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THE INFLUENCE OF MOISTURE CONTENT AND SOIL

SUCTION ON THE RESILIENT MODULUS OF A LATERITIC

SUBGRADE SOIL

Alexandre B. Parreira1

and Ricardo F. Gonalves2

ABSTRACT

Any mechanistic analysis of pavements requires the knowledge of resilient moduli (M R) of all theircomponents. The specimens used in the laboratory for resilient modulus determination are usuallycompacted and tested at the optimum water content and maximum dry density, but the pavement subgrade,although compacted close to these conditions, will support moisture variations as a result of seasonal andenvironment fluctuations.

This paper presents results from an experimental study in which one typical lateritic soil encountered inroadbed in Brazil was tested under triaxial cyclic loading and evaluate the relationship between MRvariations and water content and suction level variations.

It was found that the lower moisture content results in the greater resilient modulus, and this decreases asthe moisture content is increased, for any level of deviator stress. The filter paper showed good performanceto assess even high suction levels and the soil characteristic curve illustrates that as the soil gets more wetted,the suction decreases. The cyclic loading triaxial test does not modify the soil suction and the resilientmodulus increases so that the soil suction is increased. A good relationship was obtained between resilientmodulus, deviator stress and suction level.

INTRODUCTION

The resilient modulus (M R) is analogous to Youngs modulus and can be defined as the ratio of ciclicdeviator stress to recoverable axial strain. Any mechanistic analysis of pavements requires the knowledge ofresilient moduli of all their components. The AASHTO Guide for Design of Pavement Structure (1993) usesthe resilient moduli to characterize material properties of each layer in the pavement section and presents a

procedure to consider variations of roadbed moduli with the seasonal changes in their water content alongthe year.

The specimens used in the laboratory for resilient modulus determination are usually compacted andtested at the optimum water content and maximum dry density, but the pavement subgrade, althoughcompacted close to these conditions, will support moisture variations as a result of seasonal and environmentfluctuations. Fine-grained soils, principally, exhibit a decrease in modulus as the water content is increased,leading to increased deflections in the pavement subgrade and most pavement failures occur as a result of

wet and dry moisture content cycles of subgrade layers (Mohammad et al. 1995, Drumm et al. 1997).A large portion of Brazilian territory, as most of tropical regions as the south hemisphere of the world, iscovered with soils generically referred as lateritic. Due to their geological formation their clay fraction isrich in oxides and hydroxides of iron and aluminum. These areas present a high annual mean airtemperature, generally over than 200 C. The rainfall is high, but limited to 1800 mm (700 in.) per year, andthe exposition to the sun is high. Due to the high temperature and sun exposure, the evapotranspiration iselevated. The majority of compacted subgrades at these regions shows the moisture content below optimumcontent during the service life of the pavement. Moisture contents above optimum are unusual and there islittle discrepancy when they occur.

The purposes of this paper are to present laboratory results concerning the resilient modulus of a typicallateritic soil encountered in roadbed in Brazil and evaluate the relationship between MR variations and watercontent and suction level variations.

1 D.Sc., Professor, University of So Paulo, So Carlos, Brazil, e-mail: parreira@usp.br2 MSc., University of So Paulo, So Carlos, Brazil, e-mail: efg@netgo.com.br

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PROPERTIES OF THE SOIL STUDIED

The soil studied was collected within the domain band of SP-215 State Highway, at the 139 th km + 500m,between the cities of So Carlos and Descalvado, located in So Paulo State, Brazil. Table 1 presents theHRB, USCS and MCT classifications, the liquid limit (LL), the plastic limit (LP), the plasticity index (IP),the grading data and the specific gravity of this soil.

Table 1 : Classification, LL, IP, gradation and specific gravity of the soil testedSOIL CLASSIFICATION

HRB USCS MCT

LiquidLimit (%)

PlasticLimit (%)

PlasticIndex (%)

Clay(%)

Passing#200 Sieve

(%)

SpecificGravity(g/cm3)

A-7-6 CL LG' 46.4 28.8 17.6 38 52 2.66

The MCT classification, developed for identifying lateritic soils, indicates that the soil studied is a sandy-clay lateritic soil (LG). The conventional knowledge based on the HRB soil classification does notrecommend this type of soil as base material, even as subgrade. Nevertheless, experience has shown thatLG soils perform well in pavement construction when appropriately used.

EXPERIMENTAL PROCEDURES

The experimental phase involved compaction and CBR tests, specimen preparation, the soil suctiondetermination and the cyclic loading triaxial tests.

Compaction and CBR Tests

The compaction tests were carried out to determine the optimum moisture content (w o) and the maximumdry density (dmax) at the Standard Proctor Energy. The values of optimum moisture content, dry density andthe CBR at optimum moisture content are 19.5%, 1.66 g/cm3 and 22%, respectively.

Specimen Preparation

The specimens submitted to cyclic loading triaxial and determination tests of soil suction, cylinders with adiameter of 51 mm and a length of 102 mm, were molded by static compaction in 3 layers. In all cases, adegree of compaction equal to 100% was reached, regarding the Standard Energy. In addition, the moisturecontent of the specimens did not vary more than 0.1 percent from the desired moisture content. The wetting

process of the compacted specimens was made by capillary using a saturated porous stone disc. Thespecimens, after the preparation, were kept sealed in an airtight plastic bag for 48 hours before testing.

Cyclic Loading Triaxial Tests

The cyclic loading triaxial tests were performed using a MTS closed-loop servo-electrohydraulic testingsystem which is capable of applying repeated load in haversine waveform with a wide range of loadduration. The axial deformations were measured by LVDTs mounted inside the triaxial cell. The specimenswere submitted to cyclic loading triaxial tests following the loading sequence according to the specificationsof the AASHTO Designation TP46-94 (1996). At least 3 repeated tests were conducted for each of thedifferent conditions analysed in the study.

Soil Suction

The suction in the specimens was determined through the paper filter method according to the procedureproposed by Chandler and Gutierrez (1986). A 125mm diameter Whatman 42 paper filter was used. Thecalibration equations proposed by ASTM D5298 (1992) were adopted to calculate soil suction.

Soil Conditions Analysed

Studies 1, 2 and 3, which follow, present the moisture conditions under which the specimens were tested:Study 1: Specimens compacted at optimum moisture content and maximum dry density. One part of the

specimens was submitted to drying to wot-2%, and the other part was wetted to wot+1%. This studysimulates the subgrade moisture variation after the pavement construction, in the region.

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Study 2: Specimens compacted at optimum moisture content and maximum dry density. Here, thespecimens were submitted to drying during 48 hours. One part of the specimens was again wetted to wot.This study simulates the situation when the soil studied is used as base material of low volume roads. At thiscondition, the base is allowed to dry before being primed.Study 3: Specimens compacted at wot-2% and wot+1% and maximum dry density. This study allows theassessment of the effect of the moisture content at compaction on the resilient modulus.

PRESENTATION AND DISCUSSION OF RESULTS

Figure 1 shows the relationship between the resilient modulus and the deviator stress of specimenscompacted at optimum moisture content (wot=19.5%) and which afterwards had their moisture varied asfollows: wetted up to 20.5% and dried up to 17.5% ( Study 1); dried during 48 hours (wfinal=8.3%), andsubsequently wetted until wot (Study 2).

Taking as a reference the resilientmodulus of the specimens compactedand tested at optimum moisture content,

it is observed that the 1% moisteningresults in a 25% decrease of such value.It is also observed that the 2% dryingcauses a 24% increase and the drying of11.2% (48h) leads to a 54% increase inthe values of the resilient modulus whencompared to the ones of molded andtested specimens at wot. The 48-hourdrying of the specimens compacted atoptimum moisture content followed by amoistening until wot brought about a55% decrease in the value of the

resilient modulus compared to the MRvalue of the tests at wot.

Some authors have carried out tests with specimens compacted at w ot, in which the moisture conditionswere subsequently altered. Results found were similar to the ones observed in this study. Drumm et al.(1997), upon the analysis of A-7-5 soil, have observed that for a 0.7% moistening and 1.3% above w ot, the

resilient modulus reduces in 27% and50%, respectively. Thadkamalla eGeorge (1995), upon the test of an A-7-5 soil, have observed that for specimenscompacted at optimum moisture content

and wetted until maximum saturationhas been reached, the resilient modulusdecreases 62%, on average. In the

present study, the great MR decrease ofthe specimens dried during 48 hoursand wetted until wot can be explained byexaggerate laboratory drying, 11.2%,that could cause shrinkage cracks inspecimens. The real water contentreduction during the pavementconstruction is about 2%, in this case.

Figure 2 enables one to compare theresilient modulus in specimenscompacted at optimum moisture content

Deviator Stress (kPa)

Resilient

Modulus(

MPa)

wot = 19.5%

19.5% 20.5%

w = 20.5%

100

200

300

400

600

800

1000

10 40 5030 60 7020 80 100

Figure 2 : Resilient modulus and the deviator stress - Study1 and Study 3

wot = 19.5%

19.5% 20.5%

19.5% 19.5%8.3%

19.5% 17.5%

19.5% 8.3% (48h)

Deviator Stress (kPa)

Resilient

Modulus(

MPa)

100

200

300

400

600

800

1000

10 40 5030 60 7020 80 100

Figure 1 : Resilient modulus and the deviator stress - Study

1 and Study 2

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followed by moistening up to 20.5% (Study 1) with the resilient modulus of specimens compacted at 20.5%moisture and maximum dry density(Study 3). It is observable that thespecimens in study 3 showed lowerresilient modulus values compared tothose in Study 1. The difference was17%.

Figure 3 enables to compare theresilient modulus of specimenscompacted at optimum moisture contentfollowed by drying of up to 17.5%(Study 1) with the MR of specimenscompacted at 17.5% moisture andmaximum dry density (Study 3). It isobserved that the specimens compacted2% below optimum moisture contentshowed values of resilient modulus

33% higher than those compacted atoptimum moisture content and dried2%.

With Figures 2 e 3, the variation effect of compaction moisture in the resilient modulus value forspecimens compacted with the same dry density (d max) can be analysed. For specimens compacted at thedry branch of the compaction curve (w=17.5%), 67% higher resilient modulus values are observed, and forspecimens compacted at the wet branch (w=20.5%), 33% lower resilient modulus values are observed, when

both were confronted with resilient modulus values of specimens compacted at optimum moisture content.In this case, the overcompaction influences the particle orientations producing flocculated and dispersedstructures on the dry and the wet side of optimum, respectively.

Figure 4 shows the relationshipbetween the resilient modulus andmoisture content at the four levels ofdeviator stress (d) developed in thesetests. The moisture values of thespecimens relate to the followingconditions: w=8.3% (dried 48h),w=17.5% (dried until wo t-2%),wot=19.5% and w=20.5% (moisteneduntil wot+1%).It is observed that an increase in the

moisture content results in a decrease ofthe resilient modulus value for any levelof deviator stress applied. Suchanalogous behavior was also observed

by Edil and Motan (1979) and Phillipand Cameron (1995).

Figure 5 illustrates the characteristic curve of the soil studied. The moisture contents were reachedthrough drying and moistening processes of the specimens compacted at optimum moisture content. It isworth noting that as the soil becomes more moistened or drier, the suction, respectively, decreases orincreases. It is observed that the specimens molded with the purpose of determining the characteristic curve

and the specimens submitted to cyclic loading triaxial test show a common characteristic curve. Such fact

Deviator Stress (kPa)

Resilient

Modulus(MP

a)

wot = 19.5%

19.5% 20.5%

w =15.5%

100

200

300

400

600

800

1000

10 40 5030 60 7020 80 100

Figure 3 : Resilient modulus and the deviator stress - Study 1and Study 3

Water Content (%)

Resilient

Modulus(MPa)

0

200

400

600

800

1000

1200

5 15 2010 25

s d = 24.8 kPa

s d = 49.7 kPa

s d = 37.3 kPa

s d = 62.0 kPa

d= 24.8 kPa

d= 37.3 kPa

d= 49.7 kPa

d= 62.0 kPa

Figure 4 : Resilient modulus and moisture content at thefour levels of deviator stress

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indicates that the procedures of the cyclic loading triaxial tests do not modify the suction level of thespecimens tested.

Figure 6 shows the relationshipbetween the resilient modulus and soilsuction at the four levels of deviatorstress developed in the cyclic loadingtriaxial tests. Suction values weredetermined through the soil characteristiccurve for specimens molded in optimummoisture content and submitted to watervariations, reaching the followingmoisture contents: w=8.3% (dried 48h),w=17.5% (dried until w o t-2%),wot=19.5% e w=20.5% (moistened untilwot+1%). For all analysed deviatorstress, it is observed that the resilient

modulus increases with suction increase.Philip and Cameron (1995) also observedthe same behavior. Such growth is

proportional to the increase of thedeviator stress, contrary to what was

observed by the authors mentioned previously. In this study, for the same suction, the resilient modulusincreases with the increase of the deviator stress.

In the current research, after M Rand suction level were determined for each specimen in Study 1 andStudy 2, statistical correlations between the resilient modulus, suction, and the stress state expressed by

bulky stress, confining pressure, principal stresses and combinations of these variables, were analysed. Thebest correlation found is described by Equation 1, where: MR = resilient modulus (MPa), d = deviator stress(kPa) and Su = suction (kPa).

MR= 14.10 d0.782 Su0.076 (1)

This prediction Equation 1 has a coefficient of determination (R2) of 0.92, that indicates a good agreementbetween the variables.

CONCLUSIONS

From the analysis of the results, onecan come to the following conclusions:the lower moisture content results in

the greater resilient modulus, and thisdecreases as the moisture content isincreased, for any level of deviatorstress; the filter paper showed good

performance to assess even highsuction levels; the soil characteristiccurve illustrates that as the soil getsmore wetted, the suction decreases; thecyclic loading triaxial test does notmodify the soil suction; the resilientmodulus varies with the variation ofsoil suction, where the resilientmodulus increases so that the soilsuction is increased. A good

1

10

100

1000

10000

100000

0 5 10 15 20 25

Water content (%)

Suction

(kPa)

specimens subjected to cyclic tests

specimens not subjected to cyclic tests

Figure 5 : Soil characteristic curve

s d = 24.8 kPa

s d = 49.7 kPa

s d = 37.3 kPa

s d = 62.0 kPa

Suction (kPa)

Resilient

Modulus(MPa)

100

200

300

400

500

600

700

800

900

1000

1 10010 1000 10000010000

d= 24.8 kPad= 37.3 kPad= 49.7 kPad= 62.0 kPa

Figure 6 : Resilient modulus and suction at the four levels of

deviator stress ( d)

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relationship (Equation 1) was obtained between resilient modulus, deviator stress and suction level for theStudies 1 and 2.

ACKNOWLEDGMENT

Our many thanks to Brazilian research agencies FAPESP and CNPq for the financial support which madethis study possible.

REFERENCES

AMERICAN ASSOCIATION OF HIGHWAY AND TRANSPORTATION OFFICIALS (1996) AASHTODesignation TP46-94. Standard test method for determining the resilient modulus of soils and aggregatematerials. Washington, DC.

AMERICAN ASSOCIATION OF HIGHWAY AND TRANSPORTATION OFFICIALS (1992). Standardtest method for measurement of soil potential (suction) using filter paper No D5298-92. American Societyfor Testing Materials.

Chandler, R. J. and C. I. Gutierrez (1986). The filter-paper method of suction measurement. Gotechnique,

36, Vol. 2, pp. 265-268.Drumm, E. C.; J. S. Reeves and M. R. Madgett (1997). Subgrade resilient modulus correction for saturationeffects.Journal of Geotechnical and Geoenvironmental Engineering, Vol. 123, n. 7, pp. 663-71.

Edil, T. B. and S. E. Motan (1979). Soil-water potential and resilient behaviour of subgrade soils.Transportation Research Record, 705, pp. 54-63.

Mohammad, L.N; Puppala, A.J. and Alavilli, P. (1995). Resilient properties of laboratory compactedsubgrade soils. Transportation Research Record, 1504, pp. 87-102.

Phillip, A. W. and D. A. Cameron (1995). The influence of soil suction on the resilient modulus ofexpansive soil subgrades. Proceedings Conference International on Unsaturated Soils, Paris, Frana,Vol. 1, pp. 171-176.

Thadkamalla, G. B. and K. P. George (1995). Characterization of subgrade soils at simulated fieldmoisture. Transportation Research Record, 1481, pp. 21-27.