Technological advancements in tunnelling and construction would certainly perplex artists who created sketches for a potential tunnel through the Bosphorus, roughly 150 years ago. Travelling between the two sides of Istanbul had been limited to boats until the 20th century when two different bridges were eventually built to take cars across the strait. Even now, ferries taking commuters from the more residential Asian side to the commercial and touristic European side and back, are packed in the mornings and evenings.
Turkey’s Ministry of Transportation, the General Directorate of Railways, Harbours and Airports Construction (DLH), wanted a solution that would reduce traffic congestion and target its side effects, in particular the pollution that accompanies road traffic. Updating the city’s mass transit system became a necessity and the DLH narrowed its sight on a rail tunnel, finally fulfilling the Turkish dream of tunnelling under the Bosphorus, says Masahiko Tsuchiya, design manager on the Marmaray Project for Taisei, which won the now USD 1,800M contract with a joint venture partners Gama and Nurol in 2004.
The immersed portion of the tunnel measures 1,387m and will eventually be met on the European side with tunnel bores made by two Hitachi Zosen slurry TBMs of 7.8m diameter. On the Asian side the immersed tunnel has already connected with bores made by two more Hitachi Zosen slurry TBMs (Figure 1).
Due to seismic activity in the region, the tube had to be designed to handle an earthquake of up to 7.5 on the Richter Scale. This was achieved by fitting flexible joints made up of two omega-shaped rubber gasket rings sandwiched between steel segmental linings at each connection point. Other considerations for the construction included immense water pressure at the alignment’s deepest point, 60m below water, and the strait’s rapid, dual-layer current. The top layer of the current travels north to south at 5 knots maximum (2.5m/second), but the bottom layer of the current travels south to north at 2 knots maximum (1m/second).
In August 2005 Taisei started to prepare the seabed trench that would create a stable place for the tube elements to be installed. A dredging machine from Japan was used to evacuate the trench, working at a rate of about 30 cubic metres per bucket. Approximately 1M cubic metres of undersea deposits were dredged, says Tsuchiya. Any contaminated material excavated was taken to a confined disposal facility and clean material was disposed of into the Cinarcik ditch in the Marmara Sea, with the approval of Turkey’s Ministry of Environment and Forests. After completing the dredging in July 2007, a subsea grader was used to create a foundation layer, which was necessary before tube elements could be installed.
Most of the ground below the Bosphorus Strait consists of marine deposits with a small area of Belgrad formation. Two areas within the immersed tunnel alignment required ground improvement. One area, where the risk of liquefaction existed deeper than 4m from the tunnel’s base elevation, Taisei used compaction grouting. A mixture of water, cement, sand and additives, for durability and cohesion of the grout material, was injected through steel pipes dug by four boring machines. A total of 3,300 piles were made, down to 8m at the greatest depth. The other area within the immersed portion of the alignment that posed a risk was less than 4m deep from the base and only required dredging and replacement material.
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By GlobalDataFloating the elements
The 11 immersed tunnel elements each measure 15.3m wide and 8.6m tall with two compartments for the rail lines. Each compartment has a 1.4m escape way and a squeeze way for maintenance. The length of the elements varies, with eight measuring 135m, two at 98.5m and one measuring 110m. Their weight ranged from 18,000t to 14,000t for the smallest. To withstand the high water pressure of the Bosphorus, waterproof steel plates 7mm thick are used on the bottom and both sides of the element, with corrosion protection. Because a steel membrane couldn’t be used on top of the tube, it is connected to a waterproof sheet, which is then covered in a layer of protective concrete. The water can’t damage the tube, and neither could a sinking ship, explains Tsuchiya.
Tube elements were prepared in two phases: manufactured at Tuzla Port, in southeast Istanbul, towed 10.8km and tested at nearby Buyukada Island. Two facilities were used at Tuzla, owned by DLH, to speed up the fabrication process, explains Tsuchiya. First the steel structure of the base part was built in dry docks and then floated to a pier for construction of the upper part, allowing the next element to start. After testing and final preparations for the immersion at Buyukada, one by one, elements were towed another 28km to their installation site.
The Marmara Sea is too shallow to use a route along the shore between Tuzla and the tunnel. Instead, elements were towed around Buyukada and other nearby islands for a total of approximately 40km. It took roughly one night to tow each element and about one day to then immerse it, Tsuchiya says. The first element, numbered E11, was immersed on 23 March, 2007. This piece would later host the connection with the TBM mining from Uskudar Station, saving the deepest point of work (between elements E2 and E3) for last. The final element, E1, which will connect with TBMs from Sirkeci Station, was immersed on 23 September, 2008.
The immersion
Much of the immersion work on the Marmaray Project depended on the weather and sea current conditions. The employer’s requirements said the immersions could only be carried out when the strait’s current was less than 3 knots (1.5m/second), leaving the contractor to find small windows of time to safely float and immerse each element. Monitoring systems like GPS and ultrasonic waves were used to identify the element’s location and place them in the trench. The more traditional method of using divers to guide the immersion was ruled our because of the tunnel’s depth. Diving would be inefficient as it could take nearly an hour to swim down or back up. In particular, it was just too much of a risk to have humans diving with the water’s intense pressure, says Tsuchiya.
The Bosphorus is a busy marine crossing with ferries and other local and international traffic during the day. A temporary steel tower, weighing about 200t, 3m wide, 8m long and 35m tall, was installed in the Bosphorus as the only access shaft to the tunnel before the TBMs arrived. A portion of the shaft had been attached to element E11 before it was immersed in March 2007 and the rest of the shaft was lowered down later that spring. Once attached to E11 the contractors were able to gain access to each immersed elements via the shaft. A temporary jetty and bridge connected the top of the access shaft tower to the mainland. With the strait’s strong current and traffic, a barrier had been set up to prevent ships from hitting the tower, in the event one loses control.
The immersion barge, which needed to stand in the Bosphorus holding elements weighing up to 18,000t, had hulls 90m long and 8m wide, with four lifting lugs each with a 250t lifting capacity. The barge lowered an element with the lifting hooks attached to the lifting lugs.
The elements were buoyant before the water ballast was poured into them. “The placing barge was clinging to the tube, which was buoyant itself during towing of the tube,” says Tsuchiya. “Just before starting immersion of the tube, we ballasted to 3 per cent of the tube weight by pouring sea water into ballast tanks installed in the tube via plumbing pipes and valves which could be controlled remotely in a commander’s room of the placing barge.”
Three per cent of the tube weight is approximately 540t, which could safely be lifted by the barge and its four lifting lugs.
Once it touched down, the ballast tanks were increased to 5 per cent. Tsuchiya explains the heavier ballast was preferred to minimise lateral shift of the elements from the rapid flow of the Bosphorus.
The elements were fitted with jacks to provide horizontal and vertical movement—on both ends of E11 to adjust it’s orientation and on only one end of subsequent elements to line up—and were lowered within about a metre’s distance to the previously immersed element. After verifying its location, it was brought horizontally using guides to meet with the previous element. The Gina gaskets on the elements have a soft rubber nose that made contact with the previously installed element, and a pulling jack on the existing element was extended to draw in the new element.
As the bulkheads at each element are slightly recessed, when two elements were pulled together a chamber of water was created between them approximately 2m wide. A tap in the bulkhead of the previously immersed element was opened. About a cup of water came out and the elements were drawn close together by the unbalanced hydrostatic pressure. The remaining water needed to be pumped out.
Once an element is connected to the previous one and the connection was sealed off from the water, the foundation concrete was completed. Lock filing was applied to avoid any horizontal movement to either side, and only after the backfill for each element was finished, another one could be immersed (Figure 3). Ballast tanks were emptied after installation. Each element required about a month of work, says Tsuchiya. When the final element (E1) had been locked in place, the tunnel was covered in backfill and a protective armor was applied to avoid damage from anchors or sinking ships.
All of the elements are completely embedded so the seabed’s profile is the same as it was before the construction started. In areas where the backfilling is less than 4m, armor stone has been placed for additional protection. Maintaining the same seabed profile was required for environmental purposes, including fish migration.
Figure 1 – A placing barge lowered elements into a pre-excavated trench. TBMs join the immersed tube at E11 Figure 3 – Immersed tunnel elements beneath the Bosphorus Strait A temporary steel shaft is connected to element E11 The placing barge and E11 before immersion Element E11 is immersed by ballasting 3 per cent of the tube’s weight Backfill is added for protection and to return the seabed to its original profile
