地下水的存在给工程带来了巨大的危害。分析研究了地下水引起的接触面积的变化，并推导了饱水情况下平面滑动的临界角变化公式。λ越小，发生失稳破坏的临界角θ就越小。而地下水压力p的增加有助于接触面积A*的减小，造成了在水压作用下土体更容易发生失稳破坏。在地下水的作用下，剪切破坏的摩擦系数是一个变量。土体软化后，有利于边坡失稳。

Ground water is almost always to bring a huge harm to the foundation projects. The change of contact area of earth caused by ground water is analysed, and the formula to calculate the critical angle of planar slide under the saturation condition is derived. The calculated result show that, the smaller the λ (real contact area A of earth mass to slide area A), the smaller the critical angle θ of failure. However, the real interface area A will be lessened with the increase of ground water pressure, which makes the slope resily lose its stability.