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It's the devil in disguise

UNDERGROUND CONSTRUCTION

Excavation has begun on the Faido multifunction station for the Gotthard Base Tunnel in the Swiss Alps, an area with a troubled tunnelling history. Paul Wheeler reports.

Switzerland's £3bn Gotthard Base Tunnel, which consists of two 57km long bores, is the world's biggest tunnel project under construction.

It is named 'base tunnel' because it is 600m deeper than the road and rail tunnels that traverse the St Gotthard Pass and nearly four times their length.

At 2km down, it is also deeper than any previous tunnel through the Alps. Its builders will have to contend with unprecedented rock stresses and temperatures.

The Gotthard Base Tunnel, together with the Zimmerberg and Cenari base tunnels to the north and south respectively, will form more than 90km of highspeed railway line in an almost flat trajectory between Zurich and the Swiss-Italian border.

When complete, in about 10 years, trains twice the weight of today's will be able to run at twice the speed through the tortuous passes to Milan and beyond.

GE recently visited the Faido section of the project - one of its five major contracts - where construction has just begun on one of the tunnel's two 'multifunction' stations. These will provide emergency platforms, with access to evacuation routes and train crossover chambers. Like the Channel Tunnel's crossover chambers, these are probably the most complicated components of the project.

While most of the running tunnels are formed by tunnel boring machines, the complexity of the multifunction stations means section contractor Consorzio Tunnel AlpTransit-Ticino (TAT) is forming the labyrinthine network of tunnels by drill and blast.

Lying 1.2km below the mountaintop, the station is accessed by a 2.65km long, 12° downward adit driven into the mountainside.

Even at this depth - half of that to be encountered elsewhere on the project - the tunnel is humid and damp. Groundwater trickling into the tunnel is as hot as bath water.

Work on the Faido station is scheduled to take about three years. It needs to be complete before the twin running tunnels pass through the area, which means this work is very much on the critical path of the project.

Excavation began in March and already the complexity of layout is apparent. TAT is fortunate that the Faido site is in a block of gneiss and progress using drill and blast methods is good. TAT is working around the clock, with three eight-hour shifts. The aim is to achieve a blast and mucking out cycle within each shift.

An Atlas Copco drilling jumbo forms the blast holes. TAT is using a novel pumped emulsion explosive, mixed and charged in a mobile unit at the face. This is much safer than conventional drill and blast in that you do not need to handle, store and move explosives on site, says TAT project director Daniel Sporri.

Mucking out is carried out by a fleet of Caterpillar equipment modified in the factory for working in a tunnelling environment.

This marks a significant development for Caterpillar, whose equipment has been used on tunnelling projects for many years, but always with after-market modifications.

Inevitably in the harsh environment, equipment gets knocked around and extra protection of mechanical parts is vital.

There is also an issue with exhaust emissions, since air quality has to be carefully managed.

Caterpillar has set up a tunnelling sector to provide factory fitted modifications and a better focus on what it believes will be a big growth market over the next 10 years.

The approach seems to be paying dividends. At Faido, for example, TAT is using 18 Caterpillar machines including six wheel loaders, eight articulated dumpers, three excavators and a tool carrier.

Each shift produces a tunnel advance of between 1m and 4m depending upon the face dimensions and ground conditions, says Sporri.

Rock bolts, reinforcing nets and shotcrete provide primary rock support. Bolts are typically installed on a 1.5m square grid and are between 3m and 6m long.

Steel fibres reinforce the shotcrete. Waterproofing and cast insitu concrete lining will follow behind.

Construction of the tunnel will produce 24Mt of rock spoil, which in the age of construction sustainability is viewed as a resource.

In the past it has not been possible to use cuttings from TBMs for concrete, because the cuttings are too small to meet standard concrete specifications. In 1993, the Swiss government initiated a research programme to develop new methods of mixing and testing concrete using TBM cuttings.

As a result spoil is now being processed and used on site in the manufacture of concrete for the Alptransit project tunnel linings.

Geology and history heighten challenge Technically demanding and challenging tunnels are nothing new to Switzerland's St Gotthard Pass.

During construction of the original alpine rail tunnel in the 1870s, the contractor went bankrupt.

Bad ground and flooding killed 310 workers and injured 877 others.

The more recent 16km highway tunnel, completed in 1980, also proved to be a major engineering challenge.

Although the route was chosen to avoid known areas of poor ground, the tunnel's cost more than doubled.Shockingly for such a recent project,19 workers died during construction.

With this troubled history of tunnelling through the St Gotthard Massif, no one expects the Gotthard Base Tunnel to be straightforward.The alignment will transect diverse and demanding geological conditions from very hard granite to rocks historically described as 'butter-soft'and 'sugar sand'Ominous portents indeed.

In fact much of the alignment lies within the Aar and Gotthard massifs, which consist mainly of gneiss and granite and provide good hard rock tunnelling conditions, albeit at very high pressure given the great depth of cover.About 50km of the 57km route will be constructed using TBM.

Elsewhere sediments have been tectonically compressed and are considerably fragmented.Geological investigations have been intensive and have focused on two areas of concern.

A 5.5km long test tunnel was driven into the Piora syncline to investigate a narrow (only 230m across at the tunnel's level) but potentially difficult band of rock where the so-called 'sugar'dolomite was expected.

This decomposed sedimentary rock has caused major construction problems for tunnels located above the alignment of the base tunnel.

But four major test drillings, curving down from the end chamber of the test tunnel, all proved the opposite.The rock is a stable dolomite marble or dolomite/ anhydrite and apparently not subject to water pressure.So good was the outcome that an extra shaft and chamber were cancelled, as was further drilling for investigation and grouting.An engineer for SBB Swiss Federal Railways described the outcome as 'almost miraculous' Another area of anticipated difficulty is through the 3km-wide Tavetch sedimentary zone centred on Sedrun where the second 'multifunction station'is being formed, this time accessed by an 800m deep shaft.

Here drill and blast will be used for the running tunnels.Steel arches and a heavy application of shotcrete is likely to be standard reinforcement, making progress through this section slow.

In good conditions, daily advance rates of more than 20m should be achieved using TBMs - the problem with a TBM advance is the lack of flexibility.If conditions worsen considerably or even just differ from those anticipated, major problems could ensue.

Drill and blast in good conditions, such as those encountered at the Faido multifunction station, is not as fast as TBM work.An advance of 4m in an eight-hour shift - 12m a day - is considered excellent.However in extremely poor conditions, such as at Sedrun, drill and blast offers the less risky way forward.

Given the hazards, the scope of the project and historical problems encountered in the area, completion of the Gotthard Base Tunnel on time and budget without major mishap would unquestionably be a major achievement in the assessment and management of risk in tunnelling projects worldwide.

Alpine consortium

Consorzio Tunnel AlpTransit-Ticino (TAT) is a Swiss/ German/Austrian/ Italian joint venture led by Zschokke Locher, with Alpine Mayreyder Bau, CSC Impresa Construzioni, Hochtief and Impregilo.

It has two of the five major contracts that make up the Gotthard Base Tunnel project.Its work covers the Faido and Bodio sections, which at 30km long essentially make up the southern half of the tunnel.

Work, which should take 10 years, includes construction of the two TBMexcavated single-track running tunnels, with connecting galleries every 325m, plus the Faido multifunction station.

The project value for the Bodio and Faido contracts is £300M and £350M respectively.

At the moment TAT has 130 people working from Faido on the multifunction station, but once work starts on the running tunnels this will rise to about 350.

Integrated transport vision

The 90km of tunnel in the Zimmerberg-GotthardCenari route, together with the 35km long Lochtsberg tunnel which forms a second more westerly high-speed rail route through the Swiss Alps, are collectively known as the Alptransit project.

What is less well known is that these £6bn government-funded projects form less than half of a comprehensive and visionary upgrading of the Swiss rail network that should reap tremendous business, social and environmental benefits in 20 years.

While the main motivation is to transfer freight traffic from road to rail, Switzerland is also keen to link itself into the evolving European high-speed passenger rail network.

The Swiss reckon that if they can offer city centre-to-centre journey times of less than four hours, then rail is the preferred transport mode over air.

Completion of the Gotthard route will cut an hour off the Zurich to Milan journey time, and once Italy constructs a planned high speed link on its side of the border, Zurich and Milan will be just over two hours apart by train.

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