Development of a new tensile stress model for expansive soils

The problems associated with drying shrinkage are worldwide, and cause great damage to highways and buildings constructed on expansive soils. Literature review indicates that the development of shrinkage crack is governed by the principle of unsaturated soil mechanics. Suction increases as water content decreases. This leads to the increase of tensile stress, and cracks occur when tensile stress exceeds tensile strength. Currently, there is not an available model which describes the relationship between suction and tensile stress directly.In this research, a new model for predicting tensile stress from suction is developed based on the Summarac's model (2004), Mitchell's model (1979) and assumption that soil is elastic and isotropic before cracking. The validity of the new model is demonstrated using finite element method (FEM) and experiment. In the analysis of FEM, the "Heat Transfer Model" in Abaqus is applied to calculate the profile of suction, and the "3-D Stress Model" is used to predict the distribution of tensile stress in the subgrade. The soil specimens were tested using two initial moisture boundary conditions for their drying shrinkage characteristics. The objectives in selecting the two initial moisture cases were to simulate the field conditions. In the first case, the specimens were tested in the drying experiment from close to a saturated moisture content. The purpose was to simulate a heavy rainfall in the subgrade soil. In the second case, the specimens were tested in the drying experiment from an initial optimum moisture content. The objective was to simulate the subgrade soil condition after compaction.There are several applications of the new model in geotechnical practice such as the prediction of crack initiation from the soil water characteristic curve (SWCC), the estimation of crack depth for a given drying time, and the estimation of drying time for a specific crack depth. In this research, the depth of crack developing outside the pavement slab is estimated using the new model, and the performances of vertical moisture barrier and horizontal moisture barrier in presence of outside crack are analyzed and compared.The new model incorporates the principles of unsaturated soil mechanics for the analysis of the formation of cracks in expansive soils, and provides theoretical foundation for future design and treatment of subgrade soils