West of Dubai, work is under way on a 75km long man-made waterway costing over £6bn. Bernadette Redfern visits the Arabian Canal and interviews project director Ian Raine.
To reach the site of the first phase of work on the enormous Arabian Canal involves driving over desert for about 15 minutes in the type of 4x4 that costs £25 to drive into central London. The route of the enormous waterway begins at the coast of the Arabian Gulf west of Dubai city centre. It sweeps inland past the site of the new Dubai World Central Airport before turning to run parallel with the shoreline and eventually turning back again to meet the Gulf coast more than 25km later, emerging as part of the neighbouring Dubai Waterfront development.
Unlike most canals this is neither for movement of water or freight. Instead it is a catalyst for waterfront real estate developments along its banks. To date a single phase of excavation has been awarded to local contractor Tristar, which began work on 28 September. But before the contract was awarded client Limitless carried out an enormous 700m long, 200m wide and 45m deep trial excavation to test the excavation method.
“Trial excavations of this scale are quite exceptional, but were considered necessary given the lack of engineering precedent for the behaviour of the local weak rocks and groundwater regime,” says Mott MacDonald ground engineering manager Peter Sharp. The consultant is advising Limitless on the geotechnical conditions for the first phase of the scheme. “There were some instances of very hard layers that proved to be problematical for conventional ripping equipment,” adds Sharp. “The trial included complementary instrumented drilling to confirm rapid means to identify such layers. With the trial findings, future contractors will be in a more informed position to allow for such layers in their excavation strategies.”
Limitless project director Ian Raine says the trial has also given the project team confidence that most of the waterway will not need to be lined as the conglomerate sandstone in the lower layers has low permeability. “We want to avoid lining the canal as far possible,’ he says. “Our trial has told us there are a lot of rocky areas so we should be okay. It is a 75km waterway so the cost associated with lining the whole thing would be high,” he says.
Tristar has decided to use conventional excavation methods for its section of the canal. Bulldozers fitted with rippers eat into the rock while other dozers with buckets attached lift the fill into trucks that hold between 20m3 and 40m3 of material. The first phase involves excavating more than 200M.m3 of fill over a 9km length of canal. “We have about 300 vehicles on site and will be increasing this over the next few weeks,” says Tristar general deputy executive manager Subah Khattar.
The depth of excavation will be 5m below sea level and the height of the canal bed is currently at plus 9.5m. “We have borehole results on the strata, however, it is not 100% representative and we are starting to get hard rock. This cannot just be used as fill so it is being piled up for processing,” says Khattar.
Surrounding the canal is over 12,500ha of desert that is set to become part of a new canal-side city. The excavated material is to be used as fi l for the site. “There will be more than 1bn.m3 of excavated material and this will be used to create a new topography surrounding the canal with man-made hills of up to 250m,” explains Raine.
As well as the challenge of creating these huge hills, the contractors face challenges excavating the canal itself. The ground it cuts through can be as high as 70m above sea level, so contractors will have to dig deep to maintain a uniform canal depth. “For areas of filling to create new hills, a significant challenge will be to achieve good and consistent compaction. A series of trials are under way, with a view to identifying the most efficient compaction techniques,” says Sharp. “At the same time, we are trialling intelligent compaction control systems, combining real time dynamic stiffness measurements beneath rollers and GPS with conventional density control approaches. “This is to find a practical and efficient means for implementing the very extensive quality control programme that will be needed for a fi ling operation of this scale.”
For now, contractor Tristar is spreading the fill in layers of up to 300mm for which it has demonstrated that it can achieve compaction rates that are 95% of the maximum achievable density measured under laboratory conditions during the trial. “We are using lightweight rollers now. If we increase the compactive effort with heavier rollers we think we can spread thicker layers of up to 1m.” Tristar says that excavating the first phase of 200M.m3 will take between three and four years, giving a rate of 150,000m3 to 190,000m3 per day. If Limitless is to achieve the 2012 deadline, future packages will have to be excavated more quickly. Contracting joint venture Leighton Van Oord is confident this can be achieved.
At another site, 30km along the canal from phase one, the contractor is engaged on a second trial that involves using heavy duty equipment usually reserved for quarrying. Controlled explosions are used to dislodge the material, which is then loaded on to vehicles that each carry 190m3 of fill. This greatly reduces the number of vehicles required and increases the excavation rate. Project sources say this is achieving about twice the productivity of the first phase of work.
According to Limitless, the contractor that wins the second phase of construction, which went out to tender on 28 October, will need to be excavating at rates of more than 300,000m3 per day, which would equate to about 33 months for the 300M.m3 dig. Phase two is expected be awarded in January 2009. “The biggest challenge is the speed of delivery,” says Raine. “Construction is already under way. We will award four earthworks packages and there will be 10 in total. Seventy per cent of the canal is in the Limitless area. All packages will be awarded by the end of 2009.”
Just who is responsible for the other 30% of the canal remains unclear, but Dubai Road Transport Authority (RTA) is one of the clients. Whoever takes over has an unenviable task – the canal intersects the Sheikh Zayed Highway, the main artery between Dubai and Abu Dhabi. “The road will have to be lifted, there is no other way,” says one project source, but for now both Limitless and the RTA are silent on how this will be done.
Another major challenge is ensuring that the canal does not become a huge, stagnating pond. Raine says this will be achieved using two tidal gates. “The natural rise is about 2m,” he says. “We have had several firms conduct tidal models and they are all coming to the same conclusion: that the canal will need seven to 10 days’ flushing time [the time needed to replace its water volume at the rate of fl ow through the canal]. We will also have strict quality controls on water discharge.” Also under consideration is the effect that the canal will have on the existing groundwater table, which sits up to 25m above sea level. “In some cases the water table may be drawn down by up to 30m and further studies are being commissioned to assess the hydro-geological impact of the canal,” says Sharp.
The project still has a long way to go. Relocating the highway, tackling the effects of drainage and determining how the tidal gate system will operate are all hurdles to be cleared if the Arabian Canal is to work. The fact that construction has started before these problems were worked out might be unusual in many countries, but not in Dubai. The desire to demonstrate progress is often stronger than the desire to ensure that fully engineered solutions are in place: the clock is ticking and 2012 is not very far away.
Once complete, the Arabian Canal will include residential communities and marinas. The amount of earth excavated over three years will be five times as much as the Panama Canal.-75km length of the waterway
-300 vehicles on site
-12,500ha of desert set aside to become a new canalside city
-1bn.m3 of excavated materials will create a new topography
-2012 completion date for the canal