Flooding Begins on Subsea Palm Jumeirah Tunnel in DubaiWork on the Palm Jumeirah infrastructure continues apace, with one of its key transport links - the subsea tunnel - entering its final stages, with a scheduled completion date of early May. The six-lane tunnel, which is 40 m wide and 25 m below sea level, includes two cells for traffic. Costing AED450 million, the tunnel connects the trunk of the island to the crescent and consists of three lanes in each direction with a service tunnel in between them. Approximately 200,000 cubic metres of reinforced concrete, 260,000 cubic metres of sand backfilling and 50,000 tonnes of rock armour were used to build it. Taisei is the main contractor. Taisei will be responsible for any repair or work to the tunnel for the first 11 years of its operation. Al Naboodah is the civil subcontractor. Kinden is the mechanical, electrical and plumbing (MEP) subcontractor. Parsons is the project consultant and Parsons Brinckerhoff is the project coordinator. The chief designer is Halcrow. Visit www.taisei.com, www.alnaboodah.com, www.kinden.co.jp/english/index.html, www.parsons.com, www.pbworld.com and www.halcrow.com Construction of the tunnel got underway in October 2004. A 3 km 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 30 m-long interlocking slats of steel into the sea floor. Once this was completed, 4.3 million cubic metres of seawater was discharged at a rate of 1,000 cubic metres/hour from the cofferdam in 45 days. The cofferdam construction took place in conjunction with a concerted effort to minimise the ecological impact to existing marine life. 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. The tunnel itself measures 1.4 km in length, and is comprised of 50 25 m-long segments, which are then sealed together. The slab on the top of the tunnel measures 1.5 m while the base slab is 2.5 m. 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 Jumeirah. 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 4,000 cubic metres per day) has commenced. Once the water level is equal to that of sea level - it currently stands at -4 m - the sheet piles will be removed. A completion date, when both the inside and outside is finished, is anticipated for 8th May. 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. 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. In these hard areas water jets were used in conjunction with the vibro hammer to drive in the piles. The tunnel will be 25 m below sea level at its deepest point. Promat has supplied fire protection for the tunnel. Click here. Visit www.promat.co.ukAt present several piers that will carry a monorail system are being constructed on the tunnel. Late design changes 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. The roof slab had to be redimensioned and reinforced with steel. The changes include increasing the thickness of the top slab by 10-20 cm and, to stabilise the weight further, lightweight polystyrene sheets are being placed over the tunnel before backfilling 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 and subsequent re-design of the tunnel caused a further delay of two months. More in E-News Weekly 23/2006, 10/2006 & 37/2004. Visit www.thepalm.ae 13/07.