Geotechnical considerations in an unlined high pressure tunnel at Lower Kihansi in Tanzania: Fid-Bau Portal

Geotechnical considerations in an unlined high pressure tunnel at Lower Kihansi in Tanzania

Marwa, Ernest M. M.

Abstract The Lower Kihansi unlined high-pressure tunnel is the first of its kind to be constructed in Tanzania. The pressure tunnel consists of a 500 m vertical shaft and a 2.195 km inclined headrace tunnel. The cross sectional area of the shaft is 25 $ m^{2} $ and that of the headrace tunnel is 30–37.5 $ m^{2} $. The headrace tunnel slopes 1:7 towards the powerhouse cavern. The pressure tunnel acts as waterway towards the underground hydroelectric power generation plants with a maximum generating capacity of 180 MW. The Kihansi River has been deviated through the shaft and headrace tunnel from an elevation of 1,146–300 m above sea level. The maximum water pressure created by this deviation is 8.5 MPa. The decision not to steel line the pressure tunnel was reached after the excavation and documentation of the underground rock mass. The hydraulic jacking and hydro-fracturing tests confirmed the rock to have a minimum acceptable confining stress of 9.6 MPa, capable of withstanding the expected water pressure in the tunnel. The permeability of the rock mass is relatively low and any poor zones were sealed by grouting. The discontinuities had a favourable orientation with respect to the tunnel axis such that rock bolts and steel fibre reinforced shotcrete could be used to provide the necessary support. No failures occurred and the decision not to line the Kihansi high-pressure tunnel has proved both technically acceptable and economical.