Unsaturated soil mechanics principles to remove and replace mitigation for expansive clays

The efficacy of removal and replacement as a mitigation alternative for expansive soil is discussed in view of numerical unsaturated flow/deformation results. Observations with respect to selection of depth and replacement soil type (similar, more, or less permeable than underlying expansive clay) are presented in the context of unsaturated flow and stress/deformation principles. Comparisons are made between various remove and replace options on the basis of extent and degree of wetting (changes in soil suction) and on ground surface total and differential movements. Selection of optimal removal and replacement depth and replacement material type requires consideration of unsaturated flow principles and understanding of the role of both net normal stress and matric suction in unsaturated soil response. Complexities in the unsaturated flow process which impact the efficacy of the remove and replace method are primarily because of the highly nonlinear nature of the unsaturated soil storage function (SWCC) and hydraulic conductivity function. Further, because the unsaturated hydraulic conductivity of soils depends on the value of soil suction, placement water content of the replacement layer has significant effect on performance. The net result is that the impact of various remove and replace options is not always intuitive. The findings presented, although not encompassing of all possible removal and replacement configurations or surface flux conditions, set the stage for application of remove and replace mitigation methods based on fundamental unsaturated soil mechanics principles.