Influence of Wet-Dry Weathering on Hydraulic Conductivity and Infiltration Rate for Fine-Grained Soils and Weathered Shales

Soil hydraulic conductivity and accompanying infiltration rates are primarily a function of mineralogy, biogeochemistry, particle size, density, and particle arrangement. Soil hydraulic conductivity is typically thought of as a constant value temporally; Once established it will continue to be that value forever. However, historic and contemporary failures of slopes and earth structures have shown that it can change significantly with time due to external environmental stresses. Seasonal weathering cycles (such as wet-dry) act as external stresses on a soil mass and earthen structures. As seasonal weathering cycles impact the micro and meso-scale intrinsic and engineering properties of soil masses on a temporal scale, the net hydraulic conductivity in the active zone becomes altered. This causes bulk engineering behavioral properties to vary and impact the global stability of earthen structures over time as alterations from seasonal weathering cycles compound upon one another. For this research study, the seasonal weathering mechanism of wetting-drying desiccation cycling was used to measure degradation of soil hydraulic conductivity (K) over time. A highly plastic silty (MH) weathered shale and sandy lean clay (CL) soil were used to investigate how seasonal wetting-drying cycles impact the hydraulic conductivity of each soil with increasing cycles. To accomplish this, each soil was molded at its optimum moisture content in accordance with ASTM D698 then exposed to 0, 1, 2, and 10 wetting-drying cycles. After each weathering cycle, each soil was saturated by use of a gentle back pressure. After saturation, hydraulic conductivity measurements of each soil was then measured in accordance with ASTM D5084 using the constant head method. Increasing wet-dry cycles resulted in a 70% decrease and over a 5,000% increase in hydraulic conductivity for the CL and MH material, respectively.