Challenge

The Northern Collector Tunnel is part of a master plan development strategy for new water sources for Nairobi and 13 satellite towns up to the year 2035.  Prior to the project, the estimated water deficit in Nairobi City was 125,000m3/day.

The World Bank-funded project comprised Kenya’s longest tunnel, designed to harvest flood water flows from three rivers in Murang’a County into the existing water supply infrastructure at Ndakaini dam, the main water source for Kenya’s Capital, Nairobi.

The project faced several challenges, including:

• Geological complexity: Shaft sinking and tunnelling was constructed through a variety of geological formations, including hard rock (basalts), soft rock (ash tuffs), and soft ground (completely weathered bedrock, alluvium and landslip material). Tunnelling was carried out using a variety of excavation techniques including drill and blast, roadheader and soft ground excavation.
• Environmental constraints: The tunnel is in an area with a high-water table and several protected areas. This required careful planning through control of water ingresses during construction to mitigate potential environmental impacts.
• Social constraints: The tunnel passes under an area with a large population, so consideration was needed to mitigate disruption to the local community.

Athi Water Works Development Agency (AWWDA) engaged SMEC to complete detailed design and tender documentation under Phase I, and construction supervision under Phase II. SMEC’s scope of work included detailed design, preparation of tender documents, project monitoring and construction supervision.

Solution

The project comprised the construction of a 12 km long water transfer tunnel to intercept three rivers north of Nairobi and transfer the intercepted water to the Ndakaini Dam, the main water source for Nairobi.  In addition to the engineering design, SMEC contributed to the geological survey, environmental impact assessment and community engagement. The scope of work for the project, undertaken in two phases, includes:

• A 12 km long, 3m finished diameter, bulk water transfer tunnel from Maragua, Gikigie and Irati rivers
• Three run-of-river intake weirs with an outlet to the dam
• Associated stilling and de-silting basins
• Fish ladders at Maragua, Gikigie and Irati Rivers
• An outtake sluice and stilling basin at Githika Outfall.

Tunnel alignments were amended to incorporate 2No additional 50m deep shafts (formed at approx. 10m diameter) at Kaanja and Irati to open up new working faces to speed tunnelling and help recover delays.

Responding to the geological conditions SMEC developed the following:

• Temporary support comprised shotcrete linings with mesh and rock reinforcement and steel ribs
• On completion of excavation the tunnels and shafts were lined with reinforced concrete
• Complex Intake structures were installed on the Maragua, Gikigie and Irati rivers ensuring that required environmental river flows continued downstream to meet socio-economic and environmental needs. An outfall structure directed flows into Ndakaini reservoir.

Impact

The Northern Collector Tunnel will increase the city’s water storage capacity by 100 million cubic meters p.a. and will provide a reliable source of water for the city’s growing population.

Approximately 40% of flood water from the three rivers will be captured and diverted to the tunnel and then to Ndakaini Dam before being piped to Nairobi.