TARBELA-DAM - RESOLUTION OF SEEPAGE

Tarbela Dam, a masterpiece of engineering, is constructed on a deep al luvium foundation composed of cobbles, gravels and fine sand up to 213 m depth. Usually, the voids within the cobbles and gravels are filled with fine sand, however, in places unchoked cobble gravels referred t o as 'open work' exist. To control the underseepage, measures like con struction of a positive cut-off to the bedrock with grout curtain, cut -off trench, or slurry trench were not feasible choices because of the great alluvium depth and difficulty in drilling through dense hard co bble gravels. It was the first time that a 146.30 m high dam extending about 2743.20 m across the main river valley was designed with an ups tream impervious blanket in continuation with the core to control the seepage and exit gradients. In 1974, when the reservoir was filled for the first time, approximately 60 m of the upstream portion of Tunnel 2 collapsed, leading to erosion of the surrounding rock and overlying fill material.(1) About two hundred relief wells at the downstream toe of the dam discharged more than 0.028 m(3)/s each. Consequently, the reservoir had to be emptied. After emptying the reservoir, 362 sinkhol es and 140 cracks were observed in the exposed upstream blanket. As a remedy, the sinkholes and cracks were capped with additional blanket m aterial and the blanket was thickened and extended upstream. In order to facilitate drainage and reduce uplift pressure, additional relief w ells were installed. After refilling the reservoir, the development of sinkholes in the blanket was monitored by side-scan sonar and Ocean R esearch Equipment (ORE) profiling. Bottom dump barges were mobilized s o that any sinkhole that might develop could be treated. During the 19 75 filling, 350 new sinkholes were formed in the blanket. These were a gain treated by barge dumping. The remedy proved effective and the dev elopment of sinkholes decreased substantially and ultimately ceased by 1978. Seepage problems encountered in both the left and right abutmen ts of the main dam were dealt with by additional grouting from the adi ts coupled with downstream drainage, The drains which issued fines wer e grouted and replaced with adjacent drains equipped with filters. The paper addresses the seepage problems, control measures, their effecti veness, and the to-date performance of the dam.