Hard compacted sand beds and some gypsum are the biggest problem for the drives on the Metrosur, as well as running sand.
The geology is similar to Madrid itself, with perhaps more sand than the clay and sandy clay in the city.
'In general it's good and usually quite stable, ' says Frinciso Trelles from the contracting joint venture of ACS and Vias, which has the 30 month contract for the Alorcon section. This includes the large transfer station for the Line Ten link to Madrid.
'But the sand can be abrasive and some of it is running sand. There are also some water pockets but it drains out quite quickly.'
The 9.43m diameter Herrenkecht machine erects a seven segment precast concrete lining with a cement grout filling the annulus. Segments are 1.5m wide and 320mm thick with a neoprene seal. Internal running tunnel diameter is 8.4m.
Muck out is by a screw conveyor to a short belt which loads a 17.5m3 rail truck.
Muhlhauser from Germany supplied these and Schoma locomotives are used.
'This material is no good for a long conveyor' says Trelles. The trains dump the spoil in a 'swimming pool' area where it is loaded into MAN and Mercedes tippers for the 30km haul to a disposal site Progress graphs for the MetroSur show good progress, the Herrenknecht machine on this job driving 3,000m in the first 245 days. That includes pauses when it was transported through two stations. Stations are built within diaphragm wall boxes with 'eyes' to allow the machines through.