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Boreholes get a fix on Greek bridge

CONTRACTS

ARUP GEOTECHNICS used a range of borehole geophysics techniques during site investigations for the new Metsovitikos Bridge on the Egnatia Odos highway project in northern Greece.

The 780km road, part of the European Union's Trans- European Transport Networks programme, will provide a major east-west link between the Adriatic port of Igoumenitsa to Thessaloniki then on to Kipi on the Turkish border. Some sophisticated geotechnical engin- eering allows the road to traverse extremely rugged terrain.

Design of the bridge was awarded to Arup, together with architect Wilkinson Eyre after an international design competition.

It spans a steep sided gorge, 140m above the Metsovitikos River, with the road running in tunnels on either side. The deck will be suspended from anchors installed on the hillside.

The complex geology of the Pindos region comprises a suite of westerly verging thrust faults that bring deep ocean floor basalts and associated sedimentary rocks into contact with the flysch basin sediments.

The flysch at the bridge site is made up of interbedded sandstones and siltstones with a highly tectonised unit on one side of the valley. There the flysch contained debris flow strata representing massive submarine landslides that mixed the flysch sediments with rocks from the ocean floor.

The ground investigation involved seven rotary boreholes at each of the four anchor points, and further rotary holes across the site for slope stability assessment. Boreholes were up to 60m deep and were continuously cored and logged using wireline geophysics.

Geophysical results were used for stratigraphical correlation, to determine rock mass stiffness and to assess rock mass structure.

Instruments included a sonic tool to measure P and S wave velocities and an acoustic borehole televiewer that uses ultrasonic signals to map borehole walls, which was compared with recovered core. Crosshole seismic surveys were also carried out in two boreholes at each anchor point to produce tomographic images of the rock profile.

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