PERMANENT DEFORMATION CHARACTERISTICS OF SUBGRADE SOILS DUE TO REPEATED LOADING

Abstract

Various procedures are examined for controlling or estimating the contribution of the subgrade to the total permanent deformation that occurs in the pavement structure as a result of repeated traffic loads. The use of layered elastic analysis to estimate the vertical compressive strain at the subgrade surface is suggested as one procedure to control such deformation. A summary of available strain data is included. If these criteria are used for design purposes, the same range in stiffness for the asphalt-bound layer and the same values of Poisson's ration must be used as were used to develop the criteria. Layered elastic analysis is also suggested for estimating the amount of permanent deformation caused by the subgrade. This analysis is based on relationships among permanent strain, applied stress, and number of stress applications based on repeated-load triaxial compression tests; and these relationships are discussed. Data also indicate the importance of stress history effects. Time hardening and strain hardening permit estimates of cumulative loading effects from the results of tests at single stress levels. Although neither method predicts precisely the accumulation of permanent strain under different stress sequences, both bound the measured data and therefore have the potential to assist in the estimation deformation or rutting from cumulative traffic loading.