Kenya is planning to build the second longest tunnel in Africa after South Africa’s 13.4 kilometre Hex-River rail tunnel, as it embarks on the second phase of the standard gauge railway.
Nairobi recently signed a contract for the SGR phase from Nairobi to Naivasha, which will include a 5.3-kilometre tunnel.
President Uhuru Kenyatta and his Chinese counterpart Xi Jinping signed the contract for the $1.4 billion project, setting the stage for its construction in 2017.
Engineers from China Road and Bridge Corporation, the contractors of the standard gauge railway, will spend about 18 months drilling through the Great Rift Valley escarpment to construct the 5,300-metre tunnel.
Globally, Japan has the longest railway tunnel, the Sei-kan tunnel, at 53 kilometres. The world’s longest railway tunnels are in Asia, with Europe having the other three among the top 10.
Kenya’s tunnel will take close to 15 of the 54 months required to construct the 120-kilometre line linking Nairobi to Naivasha.
Kenya Railways chief executive Atanas Maina said the Naivasha line will follow the pipeline corridor inside the Ngong Forest to the edge of the Rift Valley escarpment.
“Due to the rugged terrain, the total length of bridges will be 28.21 kilometres and the total length of tunnels will be 8.5 kilometres, accounting for over 30 per cent of the route length. One of the tunnels at the edge of the escarpment will be 5.3 kilometres and will take months to drill,” Mr Maina said.
The EastAfrican has learnt that a consortium of geotechnical experts and engineers went through peer reviews of how to best do this and are conversant with the monumental task ahead. The timeline of the tunnel construction, though prefixed at 15-18 months, will depend on the contractor, China Road and Bridge Corporation.
“The duration will be determined by the machinery they will be using. The most technologically sound machines are drills that bore through the escarpment, brings out the crushed rocks and soil and at the same time cast the concrete shell to support the soil. By the time the drilling is complete, the casting will have been done and all that is left is building the concrete structure to support the tunnel,” a source privy to the technical details of the SGR line study told The EastAfrican.
The source said that the Chinese contractor’s engineers have already identified the sections of the escarpment where the tunnel will be dug.
“From the proposed project designs, we are going to see a bored tunnel, built using cut-and-cover methods that will involve reinforcements to ensure stability at all times. The use of the tunnel-boring machines, will be one of the most intensive elements of this stretch as it requires engineering precision so roof stability is ensured and safety of the tunnel isn’t compromised. The laying of the tracks will be the last thing to be done,” the source said.
Hesbon Omondi, a civil engineer, said that the choice of the tunnel will make the works easier.
“By the time they proposed a tunnel, they were sure that was the best option. For a railway line, there is a gradient that must be achieved and they have to cut through the escarpment. It’s not like a road, where you can have steep sections. Cutting through the whole escarpment and dumping the soil is a tough option hence the option of boring through the escarpment,” Mr Omondi said.
The major challenge though will be supporting the roof.
Moses Makideu, a lead civil engineer at Prescon, said that one of the challenges he foresees is the escarpment terrain, which will require engineering ingenuity.
“It was cheaper for the colonial railway to cross the Rift Valley since the line followed the contours of the hills. Unfortunately, for the high-speed SGR, the line cannot meander much, as it should generally follow a straight line and attain a certain gradient, and to achieve this means the construction of long viaducts, deep tunnels and steep embankments,” Mr Makideu said.
Experts predict that the tunnel will cost as much as $6 million per kilometre and require a workforce of more than 300, due to the intensity of the work.
Building a railway tunnel is one of the most demanding engineering jobs. The 50km underground Channel Tunnel that links the UK and France is an example of how modern equipment and technologies were used to achieve one of the world’s great railway engineering feats.
The engineers used tunnel-boring machines to dig and rotating cutters to chip way the soil, which was carried out on a conveyor.
Hydraulic rams shifted the position of the cutter gripper pads holding up the tunnel as it was being excavated, with reinforced concrete supports being slotted in as the work progressed.
The Naivasha tunnel will require round-the-clock lighting, emergency exits, air ventilation and a round-the-clock security especially with the heightened terrorism levels in the country.
The dark one-kilometre tunnel at Limuru has been a security nightmare not only for the area residents but also train passengers because the train slows down inside it and commuters lose their belongings to bandits.
Since the tunnel is not lit, the train is usually engulfed in darkness as it passes through this stretch.
According to Mr Omondi, the issues of safety, ventilation, and lighting will be addressed through the construction of support tunnels.
“Normally, you are not allowed to do a single tunnel. You have to construct some support tunnels on both sides,” Mr Omondi said.
“In case of any emergency inside the tunnel, you have exit doors leading to the support tunnels. These support tunnels also house the air conditioning units to bring air into the main tunnel. They also have stairs leading to the top of the escarpment that act as an exit,” he added.
In terms of lighting, the main tunnel will be fed through electricity from a connected line but the support tunnels will have standby generators as backups, in case of any emergency, especially fire in the tunnels. The proposed design will also restrict public access to the tunnel for security purposes.
The speed of the train in this section will be 80kph for cargo trains and 120kph for passenger trains.
However, Mr Makideu predicts that the elevation and gradient will slow these trains in this section.
“With the challenges of elevation and depressions, once complete, we should expect the train to probably have a maximum speed of 40 kilometres per hour inside the tunnel,” he said.