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Living with the past

Tunnelling Athens metro

Extending Athens' metro system is fraught with difficulties, as Adrian Greeman describes.

Athenians have been using their new metro system with enthusiasm since its marble floored stations opened last year, and it has proved to be a boon given the ever more congested traffic of the sprawling Greek capital. And despite their traditionally cynical view of the world, they are pleased with a programme of new extensions for Greece's Olympics 2004.

But one station, Monastiraki, remains unused and almost unexcavated. This is the last station on the Blue line, or Line Three, running from central Athens to the north east, and one of two just completed. The original Line One was built 95 years ago.

Work here encapsulates the major difficulties facing contractors on the entire project as it proceeded over the last decade.

Appalling ground conditions combined with the plethora of ancient artifacts - particularly wells - created problems both with excavation and delays while archaeological remains were investigated Archaeology was a significant design factor along the whole 17.6km of the two new lines. In a crucial 11.7km of bored tunnel serving the most central areas this meant a depth for TBM operations of around of 10m to 20m, measured to the final tunnel crown. Remains usually lie in the first 5m of ground. Away from the historical centre cut and cover methods sufficed.

'At Monastiraki, archaeology is deeper, up to 7m, and more than 3,000 years old, ' says Stelios Koukoutas, site manager for tunnels and underground works at Attiko Metro.

Monastiraki is sited in the centre of ancient Athens, close to the Acropolis. Just what can be expected underground can be seen from a giant photograph now forming part of the decor at nearby Acropolis station. It shows a massive collection of ancient pots pouring into the excavation on another part of the line, the result of hitting a 20m deep domestic well.

'We have hardly moved a metre here without hitting a well, ' says site manager for client Attiko Metro, Ioannis Karakitsios. The number is such that there is a resident archaeologist on site.

As well as causing delays for investigation and releasing mud and water into the excavation, the wells cause many problems geotechnically, because they disturb and disrupt already complex ground. The city sits on a basin of Athenian schist, a flysch-like sediment. which is a mix of clayey and calcerous limestones, greywackes, siltstones, shales, limestones and marls with intermingled partially metamorphosed rock.

The two TBMs which made the main drives - 9.5m diameter Mitsubishi-designed rock cutter machines named Jason and Persephone - handled the varying rock well on much of the route but frequently had problems with 'ravelling' leading to overbreak above the cutter head, which twice ran to surface.

The first incident saw a street kiosk at Panepistimiou Street swallowed up; the second left a hole in the National Gardens.

Fortunately neither caused major injury or fatalities.

Greatest difficulties were met near ground structures because the Athenian schist, though fairly impermeable, is weakened by water from the wells and from leaking supply pipes.

But leaving Syntagma heading for Monastiraki and the planned terminal station Keramikos, conditions were so bad that the TBM had to be abandoned.

The TBM was dismantled and the tunnel finished using NATM to create a line end overrun until Monastiraki was finished. The station was taken out of the overall turnkey contract by the 19-firm Olympic Metro international consortium and is going ahead under a separate contract (NCEI [CEI] July 1997).

The final station, Keramikos, has been cancelled altogether.

The site lies in the ancient city graveyard and the remains of numerous ancient luminaries and philosophers are thought to lie there. An extension line will now pass either side of the area.

The cancellation of Keramikos station has entailed alterations to the design of Monastiraki, which will now serve as the terminal station for Line Three until the extension is built. Terminal stations have to be longer with train turnaround sidings. Some 8m more at tail end and 26m more towards Syntagma are to be added.

Initial work has already been done. Because of ground conditions and sensitive buildings above the station, an unusal construction was chosen. An 'umbrella' of concrete- filled, interlinked microtunnels forming a vault was completed in 1997 by Dutch company Smet Tunnelling using two Herrenknecht 1,250m microtunnellers.

Vault extensions are now being made by NATM, using as Karakitsios puts it 'every method in the book'. This includes fibreglass face nails, forepoling with 12m steel tubes, grout umbrellas, adits, lattice girders on the side and invert, micropiling, passive steel side wall anchors and steel mesh and fibre reinforced shotcrete between 200mm and 500mm thick.

'We began excavating in July last year and are working in three phases with an arch heading, bench and invert, ' he says.

Phase two is under way and whole must be completed by October when a reinforced concrete lining will be installed.

An extra complication is that Monastiraki sits close to Line One and its exit tunnel curves beneath the original station.

This forms part of a clutter of buildings that restricts access to the site to a single shaft 15m by 21m and 29m deep. Traffic around the site is continuous.

Meanwhile a 460m long twin track tunnel connection between Monastiraki and Syntagma is being excavated under NATM.

Breakthrough was achieved on 9 March this year. The job includes sections of parallel single track running on 150m into the station and 260m on the far side with a 194m final triple track section forming a turnaround area.

Olympic programme

Four additions totalling Drachmas 174bn ($448M) are under way and others are planned. First is from Sepolia depot, where a Dr12.5bn design/build contract was let last July to a joint venture of Austria's Alpinebau, Germany's Wayss & Freitag and Greek firm Attikat for 3km of tunnel to Thivon. Stations will be let separately.

Second and most important for the Olympics is a 5km link from Ethiki Amyna to Stavros, where passengers will connect to a new Athens ring highway link, with buses and taxis to the new airport at Spata. Three intermediate stations will be let later.

A complex extension from Monastiraki to Egaleo will add another three stations. Eight contractors are competing for the estimated Dr46bn contract which will be done by TBM or NATM.

A final extension will take Line Two 1km south from Dafni to a new station at Ilioupoli.

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