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.

In May 1978 at the construction site of the Kurpsa hydroelectric station the Naryn River was switched into a temporary tunnel constructed on the right bank.Cofferdams are being constructed in the channel for passage of the river discharges during the construction period: an upper one with a height of 33 m and lower one with a height of 20 m, and the temporary tunnel operating in a variable regime (Fig. i). The design maximum discharge (3680 m'/sec) of 0.01% probability with consideration of transformation of the flood in the Toktogul reservoir passes through the dewatering outlet (1030 m'/sec), tunnel spillway (1680 m'/sec), and units of the hydroelectric stations (970 m'/sec).The discharge capacity of the temporary tunnel in the contract design was assumed equal to 1800 m'/sec (1% probability). However, since the Toktogul reservoir by the start of construction of the Kurpsa station was not filled, the design capacity of the tunnel was reduced to I100 mS/set. MAIN DESIGN VOLUMES OF WORKS ON THE TUNNELOpen excavation, I0" m' 9.5 Open concrete, i0 a m a 4.2 Underground excavation, l0 s m' 68.5 Underground concrete, 10' m" 10.1 Gunite 8-14 cm thick, 103 m s 1.6 Concrete of plugs (in access tunnels), I0" m' 0.5 Borehole drainage, i0 a m 2.3 Steelwork of entrance portal, tons 284.1The temporary tunnel with a total length of 634 m occurs in bedrock composed of intercalating strata of thin-, medium-, and thick-layered sandstones and argillites whose beds intersect the river valley in a transverse direction with a 40-70 ~ dip toward the upstream pool. Sandstones account for 70-75% of the thickness of the rock strata, and argillites 25-30%. The rocks are broken by tectonic joints of various systems and orders.According to the data of englneerlng-geologlc surveys, the strength coefficient on M. M. Protod'yakonov's scale ranges from 2 to 8, cohesion in the rocks is from 0.12 to 0.3 MPa, the rocks are not slakable. The open width of the tectonic joints ranges from 1 to 300 mm, the joints are mainly filled with gouge. Zones of increased fracturing with a thickness from 0.3 to 5 m are traced along the joints.The temporary tunnel is made up of the following structures: entrance portal, tunnel, and exit portal.The construction and assembly works were carried out from March 1977 to May 1978 --14.5 months.The tunnel was constructed by the mining method by driving with two benches. Two approach tunnels were constructed to accelerate the works: No. 1 in the region of the exit portal with a cross section of 22 m a, length 30 m, and No. 2 in the region of the upstream cofferdam with a cross section of 25 m 2, length 163 m. Tunnel No. 2 was originally made at

Cavitation damage to overflow spillways, tunnel spillways, and dewatering outlets has been observed at many high-head hydrostatlons. Damage to individual excess flow release structures was so considerable that it prevented normal operation and required repair after each flood. In such structures, operating at heads up to I00 m, the depth of erosion of the concrete reached more than 3 m and cavitation encompassed areas measured in tens and hundreds of meters, and the volume of concrete carried away amounted to hundreds of m s . Cavitation penetrated through the 1-2-m thick concrete linings used in the tunnel spillways, on the chutes, and in the diversion flumes, after which erosion of the adjacent rock began, accelerating further damage to the lining. Repair of the cavitation damage was a laborious job durin 8 a limited period, since, as a rule, it ought to be finished between floods and, desirably, in the summer. Under rigorous climatic conditions, when the warm season is relatively short and a considerable part of it is taken up by the passage of the flood, it is necessary to construct special heated enclosures for high-quallty performance of repair work in the winter.The situation requires that special attention be devoted to the problem of cavitation erosion in the designs of excess flow release structures and that the measures necessary for preventing damage be specified.

The Commission of the Ministry of Power and Electrification of the USSR (Min~nergo) inspected the temporary diversion tunnel of the Kurpsa hydroelectric station, made with a lightweight lining, after its operation for 2 years and 8 months.The tunnel was originally designed with the following parameters:tunnel length, 634.1 m; inside cross section, 76 m=; type of lining, monolithic concrete with a thickness from 40 to 70 cm; roughness coefficient, 0.071; discharge capacity, ii00 m3/sec; concrete of the invert, grade 300, and of the walls, ceiling, and rings, grade 200.To accelerate construction, the design was revised for the purpose of lightening the lining.This resulted, as an experiment, in the monolithic concrete lining on a tunnel section 371 m long being replaced by a Gunite lining with concrete grade 300 with a thickness from i0 to 15 cm. The monolithic concrete lining was left in the invert, in the portal sections, and at places of tectonic disturbances of the rock foundation.The roughness of the surface for calculations by the Chezy-Manning formula was assumed to be 0.013 in sections with the concrete lining and in sections with a Gunite lining, 0.030 on the roof and 0.025 on the walls.The cross section of the tunnel in connection with the change in the type of lining was increased up to 85-100 m 2 to ensure passage of the design discharge.

The book under consideration is devoted to the extremely critical and real problem of constructing dams from earthen materials in the Far North. This problem has very abruptly confronted builders, designers, and researchers in connection with the extensive nationaleconomic exploitation of the north and northeast regions. The publication of this book, which is comparatively small in volume but filled with factual data, is therefore extremely useful.The book is written by persons who participated in the construction of the complex layout of engineering structures making up the Ust-Khantaika hydroelectric plant and contains a description of the basic structural solutions and new technological procedures employed in raising the dam. In this case, the authors attempted to analyze this experience, compare it with available domestic and foreign construction experience, and state the basic nodal problems that still require further development and study. The orderly format of the book and clear-cut accounting on a good engineering level promote utilization of the perused material.The first two chapters are devoted to a description of construction conditions and general information about the hydraulic facility and to the special features of the structural solutions adopted for the dams, whose foundation is composed of both rock (the channel dam) and loose thawed soil with inclusions of permafrost lenses (the right-bank terrace dam).The third chapter, which, in our opinion, is one of the most interesting, presents data on procedures for preparing cohesive soils for placement in the antiseepage components of the dams. The principal method employed to prepare cohesive soils for winter placement in the body of the dam was first developed and implemented during construction of the Viluisk hydroelectric plant; however, certain natural climatic characteristics of the region encompassing the Ust-Khantalka plant required changes in a number of technological operations, which are outlined in detail in this chapter. In the Far North where the useful stratum of quarries is extremely thin and usually limited to the seasonal-thaw layer, and the soils themselves are of poor quality in terms of gradation and are frequently saturated, the construction of an earth dam from a quarry operation is virtually impossible.