The 66 m high and 1500 m long stone masonry and earth fill Kadana dam across the Mahi river in Gujarat, India, holds behind it 1545 hm3 to extend irrigation facilities to 11,492 ha through a direct canal on the left bank besides firming up irrigation in the existing system covering 2,71,000 ha on the right bank under the pick-up weir 65 km downstream. The project also envisages construction of 4 reversible turbine units each of 60 MW capacity.
The straight gravity dam constructed in uncoursed rubble masonry, located in the river portion comprises 405.7 m long spillway with roller bucket flanked by 106 m long power section and 70.4 m long transition section on its left and 39 m non- overflow portion on the right side. the earthfill dam wraps around the left transition section and extends 762 m to tie with the left abutment Ridge.
Construction of the dam began in 1968. However consequent upon the unprecedented floods of 33,000 m3/s in 1973,the revised hydrological studies necessitated the construction of the l13 m long and 28 m high Additional Spillway on the right bank to discharge 9070 m3/s through 6 radial gates, besides the spillway at the main dam designed to negotiate 36,800 m3/s through 21 bays spanned by radll gates each of 15.54x14.18 m size. Partial filling of the reservoir commenced in June, 1976 when the main Spillway was about 6 m below its crest level.
The highly folded rock formations in the area comprising phyllites, slates, mica-schists and quartzites belong to the Pre-Cambrian age. The geological structures have influenced the topography of region. The rocks have undergone two episodes of deformation.
Air-photo studies indicated a 16 km long lineament crossing the river 11 km upstream of the dam site. On field checking, however, it was established as a 30 m wide and 3 km long steeply dipping fault on the left bank and beyond which it graded into shear. Besides, a hot spring (39°C) and three other sympathetic faults also occur close to this major fault in the submergence area.
The surface geological mapping of the dam vicinity revealed that intricately folded quartzites, quartzose phyllites and schistose phyllites, trending in N 60·Y-S 60.Y direction and dipping 65. in southerly direction to vertical were traversed by shears, closely spaced joints and faults. About 19 faults, having steep upstream dips were recorded in the initial stage of investigations.
The pre-construction stage subsurface exploration by 4B drill holes ranging in depths from 7 to 91.5 m, aggregating to 1675 m, nine test pits aggregating to 17 m depth in the earth dam reach and one 15 m long trench into the right abutment ridge was carried out to assess the foundation conditions. In addition, 36 core holes aggregating to 1077 m depth were drilled during construction for the exploration of faults, power house and additional spillway etc.
