The construction of high-head hydroelectric complexes on rock challenges hydraulic engineers and researchers with the problem of the correct evaluation of the deformation properties of these rocks in structure foundations, primarily concrete dams�9 The main design index of deformability of rock foundations used in the design index of deformability of rock foundations used in the design of concrete dams is the modulus of total deformation Dst. * In the USSR and abroad this modulus is determined by the method of experimental static loading of plates of different stiffness carried out in exploratory trenches and pits of structures at the developmental, technical design, and working design stages. The values of Dst thus obtained show marked variations. Experimental values of Dst have a conditional character, since the areas of the plates used in investigations (of the order of I m 2, sometimes exceeding 2-4 m 9-) are not comparable with the areas of the dam foundations. The depth of the active zone of deformation of the plate base, according to solutions of the theory of elasticity, is limited to a magnitude equal to 3-3.5 of its diameter, which is several meters. Thus, the values of Dst obtained by laborious and expensive experiments characterize the deformation properties of rocks in a very limited voIume of the mass, equivalent to a geometric point in the foundation of a large dam. The character of deformability of an anisotropic and inhomogeneous rock foundation to the depth of the active zone for 100-200 m high dams is evaluated by a large number of experimentally determined points. The number of experimental determinations of Dst is usuaUy limited to a few dozen points distributed on two to three levels of the exploratory trench at one dam site and gives only an approximate measure of deformation of the foundation. Thus, the present method of investigating the deformation properties of rocks in a mass by static loading of plates of limited size and at a limited number of points is approximate. Usually the large scatter of the values of Dst gives a probable error of design values, determined by mathematical statistics, in the order of ~20-30%.