Softening of Fill Slopes due to Moisture Infiltration

Fill in slope areas is generally placed and compacted near optimum moisture, which is often well below saturation. At optimum moisture content, compacted clayey fill has high shear-strength characteristics because of negative pore pressures. After construction of the slope, additional moisture is introduced into the fill by irrigation, rainfall, or other sources. As water infiltrates the soil, the slope softens as the pore spaces fill with water and the pore pressures tend toward zero. The results of this study indicate that compacted clays are most susceptible to the slope-softening process while granular soils are relatively unaffected. If the factor of safety of a compacted clay slope is overestimated, as is commonly the case with surficial instability, then significant deformation of the slope will occur as moisture infiltrates the soil. The requirements for building setbacks should take soil types into account, with building setbacks being greater for compacted clays than compacted granular soils.