Successful underpass of the Bosphorus

A deep undersea alignment and the potential for meeting high hydrostatic pressures didn't stand in the way of the first ever bored tunnel under the Bosporus. Excavated, using a 6.11m diameter Herrenknecht EPBM, the 3.4km long tunnel will supply the European part of Istanbul with water from the Asian side. Russian tunnelling company OAO Mosmetrostroy completed the benchmark project.

The TBM started boring the 3.4km long tunnel from a working portal in the Sariyer quarter on the European side of Istanbul and holed through into the 140m deep reception shaft in the Asian city quarter of Beykoz in mid-April 2009. For the first 2.3km, the alignment took a steep 7.45% downward gradient to pass under the deepest part of the Bosphorus. On this steep decline, the seabed overburden was as little as 35m in places with the sea level rising to twice that at up to 70m deep. The remaining 1.1km ran almost horizontally to the Asian side reception shaft.

At its deepest point, the EPBM operated at 135m below the sea level, presenting the greatest challenge on the project.

Working portal in Europe

The TBM had to withstand water pressures of up to 13.5 bar and its segmental lining was designed with gaskets tested to withstand pressures of up to 20 bar. The molds for production of the lining segments were supplied to Turkey by Herrenknecht Formwork GmbH.

Advancing at up to 20m/day the machine progressed the 3.4km long x 6.11m diameter tunnel according to schedule and towards a safe breakthrough on Monday, April 13. To recover the EPB from the 8m diameter reception shaft, the machine was designed in small shield segments. A steel pipeline will be backfilled into the segmentally lined tunnel to finish the water conveyance conduit. The contract is expected to be completed in Spring 2010.

The 'Melon 7' tunnel is the key element of a large-scale project to secure and improve long-term drinking water supply for the 10 million inhabitants of Istanbul. The water is also required urgently on the European side of the city for agriculture and regional industries. For this purpose, the River Melon has been dammed on the Asian side some 170km outside the gates of the city. From this high-rainfall area, water will be channeled to Istanbul in several stages with the bored tunnel providing the crossing beneath the Bosporus to the European side.

Herrenknecht EPBM

Breakthrough of the Herrenknecht S-391 EPB shield marked successful excavation of the first bored tunnel beneath the Bosporus and the first bored tunnel connection between two continents. The other major tunnelling project in Istanbul to link the European and Asian parts of the city, the Marmaray railway connection, incorporates an immersed tube crossing of the Bosphorus. In view of the challenges mastered, the bored tunnel is an excellent reference for mechanized tunnelling.

In March 2009, Istanbul hosted the 5th World Water Forum where experts, politicians and citizens discussed the future possibilities and precise plans for sharing and using more effectively and fairly the world's scarce water reserves - one of the greatest challenges of the 21st Century. During the event, the Director General of the National Water Commission CONAGUA of Mexico, Jose Luis Luege Tamargo, visited the Melon 7 construction site and referred to the world's largest wastewater project, which is located in Mexico City. Herrenknecht is delivering three tunnelling machines to complete a total 30km of the extensive Emisor Oriente project. The 4th World Water Forum was held in 2006 in Mexico.

References:

Emisor Oriente project, Mexico City - TunnelTalk, Feb 2009

Herrenknecht
Marmaray Railway Project