It is said that when the young Theseus was first seeking his father Aegeus, the king, he travelled from Corinth to Athens. At the cliffs of Kakia Skala, the “evil stairs” close to Megara, he met the robber Skiron who would force passers-by to kneel and wash his feet and then kick them over the edge to be eaten by a giant turtle. Theseus instead pushed Skiron over.
The modern motorway journey from the Isthmus to Athens is less fraught and the seafood less aggressive – but the limestone cliffs of Kakia Skala still present a major obstacle. Here, some 50km from the capital, the dual three-lane highway narrows to a two-way national road and winds for 8km along picturesque but plunging sea-edge slopes up to 650m high.
Besides slowing traffic and creating numerous jams, the bottleneck is also dangerous. Dealing with this section has been a key outstanding task for the 1990s PATHE project, the Greek government’s scheme to build a 780km long motorway from Patra in the west via Athens to Thessalonika and Euzoni in the north.
The 45º to near-vertical escarpments are also a major obstacle for a new express railway project. The new line, from Athens to Corinth, is under construction by government infrastructure body Erga Ose and is due to open next year. Like the road, there is a special need to finish the project in time for the 2004 Olympic Games.
An initial scheme to upgrade the double road-rail route involved some relatively short stretches of tunnel to cope with the very steep slopes and headlands. Most of the 8km road and rail link was to be on viaduct and “belvedere” or balcony structures; the road would run higher up the slopes than the rail line. “We had four small tunnels to do, but mostly there were nine large viaducts,” explains Ioannis Kavvadas, assistant project manager for Greece’s largest contractor AKTOR, which is carrying out the bulk of the road project. AKTOR is also in JV with Aegek and Alte on the railway below.
Seismic redesign
Originally, the tunnel lengths were to be no more than 1000m in total for the road, and three lengths of 667m, 420m and 1537m for the railway. The rail tunnels are 14m wide and 10.7m high for twin tracks, and the road tunnel cross sections are larger, 19m wide and 12m high, carrying three lanes and a hard shoulder in one direction. However, the project has been substantially changed. Now the new motorway has been realigned further back into the slopes and will include five tunnel lengths, three for the inner, Corinth bound carriageway, and two for the outer Athens direction (Figure 2). Additional work now also includes eight escape passages bored every 300m, with a total length of 880m, using an excavated area of 25m². There are also about 35 retaining walls, one small monolithic bridge, six culverts and a major flood control structure.
“The reason for the changes was the earthquake in Athens in 1999,” explains Kavvadas. In September that year Athens suffered a major shock and the project’s design parameters were made significantly more stringent. The changes required a rethink of the design and additional tunnelling works, to take the alignment further back into the cliffs; while PATHE, through its designers Odomichaniki and OK Meletitiki reworked the design, construction continued on the unchanged intermediate sections. New tunnels were needed because “viaduct was too difficult to do to new standards on the slopes, which have a lot of debris and partially cemented scree,” according to project coordinator Andreas Polakis at PATHE’s construction division in the Ministry of Environment, Physical Planning and Public Works. There was concern about the stability of viaduct piers in an earthquake. “There is also a potentially active fault running right through the area,” adds Dr Georgios Tsisfoutidis, a geotechnical supervising engineer from Erga Ose, for the railway sections. Like much of the Greek region there has been substantial tectonic and seismic activity at Kakia Skala.
For the railway just the first 3km has been affected by redesign, with two tunnels and an intervening viaduct section all merged into one longer bore of 2,380m, and set deeper into the hillside.
All these changes were made somewhat easier due to the fact that the rail and road projects have been treated as a single scheme from the start. Recognising that the two route lines ran very close, the government suggested Erga Ose should second the design and project management to PATHE, which had began its work a little earlier. It made sense to keep the design coordinated as well as the following construction. The two routes do run almost parallel, though the rail is 20m to 30m lower down the slopes. At one point a rail tunnel crosses underneath a road tunnel above.
A US$197M contract for the scheme was let in January 1999, for the whole combined project. With the new alignments the current project cost is cited at US$334M; split into US$219M for the road works and US$115M for the rail. Much of the rail tunnel is now 95% complete, and the road bores are being fitted out with lighting and ventilation. One shorter tunnel, not on the critical path, is still being excavated. The road and rail work was split by the JV, with the three contractors each taking a different section. AKTOR has the largest and most difficult road and road tunnel works, and Aegek and Alte work on the railway below. All of them used conventional drill and blast techniques with variable support.
Ground conditions
All the tunnels face similar geological challenges. Kakia Skala comprises primarily two main types of limestone, a 100M-65M year old Cretaceous limestone, often highly tectonised and weathered and therefore often with a fairly high clay content in the rockmass, and a much older 230M-200M year Triassic limestone. The latter is “more thickly layered and solid,” says AKTOR engineering geologist Georgakopoulos. “You can see the fault structures in the Triassic rock whereas the Cretaceous is obscured,” he says. The hardest rock is around 100MPa, but both types have suffered from the stretching and compressions imposed by the collision of crustal plates and are often weathered. Faulting is frequent in the Triassic rock and there is general tectonisation throughout.
According to project adviser Dr Evert Hoek, former Imperial College professor and independent international consultant, the rock is not exceptionally difficult for Greece. The main problem is the dormant fault line running through the headlands, more or less along the line of the tunnels, which creates fracturing around it to about 60m distance; this fault was the main reason for the redesign as it is still active. “Not too difficult” still means some awkward battles however. In the Triadic rock for example cracking can create a “sugar lump” structure to the rock, made worse in places by weathering so that it is “virtually a soil”. In other areas loose blocks are embedded in weathered fill. Karstic voids are found in both rock types, though the motorway is more in the Triassic rock and the railway more in the Cretaceous.
Tunnel construction
On the motorway the first and the third tunnels have proved toughest with extensive forepoling needed, particularly to work through the first Athens-end tunnels, the 840m tunnel on the inner carriageway and the shorter 220m tunnel on the outside.
The contractor bought a fair amount of new equipment for the project including several Tamrock jumbos. Two three boom Paramatic machines worked together to tackle a large 110m² top heading, 8m deep, with vertical Tamrock drills used for the follow-on 4m deep bench, which had a 90m² cross sectional area. Work was carried out from both ends of the longer tunnels, “we had six jumbo rigs going at one point,” says Kavvadas.
The blasting round was often limited to about 1m in the Athens-end tunnels and the long tunnel at the Corinth end of the project, because of the rock conditions. There were a lot of karstic voids, some man sized. There were also karstic faults filled with stiff claystone or loose blocks. “But fortunately there were no major falls or serious accidents,” says Kavvadas.
An Italian Casagrande machine installed forepoles. Support varied in five categories, according to rock condition inspections, and included rock anchors up to 6m long, shotcrete to 250mm thickness and in a few places steel ribs every 0.75m, though mostly these were used at 1.25m spacings. “We used Swellex bolts quite a lot for general purpose work, though there are some conventional bolts and some places where we put in self-drilling bolts with sacrificial bits.”
“The ribs used elephant footings, which we supported with steel sections once the benching was coming through,” says Kavvadas.
In the central tunnel the Triassic rock was much sounder and longer blast rounds, up to 3m, were possible. But again in the third Corinth carriageway tunnel the more difficult rock meant shorter rounds, particularly in fault areas, which tended to cross the tunnel line every 30m.
Excavation has also had its problems at the portals on the east end, where the oblique angle of tunnel to the rock face meant difficult excavation. Rather than cut back the rock completely one of the designers, OK Meletitiki, produced a “false portal” solution with the tunnel entrance built up using a pyramid step form to create full profile. For the left side tunnel this was a 30m length and for the right a 100m length. A concrete fill was used which could be subsequently excavated by hydraulic hammer. “On the second tunnel, the right side, we used a polystyrene filler which could be simply pulled out,” says Kavvadas.
But there are other difficulties. Most of all the contractor has battled with extremely tight spaces for the work sites, which sit alongside the main national highway. The highway, the main spine road for Greece, must maintain two lanes running in each direction at all times. Conditions were most cramped early on, though things have improved as the new road has been excavated. When some of the road is put into service the cramped conditions will be back says Kavvadas. “It has been very problematical for access,” he explains. Trucks must find a gap in the stream of traffic in order to haul spoil away from site. “On Fridays, weekends and public holidays it is impossible to use the road at all,” he adds “because of the commuter traffic leaving and returning to Athens”.
On top of this a great number of traffic diversions were needed. “Also, each and every time we blasted a round we had to ask and wait for the traffic police to stop the traffic for safety reasons. That could be three times a day and although the hold up was only a few minutes the wait might not be. You can imagine how disruptive this can be.”
Truck movements are continuous, because although more than 50% of the spoil can be used for concrete aggregate and embankments, much of it must travel 6km to a site just off the main highway towards Athens, to be crushed and cleaned as required. Any waste spoil is taken to an old quarry near the ancient town of Megara, which is to be landscaped and handed over to the town as parkland at the end of the project. Some 2.5Mm³ is excavated externally and 1.5Mm³ from the road and rail tunnels; the tunnels’ contribution to the road is 1,250,000m³ and 900,000m³ respectively.
Excavation on the rail tunnels has been tackled much as above, though with smaller tunnel diameters to deal with. Work on the railway was split, with the slightly more experienced Aegek tackling the more difficult eastern end of the long tunnel and Alte working from the other end in sounder rock. Aegek also tackled the other tunnel going in the opposite direction from the same worksite.
Aegek made slow going in the first period, because of heavy karstification, and in the first 700m or so made a maximum of 3m progress a day on the top heading. The Cretaceous limestone had a large if irregular block structure, with 1m blocks often embedded in loose material of terra rossa and rock chips. According to Dr Giorgios Tsisfoutidis, the blocks acted like keystones in some places and could bring down more material. Rockbolting and spiles were used to hold blocks and there were early attempts to grout the voids and loose material. “But after losing some 90t of grout we decided it was heading straight out to sea,” says Tsisfoutidis.
Again extensive forepoling was needed and substantial use of steel ribs with elephant footings. “We even had to anchor these footings back because of the danger of voids beneath, when the bench was taken out” says Tsisfoutidis. Voids also created problems for the blasting, with so many exits for the exhaust gases that it sometimes had little effect. Instead Aegek worked its way forwards with a Rammer G100 hydraulic hammer mounted on a Cat excavator. “There was some 25% blasting needed but only where appropriate” says Tsisfoutidis. At the Alte end of the tunnel the problems were less, though rock which was fractured down to the “sugar cube” structure was also encountered. Alte used Tamrock jumbos and Toro loaders, much as AKTOR on the road above. Surprisingly in Greece, Aegek has opted for Atlas Copco H135 jumbos on its second tunnel, though it also had a Tamrock 206 at work.
Current progress
Progress on most of the project has been quite high says Polakis from PATHE, with 65% of the total works completed. Excavation and support is complete on 4000m of the road tunnel, and all of the rail tunnels, some 4200m. The remaining works include E&M works, now underway. About 3000m of the road tunnel lining is completed and the rest underway; the final tunnel lining is a 2mm thick PVC Italian Flag waterproof membrane, plus geotextile, and 500mm of cast in-situ concrete. “Water is not a problem – mostly we would see it drain straight through to the sea,” says Tsisfoutidis.
AKTOR is currently pushing to complete early if possible with some acceleration measures, working three shifts and bringing in plenty of equipment for the lining work. Four steel forms from SAME of Italy and four final lining moulds are in use. The company has to complete viaduct structures and there is also a substantial amount of cut and cover to do. A cutting section is also underway, 1400m long through scree material. To concrete the cut and cover sections the contractor is using precast roof panels as permanent formwork. “It means the steel tunnel forms do not need to be held up for 28 days while the arch reaches full strength” explains Kavvadas. Inside the tunnel the forms can be struck after 10 hours, since the lining does not have an immediate load bearing function.
The E&M design for the tunnels is being done by TEKEM, with most of the installation carried out by AKTOR, although lighting is supplied by Philips of Holland. Work includes an operations control building, substations, air pollution measurement, longtudinal ventilation, fire detection and hydrants. There is also telecommunication, CCTV, and traffic control.
Related Files
Fig 2 – Map showing the alignment of the Kakia Skala rail tunnels, with the motorway tunnels shown inset. The new road is located higher up the slopes, above the rail line, on a similar alignment
Fig 3 – Typical rail tunnel cross section showing the train envelop. Anchors were typically 5m-6m long
Figure 1 – Map of Greece, showing the location of the project
