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The two biggest TBMs in the world are in Madrid, the first already driving west while the second nears working readiness. Damon Schünmann reports.

The world's two largest TBMs are now on site at Madrid's ring road redevelopment project (GE July 2005). At over 15m diameter they are on the same scale as - and have a vague resemblance to - the gigantic boring worms from Frank Herbert's sci-fi novel Dune.

The project to upgrade, and in parts bury, the M-30 means the Spanish capital's skyline is filled with more than 80 foundation rigs.

But it is underground that some of the most impressive work is taking place on the route, now known as Calle (street) 30.

Split into a number of large contracts by client the city council, the E2.5bn (£1.7bn) southern bypass project is intended to ease congestion at the southern junction, the city's major north-south and east-west connection.

The road handles 260,000 vehicles a day, so burying the east-west portion in a 4.2km tunnel should have a marked effect in reducing traffic jams. The TBMs will parallel bore about 3.5km in twin tunnels, a maximum of 75m below ground, while the rest will be cut and cover.

When GE visited in February, the first of the monsters, a Herrenknecht Mega S-300 TBM, had travelled about 650m along the line of the northernmost of the tunnels to create a three-lane route.

The tunnels' circumference will accommodate a route for emergency vehicles below the main carriageway and an overhead ventilation shaft.

Cross shafts will connect the two tunnels every 200m.

Raquel Muinos, site visit coordinator for the southern bypass contracting joint ventures, says the Mega S-300 earth pressure balance machine is not just one of the world's two biggest, it also has some unique features.

'The Herrenknecht is the first TBM to have two cutting wheels that can rotate in opposite directions, ' she says. 'The inner wheel can also extend forward of the outer by 400mm to create a pilot hole for the outer to follow up on.' It is also one of a kind in having three screw conveyers to transport spoil away from the pressure chamber, one for the central wheel and the other two for the exterior.

This £226.9M contract for the 3.56km northern tunnel is a joint venture between main contractors Ferrovial Agroman and Acciona Infraestructuras. Ground conditions are mainly hard clays with some gypsum.

'The tunnel linings are built in 'nine plus one' - 2m long segments with the plus being a smaller wedge shaped piece, ' says Muinos. 'They are 600mm thick steel reinforced concrete with polypropylene fibres.

These are designed to prevent concrete spalling from a fuel truck fire for two hours.'

At 3.68km the southern route, which is yet to start, will be slightly longer than the north tunnel, as it is on the outside curve. This is the task of the second monster, a Mitsubishi/Duro Felguera earth pressure balance TBM weighing in at more than 4000t. It will be operated by an FCC Construccion/ Dragados JV on its £280.9M part of the scheme.

Colcrete Eurodrill has supplied five BN17-48 Seepex UK pump sets to handle bentonite on the TBM. The pumps can operate at up to 48bar for the slurry to act as a spoil conditioning agent.

However this will be a backup that may not be used. 'We are not using bentonite at the moment [on the Herrenknecht TBM] as we don't have much sand here; it's mostly clay and gypsum, ' says Muinos.

The second TBM is being assembled in its launch shaft close to the site of Madrid's Crystal Palace, a glass and steel structure a least half a century old which houses a botanic museum.

This tunnel will pass under its foundations while the north tunnel will go beneath one corner.

To protect the museum, subcontractor Geocisa is pre-grouting to raise the ground up to 20mm and will use compensation grouting from four shafts once tunnelling begins.

The south tunnel TBM will launch through unreinforced piles below a waling beam. 'The ground behind the piles in the launch area is treated with tubes a manchette grouting, on a 1.5m grid, and mortar piles, ' says Empirica geotechnical consultant Juan Miguel Alcaide Alonso, who is contracted to Madrid city council.

'These protect the ground from settlement and chimneys forming at the crown of the tunnel that can go up to ground level. But once the TBM's shield has fully entered the tunnel the pressure at the cutting face can be controlled [to protect against this].' Muinos says all work on the southern bypass must finish next year, in time for mayoral elections in the spring. Madrid mayor Alberto Ruiz-Gallardon made election promises about transport infrastructure which he intends to fulfil before the city goes back to the polls.

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