Design methodology of ash dyke using geosynthetics considering hydraulic conditions

Ash dyke is an earthen embankment which is constructed at every thermal power plant for disposal of fly ash slurry. Seepage of water takes place into the dyke from the fly ash slurry. In order to control the seepage inside the dyke, conventionally ash dyke is provided with chimney and blanket drains, which required sand as a drainage material. Today sand is not often available at various sites in sufficient quantity and required quality. Hence, for dyke construction sand is transported from other locations. Transportation cost of such a huge quantity of sand from long distance can substantially increase the cost of construction. The deficit in availability of sand in dyke construction motivates to find an alternative solution. The various geosynthetics material like geotextile, geonet, geomembrane, geocomposite and perforated pipes are available. These materials are being used in various geotechnical structures like retaining wall, filter for rock toe, drainage trench for highway, liners for landfill and many more applications. In all the above structures functions performed by geosynthetics are similar to the function of chimney and blanket drain in ash dyke. The objective is to design ash dyke system such that, it does not require substantial amount of sand in dyke construction. Instead, it will use geosynthetics materials for proper functioning. Two alternative designs of ash dyke system have been proposed, and both of these have application of geosynthetics material to control the seepage in dyke body. These alternative designs are ash dyke with vertical drainage composite and ash dyke with horizontal drainage composite. The design of Ash dyke with vertical drainage composite is similar to the ash dyke with chimney drain. It consists of vertical drainage composite, drainage trench, horizontal drainage system and horizontal drainage composite. In this design a vertical drainage composite is placed at centre of the dyke to collect seepage water, which is connected to the drainage trench. A horizontal drainage system consisting of lateral pipes having initial portion perforated is connected to drainage trench, which collects seepage water from drainage trench and drain out from the dyke body. A drainage composite is also provided in downstream side of the dyke to collect seepage from foundation soil. Ash dyke with horizontal drainage composite consists of horizontal layers of drainage composite and down slope drainage system. Drainage composite in the dyke is installed in layers at different levels and top drainage composite is extended vertically up to height of fill slurry to collect seepage water. Arrangement of horizontal composite is depends upon the quantity of seepage. These drainage composites are connected to the downslope drainage system to drain out the seepage water. In both the alternate design raising of ash dyke is done by upstream method of raising. The methods proposed in this study have been developed to design each component of both the alternative ash dyke system by using the theories developed in the literature for the design of geosynthetics in above mentioned other structures. Also detailed specifications and various factors to be considered in the design of each component of ash dyke have been proposed. A step by step construction procedure for both the alternative designs has been proposed and a design example problem is solved for future reference.