Seismic survey for piercing of an underwater tunnel for Koyna Hydroelectric Project, India

Abstract

Stage IV of the Koyna Hydroelectric Project is being constructed at Navaja, 12 km north of the existing Koyna Dam, to generate additional power of 1000 MW. The area mainly comprises compact and non-vesicular amygdaloidal and chlorophaeitic basalt, red or black tachylitic basalt and volcanic breccias of different kinds. The intake tunnel which uses lake tapping technique for the first time in India is to be pierced under the Koyna Lake in a single blast. Seismic refraction and reflection studies were carried out in the Koyna Lake along two alternate dam alignments and in the area where the lake tapping tunnel opens out to determine the lake-bed levels, probable rock levels and rock quality. The proposed dam is to be constructed for lowering the minimum draw down level for the intake tunnel to generate power even during the lean season. Seismic refraction results revealed a silty sandy top layer on land (Vp=300–900 m/s) and a saturated sand and gravel bed in the river portion (Vp=1700–2000 m/s). Rock was found shallower in the western part and deeper on the eastern side with bedrock levels varying from 646 m to 602 m. The compressional wave velocities of the bedrock ranged from 4200 m/s to 5500 m/s which indicated that the rock was of good quality. Seismic reflection studies revealed three prominent reflectors corresponding to the lake-bed, weathered and good rock. While seismic refraction survey on land delineated the weathered rock (Vp=1000–1400 m/s) the same however, did not delineate the weathered rock underwater due to the poor contrast between overburden and the weathered rock. The depth contours of lake-bed and bedrock were drawn. The lake-bed levels varied between 650 m and 612 m while the same for bedrock ranged from 646 m to 602 m. The bedrock levels, correspond to rock having RQD greater than 95% indicating rock to be of very good quality. The rock levels as evaluated by seismic studies correlate well with those inferred from the few boreholes drilled. From the seismic studies it was concluded that rock at site is generally of good quality. Also, the bedrock levels evaluated along two alternate dam alignments are same indicating that there is no difference in foundation level of the proposed alignments.