Crossing the Bosphorus and beyond

The construction of the underground and undersea crossing of the Bosphorus faces a diverse range of challenges from the excavation and preservation of archeological treasures from one of Europe’s oldest civilisations to designing and constructing the deepest immersed tube tunnel in the busiest and narrowest shipping lane in the world in accordance with the highest international earthquake standards (figure 1).

The client for this impressive undertaking is the Department of Transport Turkey (DLH) supported by the Engineer, Avrasyaconsult, which is a consortium led by Pacific Consultants International and including, Yuksel Proje, Parsons Brinckerhoff, JARTS and Oriental Consultants.

Using FIDIC two party contracts with Avrasyaconsult acting as the Employer’s Representative, the work is let in three design-and-build contracts. BC1 covers the 14km of two-track bored, immersed tube and cut and cover tunnels and was awarded to the Japanese and Turkish joint venture of Taisei, Gama & Nurol. CR1 covers all infrastructure and tracks on the 62km of three-track surface alignment, including all systems, surface stations, control centre and maintenance facilities; it was awarded to the French, Turkish and Japanese joint venture of Alstom, Dogus & Marubeni. CR2 covers all rolling stock and will be awarded this year.

Finance for the US$3bn project is provided by the Japanese Bank for International Co-operation, the European Investment Bank and the Council of Europe Development Bank. The construction programme runs from 2004 to 2011.

Scope

The project scope includes 76km of triple track, the third track being for inter-city and freight; 3 depots; 2 workshops; an operations control centre and 44 ten-car passenger trains. The tunnels comprise 1.4km of immersed tube tunnel up to 58m deep, 9.8km of twin bored tunnel and 2.4km of cut & cover tunnel. There are three large underground stations and 37 surface stations. All tunnels and stations are designed to remain operational after an earthquake of 7.5 moment magnitude.

The installed systems include ventilation designed for a 100MW freight train fire, probably the largest in the world, a 25kv AC traction power system and the newest signaling and communication systems.

The railway will run 3,500 person capacity trains and operate with a moving block signalling system. Step-free access to stations and trains will be provided. The track will comprise continuously welded rail and the systems have been designed in accordance with NFPA 130.

Archaeology

Istanbul was founded in about 658 B.C. by a Greek expedition and was named Byzantion after the leader of the colony, Byza. The city was renamed Constantinople after the Emperor Constantine following the Roman conquest in about 330 A.D. It subsequently became the capital of the Eastern Roman Empire. In 1453, the city was conquered by the Ottoman Empire, but its name was not changed officially to Istanbul until 1930.

Consequently the city is the site of numerous historic buildings, ruins and other artifacts of Ottoman, Byzantine, Roman and Greek civilisations. Project excavations have exposed ten sunken boats, ancient walls and pottery; chapels, cisterns, baths and villages have been discovered; and the port of Constantinople, also named the ‘Eleutherios harbour’ in historical records, one of the busiest ports in Byzantium, has been uncovered. archeological investigations at Yenikapi station involve 130,000m3 of hand-excavated spoil.

Geology

The geology of the tunnel alignments includes interbedded clay and limestone, water-bearing sand and stiff to hard clay between Yedikule and Yenikapi cut and cover stations. The remainder of the bored tunnels will be driven in the Trachya Formation, which comprises interbedded mudstones & sandstones with unconfined compressive strengths between 20 and 100MPa intruded by dolerite dykes of up to 200MPa. The sea bed in the Bosphorus comprises soft marine sediments (figure 2).

Ground investigation was carried out in several phases. A design phase in 1985 to 1987 comprised 20 marine boreholes, 37 land boreholes, penetrometer testing, geophysics and bathymetric surveys. Pretender investigations were carried out in 2002 to 2003 and comprised 7 marine boreholes with cone penetrometer tests, 26 land boreholes and both land-based and marine geophysics. In 2004, the contractor carried out borehole investigations with maximum spacings of 200m in soft ground, 300m in hard rock and 50m at the stations.

Immersed tube tunnel

The 11 immersed tube concrete tunnel elements are 15.3m wide, 8.75m high, 135m long and weigh 19,000 tonnes and are cast in a fabrication yard 40km from the immersion location. The lower half of the unit is cast in a dry dock and the upper half cast afloat. The gravel foundation is levelled in the trench by a robot grader prior to immersion of the tube element. The units are floated out underneath a catamaran placing barge, a number of anchors are fixed at the sea bed and the unit lowered into position.

The Employer’s Requirements (ERQ) dictate that the immersion operation shall be carried out only when the speed of current through the strait is less than 3 knots. However, the speed varies a lot even in a very short period. Therefore, the ERQ requested that for at least one year continuous current data was collected to develop software to forecast the current velocity at any point for the coming period of immersion. Factors influencing current speed include: atmospheric pressure in the Black Sea and in the Sea of Marmara; wind; rainfall quantity; and salinity. These factors can cause sea level difference of a maximum of 50cm and an average of 25cm. The contractor developed the programme, which models the currents with an accuracy of 90% and which demonstrates the stratified current in the strait.

For the connection between immersed tube and bored tunnel, the TBM will be driven from rock into a transition zone of low-strength sand-cement, placed by tremie methods, and into circular steel sleeves provided at the ends of the immersed tunnel. The TBM will be sealed into the immersed tunnel and dismantled (figure 3).

Bored tunnel

The finished tunnel diameter is 7.04m. Cross passages will be spaced at 200m. The segmental lining of a six-piece ring plus key will be constructed of concrete grade C50 reinforced with 132kg/m3 of steel, the segments are 300 and 320mm thick, 1500mm wide and each one weighs 4.4 tonnes. There is no fire protection in the lining concrete – a 50mm spray-on cementitious material with fire inhibitors is provided at critical areas.

For the drives between Yedikule and Yenikapi stations, one Lovat EPBM has been designed to cut through the stiff to hard clay with sand layers. It has a cut diameter of 7.994m, and shield diameter of 7.956m giving a 38mm tail clearance. The overall length is 10m; the maximum thrust is 64,000kN and the maximum drive power from seven motors is 2100kW. Muck disposal is via a continuous screw to an open trough conveyor at a rate of 800 tonnes per hour. Accelerated OPC grout is pumped through the trailing shield and Condat CLBF4/M soil conditioning foam is added to the spoil.

The initial 100m excavation of the Lovat drive was completed in November 2007, but expropriation and archaeology issues are delaying the main drive. The anticipated advance rate is 12m/day, penetration speed of 25mm/minute and a target time per ring installed of two hours. Predicted surface settlements are 20mm for overburden less than 10m deep, 10mm for overburden 10 to 20m deep and less than 5mm for overburden >20m deep. The programme allows 18 months for two 2540m long tunnels, comprising 7 months for each drive with 4 months to re-mobilise for the second.

For the remaining drives, four slurry TBMs have been manufactured by Hitachi Zosen. The excavation diameters range from 7900 to 7930mm. They are designed to cope with a water pressure of 8bar. The maximum thrust is 75,000kN and the maximum drive power from eight motors is 2000kW. The machines can cope with a curve radius of 300m. The cutter head has 56 rock cutting discs and 108 soil picks. The cutters are made of tungsten carbide in a steel matrix. Accelerated OPC grout is pumped through the segment ports. Bentonite is added to the spoil to improve pumpability in the sands and an organic thickening agent is used to improve the slurry viscosity.

At present, TBMs 2 and 3 are awaiting the completion of archeological excavations at the stations on the European side; TBMs 4 and 5 have completed 50% of the tunnels on the Asian side. Some abrasion and pumping related delays have occurred. The anticipated advance rate is 12m per day: the actual rate is 6 to 14m per day with a record advance of 70m in one week and 15m per day. The target time for an installed ring is 2 hours. The predicted surface settlements are 10mm for overburden less than 10m deep and less than 5mm for overburden more than 20m deep.

Underground stations

At Uskudar on the Asian side the cut and cover station box is 275m long, 32m wide and 29.5m deep. Soil improvement of soft sand comprised 10m deep by 3m diameter jet grout columns installed by specialist sub-contractor Bauer of Germany. Excavation support was provided by a 1.5m thick diaphragm wall with three 1m diameter reinforced concrete struts. The excavation is 60% complete in a programme stretching from July 2006 to July 2012.

On the European side the Yenikapi cut and cover station is 250m long, 32m wide and 20m deep. Support is provided by 0.8 or 1m diameter secant piles in a 25m deep wall with 35 tonne tie-back anchors. In a September 2006 to December 2012 programme, excavation is 20% complete.

Sirkeci station on the European Side is 235m long and 60m deep (figure 4). Support comprises 150mm deep steel arches in 300mm thick sprayed concrete and 4 – 6m long rockbolts; 1000mm thick concrete final lining is applied over a waterproofing membrane. The west shaft is 50% complete and the east shaft piling 90% complete. Completion is due in October 2012.

Summary

The benefits of the new railway include the increase in train journeys from 3% to 27% of all passenger journeys and a direct rail, passenger and freight, connection between Asia and Europe. The crossing of the Bosphorus will take 4 minutes and the project will provide an economic benefit of US$600M per annum. Modern stations and air-conditioned rolling stock will link via seamless interchanges to the planned tram and existing metro systems. Environmental benefits include reducing vehicle emissions, no permanent impacts and no negative visual impacts – the spectacular views over the Bosphorus and Istanbul’s historic skyline will be preserved.

Fig 1 – Plan map of the Bosphorus crossing alignment Fig 1 Fig 2 – The tunnel alignment with TBM drives and immersed tube locations Fig 2 Fig 3 – Schematic of the docking process between TBM and immersed tube Fig 3 Fig 4 – Sirkeci station diagram Fig 4 One of the Hitachi Zosen TBMs being used under Istanbul Hitachi Zosen TBM TBM docking element on the immersed tube element TBM docking element The drive portal on the Asian side of the project Drive portal Questions from the floor

A representative of Tube Lines asked how the tunnels will be ventilated. Mark Gilbey of PB responded that the ventilation exhaust components include over-track exhaust systems for the trackways, under-platform exhaust for the station platforms, tunnel ventilation fans at both ends of each station and jet fans at selected locations.
David Powell of Mott MacDonald asked what specific measures were installed at the interfaces between the immersed tube and bored tunnels to cope with differential settlement and potential leakage. Daniel Horgan responded that the immersed tunnel was designed with two seismic joints comprising water-proofed concrete and 6mm steel plate â€“ these allow a certain level of rotation.
Bob Peters, retired, asked whether in a large earthquake the scheme could be recovered in seven days. He had been involved with work, which had predicted the earthquake along the North Anatolian Fault and the very high horizontal forces responsible for the collapse of an NATM-built tunnel in Bolu. The next major earthquake is likely to be in Istanbul â€“ is the final lining reinforced and what measures have been taken to guard against the effects of earthquakes? Daniel Horgan responded that the tunnels can withstand a substantial earthquake in which 90% of the buildings in Istanbul would fail. However, an earthquake during construction would cause significant damage. Bob Peters also asked what was the driving force behind the archeological excavations. Daniel Horgan responded that a Heritage Committee had been set up and given supreme powers to preserve archeological artifacts.
Neville Harrison, consultant to Mott MacDonald, asked who pays for the archaeological investigations. Daniel Horgan noted that contract BC1 includes a provisional sum to cover this.
Richard Loudon, Travers Symonds, asked what ground improvement was in place for the foundation of the immersed tube elements. Douglas Madsen responded that soil replacement was recommended by the contractorâ€™s design.
Haydn Davies, London and Continental Railways asked why steel and polypropylene fibres were not used in the tunnel segments and noted that only one structural repair was needed on the CTRL. Douglas Madsen responded that the contractorâ€™s design called for bar reinforcement cages and a different type of fire resistance.
Rapporteur: Rory McKimm