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Pinning down the Bosphorous


Attracted by the most stunning views in Istanbul, wealthy investors are developing and protecting property on the banks of the Bosphorous, one of the world's most dangerous and busiest shipping lanes.

The Bosphorous (Bogazici in Turkish, meaning the strait), links the Black Sea with the Sea of Marmara and as such separates Europe from Asia. Istanbul sits across the channel, a former river valley flooded during the Tertiary period, and one of the most hazardous in the world.

Up to 4. 5km wide along its winding 31km length, it narrows to just 700m in places, with strong currents making it act more like a river. This, combined with some 1,350 vessel movements each day makes it extremely difficult to navigate.

Work is under way to improve navigation, however. Turkish geotechnical contractor Temeltas is installing piled foundations for eight new radar towers to control and monitor marine traffic along the strait.

For one at Skutari on the Asian bank, the firm is installing 32, fully cased, 650mm diameter bored piles through 18m of soft alluvium and 2m to 3m into fractured greywacke.

The £1. 4M contract also involves building a contiguous bored pile retaining wall on the channel bank that will protect the foundations from erosion by the strong currents.

Greywacke - a mixture of claystone, siltstone and sandstone - is the dominant rock type in the region, forming the valley sides of the Bosphorous, which are up to 200m high.

Soft and loose sediments, mainly sandy clay and silt with shells, overlie the rock at the shoreline, often covered with fill. The thickness of these deposits is highly variable along the strait, up to 30m in places.

Groundwater is at sea level.

The Bosphorous shore area is also one of the most affluent in Istanbul and the many large detached houses have some of the best views of the strait.

With the facades protected by strict architectural controls, basements are often added to gain space.

In carrying out ground improvement beneath a number of houses along the Asian bank, contractor Zetas is aware of the potential of liquefaction of the soft sediment and fill during earthquakes in this highly seismic area.

Jet grouting is used, and often working in extremely restricted headroom, the firm's rig can operate with a clearances of just 2. 2m, says director Professor Turan Durgunoglu.

Two such projects involve installing a watertight temporary retaining system around base- ments to install foundations and permanent retaining systems.

Where retaining walls are close to the shoreline (often just 1m to 2m away), Zetas is again using its small rig to install jet grout columns.

Typically, two rows of 800mm diameter jet grout columns, form the walls of the basements and short intersecting jet grout columns form the impermeable basement floor.

'The system is very fast and economical, requiring much smaller equipment compared to piles or slurry walls, ' says Durgunoglu. 'Jet grouting has performed excellently and produced almost dry working conditions for the basements. ' At one of the sites the firm has even constructed a basement 1. 2m below sea level.

One of the largest jobs on the shoreline is construction of the Kurucesme Les Ottomans hotel.

Work includes the excavation of a deep basement with an anchored retaining wall and the installation of bored cast insitu piles.

Previous occupant of the site was a villa supported on some 500 cast insitu bored piles. These have been demolished, as well as some historic wooden piles and about 2,000m 2of diaphragm wall.

One of Turkey's largest contractors, STFA, is carrying out the work for client Unit Turizm Yatirimlari.

The retaining wall around the hotel basement is made up of 51,000m of 800mm diameter jet grout columns, which vary in length from 7m to 26m. At the water's edge the jet grout wall retains between 10m and 12m of water in the channel.

An anchored bored pile retaining wall supports the excavation in the deepest area, with some 1,800m of 800mm and 1,200mm diameter bored piles installed with 3,600m of anchors supporting between 40t and 70t.

Geology of the site comprises greywacke bedrock, which slopes steeply towards the Bosphorous, overlain by silt with shells and then fill.

Where the greywacke is near the surface the retaining wall is anchored into it, but where rock head is deeper the anchors are fully grouted. Elsewhere, ground is being improved by jet grouting, to provide support and reduce settlement.

Some 55,000m 3of excavation will have taken place by the time the £1. 5M groundworks contract is completed at the end of September.

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