EFFECT OF PARTIALLY CHOKED PRESSURE RELEASE SYSTEM ON EXIT GRADIENT AND UPLIFT PRESSURES

Abstract

Percolation of water below hydraulic structures founded on permeable soils causes two fold adverse effects namely uplift pressure and piping. Besides designing such structures for forces duE- to surface flow, their stabil-ity against forces caused by seepage wat'r has also to be ensured, Adequate factor of safety arainst such forces is achieved by providing a suitably designed pressure release system including an inverted filter and pressure release pipes at the end of structures. These, when partially or fully choked, may seriously affect the stability of the structures. study on the effect of choking to various degrees of both the filter and the pipes on uplift pre-ssures and exit gradient has been presented. Boundary conditions, which govern the seepage flow, restrict the analytical solution to some simple cases, y series of experiments has, therefore, been conducted to carry out the present study, for which eleetrical analogy technique has been adopted, The results of the experimental studies have been compared with the analytical values for some cases, Series of curves have been drawn to indicate the effect of choking of the filter with fixed effectiveness of the pipe and effect of cho3iing of tlr i~ipP wa. l? fixed effectiveness of the filter,Curves have also been presented which give the cumulative effect of simultane-ous choking of both the filter and the pipe for each of some selected salient points on the foundation profile. Analysis has been made of conditions which may result in excessive and unbalanced pressures thereby endangering the safety of the struture. The studies indicate that the choking of the pipe increases the uplift pressures significantly. It is also found that provision of both the filter and the pipes substantially decreases the chances of arising unsafe conditions due to choking of the system as compared to the case when only filter is provided.