Procedures to predict vertical differential soil movement for expansive soils

Abstract

Damage to lightly loaded structures, paving and service piping in areas of expansive clay soils has occurred throughout the world. The cause of this damage has been the inability to accurately model expansive soil movement so that foundations are adequately designed to withstand the movement. The amount and rate of differential soil movement for expansive soils is due to a combination of soil characteristics, namely: suction compression index, unsaturated permeability, and diffusivity. Currently, geotechnical engineers run tests to measure the soil properties required to estimate differential soil movements. However, there seems to be apprehension toward attempting these soil movement calculations due to the perceived complexity of the calculations or a simple lack of understanding of the theory. The procedures delineating the step by step process used to calculate suction profiles and volume strains of expansive soils is presented. These procedures include the methodology to predict soil heave and shrink underneath shallow foundations which generate maximum center lift and maximum edge lift slab distortion modes. The main contributions of this research are: equations and procedures to calculate the equilibrium suction profile and depth to constant suction for a particular soil profile and location, equations to calculate the horizontal velocity flow of water in unsaturated soils, the methodology to predict differential soil movement shortly after a slab has been constructed and before the soil under the slab has reached an equilibrium moisture content, and the procedures to apply differential soil movement theory to soil profiles with shallow foundation design.