Classification Procedures For Expansive Soils
Classification procedures forexpansive soilsA. Sridharan, PhD, DSc and K. Prakash, PhD&Most of the national codes of practice donot give characterization and classi cationof expansive soils, in spite of expansivesoils being distributed very widely overalmost all geographical locations in theworld, causing distress to the structuresfounded on them and discomfort to theusers. A simple user-friendly approachbased on the free swell ratio, de ned asthe ratio of the sediment volume of soil indistilled water to that in carbon tetrachloride or kerosene, is formulated considering the compatibility of the resultswith oedometer free swell tests and thesoil clay mineralogy. Statistical illustrations are provided which clearly indicatethat while the assessment of soil expansivity based on index properties is anoverestimation, there is a consistency inthe classi cations based on oedometer testresults and the proposed approach. It isrecommended that simple approaches suchas the one proposed here to classify theexpansive nature of soils are entered intostandard codes of practice.Keywords: geotechnical engineeringIntroductionA soil classi cation is a systematic method ofcategorizing soils into various groups andsubgroups according to their probable engineering behaviour but without detailed descriptions. Most of the earlier classi cation systemswere based on grain size distribution (e.g. MITclassi cation) and soil texture (e.g. texturalclassi cation). However, with the introductionof Atterberg limits, new classi cation systemshave come into existence.2. It is possible to trace many soil classi cation systems such as the Casagrande uni edsoil classi cation system (USCS), the AmericanAssociation of State Highway and Transportation O cials (AASHTO) system, the pedologicsoil classi cation system, Federal AviationAgency (FAA) system, US public roads administration (PRA) system, and the textural classi cation system. Currently, the USCS and theAASHTO system are in use in civil engineeringpractice.3. The USCS was originally developed byCasagrande 1 and was later modi ed by the USBureau of Reclamation (USBR) and the USArmy Corps of Engineers, to enhance itsapplicability to many more elds. Manynational standard codes of practice such asASTM designation D2487-93, 2 BS 5930 3 and IS1498 4 follow this modi ed version of the USCSas it stands or with slight modi cation.4. Both systems, namely modi ed USCS andAASHTO, base their classi cation of soils forengineering purposes on particle size characteristics, liquid limit (w L ) and plasticity index(I p ) of soils. The subgrouping of coarse-grainedsoils is done with the help of parameters suchas uniformity coe cient (C u ) and coe cient ofcurvature (C c ) to account for the gradation ofsoils. The subgrouping of ne-grained soils isentirely based on a plasticity chart (i.e. I pplotted against w L ). In addition, some codes ofpractice give some useful criteria that need tobe followed to obtain a rough estimation of soilcharacteristics such as angularity (of coarsegrained soils), moisture content, consistency,cementation, dry strength, plasticity, dilatancy,toughness, organic content (e.g. ASTM designation D2488-93, 5 IS 1498 4 ). However, apartfrom IS 1498, 4 these systems do not have thecriteria to assess the expansivity of the soil.Proc. InstnCiv. EngrsGeotech. Engng,2000, 143, Oct.,235 240Paper 12075Written discussioncloses 21 December2000Manuscript received8 June 1999;revised manuscriptaccepted 5 June 2000Soil expansivity5. Expansive soils are found extensively intropical areas. The presence of expansive soilsgreatly a ects the construction activities inmany parts of south-western United States,South America, Canada, Africa, Australia,Europe, India, China and the Middle East. 6More and more expansive soil regions are beingdiscovered each year with an increase in theamount of constructional activities, particularlyin the underdeveloped nations. These soils arecharacterized by the presence of a large proportion of highly active clay minerals of themontmorillonite group which are responsiblefor the pronounced volume change capability ofthe soils. A number of soils such as volcanicash soils 7 and diatomaceous earth 8 cannot begrouped by using the existing classi cationsystem only; however, their distribution isrestricted to certain areas. Unlike these, theexpansive soils are distributed geographicallyvery widely, covering large areas. Hence, identi cation and classi cation of such soils isessential.6. Many criteria are available to identifyand characterize expansive soils, such as liquidlimit (Table 1), plasticity index (Table 2),shrinkage limit (Table 3), shrinkage indexA. Sridharan,Professor of CivilEngineering,Department of CivilEngineering, IndianInstitute of Science,BangaloreK. Prakash, AssistantProfessor,Department of CivilEngineering, SriJayachamarajendraCollege ofEngineering, Mysore235
SRIDHARAN AND PRAKASHTable 1. Soil expansivity prediction by liquid limitDegree ofexpansionLowMediumHighVery highsoil. This does not account for variations ofdensity.9. IS 1498 4 gives a criterion to predict theexpansivity of soils, based on the free swellindex. 16wL: %Chen6IS 1498 453030 4040 6046020 3535 5050 7070 90FSI Table 2. Soil expansivity predicted by plasticity indexDegree ofexpansionLowMediumHighVery highIp : %Holtz and Gibbs52012 3423 4543210Chen 6IS 1498 40 1510 3520 5543551212 2323 32432(Table 3), free swell index (FSI) (Table 3),activity and per cent free swell.4,9 147. Chen 6 observed that there was no conclusive evidence of the correlation betweenswelling potential and shrinkage limit. Sridharan and Prakash 15 have also shown that theshrinkage limit can not be satisfactorily used topredict the swell potential of a soil and that themechanisms governing the shrinkage and swelling are entirely di erent.8. Holtz and Gibbs 10 proposed the per centfree swell test. It consists of pouring slowly10 cm 3 of oven dried soil (passing a 425 mmsieve) into a 100 cm 3 measuring jar lledwith distilled water and noting the volumeof the soil after it comes to rest at thebottom of the jar. The free swell is thenreported as the increase in the volume of thesoil expressed as a percentage of the initialvolume. The major drawback of this method,which is crude, 6 is that measuring 10 cm 3 ofsoil is not that easy and the proceduretherefore introduces personal judgement asone more factor. It is normal to quantify10 cm 3 as the volume occupied by 10 g ofVd ÿ Vk 100Vk1 where V d is the sediment volume of 10 g ofoven dried soil passing a 425 mm sieve placed ina 100 ml graduated measuring jar containingdistilled water, and Vk is the sediment volumeof 10 g of oven dried soil passing a 425 mmsieve placed in a 100 ml graduated measuringjar containing kerosene.10. However, this method gives negativefree swell indices for kaolinite-rich soils andmay underestimate the expansivity of montmorillonitic soils, if the soils contain a signi cant amount of kaolinite clay material. Toeliminate this di culty, Sridharan et al. 17 havede ned the modi ed free swell index (MFSI) asthe ratio of equilibrium sediment volume (V d )after 10 g of oven dried soil is mixed thoroughly with the distilled water to form a soil water suspension of 100 ml initial volume in a100 ml measuring jar and allowed to settle, tothe dry weight of the soil. ThusMFSI Vd102 11. Sridharan et al. 18 have observed that thesediment volume occupied by unit weight ofdry soil in distilled water together with thatin carbon tetrachloride provides useful information about the soil expansivity and natureof the soil typeÐexpansive/non-expansive/combination of both (Table 4).12. The predictive capability of the MFSI isevident from the following statistical illustrations. About 32 soils from various parts ofIndia, 16 of them kaolinitic (25% 4 w L 4 100%)and the remaining 16 montmorillonitic(47% 4 w L 4 124%), were considered for theanalysis (data from Sridharan et al. 19,20 ). Thesesoils are placed on the plasticity chart as shownin Fig. 1. It can be noted that both the kaoliniticTable 3. Soil expansivity predicted by other measuresDegree ofexpansionColloidcontent 10 :% minus0 001 mmShrinkagelimit 10 : %Shrinkageindex 4 : %Free swellindex 4 : %Per centexpansion inoedometer*as per Holtzand Gibbs 10LowMediumHighVery high51712 2718 374274138 186 1251051515 3030 6046055050 100100 200420051010 2020 30430* From dry to saturated condition under a surchare of 7 kPa.{ From compacted, saturated condition under a surcharge of 7 kPa.Note: Shrinkage index (plastic limit 7 shrinkage limit).236Per centexpansion inoedometer{as per Seedet al. 120 1 51 5 5 05 25425
CLASSIFICATIONPROCEDURES FOREXPANSIVE SOILSTable 4. Soil expansivity classi cation based on MFSI 18MFSI: cm 3 /gSediment volume incarbon tetrachloride:cm 3 /g51 51 5 2 01 5 2 02 0 4 044 01 10 3 0041 1 and 5MFSI41 141 141 1Clay typeSoil expansivityNon-swellingMixture of swelling and oderateHighVery High11·1%Montmorillonitic soils10011·1%11·1%11·1%Kaolinitic soils44·4%A-line80Plasticity index: 080Liquid limit: %10055·6%Medium120Fig. 1. Position of the soils analysed on theplasticity chartand montmorillonitic soils lie above and belowthe A-line. Hence, nothing can be inferredregarding their expansive nature just by theirposition on the plasticity chart.AnalysisÐphase 113. In the rst phase of analysis, 18 soils 19(nine kaolinitic and nine montmorillonitic) areconsidered. The degree of expansivity of thesesoils has been assessed based on their liquidlimit, 6 plasticity index, activity, 14 percentageswell in the oedometer test 10 and MFSI according to Table 4. Winterkorn and Fang 21 andChen 6 suggest that the most useful and reliableassessment of swelling capabilities for expansive soils can be obtained from conventionaloedometer tests. According to USBR, 10 thecriterion for expansiveness of a soil is the totalvolume change of a soil from air dry to asaturated condition under a surcharge of 7 kPa,in an oedometer. Hence, the results obtainedfrom oedometer swell tests conducted on air drysoils assessed as per Table 3 are taken as areference point.14. Figure 2 shows the comparison of suchan assessment for kaolinitic soils. While theoedometer tests assign a low degree of expansivity for those soils true to their mineralogy(note that kaolinite is a non-expansive claymineral), the criteria based on liquid limit,plasticity index and activity show a largepercentage of the non-expansive soils to have a(d)(e)low to very high degree of expansivity. Plasticity index, activity and liquid limit predict ahigh to very high degree of expansivity inabout 55 5, 66 6 and 77 8% of cases respectively. However, the evaluations based on theMFSI given in Table 4 are in better agreementwith those from the oedometer tests.15. Figure 3 illustrates the comparison ofthe evaluation of the degree of expansivity ofmontmorillonitic soils by the di erent criteria.It can be noted that liquid limit, plasticity indexand activity greatly overestimate the soilexpansivity in comparison with those observedin oedometer tests. As Chen 6 quotes, while itmay be true that a high-swelling soil willmanifest a high index property, the converseneed not be true. On the other hand, the22·2%22·2%Fig. 2. Prediction ofsoil expansivity ofkaolinitic soils by:(a) liquid limit;(b) plasticity index;(c) activity;(d) oedometer test;and (e) MFSIFig. 3. Prediction ofsoil expansivity ofmontmorillonitic soilsby: (a) liquid limit;(b) plasticity index;(c) activity;(d) oedometer test;and (e) high11·1%Medium22·2%22·2%(d)(e)237
SRIDHARAN AND PRAKASHTable 5. Proposed expansive soil classi cationOedometerper centexpansion*Free swell ratio511 55 1515 2542541 01 0 1 51 5 2 02 0 4 044 0Clay typeSoil expansivityNon-swellingMixture of swelling and oderateHighVery high* From air dry to saturated condition under a surcharge of 7 kPA.predictions of soil expansivity based on theproposed MFSI are again quite satisfactory.16. It is not untimely to mention that themechanisms controlling the liquid limit ofkaolinitic and montmorillonitic soils areentirely di erent from each other. While theliquid limit of a kaolinitic soil is controlled bythe particle arrangement and the interparticleattractive forces, that of a montmorillonitic soilis controlled by the double layer thickness. 20,22Hence, higher liquid limits do not necessarilymean expansive montmorillonitic soils. This isthe reason why the liquid limit and the relatedindex properties, without any consideration ofclay mineralogy, cannot satisfactorily indicatethe soil expansivity. Instead, they may give analtogether di erent and wrong picture.17. It has been observed by the authors thatthere are instances wherein the values ofsediment volumes, obtained from free swelltests, in water and carbon tetrachloride do notfall into any of the categories listed in Table 5,thus resulting in ambiguity. This limitation canbe avoided by the use of the ratio of equilibrium sediment volume of 10 g oven dried soilpassing a 425 mm sieve in distilled water to thatin carbon tetrachloride, which is de ned hereinas free swell ratio'. It is the authors' experiencethat while dealing with the Indian black cottonsoils, the expansion predictions by the oedometer test proposed by Holtz and Gibbs 10 areslightly underestimated. With this experience,considering the clay mineralogy of the soils, theauthors propose a criterion for classifying thesoils based on their expansive nature as shownin Table 5.18. Figure 4 indicates the statistical predictions of soil expansivity of 18 soils, consideredin the rst phase of the analysis, by theproposed classi cation based on expansion fromoedometer tests and free swell ratio. It can beobserved that the predictions based on the twomethods proposed match very well and also thatthe predictions based on the proposed free swellratio (i.e. Fig. 4) are in line with those from theconsideration of Table 4 (i.e. Figs 2 and 3).AnalysisÐphase 219. Seed et al. 12 have also proposed anexpansivity classi cation based on per cent238Modified percentexpansion11·1%Free swell 22·2%22·2%11·1%55·6%(b)NegligibleHighLowVery high44·4%Moderateswelling potential observed by compacted soilsat maximum dry density and optimum moisturecontent conditions, under a surcharge of 7 kPa(Table 3). In order to prove the enhancedcapability of the proposed free swell ratio inclassifying the soils, the second phase ofanalysis is done as follows.20. Fourteen soils, 23 seven kaolinitic andseven montmorillonitic, compacted at standardProctor's maximum dry density and optimummoisture conditions, were allowed to swellunder a surcharge of 7 kPa. Using the criteria ofSeed et al. 12 and that based on the proposed freeswell ratio, the 14 soils are classi ed withrespect to their expansive nature; the resultsare shown in Fig. 5. A very good correlation isobserved between the predictions from twoentirely di erent testing procedures whichindicates the acceptability of the free swellratio for predicting the soil expansivity.21. The determination of sediment volumeof highly expansive soils in distilled water canbe problematic, as such soils take a prohibitively long time to settle. This di culty can beFig. 4. Prediction ofsoil expansivity byproposed oedometerper cent expansionand free swellratio criteria for:(a) kaolinitic soils; and(b) montmorilloniticsoils
CLASSIFICATIONPROCEDURES FOREXPANSIVE SOILSFree swell diment volume in 0·025% NaCI solution: cm3Oedometer (compactedstate)4234261842·9%101045 182634Sediment volume in water: cm3(b)HighModerateVery highFig. 5. Prediction of soil expansivity of soilsfrom the data of soils compacted at optimumconditions and from free swell ratio for:(a) kaolinitic soils; and (b) montmorillonitic soilsovercome by using 0 025% NaCl solutioninstead of distilled water, which reduces thesettling time markedly without a ecting the nal equilibrium sediment volume (Fig. 6). Ithas been observed 23 that the soils have takenabout 24 216 h to reach equilibrium in waterdepending upon the type of soil. However, thesame soils took about 4 24 h to reach theequilibrium in 0 025% NaCl solution. Likewise,kerosene is a more easily available, cheapernon-polar liquid which is relatively less volatilethan carbon tetrachloride. The equilibriumsediment volumes obtained in these two liquidsare almost the same (Fig. 7), with a maximumerror of 5%. Hence, use of kerosene instead ofcarbon tetrachloride is suggested, owing to itsgreater accessibility.Conclusions22. The previously described analysis hasclearly indicated that the index properties suchas liquid limit, plasticity index and relatedparameters cannot satisfactorily predict the soilexpansivity, as they do not consider the e ectof clay mineralogy. The free swell ratioapproach, in addition to predicting the soilexpansivity more realistically and satisfactorily, gives additional information about thenature of the clay mineralogy of soils.23. The free swell index test discussed inthis paper involves a very simple, user-friendlytesting procedure, requiring negligible instrumental sophistication. The test results in theform of free swell ratio proposed can be used to14Sediment volume in kerosene: cm3Low42Fig. 6. Comparison ofequilibrium sedimentvolumes in 0 025%NaCI solution withthose in distilledwater 23121045 88101214Sediment volume in carbon tetrachloride: cm3Fig. 7. Comparison ofequilibrium sedimentvolumes in kerosenewith those in carbontetrachloride 23obtain quite reliable information about thedegree of soil expansivity and about the soiltype (Table 5). As the sediment volumes in thenon-polar liquids carbon tetrachloride and kerosene are almost equal, the applicability of thetest can be further enhanced by using commonly available kerosene in place of carbontetrachloride. The authors feel that the inclusion of such a simple test to evaluate theexpansive nature of the soils in standard codesof practice of various national organizationswould be useful.References1. C A S A G R A N D E A. Classi cation and identi cationof soils. Transactions of ASCE, 1948, 113, 901 992.2. A M E R I C A N S O C I E T Y F O R T E S T I N G A N D M A T E R I A L S .Standard Classi cation of Soils for EngineeringPurposes (Uni ed Soil Classi cation System).ASTM Designation D2487-93. Annual Book ofASTM standards, 1995, West Conshohocken,4.08.3. B R I T I S H S T A N D A R D S I N S T I T U T I O N . British Code of239
SRIDHARAN AND PRAKASH4.5.6.7.8.9.10.11.12.13.Practice for Site Investigations. BSI, London, 1981,BS 5930.B U R E A U O F I N D I A N S T A N D A R D S . Indian StandardClassi cation and Identi cation of Soils forGeneral Engineering Purposes. BIS, New Delhi,1970, (Rea rmed 1987), IS 1498.AM E R I C A N SO C I E T Y F O R TE S T I N G A N D MA T E R I A L S .Standard Practice for Description and Identi cation of Soils (Visual Manual Procedure). ASTMDesignation D 2488-93. Annual Book of ASTMstandards, 1995, West Conshohocken, 4.08.C H E N F. H. Foundations on Expansive Soils.Elsevier, Amsterdam, 1975.S U Z U L K I A. and K I T A Z O N O Y. Engineering classi cation of volcanic cohesive soils. In Report onVolcanic Ash Soils in Japan: Properties andPractical Use. 1998, pp. 39 43.N A G A R A J T. S., O N I T S U K A K., T A T E I S H I Y. andH O N G Z. Is diatom earth a collapsible material?Proceedings of the International Symposium onProblematic Soils, Sendai, Japan, 1998, vol. 1, 257 260.A L T M E Y E R W. T. Discussion on engineeringproperties of expansive soils. Transactions ofASCE, 1956, 121, 666 669.H O L T Z W. G. and G I B B S H. J. Engineering properties of expansive clays. Tran
(Table 3), free swell index (FSI) (Table 3), activity and per cent free swell.4,9–14 7. Chen6 observed that there was no con-clusive evidence of the correlation between swelling potential and shrinkage limit. Srid-haran and Prakash15 have also shown that the shrinkage limit can not be satisfactorily used to
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Estimation 1. Introduction Light structures, such as highways and railroads, built on expansive soils are prone to damages from the swelling of their underlain soil layers (Nelson and Miller, 1992; Pedarla et al., 2011). Hence, it is important to evaluate the total volume change (swelling or shrinkage) potentials of these soils.
3. Design of Micropiles in Expansive Soils Micropiles have been used to underpin foundations since the early 1950s and they are increasingly being used for underpinning foundations experiencing heave due to expansive soils. Despite their increasing usage, there is a lack of published literature regarding micropile design,
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Some clay soils are well structured and relatively easy to manage. Sodic clay soils, by contrast, are more likely to be poorly structured and difficult to manage. What are sodic soils? Sodic soils are said to be 'dispersive' which means they tend to lose their structure when wet by rain water. (The cloudiness of
