SEASONAL VARIATION OF SUBGRADE SUPPORT VALUES

Abstract

Seasonal variations in climatic conditions have long been recognized to have a significant impact on the structural response of flexible pavements. Of particular significance is the effect of subgrade moisture content on subgrade support values and potential roadway damage. The Montana Department of Transportation is conducting a project to evaluate the response of subgrade resilient modulus to seasonal changes in subgrade moisture content. Moisture content is being measured through permanent subsurface instrumentation while deflection tests are periodically being conducted to establish subgrade resilient modulus values. This report describes work performed in support of this project. The purpose of this study was to provide laboratory data on subgrade support values for the instrumented sites. These data will illustrate the variation in subgrade support with changes in moisture content and sample dry density. This information will be used to verify the accuracy of the data generated in the field and might also be used to help guide the backcalcuation procedure. Laboratory testing has consisted of resilient modulus, triaxial compression, CBR and R-Value tests. Results have been generated for various combinations of sample dry density and degree of saturation. These results have been related to resilient modulus whenever possible. The results tend to show that ultimate strength related parameters are most sensitive to changes in dry density and moisture content for the cohesive subgrade soils and that deformation related parameters are most sensitive for the more non-cohesive subgrade soils.