Aménagement Caprivari-Cahoeira étude en charge

Abstract

This article refers to the design studies and work done in constructing the pressure shaft in the Capivari-Cachoeira hydro-power scheme, which has an installed output of 250,000 kW. The shaft operates under pressures of up to 85 kg/cm2. Its diameter is 3.0 m in the upper unlined portion and 2.90 m in the lined portion below it. The article begins with a theoretical study of stresses in indefinite porous ground out of which a cylindrical cavity has been hollowed out and filled with water under a constant pressure. The ground thus contains a flow, the resulting loss of head being simply considered as an additional volume force acting at all points in the ground. The calculation result confirms the Bratz theorem which states that the forces acting at any point are defined simply by adding the percolating flow pressure and the stresses in the ground, assuming pressure to be only applied on the surface. The discrepancies between the resulting conclusion and the results of two other investigations are explained by calculation : the strain assumptions differ in each case, but each has its own particuar field of application. A theoretical study of the effect of grouting on the percolating flow characteristics is then described. Next, the results of pressure tests on a cavity under conditions which are seldom if ever attained in practice are discussed. The cavity was 15 m long, with a diameter of 2.20 m and was subjected to a pressure of 65 kg/cm2. The test was also appllied to the cavity with unlined sides, then with a concrete lining grouted under high pressure. The load with stood by the rock was found to he practically the same in both cases, with little difference between the respective leakage rates. The analysis of losses over a period of time confirmed the results of a theoretical interpretation of these effects, it having been found that the filling of the ground with water appeared to reduce the losses, i.e. an effect which is generally ignored in standard tests on small-diameter wells. These tests were completed by tests in which small-diameter boreholes were put under pressures of up to as much as 100 kg/cm2 by the classical Lugeon test method. The results obtained are compared with the experimental cavity data. This enabled the upper limit of the lined shaft portion to be determined, which was found to be at a point at which the maximum static head was 400 m of water. Finally, the calculations whereby the lining thickness in the lower shaft portions were determined are discussed. These calculations as conventiona1 as regards internal pressures, but an original solution was found to the outside pressure problem which consisted in sinking a shaft running parallel to the lined shaft and providing drainage borings to eliminate dangerous water pressure in the ground when emptying the system.