Analyzed herein are the engineering-geological conditions within the area of the Tehri HPP underground structures which have been designed at the "Hydroproject" Institute. Complicated natural conditions, including high regional seismicity, required a complex of up-to-date geo-mechanical and geophysical studies and computational analyses to be performed providing the basis for elaboration of layout and design of power house and transformer hall. Particular attention in the paper has been given to the criteria of strength and stability of the rock mass surrounding the underground structures.
Trustworthiness of safety prediction for such large underground ca- caverns in rock masses as HPP machine halls is a multi-factor function. Of greatest importance, in our opinion, are those listed below:
adequate information about the properties of the surrounding rock mass.
properly selected mathematical model for analysis of strain-stress state (SSS) and stability of the rock mass;
validity of the reliability criteria applied.
At present, tens of large HPP and been underground machine halls have been constructed all over the world; various methodologies and reliability criteria were used in designing thereof. Complexity, often unique characteristics of the structure as well as a great diversity of natural and engineering-geological conditions for the would-be and already constructed structures still pose new problems for the designers and scientists. Recent trends are towards decrease in the scope of pre-design surveys of site investigations at the areas underground structures and the primary emphasis is placed upon the surveys, in-situ observations and tests just in the progress of excavation which provide the basis for revision of excavation procedures and constructions. Such approach proved to be good in practice and made it possible to shorten the construction period and save funds. However, the inadequate scope of surveys at the designing stage can raise difficulties in the course of construction (as it took place at the construction of underground Fortuna HPP in Panama) and in the long run - in wasting time and money. Therefore, it is of great importance to specify the proper scope of surveys and site investigations, which would allow serious mistakes to be eliminated; decision of the problem is a kind of art. As results from the analyses of the publications of International Symposiums and Conferences, the finite element method (FEM) as applied to elastic-plastic stage has been mostly practised in studying the SSS of large underground caverns. As a rule, solution of two-dimensional problem, Mohr-Coulomb criterion of plastic flow and, in the latest years, Hooke-Brown criterion were applied. Criteria of reliability of large underground excavations are mostly complicated and ambiguous. There are some proposals to take some certain values of relative deformation or rock mass deformation velocity in the cours of excavation as the reliability criterion (Deere, Isaza, Ferro & Giussani 1986). But these criteria can be applied only for limited categories of rocks and cannot be considered as general ones. Often the reliability of an excavated cavern is considered as a function of sizes of plastic zones (zones of ultimate SSS) surrounding the cavern. In some other papers this factor is ignored, and interfacing of plastic zones in the area between the machine hall and transformer halls is allowed, and rock bolts are fully installed in the plastic zone. Considered below are the issues associated with study and validation of reliability of machine and transformer halls caverns at the Tehri HPP (India), designed at the "Hydroproject" Institute (Moscow, Russia).