated with shrink-swell potential to evaluateeach soil's propensity to be expansive, and (iv) to de-velop shrink-swell risk categories for soils within differ-ent parent materials.

MATERIALS AND METHODSStudy Design

Twelve map units in four physiographic provinces in Vir-ginia comprised the study. These clayey soils have estimatedmoderate to very high shrink-swell potential with varyingamounts of expanding 2:l's, interlayered 2:l's, mica, and ka-olinite. Estimated shrink-swell potential for each of the soils

for initial placement into expansive soil classes was obtainedfrom the USDA-NRCS database (Table 1) (Soil Survey Staff,1997b). All soils are classified (Soil Survey Staff, 1994) as fine,with mineralogy classes encompassing kaolinitic, vermiculitic,smectitic, and mixed families (Table 1).

Map units in this study were delineated in accordance withprocedures of the National Cooperative Soil Survey (Soil Sur-vey Staff, 1993). Three delineations in each of the 12 mapunits were selected from soil survey maps. One pedon withineach delineation was excavated to 1.8 m for a total of threepedons per map unit. The most representative pit face wasdescribed and sampled. Data from the mid-argillic horizonare presented here since this is the depth at which foundationsare typically installed (0.5-0.9 m).

Laboratory AnalysisSamples were air dried, ground, and sieved to remove

coarse fragments >2 mm. Laboratory analyses include parti-cle-size distribution, CEC, Atterberg limits, PVC, and claymineralogy. Particle-size distribution was accomplished by thepipette method (Gee and Bauder, 1986) and CEC by the sumof cations method (NH4OAc, pH 7 and BaCl2-TEA, pH 8.2)(Thomas, 1982). Atterberg limits (liquid limit, PI) were mea-sured by ASTM method D4318 (American Society for Testingand Materials, 1993). Coefficient of linear extensibility wasdetermined by the method of the National Survey Center (SoilSurvey Staff, 1996). Potential volume change was determinedby the method of Lambe (1960). Shrink-swell potential wasdetermined on each sample on the basis of PVC data. Shrink-swell potential classes are low (<81 kPa), moderate (81-153kPa), high (153-225 kPa), and very high (>225 kPa) (SoilSurvey Staff, 1993). Mineralogical composition was deter-mined by x-ray diffraction and thermal methods. Free Fe ox-ides were removed with dithionate-citrate-bicarbonate (Mehraand Jackson, 1960). Sand was removed by sieving, and the clayfraction was separated from silts by centrifugation (Jackson etal., 1950). Oriented mounts of the clay fraction were preparedby the method of Rich (1969) and saturated with KC1 andMgCl2-glycerol (Whittig and Allardice, 1986). Clay mineralswere identified with a Scintag XDS 2000 x-ray diffractometer(Scintag, Santa Clara, CA) with Cu-Ka radiation. Thermalanalysis of the clay fraction was accomplished with a Dupont990 Differential Scanning Calorimeter (Dupont, Wilmington,DE). Quantitative estimates of kaolinite and gibbsite wereobtained by measuring endothermic peak areas. Quantitativeestimates, to the nearest 5%, of other clay fractions weredetermined by proportioning integrated peak areas of x-raydiffractograms, using kaolinite as an internal standard. Swell-ing 2:1 minerals were estimated by summing the smectite con-

270 SOIL SCI. SOC. AM. J., VOL. 64, JANUARY-FEBRUARY 2000

800

700

600

500

400

300

200

100

0

MODERATE677

HIGH

584

• illi!'t°p~.%-f

VERYHIGH686 676

615

Jig

IIpit

Cecil Mayodan Creedmoor Frederick Iredell WaxpoolDavidson Craven Peawick Kelly Jackland Carbo

Fig. 1. Relationship between clay content and estimated shrink-swellclass. Means as indicated by the same letter are not significantlydifferent at the 0.05 level of probability.

tent and one-half of the vermiculite content, since expansionis limited to two water layers for vermiculite.

Statistical AnalysisSignificantly different means of Bt horizon soil properties

were separated by least significant difference at the 0.05 level.The Pearson product moment correlation coefficient betweenvariables was used to examine the relationships betweensoil properties.

RESULTS AND DISCUSSIONSoil Properties

Average values for soil properties for each map unitare presented in Fig. 1 through 7, and all data for eachprofile are given in Table 2. The largest (most extreme)variance within individual map units was observed inclay content and liquid limit (Table 2). The low claycontents and liquid limits corresponded with low valuesfor other measured soil properties. Most of the followingdiscussion refers to average values for soil propertiesfor each of the map units (Fig. 1-7). Shrink-swell classesindicated in the figures (moderate, high, very high) arethe USDA-NRCS shrink-swell potential classes (SoilSurvey Staff, 1997b).

The Bt horizons of all 12 soils were clayey with clay

CEC (cmolc kg-1 soil)MODERATE HIGH

17.3 : 17.2

m

VERYHIGH

26.3 26.4


Fig. 2. Relationship between cation-exchange capacity (CEC) andestimated shrink-swell class. Means as indicated by the same letterare not significantly different at the 0.05 level of probability.

Limit Limit ( % H2O)100

90

80

70

60

50

40

30

70

MODERATE HIGH VERYHIGH

79 80

71

61

::J'y:

Sif"xft;s?M

55

ipb'-'!

^prf,>i

Iftti'li

56

Kft

ilfRj

JiCSaiIW

'iC^

Sip:i||M;

I»ftsit?*:;

51

y&'^t'.y^t

M

11;lift?«;r:«w

62

«:;:<:«

i.vs -„-:>!

sea;1B:W'

;:-"••"?;•• =

:"ilr:"./i

nSBS

,;•„>" S',1!.

51III

••.*'^: .••

pri-v]

=*syeaIS?;

S--IP;

Cecil Mayodan Creedmoor Frederick Iredell WaxpootDavidson Craven Peawick Kelly Jackland Carbo

Fig. 3. Relationship between liquid limit and estimated shrink-swellclass. Means as indicated by the same letter are not significantlydifferent at the 0.05 level of probability.

contents ranging from 363 to 693 g kg"1 (Fig. 1) and 182to 790 g kg"1 for individual profiles (Table 2). Estimatedhigh and very high shrink-swell soils had similar claycontents, except Creedmoor, which had a much lowerclay content (Fig. 1). Moderate shrink-swell soils, Ceciland Davidson, also had high clay contents similar to themore expansive soils. Thus, no apparent relationshipbetween clay content and susceptibility to shrink-swellbehavior was observed.

A partial trend in shrink-swell behavior can be ob-served with CEC. High and very high shrink-swell soilsgenerally had higher CECs than the moderate soils (Fig.2). However, the Frederick map unit had significantlylower CEC and behaved or belonged in the same classas the moderate shrink-swell soils if only CEC is usedto estimate shrink-swell potential. Likewise, Mayodanand Craven, which occur in the moderate to high shrink-swell class, had significantly higher CEC, comparablewith Creedmoor, Peawick, Kelly, and Iredell in the highand very high classes.

Liquid limit is the highest in the very high shrink-swellclass, intermediate in the high class, and lowest in themoderate class (Fig. 3). Liquid limit demonstrated goodcorrelation with expected shrink-swell class, yet differ-entiating the moderate from the high shrink-swell classand the high from the very high class was difficult.

Plasticity Index (% H2O)50

40

30

10

MODERATE HIGH VERYHIGH43


Fig. 4. Relationship between plasticity index and estimated shrink-swell class. Means as indicated by the same letter are not signifi-cantly different at the 0.05 level of probability.


Coefficient of Linear Extensibility0.20

0.10

0.05

0.00

MODERATE

0.03

a

HIGH VERYHIGH

0.16

0.12 :

0.14

Cecil Mayodan Creedmoor Frederick Iredell WaxpoolDavidson Craven Peawick Kelly Jackiand Carbo

Fig. 5. Relationship between COLE and estimated shrink-swell class.Means as indicated by the same letter are not significantly differentat the 0.05 level of probability.

Swelling 2:1 Clay Minerals (g kg-' soil)

500

400

300

200

100

(1

MODERATE i HIGH \ VERYHIGH

. 87

a

i i 513

432

373 i 380

175 182 175 r̂ -1

103

ba

cba

eb

cba

c

217

C

d

: 348

«te 'd

ed

e

Cecil Mayodan CreedmoorDavidson Craven

Frederickiwick Kelly

Iredell WaxpoolJackiand Carbo

Fig. 7. Relationship between swelling 2:1 mineral content and esti-mated shrink-swell class. Means as indicated by the same letterare not significantly different at the 0.05 level of probability.

Plasticity index, used widely by the geotechnical com-munity to assess shrink-swell potential, showed littlecorrelation with expected shrink-swell class (Fig. 4).However, two distinct groups, which overlap the prede-fined classes, were indicated. Kelly, Iredell, Jackiand,Waxpool, and Carbo had much higher Pis than theremainder of the soils.

Coefficient of linear extensibility and swell index aredirect measurements of shrink-swell potential (Fig. 5and 6). No discernable relationship was observed be-tween estimated shrink-swell class and COLE for the12 soils (Fig. 5). However, as with plasticity index, twodistinct groups separating soils into moderate or high(Cecil, Davidson, Craven, Creedmoor, Peawick, Freder-ick) and high or very high (Kelly, Iredell, Jackiand,Waxpool, Carbo) were observed. The other direct indi-cator of shrink-swell potential, swell index, showed ahigh correlation with estimated shrink-swell class (Fig.6). All four moderate class soils had moderate swellindices (81-153 kPa). Cecil and Mayodan, although hav-ing measured swell indices of moderate, bordered thelow class (<81 kPa). All four very high soils had averageswell indices that placed the soils in the high shrink-swell class (153-225 kPa), although Jackiand, Waxpool,

Swell Index (kPa)250

200

150

100

50

MODERATE : HIGH i VERYHIGH; 211 210

i : 187

85

ba

151

dcb

I 148 147 \

10182

aba

dcba

dcba

124

C

ba

100 i

ba

<lc

A A d

Cecil Mayodan Creedmoor Frederick Iredell WaxpoofDavidson Craven Peawick Kelly Jackiand Carbo

Fig. 6. Relationship between swell index and estimated shrink-swellclass. Means as indicated by the same letter are not significantlydifferent at the 0.05 level of probability.

and Carbo were borderline to the very high class(>225 kPa).

Swelling 2:l's (smectite, 1/2 vermiculite) had the high-est correlation with shrink-swell class, as expected (Fig.7). Smectitic Waxpool, Jackiand, and Iredell had highsmectite contents, as does the mixed mineralogy Carbo.The kaolinitic Cecil and Davidson soils had the lowestsmectite contents, whereas soils with mixed mineralogy,in both the moderate and high classes, had intermediatesmectite contents. The Creedmoor soil averaged 363 gkg"1 clay but had similar smectite content, on a whole-soil basis, to the high clay Frederick. Both had similarswell indices of =120-150 kPa, further supporting theuse of whole-soil smectite content rather than clay con-tent when estimating shrink-swell behavior.

Correlation of Shrink-Swell PropertiesAll shrink-swell indices measured were positively

correlated with each other. Swelling 2:l's, CEC, andliquid limit (indirect measures of shrink-swell potential)were significantly and positively correlated with swellindex, a direct measurement of shrink-swell potential(Table 3). Values for COLE and PI were not signifi-cantly correlated with shrink-swell properties, althoughother studies have indicated as such (Anderson et al.,1973; Franzmeier and Ross, 1968; McCormack andWilding, 1975; Schafer and Singer, 1976). The lack ofcorrelation of COLE and PI in this study may be dueto Al interlayering or high Fe oxide coating of the claysinhibiting swelling.

Expansive Soil IndexThe absolute values of the four soil indices most corre-

lated with predicted shrink-swell potential in this studywere swelling 2:l's, CEC, swell index, and liquid limit.Thus, these soil indices were summed into an ExpansiveSoil Index (ESI) given by the following equation:

ESI-1 = swelling 2:l's + swell index + liquidlimit + CEC

The ESI-1 ratings >500 indicate high and very high


Table 2. Physical, chemical, and mineralogical data of control sections (Bt horizons) and (ESIs) for each profile in the map units.Liquid Plasticity Swell Swelling

Map unit Profile Clay CECf limit index COLE$ index 2:l's ESI-1§ ESI-2§ ESI-3§

Frederick

Carbo

Cecil

Iredell

'soil kPa g kg~' soil

759790375713704662

569677506681712665

11.513.55.4

25.023.028.5

7.09.19.0

17.020.015.0

676840857381

606157707370

Valley and Ridge9

167

391629

Piedmont171819374235

0.050.040.020.080.050.03

0.030.030.030.090.110.07

162102107214254136

769484

265159138

220280150525565450

6075

125465280300

Triassic Basin (diabase, metamorphic shale)

461464302849915696

203239275817532523

241184152324350246

143164150352223223

798245

11096

110

677066878585

Jackland

Waxpool

Davidson

Kelly

123123123123

688679662680641525662690679709589677

28.026.724.329.027.622.69.011.08.037.933.412.8

659082767261535756714571

374844403917171916383339

0.140.160.170.180.120.120.040.050.040.110.100.14

17124022325422814815515914010865128

3804203404754553659512590405325390

644777669834783597312352294622468602

264357329359328232217227204217143212

93117106105100846268641097884

Triassic Basin (sandstones/shales)

Creedmoor

Mayodan

Craven

Peawick

123123

123123

368182539282383541

356486418348648572

19.55.126.913.014.424.6

9.513.413.918.325.821.6

615667615157

475953365858

17931241317

Coastal Plain10171772021

0.050.010.090.020.050.08

0.020.040.030.100.040.08

76178189829568

10212576141178121

185115225170180175

125250170165200230

342354508326340325

284447313360462431

157239283156160150

159197143195262201

816194746582

577267548480

t Cation-exchange capacity.I Coefficient of linear extensibility.§ ESI-1 = swelling 2:l's + swell index + liquid limit + cation-exchange capacity (CEC); ESI-2 = swell index + liquid limit + CEC; ESI-3 = liquid

limit + CEC.

shrink-swell potential (Table 4) and would require spe-cial design of foundations, such as adding reinforcingbars to footings or installing moisture barriers, to de-crease potential expansive soil damage. An ESI-1 <500describes soils with moderate to high shrink-swell po-

Table 3. Correlation coefficients for soil properties.

CECtSwelling 2:l'sPlasticity indexLiquid limitSwell indexCOLE*

Clay

0.360.490.520.610.410.41

CEC

0.70*0.630.450.320.65

Swelling2:l's

0.530.72*0.89*0.61

Plasticityindex

0.79*0.490.76*

Liquidlimit

0.600.71

Swellindex

0.51

* Significant at the 0.05 level of probability.t Cation-exchange capacity.t Coefficient of linear extensibility.

tential. Special design features are suggested to reduceshrink-swell risk, although the design of such featuresmay not be as extensive as required for foundationsconstructed on higher shrink-swell potential soils.

Identifying and quantifying swelling 2:1 minerals isdifficult, time-consuming, and expensive to routinelymeasure. Only a few laboratories are equipped to makethese types of quantifications. Table 3 showed that highvalues of swell index, liquid limit, and CEC corre-sponded with high amounts of swelling 2:1 clay minerals.Thus, an alternative ESI, termed ESI-2, was proposedthat used only the absolute values of swell index, liquidlimit, and CEC. The ESI-1 and ESI-2 were highly corre-lated (R2 = 0.92) and gave similar shrink-swell riskseparations of the soils (Table 4). However, separationswere not as distinct as when swelling 2:l's are in the


Table 4. Expansive soil indices (ESI).

Soil series

CecilDavidsonMayodanCravenCreedmoorFrederickPeawickKellyIredelllacklandWaxpoolCarbo

ESI-lt239319330348401409418564624697738820

ESI-2J

152216155166226192219191276317306307

ESI-3§

686574657968739088

10596

105

Shrink-swell risk

Moderate

Moderately

High

Very high

high

Table 5. Shrink-swell potential risk of soils as related to parentmaterial.

Shrink-swell risk Parent material Soil series

t Swelling 2:l's + swell index + liquid limit + cation-exchange capacity(CEC).

$ Swell index + liquid limit + CEC.§ Liquid limit + CEC.

equation. An ESI-2 of =250 did appear to separate soilsof high risk from soils of very high risk. Demarcationof moderate risk soils from high risk was not as distinct.Thus, conservative estimates of shrink-swell potentialare recommended. Although swell index is an easy pa-rameter to measure, there is little swell index (PVC)data in soil survey databases. However, extensive dataon liquid limits and CEC are contained in many soilcharacterization databases. Thus, a third expansive soilindex, ESI-3, can be formulated using only liquid limitand CEC as shrink-swell predictors. Fewer categoriesof estimated shrink-swell risk would be entailed whenusing this index.

We now have developed three ESIs, each requiringa different level of data input and applying a differentlevel of shrink-swell predictability. What ESI ratingshould be employed for various intensities of site assess-ment? We suggest the following guidelines:

ESI-3 — Liquid Limit and Cation-Exchange Capac-ity. Employed when general information is needed, suchas performing feasibility studies for a proposed subdivi-sion or highway, ESI-3 would be sufficient to screensuitable areas from unsuitable areas. Liquid limit andCEC are indirect indicators of shrink-swell potential.

ESI-2 — Swell Index, Liquid Limit, and Cation-Exchange Capacity. Used when site-specific informa-tion is needed, application of ESI-2 would include suit-ability of a site for home foundations or on-site waste-water disposal. Swell index is a direct measure ofswelling pressure.

ESI-1 — Swelling 2:l's, Swell Index, Liquid Limit,and Cation-Exchange Capacity. The ESI-1 rating wouldbe used when data is needed in litigation court cases orwhen additional information is required for foundationsor other structures designed to reduce potential damagefrom shrink-swell soils.

Parent Material Correlationwith Shrink-Swell Indices

Many other soils are formed from the same or similarparent materials as the soils described in this study.Thus, it is possible to extrapolate these data and ESIratings for similar soils on the basis of parent material.

Very high

High

Moderately high

Moderate

Mafic rocks Jacklandpoorly drained to Waxpoolmoderately well drained Iredell

Carbonate rocksshallow to rock Carbo

Triassic rocksthermally altered shales Kelly

Carbonate rocksdeep to rock Frederick

Triassic rocks Mayodanshales and sandstones Creedmoor

Coastal Plain sediments Cravenclayey fluvial/marine Peawick

Mafic rockswell drained Davidson

Felsic rockswell drained Cecil

Soils with restricted drainage formed from mafic rocks,soils formed from metamorphic shales, and shallow soilsformed from carbonate parent material are at very highrisk for expansive soil behavior (Table 5). Moderatelyhigh to high risk was assumed in soils formed frommetamorphic shales, soils derived from Triassic sand-stones and shales, deep soils formed from carbonaterocks, and Coastal Plain clayey fluvial and marine sedi-ments. Moderate risk can be correlated with soilsformed from well-drained felsic and mafic parent ma-terials.

CONCLUSIONSExpansive soils cause billions of dollars of damage to

homes and property in the USA each year. Damagecan be avoided or mitigated if the propensity of a soilto shrink and swell is known before construction. Pre-dicting shrink-swell potential accurately requires boththe knowledge of which soil properties influence shrink-ing and swelling and the magnitude of these parameters.Recognizing the need for quantitative soil information,we developed an expansive soil rating system to assessshrink-swell potential of 12 clayey soils formed frommajor parent materials that occur throughout Virginia.Expansive Soil Indices are obtained by summing theabsolute values of swelling 2:1 minerals, swell index,liquid limit, and CEC. Three levels of precision areavailable, with highest precision obtained with all fourproperties. Intermediate precision is available by usingswell index, liquid limit, and CEC. Lower precision, butrapid assessment, can be achieved by using only liquidlimit and CEC. The ESI system has the flexibility ofallowing for the classification of other soils formed fromsimilar parent materials, enabling in rapid, quantitativeassessment of shrink-swell potential of a soil.


tors affecting the coefficient of linear extensibility in Vertisols. SoilSci. Soc. Am. Proc. 37:298-299.

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An Expansive Soil Index for Predicting Shrink–Swell Potential