Relaxation and creep properties of thin interbedded clayey seams and their fundam ental role in the stability of dams

In many cases gravity dams must be constructed on complex multilayered rock foundations, which consist of consecutive layers of mudstone, argillaceous silt stone and sand stone. These layers are in many cases separated by thin interbedded seams of clay, which may be continuous or nearly continuous over very large extensions. In such foundations the relaxation and creep properties of these weak seams play a fundamental role in the stability of dams and they must deserve our main concern in field and laboratory testing. In this paper relaxation tests are described, carried out according to the multistep method after Tan. The results are analysed and constitutive equations are set up; a method is proposed for the determination of the longterm strength. In the case of nearly horizontal layers the designing principle is suggested that the horizontal stresses must be transferred to deeper rock layers and four types of foundation are recommended.