Retrofitting of Existing Earthen Fly Ash Dam by Finger Drains and Gabion Wall

Abstract

Coal-based power plant generates huge amount of fly ash as residual waste for ash ponds with significant dam heights that have been constructed over a period of operation. Sometimes in long run, seepage may threaten the stability of dam due to internal erosion or clogging of filters. This paper presents a case study on fly ash dam where seepage and distress of dam section had a concern for the owner. The entire pond had raised by upstream method of construction with earthen as well as pond ash material. One stretch of dam settled including starter dam and moved horizontally under heavy seepage during monsoon. The root cause of problem had been identified through new geotechnical investigations and engineering solution was proposed to stabilize dam section. The rectification measures involved revival of old rock toe, strengthening of dam by constructing gabion wall at toe of starter dam with geotextiles and graded filters. The internal drainage was streamlined with provision of PVC perforated finger drain wrapped with non-woven geotextile material inside trenches filled with graded filters. The networks of finger drain at different elevations were ultimately channelized to drain out trapped seepage water and release the pore water pressure on dam section. The revived existing rock toe with new gabion wall had stabilized the toe and stopped the starter dam movement further. New piezometers and settlement monuments were installed on entire dam section at different elevation for monitoring. The proposed solutions had appropriately addressed seepage problems and strengthened dam toe to improve the stability.