Kenya is set to become the second country in Africa, after South Africa, to have the longest railway tunnel, a 7.14km civil engineering feat on the escarpment between Nairobi and Naivasha.
Engineers from China Road and Bridges Corporation (CRBC), the SGR contractor, are already nine months into the construction of the tunnel, which comprises three sets: a 4.5 kilometre stretch with a buried depth of 108 metres; a 1km one with a buried depth of 46 metres, and another 1.64km stretch.
The engineers at the 4.5km tunnel at Em Bulbul in Kajiado County, south of Nairobi, are using the New Austrian Tunneling Method (NATM), which involves full-scale excavation.
Initially, it had been thought that the tunnel would be excavated with boring machines but the rock and soil structure couldn't allow for it.
According to James Karanja, deputy lead consultant project engineer from the China National Abrasives Industry Corporation (CAEC), the tunnel has a clearance width of 6.4 metres and will pass through five fault zones, two of them being active fault zones.
"We are constructing the concrete in 10-metre segments to allow for movement, especially during earthquakes. This ensures that it can withstand such natural forces," said Mr Karanja.
Its construction will adopt the NATM, which Geoffrey Baraka, a tunnel structural engineer, said that given that the tunnel is a single track tunnel with narrow construction space, provides a tight construction organisation.
"With the NATM, the first step is to do the curved heading excavation and primary support of the tunnel. This excavation is done by drilling holes into the rock face then charging the holes with explosives and then blasting. After this, we do the ventilation before the short firer re-enters," Mr Baraka said.
Thereafter the soil and rocks are scrapped and then loaded onto trucks and moved out of the tunnel.
Then comes the initial shotcrete manually sprayed on the surface of the tunnel and then ring-I-beams are erected as support to prevent the tunnel from collapsing.
"After that we wield the reinforcement meshes on the hollowed out sections of the tunnel with longitudinal connection steel bars. Then we install the advance small ducts, radial rock bolts and foot anchor bolts, then we inject the grouting," Mr Baraka said.
The tunnels have also been waterproofed using geotextiles and waterproof sheets on the sidewalls to prevent water from sipping in, but instead to invert and drain out through a duct, which leads out.
Kilian Kimani, the materials engineer at the tunnel, said that these water sheets prevent seepage after the construction, ensuring that the structure remains firm and sound.
"These sheets are placed at an incline on the sidewalls to the water stops, which then goes down the inverted tunnels and is drained out through a dedicated pipe," he said.
Construction of the tunnel is projected to last 24 of the 42 months projected for all SGR projects and so far they have done one kilometre in the past eight months.
"We have managed to do 350 metres on the entrance at Em Bulbul. We have been slow on this side because it is mainly soil we are dealing with which is more sensitive and difficult to handle. This has seen us do a maximum of two metres a day," said Mr Karanja.
Step by step
On the exit of the 4.5 kilometre tunnel, the contractor has done 700 metres, with the formation being mostly rock, which they say is easier to blast through and reinforce.
This has seen them do a maximum of four metres daily. With poor air circulation inside the tunnel, they are using an adit to pump in oxygen into the tunnel.
"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 are also poisonous carbon monoxide, carbondioxide 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.
Safety, security and continence are the major concerns being dealt with at the Nairobi-Naivasha SGR extension. The tunnel will require emergency exits, air ventilation and a 24-hour lighting system and security, especially with the heightened terrorism levels in the country.
These will be installed by the contractor as soon as the tunnel is completed, to provide air ventilation and circulation for the maintenance workers who will be based inside.
"We have already completed the evacuation tunnel, which is right in the middle of the 4.5km main railway tunnel. This will be strictly for use by vehicles to access the tunnel in case of an emergency," he added.
For lighting, the main tunnel will be connected to electricity from a dedicated line but will also have standby generators as backup in case of emergencies, especially fire.
The tunnel has side walkways, with refuge access in case one needs to take cover as the train approaches, while they are in the tunnel. The refuge access will also house the signal equipment and electronics that will be used by the trains, and rail networks.
"We also have a total of 147 cave-ins (small caves) within the tunnel at 30 metres intervals to allow for these refuge access. They will also be used to hold some air ventilation units to improve the airflow within the tunnel," Mr Baraka explained.
On the downside, keeping with Chinese cultural traditions said to abhor having women deep underground in such sites, no female workers are allowed inside the tunnel.
Currently, Japan has the longest known railway tunnel at 53 kilometres. Seven of the world's longest railway tunnels are in Asia, with Europe having the other three amongst the top 10.
However, the engineering plans for the section past Naivasha at the Great Rift Valley Lodge are already setting expectations high that something of an engineering feat is in the offing.
Two years ago, engineers had predicted that the SGR contractor would use drilling machines that would bore through the escarpment, bring 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 must have been done and all that is left is building the concrete structure to support the tunnel. This cut and cover method was however abandoned.
Engineers predict that the tunnel project will cost as much as $6 million per kilometre or a total of $42 million to compete with a workforce of more than 100, due to the intensity of the work.
Kenya has one existing tunnel, the 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.
This nevertheless required expertise as the topography of the area is gently hilly, which posed a challenge to the construction of the old railway line.
This made the engineers then to elevate some parts of the railway line and also have a tunnel but most of the railway line in the escarpment meanders along the foot of the hills.
Building a railway tunnel is one of the most demanding engineering jobs, which requires the best technology.
The 50km underground tunnel Euro tunnel that links England with France, is an example of how modern equipment and engineering technologies were used to achieve one of the world's railway technological feats.
The engineers used tunnel-boring machines to dig, rotating cutters to chip way the soil, which was then carried on a conveyor belt into railroad trucks.
The robotic precision of the engineers also saw hydraulic rams shift the position of the cutter gripper pads holding up the tunnel as it was being excavated, with reinforced concrete supports fitted into the places that had been chipped.