Scientia Iranica A (2014) 21(4), 1231{1240

Sharif University of TechnologyScientia Iranica

Transactions A: Civil Engineeringwww.scientiairanica.com

Study on the failure behavior of three di�erentstabilized problematic soils

E. A akia;�, P. Sedighib and A. Eslamia

a. Department of Civil Engineering, Amirkabir University of Technology, Tehran, Iran.b. Department of Civil Engineering, Central Teharn Branch, Islamic Azad University, Tehran, Iran.

Received 22 July 2012; received in revised form 4 May 2013; accepted 29 April 2014

KEYWORDSFailure criteria;Cement treatment;Shear strength;Soaked condition;Problematic soils.

Abstract. In this study laboratory testing of e�ectiveness of cement treatment has beenmade on geotechnical parameters of problematic soils encountered in southern coast line ofCaspian Sea, Iran. Gorgan loess, Rasht clay, and Anzali sand were selected in this research.Addition of cement was found to improve workability and increased uncon�ned compressivestrength, and elastic modulus of soils signi�cantly. Triaxial test results indicated thatcement treatment not only improved shear strength remarkably, but also it changed thetype of failure greatly from ductile to brittle behavior. The large scale direct shear testresults showed signi�cant improvement in shear strength and shear modulus. Besides, thebrittle behavior of cement treated samples was observed. Eventually, it was found thatthe trend of failure envelope of cement treated samples was non-linear, and some failurecriteria such as modi�ed Gri�th theory, Hoek-Brown theory, and the Johnston criterioncan describe the soil cement behavior satisfactorily.

c 2014 Sharif University of Technology. All rights reserved.

1. Introduction

In recent years, due to population growth, suitable landfor construction has been hard to locate. For improvingand optimum use of available soils, great competitionamong civil engineers has been created. Distributionof problematic soils, those with high moisture andlow e�ciency pose a lot of di�culties to constructionprojects. All improvement techniques seek an increasein density and shear strength, providing stable con-dition, reduction of soil compressibility and controlground water ow, or increasing the rate of consoli-dation [1]. The soil-cement technique has been usedsuccessfully in pavement base layers, slope protectionfor earth dams, as a base layer to shallow founda-

*. Corresponding author.E-mail addresses: ea aki@aut.ac.ir (E. A aki);sedighi.p.eng@gmail.com (P. Sedighi); afeslami@aut.ac.ir(A. Eslami)

tions and to prevent sand liquefaction [2-5]. Manyresearchers have studied e�ciencies of soil-cement [6-9]. A recent study has made an extensive laboratorytesting of cement treatment e�ects on shear strengthparameters and also estimating failure envelope ofsoils encountered in southern coastline of CaspianSea.

2. Testing soils

In this study, three di�erent soils including Gorganloess, Rasht clay and Anzali sand from southerncoastline of Caspian Sea, have been selected as shownin Figure 1, while the sampling procedure has beenconducted in the shallow depths. Soils in the northernparts of Iran are mainly classi�ed as clay, silt, loess andsand. Due to the unique geological condition, waterlevel in these areas is high and soils are mostly satu-rated. Table 1 presents a summary of the geotechnicalproperties of these soils.

1232 E. A aki et al./Scientia Iranica, Transactions A: Civil Engineering 21 (2014) 1231{1240

Figure 1. Three di�erent zones in southern coast line ofCaspian Sea.

Table 1. Geotechnical properties of the testing soils.

Soilproperties

Rasht Anazli Gorgan Standardmethods

Soil group CL SP CL-ML ASTM D422-63

Passingno. 200

83 1.5 98.5 ASTM D422-63

Speci�cgravity

2.72 2.68 2.7 ASTM D854

LL (%) 48 | 25 ASTM D4318

PL (%) 26 NP 20 ASTM D4318

PI (%) 22 | 5 ASTM D4318

In situdensity(g/cm3)

1.89 2.08 1.63 ASTM D1556

Naturalwater

content (%)30 14 18 ASTM D2216

Maximumdry

density (g/cm3)1.58 1.85 1.7 ASTM D698

Optimumwater

content (%)22 7.5 14 ASTM D698

3. Laboratory testing methods

Laboratory works were carried out on the mixtureof soils with di�erent percentages of Portland cementtype II by 2.5, 5 and 8% of dry weight of soil.Laboratory tests such as standard proctor, Uncon�nedCompressive Strength (UCS), Consolidated-Drainedtriaxial (CD), and large-scale direct shear tests wereperformed on both non-treated and cement treated

samples. Soil-cement mixtures were tested to reach anoptimum ratio [10-14].

To determine the optimum moisture content andmaximum dry density, series of standard proctor testswere carried out on non-treated and cement treatedsamples of three types of soils, based on ASTMD558 [15] and ASTM D698 [16] standards.

Uncon�ned compressive strength test is the mostcommon test in soil stabilization and plays a great roleas an index for quantifying the soil improvement thathas been carried out on the specimens based on ASTMD2166 [17]. Treated specimens were also prepared atan optimum moisture content and 95% of maximumdry density for each mixture of 2.5, 5 and 8% cementcontent by weight, using split mold with inner diameterand height of 5 and 10 cm, respectively. Materials werecompacted in the molds in 7 equal layers to achievedesirable density, and it was carefully tried to preparethe most uniform specimens and less disturbance whilebringing them out of the mold. For cohesive soils,hydraulic jacks were used to bring the samples outof the mold. However, for non-treated Anzali sandsplitting the mold without disturbing the specimen wasnot possible. The prepared specimens divided intotwo series. One series of specimens was wrapped ina plastic sheet and kept for 7 days in a controlled roomtemperature. The second series was kept for 3 daysin a humid room at a temperature of 25 degrees andthen was immersed in water under soaked conditionfor 4 days before testing. It should be noted that thesoaked condition was not applicable for the third seriesof non-treated specimens.

For a detailed analysis of shear strength of bothnatural and cement stabilized specimens, series ofConsolidated-Drained triaxial tests (CD) were alsoconducted on all three types of samples. Con�ningpressure varied from 50 to 500 kPa. The diameterand height of the split mold were 3.8 and 7.6 cm,respectively. The specimens were then embedded ina triaxial chamber and backpressure was applied untilsaturation was reached. Specimens were consolidateduntil the height of water in burette did not rise.Finally, specimens were sheared at a deformation rateof 0.08 mm per minute (strain controlled condition).B values are dependent on the compressibility of soils.Cement treatment decreases the compressibility of soilssigni�cantly. B value for cement treated sand wasobtained to be about 0.7, was equal to 0.8 for cementtreated loess and clay soils, and 0.9 for both non-treated loess and clay.

Direct shear test was carried out according to theASTM D3080 [18] as a standard procedure for soiltest under consolidated drained condition. Large scaledirect shear apparatus with a 300 � 300 � 150 mmshear box was used in this study. This test was carriedout on the non-treated and cement treated (5 and

E. A aki et al./Scientia Iranica, Transactions A: Civil Engineering 21 (2014) 1231{1240 1233

8%) sandy soil only. Materials passed sieve No 3=400

and specimens were prepared at 95% of maximum drydensity and optimum water content in a steel split mold(300� 300� 100) and cured for 5 days, then placed ina shear box for 48 hours in water to get full saturation.Vertical pressure was applied for 5 hours and thenspecimens were sheared at the rate of 0.5 mm/min.Vertical pressure varied from 50 to 200 kPa.

4. Test results and discussion

4.1. Uncon�ned Compressive Strength (UCS)The e�ect of cement treatment on uncon�ned stress-strain behavior of clay soil for unsoaked and soakedconditions is shown in Figure 2. It is observedthat the peak axial stress increases signi�cantly dueto cement treatment, but the corresponding straindecreases from approximately 3.5% to 1.5%. E�ectof cement on uncon�ned compressive strength andmodulus of elasticity is shown in Figure 3 for bothsoaked and unsoaked condition. It is observed thatthe soaked samples with 8% cement content exhibitgreater uncon�ned compressive strength compared tothe unsoaked samples, but the modulus of elasticity forunsoaked samples is greater compared to the soakedsamples. Table 2 presents a summary of uncon�nedcompressive strength and modulus of elasticity of theall 3 types of soils for both the unsoaked and soakedconditions.

4.2. Consolidated-Drained triaxial test (CD)Deviator stress versus axial strain, and volumetricstrain versus axial strain curves for non-treated and8% cement treated Gorgan loess at di�erent con�ningpressure are presented in Figure 4. The stress-strainand volume change of non-treated loess typically showsductile behavior. This was also observed in the bulging

type failure. It can be seen that deviatoric stressincreases with an increase in con�ning pressure andcement addition. By addition of 8% cement, peakdeviator stress occurred at about 1.5-2% strain incre-ment and progressive softening observed up to 15%

Figure 2. E�ect of cement treatment on stress-straincurves of Rasht clay: a) Unsoaked; and b) soaked.

Table 2. Summary of uncon�ned compressive strength test and modulus of elasticity of non-treated and cement treatedsoils.

Soiltype

Cementcontent (%)

UCS (kPa) E (MPa)Unsoaked Soaked Unsoaked Soaked

Gorganloess

0 87.6 0 5.7 02.5 515.6 151.4 42.8 11.85 1480.6 1166.1 93.2 688 2137.4 1816.4 163.3 108.9

Rashtclay

0 112.3 0 4.3 02.5 367.8 75 15 05 1299.7 1032.6 76.1 40.38 1730.8 1919.5 99.3 75.2

Anzalisand

0 5 0 0 02.5 103.4 78.1 9 5.15 909.3 715.2 63.5 50.58 1603.6 1494.7 102 87.7

1234 E. A aki et al./Scientia Iranica, Transactions A: Civil Engineering 21 (2014) 1231{1240

Figure 3. E�ect of cement treatment on Rasht clay: a)Uncon�ned compressive strength; and b) modulus ofelasticity.

axial strain. These cement treated specimens failedwith either single or double shear band (Figure 5).For specimens with 8% cement, failure strain wasabout 1.2-1.5% and specimens failed with splitting typeat low con�ning pressure, and with planar type athigh con�ning pressures. Volumetric strain-axial straincurves showed a behavior similar to Over-Consolidated(OC) soils. Initial compression up to the failurepoint and then expansion were observed up to 15%axial strain. Results of triaxial tests on Anzali sandand Rasht clay followed the pattern of Gorgan loess.The strength parameters of all specimens which wereobtained from Mohr circles and failure envelope arepresented in Table 3. Cement treatment increasedthe cohesion parameter remarkably, but friction angleincreased initially and then decreased (or remainedconstant) by increasing the cement content.

4.3. Large scale direct shear testShear stress versus horizontal displacement curvesfor non-treated and 5 and 8% cement treated sandysamples for vertical pressures equal to 200 kPa areillustrated in Figure 6. It is observed that peakshear stress increases signi�cantly due to the cementtreatment and progressive softening observed after-ward, but the corresponding shear strain decreasedfrom approximately 3.5% (displacement = 10.5 mm)to 1.7% (displacement = 5 mm). Thus, cement treatedsoils exhibited brittle behavior compare to non-treated

Figure 4. Results of consolidated-drained triaxial testsfor non-treated and 8% cement treated of Gorgan loess.

soils. Table 4 presents the shear strength parametersof non-treated and cement-treated samples of Anzalisand based on Mohr-Coulomb criteria. Results showedthat cement treatment led to a high increase in bothcohesion and friction angle parameters. It is alsoobvious that shear modulus corresponding to the peak

E. A aki et al./Scientia Iranica, Transactions A: Civil Engineering 21 (2014) 1231{1240 1235

Figure 5. Double shear band failure of 5% cementtreated loess.

Figure 6. Results of large scale direct shear test fornon-treated, 5 and 8% cement treated Anzali sand in �v =200 kPa.

Table 3. Shear strength parameters of all three types ofsoils obtained from triaxial test.

Soil type Coment (%) C0(kPa) �0 (deg)

Gorgan loess0 15 235 157 378 244 38

Rasht clay0 25 285 121 388 246 36

Anzali sand0 | |5 76 438 198 43

shear stress increased signi�cantly due to cement sta-bilization.

5. Failure criteria

Cement treated soils in low-con�ning pressure exhibitbrittle failure and, at very high con�ning pressures,

Table 4. Shear strength parameters of non-treated andcement treated samples of Anzali sand from large scaledirect shear test.

Coment content(%)

C0(kPa) �0 (deg)

0 7 345 122 478 223 51

mostly show plastic failure [19]. In other words, Mohr-Coulomb failure envelope for cement treated soils isnon-linear. Results show that the Mohr-Coulombcriteria, which is presented by Terzaghi [30] and is thebasic of soil mechanics development, is valid for onlylimited range of stresses. Using the shear strengthparameters of Terzaghi equation (�f = C + � tan')may lead to some anomalies [9]. Several failure criteriasuch as Gri�th theory, modi�ed Gri�th, Johntsoncriteria and Hoek-Brown failure criterion have beensuggested for predicting the failure envelope of soil-cement samples. It must be noted that applicationof each failure criteria depends on material type andstress conditions.

5.1. Gri�th and modi�ed Gri�th crack theoryGri�th [20] proposed that the failure of brittle mate-rials is governed by the initial presence of microcracks.Under uniaxial and biaxial compression, neglecting thein uence of friction on the cracks when closed, andassuming that elliptical cracks will propagate from thepoints of maximum tensile stress concentration, a stresscriterion is obtained as:

(�l � �3)2 = �8�t(�l � �3) (1)

for �1 + 3�3 > 0

if �1 + 3�3 < 0 then �3 = �t;

where �1 and �3 are the two principal stresses and �tis tensile strength of material. It can be seen fromEq. (1) that the Gri�th stress criterion predicts astrength ratio of RG = �c=�t = 8 [21]. When thetensile strength test data are not available, the generalapproach to estimate rock tensile strength makes use ofthe correlation between uniaxial compressive strength,�c, and tensile strength, �t, and applies the generallyagreed relationship of �c = R:�t, where R � 10 [22].Murrell [23] showed that the Gri�th criterion can berepresented in the Mohr plane in term of shear stressand normal stress as:

�2 + 4�t�n � 4�2t = 0: (2)

To allow for the frictional resistance on initially closedcracks, McClintock and Walsh [24] proposed a modi�-cation of Gri�th's criterion. Following Brace [25] the

1236 E. A aki et al./Scientia Iranica, Transactions A: Civil Engineering 21 (2014) 1231{1240

modi�ed criterion in terms of stresses, homogeneous atin�nity, can be written as:

�l�(�2 + 1)

12 � �

�� �3

�(�2 + 1)

12 + �

�= 4�t; (3)

where � is the coe�cient of joint friction. Brace [25] hasshown that the fracture criterion, modi�ed to accountfor the e�ects of crack closure in compression whichcan be represented by a limiting Mohr envelope. Thisline is straight having the equation in Mohr plane andexpressed by Eqs. (4) and (5):

�2 + 4�t�n � 4�2t = 0 for �n < 0; (4)

� = 2�t + ��n for �n > 0: (5)

5.2. Johnston failure criterionAn extensive study by Johnston and Chiu [26] onMelbourne mudstone resulted in a new failure criterionfor soft rocks, given by:

�0IN = (MB�03N + S)B ; (6)

where M and B are intact material constant, S is thearameter that accounts for strength of discontinuitiesof rock or soil, with S = 1 for the intact material. Also,�01N and �03N are e�ective principal stresses at fail-ure, normalized by uncon�ned compressive strength,�c.

Based on a broad range of data for clays androcks, Johnston [27] suggested that Eqs. (7) and (8)can be used to determine the B and M parameters as:

B = 1� 0:0172(log �c)2; (7)

M = 2:065 + 0:276(log �c)2: (8)

5.3. The original Hoek-Brown failure criterionThe Hoek-Brown failure criterion is an empirical cri-terion developed through curve-�tting of triaxial testdata. The conceptual starting point for the criterionwas the Gri�th theory for brittle fracture but theprocess of deriving the criterion was one of pure trialand error. The original Hoek-Brown criterion wasproposed in [28] and is de�ned as:

�l = �3 +pm�3�c + s�2

c ; (9)

where m is a constant depending on the characteristicsof the rock mass, s is a constant depending on thecharacteristics of the rock mass, �c is the uniaxialcompressive strength of the intact rock material, �1is the major principle stress at failure, and �3 is theminor principle stress at failure.

The measured shear stress and normal stressvalues for non-treated, 5%, and 8% cement treated

from triaxial tests are shown in Figure 7. Failureenvelope lines including Mohr-Coulomb (obtained fromMohr circles), Gri�th (Eq. (2)) and modi�ed Gri�th(Eq. (5)) are also drawn in this �gure. Based onthe Mean Squared Error analysis (MSE), to de�ne �coe�cient, it is obvious that predicted shear strength,using modi�ed Gri�th theory has better agreementwith measured shear strength compared with Grif-�th theory. It is clear that Mohr-Coulomb criteriaresults for cement stabilized soil are not applica-ble.

Measured normal stresses (�3 and �1) and thosepredicted by Johnston's theory (Eq. (6)) and alsoHoek-Brown failure criterion (Eq. (9)) for non-treatedand 5% and 8% cement treated specimens for allthree types of soils are shown in Figure 8. Valuesof S parameter were determined by MSE analysisto obtain the lowest MSE value. For Hoek-Brownfailure envelope m and s parameters were determinedby trial and error regarding to the suggested rangeof values by Sjoberg [29]. It has been observedthat both Johnston and Hoek-Brown criteria basedon MSE analysis have a good compatibility with thecement treated test results. But Hoek-Brown cannotbe used for non-treated samples under high con�ningpressures.

6. Conclusions

This study made an extensive laboratory testingfor evaluating the e�ectiveness of cement treatmenton shear strength parameters of soils encounteredin southern coast line of Caspian Sea. Test re-sults included uncon�ned compressive strength andconsolidated-drained triaxialand large scale directshear. Also failure envelope trend of cement treatedsamples was discussed by some criteria such as Mohr-Coulomb, Gri�th, modi�ed Gri�th theory, the crite-rion suggested by Johnston, and Hoek-Brown failurecriterion. According to the test results, the followingconclusions can be stated:

� Addition of cement led to signi�cant increase inuncon�ned compressive strength and modulus ofelasticity for both soaked and unsoaked samples.Also reduction in failure strain and brittle type ofrupture was observed.

� Triaxial test results indicated that cement treatmentimproved shear strength remarkably, but it changesthe type of failure greatly from ductile to brittlebehavior. 5% cement treated samples displayed pla-nar and 8% cement treated showed both planar andsplitting type of failure according to the con�ningpressures.

� It was derived that cemented treatment increases

E. A aki et al./Scientia Iranica, Transactions A: Civil Engineering 21 (2014) 1231{1240 1237

Figure 7. Triaxial test results and predicted failure envelope using Gri�th, modi�ed Gri�th and Mohr-Coulomb theories.

the cohesion parameters due to the adhesion andbounding among the cemented soil particles.

� Con�ning pressure has an e�ective in uence on soilbehavior and it is essential to perform triaxial tests

beside other simple and common tests such as UCS,CBR and DST.

� Based on MSE analysis, modi�ed Gri�th theoryand Johnson criteria are compatible with obtained

1238 E. A aki et al./Scientia Iranica, Transactions A: Civil Engineering 21 (2014) 1231{1240

Figure 8. Triaxial test results and predicted failure envelope using Johnston criteria and Hoek-Brown failure criterion.

results. Therefore, the achievements are suit-able for predicting the failure envelope of non-treated and cement treated soils. Also Hoek-Brown failure criterion can well describe the failuretrend of cement treated soils, but it is not beingused for non-treated soils. Moreover, using Mohr-Coulomb criteria for cement treated soils led to

a number of errors and high values of frictionangles.

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Biographies

Esmail A aki received his Bachelor and Masterdegrees in Engineering Geology and Geotechnics fromMcGill University, Canada, in 1980, and his PhD inGeotechnical Engineering from University of NewcastleUpon Tyne, U.K. in 1996. From 1980 to 1984, he wasworking in Geotrchnical department, Laboratory ofsoil mechanics, Ministry of Road and Transportation.

1240 E. A aki et al./Scientia Iranica, Transactions A: Civil Engineering 21 (2014) 1231{1240

From 1984 to present he has been working as a memberof academic sta� in the department of Civil andEnvironmental Engineering, Amirkabir University ofTechnology. He is now teaching Engineering Geology,Advance Engineering Geology and Site Investigationcourses. He has three published books and severaljournal and conference papers.

Pouya Sedighi. His biography was not available atthe time of publication.

Abolfazl Eslami received his PhD degree in Geotech-nical Engineering from the University of Ottawa,

Canada, in 1997. He is an Associate Professor inGeotechnical Engineering at Amirkabir University ofTechnology (AUT). He was Assistant Professor in CivilEngineering Department at University of Guilan from1997 to 2008. His research interests are mainly indeep and shallow foundations, soil modi�cation, in-situ testing in geotechnical practice, supported andunsupported excavations. His publications are morethan 15 ISI journal papers, 22 national journal papers,30 international conference papers, 15 national confer-ence papers and 5 text books and handbooks in foun-dation engineering, deep foundation and geotechnicalengineering.