Abstract
The use of unlined/shotcrete lined pressure tunnels and shafts are cost-effective solutions for a hydropower project and are being implemented worldwide. To implement this concept, the ground conditions at the area of concern should be favorable regarding minimum principal stress magnitude, which should be higher than hydrostatic water head acting on the tunnel periphery. In addition, the rock mass should be relatively unjointed or joints in the rock mass should be relatively tight. Among the most important issues in the design of unlined/shotcrete lined pressure tunnels is the extent of hydraulic jacking and water leakage out of the tunnel during operation. This manuscript first presents fluid flow and potential hydraulic jacking assessment of two selected locations of the headrace tunnel of Upper Tamakoshi Hydroelectric Project (UTHP) in Nepal using the UDEC. It is noted here that the 7960 m long headrace tunnel will experience a hydrostatic water head that will vary from 2.9 to 11.5 bars (0.29–1.15 MPa). The headrace tunnel is supported by sprayed concrete (shotcrete) in combination with systematic rock bolts in the tunnel walls and crown. The invert of the tunnel and few hundred meters downstream end (at surge shaft area) of the headrace tunnel is being concrete lined after the completion of all other works. The qualitative fluid flow assessment carried out using UDEC indicated considerable pressure built-up in the joint systems suggesting potential hydraulic jacking. This was especially the case at the downstream segment (downstream from chainage 7100 m) of the headrace tunnel. The manuscript further presents the quantitative results of water leakage estimation from the headrace tunnel carried out using Panthi (Panthi KK (2006) Analysis of engineering geological uncertainties related to tunnelling in Himalayan rock mass conditions. PhD Thesis, NTNU, Trondheim, Norway;Panthi, Note on estimating specific leakage using Panthi’s approach, NTNU, Trondheim, 2010;) approach. The leakage assessment carried out indicated an average specific leakage of about 2.5 l/min/m tunnel, which may result in over 210 l/s leakage from the headrace tunnel. The evaluation also indicated that the outer reach (860 m downstream segment) of the headrace tunnel after chainage 7100 m seems extremely vulnerable and over 80 l/s water leakage may occur only from this headrace tunnel segment during operation of the hydropower plant.
Highlights
Optimum, cost-effective and long-term stabile headrace tunnels are the key to the successful development of hydropower plants
In terms of stress criteria, the tunnel alignment at Location A was found to be safe against hydraulic jacking whereas tunnel segment at Location B was found to be vulnerable for hydraulic jacking
Since fluid flow analysis carried out using UDEC only indicates potential hydraulic jacking and possible water flow through joints but not the leakage amount from the tunnel, a leakage assessment was carried out using Panthi (2006) approach discussed at Sect. 2 (Brief review on fluid flow and leakage)
Summary
Cost-effective and long-term stabile headrace tunnels are the key to the successful development of hydropower plants. The interaction between water pressure and joints in the rock mass will govern the fluid flow capacity, which is termed as hydraulic conductivity This manuscript presents the results of fluid flow behavior assessment carried out for the joint systems in the rock mass at two selected locations of the headrace tunnel alignment of UTHP project. Since fluid flow analysis using UDEC provides a qualitative assessment on potential leakage and the detail rock engineering assessment along the headrace tunnel indicated that the geological features such as small scale weakness and shear zones or shear bands and joints in the unfavorable direction from where water may leak during hydropower plant operation, it was felt essential to carry out a study on the potential leakage from the headrace tunnel during operation when maximum up to 115 m (11.5 bars) hydrostatic water head will be acting at the headrace tunnel. Panthi (2006, 2010) approach of leakages assessment was used to estimate the water leakage out from the headrace tunnel of UTHP project
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