The use of unlined/shotcrete lined pressure tunnels and shafts are very cost effective solutions for the hydropower project and therefore are being implemented worldwide. However, the ground conditions at the area of concern should be favorable regarding both minimum principal stress magnitude and rock mass strength, which should be higher than the hydrostatic head acting on the tunnel periphery. In addition, the rock mass should be relatively unjointed or joints in the rock mass should be tight enough. The most vulnerable issue in the design of unlined high-pressure headrace tunnel is to insure the potential leakage out of the tunnel during operation is within acceptable limit. This manuscript assesses the potential leakage extent from a shotcrete lined high-pressure headrace tunnel of the Upper Tamakoshi Hydroelectric Project (UTHP) in Nepal. Most of the headrace tunnel at UTHP is supported suing sprayed concrete (shotcrete) in combination of rock bolting in the walls and the crown and concrete lining in the invert. The downstream stretch (at surge shaft area) of few hundred meters headrace tunnel will be supported with full concrete lining. The approximately 8 km long headrace tunnel will face a maximum hydro-static pressure head of up to 120 m during power plant operation. The preliminary results of the leakage assessment using approach suggested by Panthi (2006) indicates that the average specific leakage from the headrace tunnel will be around 2.7 l/ min/m tunnel. The evaluation concludes that the outer reach of the headrace tunnel after chainage 7300 m is extremely vulnerable for excessive water leakage to occur during operation. The joint set dipping towards the valley side slope of Gongar Khola seem very critical for potential water leakage, suggesting remedial measure before water filling in the tunnel.
The fluid flow characteristic in most of the rock mass is mainly governed by permeability of the joints and discontinuities. In an unlined or shotcrete lined pressure tunnel, water gives pressure (Pw) to the rock mass equivalent to a hydrostatic head (H). The interaction between the water pressure and joints in the rock mass will therefore govern potential of fluid flow capacity, which is termed as hydraulic conductivity. Basnet and Panthi (2018) used the Norwegian confinement criteria to study the applicability of shotcrete lined tunnel at Upper Tamakoshi Project. The Norwegian confinement criteria showed that the whole headrace tunnel alignment is safe against hydraulic jacking with factor of safety exceeding 3.5. The stress state analysis on the other hand showed some critical locations where the factor of safety is less than minimum required factor of safety of 1.3. The low level of factor of safety was mainly confined at areas where weakness zones are located and also on the downstream stretch of the headrace tunnel. The detail rock engineering assessment concluded that the geological features such as small scale crushed zones, shear bands and some joints in unfavorable direction from where water leakage is likely to occur during operation of the project. Therefore, it is realized that the stress state analysis carried out for the UTHP was not sufficient to address the behavior of these geological features when exposed to high hydro static water pressure. It is therefore felt necessary to carry out study on the potential leakage from the headrace tunnel during operation phase when the headrace tunnel will have to sustain maximum up to 120 m water column. Panthi (2006) suggested a semi-empirical approach to assess potential leakage from water tunnels, which is used in this article to assess extent of leakage out from the headrace tunnel of UTHP project
