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At the cutting edge

Over 40km bored and cut and cover tunnels thread through the hills along the highway's length.

Rock across the north of Greece is a combination of fleisch, clays, sedimentary rock, gneiss, granite and marble, full of joints and faults. Where exposed faces are required - in cuttings for instance - collapse and slip has been a recurring nightmare, requiring a wide variety of stabilising techniques from rock bolting to the construction of retaining walls and hydro seeding.

In tunnels, however, the rock has lent itself well to excavation by drill and blast and in places by road header. Progress is typically 2m/day to 8m/day. In areas of weak rock fore-poling provides temporary support. Rock bolting has been extensive, but arch support has barely been used.

Lining is New Austrian Tunnelling Method sprayed concrete. Depending on rock type, thickness of the permanent lining is 50mm to 200mm.

'We're trying to get the contractors and designers to use modern methods of spread and deformation measurement,' says Brown & Root tunnels expert Gerald Bonner. A secondary lining safeguards structural performance - 'when ground is soft we wouldn't necessarily have confidence in the primary lining alone,' says Bonner - and allows for easy refurbishment.

Tunnelling is being pushed forward across the scheme to maximise absorption of EU funding.

Traffic volume is predicted to grow dramatically and all tunnels are designed as twin bore dual carriageway. In lengths of over 500m cross- over adits allow emergency evacuation.

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