BEHAVIOUR OF GEOTEXTILE FILTER IN EARTHEN DAMS

Abstract

Filters are provided in the earthen dams to lower the phreatic surface within the dam while preventing the emergence of water from the downstream side of the slope, where flow could trigger erosion and safety of the structure may be jeopardized. For the filter to function properly it has to satisfy few requirements which are of primary importance. In projects where granular material is not readily available or where the appropriate aggregate sizes cannot be obtained in sufficient quantities the design and construction of filter can be a tedious and uneconomic job. In such scenarios geotextile filter constitute an attractive solution and can be used as a replacement for granular filter. This study has concentrated on functional comparison of conventionally adopted granular filter and geotextile filter. An attempt has been made to first design a suitable granular filter and then a geotextile filter for the given base soil (fine sand). Granular filter was designed following basic specifications ensuring no significant infiltration of base soil particles and seepage forces within the permissible limit. Geotextile filter was designed based on the test results of Apparent Opening Size (095) and Pennittivity. These fests were performed on the geotextile samples available and samples satisfying retention (for steady state condition), permeability and anticlogging criteria were chosen. To compare the seepaje rate, an earthen dam model was constructed in a steel tank using line sand as base soil and relative density was maintained to be 65%. At upstream head, water was fed and seepage quantity was collected from the downstream end. The filtration quantity throuZgD h downstream side of the earthen dam model was compared in three cases: without any filter, with granular filter and with geotextile filter (with three different geotextile samples which were chosen based on geotextile property tests from all the geotextile samples available). The effect of using geocomposite in place of geotextile was also studied and was compared with the above mentioned model tests with granular and geotextile filter. Model test results were also verified using SEEP/W - GeoStudio 2007 software.