Groundwater flow and the stability of heterogeneous infinite slopes underlain by impervious substrata

The effects of steady groundwater flow on the stability of heterogeneous infinite slopes underlain by impervious substrata were investigated using a series of computer simulations. A preliminary one-dimensional analysis shows that the magnitude of hydraulic head perturbation that can be attributed to flow across an idealized heterogeneity is controlled by both the size of the heterogeneity and the hydraulic conductivity contrast. In the limit, a perfectly impermeable heterogeneity will give rise to hydraulic gradients that are controlled by the normalized length of the heterogeneity, whereas an infinitely permeable heterogeneity will cause the hydraulic gradient to vanish across the heterogeneity. A series of two-dimensional finite-difference models shows that individual heterogeneities occupying 10% of the slope area can significantly perturb flow fields and influence local factors of safety in hypothetical slopes. Although local factors of safety are reduced enough to indicate the development of small secondary slides as a consequence of flow through or around heterogeneities, overall factors of safety remain unchanged. When heterogeneities are closely spaced, however, their effects on the stability of the model slopes are cumulative and in one case the overall factor of safety was increased by about 2%.