EFFECT OF INTERMEDIATE DRAINS ON STABILITY OF STRUCTURES FOUNDED ON SOIL OF FINITE DEPTH

Abstract

The stability of hydraulic structures founded on permeable soil has to be ensured against forces caused by percolating water. The seepage water exerts uplift pressure below the structure and tends to remove the soil particles at the exit and which may ultimately lead to piping. -Intermediate filters or drains can be provided below the floor of hydraulic structures founded on permeable soil to reduce uplift pressures resulting in appreciable savings. A solution was obtained by Chawla (6) for a drain of any dimension located anywhere between two cutoffs of a flat floor founded on permeable soil of infinite depth. Until now no solution was available to determine the effect of plane drainage located anywhere between two cutoffs of a flat floor founded on permeable soil of finite depth. In many cases an impermeable layer exists at shallow depth and downstream sheetpile cutoff may extend upto impermeable layer due to inadequate subsurface investigations or as a safe gaurd against scour. In such a situation uplift pressures equal to upstream water head would develop below the central portion of the floor unless the upstream sheet pile is also driven upto the impermeable layer or an alternative arrangement is made to release the pressure. It is impossible to provide a vertical cutoff completely free ffom leakage, unless it is made of concrete and is monalithic with the floor. In steel sheet—piles leakage may occur due to improper interlocking or corrosion of sheet—pile and therefore even (iv) providing a complete cutoff at the upstream end of the floor to intercept seepage cannot be relied upon. In such a situation, an intermediate filter could be provided to release the uplift pressures. In the present work, the problems of seepage below a flat floor, with a partial cutoff at the upstream end and a partial or complete cutoff at the downstream end, founded on permeable soil stratum of finite depth with an intermediate filter of any dimension located anywhere in between have been solved with the help of conformal mapping. In addition the solution has also been obtained using finite element method for the seepage below a flat floor with two partial cutoffs and an intermediate filter founded on subsoil underlain by sloping impermeable layer. The uplift pressures at key points end exit gradient at the end of the floor have been calculated for various combina-tions of the parameters involved. The results have been plott-ed in the form of curves which can be corveniently used for design. The results indicate that the uplift pressures on the floor and exit gradient at the end of floor reduce considerably with the provision of an intermediate filter of even very small length. Further reduction in pressure and exit gradient with increase in length of the filter is less as compared to the initial reduction. The uplift pressures decrease on the down-stream and increase on the upstream side of the ,filter as the location of filter is shifted downstream. (v) The uplift pressures on the floor and exit gradient at the end of floor decrease with decrease, in the depth of pervious stratum and increase with increase in the inclination of the impervious layer. The uplift pressures decrease with increase in the depth of upstream cutoff. The effect of depth of the downstream cutoff on uplift pressure is very small, however, the exit gradient reduces significantly with increase in depth of the downstream cutoff, An illustrative example of Narora barrage, using the deSign curves developed in the present work has also been included in Appendix A-1.The redesign with intermediate drainage shows 40, reduction in sectional area of concrete