Changing ground, a dormant fault, karstic voids and tectonic shattering are among the problems for contractors on a major tunnel on the Greek railway system, reports Adrian Greeman.
Throughout history the route between Athens and Corinth, the gateway to the Peloponnese peninsula, has been one of the most important in Greece. In ancient times it led on to Sparta; today the Adriatic port of Patras lies beyond.
A new motorway is almost finished and now the slow, single track, metre-gauge railway is being replaced with a modern electrified 200km/h line, double tracked at the wider, conventional European gauge. This will extend for 99km as far as Kiato where an upgrade to the metregauge system will run on to Patras part of a major overhaul of the national rail system (see panel story).
The approach to Corinth is not easy. About 30km beyond Athens, the Attica flat land gives way to the more typical hill tumble that gives Greece its islands and its mountain homes for the gods. Several tunnels between 800m and 2km long and high viaduct links are needed.The most difficult is at Kakias Skalas, where a 400m high limestone escarpment plunges into the sea. Here even the motorway has had to wait for a connection, and for 8km it shrinks back to the single carriageway of the previous national road, winding around 600 slopes.
Tunnelling and bridge work is killing two birds with one stone. A twin-bore tunnel and viaducts will complete the motorway while some 20 to 40m below, double-track tunnels for the railway are being driven on a roughly parallel line.The alignment includes two tunnels for the railway, the first 2380m long and the second 1537m.
Motorway authority PATHE is sharing the work with the specially created railway organisation Ergose, formed to build all new railway infrastructure in Greece.
'But it did not make sense to split the design, ' says Dr George Tsifoutidis, a British trained geotechnical engineer with the Ergose, 'so it has been coordinated by the road authority, which began its project before ours.'
Both road and rail has been let in a single 70bn Drachma (£115M) contract under way since January 1999.As is usual in Greece, the four firms in the joint venture subdivided the work, with the experienced tunneller Aktor and Tev tackling the higher and larger road tunnels, while the equally experienced Aegek works from the east on the rail tunnel. Alte, a large general contractor new to tunnels, is handling the other end. It is also responsible for a second rail tunnel.
Originally the scheme had three tunnels along the alignment, those for the rail subparallel with the highway.
The rail tunnel crosses the motorway in a couple of places about 20m below.
'We started with a 667m long twin track tunnel and one 420m long, with a long viaduct passing across the steep headland in between, ' explains Tsifoutidis. A longer 1537m long tunnel passed through another difficult headland further west. Work included several smaller viaducts.
'But after Athens' earthquake in September 1999 the designers became worried about founding high viaducts on slopes of up to 700, particularly because a fault system passes through here, ' he says.
The fault is last known to have moved 2,500 years ago and is dormant in engineering terms.
Everywhere in Greece the geology is heavily affected by recent tectonic activity, part of the Alpine events as Africa crashed into Europe, explains Tsifoutidis. Rock went through an earlier compression followed by a dilation and tension cracking, the result of a subduction zone to the south of Crete, which created a volcanic dome to the north the explosive island of Santorini is the most obvious result.
Because of the fault it was decided to run the first 3km of the alignment further back into the hill, explains Tsifoutidis, creating a single longer 2,380m tunnel, designated S1/S2. Three of the above ground structures were eliminated. The remaining part of the alignment is unchanged and includes the S3 tunnel.
But Dr Evert Hoek, former professor at Imperial College in London and the University of Toronto, says the fault is still a significant factor for tunnelling. He is tunnelling and geotechnical consultant to the client.
'The fault runs more or less along the line of the tunnels and is about 2m thick.But it has had an impact on the rock structure 60m either side, ' he explains.
As a precaution tunnels are 50mm wider than needed to allow for possible future seismic movement.But despite the fault-induced fracturing and general tectonisation of the rock, Hoek believes the geology is otherwise not overly difficult for Greece.
Conventional drill and blast is used for both tunnels with a fairly heavy use of steel sets and bolts, including self-drilling Bellari rock bolts, and shotcrete for support in five categories according to the class of rock encountered.
Alte is using two Tamrock jumbos for its face work. A loader and backhoe work at each front and spoil is taken 6km to an old quarry which is being filled and will be landscaped for the local town of Megara, a community dating back to 1400BC.
The east side of the tunnel is more difficult. The Cretaceous limestone is in large blocks over 1m sided, sometimes embedded in softer material, the result of the collapse of karstic solution features.Voids are typically filled with loose material, terra rossa and chips of rock.
'Blocks act like wedges or keystones and can bring other material down if they fall out, ' says Tsifoutidis.'They are not to be ignored.'
Aegek has to work forwards a metre at time and best daily progress is about 3m.So far it is in about 500m on the top heading, with a bench following some distance behind. Meanwhile Alte, working from the other end, has done about 1900m.
'Aegek started with drill and blast but it proved impossible for most of the work because the blast gases simply escape through the rock voids, ' explains Tsifoutidis.
'And these are not small voids but big enough for a man.
'We did try grouting but after losing some 90t of grout we concluded it was simply going straight through to the sea, ' he adds.
The whole rocky headland is dry, even for Greece, with a very low water table, because water drains straight out of the cliff.
So Aegek is using a 4t Rammer G100 hammer mounted on a Cat 245 to work carefully on the face in the top heading, explains the contractor's engineer at the site, James Siampos.
'There has been a little drill and blast, ' says Tsifoutidis, 'but only for about 25% of the rock' The strategy is to keep as much rock in place as possible using spiles through the blocks, and steel arches.There are five categories of tunnel support, most using 5m long grouted passive anchors and 4m long Superswellex rockbolts in the crown. For severe conditions, support includes 1m steel arches for the top heading only and for the most difficult ground is taken right round to the bench invert. A 50kg/m 250mm thick according to the support category.
'A particular problem is supporting the half arches, ' says Tsifoutidis.'If they happen to be founded were there is a void they can be left hanging in mid air after benching.'
Arches are therefore tied to the wall with two rock bolts at their feet.
Aegek is using both a Tamrock 206 and Atlas Copco H135 jumbos for its drilling on the second tunnel, which is fairly straightforward although rock condition changes continually.
The tunnels are due to be finished in 2003.