The results of the stability study of dam foundation

including the effects of ground stresses on the stability of base masses and long-term stability and strength of weak intercalations

especially of clayey ones

are presented. It is proved that the considerable tectonic stresses exist even in red rock layers with slight folds and a gentle terrain and that the stability of masses are directly affected by local stress relief. It is found that sedimentary rock behaves like rheology. A method of deter-mining long-term strength of rheological rock is suggested.