Simplified Laboratory Assessment of Subgrade Performance Parameters for Mechanistic Design of Pavement Foundations

Abstract

With the increasing agenda for sustainability, the United Kingdom is attempting to move away from the empirical design of pavement foundations to develop a performance specification approach that facilitates analytical design. The measurement of the subgrade performance parameters of resilient modulus and resistance to permanent deformation is required for analytical design. These parameters ideally should be assessed concurrently under loading and environmental conditions similar to those the materials will experience in the field. To date, measurement of these parameters is largely confined to research laboratories using cyclic triaxial testing with advanced on-sample strain measurement. This apparatus is considered too complicated for routine commercial use; hence, the implementation of laboratory performance evaluation for routine pavement foundation design is potentially limited. A previous program of cyclic triaxial testing on clay subgrades indicated a series of useful correlations between strength and permanent deformation behavior (via a threshold stress) and material resilient modulus at this threshold. The previous work is reviewed; with these correlations, data from tests performed on three different clay materials to develop simplified equipment and procedures for the routine measurement of the required design parameters are presented. Simple pseudostatic tests can measure a subgrade modulus for a simplified performance-based design. The previous data (in the light of the recent work) were reevaluated to show a boundary correlation that may allow a shear strength–based parameter to control (in design) the onset of permanent deformation, and the ways long-term subgrade water content changes can be accommodated are detailed.