Location: Gerukamukh near North Lakhimpur, Assam

Brief of the Problem: Probable Instability of Gravity Dam due to High Water Pressure

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.

The concrete gravity dam was designed to be 116 m (381 ft) tall. Its length is 284 m (932 ft), and the dam has a structural volume of 2,250,000 m3 (2,942,889 cubic yds). The reservoir created by the dam has a gross storage capacity of 1.37 km3 (1,110,677 acre·ft).

The project in itself is very sophisticated and unique and monitoring instruments are required to keep a check on its stability and to get the information of any kind of alarming situation. For continuous monitoring of the gravity dam a high precision instrument known for its monitoring of the tilt is required. Therefore, three Inverted Pendulums are added to the design. The uniqueness of the suggested inverted pendulum is that the starting point is 90 meters deep from the foundation level of the dam.

Engineered Solution: Monitoring using Inverted Pendulum to Regulate the Controlled Flow in the Dam

The water pressures may create the movement in the gravity dam. The primary task was to monitor and get an idea of displacement/movement if it is going to occur any in the future. To ensure the deviation is within allowable limits continuous monitoring system is required. Therefore, the client has designed Inverted Pendulums to be fixed into 300 mm dia. hole at 90 m depth so that accurate and precise information can be collected during the operational time of the dam.

The initial task of drilling was performed by Geo Spar to install the pendulum system along the dam axis on the left and right sides. At the operation stage of the dam, the inverted pendulum will work in such a way that as soon as the deviation gets higher than the permissible limits, water will be released from the dam, and hence the body of the dam will be prevented from undergoing any excess deviation.

Difficulties faced during execution:

Installation of an Inverted pendulum system initiated with the drilling of a vertical hole of 300 mm diameter up to a depth of 90 meters. While drilling a sophisticated guided system was required to drill a hole without any deviation in its verticality. Vertical alignment is significant because a slight deviation from a 90-degree angle could lead to cumulative error and adds to a higher deviation at 90-meter depth. Overcoming all this difficulty and challenges our drilling operation was quite successful with just a marginal 0.2% drilling error. Certification of verticality of the hole was done by AIMIL Ltd. with the use of an inclinometer.

The technology used for Execution:

The drilling operation was carried out in a tough geological profile which was carrying highly abrasive and high strength rock. The selection of drilling technology, machines, and tools were a big challenge for a work that required such precision and was to be done at such depth. This kind of work in such geology has never ever been done by any organization in India before 2008.

Our company adopted two drilling methods simultaneously. One was rotary and another was percussive. We started with rotary drilling up to a depth of 40 mtrs – 45 mtrs with centralizers and guides at every 3-meter interval to control the alignment. Thereafter drilling speed got highly reduced because of the difficult geology and then the percussive system was implemented with the same centralizers and guides. The drilling operation was carried out at a very slow pace which in turn took 3 months to complete one hole. Ultimately it took us a period of 12 months to finish 3 holes with proper for verticality at different intervals as required.

Achievements:

We feel proud that we have played an important part in transforming NHPC’s and SOMA’s vision into reality by drilling these three holes and took it upon us to get this difficult task get done with precision and accuracy.

Constant monitoring with this inverted pendulum device will help to control the wall deviation within permissible limits produced by the water and its dynamic effect on the body of the wall.