Boundaries busted for highway under the Bosphorus 24 Sep 2015

It is a joint venture of Yapi Merkezi of Turkey and SK Engineering & Construction of Korea that is progressing the build, operate and transfer (BOT) concession for the Turkish Government’s Ministry of Transportation to create a new undersea highway crossing of the Bosphorus and improve traffic connections across the strait for the city of Istanbul. The existing car ferries and toll bridges are heavily congested and the third bridge, planned to open by the end of 2016, is designed as a bypass of the city to the east. The immersed tube railway tunnel – completed and opened in 2013 – provides additional metro and rail passenger capacity between the European and Asian sides of the Bosphorus and a new project for combined road plus metro bored tunnel is being discussed. It is extra road capacity for the city, however, that continues to drive immediate demand.

Fig 1. Fixed traffic links across the Bosphorus and their locations

The Eurasia road tunnel crossing of the Bosphorus was planned by the Turkish Government some ten years ago, following two independent feasibility studies - one by the Japanese engineering firm Nippon Koei, which was engaged at the time on the immersed tube rail link project, and the other, in greater detail, by French construction company Vinci. Once adopted, the Turkish Government placed the same urgency on the project as it places on any proposal for adding transportation links across the natural waterway.

Since the first fixed link across the Bosphorus was built in the 1970s, the economic model for developing new toll highway connections has proven attractive. When the bored tunnel procurement process began in late 2006, seven or eight groups submitted expressions of interest. These were shortlisted to two: the Yapi Merkezi SK E&C (YMSK) proposal and one other. The main selection criterion, other than the technical qualification criteria, for the Government was the length of the concession period. On this basis, the rival bid, with its proposal for a concessionary period some eight or nine years shorter than the 25-year operation term proposed by YMSK proposal, was selected.

On further review irregularities in the rival’s bid had it disqualified, promoting the YMSK partnership to the winning position. During negotiations YMSK maintained its 25-year concession period and in 2009 the partnership created the 50/50 Avrasya corporation as the concession holder and the 52/48 YMSK JV to complete the project’s five year design-build construction contract. The partnership, it was explained during a meeting with senior managers and directors of the project in Istanbul, was established when SK E&C of Korea approached Yapi Merkezi to be its Turkish partner in a proposal for the concession. SK was also working on proposals to be involved in construction of the third bridge across the Bosphorus in Turkey.

Concession economics

Under the concession agreement, only the Government can set the toll. This is set at the equivalent of US$4 plus 18% VAT when completed and opened to traffic. This is charged in both directions and similar to the car ferry services across the strait. Through the two years of preliminary investigation and design studies, the total investment for constructing the new highway tunnel crossing was set at US$1.245 billion. “This is for everything,” said Basar Arioglu, Chairman of the Board of the Turkish construction company Yapi Merkezi and also of the Avrasya Tuneli concession company during the meeting with TunnelTalk. “It includes also the financing costs through the five-year construction drawdown period.”

All of the financing is raised 100% privately. US$960 million is made available as credit facilities. Of this, 48% or $460 million is from Korean financial institutions and $500 million from mainly European Union investment facilities and Turkish banks. The remaining $285 million is equity invested by the YM and SK partners.

Attracting enough traffic for the economics of the concession to work is said not to be an issue. “We are building a highway of two 60km/hour traffic lanes in each direction and for cars and mini buses only – no heavy goods trucks, motorbikes, bikes or pedestrians and all traffic studies of recent times confirm there is pent up demand just waiting for availability of the new facility,” said Arioglu. “The two operating bridges, with a toll of about US$1, including VAT in each direction, carry some 450,000 vehicles/day and are heavily congested, taking up to an hour to travel in one direction in peak periods. The new bridge is at the opposite end of the Bosphorus from us and will be used mainly for freight traffic to bypass the city. The opening of the Marmaray immersed tube railway crossing in 2013 has made no significant improvement to traffic congestion on the bridges.”

In sharing traffic usage risk with the Government, the Government provided a guarantee of 25 million journeys through the toll highway tunnel per year, with a 70/30 share of profits in favour of the concessionaire for usage above that. “It is highly unlikely that traffic will ever be below 25 million cars per year,” said Arioglu. “That is about 68,000 vehicles/day in two directions and the estimate of usage when opened at the end of 2016 is for a total of 100,000 vehicles/day.”

With these predictions and negotiations in place, YM and SK started negotiations with Lenders for the financing of the project. Such negotiations took until the end of 2012 when the loan agreements were signed with financial close in March 2013.

In this period there were also some objections from environmental groups who suggested that the highway would attract extra traffic and increase air pollution, and destroy the historic environment of the old city on the European side of the connection. Following assurances that the construction program would respect archeological investigations within the project’s earthworks, and that high priority would be placed on ventilation, public safety and traffic management at the connections with existing highway infrastructure at each end of the alignment, the environmentalists stood down and work progressed.

Bored tunnel versus immersed tube option

The most technically demanding section of the new 14.5km long highway between Kazlicesme on the European side of the strait and Goztepe on the Asian side, was planned from the start as a single-tube, double-deck, four-lane, large-diameter TBM bored tunnel under the sea channel. This connected either side to cut-and-cover access boxes and U-section transitions to link with existing highway infrastructure and with a 1,000m-long section of twin-tube NATM work through the rise on the Asian side (Fig 2).

Herrenknecht Mixshield ready for delivery from Schwanau

Detailed design engineer of record for the design-build construction contract is Parsons Brinckerhoff of the USA with associate consultants SU YAPI Muhendislik for civil and structural design work on the underground transition facilities and tunnel portals, and IGT Geotechnik und Tunnelbau of Austria for the NATM design work. HNTB of the USA was engaged by the concession company Avrasya as the project’s independent design verification engineer and Arup is retained by the lenders as their technical advisor and representative engineer. Marsh has acted as the broker and insurance advisor to YMSK partnership of the construction works and JLT is the advisor for the Lenders in that respect. Munich Re is the lead insurer.

Parsons Brinckerhoff was engaged as lead designer also on the Marmaray immersed tube railway crossing, and geotechnical engineer Fugro GmbH, also working on that project, was engaged by YMSK to complete an additional 16 offshore boreholes along the alignment of the planned highway bored tunnel to supplement geological data supplied by Government studies as part of tender documents. These were completed from jack-up barges and using dynamic ship drilling.

The feasibility studies completed before tender had confirmed a bored TBM tunnel as being less expensive than an immersed tube crossing of the strait. An immersed tube for the Marmaray crossing was required to maintain gradient limitations for mainline freight rail operations, but even so, the stations on either side of the crossing are deep with several flights of long escalators needed to reach platform level from the surface. The immersed tube was also a major marine operation to float and sink the 11 elements onto the seabed some 58m below sea level and across one of the busiest seaways in the world. The immersed tube was estimated initially at approximately US$850 million but is said to have more than doubled in cost during the construction phase, making it considerably more expensive than the bored tunnel estimate. Arioglu declined to say how much of the total $1.245 billion cost of the Eurasia project is construction cost, saying only that it is less than $1 billion.

TBM assembly indeep access box

With the additional geological investigation of the strait completed, and after investigating five alignment alternatives between the set route between Kazlicesme and Goztepe, the bored tunnel was aligned at 106m below sea level at its deepest point and below a maximum 62m water depth and a minimum 30m of cover for the underwater crossing. The 3.4km bored TBM tunnel also passes through two horizontal curves of 2,000m and 1,800m radius and a vertical curve of 4,330m radius as it maintains its constant 5% gradient either side of the deepest point (Fig 2). Once completed, the highway tunnel will be 5.4km from portal to portal with a total 1,000m of cut-and-cover approach ramps either side and a 1,000m length of twin-tube NATM tunnelling on the Asian side from the 165m-long TBM assembly, launch and working shaft to the cut-and-cover ramp section.

To accommodate the specified double-deck, four-lane undersea section of the project, a TBM of 12.5m o.d. would have been adequate. Mindful that even a small increase in the diameter of a large TBM increases dramatically the technical demands, the YMSK JV and its designer Parsons Brinckerhoff opted to increase the tunnel dimensions to 13.7m o.d. to incorporate improved safety elements, such as fully protected escape routes every 200m between the decks and to provide larger fans for the deeper tunnel, the depth of the tunnel having increased by about 25m from tender stage.

When complete, the highway tunnel will have a ventilation building either side of the strait. Fans in the two ventilation shafts will run constantly to eject exhaust air high above the vent buildings. In the tunnel, the piston effect of the 60km/hr traffic flow is expected to provide much of the air movement of 20km/hr, resulting in four complete changes of air in the tunnel every hour. The longitudinal ventilation fans along the full length of the tunnel and on both decks will be activated automatically, based on air speed and quality measurements.

Herrenknecht Mixshield

When it came to procuring a TBM for the project, the YMSK JV selected a 13.7m o.d. slurry Mixshield system from Herrenknecht AG. Manufactured at the company’s factory in Schwanau, Germany, the machine is a sister, in design terms, of the Mixshield used on the fourth bored traffic tunnel under the Elbe River in Hamburg, Germany in the early 2000s (see Reference list below). “That was a cutting edge breakthrough for TBM design and application,” said Gert Wittneben, who was TBM drive manager for that project and served as TBM drive manager for the Eurasia project. “Features used on the Elbe tunnel TBM have been duplicated here in Istanbul, including, vitally, man access into free-air spokes on the cutterhead in which all the disc cutters and many of the cutter knife bit tools can be replaced without needing a compressed air working environment.”

TBM operators’ cabin with screen of DCRM system bottom right

Recent technological advances are utilised by the Istanbul TBM, the most important of which, for Wittneben, is the disc cutter rotation monitoring system (DCRM). Developed by Herrenknecht, the system is based on magnetic signaling. As the disc rotates it passes a magnetic sensor which relays the signal to a monitoring screen in the operator’s cabin. “This was a big part of our success under the Bosphorus,” said Wittneben. “On the operator’s screen we needed to see green concentric circles to indicate that all discs were turning. We also adopted a policy of changing immediately, in the free air spokes of the cutternead, any disc that was not turning – rather than waiting for a scheduled maintenance shift. We also had wear detector bars filled with hydraulic oil on the cutterhead body, but avoiding critical wear of the TBM structure is largely a function of keeping the discs and tools in good shape. Changing the discs as necessary also gave TBM shifts a task in addition to the excavation and ring-building routine.”

Working in the free-air man-entry spokes of the cutterhead

The cutterhead was dressed with 35 x 19in double ring monoblock disc cutters and 194 cutting knifes or ripper tools. Some of these were supplied by E-Berk of Turkey, which has supplied cutterhead tools to projects across the world. In total, 400 discs were replaced on the 13.7m diameter cutterhead during the course of its 3.34km drive.

In addition to free-air access to the cutterhead tools through the man-entry spokes, there was also the need for four compressed air interventions into the excavation chamber. These called for saturation diving under compressed air pressures of up to 12 bar. Specialist hyperbaric company Nordseetaucher of Germany was engaged for these operations. For each intervention session, up to four divers lived in the pressurised accommodation chamber on the surface for up to 15 days. From the accommodation chamber, it was into a pressurised shuttle or pod to be transported down through the working shaft and to the TBM for docking onto the lock door on the TBM bulkhead. At the end of each intervention process it took up to seven days to decompress safely from the high working pressures.

“These are specialist operations, and with some risks involved,” said Wittneben. “We are happy to report that we needed only four saturation diving operations during the drive.” Two of these, explained Wittneben, were to mend damage to the grill in front of the slurry circuit intake to prevent boulders entering the system. “The operations went smoothly, and in fact we recorded no serious accidents or injuries on the TBM drive at all. It was a safe operation from launch of the TBM on 19 April 2014 through the 16 months (490 calendar days) production phase, working 24hr/day, seven days/week, right through to breakthrough on 22 August 2015.”

Saturation divers lived for up to 15 days in the pressurized accommodation chamber (left) which was monitored around the clock when in use (left middle) with the divers travelling to work in the pressurized shuttle or pod (right middle and right)

Rock conditions along the alignment are highly fractured and the risk of face collapses was high, particularly on the 5% up-gradient slope that provided a 75cm look-up across the 13.47m diameter cutterhead face (with a 75cm overhang on the down slope). Even under compressed air, with the bentonite slurry cake sealing the face, the risk was of the air disappearing into wide cracks, explained Witttneben. Faults in the bedrock are also filled with clay, which carried the risk of becoming compacted in the excavation chamber or clogging the cutterhead. “It was a very challenging and exciting project,” said Wittneben. “You could not predict what was coming next.”

The TBM was equipped with the possibility of fitting a drill rig, and advance probing and pre-grouting drill ports were mounted on the shield, but in the event these were not required. There was also the possibility to extend the cutterhead some 400mm ahead of the shield but this distance was maintained at 100mm throughout the drive.

When TunnelTalk visited the project just after breakthrough, Wittneben hosted a tour of the machine. We entered the TBM through a cutterhead opening on the European side of the project, into the excavation chamber, then into the man-entry free-air cutterhead spokes (where workers were busy removing the tools as part of the machine dismantling process), then on through the bulkhead to the Asian side of the project and into the TBM ring-build area, back along the back-up to the operators’ cabin, and finally back again the same way to the European side. This was a very special tour, and one that very few in the industry have had the opportunity to appreciate.

Turkish Prime Minister Ahmet Davutoğlu (middle), joins Vice Chairman Erdem Arıoğlu of Yapı Merkezi (right) and Martin Herrenknecht (left) at the breakthrough ceremony

The TBM in the reception shaft was said to look in good shape overall. The extra wear plates, and other installed features, had protected the cutterhead from excessive wear although some of the ripper tools on the areas of the cutterhead that were not accessible from the free-air man-entry spokes were missing and the damage they caused was evident. In addition to damaging the grill in front of the slurry intake chamber, they had caused evident damage where they had jammed between the rotating cutterhead and the forward shield. The thick steel of the shield had been bent and junks ripped off.

After the tour, Wittneben and his colleague, Alper Bulutlar took the short-cut back through the TBM and along the tunnel to their offices on the Asian side.

Seismic accommodations

In addition to significant TBM advances, the project also required special design of the lining and the internal structure of the double-deck highway. Turkey lies in a geological zone of high seismicity and the tunnel runs parallel to the Anatolia plate-boundary fault some 16km offshore in the Marmaray Sea. All structures on the project are designed to withstand a 7.25 moment-magnitude earthquake. To protect the bored tunnel there are two seismic joints built into the precast concrete segmental lining at the geological transitions from the hard rock of the Trakya Formation to the soft sedimentary deposits at the bottom of the Bosphorus Strait. These joints allow movement of 75mm in contraction, 75mm in expansion and 50mm shear. “The machine’s vacuum segment erector was modified to a mechanical erector to install the steel segments of the seismic joint,” explained Ms Bahriye Yaman, Chief of TBM Methods for the YMSK JV.

Section of a seismic joint with its shear pins

As the TBM progressed and erected the eight double-gasketed and bolted 600mm thick segments, plus key, in each 2m wide ring of rebar reinforced lining, the corbels for the upper and lower decks of the highway were cast in-situ as a rolling programme. Behind them, and at about 400 rings into the drive, the in-situ casting of the upper deck (to keep the invert free for TBM service vehicles) started, and is progressing at the rate of one cast in the 12m long form/day. The precast elements of the lower deck will be installed following breakthrough of the TBM. To accommodate seismic movement the decks are freestanding on the corbels and the corbel units are as wide as each ring of segments and isolated from one another. With no rigid connections, the tunnel structure will move like a wave in the event of an earthquake.

The largest of the rebar-reinforced segments, designed to meet seismic concerns, weighs 15 tonne. After every 400 rings, a special rebar-corrosion monitoring ring is installed, the segments of which are fitted with sensors and data probes. CBE supplied the moulds and casting system for the segment production and Dätwyler suppled the gaskets. As the deep undersea drive progressed, VMT guidance systems kept the heading on course, and monitored an array of TBM-operating functions.

Open cut and NATM works

Two other sections of significant civil works are the open cut transition and working access boxes on each side of the TBM drive, and the 1km of twin NATM tunnels from the TBM access box to continue the transition to the surface portals on the Asian side.

The TBM assembly and working access box is large at 165m long x 38m deep 26m wide. The secant piled supported structure is further supported by a dense pattern of ground anchors. Once the TBM was away on its drive in April 2014, work started at the opposite end of the box on the NATM tunnels. With a complex transition from the double-deck TBM-drive highway to the side-by-side NATM configuration to the surface now to be built in the access box, it might have been easier to reduce the open box structure, create two working shafts – one for the TBM and another to advance NATM in two directions – and mine the transition for the most part. As it is, NATM advanced from the access box and from the surface portal to accelerate progress.

To start the NATM tunnels, the lower one was advanced first, explained Korhan Demir, JV Technical Office Chief for the cut-and-cover and NATM works. The upper tube started once the lower heading had advanced about 18m. “There is a very narrow pillar of about 4m maximum between the two tubes at their start,” explained Demir, “and the first 30m of both took some time using up to six sequential headings in the 85m2 tubes. We planned continuous pipe umbrella pre-support on 25m-long rounds but this proved difficult for the subcontractor to install with accuracy and the length was reduced to 18m long, which increased the price by about 25%. Minimum cover over the NATM tunnels is 9m and they pass under a busy local road and very close to the operating rooms of an eye clinic, which were very sensitive to vibration. Excavation and drilling for rockbolts and the pipe umbrella stopped during the times of eye operations. We had a license for drill+blast excavation but did not use it. Excavation was by excavators and hydraulic breakers.”

Once through the initial rounds, excavation followed a top-heading, bench and invert sequence with the bench about 6m ahead of the top-heading and the invert between 25m and 90m behind it. The construction works are all based on self-certifying processes and the NATM subcontractor also completed his own surveying. Any corrections to his own work were at his own cost.

NATM excavation advanced from portals in the opposite end wall of the TBM open cut access box

The NATM works are designed by the project’s Austrian design consultant IGT Geotechnik und Tunnelbau to Euro Code 7, Austrian best practice, and ACI codes. “We used two layers of wire mesh and shotcrete to support the excavations and installed also lattice girders,” said Demir. “One layer of mesh would have met design specifications but we included the second as a redundancy layer for the trimming process. The subcontractor’s crews, mostly with a mining background, were thicker with the shotcrete, rather than thinner, for the 30cm primary lining specification (minimum 25cm for quality control) and so trimming has been needed, sometimes back to the outside layer of wire mesh. The lattice girder was included only to assist profiling on the 5% slope and curves of the tubes.

“We could have used steel-fibre reinforced shotcrete which would have reduced the time and materials cost compared to two layers of wire mesh but there is only the Model Code available to design for fibres. Fibre shotcrete also requires good profile control. Contractors are not keen to use fibre shotcrete for filling excavation underbreak.”

The NATM tubes achieved their in-tunnel breakthrough in April and May 2015. When TunnelTalk visited the site in August, final lining of the tunnels was in full swing casting the invert ahead of the arch and with sheet membrane waterproofing advancing ahead of the concrete pours. Spray-on waterproofing systems were considered but only briefly it was said by Demir. “Spray waterproofing is only included in the British Standards and the Euro Code only allows sheet-waterproofing. The BSI standards used to be the codes to go to. The Euro Code is now being used although it is based primarily on geotechnical design rather than being tunnelling specific.”

Concreting works, using 10m long forms, was progressing well with one cast in every 36 hours and with form strike time of 12 hours. “Continuing on current progress, the final lining should be completed by December or January,” said Demir. This includes the cross passages between the 960m-long NATM tubes, some of them quite steep between the upper and lower tubes as they come to the surface. There is a form for the cross-passages, but for the steep ones, custom timber forms will be used. In the meantime, and with the TBM drive also complete, work on the complex structure in the open cut transition box had started.

On the European side, the open cut transition is not as deep and is supported by levels of bracing. During the visit, Ufuk Kukul, Section Chief for the European side works, explained that archeological studies of the excavations near the old city of Istanbul had caused some delays. “But it is an important process for these construction projects and we have accommodated the archeological teams,” said Kukul. “They have now finished their work and we are on schedule to complete the works by our due date.”

The project’s programmed opening date was for August 2017, but better progress through the construction phase than projected, particularly on the 3.34km long TBM drive, has brought this forward to the end of 2016. When completed, Egis of France will operate and maintain the toll highway through 10 years of the investment payback period and will have first refusal to extend the contract. Once opened, drivers will take just five minutes to cross the Bosphorus at 60km/hr, compared to a 30min to 60min crossing using the bridges or a 15min crossing on a ferry.

Future tunnelling works in Turkey

As well as the proposed and truly ambitious triple-deck road and metro bored tunnel crossing under the Bosphorus, Turkey is progressing and planning an extensive programme of underground projects. Istanbul and the capital city Ankara are progressing and planning further extensive extensions of their metro systems over the coming years and a high-speed railway is being advanced between Istanbul and Ankara. There are several hydro projects in construction and others are planned to increase energy production in the country. A program of highway and railway expansion requires significant lengths of tunnelling through the mountain range that divides Turkey on the east-west axis.

Proposed triple-deck road and metro bored tunnel

All these projects continue to improve the economic growth prospects of Turkey and support the nation’s aspirations to join the European Union. The support of the EU through its European Development Bank and other financial institutions is instrumental in realising major infrastructure projects and encourages inward investment into Turkey from other countries including Korea and Japan. The possibility for accession to membership of the EU has cooled in recent years, but the political, social and economic discussions of the process continue.

With an intense programme of tunnelling and underground space excavation to advance, there is great opportunity for civil engineers in the tunnelling industry in Turkey. Many young civil engineers, like Bahriye Yaman, have continued their education and gained experience working overseas before returning home to work in Turkey. Yaman attended university in the UK and worked for a UK design and project management company before returning to Turkey to take up her position on the Eurasia project.

In the meantime, education opportunities in tunnel engineering have improved in Turkey and there are several training courses and tunnelling conferences organized in the country. In August, Tunnelling Association of Turkey, which is a long-term member nation of the International Tunnelling and Underground Space Association (ITA), hosted the Tunnel Expo conference in Istanbul. It also conducted the fourth in a series of tunneling short courses, which attracted more than 200 attendees, most from Turkish and Middle East tunneling contractors. Tutors included many experts and leading professionals from Turkey as well as international guest speakers Dr Levent Ozdemir, Lok Home, President of the Robbins Company, which has several TBMs working on projects in Turkey, and Dr Guner Gurtunca from the USA and Dr Nick Barton from Norway.

With positive political support of major infrastructure projects that include extensive tunnelling and underground development and the financial support for BOOT and PPP procurements being attracted from the world over, Turkey will be a tunnelling hotspot for a foreseeable future.

Gallery