Abstract:
The Punatsangchhu-II Hydroelectric project (1, 020 MW), on the Punatsangchhu river is spread along the Wangdue – Tsirang National Highway in Bhutan in the central Himalayan region, about 80 km east of Bhutan’ s capital Thimphu. The project includes a complex system of rock tunnels and caverns. The power house complex has been excavated within rocks of the Thimphu Gneissic complex comprising mainly quartzofeldspathic gneiss, biotite micaceous quartzite with leucogranite and pegmatite patches and veins. In March 2016, a huge rock mass collapse occurred in the already excavated downstream surge chamber, as a consequence of an active shear zone. The total volume of muck generated in the collapse was estimated to be approximately hundred thousand cum. Designer required the fallen debris to be improved to low grade concrete of M10 strength so as to install 100T capacity Cable Anchors supporting slender RCC walls. BAUER was entrusted the job as a specialist sub-contractor. The job involves nearly 18, 000m of drilling and systematic grouting followed by 19, 000m of permanent tieback strands anchors with working load up to 1, 000KN each. Along with various equipment for the injection works, up to five new drilling rigs have been mobilized from Europe for this work. The engineering challenges of this project was further compounded due to the cramped working environment and the difficulties related to drilling. With a width of approximately 18 m and a height of 8 m, the work area is extremely small, creating an extremely challenging environment for equipment and personnel, while overcoming the technical challenges. The paper describes the drilling and grouting solutions adopted and the unique and unprecedented geotechnical challenges of this project.