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Construction of large hydraulic engineering installations and particularly high-head dams in folded mountain rock has placed fresh requirements on engineering geology. The calculation of dams on rock foundations, i.e., on elastic homogeneous and isotopic media with such heavy loadings on the foundation as represented by modern installations, results in big errors in evaluating stability and stress of the installations, in stability calculations for large rock slopes and the banks of reservoirs, in addition to several other important problems. Consequently, various types of models are being used increasingly in engineering geology for reproducing the different properties of the foundations. At a certain stage of accumulation of data, the investigator finds it necessary to generalize, in order to appreciate the nature of the relationships and interaction between the separate factors, the foundation and the engineering installation, etc. These interrelationships and interactions are represented in the form of circuit diagrams, in the natural physical or mathematical form, i.e., in the form of models providing a simplified representation of the object studied. In order to resolve engineering-geologic problems, models are set up which, depending on the form in which the data relating to the natural phenomenon or installation is expressed, may be to scale (graphical, natural, or of equivalent material), physical (hydrochemical, optical, etc.), or conceptual and mathematical (determinate, statistical).Engineering-geologic models must conform to the following requirements: simplicity of construction and sufficiently accurate representation over the whole range of measurements; a high degree of adequacy of the natural system or structure modeled; suitability for use in the design of planned installations and their foundations by existing methods of the mechanics of deformed media and hydromeehanies.The scale model is the most common and useful, the prototype of the object being subdivided into sections, reduced for ease of investigation and simple representation. Included are models of dam foundations, bridges, reservoir banks, etc.Physical models (electro-hydrodynamic, hydrochemieal, optical) simulate the geologic phenomena on the basis of generalization of their physical nature. This type of modeling is based on the principles of similarity, for example, between the laws of motion of water and an electric current.Conceptual models represent the conceptual image of some natural phenomenon, i.e., schematic models expressed in the form of various diagrams, mutual relationships, and variants. Such models express certain phenomena in the form of a hypothesis. An example of this type of model in soil mechanics is that of various types of rock. Conceptual models are qualitative and help largely in the study of complex geologic processes and bodies, and are a necessary stage in their study.Approaching the investigation of a given phenomenon by means of scale, physical, and conceptual models, data is obtained, allowing generaliz...

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During excavation of the pit of the main structures of the Toktogul hydro development on the Naryn River, bottom release fractures filled with soft sandy loam material were found in the crystalline limestones of the valley bottom. These fractures, having a considerable length and opening, can be the cause of deformations of hydraulic structures as a result of normal and tangential stresses.The widely known term "flank release fractures" was introduced in 1953 by A. G. Lykoshin [1]; he explained the mechanism of load release in the flanks of river valleys. In 1959 G. K. Bondarik proposed to call the "exfoliation fractures" distinguished by Lykoshin in the valley bottom "bottom release fractures" [2]. The key to understanding the conditions of formation of release fractures is an analysis of the stress state of the rock mass.As is known, a rock mass is in a state of combined stress composed of tectonic stresses and the weight of the overlying rocks [3][4][5][6]. When driving mine workings the stresses around them are redistributed, which, according to Zh. Talobr, leads to the formation of three concentric zones: decreased, increased, and unchanged stresses [4]. It is natural to assume the presence of such zones in the flanks and bottom of a river valley, where redistribution of stresses is related with erosion downcutting. As a result of investigations* of the rock mass at the Sayan-Snsh ' (1963-1964) and Toktogul (1965) hydroelectric stations, V. M. Kutepov obtained values of natural stresses which permitred distinguishing zones in river-valley flanks corresponding to those around mine workings noted by Zh. Talobr [2]. Horizontal tensile stresses causing, according to Lykoshin [1,7], the formation of release fractures were also found in the zone of decreased stresses. Modeling of the stress state and seismic investigations of the rock mass in the area of the Toktogul station1" also confirmed the presence in the valley flanks of zones of increased stresses duplicating the outlines of the valley. The results of the investigation showed stress concentration near the foot of the slope.Thus, we obtained the following picture of the stress state of the rock mass at the site of the Toktogul hydro development (Fig. 1). A zone of decreased stresses is found in the near-surface part of the mass. Exceptions are inthe near-slope part of the valley bottom, where there are areas with increased stresses related to the effect of the weight of the flanks. The zone of decreased stresses with distance from the surface is replaced by a zone of increased stresses. This zone duplicates the configuration of the erosion downcutting. Deeper into the mass is a zone of unchanged stresses. In the zone of decreased stresses there is additionally a zone of tensile stresses. This is the zone in which flank and bottom release fractures form. The thickness of the zone of development of bottom release fractures is maximum in the middle part of the channel of the Naryn River and is 10 m.In the presence of intense tectonic jointing of rocks, which...

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