131. Dispensing with the Structural Supports of Pressure Tunnels by Impro ving Rock Cover – A Few Examples

In the paper, case studies of three projects wherein simple techniques were adopted to improve the rock cover to dispense with the costly structural supports of pressure tunnels and shafts are discussed. In the original lay-out of Tuivai hydroelectric project, Mizoram, the pressure tunnels transforms into an inclined shaft midway between surge shaft and powerhouse. Shortage of rock cover was noticed for the tunnel and shaft for a length of over 800 m. by shifting the inclined shaft close to surge shaft, the cover can be improved remarkably. The alignment of 4.5 km long HRT of Athirappilly hydroelectric project in Kerala crosses two short cover valleys for which additional supports were proposed in the layout. It was recommended to adopt a variable slope of 1 in 15, 1 in 70 and 1 in 200 in place of an uniform slope of 1 in 100 to place the tunnel at deeper levels and to improve the rock cover. Shortage of lateral cover for the pressure shaft of Pykara ultimate stage hydroelectric project in Tamil Nadu has resulted in design of exorbitant thickness of steel liner. The inclination of the shaft was altered from 45o to 60 o to augment the rock cover and minimize the thickness of steel liner. Thus, a discreetly designed and flexible layout with an option of modification to suit the geological setting will be of great help in minimizing the supports.