Punatsangchhu I, Hydroelectric Project, Bhutan
|The Punatsangchhu Hydroelectric Powerplant project is located in the Southern Himalayas, about 80 km east of Bhutan’s capital Thimphu. The earth dam reaches across the Punatsangchhu River, delimited by steep hills on both sides.
For the excavation of the pit for the main dam construction, two temporary cofferdams upstream and downstream the pit are required across the Punatsangchhu River bed, to seal off ground- and river water ingress.
The V-shaped river gorge is filled with heterogeneous river deposits, predominantly consisting of highly permeable sand-gravel packages, intercalated by cohesive layers. Boulders in size up to few meters are also frequently occurring.
Bedrock reaches from exposed surface down to as deep as 93 m below actual working platform elevation, with UCS strength of occasionally more than 100 MPa.
|Exactly those highly permeable non-cohesive soil layers needed to be sealed for the purpose of creating a watertight barrier underneath the upstream cofferdam. After evaluation of different sealing methods, a two phase Cutter excavated Cut-off Wall (CoW) was selected as the most appropriate and reliable methodology for construction of the sealing element.
Pretreatment works were carried out by means of gravity- and Tube à Manchette grouting, to seal the open void soil layers and to fix boulders and big cobbles, thus mitigating the risk of collapses of excavated CoW trenches, and to minimize material overconsumption. Pretreatment reached down to as deep as 96 m below working platform.
The Cut-off Wall was constructed using one BAUER DHG-C hydraulic grab, and two BAUER BC 40 trench cutter units. The nominal width of the CoW is 1,200 mm with panel’s length of 2.80 m and depth down to maximum
93 m, always embedding min. of 0.60 m into the solid bedrock. The biggest challenges hereby were the cutting through the numerous boulders, and the embedment of the CoW into the steeply inclined, partially vertically dipping bedrock contour.
Amongst the technical challenges for constructing a CoW under such difficult geotechnical conditions, the site logistics were one of the most significant issues:
All machineries, equipment and tools had to be shipped to Kolkata/India, followed by road transports to the site, over narrow mountainous roads into a quite remote area. Despite all the challenges, Bauer managed to complete the project on time and to the full satisfaction of the client.
|Scope of work: • Soil pretreatment: 5,400 lm / 2,050 m³ gravity & TaM grouting
• Cutt-of Wall: 8,062 m² / 1,200 mm