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DUBAI FOCUS - The palm projects are being built from dredged material and will greatly increase Dubai's coastline as well as build upon its already burgeoning commercial and leisure infrastructure.

Mention Dubai, and many people immediately think of The World. This is the gigantic offshore project that has created artificial islands in the shape of a world map that rich buyers can invest in and where they can spend their holidays.

Similar schemes are the three huge palm projects that, like The World, will consist of land created from dredged material. These will also house business and holiday infrastructure.

One of the main drivers for this seaward expansion is the fact that this tiny emirate only has a 72km coastline. Even the smallest of the palm islands, with its own coastline of 78km, will more than double this, while adding about 560ha of land.

This is the Palm Jumeirah, which measures about 5km in length and width.

Developer Nakheel has more than £30bn of projects under development and some of this vast sum has gone into the other two, even larger, palms. Over to the west is Palm Jebel Ali, which is already about 80% complete and will measure 7km long by 7.5km wide. In the east, work is beginning on the largest of the three developments, the enormous rugby ball-shaped Palm Deira, which will reach a staggering 14.3km into the Persian Gulf and measure 8.5km in width.

Palm Jumeirah is about a year behind schedule, but the end is now is in sight and private villas on the fronds are rising up in a variety of multicultural styles. The delay was partly due to subsidence that became apparent when the dredged material began sinking. This has been dealt with by using vibro-compaction (VC), that has also been employed to prevent liquifaction in the event of an earthquake.

British contractors have been involved with various schemes across the palm and locally based Dutco Balfour Beatty (DBB), a 51%:49% controlled outfit, has been kept busy.

Balfour Beatty subsidiary Pennine has returned after completing frond work and is now compacting the ground at two points on the rock armoured crescent that protects the fronds.

Split into 40 contracts, running clockwise, the subcontractor is using VC (also known as vibro-fl otation) at C14 at the top left of the crescent, on a job that also features DBB and piling expertise from its Stent subsidiary. But Stent works in Dubai under the DBB brand rather than having its own identity.

'Pennine is working on plots C14 and C40 to prevent liquefaction.

DBB is doing the enabling works at C14, including rotary bored piling and is also tendering for the contract to build the Missoni hotel here, ' says Pennine regional manager Don Godenzie.

He explains that the elements needed for liquefaction are loose fire sand, which is waterlogged, and an earthquake. 'The dredged sand is placed in a loose state and the purpose of VC is to make it more dense. We can't do anything about the water or it being fine sand, so the idea of VC is to make it medium dense or better.' The Nakheel specification is for a magnitude six event with a peak ground acceleration 0.15g at the [sandstone] rock head, which on this job would be 0.2g at the surface.

To ensure this target is met, two Pennine rigs are busy 24 hours a day working across the 45,000m at depths up to 18m down to the sea bed. The process sees a vibro-flot penetrating the ground aided by water jets to where the VC rearranges and compacts the sand particles into a more dense state. Site workers introduce sand at the surface as the vibrating poker withdraws in 0.5m to 1m lifts. Nakheel requires the treated ground to settle to at least 5% of the total treated depth, but Pennine says it is achieving up to 10%.

Godenzie describes the differences between this and vibro-column work: 'With stone columns we keep the water on, even when the poker is at depth to keep the hole open and the stone is introduced all the way down to the bottom of the hole.

With vibro-compaction the sand does not have to get to the bottom of the hole and you want the hole to collapse so the material compacts.

Therefore, the water pressure can be reduced once the poker is at depth.' When work is going well, tell-tale cracks in the ground appear around the circumference of the vibro-flot at a few metres distance.

The Pennine crews are treating the ground at 3.5m triangular centres, but favourable CPT results mean that the company hopes to increase some of these to 3.75m and 4m spacings on a 5850 compaction point grid.

The subcontractor came on site at C14 in November and should finish in February. It began slightly earlier, in October, at its other plot C40, which is located on the crescent at its most clockwise extremity. This lucrative end plot will house apartments and a hotel. Work in this area was due to fi nish in December, by which time Pennine's other two rig crews will have worked across a 70,000m 2 plot.

The ground here is not so good and so compaction points are only 3m apart. 'While the upper sand is actually better than at C14, the silt below is worse, ' says Godenzie.

'But the silt is less likely to liquefy anyway and if we get clay it does not liquefy at all.

'We have now tested 3.25m and 3.5m and the results were good so we are now going through the submission process to increase the spacings, as we get paid per m 2.'Another indication of the size of Palm Jumeira - as if one were needed - is that treatment depths average about 15m, because the plot is several kilometres closer to land. DBB is not involved at this second Pennine site, where piling is being done by Arabian-Forasol and the engineer is LC Consulting.

'It's pretty straightforward for a VC job, ' says Godenzie. 'The only thing is sequencing and programming to stay ahead of the piling.'

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