The problem of determining the best profile for a c, φ soil slope is tackled, with reference to rigid rotational failure mechanisms. By means of limit analysis, upper bound values of collapse load are determined. A new mathematical function is derived to compute the external work made by the weight of a double spiral shaped soil region sliding away. It is found that a log spiral front is associated with a larger safety factor than a planar one. The obtained results may also explain the log-spiral shape of perimeter walls of some typical Japanese castles.