Not getting into the flow

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BIG JOB: David Aldis, chief resident engineer, Halcrow, says the vast scope of the project was creating new challenges.

Working near water brings with it a whole host of problems. Hugo Berger visits Dubai's creek extension to see first-hand the issues water raises.

Dubai's geographic position - with a long coastline on one side and an arid desert on the other - does not make it the ideal city for finding prime development sites.

Once the Gulf coast had been fully developed, there was nothing left but inland tracts of land.

The municipality has taken matters into its own hands and imaginatively found a solution to the dilemma. At this very moment, vast swathes of land are being dug out to make way for man-made lakes, marinas, islands and canals.

Projects such as the Arabian Canal, The Waterfront and the numerous island schemes aim to push up land values in an area where space is finite.

The construction of any building has, by its very nature, hazards associated with it. But these dangers are vastly increased when building in the vicinity of water.

In Dubai, there have been a number of high-profile cases where under-construction foundation walls have been breached by nearby water bodies.

Last year alone there were two major incidents which hit the headlines.

In February, a wall collapsed at Infinity Towers in Dubai Marina, which led to dramatic pictures of workers running for their lives as a torrent of water pouring in beamed around the world.

And in December, the foundations of the D1 Tower in Dubai Creek were flooded.

Although these incidents are still under investigation, some have suggested the accidents were caused by a failure in the quay wall construction.

Recently, Construction Week visited the site of a quay wall construction project at the Dubai Creek extension to meet with Halcrow, the main consultant behind the construction of the quay walls.

Halcrow has had a presence in the UAE for about 50 years, and has worked on some of the most innovative jobs in the city, including the original dredging of Dubai Creek.

David Aldis, chief resident engineer, Halcrow, says that although he cannot comment on the recent quay wall breaches in other parts of Dubai, there were techniques to prevent similar incidents occurring.

"Anyone who builds within a certain distance of the wall has to build a full retaining wall around their structure in order to keep the ground water and the sea water out," he says.

"The quay wall is a barrier and the wedge is there to relieve hydrostatic pressure, but it does not hold back any water at all.

"Around the rock wedge you have geo-textile which keeps the sand out of the rock wedge to stop any subsidence behind the quay wall. There have been issues when contractors have dug basement levels behind quay walls and caused them to collapse.

"Our quay walls have never failed in themselves but it's very easy to point the finger at the quay wall.

The first phase of the creek extension, taking the waterway 10km from the creek to Sheikh Zayed Road, has been underway since mid-2005. At present it is about 80% complete.

The creek walls are built the same way with a construction technique engineered by the company; the first stage is dewatering, with 500mm wells dug to a depth of 10m to lower the water table.

This is followed by bulk excavation and the placement of crushed rock foundations at a depth of 50mm to 150mm. The lower level of the blockwork is cast in-situ, with three levels of pre-cast blocks placed above.

Behind the wall, a wedge is dug which has a geo-textile surface placed on it and is then filled with 75mm to 250mm size rocks.

An anti-scour rock is placed on the front of the key wall, and then mooring ballards are installed.

Aldis says one of the challenges during construction has been convincing developers not to build too close to the quay wall.

As the wall is only a façade, which can let water through to the rock-filled wedge behind, it is imperative that building work takes place at least 10m away from the water's edge.

Aldis says the vast scope of the project was creating new challenges.

The developers obviously want to use as much land as they can because it's the use of land which makes the development viable so they want to build as close to the quay wall as possible," he says.

"If they need to build within the active wedge of the wall, they have to pile so that the weight of the structure is taken down below the toe of the quay wall.

Normally, developers are very good and they ask where they can build, but the main problem is confusion in respect of the quay wall as it is not impermeable.

"Some contractors don't realise this and they think they're building behind a watertight wall.They're not - they're effectively building in the middle of a channel and water can flood in.

"Even if you're digging in the dry, because we are fairly near the sea, the water level is about one or two metres below ground level.

"In order to keep that dry you have ground water pumps which then pump into a pipe which discharges into the sea or the creek.

Construction Week was taken by Aldis to the site of a circular pool in the Creek extension by Business Bay where a section of quay wall is under construction. A lone wall stands at the edge of a huge desert ditch, making it hard to believe that one day the area will be a vast lake, surrounded by a thriving business district.

As with most projects of this size, original tenants may need to vacate inbetween construction work on site. In this instance, this section is near a former army base and Halcrow has been waiting for the base to be vacated and demolished by the military before work can begin.

Aldis says that the Army is only one of numerous other agencies which Halcrow is having to work alongside.

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