Courtesy of Construction Review Online
Kenya has become the second country in Africa, after South Africa to boast of having the longest railway tunnel. The tunnel is estimated to be 7.14km stretching second to the Hex River Tunnels in South Africa which stands at 13.6km.
This is the first of the three sections of the 7.14 km SGR tunnel of the second phase of the standard gauge railway running from Nairobi to Naivasha covering a total distance of 120 km.
The Ngong tunnel is designed as a single track railway tunnel comprising of three sets: a 4.5 kilometer stretch with a buried depth of 108 meters; a 1km with a buried depth of 46 meters, and another 1.64km stretch. In the construction process, the contractor China Construction Company 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)
To hasten the process, the contractor was at hand to ensure timely completion of the project by working on the tunnel from three points that are; the entry point of the locomotive, the exit of the tunnel and from the center.
Section Office Number one of the SGR, which was tasked with the construction of the tunnel had over 3300 staff majority of whom worked on the tunnel construction.
The construction adopted the New Austrian Tunneling Method (NATM). According to Geoffrey Baraka, a tunnel structural engineer, given that the tunnel is a single track tunnel with narrow construction space, it provides a tight construction organization.
NATM has become the world’s most popular underground construction method. Also known as sequential excavation method, this technique maximizes on the use of principles as opposed to a technical method. The method was first developed in Austria and has been adapted globally in constructing underground passages for rail and road; dug through hills, rivers as well as other roads.
While using the NATM, the first step was to do the curved heading excavation as well as putting in place the primary support of the tunnel. The excavation was done by drilling holes into the rock face then charging the holes with explosives, which are then blasted. Following this process, ventilation is done before the short firer re-enters.
Soil and rocks that have been blasted were then scrapped and loaded onto trucks and moved out of the tunnel. The initial concrete is then sprayed on the surface of the tunnel and ring-I-beams are erected as support to prevent the tunnel from collapsing. Reinforcement meshes were then wielded on the hollowed out sections of the tunnel with longitudinal connection steel bars followed by the installation of the advance small ducts, radial rock bolts and foot anchor bolts. Once this was done, the grouting was injected.
Geotextiles and water proof sheets were placed on the side walls to keep the tunnel waterproof. This prevents water from sipping and instead inverts and drains out the water through a duct, which leads out.
The water sheets also prevent seepage after the construction is done, which ensures that the structure remains firm and sound. The 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.
The contractors were doing maximum of four meters a day and due to the poor air circulation in the tunnel, an adit was used to pump in oxygen into the tunnel. There was a lot of dust and gases inside the tunnel caused by the drilling, blasting and loading of the excavated materials. Dangerous fumes from exhaust pipes of the trucks and the equipment on site also contribute to the poor air circulation.
It is also necessary to pump in oxygen from outside as the rocks within the tunnel contain carbon monoxide and carbon dioxide, which are both dangerous gases and can cause anoxia, if the oxygen levels fall below 18%.
Safety, security and continence were among the major concerns being dealt with at the Nairobi-Naivasha SGR extension. The tunnel required emergency exits, air ventilation and a 24-hour lighting system and security, especially with the heightened terrorism levels in the country.
These emergency exits were installed by the contractor to provide air ventilation and circulation for the maintenance workers who were based inside. An evacuation tunnel, which is right in the middle of the 4.5 kilometer main railway tunnel was also constructed.
To provide lighting, the main tunnel were connected to electricity from a dedicated line but will also have standby generators as backup in case of emergencies, especially fire.
In regards to safety, 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. This refuge access will also house the signal equipment and electronics that will be used by the trains, and rail networks.
There is a total of 147 cave-ins (small caves) within the tunnel at 30-meter intervals to allow for the refuge access. The cave-ins were used to hold some air ventilation units to improve the airflow within the tunnel.
Construction safety precautions
According to Steve Zhao, spoke person of the SGR Phase 2A safety of the construction workers were highly prioritized as all employees were required to wear protective gears at all times. Besides, the air inside the tunnel is sampled and analyzed on a monthly basis to ensure workers are not exposed to harmful gases. Also as a precaution only male workers were allowed in the tunnel, while females work on other sections.
Apart from the Ngong tunnel construction, two more tunnels measuring 2.64km were constructed along the 120km Naivasha- Nairobi SGR Route.