Fluid Flow and Leakage Assessment Through an Unlined/Shotcrete Lined Pressure Tunnel: A Case from Nepal Himalaya

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.

Hydraulic Transient Impact on Surrounding Rock Mass of Unlined Pressure Tunnels

“…where q is the flow rate per width of joint, a is the aperture, ∆p is the pressure difference, l is the length of joint, and µ is the kinematic viscosity of fluid. Fluid flow through joints is, in general, non-laminar and unevenly disturbed in the joints due to the roughness and infilling conditions of the joints [20]. The connectivity between joint systems in the rock mass, joint persistence, joint spacing and the infilling condition governs the permeability and extent of flow.…”

Engineering Geology in Hydropower Engineering

“…In Equation 4, H is the hydrostatic water head (Figure 4), Jn is joint set number, Jr is joint roughness number and Ja joint alteration number as described by Barton et al (1974) in the Q-system of rock mass classification. The joint permeability factor (fa) given in Equation 4 can be estimated using Equation 5as recommended by Panthi and Basnet (2021).…”

“…Figure 5. Typical topographic condition surrounding an unlined pressure tunnel / shaft(Panthi and Basnet, 2021).…”

Underground Hydropower Plants

“…The rock mass behaves differently when it is exposed to the water pressure. Therefore, the interaction between water pressure and rock mass is an important issue for unlined or shotcrete lined pressure tunnels (Panthi and Basnet, 2021).…”