Palm tunnel to link trunk and crescent

In order for vehicles to travel between the trunk and crescent of the Palm Jumeirah, an undersea tunnel is currently under construction. Zoe Naylor steps onto the island, to report from the ‘eighth wonder of the world’.

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By  Zoe Naylor Published  May 20, 2006

|~|122proj200.gif|~|The 1.4km-long tunnel (inset) will connect the trunk of the Palm Jumeirah to the crescent. A cofferdam has been built on both sides of the site to enable water to be drained away to make way for construction of the tunnel.|~|Billed as the ‘eighth wonder of the world’, Nakheel’s Palm Jumeirah is certainly a feat of engineering. Around 90 million m3 of reclaimed sand has been used to create the landmass upon which myriad villas, hotels and leisure facilities are now taking shape. One of the latest construction challenges to get underway on site is the Palm Vehicular Tunnel, a US $122.5 million (AED450 million) undersea tunnel that will connect the tip of the island’s trunk to the crescent. “The total length of the tunnel is 1.4km — around 1km will be under the sea, the rest is for the onshore approaches,” says Najdat Othman, resident engineer at Parsons, (supervising consultant for the Palm Vehicular Tunnel). Taisei is the main contractor on the project, Al Naboodah is the civils subcontractor, Halcrow is the design subcontractor and MEP subcontractor is Kinden. The 38m-wide tunnel will comprise three cells: two vehicular, which will carry three lanes of traffic each way; and one used for emergency evacuation and services. From the top of the tunnel to the surface of the sea there is a 10 m-deep navigation channel, while from the sea level to the bottom of the tunnel’s foundations is a distance of 23m at the deepest point. The tunnel has been excavated using a dry cut and cover technique, using a sheet pile cofferdam. “We built a cofferdam in the sea and isolated the area in which the tunnel will be built. Then we discharged the water, dried the area and started construction of the tunnel,” says Offman. The cofferdam itself is 2.4km long. Around 4,000 sheet piles have been used (each around 30m long) and driven into the seabed by vibro hammers. “The bottom of sea is a mixture of rock and sandy materials,” explains Othman. “Sometimes where the area was very hard we used water jets in combination with the vibro hammer to help drive in the sheet piles.” The cofferdam sits +3.3m DMD (Dubai Municipality Data) above the sea level and has a maximum water reach at high tide of +2.5m. And according to Othman, the contractor worked 24 hours a day to construct it: “Once completed, we used nine large pumps to discharge 5.5 million m3 of water.” The capacity of each pump was over 1,000m3 per hour, which meant running them 24 hours a day for 47 days to discharge the water. This equated to around 117,000m3 of water discharged on a daily basis. Running parallel to this work was a marine life rescue operation to prevent fish and other marine creatures from becoming trapped inside the dam after it was closed. This included catching the fish alive and relocating them in the waters on the both sides of the cofferdam. A vital aspect of the tunnel project is the daily monitoring of the cofferdam: “We’re working in the sea, which means we are being protected by the cofferdam,” says Othman. To ensure the integrity of the structure visual inspections are carried out to assess any movement of the sheet piles and changes in water level. “Sometimes there is a little movement in the sheet piles at high and low tide, but this is perfectly within the acceptable range and we’ve allowed for tolerances of the movement within the sheet piling,” adds Othman. In terms of the construction of the tunnel, around 60% is completed, according to Othman: “Work has now progressed onto the construction of the concrete box i.e. the foundations, walls and roofs for the tunnel.” The concrete is protected with waterproof systems that help keep the structure watertight and avoid any leakages. “We use special HDPE [high density polyethylene] membranes to protect the tunnel from the seawater, and there will also be rock armouring on top of the tunnel which will protect the waterproofing system from sinking ships or ships’ anchors.” The roof of the tunnel is all cast in situ. This means fewer joints than if using precast beam units, which is vital for keeping the concrete watertight. Unimix is supplying the concrete, and to ensure there is a continuous supply of concrete there is also back-up support from the plant at Jumeirah Beach Residence if needed. Around 185,000m3 of concrete will be poured during the project. “The base slab of the tunnel is 2.5m at the thickest part, which requires some sizeable concrete pours — we’ve poured around 2,500m3 in a single go so far,” says Othman. After the base slab, work begins on the walls and then the roof: “We are in-situ casting a 1.5m-thick roof slab which is more challenging than you would often find on tunnels that are constructed using precast beam units.” Peri is providing the shuttering system for the tunnel: “It’s not an easy job since the tunnel has different grades — the approach has a 5 or 6 % slope so casting the roof slab was not straightforward.” According to Othman one of the most important elements was planning for the easiest movement of the shutters: “We have a limited number of shutters, so we use four sets of shutters for the walls, six sets of the roof and four sets for the base.” In terms of concrete work, most of the base slab has now been cast and the full tunnel structure should be completed by the end of the year. The next stage of the project will be the M&E commissioning which is likely to start at the beginning of 2007. The overall project’s anticipated completion date is early May 2007. “MEP work is now at the conduit installation stage which runs parallel with the concrete work i.e. while we’re casting the concrete we’re also providing the conduits and access doors. “All electrical wires and facilities are selected to withstand the severe the conditions such as the high saline content in the air,” adds Othman. Constructing an undersea tunnel is never a simple task. It demands a range of innovative construction techniques as well as a team that can handle the challenging working environment — especially with the hot Gulf summer temperatures around the corner. But as the Palm Vehicular Tunnel edges towards completion, it’s all just par for the course for the project’s site team.||**||

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