Effect of Suction on Resilient Modulus of Compacted Fine-Grained Subgrade Soils

Resilient modulus of compacted subgrade soils is the primary mechanical property required in the mechanistic–empirical design of pavement structures. In most cases, the resilient modulus is determined of a specimen prepared at a single compaction condition (i.e., near optimum water content and at a specified percentage of maximum dry unit weight). However, in the field, the resilient modulus changes in response to changes in moisture content and corresponding changes in matric suction. The relationship between resilient modulus and suction is described for four fine-grained compacted subgrade soils. Resilient modulus tests were conducted in accordance with NCHRP 1-28A on test specimens prepared from each soil and conditioned to different matric suctions. The summary resilient modulus increases with increasing matric suction. This relationship is quantified empirically by using a modulus ratio, defined as the ratio of the summary resilient modulus at a particular suction to a reference summary resilient modulus. Two reference summary moduli were considered: at optimum compaction conditions and at saturation. The modulus ratio is linearly related to the logarithm of matric suction for all soils.