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 powerhouse complex has been excavated within rocks of the Thimphu Gneissic complex comprising mainly quartz-feldspathic 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 (DSC) because of an active shear zone. The total volume of muck generated in the collapse was estimated to be approximately 100.000 cum. The designer required the muck fallen into the cavern to be improved to low grade concrete of M10 strength to install 100 T capacity Cable Anchors supporting slender RCC walls. BAUER was entrusted the job as a specialist sub-contractor. The job involves nearly 19.960 m of drilling and systematic grouting followed by 19.900 m of permanent tieback strands anchors with working load up to 1.000 KN 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 a challenging environment for equipment and personnel, while overcoming the technical challenges. The paper describes the drilling, grouting and anchors solutions adopted and the unique and unprecedented geotechnical challenges of this project.
