By Christopher Sell
The cofferdam on the Palm Jumeirah's sub-sea tunnel is currently being flooded and is scheduled to be completed this week. With work progessing to open the access route between the crescent and the trunk, Christopher Sell was invited onto the site to witness another stage of Nakheel's offshore show.
As 66 families have now made the ‘eighth wonder of the world' their new home, work on the Palm Jumeirah infrastructure continues apace, with one of its key transport links - the sub-sea tunnel - entering its final stages, with a scheduled completion date of early May. Costing US $123 million (AED450 million), the tunnel, which connects the trunk of the island to the crescent, consists of three lanes in each direction with a service tunnel in between them. Approximately 200,000m
of reinforced concrete, 260,000m
of sand backfilling and 50,000 tonnes of rock armour were used to build it.
Taisei is the main contractor, Parsons is the project consultant and Parsons Brinckerhoff is the project coordinator. The chief designer is Halcrow.
Construction of the tunnel got underway in October 2004. A 3km cofferdam was needed to enable construction (a cofferdam is used to exclude water from an area that is normally submerged). To build the dam, Taisei inserted 4,000 30m-long interlocking slats of steel into the sea floor. Once this was completed, 4.3 million m
of seawater was discharged at a rate of 1,000m
/hour from the cofferdam in 45 days.
The tunnel itself measures 1.4km in length, and is comprised of 50, 25m segments, which are then sealed together. The slab on the top of the tunnel measures 1.5m while the base slab is 2.5m. Special HDPE (high density polyethylene) membranes are used to protect the tunnel from the seawater, while there will also be special rock armouring on top of the tunnel to protect the waterproofing system from sinking ships or anchors. To ensure that concrete was delivered on site consistently with no delays, a batching plant was set up. This also served a secondary benefit of reducing through traffic on the Palm. With six teams working at full capacity, each segment took about one month to complete.
With work on the tunnel now complete, back discharging of water (at a rate of 4,000m
per day) has commenced. Once the water level is equal to that of sea level - it currently stands at -4m - the sheet piles will be removed. "We anticipate a completion date, when both the inside and outside is finished, of 8 May," says Dr Tamer Al Hafez, senior geotechnical and tunnelling engineer, Nakheel.
A key concern during the construction of the tunnel was the cantilever effect on the sheet piles as a result of the pressure disparity between the water outside the wall, and the water inside the dam. This was assessed on a daily basis, adds Al Hafez.
A pile driver on a floating barge was used to install the sheet piles. While the top half of the seabed consisted of sand, the next layer was a very hard rock base, which delayed the schedule by one month because of the difficulty in getting the piles secured, according to Al Hafez.
In these hard areas water jets were used in conjunction with the vibro hammer to drive in the piles. The tunnel will be 25m below sea level at its deepest point.
When questioned whether this extra vibration causes any damage to foundations on the Palm, Shaun Lenehan, senior manager, environmental group, Nakheel explains that the project benefited from a lack of development in the immediate vicinity.
"When this was happening, there was nothing else in the neighbourhood. The villas were far less advanced. What we do is look at cracks in buildings beforehand and afterwards and measure the difference. You usually do a dilapidation study on existing structures before you do vibro compaction, but this was a remote site. All of the construction that has happened subsequently has undergone ground improvement as well."
The engineering task was compounded further by late design changes that saw the Palm's monorail system integrated with the tunnel, applying loads and vibration factors that had not been accounted for in the original design. "We had to take into consideration the vibration of the monorail above it. We also had to re-dimension the roof slab and reinforce it with steel, which we didn't have originally. At the beginning I said ‘no way'," says Al Hafez. However, after consultation with its designers, and an analysis of the monorail loads, Taisei concluded that the integration work was feasible provided changes were made. Al Hafez adds that Taisei will be responsible for any repair or work to the tunnel for the first 11 years of its operation.
The changes include increasing the thickness of the top slab by 10-20cm and, to stabilise the weight further, lightweight polystyrene sheets are being placed over the tunnel before sand and rock are used to firm-up the piers. The polystyrene, which has not been used before in this context, is covered with waterproofing material to protect it from the harsh saline environment.
The re-routing of the monorail - which stemmed from the relocation of a mosque - and subsequent re-design of the tunnel, caused a further delay of two months. "Putting the monorail at the end of the spine reduced the distance it runs over water," says Lenehan.
"At any possible opportunity the intention was to run the monorail over existing road rather than running parallel with it, in order to rationalise transport corridors."
Lenehan is quick to dispel any rumours of ecological damage as a result of the cofferdam, citing numerous studies and tests that were carried out during the build. "When we originally designed the Palm we undertook some hydraulic investigations which showed the water circulating around the Palm. We knew we were going to have this temporary obstacle to the circulation patterns, so in the meantime we have been monitoring the water quality either side [of the dam]."
He adds that Kerzner, which is developing the Atlantis project on the crescent, has been monitoring water quality, which has assisted Nakheel. "So that was above and beyond what we normally would have otherwise done," he says.
The cofferdam construction took place in conjunction with a concerted effort to minimise the ecological impact to existing marine life. Lenehan says that as the water level dropped inside the dam, all marine life was collected, stored in an oxygenated tank and put out to sea. In all, 2,000 specimens were caught.
He adds that even now, any contractor working on the Palm is subject to stringent regulations and cannot drain any water without obtaining a permit first: "If they do any de-watering from excavations on the Palm, they have to sample it and show us the results. If there is a problem they have to treat it and when they discharge it, it has to be a certain distance from the shore. This is standard for everyone; we have gone to a lot of trouble to protect water quality. We are tough on all utilities, hotels and residents. There are nearly zero emissions going into the water from the developments."
The Palm Jumeirah certainly represents an engineering spectacle, and to build a tunnel within the confines of this development demonstrates tenacity. It shows that to achieve something spectacular, chances have to be taken. Come 2012, it will be fascinating to see whether this particular gamble has paid off.For all the latest construction news from the UAE and Gulf countries, follow us on Twitter and Linkedin, like us on Facebook and subscribe to our YouTube page, which is updated daily.