One of the fundamental design criteria of dam construction is the necessity to “anchor” the dam to the rock – on all sides. Thus, the first step of dam construction is exposing the stable rock formation – in the river bed and the banks.
This activity calls for removal and carting huge quantities of soil – called overburden in the hydel power industry parlance. And the quantities are so enormous and terrain so difficult, that the task becomes daunting and cost intensive.
More importantly, this also means that centuries old trees will have to be cut.
At Malana II project it was discovered that the stable rock formation in the left bank was much more “deeper” than earlier investigations had indicated. Excavation by conventional methodology involved removal of 200000 M3 of additional overburden – resulting in increase in the construction time and loss of significant nos. trees.
It was at this point that the innovative idea of a “DRIFT/KEYWALL” was successfully adapted –limiting the budget, build the dam in time and more importantly save the trees and protect the environment.
It was dangerous, challenging but satisfying & fulfilling.
The proposal was to construct a key wall that would anchor the dam to the rock on the bank side in the overburden of around 62 M length. The methodology adopted for this was construction of a series of horizontal “walls” (4m high x 3M wide) one on top of other with “interlocking” arrangement like shear keys between each “wall” till dam top – thus resulting in one big key wall.
Considering the conditions of the area and the criticality of the key wall the construction had to be cautious and careful – both from strength point of view and safety of personnel. Intermediate steel support, RCC laggings were provided to retain the back filling concrete in extreme poor rock condition. Since the strata had very less standup time, fast operations and quick support systems were needed. The major challenge was mechanization of mucking for which conveyor system was adopted.
The idea was mooted by the designers of the project M/s Energy Infratech Private Limited. Abir rose up to the challenge.