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10 February 2009

Desert waterway

The 75km Arabian Canal is one of the UAE’s most ambitious and challenging construction projects yet. Mott MacDonald is geotechnical designer.

“The Arabian Canal is the biggest and most complex civil engineering project ever undertaken in the Middle East,” said developer Limitless, unveiling plans for its 75km long waterway in 2007. The first excavations showed that this was no vain boast. Two trial excavations dwarfed the earth moving plant used to create them – and these 700m long by 50m wide sections were modestly proportioned compared to parts of the canal, up to 150m wide, now being cut. Mott MacDonald is leading geotechnical design.
Excavation on this scale hasn’t been attempted in the region before

Connecting to the sea at either end, the waterway will arc inland from Palm Jumeirah to Dubai Waterfront at Jebel Ali. Limitless plans to develop a ‘city within a city’, with landscaping and mixed use development carving a green swathe through the desert. With a navigable depth of 6m, the world’s largest pleasure boats will be able to cruise the canal’s length.

Inland, the desert rises to 70m above sea level. Deep valleys will have to be dug, requiring an estimated 1 billion m³ of rock to be moved. “Excavation on this scale hasn’t been attempted in the region before,” states Mott MacDonald’s Peter Sharp. “We needed to find out how easy or difficult it would be.” Small efficiencies gained in design and construction can translate into huge cost and programme savings. We oversaw unusually extensive field trials in advance of construction to get best value from the geotechnical engineering.

Some 80 deep boreholes were drilled along the route of the canal, showing ground to be composed of weak siltstones intermixed with sandstones, cemented aggregates known as conglomerate and bands of stronger rock, like concrete.

For the first trial, contractor Samsung used rippers to tear up the ground. “What you want for good productivity is chunks of rock that lift up with the claw and can then be easily manhandled into tipper trucks by excavators,” Peter explains. “But there were areas of hard rock where the claw was just scraping along the surface, so the rock wasn’t breaking up. Then there were other areas of very weak rock where the claw left a neat, narrow furrow – it wasn’t ripping up enough rock. Productivity overall was less than ideal.”

Contractor for the second trial, Leighton Van Oord JV, used excavation techniques “you’d normally find in a quarry operation, not on a construction project”, Peter says. Low energy explosive blasting broke up the rock, which was then loaded by gargantuan excavators into quarry size dump trucks which, when full, weigh 250 tonnes.

A combination of the two techniques is being used by contractor Tristar, appointed in September to deliver the first 10km phase of the canal development.

Peter notes another key objective is to minimise the use of rock armour and concrete walling. “A key aspect of the excavation trials was to study the natural stability of the weak rocks, which will help determine the angle of the canal banks. The ambition is to avoid lining the canal wherever possible. A combination of engineered revetments and beaches are needed locally to protect slopes against wave erosion, but the slopes as a whole are self-supporting.”

Across the project, groundwater will have to be pumped away during construction to make sure that the cut does not fill, bogging down machines before they have reached the required depth. Studies are ongoing to assess how the canal will impact groundwater levels and flows.

The trials were essential to working out how best to compact and stabilise spoil removed to create the canal. “Limitless wanted to balance cut and fill and use all of the material on site. With a billion cubic metres of spoil to shift, even a small imbalance would pose a massive logistical challenge for off-site disposal,” Peter explains. Nearly all adjacent land is earmarked for future development. Accordingly, spoil will be used to introduce new topographical features to the Dubai landscape – hills of up to 250m high.

Another challenge will be to achieve good compaction. “Finding an efficient method to assure compaction quality on a fill operation this large has a huge bearing on the speed with which work is carried out,” Peter says. Leading edge technology is being examined: compaction is carried out by heavy rollers fitted with intelligent density control systems. These measure dynamic stiffness of the rolled ground as they go. The rollers’ locations are plotted with the aid of GPS tracking and data is relayed in real time so that a detailed picture of compaction can be made. A key aspect is the GPS tracking of truck movements to assure compaction is even.

It is anticipated that construction of the canal will take place in 10 phases, completing in 2012.
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