Permanent Deformation Characterization of Subgrade Soils from RLT Test

Abstract

AASHTO's practice for designing flexible pavements requires resilient modulus of the subgrade soils. The resilient characteristics are assumed to account for the permanent deformations of soils. However, this approach can provide misleading characterizations. Soils such as silty sands, silty clays, and sandy clays possess good resilient characteristics. Nevertheless, they still undergo large plastic deformations under traffic loads. Hence, both elastic and plastic strain responses are necessary for a complete characterization. This paper presents an evaluation on the repeated load triaxial (RLT) test method for providing the permanent strain response of the soils. RLT tests were conducted on three soil types: (1) Well-graded sand; (2) silty clay; and (3) heavy clay. Tests provided the plastic strain data at various confining deviatoric stresses and loading cycles. Data were analyzed with a plastic strain formulation that accounted for stresses and loading cycles. The formulation has reasonably represented the plastic strain response in these soils. Formulation predictions agree with the measured permanent strain results of a field site subjected to accelerated loading. Future research needs are also discussed.