Phase relationship equation for moisture induced shrink and swell of soils

Abstract

Expansive soils are the cause of significant structural damage every year. These problems result from differential volumetric changes in the soil structure. In this study, a procedure is developed for using the Phase Relationship Equation for Moisture Induced Shrink and Swell (PREMISS). The equation is based on soil weight-volume relationships and the continuity between the changes in air and water volumes within the soil. This equation uses seasonal soil moisture content changes over the depth of the active zone from local databases and soil properties prior to shrink or swell to predict the expected vertical movement beneath foundations. In developing the procedure for using PREMISS, two soil properties were investigated: the swell limit and the shrinkage limit. These limits are the soil moisture contents that correspond with extreme volumetric conditions. Evaluation of laboratory swell test results show that the swell limit probably does exist, and a procedure for measuring the swell limit of a soil is developed. In the shrinkage limit investigation, the Submersion Method shrinkage limit test is proposed. This method measures the undisturbed shrinkage limit as opposed to the remolded shrinkage limit measured by current ASTM Methods. There are two forms of PREMISS. One is the swell form, and the other is the shrinkage form. The manner in which the change in air volume relates to the change in water volume differs for shrink and swell. During swell, the air volume is decreasing, but the air volume change is limited by the initial volume of air. During shrinkage, the air volume is increasing, and the change in air volume is limited by the initial volume of water. How these volume changes occur during soil moisture changes is examined in this study. Evaluation of laboratory swell and shrinkage test results indicates that there is a linear relationship between the changes in air volume and the changes in water volume during shrink and swell. This linear relationship is consistent with PREMISS.