DRAWDOWN PORE PRESSURES IN EARTH DAMS

Abstract

The prediction of drawdown pore pressures in earth dams, is a major problem for which the available solutions still lask finality. The effect of drawdown is two fold. Firstly it establishes new boundary conditions for the flow of water with changed phreatic surface and secondly the changes in stresses due to drawdown, cause time dependent change in the pore pressures. The various factors that affect the drawdown pore pressures are permeability, ani so tropy , drainabili ty, , comp re s si-bill ty, air content of soil, capillary action, geometry of dam section, rate of reservoir lowering etc. There is no unanimity among the various researchers about the magnitude of pore pressures to be accounted for at the end of drawdown in the de sign of an earth dam. One view is that only the pore pressures given by gravity flow net assuming the fill as rigid, need. be considered whereas the another view is that compressibility plays an important role and the residual pore pressures being due to The process of diffusion, would be hi gher than those given by flowne t analysis. So far, the analysis of observational data is inconclusive as tothether the pore pressures correspond to incompressible or compressible behaviour of the soil, and in the latter case how the effect of compressibility should be accounted for....