CHARACTERIZATION OF GRANULAR MATERIAL

Evaluation of the resilient modulus of granular material as used in design and structural evaluation of flexible pavements is addressed. It is shown that the well-known equation relating the modulus to the sum of principal stresses does not properly describe granular behavior: the predicted response is not compatible with laboratory test results that show a strong dependence of the modulus on the stress ratio from which the equation is derived. A general law that includes the effects of shear strains is shown to be in good agreement with test results. The response of nonlinear materials is sensitive to their state of stress during loading. A literature review covers case histories of full-scale retaining wall models with compacted backfill, where residual stresses induced by compaction were measured. A proposed theory based on limit equilibrium is reported to give good estimates of residual stresses. It is found that granular base and subbase materials, compacted with heavy rollers, may develop relatively high residual horizontal stresses. The general law for characterizing granular materials is used with different postulated residual stresses in pavement analyses. The results obtained appear to be in good agreement with all aspects of granular material behavior, provided that a residual stress of the order of 1 to 2 psi is assumed to be induced by compaction.