Twin bore tunnels are being constructed at the southernmost point of the ring road to free up one of the busiest road junctions in Spain. The world's two biggest tunnel boring machines (TBMs), each with a diameter of 15m, are being used.
The tunnels will carry three lanes of traffic each, with a fourth lane for emergency vehicles below the main deck, in the bottom third of the each tunnel, and ventilation in the upper third.
Cross shafts will connect the two tunnels every 200m.
Madrid's ground is mostly sands and gravels from eroded granite mountains. In the south, over the 3.7km long section of bored tunnel, there are coarse sands, clays and gypsum.
The TBMs will pass directly beneath Madrid's Crystal Palace, a 50 year old glass and steel structure housing a botanic museum. It is sensitive to ground movement and must be protected by pre-grouting, followed by compensation grouting when the drive passes underneath.
'We are pre-treating until the ground is tensed and ready for a load, ' says Geocisa design engineer Ricardo Oprandi. 'Not only is it the biggest TBM job in the world, but it is twin bore so the ground movement could be significant.' Geocisa has sunk four shafts from which horizontal grout lines radiate. There are three tiers of lines extending 40m into the ground. The deepest of these are sleeved in PVC instead of steel, so that if a TBM should hit one, it will not damage the teeth of the cutting-face.
Geocisa grouting manager David Ruiz says: 'We are lifting the ground below the Palace up to 20mm. In a normal building, in normal ground, we would do about 4mm. But we are expecting a lot of ground movement here.
'It's been an interesting job but it will get more so once boring starts. We may have to compensate grout to protect the Palace and also grout any chimneys formed by the TBMs [caused by over-excavation at the tunnel's crown].
'The secret is to have the grout lines quite close together so we have overlapping zones of influence, ' says Oprandi.
Grout injection is automatically controlled and logged by a system the Spanish subcontractor has developed.
The data logger controls pressure and volume, and will cut off if levels are exceeded.
'There used to be problems with grouting works of proving how much had gone into the ground. But now control can be handed to the client via the data, ' says Oprandi.
One of the TBMs, a Herrenknecht Mega S-300 earth pressure balance machine, is now nearly 1km into the south tunnel drive. The Herrenknecht is claimed to be the first TBM to have inner and outer cutting wheels which can rotate in opposite directions. The inner wheel can also extend ahead of the outer one by 400mm to create a pilot hole.
The tunnel is being driven by a joint venture of contractors Ferrovial Agroman and Acciona Infraestructuras under a £226.9M contract.
Tunnel linings are of precast segmental type. Rings are 2m long and 600mm thick, and consist of nine segments.
The lining is designed in steel reinforced concrete with polypropylene fibres capable of surviving a two hour fuel fire.
The southern route has yet to start and will be slightly longer than the north tunnel, as it is on the outside curve. An FCC Construccion/Dragados JV will operate a Mitsubishi/Duro Felguera earth pressure balance TBM on its £280.9M part of the scheme.