Design and analysis of flexible pavement systems depend on soil layer characterization, traffic loads and number of passes. Currently, the AASHTO design method for flexible pavements uses resilient characteristics of subsoils to characterize and determine the structural support of each layer and then design the layer thickness. Resilient properties are used in the characterization since they are assumed to account for plastic deformation of subsoils. This assumption is known to provide misleading characterizations in mixed soils such as silty clay and silty sand. Hence, there is an important research need to evaluate the plastic deformation behaviors of subsoils in order to evaluate the plastic deformation effects on resilient strain response of the soils. A research study was initiated to establish a test procedure to measure plastic strain potentials of subgrade soils using a repeated load triaxial device. This test was performed on soil specimens by subjecting them to a deviatoric load for 10,000 cycles. The plastic strains are monitored during test cycles and the accumulated plastic deformations were determined. This thesis presents these plastic strain results conducted on five soils including natural clay, kaolinite clay, sand, coarse sand and silty clay. Effects of compaction moisture content and dry unit weight, confining pressure and deviatoric stresses on the plastic strains of soils are addressed. A plastic strain formulation model was developed and evaluated for predicting the plastic strains of soils. Evaluation studies indicated that the predictions are reasonable and can be improved by including more test data on soils. The plastic strains recorded in the present 10,000 cycles test did not address affect the resilient response of subsoils.
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