Numerical Analysis of Horizontal Moisture Barriers in Pavements Constructed on Expansive Soils

Expansive soils experience large volume changes during the process of drying and wetting, and thus cause many problems to civil infrastructure such as roughness and cracks in pavements and uneven settlements in light-weight structures. Horizontal moisture barriers have the potential to keep the water content stable under the covered areas by increasing the length of water diffusion. The analysis and design of horizontal moisture barriers are currently based mostly on experience and empirical methods, and for instance, the influence of cracks developing outside the covered area on the performance of moisture barriers has not been investigated in detail. In this paper, the effects of parameters including unsaturated diffusion coefficient and drying times are analyzed using analytical and numerical methods. Variation of suction and settlement due to the shrinkage of the soil beneath and within the close proximity of the pavement (or covered area) are determined to evaluate the performance of moisture barriers. By comparing the changes of suction and settlement in a parametric study, the contributions of diffusion coefficient and drying times on the performance of moisture barriers are studied. An important parameter exerting critical effect on the performance of moisture barriers is the development of cracks forming outside the covered areas as a result of drying shrinkage. Outside the covered areas, shrinkage cracks can easily initiate and propagate during the drying time, and then become pathways for water infiltration and evaporation. The paper investigates the effects of outside cracks on the performance of moisture barriers. The analysis indicates that the performance of horizontal moisture barriers not only depends on the soil properties, but also depends on the climatic effects. For the same moisture barriers installed in different climatic zones whose soil properties are exactly the same, the performance of the barriers may be different because of the different drying times depending on the climate. The analysis also shows that the performance of horizontal moisture barriers is significantly degraded in the presence of the outside cracks. When the depth of outside crack reaches a specific value, the function of the horizontal moisture barriers can significantly be diminished.