TECHNOLOGY
densely populated area. There are underground plans, but these are not always accurate— so we have to do physical investigations, often by means of digging a trench by hand,” he says.
• Changing ground conditions often affect a large site, meaning the design has to be adapted to those changed conditions.“ Clients sometimes do not want to spend too much money on the geotechnical report upfront, and so it is not uncommon to find different or unforeseen conditions.” He gives the example of a new head office in Sandton on which Franki Africa performed the lateral support works. The design provided for a certain level of soft material followed by hard rock. When the excavation uncovered a further layer of soft material where there should have been rockface, it meant redesign and additional pillars and support. Such errors can occur for reasons of time-saving, cost-saving, or limited experience or knowledge of the ground conditions in the location, says Ferreira.
• Existing large buildings are another challenge, when earthworks on the new site cannot be permitted to cause any movement to a neighbouring building’ s foundations. Ferreira says this requires a careful layer by layer descent with monitoring at each new level.“ If we do detect any movement as we descend, it will require redesign to put in additional support to ensure no further movement. A further complication can be that sometimes buildings in reality are not as they’ re shown in the plans.”
Franki Africa is also active in most parts of southern Africa, and Ferreira notes that although it often takes a long time to get projects under way,“ once we get the go-ahead it is not much different to construction in South Africa, with the exception if there is a mechanical breakdown due to logistical challenges”.
26- CEC August 2018
The past: Gautrain
“ A tunnel can be built in three ways,” says Van der Merwe,“ by means of a‘ cut and cover’ in which a roof is subsequently built over the channel; drill and blast; and a TBM.” GMA used all three types where appropriate, but Van der Merwe says the agency would look more to TBMs in future extensions of the Gautrain.“ TBMs minimise your risk. With drilling and blasting you don’ t know what you’ re going to get. You do with TBMs, but they carry the disadvantage of having to repeatedly change the heads, as you cannot use the same cutters for soft and hard materials.” A TBM is custom designed for every job. It excavates and lines the tunnel at the same time and is buried alongside the tunnel when the job is done. TBMs are undoubtedly expensive. For phase one, GMA bought one from Herrenknecht for R450-million and today it lies buried in the ground below Johannesburg( after being stripped). However, technology has improved even since phase one, says Van der Merwe.“ TBMs are becoming more common with improved technology, while users are more skilled today. The technology of rail is also improving and in the next expansion, we are looking at technology in which the braking mechanisms of our trains can generate sufficient power to run a fleet of electric or hybrid buses. Rolling stock is becoming lighter and more power efficient and in the new line, we will select rolling stock based on this power efficiency.” According to ridership projections, the GMA had the building of a second tunnel penned in at the thirteenth year of operation. Ridership volumes are already at the projected 13-year levels and the single tunnel is creating a bottleneck. He has no doubt the project will get the go-ahead.“ What’ s good about infrastructure is that it creates a ripple effect in the economy through creating jobs, skills, and careers.” Because of its size and cost, phase two will be built in stages, with the first being two sections: Little Falls to Cosmo; and Cosmo to Randburg and Sandton,
Franki Africa
Franki Africa’ s group business development manager Victor Ferreira explains what happens when clients do not spend enough on upfront geotechnical reports.
with an extra tunnel to Marlboro. Section two will run from Little Falls to Roodepoort to Yabulani in Soweto and will also include tunnelling. Another reason it will proceed is the passenger volumes a train can carry. For a freeway to carry as many people as a railway line might require a freeway as much as 50 lanes wide— a clear impossibility.
The present: Doha Metro
One of the biggest and most audacious underground infrastructure projects under way at the moment is the Doha Metro. Brilliantly master-planned by client Qatar Rail, it took just 26 months for the first phase, providing the capital of Qatar with three citywide metro lines with 111km of ultramodern tunnel systems. According to Qatar Rail, the overall completion of the project‘ Doha Metro, phase 1’ recently stood at 72 % of the work, including 81 % of construction works and 59 % of systems works. That accomplishment outshone anything previously achieved in urban metro tunnelling worldwide. From August 2014 to September 2016, on three main lines, four international joint ventures consistently pushed forward a total of 21 TBMs( compared to the one Gautrain employed), designed and equipped by Herrenknecht specifically for the project. At peak times, 20 TBMs