The stability of an arching slope in deformable materials above strong rocks strongly de-pends on the shape and width of the span. Equations for a free surface problem that incorporate these two parameters were derived using a simplified two-dimensional arching slope model, and were vali-dated using physical model tests under 1g and centrifugal conditions. The results are used to estimate the maximum excavation width for a weak claystone slope in a lignite mine, for which we calculate a safety factor of 1.31.