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 funda-nental 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.
The foundation rock has often the following geomechanical characteristics The foundation rock consists of multilayers of sandstones, argillaceous siltstones and mudstones. These layers are sloping downwards under an angle of a few degrees only;
Each rock layer contains horizontal planes of weakness; this discontinuities may be continuous or interrupted and are generally filled with micas and muscovites and in some places also with granulated clays;
Most unfavourable are the interbedded weak zones, which are continuous with horizontal extensions of thousands to ten thousands of square meters; these intermediate layers are alternatingly composed of thin clayseams, fissured mud rocks (usually with a thickness of 10 to 50 cm), fractured cleavage boundary zones and then thin layers of fat clay. These soft seams contain the clay minerals illites and spurs of montmorillonites with a total base exchange capacity of circa 20 to 30 me/100 gr. and a surface area of 200–300 m2/gram.
Despite their small thickness, which in soma intermediate reams are only 0.5 to 5 mm they play a fundamental role in the strength characteristics of the foundation rock as they ma nhave continuous large extensions of thousands to tenthousands of m2 with ondulations of very small amplitudes and large wavelengths;
The above discussed clay seams are the more dangerous as they show excessive creep and relaxation properties like clays which are known to be influenced by physico-chemical changes in the percolating ground water.
From excavations and investigations in vertical shafts and boreholes it can be investigated how many of such intermediate weak layers can play an important role in the deformation and stability of the foundation.
For design purposes it is important to know the deformation characteristics and long term creep strength of these weak layers. For example on behalf of the Ghe Zhou dam the first large project it the Yangtze river, creep tests on samples with. an area of 50x60 cm2; large dimension field tests on blocks of 170x'30×1170 cm3, repeated loading tests for measuring the residual strength