The Subansiri Lower Dam, officially named Subansiri Lower Hydroelectric Project (SLHEP), is an under construction gravity dam on the Subansiri River in NorthEastern India. It is located 2.3 km (1.4 mi) upstream of Gerukamukh village in Dhemaji District and Lower Subansiri District on the border of Assam and Arunachal Pradesh states. The concrete gravity dam is designed to be 116 m (381 ft) tall, measured from the river bed and130 m (430 ft) from foundation. Its length will be 284 m (932 ft) and the dam will have a structural volume of 2,250,000 m3 (2,942,889 cu yd). The reservoir created by the dam will have a gross storage capacity of 1.37 km3 (1,110,677 acre·ft), of which 0.44 km3 (356,714 acre·ft) can be used for power generation or irrigation. The gravity dam is prone to high instability due to the water pressure and deviation cause due to the water pressure should be within limits, to ensure the deviation within limits continuous monitoring is required. Therefore, government decided to put a pendulum arrangement on the dam walls to measure the deviation causing due to water pressure so that water can be realized whenever deviation is getting out of limits.
The pendulum system is installed on the wall by Spar Geo Infra Pvt Ltd accurately for measurement and keeping monitoring continuously over the deviation of wall caused due to water pressure. As soon as deviation is observed getting greater than permissible limits water is released from the dam and walls are prevented from undergoing excess of deviation.
Difficulties faced while execution:
For installation of pendulum system vertical hole of 300 mm diameter has to the depth of 100 meter. While drilling hole maintenance of vertical alignment is major issue because slight deviation from 90 degree angle will lead to great deviation at 100 meter depth.
Extra Technology used:
Pendulums are simple, and very reliable and accurate systems used to monitor internal lateral deformations of concrete dams, dam foundations and abutments, tall industrial buildings and bridge piers. The inverted pendulum uses the same readout units, and includes a stainless steel wire anchored in the structure foundation with a float fixed at its upper end. The float, which is free to move in a tank, tensions the wire and keeps it vertical. Once anchored in stable point in foundation it measures absolute deformation of the structure and it’s used as a reference for surface geodetic surveying.
The wall deviation is kept under permissible limits of deviation by constantly monitoring the deviation produced by water on wall. The accuracy and reliability of pendulum system caused the prevention of generation of high deviation.