The shape of archy soil holes developed in uniform or typical double-layer dike foundation is simplified as three-hinged arch in this paper to deduce the approximate reasonable arch axis equation for soil arch by applying the solving method of reasonable arch axis of three-hinged arch from structural mechanics. Furthermore, the conditions for soil arching effect presented by Karl Terzaghi is employed to define the minimum arch height and the minimum arch thickness, and to analyse the rationality of arch parameters and the conditions for stablized soil arch. The mechanism of soil collapse presented in this paper can be taken to estimate the stability of soil holes.