India's present energy need from coal reserves contribute about 55% of the total power generation. Indian coal has high ash contents and presently about 200 million tonnes of ash is generated annually. The generated ash has become a subject of worldwide interest in recent years because of its diverse uses in manufacture of cement, bricks and concrete block, filling of underground cavities, mine voids etc. The present ash utilisation is about 69% and the balance ash is stored safely in ash ponds. The utilization of ash is strongly related to economy of its use, which changes with time. There may not be users in sight today, but after some time there could be bulk users. The most economic and commonly used method to dispose ash is by hydraulic transport, in the form of slurry, to the ash pond. At the ash pond, storage space is created by constructing dyke embankments all around, within which ash particles will be allowed to settle and the decanted water is allowed to escape for recirculation back to plant. Based on the type of the soil available for the embankment construction, mostly, it is a homogeneous section with internal drainage arrangement of sand chimney and sand blanket. The dyke embankments are designed as water retaining structures as per relevant Indian Standards applicable for earth dams. For internal drainage in these embankments, natural river sand or crushed stone sand is generally used. To create sustainable and environment friendly infrastructure, NTPC has recently explored the use of bottom ash as filter material with respect to pond ash as base material. Based on laboratory tests, it is found that bottom ash possess the required filter ability, internal stability, drainage capacity, self healing properties and does not segregate. Accordingly, bottom ash has been adopted as an alternate filter material in internal drainage system of dyke embankments of various plants of NTPC. The above have been successfully implemented and found to be safe under operating phase. This use of bottom ash as a filter material has also saved the time and cost over runs and most importantly is echo-friendly. This paper highlights the use of bottom ash as filter material in water retaining structures such as dyke embankments/earth dams in a sustainable manner to entire world.
