Engineering-geologic justification of the planning of antiseepage measures

Abstract

The impervious curtains, screens, and upstream aprons of most of the existing dams were designed and constructed by taking into account, to a greater or lesser extent, the engineering-geologic and hydrogeologic conditious which characterized the seepage through the rock in contact with the structure. However, in most cases, the evaluation of these conditions was mainly of a qualitative nature. The writer does not know of any dams for which the parameters of the impervious elements, and primarily of the impervious curtains, could have been determined analytically. In the best cases modeling by the electrohydrodynamic analogy method was used, but. as is natural, with significant averaging of the seepage characteristics and schematizing of the boundary conditions. Since this approach is typical, it may be concluded that the main factors adopted as the bases for the design of impervious elements are the following: general characteristics, interpretation, and purposeful evaluation of the engineeringhydrogeologic conditions. The main type of investigation for this purpose is the hydraulic verification of the wells, which makes it possible to disclose the zonal distribution, and sometimes the value of the permeability, of saturated rock, and to evaluate the conditions and need for grouting the saturated rock. as well as the unsaturated rock in the aeration zone.Impervious curtains can be divided into two types: a) foundation curtains, located within the boundaries the river-valley portion limited in the vertical direction by the normal water level, and b) bank curtains, between the normal water level and the bank rock masses. The main hydrogeologic characteristics determining the parameters of foundation curtains are: a) presence and depth of practically impervious or very weakly pervious layers into which the curtain may be closed, thus making it complete, and b) presence or absence of a regular decrease of the permeability as the depth increases, associated with a decrease of the jointing or karat characteristics of the rock. These two factors almost always determine the depth of the curtains, with the exception of some relatively infrequent cases in which the curtain has a purely constructional significance; that is, when it is designed as an incomplete curtain ensuring only the required reduction of the uplift pressure. The required depth of these curtains is nsually determined by modeling (electrohydrodynamic analogy method) and is verified by rough analyses based on approximate schemes. The most difficult step is the establishment of the upper boundary of the zone which, from the standpoint of regularity of decrease of the water conductivity through the joints or the karat formations as the depth increases, is considered to be sufficiently impervious, that is, adequate as support for the curtain. Examples of the construction of curtains on the basis of this principle are the curtains at the Pavlovsk. Cbarvaksk. and Ingursk plants, and, abroad, at the Grou. and Saint Croix dams. among others. The number of complete...

Engineering-geologic substantiation on the design of cutoffs

“…An analysis of more than 500 experimental pourings and injections of water into boreholes at the Nurek hydro development site, whose foundation is composed of intercalating strata of sandstones and siltstones, and the Toktogul hydro development, where crystalline limestones are widespread, showed [2] that the magnitude of specific water absorption, calculated on the basis of discharge during the first 30 min of injection, is quite close to the results obtained upon stabilization of the discharge with an incremental duration of 3 h-The coefficient of correlation of the indicated quantities varies from 0.8 to 0.9, increasing for larger values of water absorption. Lykoshin [1] and L. P. Konyarova [3] arrived at the same conclusions. A.…”

“…* Continuation of the discussion of A. G. Lykoshin's article [1]; see also the article of E. E. dallas and the editor's comment, Gidrotekhnicheskoe Stroitel 'stvo, No. 4, 38-42 (1974).…”

“…

A number of urgent problems related with the investigation of the percolation properties of hydraulic structure foundations and the development and implementation of cutoff measures were touched upon in the article of A. G. Lykoshin [1]. His suggestions were as follows.

…”