Simplification of Resilient Modulus Testing for Subgrades

Abstract

Resilient modulus has been used for characterizing the stress-strain behavior of subgrade soils subjected to traffic loadings in the design of pavements. With the recent release of the M-E Design Guide, highway agencies are further encouraged to implement the resilient modulus test to improve subgrade design. In the present study, physical property tests, unconfined compressive tests, resilient modulus (Mr) tests and Several Dynamic Cone Penetrometer (DCP) tests were conducted to assess the resilient and permanent strain behavior of 14 cohesive subgrade soils and five cohesionless soils encountered in Indiana. The applicability for simplification of the existing resilient modulus test, AASHTO T 307, was investigated by reducing the number of steps and cycles of the resilient modulus test. Results show that it may be possible to simplify the complex procedures required in the existing Mr testing to a single step with a confining stress of 2 psi and deviator stresses of 2, 4, 6, 8, 10 and 15 psi. Three models for estimating the resilient modulus are proposed based on the unconfined compressive tests. A predictive model to estimate material coefficients k1, k2, and k3 using 12 soil variables obtained from the soil property tests and the standard Proctor tests is developed. The predicted resilient moduli using all the predictive models compare satisfactorily with measured ones. A simple mathematical approach is introduced to calculate the resilient modulus. Although the permanent strain occurs during the resilient modulus test, the permanent behavior of subgrade soils is currently not taken into consideration. In order to capture both the permanent and the resilient behavior of subgrade soils, a constitutive model based on the Finite Element Method (FEM) is proposed. A comparison of the measured permanent strains with those obtained from the Finite Element (FE) analysis shows a reasonable agreement. An extensive review of the M-E design is done. Based on the test results and review of the M-E Design, implementation initiatives are proposed.