STUDY OF TIME COMPRESSION AND PORE PRESSURE DISSIPATION AT LINGANMAKKI EARTH DAM, SHARAVATHI VALLEY PROJECT.

Abstract

The Sharavathi River in Mysore State has an hydro power potential of 1.3 Million K.W. Under the Sharavathi Valley Hydro Electric Project, the biggest power House in India capable of generating 1 Million K.W. of power has been esta- blished. The Linganamakki Dam, the main storage dam of the project, has an Earth dam, 3280 ft long and 177 ft in height at the deepest section. The dam is located in an area of intense rainfall at times even exceeding 300 inches annually. It is founded on low density poor soil. Due to the non- availability of semi pervious soils, the design of the section has been done by providing suitable filters. Composite soil of different classification groups has been used with rigid quality control. An upstream free draining rockfill has also been provided. Three Cross Arm systems and nine embankment piezometer tips have been installed in the dam. The readings of the se- ttlement apparatus and piezometers have been analysed and com-pared with the theoretical analysis based on the compressibility characteristics of the soils. The total and the time rate of settlement for the foundation strata, embankment at two sec- tions, have been computed. As data on construction pore pre- ssures is not available theoretical construction pore pressures .based on Hill's method have been calculated for the placement conditions. The trend of steady seepage pore pressures, the effect of the variation of reservoir level on the pore pressures and the drawdown pore pressure coefficients have been examined from the commencement of reservoir operation. The consolidation of the foundation layer is found to follow the Terzaghi theory though the observed value is higher than computed. The total settlement of the fill lies between 2.4 to 3.2 % of the fill height involving a total stress of 129 pounds per square inch. The post construction settle- ment has been of the order of 0.26 to 0.5 %. The observed rate of settlement is more than the theoretical rate. Most of the settlement ie. upto 90% has been experienced during con- struction period itself. Settlement on saturation of fill has been negligible. The theoretical pore pressures are found to be only 40% of the fill height due to high air content. No dissipation of pore pressures are observed during construction shut downs probably due to climatic conditions. . The phreatic line as observed is found to agree with the Basic Parabola except near -.3- the Inclined filter which is reasonable. During drawdown, though not rapid, Bishop's pore pressure coefficient, B, has values ranging from 1.30 to 2.35 which indicate considerable reduction of pore pressure.