When water conduits operate with velocities exceeding 20-25 m/sec, high-vacuum regions are created, which result in the development of cavitation, followed by heavy erosion of the concrete, and in many cases by damages to the hydraulic equipment and the steel facings of concrete surfaces.This article presents some results of work carried out at the S. u Zhuk Scientific-Research Department of the All-Union Design and Scientific-Research Institute for Hydraulic Structures (NIS Gidroproekta), which permit preparing recommendations for the prevention of hazardous cavitation erosion of elements of hydraulic structures.Cavitation on Surface Irregularities of Water Conduits. In order to prevent cavitation erosion, the allowable dimensions of irregularities on the surfaces of water conduits should be specified for each structure.The degree of cavitation under a flow is determined by the cavitation index [1?. ]')o~ --/OjWoo c% in which L,e --2g P~,is thecharacteristicpressureinkg/mZ, v~oisthevelocityinm/sec, Pv is the saturated vapor pressure of the fluid in kg/m 2, and y is the unit weight of water, in kg/m 3.

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At the end of 1970 the dam of the Atbashi hydroelectric station [1] on the Atbashi River, a left-bank tributary of the Naryn River, filled to the head of the first stage of construction. The dam, designed for a head of 75 m, is constructed in the high-mountain, difficultly accessible region of Tien Shan with extremely complex seismic and geologic conditions. The dam site is located in a narrow rock canyon, in the lower part of which is a cut in the rock 30 m deep and 8-10 m wide; in the upper part to a height up to 200 m above the water level the steepness of the rock banks is 72-76*. The rocks consist of strong but fissured karstified limestones. The rock jointing is quite substantial, the joints being mainly of tectonic origin. Bedding joints and flank release joints are also noted.Of greatest danger as possible routes of seepage were the flank release joints dipping at an angle of 60-70* toward the right flank, extending parallel to the river for tens and hundreds of meters, and having openings as great as 50 era. Most of the joints are filled with compact sand-clay material or calcite. Along the leR flank many of the joints wedge out along the trend toward the surface owing to the curved arrangement of the channel. In all limestone members there are ancient karst forms with cavities measuring from several millimeters to tens of meters. Under these conditions the permeability coefficient of the rocks varies widely (0.2-0.01 m/day) and the groundwater level is located 15-18 m below the water level in the river. Percolation of the surface waters from the channel into the water-bearing horizon under natural conditions is insignificant. A layer of alluvial and coarse-fragmental materials with a thickness of 6-8 m is situated in the river channel. The seismieity of the construction region is 8.In selecting the type of dam the complexity of delivering building materials to the region, which is difficultly accessible and far from the railroad, was taken into account and therefore a concrete dam was rejected and it was decided to construct an earth dam with shoulders of gravelly earth. With consideration of the complexity of pumping water from a foundation pit and the available experience on compacting and solidifying alluvial soils by grout injection it was decided to place the dam on an undrained foundation with subsequent grouting of the alluvial deposits. A grout core in alluvial deposits can be linked reliably with a grout curtain in a rock foundation and in the flanks.On analyzing the construction conditions it was also recognized that the construction of cut-off devices in the dam in the narrow, deep, and difficuR1y accessible canyon by conventional methods with the construction of

The construction of hydro developments with high dams of large volumes lasts, as a rule, more than !0 years, which leads to immobilization of capital investments and reduction of their effectiveness in the economy.To avoid this, capacities are started up at intermediate heads with the dams still under construction, sometimes with the use of temporary runners. For example, the staged start-up of units of a hydroelectric station whose construction is being completed made it possible to produce additionally about 33 billion kWh of electricity during five years.The staged putting of structures into operation is specified by the design. A limit level of filling of the reservoir and minimum allowable profile of the dam ensuring satisfactory operation of the structure at the acting head are established for each stage. When assigning the dam profile of each stage it is assumed that, with consideration of staged construction, the stress--strain state of the dam of the complete profile at the normal pool level (NPL) will correspond to the calculated design assumptions.Among structures put into operation in stages are the dams of hydroelectric stations: the Sayano-Shushenskoe gravity-arch dam and the Nurek eartPr-rock dam.The Sayano-Shushenskoe gravity-arch dam with a height of 242 m and crest length of 1066 m has a base width of 105.7 m and crest width of 25 m. The dam is divided lengthwise by contraction joints into sections about 15 m wide. The sections consist of 27-m-long columns. A grout curtain I00 m deep and a drainage system are constructed in the foundation of the dam.The dam is located in a canyon with steep walls (800-900 m above the waterline).Its foundation is composed of schists.The climate of the construction region is markedly continental, the seismicity is 6-7.To monitor the work and state of the dam during construction and temporary and permanent operation, it is equipped with a considerable number of monitoring and measuring instruments. A monitoring system is created on the structure, which consists of about 3000 embedded sensors for determining deformations, temperature, and seepage pressure in the concrete, a piezometric network, a number of geodetic instruments, and devices for measuring displacements of the structure and rock foundation.The staged start-up of the dam requires timely cooling of the concrete masonry to the temperature of grouting the column and radial joints, promptintervention in the technology of constructing the structure, and effective checking of the curing of the placed concrete, which under conditions of Siberia is one of the primary problems.For this purpose, additional embedded telethermometers were installed in the dam.For provision of on-site observations, regulation of the temperature regime of the dam, and provision of timely sealing of the concrete masonry of the dam, a special service was created at the construction site which works in close contact with design and research institutes. One of the tasks facing the service was a determination of the effect of the temperat...

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