在均质堤基或典型双层堤基中由于各种原因形成土洞，其轮廓线是拱形。将土拱近似为三铰拱，应用结构力学中三铰拱合理拱轴线的求解方法，推导了土拱的近似合理拱轴线方程。应用太沙基提出的土拱效应存在条件，推导了在粘性土和无粘性土的土拱的最小拱高和最小拱厚计算公式，分析了拱形参数的合理性及形成稳定土拱的条件。应用本文的土体坍塌机理研究可以用来判断土洞的稳定性。

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.