By Isaac Mwangi | Wednesday May 23, 2018
The construction of the 4.5 km Em-BulBul tunnel on the Nairobi - Naivasha Standard Gauge railway line is expected to be complete within the next 3 months.
The 4.5 kilometer stretch with a buried depth of 108 metres is the first of three sections that make up the 7.14km tunnel on the planned 120 km Nairobi - Naivasha railway. The other sections comprise of a 1km one with a buried depth of 46 metres, and another 1.64km stretch.
Once complete, the tunnel will be the second longest in Africa after the 15.5 km long Gautrain line tunnel between Johannesburg and Marlboro Portal in South Africa. It will also be the second railway tunnel in Kenya. The first tunnel is a 1km stretch at Limuru which was built in early 1900s by the colonialists using chiselling to make it possible for the line to go through the Rift Valley escarpment.
According to CRBC, the tunnel which has a clearance width of 6.4 metres is being constructed using the New Austrian Tunneling Method (NATM), which involves full-scale excavation given that the tunnel is a single track tunnel with narrow construction space which provides a tight construction organization.
The engineering, design and construction of tunnels is perhaps one of the most complex challenges within the realm of civil engineering. The diversity of the earth's geology requires that each and every tunneling project be approached and analyzed based on its own set of characteristics.
Some of the factors that are unique to tunnel construction include;
- Uncertainty in the nature and variability of ground conditions (rock quality, ground water, gas, etc) - need for adequate site investigations prior to and vigilance during tunneling
- Confined space of tunnel environment (limited access, escape, air quality control)
- Difficulty in communications (sound and signal barriers)
- Work in compressed air (soft ground)
In an earlier article published by the East African, Mr. Baraka, the tunnel structural engineer indicated that poor air circulation inside the tunnel was one of the challenges they have to deal with.
“Inside this tunnel, we have dust and gas caused by drilling, blasting, loading of excavated materials and shotcreting. We also have dangerous fumes from the exhaust gas of the trucks and all the equipment in here. Within the rocks is also poisonous carbon monoxide, carbon dioxide which can cause anoxia, if it reduces the density of oxygen to less than 18 per cent. That is why we have to pump in the oxygen from outside through the adit into the tunnel,” Mr Baraka explained.
According to the Chinese Defense Science and Technology Agency, continuous monitoring of the air quality in the tunnel is necessary to ensure safety of the workers. Measurement limits to be observed include;
- Oxygen – 19.5 to 23%
- Nitrogen Dioxide – Less than 5 ppm
- Lower Explosive Limit – Less than 10%
- Carbon Monoxide – Less than 25 ppm
- Dust – Less than 10 mg/m3 (Long term)
Other hazards encountered in the construction include Fly rock and air blast during blasting, being knocked over/crushed, rock falls, noise, accidental detonation by drilling into explosives, tripping and falling, as well as collapse as result from instability of exposed rock surface. A Safety Management Plan is therefore critical for such a project detailing the Job Hazard Analysis, safety Procedures, Controls and Review Programs that lead to improvement measures.
The construction of the tunnel will no doubt offer a great platform for engineers in Kenya to learn more on how to manage the hazards associated with such projects particularly owing to the several factors that are unique to tunnel construction.