Location: Power House, Mangan, Sikkim
Brief of the Problem: High Vibrations in Tunnel Profile after Seismic Event
The Power House of Teesta-3 Hydro Electric Project has located at Mangan roughly 20 km from its dam location in Chungthang Village. It is an underground powerhouse. An earthquake of 7 Richter magnitude scale occurred in 2011, and the powerhouse was in the earthquake zone. The seismicity caused the vibration in the geological strata and created a disturbance in the rock layers. This, in turn, endangered the stability of the powerhouse.
The powerhouse chamber is namely: bus duct and transformer chamber. The bust duct is used to carry electrical appliances and cables. There were a total of 6 bus ducts and one entry tunnel to the transformer chamber. Distance between the parallel tunnels (bus ducts) was approx. 18 m.
Engineered Solution: Retrofitting using Horizontal Cable Anchors
There was a need to provide the system which confines the zone around the powerhouse. Since the seismic vibration caused the geological disturbance, that could be avoided by the system, which not only reinforces the disturbing mass around the bus ducts but also provides the rigidity.
To serve the above purpose, a system of horizontal cable anchors were used. Anchors prestressed at a load of 100-Ton. Holes of 125mm diameter were drilled with 17.5 m depth for the installation of horizontal cable anchors. This resulted in to increase in tunnel wall stability and provided resistance against lateral deformation.
Difficulties Faced during Execution:
Since the strata were extremely difficult in terms of implementation of the cable anchors, the drilling and installation of cable anchors have to be done with extreme care.
Moreover, the fragile geology created obstruction in maintaining the horizontal direction of drilling holes. The presence of the rock bolt in between the anchoring direction led to repeated changing of drill bits. Due to this issue, it took us a little longer to complete the work.
The technology used for Execution:
The emerging technology of cable anchors for the purpose of tunnel stitching was used, and cable anchors were stressed from both sides, and after that, they were locked for better protection. To keep a hold of the correct horizontal direction, tools were imported for guidance purposes. Also, a sophisticated portable micro piling rig was used as the size of the bus ducts were small for a normal-sized machine.
Geo Spar was successful in resolving the issue of vibration permanently. Till now, the powerhouse is secured, and there has not been any other issue. We are proud to say that we were able to deliver a reliable and secure solution.