With more than half of the 7.7km long single-track, twin tube tunnels bored for the first, or ‘base’, metro line in northern Greek coastal city of Thessaloniki, the first TBM is due this month to reach Efklidi station. Tunnelling resumed after a half-year standstill in August 2010, the hold-up having been caused by archaeological discoveries and limited availability of work sites.
But these were not the first programming challenges for the tunnelling works on the 9.5km long base metro line, which will have driverless trains – similar to the system in Copenhagen. Earlier, the TBMs were faced with passing below a weak mosque structure, through areas of poor ground and overcoming unexpected hurdles of the wrong reinforcement in diaphragm walls that were the be bored through at partly-built stations.
However, since the TBM restarts last August, and despite a refit for a slight change in geology, the rate of progress achieved by the shields has picked up. They have been passing more partly-built stations and ahead must hold before breakthrough at Efklidi station, and later relaunches for the remainder of the bores to be done.
Originally, before the various challenging delays, it had been anticipated that the line would have come into service by the second half of 2010. Despite the setbacks around the underground works, though, the task of tunnelling has progressed relatively well.
When completed, the base metro line is to carry 18,000 passengers per hour in each direction – north and south – with the frequencies of the driverless trains giving intervals of 90 seconds between services. Presently, it is expected that the new line will be commissioned in 2013.
Once completed, the north-south running metro line is to be extended in stages – farther north with two branches, and a branch to the international airport in the south.
Procurement & Plan
Thessaloniki base metro line is being developed by Attiko Metro, based in Athens is also working on further development of the subway network in the capital.
In 2006, Attiko Metro awarded the design, construction and commissioning contract for Thessaloniki base line metro to a JV of Aegek (28.76 per cent), Impregilo, (26.70 per cent) Ansaldo Trasporti (21.72 per cent), Ansaldo Breda (15.60 per cent) and Seli (7.21 per cent). The contract is valued at Euro 958 million, and includes construction of the twin tunnels, 13 stations, control centre, aministration and services buildings and depot, and trains.
A separate JV, comprising two of the JV partners – Seli and Aegek – is undertaking the underground works in a subcontract, on a 60:40 basis. In total, the tunnelling and other civil engineering works being undertaken by the subcontracted JV plus Impregilo accounts for just over half of the main contract value.
The base metro line will have 13 stations. From north to south, they are: N. Sid. Stathmos, Dimokratias, Venizelou, Aghia Sofia, Sintrivani, Panepistimio, Papapfi, Efklidi, Fleming, Analipseos, Patrikiou, Voulgari and Nea Elvetia.
Along the alignment, geology includes clay, sandy clay, and sand with cover of up to 27m. Sections of ground along the route are very weak. However, combinations of tunnelling below historic buildings and the potential for archaeological discoveries were recognised to be among the prime challenges on the project.
Tunnelling works commenced in early 2009 and completion is expected in late 2012, due to a range of delays related to the type anticipated, and other construction and access hurdles. Previously, it had been expected the tunnelling work would be finished by 2010.
The 5.3m i.d. running tunnels are being bored by a pair of 6.12m diameter Herrenknecht EPBMs, each with maximum cutterhead thrust of 40,000kN, power of 1000kW and cutterhead rotational speed of up to 4 rev/min. Seli has provided the backups for the machines. The shields will always work in full EPB mode, in pressures ranging from about 1.7-1.8 bar to 2.5 bar, and with full chambers.
The precast concrete lining is assembled from 300mm thick segments to be bolted (5 + 1) rings, each 1.5m long. The excavation and lining system includes a two-component grouting system.
Tunnelling works are performed four days per week (Monday-Thursday) with maintenance on Friday mornings. On the working days, the crews work three shifts.
Tunnelling
Assembly of the first TBM was undertaken in late 2008, some months after originally envisaged. The shield was launched in January 2009 to bore the south tube of the Thessaloniki base metro line. The second machine was launched from the same 15m shaft to drive the parallel north bore in April 2009.
The south TBM drive – the tube on the right when looking southward in direction of the bores – has remained the lead shield of the pair, always a few hundred metres ahead of its sister machine. By mid-January this year, the machines had been in the ground for almost two years and had advanced 4.2km and 3.9km, respectively.
A range of challenges have been faced on the overall construction works with impacts on the tunnelling progress and causing the programme for the drives to be extended. Despite those impacts, the tunnelling works have gone relatively well.
On their first drives, which were almost 3km long, the TBMs were to pass the partly-constructed sites at stations (Dimokratias, Venizelou, Aghia Sofia) without having breakthroughs into completed boxes – the first hole through would not come until Sintrivani, the fourth location to be reached by the machines.
Along these drives, the early challenges for the tunnelling works and advancing the TBMs included securing passage below the 15th Century, seismicallyweakened Alkazar mosque, but also some difficulties with some recently-built parts of structures for the metro: unexpectedly, the wrong reinforcement – steel instead of fibreglass – had been used in a few diaphragm walls through which the TBMs would have to pass.
Resolving the construction problem with those diaphragm walls required extra time and work to overcome, but time and effort was needed for necessary remedial operations on the TBMs to deal with both tool replacement on the cutterheads and maintenance work needed. These works on the TBMs called for hyperbaric interventions, and, for example, in one week alone there were eight such actions needed, in pressures up to 1.7 bar.
Sintrivani station was finally reached in November 2009. By then, average daily progress rates for each machine was in the 13-15m range, excluding stoppages and only based on days of active excavation.
But it wasn’t until August 2010 that the machines were re-launched, three weeks apart, on drives via more part-built stations to Efklidi station. The lengthy standstill before the second drives could commence was the result of the combined effect of extensive archaeological works and discoveries at station sites, and limited availability of work sites. The diaphragm walls and slab construction works are at varying stages of completion in most locations.
Drives ahead
As the drives proceeded into this section of more difficult geology combined with old building stock, the cutterhead configuration of the TBMs was changed towards late 2010. Leaving the red stiff clay they had been boring through for about 2km to softer ground, the re-configuration of the TBMs included removal the central cutters to replace them with central rippers. Some cutterhead openings were increased, which also resulted in lower thrust forces and torque for improved TBM advance, and good control of face pressure and EPB parameters.
Since the re-start of the machines in the third quarter last year, the average daily advance for each machine has been 15.5m, which has enabled them, together, to complete more than 500m per month. The best week saw one machine cover 80m, with an advance of 22.5m on the best day.
The south TBM is due to arrive outside Efklidi station – yet to be finished – at the beginning of February. The shield will be stopped for an estimated time of about a month for the station structure to be completed. Meanwhile, its sister TBM will have closed the gap and both machines will then be able to hole through into the box station, around March.
Once departed from Efklidi, the TBMs will have a further 3km each to bore via sites for stations where only early works are underway, including the immediate next on the line – Fleming and Analipseos, but where no excavations have yet taken place. The soft ground for the remaining drives is expected to comprise sand, silt and lower clay content (only 18 per cent) with groundwater pressure around 1.5 bar.
By mid-year the first of the TBMs is expected to be at Analipseos.
Extensions
In the late 1990s, an effort to have the metro line developed under concession failed and by 2003 the procurement and funding approach had turned to employing both state and EU funding. There followed an extended procurement period that included a few stages before shortlisted parties bid and, finally, the contract was awarded.
Thessaloniki is now looking to have three extensions to the base metro line, as follows:
• Phase 1 extension – to extend the base line both to the east and west, with five stations over 5km to Mikra in the Kalamaria area to the east, and five stations over 5km to Efkarpia in the Stavroupoli areas to the west.
• Phase 2 extension – to extend the Phase 1 extension a little farther, from Efkarpia to PapaGeorgiou; and, extend the base line also west via a separate branch with three stations to Scholes.
• Phase 3 extension – to take Phase 1 further east, on an elevated section, via four stations to Makedonia airport. This section is still in planning.
Attiko Metro says it hoped to have the contract for the mostly underground Phase 1 extension to the Kalamaria area awarded in 2011.
Location map of Thessalonikimetro showing Base Line and planned extensions Twin tunnel bores on Thessaloniki metro are more than half complete but many station works remain to be done Two Herrenknecht EPBMs are driving the twin tubes at Thessaloniki metro Breakthrough at Sintravani station in late 2009. Boxes at earlier stations were not finished when passed The lead TBM was relaunched on the second drive in August 2010 Ahead, the shields face softer ground, older structures and continued archaeological works around part-build stations.