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Tunnelling by template

Italian engineers boast that the 78km rail route being driven under the Appennine hills is the world's most complex tunnelling project. Andrew Mylius visited Bologna to investigate.

Lying between the Renaissance cities of Florence and Bologna, the steep flanks and plunging goldgreen valleys of the Appennine hills make for one of Italy's most spellbinding landscapes.

Hurtling along on a new, 300km/h high speed line though, travellers will glimpse only momentary shreds of scenery.

Of the 78km route, more than 73km is underground. Trains will rush through nine tunnels, emerging into daylight to dash across just 1.2km of bridge or viaduct and 3.9km of embankment.

The £1.1bn Bologna-Florence high speed rail project is part of a bid to expand passenger carrying capacity on the Italian rail network. It is expected that routes connecting Italy with its central European neighbours via the Alps will be initiated within a decade, explains supervising engineer for the tunnelling works Claudio Gambelli, and the increased traffic would bottleneck if the existing system were not upgraded and enlarged.

Meandering through the Appennines, the present journey can take well over an hour. Completion of the link in 2005 should see travelling times cut to 30 minutes.

Italian state railway Ferrovie dello Stato has let the project, its first ever turnkey design and build contract, to main contractor Fiat. Fiat heads the Cavet consortium, bringing together contractors Consorzio Ravennate delle Cooperative di Produzione e Lavoro, Fiat Engineering, Impregilo, Impresa Federici, and Cooperativa Muratori e Cementisti di Ravenna.

Construction of 100km of access roads started in June 1996, but tunnelling got under way only in 1998. Work is currently forging ahead at eight sites along the alignment.

Including 12km of adit, some 40km of tunnel has been excavated so far, Gambelli reports.

Tunnellers are working around the clock, seven days a week.

Cavet's impressive rate of progress is being achieved despite the Appennines' complex geology. Ground includes marls in schiler layers, 'chaotic' clay and boulder complexes, weakly cemented sand and sandstone, competent sandstone, flysch, marly-sandstone, soft clays and silts.

In the Raticose tunnel, where the hills are highest, overburden exceeds 500m. By contrast, on the Borgo Rinzelli and Morticine sections there is little more than 10m of soil overhead. 'Here, cut and cover is the obvious solution, ' notes Gambelli. 'But Borgo Rinzelli and Morticine are sensitive areas. Birds must be protected.' Full face excavation is to be carried out instead. Elsewhere, the water table can be as much as 200m above the tunnel.

Gas is also an issue.

The secret of ground stabilisation in the widely varying conditions is a sophisticated, adaptable tunnel design system produced by consulting engineer Rocksoil. Very simply, 'templates' have been drawn up for rock bolting, installation of glass-fibre rods, jet grouting and compensation grouting, insertion of drainage bores into surrounding rock, pre-cutting ahead of the face, installation of steel supports, and shotcreting.

Rocksoil's system is based on prediction and preventing problems. Extensive site investigation has allowed Cavet to establish which template it will use in particular sections of tunnel. And where conditions differ unexpectedly, or monitoring reveals unpredicted ground behaviour, it is able to substitute an appropriate alternative.

Introducing an unusually high degree of certainty about the short term stability of the tunnel enables full face tunnelling in soft, complex conditions where NATM or partial excavation techniques would normally be used. In principle, rates of advance, the integrity of the final tunnel structure and safety of workers are all improved, says Gambelli.

The tunnel is designed for twin track and has a finished diameter of 11.5m. But depending on ground conditions, which affect the thickness of primary and secondary linings, excavated diameter ranges from 13m to nearly 14m - the inner lining of insitu cast reinforced concrete is between 600mm to 900mm thick, says project manager for concrete works at the Monte Bibele tunnel, Lucia Gianni.

A variety of methods are being used to drive the tunnel - a TBM is being deployed close to Florence where ground is relatively consistent and dry. Drill and blast is being used in a section of the Pianoro tunnel near Bologna, while at Raticose, roadheaders and backhoe excavators are being used in the marls and sandstones.

At Vaglia progress is being made with hydraulic breakers.

Following no more than 2m behind the cut face, temporary support is provided by steel arches at 1.5m to 2m centres. A layer of 200mm to 300mm of steel fibre reinforced shotcrete is then sprayed onto the walls.

Where ground conditions deteriorate, the tunnel crown is stabilised with rock bolts and anchors. And in very poor ground, 20m to 25m long glassfibre rods are inserted into the face along the line of the tunnel after each 3m to 4m of excavation. The rods are grouted solid and the face supported with a temporary shotcrete lining.

Excavation then continues using a backhoe excavator.

Cavet is currently using grouted glassfibre rods in the Raticose tunnel which passes through some of the most difficult ground on the line. The contractor is placing steel support in the tunnel invert as well as in the crown. The interval between supports is also reduced to 800mm. To prevent water ingress, grout has been injected around the tunnel circumference.

Gambelli says that to minimise danger of collapse, work has to be continuous. Progress is currently down to 1.5m a day and ground behaviour is monitored constantly enabling immediate response.

The caution is in response to experience last year during construction of an adit to the Raticose tunnel. Unexpectedly high rock convergence meant work was suspended after only 200m.

To prevent collapse the excavated length of tunnel had to be filled with concrete and the surrounding ground - mixed clay and boulders - treated.

The concrete was then carefully removed through probe tunnels.

Raticose tunnel has so far not called for the most extreme of Rocksoil's templates. In the most difficult ground a small diameter pilot tunnel is driven first and a series of glassfibre ground supports installed in a radial pattern to support the ground for the main excavation.

Meanwhile, where little cover exists, for example in the Borgo Rinzelli and Morticine tunnels and near tunnel portals, permeation grouting will be used.

Steel rib elements, 290mm wide and 160mm thick, are brought to the tunnel in four sections, bolted together and lifted into place. When one or two ribs are in place, shotcrete is sprayed on, followed by a geotextile and a PVC waterproofing layer. The final insitu concrete lining is cast using Cifa travelling forms. Meanwhile, a mass concrete floor is laid in the invert in half widths ensuring access to the face at all times.

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