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