MODELING OF PAVEMENT SUBGRADE BEHAVIOR UNDER MOISTURE VARIATION AND FREEZE-THAW CYCLES
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Very often deformation of pavement subgrade involves soils that are partially saturated. Many of the past constitutive models have limited application to modeling the behavior of such soils as their development was focused on either dry or fully saturated soils. This study uses the recent advances in unsaturated soil mechanics to modify the formulations of the Drucker Prager Cap (DPC) model. The effects of moisture variation freeze-thaw cycles on clayey and silty pavement subgrade behavior are studied using this modified DPC model. Simplified procedures for the determination of model parameters are developed. Laboratory tests on three types of silty and clayey soil were performed; the specimens were prepared with several moisture contents and subjected to freeze-thaw cycles. Test results show that for all three types of soils, the yield stresses of the specimens with lower moisture contents are higher at the same volumetric strain; elastic modulus, the initial and failure Cap position and the aspect ratio of Cap surface are lower when specimens have high degree of saturation. The elastic modulus and all the parameters of Cap surface of all soil types are significantly higher under frozen conditions, especially for the specimens with high moisture contents. On the other hand, the elastic modulus of soils is slightly decreased after freeze-thaw cycles while the parameters of Cap surface remained the same. The modified DPC model was implemented into the finite element program and used to simulate the behavior of pavement subgrade subjected to traffic loads under varying moisture and free-thaw cycles conditions. A three-dimensional finite element model representing the geometry properties of full-scaled APT tests was created. Partition methods were for defining layers of asphalt, base and subgrade and proper boundary conditions and loads were applied. The performance of DPC model was verified with APT test data, then the behaviors of pavement subgrades under differing asphalt and base properties, saturation levels and freeze-thaw cycles were simulated. Simulation results show that the maximum deformations of subgrade are lower when the asphalt and base have high elastic modulus, or under low moisture contents and freezing conditions.