Over 80 excavation rigs are working in Madrid as the huge, o4bn (£2.77bn) inner ring road project speeds towards its May 2007 completion date.
That the project, known as Calle (Street) 30, comes in on time is of critical importance to more than the contractors.
Madrid's mayor, Alberto Ruiz-Gallardon, made election promises for the route, formerly known as the M30, and the city's voters go back to the polls in the month for which completion is scheduled.
Calle 30 involves work all around Madrid's city centre.
There are 56km of tunnels being built in all. But by far the greatest effort is concentrated on the southern half of the ring road. 'To the south of Madrid is a heavily populated region and the M30 inner ring road formed a barrier for pedestrians getting into the city, ' says Geocisa design engineer Ricardo Oprandi.
Accordingly, large lengths of the road are being put into tunnel, with new routes into the city punched across the circumferential artery (see map), to improve city access by foot.
Western and southern portions of the project present major geotechnical challenges.
The western section crosses beneath the River Manzanares several times. Burial of the existing road in a cut and cover tunnel involves extensive piling, underpinning of historic bridges and modification of foundations.
Crossing the Manzanares involves temporarily infilling half the river's width with alluvial material. This provides a work platform, allowing top down construction of half of the tunnel: 800mm thick diaphragm walls are installed to depths of 11m to 32m in the river bed.
Insude the diaphragm wall the bed is levelled and blinded, enabling the tunnel roof to be cast. Excavation of the tunnel box can then be carried out underneath while fill is removed.
Oprandi says that above tunnel roof level, the diaphragm walls consist of very weak grout, making them relatively easy to remove.
Fill is then placed on the opposite side of the river, allowing the process to be repeated and the tunnel completed.
Spanish geotechnical contractor Geocisa site engineer Jesus Plaza says: 'We've got general problems because a lot of the time we're working on bits of river that have been filled in to create a construction platform.
When you're diaphragm walling across the river and digging very unconsolidated ground it can collapse even with bentonite. Then we have to mortar the hole, leave it to go off and re-excavate.' A different, top down construction technique is called for where the three lane route passes directly beneath one of the piers supporting Madrid's Segovia Bridge.
A jacking system is being used to counter subsidence risk.
Main contractor Ferrovial Agroman's instrumentation engineer, Fernando Sola Perez, explains the installation process: 'First we put in a diaphragm wall for the tunnel either side of the bridge. A jet grouted wall was then installed under the bridge to make it as watertight as possible (see diagram). Diaphragm walls are of 800mm diameter contiguous bored piles.
Then we lowered the ground level below the arches to get a drilling rig in and installed two rows of vertical 200mm diameter temporary cased micropiles against both inside faces of the pier.
After this we used cement pre-treatment in two horizontal rows through the pier to ensure infill material cohesion, ' adds foundation subcontractor Keller Terra production manager Emilio Lopez de la Guardia.
Twin rows of horizontal micropiles were also installed through the pier.
Jacks placed between the capping beams for the vertical piles and the horizontal piles will thrust off the vertical piles, to hold the bridge's horizontal alignment in case of settlement.
This prepares the way for excavation beneath the pier and casting of a thick reinforced concrete tunnel roof slab. This will be post-tensioned before tunnel excavation starts.
Once post tensioning is complete, the vertical minipiles and jacks will be rendered redundant - no movement of the slab is expected. They will remain in place during excavation, but once complete they will be removed.
Geocisa is monitoring bridge movement closely.
On some of the tunnels, installation of the central 800mm diameter load bearing piled walls separating opposing traffic requires close attention.
Piles are base grouted, with grout injected to 3m beneath the toe. Rig crews install a 100mm steel tube down the centre of the piles and insert a tube à manchette to grout.