Bosphorus breakthrough

A joint venture between Yapı Merkezi and SK Engineering & Construction (YMSK) has broken through on the new 3.34 km Bosphorus road tunnel project in Istanbul, Turkey. The bore was completed using a 13.66 m diameter Mixshield tunnel boring machine (TBM) from Herrenknecht in water pressures up to 11 bar – equivalent to a water depth of some 110 m.

The tunnel forms part of a new 14.6 km highway which is expected to cut journey times across the Bosphorus from the current 100 minutes by car ferry to 15 minutes. The is also expected to relieve congestion on existing bridges.

At its deepest, the tunnel runs 106 m below the water level. The water is 62 m deep at its greatest, and the tunnel basses some 40 m below the sea bed level. The water pressure and geological conditions comprising permeable sediment and highly fractured, saturated rock made for difficult tunnelling conditions.

"That meant using a TBM that could handle hard and soft ground and at high pressure" said Gert Wittneben, TBM construction manager for YMSK. "It had to be a slurry machine, a Mixshield type."

The TBM from Herrenknecht has a face with both hard rock discs cutters, mounted on six radial arms, scrapers and buckets and openings between for the slurry support and spoil removal. It also included a system to replace worn disc cutters under pressure, and Herrenknecht said 500 such cutting tools were used in the bore.

“We are proud that we could have contributed an important part to the success of this milestone project” said Herrenknecht founder & CEO Dr Martin Herrenknecht.

Başar Arıoğlu, chairman of Yapı Merkezi added, "It was a difficult path from the outset, but finally we made it despite the great challenges."