PREDICTING RESILIENT MODULUS: A STUDY TO DETERMINE THE MECHANICAL PROPERTIES OF SUBGRADE SOILS (ABRIDGMENT)

Extensive literature review, detailed regression studies, and limited laboratory testing were used to develop models for predicting subgrade resilient modulus. General models are proposed for cohesive soils and nonplastic or granular-type soils. These two models are provided in the new Forest Service Surfacing Handbook (FSH 7709.56a) to provide the practicing engineer with guidance in the absence of test results. More than 250 different soils, representing more than 3,300 modulus test points, were placed in a computerized data base for regression studies. Although these data come from a literature search and testing variations definitely existed, the final models provide a useful first estimate. However, it was strongly recommended that resilient modulus laboratory tests be obtained whenever feasible. The effect of an error in subgrade modulus estimation on the thickness obtained from the design procedure was also studied. Some laboratory tests were made for use in model verification.