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A number of special problems requiring quick and reliable solutions arise during construction of hydraulic structures. One of the main problems is the study of the state and determination of the thickness of the zone within which to remove weathered and disintegrated rocks in concrete dam foundations. The possibilities of seismoacoustic methods and their rational combination with other types of investigations when evaluating the degree of preservation of a rock foundation are discussed in this article as related to investigations in the foundation pit of the Ust'-Ilimsk hydroelectric station spillway dam.Usual engineering-geologic methods do not provide continuous tracing of the bottom of the removal zone and are open to subjectivity in evaluating rock preservation on the basis of cores from holes having a too-small diameter (110 ram) for a reliable characterization of large rock volumes. Large-diameter inspection holes (915 mm) have a number of advantages when evaluating the quality of the dam foundation in comparison with core-sample holes and in limited areas permit a more accurate determination of the thickness of the rock subject to removal. However, drilling inspection holes in diabases is very labor-consuming , is limited by the number of rigs available, and can be done only in comparatively small numbers.A determination of the bottom of the removal zone (zone of disintegrated diabases) is not always possible either on the basis of water permeability, since in rocks it seldom reflects their true jointing, In this connection the suggestion was made to use seismic methods for determining the magnitude of the removal zone. Setting up of seismic investigations was justified by the presence of relationships between the propagation velocity of elastic waves and the degree of jointing and weathering of rock that have been weli substantiated experimenrally and theoretically [1, 5,6].The foundation of the Ust'-Ilimsk station concrete dam is composed of diabase trap rocks which are characterized by a comparatively low jointing and great strength. Despite this, a zone of increased jointing due to weathering processes and natural stress relieving is observed on their surface. Both the depth of the weathered zone and the degree and character of disintegration of the trap rocks in it are variable.As a result of detailed engineering-geologic investigations in the foundation of the concrete dam of the Bratsk hydroelectric station, the weathered diabases were divided into zones A, B, and C. Analogous zones were distinguished also in the foundation of the Ust'-Ilimsk dam.Zone A (uppermost) is characterized by a considerable change in the mineral composition, ferruginization of the rock, and intense crushing, to the extent that the rock is practically converted to gravel and fragments. Its strength decreases markedly.Weathering in zone B is expressed in schist formation of the rock or in the formation of severely ferruginized areas with decreased strength along the fracture planes. As a consequence of this the rock tumed bro...

The 271-m-high arch dam of the Inguri hydroelectric plant was constructed under complex engineeringgeologic conditions. The carbonate rock mass, which serves as the foundation of the structure, is weakened by systems of tectonic cracks and discontinuities, while its surface has been altered considerably by exogenous processesTo ensure the stability of the dam, a cutoff trench from 50 to I00 m wide and 30 m deep was made around its perimeter in the rock. The depth of the cutoff was assigned proceeding from the excavalion conditions of the rock in the zone of vigorous surface weathering. Thus, relatively safe sections of the mass should serve as the foundation of the dam. The boundaries of the cutoff were established through detailed engineering-geologic studies of the rock properties from exploratory tunnels and test holes, and also from data derived from numerous seismic observations of surface profiles and in mining excavations [3]. Rock that exhibited a high degree of jointing (a crack void space n = 8-10%)0 low strength, and increased deformatibility,andwas characterized by longitudinal-wave spread velocities Vsp of less than 2000 m/see was removed [i-3]. In this case, it was assumed that the exposed seerions of the mass outside the zones of tectonic discontinuities and crushing will possess adequately high strength and deformation indicators, which will permit the direct placement of concrete on the rock. After stripping operations were conducted by blasting using blast holes and a smooth cleavage [4,5], visual Inspection of the exposed rock indicated that its quality was inferior to that expected. The seismic method was employed to evaluate the properties of the exposed rock quantitatively, Methodology Employed for Making Observations and Interpreting Their Results. The following entered into the sdsmic investigation: determination of the degree of variation of the rock in the "surface section of the cutoff, establishing the zone widths of the rock weakened by the blasting operations and studying the structure of this zone, and a more precise definition of the properties of the rock below the zone of weakness, The basic data on the properties and structure of the weakened zone were obtained from investigations of surface profiles spaced 10-20 m apart along the perimeter of the trench. The studies were conducted by the method of refraction waves in accordance with the methodology of longitudinal profiling and a multiple-point observation system. Observations were made with seismic receivers spaced 2 m apart, a travel-time bath approximately 100 m long, and an explosive interval of 10-12 m. This interval was adopted to increase the degree of detail in the weakened-zone study. Elastic oscillations were produced by blasts from small applied charges. To improve elasticwave reception, the seismic receivers were set into special bore holes 0.1-0.2 m deep and seated tightly against the walls of these holes by wedges, The studies were conducted using a portable 24-channel seismic station with an oscillograph-paper sp...