Characterization of Clayey Subgrade Materials for Mechanistic Design of Flexible Pavements

Abstract

The design of flexible pavements is based on the empirical approach and in some cases on the empirical-rational approach. Funds are being invested in developing new design methods that will principally be based on the mechanistic-rational approach. A procedure for characterizing clayey subgrade materials to provide the properties needed by the new methods is presented. The procedure addresses the following elements: first, the material should be brought to the condition that prevails under pavements. Sample preparation and a soaking procedure to simulate field condition are described. Second, after about 10 days of sample conditioning, the material is tested under repetitive loading to provide both resilient and permanent properties. In clayey soils, two samples are tested under different deviatoric stresses and for 10,000 to 100,000 load repetitions. The stress level should correspond to the stress induced by overburden and traffic load. Tests are conducted on a swelling clay, and the results are analyzed. It is seen that (a) the proposed procedure for sample conditioning is appropriate for Israeli climatic conditions, but it can be changed to fit other climatic conditions; (b) the stress level in subgrades is relatively low, so the behavior of the clayey materials can be fairly well described by linear elastic theory; and (c) the total deformation increases either linearly or exponentially with number of load repetitions (in a log-log scale).