Temporal variation of the characteristics of sands in hydraulic-fill structures of the Riga hydroelectric station

Abstract

At present when designing hydraulic-fill structures the temporal variations of the characteristics of hydraulic-fill sand caused by processes of infilling of their voids, consolidation, and development of structural bonds are practically disregarded.The still unused reserve of a possible increase of the rate of hydraulic filling, compression of the dam profile when necessary, and increase of the permissible rate of reservoir drawdown lies in these processes which substantially increase the strength and dynamic stability of sands and reduce their compressibility and permeability.Theoretical and experimental investigations performed by the Department of Engineering Geology, Moscow Civil Engineering Institute (MISI), lead to the conclusion of the need to create animproved system of engineering-geologic control, including geological, technical (hydrotechnical), and ecological aspects of control, carried out at the final stage of survey and design works and during construction and operation of the structures.In particular, control investigations and prediction of temporal changes in the characteristics of hydraulicfill sands is important when constructing hydraulic-fill dams. On the basis of these works it is possible to refine the designs and requirements of the existing specifications (in particular, the design of the structure, rate of hydraulic filling, and control values of the indices of the quality of placing the fill).In a number of cases well-founded recommendations on a possible change in the operating regime of the hydroelectric scheme, for example on an increase of the reservoir drawdown rate to increase the peak-load capacity of the hydroelectric station, can be given on the basis of long-range forecasts and long observations during construction and operation of the structures.We will show the effectiveness of the recommended approach to conduction of engineeringgeologic control on the basis of investigations of the hydraulic-fill structures of the Riga hydroelectric station.The structures of the Riga hydroelectric scheme include two earth channel dams and three levees --left-bank, insular, and right-bank --with a total length of about 15 km and volume of more than ii million m 3. With respect to design they belong to narrow-profile structures with a maximum heightof 22 m, slope of upstream face m =3-4 and of downstream face m z =3-3.5, crest width i0 m; concrete slabs are placed on the upstream slopes from the crest to the berm, the downstream slopes are sodded.The structures were constructed by trestle-free hydraulic filling of medium-grained sands by300-40 suctiondredgesin stages during 1967-1975. Hydraulic-filling was carried out in layers up to 1.0 m thick.According to the design the earth dams and levees of the Riga hydrostation belongs to homogeneous hydraulic-fill structures.The limiting values of the particle-size distribution 9 of the hydraulic-fill sands were determined by the hydraulic-fill specifications; the control values of their dry desinty Pd =1"58 g/cm 3 and permeability coefficient ...