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Through the work of the Strategic Highway Research Program (SHRP, 1987–1992) and the Federal Highway Administration (FHWA, 1992-present), the government has provided financial and technical assistance to develop and improve a laboratory test method to determine the resilient modulus properties of unbound materials. Although the work — part of the Long Term Pavement Performance (LTPP) study — has led towards the adoption of test procedure T307-99 in the current release of the American Association of State Highway and Transportation Officials (AASHTO) Tests, many skeptics insist that the method does not lend itself towards repeatable, reproducible test results.
This paper acknowledges that the work conducted by SHRP and FHWA focused rimarily upon developing a test method that would be relatively simple and highly productive with less variability inherent in the previous, existing test procedure. Variables not investigated included compaction methodology, instrumentation location and sensitivities to other influencing factors such as precision of confining pressure, waveform control, membrane thickness and porous stone properties. Additionally, the testing program did not successfully establish a precision and bias statement for the test method utilized.
The repeatability of the test is examined by utilizing eight replicated test specimen ub sampled from a homogenous Alabama soil and nineteen replicated test specimen sub sampled from a homogenous Georgia soil. Each test specimen was prepared using the five-lift static compaction method. All specimens were tested within the range of ±1 pound per cubic foot density and ±0.4 percent moisture content, thus minimizing variations of results due to material variation. Averages, standard deviations, and coefficients of variation (c.v.) were determined for resilient modulus values calculated at each load sequence, resulting in c.v.s of below 4.5%. The resilient modulus values were calculated using the constitutive model: Mr = K1(Sc)K2 (S3)K5 in order to normalize the data for comparative purposes.
The test method can promote repeatable test results, although much more testing is recommended to produce precision and bias statements within and between laboratories.
resilient modulus, soil stiffness, subgrade
President, Boudreau Engineering, Inc., Norcross, GA