Punatsangchhu-I, Hydroelectric Project, Bhutan

The Punatsangchhu-I Hydroelectric Powerplant project is located in the Southern Himalayas, about 80 km east of Bhutan’s capital Thimphu. The new 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 were required upstream and downstream of the pit 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 sizes up to few meters also frequently occur.

Bedrock reaches from exposed surface down to as deep as 93 m below actual working platform elevation, with a UCS strength of occasionally more than 100 MPa.
 
     

  These highly permeable non-cohesive soil layers needed to be sealed for the purpose of creating a seepage barrier underneath the upstream cofferdam. After evaluation of different sealing methods, a twophase cutter excavated cut-off wall (COW) was selected as the most appropriate and reliable methodology for the construction of the sealing element.

Pretreatment works were carried out by means of gravity grouting and tube-à-manchette grouting, to temporarily seal the open void soil layers and to fix boulders and big cobbles, thus mitigating the risk of localised 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 1.2 m with panel length of 2.8 m and depth down to a maximum of 93 m, always embedding a minimum of 0.6 m into the solid bedrock. The biggest challenges hereby were 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, and then by road 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