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DESIGN OF HORIZONTAL FILTER LENGTH IN HOMOGENEOUS EARTH DAMS

ABSTRACT Earth dams are constructed to store or divert water. They are economical because they are constructed with locally available materials. A homogeneous section in an earth dam is adopted for low to moderate heights. A purely homogeneous dam section poses the problem of seepage leading to sloughing and piping if it is not quite wide at base. An earth dam can be prevented from the seepage failure by providing the filters. A filter can keep the top How line or phreatic line well within the body of dam. As the phreatic line follows a parabolic shape, there is some minimum length of downstream drainage filter to prevent the downstream slope from the softening due to seepage. Since earth dams are made of fine-grained soil, the softening may occur due to the capillary rise; hence an account for the capillary rise is necessary while calculating the minimum length of downstream drainage filter. Analytical solutions are not available for determining length of filter and downstream slope cover, though graphical solutions are available for them but they are long and involve personal skills and judgement. Explicit equations have been obtained in the present work for calculating downstream slope cover and length of downstream drainage filter. Similar equation has also been obtained for minimum effective length of drainage filter. These equations arc non-linear and representative graphs have been plotted for them covering all the practical ranges of the dam geometry.