Seasonal wetting and drying affect pavements on expansive soils with two main damage types: roughness development and longitudinal cracking. the purpose of the moisture barrier is to isolate the subsoil from these climatic changes. the predominant type of damage and the function of the barrier are found to depend on the initial moisture conditions of the subsoil. for desiccated soils, the barrier must prevent the infiltration of rainfall into the shrinkage cracks to stop the development of roughness. for wet soils, the barrier must prevent excessive drying under the edge of the pavement. the barrier depth is chosen based on the maximum crack depth and the shrinkage of the pavement edge that would occur under the most severe drought intensity expected during the lifetime of the pavement. the drought intensity is chosen based on a statistical analysis of meteorologicaldata for the site. the driest matrix potential profile is obtained from the drought intensity through a simulation of unsaturated waterflow. this profile is used to determine the maximum shrinkage. the accumulated shrinkage with depth is used to calculate the total edgeshrinkage. a modified shrinkage, which includes the effect of multiple cracks, is used to simulate the propagation of the cracks. the rooting depth of native vegetation is identified as the controlling parameter on the barrier depth. a conservative estimate of the depth of grass roots is 8 ft. a successful moisture barrier must be placedto the depth of the roots in order to stop longitudinal cracking and about 25 percent deeper than the rooting depth to stop the development of roughness. this paper appeared in transportation research record no. 1137, soil mechanics considerations in arid and semiarid areas. for covering abstract see irrd no 817740.
Abstract