Estimating Slope Stability Reduction due to Rain Infiltration Mounding

Rainwater can infiltrate into slopes and earth dams. Given sufficient infiltrating rainfall intensity, frequency, and time, the phreatic line within the slope or dam will "mound" above its otherwise steady state position. Such a mound will increase downslope seepage and reduce downslope stability. This paper develops an approximate method for solving this transient seepage "mounding problem" and the resulting reduction in stability. It shows the following variables will reduce stability: increased rainfall intensity and duration, decreasing permeability-especially over the 10^-2 - 10^-4 mm/s range-and its anisotropy, increasing the slope cotangent, decreasing the depth to an impermeable underlayer, or increasing the depth to a permeable underlayer. Increasing size and effective porosity has only a time effect. In special circumstances the mounding effect can reduce slope stability by 50% and the paper includes a slope case history example.