The Sondu/Miriu hydro power project is under construction in Nyanza Province, West Kenya, about 60km south of Kenya’s third largest city Kisumu. When completed in 2003 the project will have a maximum output of 60MW from two units and an average annual energy production of 330GWh.

Nippon Koei Company of Japan and staff from GIBB of the UK are supervising the work for Kenya Electricity Generating Company Limited (KenGen). The intake and tunnel is being constructed by the Konoike/Veidekke/Murray & Roberts Joint Venture (KVM). Site work started in March 1999.

The project is a run-of-river scheme that will incorporate a gated intake structure to transfer the water into the 6.2km long, 4.2m internal diameter concrete-lined tunnel to take water to the top of the Nyakach Escarpment. The power station is situated approximately 200m below and connected to the tunnel by a surface steel penstock with an internal diameter of 3.6m. The water is returned to the river along a 4.7km channel with potential for a further 20MW station.

The tunnel is constructed through mainly good quality granodiorite but there are various degrees of weathering with some faulting. The tunnelling conditions have been good and often dry or with just minor seepage. Where inflows have occurred these have been easily managed. Thirty-seven percent of the main drives have been left unsupported while different combinations of 2.5m long grouted rock bolts and shotcrete, using a Normet shotcrete machine, have been used depending on the rock conditions. Only 20m of the main excavation have required full steel supports.

The tunnel excavation is by the drill and blast method at two faces. The two excavation teams are similarly equipped with a two-boom, wheeled drilling jumbo, a 1.9m3 and a 7m3 loader and two 20ton tunnel dump trucks for mucking out.

Access to the tunnel is by three temporary access adits, one at each end and the third near the centre. A total of 1.3km of 5x5m horseshoe-shaped tunnel has been excavated on two faces on the downstream section. Breakthrough is expected in early 2001 with full breakthrough by the end of the same year. The concrete lining will start in April 2001 using a 15m long telescopic shutter and concrete delivery will be by agitator truck. Completion is expected at the end of 2002.