Zachary Ochuodho @zachuodho
Construction of underground railways tunnel at the Ngong Hills, that is part of the Nairobi–Naivasha SGR Phase 2A project, will be completed in August.
The contractor, China Communication Construction Company (CCCC), has already done 2.5 km of the 4.5 km tunnel. CCCC spokesperson Steve Zhao said the engineers will ensure that within the five months, the entry point of the locomotive, the exit end of the tunnel and the centre shall be complete.
“So far, 1.2km is complete from the exit end, 0.8km from the entry side of the tunnel and 0.5km from the mid-section. The construction is taking place simultaneously to ensure that we complete all the work within the stipulated time,” he said.
The construction of the line to Naivasha is expected to be completed next year. Apart from the Ngong tunnel, two more tunnels measuring 2.64 km will be constructed along the 120km Naivasha–Nairobi SGR route.
Zhao said section No. 1, which had been tasked with the construction of the tunnel, currently has more than 3,300 staff, majority of whom have been working on the tunnel construction. The Ngong tunnel has been designed as a single-track railway tunnel with a clearance height of 7.6 metres and a width of 6.4 metres.
In the construction process, the contractor has employed the three-bench seven-step excavation method. Benching is the creation of a series of steps in the vertical wall of an excavation to reduce the wall height and ensure stability during construction.
Ground collapse is one of the primary risks in excavation work and the three-bench method minimises the risk of rock falling onto the excavation.
This is achieved by first blasting off the rocks in a calculated manner that will ensure the tunnel’s arch shape is achieved. Each blasting covers 2.4 metres and the three-bench seven-step excavation method divides the working face into three benches (upper, middle, and lower) and seven working faces with parallel excavation and support erection so as to achieve safe and rapid construction of large-section tunnels.