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PROPOSAL FOR RESEARCH AND IMPLEMENTATION STUDY

Simplification of Resilient Modulus Testing for Subgrades

by

Daehyeon Kim

Geotechnical EngineerINDOT Research Division

and

Nayyar ZiaGeotechnical Engineer

INDOT Materials and Tests Division

Joint Transportation Research ProgramProject No. C-36-52S

File No. 6-20-18SPR-2633

Prepared as a Part II Study in Cooperation withthe Indiana Department of Transportationand the Federal Highway Administration

U S Department of Transportation

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1. Identification

(1) Project Title: Simplification of Resilient Modulus Testing for Subgrades

(2) Principal Investigators: Daehyeon Kim and Nayyar Zia(3) Research Agency: JTRP

2. Introduction

Resilient modulus is an important material property of the subgrade soils and is an inputparameter in the design of pavements. It is based on the recoverable strains after a series ofcombination of confining and deviator stresses is applied to the soil specimen in order to take

into account nonlinear behavior of subgrade soils under the traffic loadings. Resilient modulus isdefined as:

r

d

RM

=

where MR is the resilient modulus; d is the repeated deviator stress; and r is the recoverableaxial strain.

Since AASHTO Guide for Design of Pavement Structures in 1986 recommended

highway agencies to use a resilient modulus (MR) value obtained from a repeated triaxial test forthe design of pavements, numerous efforts have been made for obtaining more accurate,straightforward, and appropriate MRvalues which are representative of field conditions. In thepast or even in the present, most highway agencies have used California Bearing Ratio (CBR)value to characterize subgrades in the design of pavements and to correlate it with the resilientmodulus value. CBR value, however, is a static property that can not account for the actualresponse of the pavement structure under the dynamic loads of moving vehicles (Mohammad etal. 1998).

Although the AASHTO design guide requires designers to use one resilient modulus valuerepresentative for a given subgrade considering seasonal variation, it is not such an easy work toobtain the resilient modulus by performing a standard repeated triaxial test due to its complex,time-consuming and costly testing procedure (Elliot and Thornton 1988, Ping et al. 2001). Dueto its complexity and difficulty, many correlations have been made between MR values fromrepeated triaxial test and measurements obtained from nondestructive field testing methods,such as Cone Penetration Test (CPT), Dynamic Cone Penetration Test (DCPT), Falling WeightDeflectometer (FWD), and Plate Load Test (Plate Load Test). Of these methods, CPT is one of

the most frequently used methods due to its economy, reliability and repeatability (Mohammadet al. 1998). Also, at small strain levels (i.e. less than 0.1%), some alternative laboratory testssuch as unconfined compression test (Drumm et al. 1990, Lee et al. 1995) and static triaxial test(Kim et al. 2001) were suggested to overcome the complexity and difficulty of the repeatedtriaxial test. A simplified testing procedure was also suggested by decreasing the number ofconfining and deviator stresses (Elliot and Thornton 1988)

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Drumm et al. 1990, Thompson and Robnett 1979). There is a breakpoint resilient moduluscorresponding to the resilient modulus at a deviator stress of 6 psi (Thompson and Robnett1979). This breakpoint characterizes the behavior of these soils under repeated loads and mightbe different depending on the subgrades tested.

It appears that there is some disagreement between researchers on confining pressure for

cohesive subgrades. It is noted that Lee et al. (1997) obtained considerable different values fordifferent confining stresses, while others showed little effect of confining stresses on themodulus of cohesive soils (Drumm et al, 1997). It is also noted that Thompson and Robnett(1979) in their study used zero confining pressure with change in deviator stress. However, theeffect of confining pressure on resilient modulus of cohesive subgrades has not been thoroughlystudied based on very careful investigation of drainage mechanism during testing and detailedsoil types depending clay contents and moisture contents.

Most researchers have shown that there is little effect of moisture content on resilientmodulus of granular subgrades (Lee et al 1995, Drumm et al. 1997), while there is significanteffect of water content on resilient modulus of cohesive subgrades (i.e. the higher the moisturecontent, the less the resilient modulus) (Drumm et al. 1997). Note that the moisture contents oftypical in-service subgrades in Indiana have increased to the moisture contents corresponding tothe 90% - 95% degree saturation (Altschaeffl et al. 1998). It is expected that the increase inmoisture contents of in-service subgrades will result in the reduction in their resilient modulus.Therefore, the increase in moisture contents of the subgrades compacted below or at the

optimum moisture content will be an important part for better evaluation of resilient modulus ofin-service subgrades.There have been two JTRP projects on the resilient modulus tests focusing on the evaluation

of the resilient behavior of typical subgrade soils in Indiana and its implementation, respectively.However, the repeated triaxial test, known as the most reasonable test for obtaining resilientmodulus has still not been implemented on a routine basis due to its complex procedure. In theproposed research project, the complex procedure of the standard resilient modulus test(AASHTO 307-99 or AASHTO 294-94) will be simplified through an extensive experimental

program for typical disturbed and undisturbed subgrade soils. The simplified procedure willcontribute to the construction of database on the resilient modulus of subgrades in Indiana.

3. Problem Statement

In order to reflect the AASHTO specification, two research projects on resilient modulus(MR) and their implementation have been completed under FHWA/INDOT/JHRP 92-32 and

FHWA/INDOT/JTRP-98/2 several years ago in Indiana. However, in reality, complexity anddifficulty to perform MR test prevented many highway agencies including INDOT fromincorporating it into a routine test. Except for few important projects, CBR test is still used toobtain resilient modulus value by using an empirical correlation between resilient modulus andCBR (Nayyar 2002, personal communication).

In the previous JTRP projects, many predominant soils (i.e. A-3, A-4, A-6, A-7-5 and A-7-

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from the traffic loadings in Indiana and to account for such reasonable stress levels in the MRtest. Over- or underestimation of the stress levels in the subgrades will lead to erroneous resultsof resilient modulus results (Houston et al. 1993). Through a resilient modulus test, as oneresilient modulus corresponding to the representative confining and deviator stress is needed fora given subgrade in designing a pavement, the complex testing procedure can be simplified for

practical design purpose.The current standard test method to determine the resilient modulus is described by

AASHTO T 307-99 which has recently been upgraded from AASHTO T 294-94 and AASHTOT 274. Most literature is limited to AASHTO T 294-94 and AASHTO T 274 but no literature onthe evaluation of AASHTO T 307-99 appears to be available. In the AASHTO T 307-99, fieldconditions are simulated by conditioning and postconditioning (i.e. main testing). Conditioningconsists of 500 to 1000 load applications at a confining stress of 6 psi and a deviator stress of 4psi. In addition, main testing is performed at three levels of confining stresses (2, 4, 6 psi) forwhich each 5 level of deviator stresses (2, 4, 6, 8 and 10 psi) are applied, resulting in 15 steps ofload applications. It classifies soil types into type 1 and type 2 materials. Granular soils andcohesive soils are categorized as type 1 and type 2, respectively. This test applies to the sameprocedure for both granular and cohesive subgrades and is done under drained condition only.However, the research on the drainage condition has been quite limited and somewhat neglected.Although the test is done under drained condition, considerably fast and repeated loadapplications (i.e. a load pulse of 0.1 sec.) may lead to undrained or partially undrained condition,

especially for cohesive subgrades.

4. Objectives

The main objective of the proposed research is to simplify the complex standard repeatedtriaxial test for obtaining resilient modulus of subgrades, to allow the repeated triaxial test to be aroutine test and to ultimately improve the design quality based on stress-strain behavior of

subgrade soils. The detailed goals of the research will be:

(1) Simplification of the standard resilient modulus testing.(2) Clarification of confining pressure effects on resilient modulus of cohesive subgrades.(3) Evaluation of postcompaction moisture content increase.(4) Construction of database of resilient modulus depending on soil types and geographical

locations in Indiana.

5. Scope and Work Plan

Since the resilient modulus of cohesive and granular subgrades is mainly dependent ondifferent factors (i.e. deviator stress and confining stress, respectively), a series of resilientmodulus testing will be performed for both typical cohesive and granular subgrades collectedfrom selected sites. For cohesive subgrades, the undisturbed samples will also be collected and

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simplified procedure and that obtained from the empirical equation suggested by Lee et al.(1997). Once the simplified testing procedure is verified, subgrades at many sites will becollected and tested for construction of the database on resilient modulus depending on both soiltypes and geographical locations. The general tasks for the proposed research are:

(1)

Literature search.(2) Determination of range of confining and deviator stresses.(3) Soil sample collection and laboratory testing for cohesive subgrades.(4) Soil sample collection and laboratory testing for granular subgrades.(5) Data analysis and interpretation of results.(6) Extensive resilient modulus testing.(7) Report writing.

Task 1: Literature search

a. Review of available standard test proceduresb. Review of studies on resilient modulus for cohesive and granular subgradesc. Review of MTS triaxial equipment

Task 2: Determination of the range of confining and deviator stresses

In this task, determination of the range of confining and deviator stresses in thesubgrages will be made by multi-layered elastic analysis (i.e. using ELSYMM5) for typical

road cross-sections in Indiana under traffic loadings.

Task 3: Soil sample collection and laboratory testing for cohesive subgrades

The main goal of this task is to develop a simplified resilient modulus testprocedure as well as obtaining some basic soil properties including consolidation tests andshear strength parameters, which will be needed for evaluation and validity of the simplifiedprocedure. The tests include:

a. Fundamental soil property tests:- Specific gravity (Gs) and natural water content (w%) tests.- Atterberg limit tests.- Hydrometer tests for grain size distribution.- Compaction tests

b. Consolidation tests.

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This task will be performed for granular subgrades following the similarprocedure as described in Task 3.

Task 5: Data analysis and interpretation of results

In this task, based on results of Tasks 3 and 4, a simplified resilient modulustesting procedure for cohesive and granular subgrades will be developed. In addition,clarification of confining pressure effects on resilient modulus of cohesive subgrades will bemade. The effects of postcompaction moisture content increase on resilient modulus ofsubgrades will also be evaluated.

Task 6: Extensive resilient modulus testing

Construction of database of resilient modulus depending on soil types andgeographical locations in Indiana will be made through extensive resilient modulus testingbased on the simplified testing procedure.

Task 7: Report writing

6. Reporting Plan

Draft Final report: draft date, 31 November 2004Final report: 31 March 2005

7. Budget

By category

Category Dec.02-Nov.03 Dec.03-Nov.04 Dec.04-Mar.05 TotalsSalaries $0 $0 $0 $0

Testing $500 $500 $0 $1,000

Travel $2,000 $2,000 $0 $4,000

Equipment $55,000 $10,000 $8,000 $73,000

Other S&E $400 $400 $200 $1,000

Publication $0 $0 $1,000 $1,000

Totals $57,900 $12,900 $9,200 $80,000

Task 1: $ 3,000Task 2: $ 2,000Task 3: $39,000Task 4: $10,000Task 5: $15 000

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Drumm, E. C., Boateng-Poku, Y. and Pierce, T. J (1990), Estimation of Subgrade ResilientModulus from Standard Tests. Journal of Geotechnical and Geoenvironmental Engineering,ASCE, 116(5), pp. 775 789.

Drumm, E. C., Reeves, J. S., Madgett, M. R. and Trolinger W. D. (1997), Estimation ofSubgrade Resilient Modulus from Standard Tests. Journal of Geotechnical andGeoenvironmental Engineering, ASCE, 123(7), pp. 663 670.

Elliot, R. P., and Thornton, S. I. (1988), Simplification of Subgrade Resilient Modulus Testing.Transportation Research Record, 1192, TRB, National Research Council, Washington, D.C., pp.1 - 7.

Houston, W. N., Houston, S. L., and Anderson, T. W. (1993), Stress State Consideration forResilient Modulus Testing of Pavement Subgrade. Transportation Research Record, 1406, TRB,National Research Council, Washington, D.C., pp. 124 - 132.

Kim, D., Kweon, G. and Lee, K. and (2001), Alternative Method of Determining Resilient

Modulus of Subgrade Soils Using a Static Triaxial Test. Canadian Geotechnical Journal, Vol.37, pp. 107 - 116.

Lee, W., Bohra, N. C. and Altschaeffl, A. G. and (1995), Resilient Characteristics OF DuneSand. Journal of Transportation Engineering, ASCE, 12(6), pp. 502 506.

Lee, W., Altschaeffl, A. G. and White, T. D. (1997), Resilient Modulus of Cohesive Soils.Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 123(2), pp. 131 136.

Mohammad, L. N., Titi, H. H. and Herath, A. (1998), Intrusion Technology: An InnovativeApproach to Evaluate Resilient Modulus of Subgrade Soils, Geotechnial Special PublicationN0. 85, ASCE, pp. 39 58.

Ping, W. V., and Yang, Z., Liu, C., and Dietrich, B. (2001), Measuring Resilient Modulus ofGranular Materials in Flexible Pavements. Transportation Research Record, 1778, TRB,National Research Council, Washington, D.C., pp. 81 - 90.

Santa, B. L. (1994), Resilient Modulus of Subgrade Soils: Comparison of Two ConstitutiveEquations. Transportation Research Record, 1462, TRB, National Research Council,Washington, D.C., pp. 79 - 90.

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9. Work Schedule

Month Year

2002 2003 2004 2005

Task 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3

Task 1

Task 2

Task 3

Task 4

Task 5

Task 6

Task 7

Review