Section two of Istanbul's Metro should further relieve the severe traffic congestion in Turkey's capital. Tunnelling is now well under way using a variety of techniques. Dan Simpson reports.
The Istanbul Metro is set to become one of the world's most heavily-used urban mass transit systems. The first section opened in October 2000 and is already handling 70,000 passengers per hour in each direction. Construction of section two is now well under way and is due to open in 2002.
Istanbul has grown rapidly in recent years. Now home to 10 million people, the city is severely congested, mainly due to a heavy reliance on road transport. This problem was compounded by Istanbul's bid for the 2008 Olympic Games (recently won by Beijing in China), which needed a modern transport system to carry spectators to and from venues efficiently.
The Greater Istanbul Municipality's solution was construction of a north-south metro system beneath the city centre.
Section one of the project runs in 7km of tunnels between the Taksim and 4th Levant districts, and includes six stations. Section two runs south from Taksim, a pedestrian area popular for shopping and public events, to Yenikapi, adding four stations.
Although most of the 5. 2km route will be in tunnel, the line will rise to the surface and run on viaduct across the Golden Horn, a natural inlet that divides the old part of Istanbul to the south and the modern districts to the north. Eventually, the metro will link in with the city's light rail system to produce an integrated rail transport system.
Turkish company Yuksel Project International and Louis Berger Consortium are the consultant and engineer for the project and the Anatolian Metro Co-operation, a co-operation of several Turkish firms, is carrying out the tunnelling and excavation works. Another Turkish company, Tekfen, was the main contractor for the first stage of the metro.
Most of section two will run in single line twin bored tunnels, with occasional larger bored single tubes, with two lines, on the crossovers. Large bored caverns form the station concourses at Sehzadebasi and Sishane stations.
The 6m diameter tunnels are excavated by road header in a horseshoe shape in six staged digs to limit ground movement. One side of the tunnel is excavated first in upper and lower drifts, followed in similar fashion on the other side. Finally the centre of the tunnel is excavated in a further two stages.
Two types of geology are encountered on the alignment. The majority of the section involves tunnelling in a fractured greywacke. Excavation by road header through this material is followed by temporary support built using a modified New Austrian Tunnelling Method technique, with a primary lining of 200mm sprayed concrete and wire mesh.
However through a medium hard, high plasticity clay, known locally as the Suleymaniye formation, at Yenikapi at the southern end of the route, umbrella grouting is being used to support material in advance of tunnelling.
The face is temporarily supported using 12m long grouted reinforcing bars. Arches of grout are formed over the crown of the tunnel, with grout injected through 114mm diameter, 9m long holes running parallel to the tunnel at 400mm spacing.
Grout injections are inclined at about 5infinity to allow a 3m overlap with the next 6m advance, with temporary support of a combination of soil nails, sprayed shotcrete and wire mesh.
On to the initial lining go lattice girders, fixed with rock bolts. An inner wire mesh is then added followed by a 400mm thick layer of concrete cast over a layer of PVC waterproofing membrane.
Access to the tunnelling work at Yenikapi station is via a 23m deep shaft, constructed from contiguous hard/soft piles. A crane above the shaft removes excavated material, transported from the face on a large tipper truck. The station itself is being built by cut and cover techniques.
Sishane station, just north of the Golden Horn, is one of the larger sites; it also houses a shopping concourse at intermediate depths. The station will be accessed by three entrances and is supported with anchored walls around its perimeter.
Internally cast insitu walls and columns provide support for the shop spaces. With a six storey building above the station, settlement was a key issue; but indications so far show there has been no problem.
The surface Unkapani station sits at the start of the viaduct on the southern bank of the Golden Horn where it was not possible to run the metro in tunnel. Due to the historical setting, the viaduct must not interfere with views across the inlet, and designs are still awaiting approval.
Tunnelling for section two is costing £110M and is due to finish by the end of May 2002.