Equipment India | June 2009

Site Report

A Spectacular Concreting Job

At a railway tunnel in Istanbul, Putzmeister concrete pumps and a concrete wet spraying machine are involved in producing the huge tunnel elements for the underwater section, in lining the tunnel with shotcrete and in backfilling tunnel formworks on land. A report.

The metropolis of Istanbul, which boasts of a population of 15 million and stands at the dividing point between Europe and Asia, is currently seeing the construction of a spectacular railway tunnel, designated the Marmaray Project. Putzmeister concrete pumps and a concrete wet spraying machine are involved in producing the huge tunnel elements for the underwater section, in lining the tunnel with shotcrete and in backfilling tunnel formworks on land.

Istanbul is home to around a fifth of the population of Turkey. Traffic planners are focussing on modernising public transport and developing the infrastructure in order to relieve the burden on the city’s chronically congested bridges over the Bosporus and the associated access roads. The Marmaray Project, which was established in the 1980s, is an integral component of this measure. In May 2004, tenders were finally approved and the project was started. An important section of the structure is a 1.4 km tunnel segment which lies on the floor of the Marmara Sea at the southern end of the Bosporus. This segment consists of enormous prefabricated concrete elements which are lowered to a depth of up to 58 m, sealed and interconnected. The tunnel connects the European suburb of Halkali with Gebze on the Asian side of Istanbul.

Work is being conducted on both sides of the waterway to establish a connection between the tunnel and the existing rail and metro network. The construction measures include, among others, 9.8 km of driven tunnel sections, 2.4 km of open cut tunnel sections, three new underground stations and 37 stations located above ground, which are currently undergoing a comprehensive modernisation programme. A 63 km existing railway line is being expanded. The client is the Turkish General Directorate for Rail, Port and Airport Construction (DLH). The project is being undertaken by a Japanese/Turkish consortium (contract section BC1) under the management of the Taisei Corporation, together with Kumagai, Gama and Nurol. The construction costs are estimated at $ 3.5 billion.

Efficient tunnel elements
Concreting of the eleven 135 m long, 15.3 m wide and 8.6 m high tunnel segments is being conducted in a dry dock on the Asian side of Istanbul in the suburb of Üsküdar. This is where Oyak Beton is utilising several of its Putzmeister truck-mounted concrete pumps (with boom sizes M 24-4, M 36-4 and M 47-5) to fill the base, wall and ceiling form-works of the caissons from the edge of the dock. Approximately 5,000 cu m of concrete in varying strength classes (up to C40) is required for each tunnel element. The upper segment is concreted with a flooded basin.
Sturdy tugboats tow the finished and sealed, reinforced tunnel segments from the dock to a floating platform in the Marmara Sea in a catamaran construction. The 30,000 tonne reinforced concrete constructions are carefully towed between the two hulls. The catamaran is manoeuvred to the exact pre-calculated position above the seabed, in which dredgers have excavated a deep channel. The special ship is held in place by thick steel coils, which are secured to 13 buoys anchored on the seabed. The catamaran houses huge cranes with cable winches, which are used to lower the tunnel sections into the artificial ditches. Cameras and measuring instruments monitor the precise alignment of the tunnel segment, which is achieved by filling and emptying ballast tanks in a similar manner to a submarine. The critical period is during the final phase where there is a risk of the large rubber seals being damaged; this is because these are required to produce a watertight connection between the segments. The top of the tunnel element is subsequently protected against damage with a 4.5 m thick sediment layer.

It has long been known that a dangerous current runs through the passage between the Black Sea and Mediterranean Sea. Strictly speaking, there are two directions in which the water flows through the Dardanelles, the Marmara Sea and the Bosporus: In the lower water layer, there is a strong current running from south to north; in the upper layer, however, the water flows in precisely the opposite direction. In order to be able to position these tunnel segments accurately at the calculated positions, despite this strong and often fluctuating current speed, the Japanese experts use a specially developed system for predicting current.

A significant challenge for the tunnel constructors is posed by more than 10,000 earthquakes that are registered in the region every year (most of which, however, are barely perceptible). The reason for these earthquakes is the Anatolian Fold, which meets at the two continental plates. It is located some 15 km from the tunnel. The safety measures include, for example, special protection for the connection points of each individual segment under water. And where the Marmaray tunnel merges with the land tunnel, particularly powerful, flexible and elastic rubber seals are used which will not break in the event of an earthquake.

Versatile construction methods
While the 1.4 km tunnel section is being established from caissons at the bottom of the Marmara Sea, entirely different tunnelling methods are being utilised at other sections of the 76.3 km Marmaray Project. Tunnelling in areas along the banks and those areas susceptible to soft ground conditions, for example, is conducted in a shield tunnelling process with tubbing support. In other sections, the inner formwork is concreted by stationary Putzmeister concrete pumps, which are made up of both modern
BSA 1408 E units and older fully functional pumps. Other sections are built using an open construction method, often involving the use of PM truck-mounted concrete pumps. For shotcreting, Putzmeister provides a compact SPM 400 wet spray manipulator. The spray arm of these machines has a working range of 2 m - 8.5 m and its spraying capacity is up to 20 cu m/h.

With an annual production capacity of 4.5 million cu m, Oyak Beton is one of the largest ready-mixed concrete producers in Turkey. Oyak Beton has its own concrete laboratory and is represented throughout the country thanks to its multiple mobile and stationary mixing plants. Besides numerous truck mixers, the company also operates a fleet of around 70 Putzmeister truck mounted concrete pumps in varying sizes. In addition to concreting the tunnel segments, the company also delivers and pumps concrete to other sections of the Marmaray Project (contract section BC1 and CR1). Oyak Beton is also currently involved in other major construction projects in Turkey.




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