Damon Schźnmann takes a look.
The latest tranche of work on Madrid's ring road has involved some careful tunnelling where the route is being taken underground in the south. Not only does the strata already contain an expanding metro network. But addition there are historical building foundations such as those of the Palacio de Cristal (Crystal Palace), a glass and steel structure built in 1887 which houses a botanic museum, along with other typical big city complications.
Contractors used a mixture of techniques for the 2.5bn (£1.7bn), 56km scheme including earth pressure balance machines, cut and cover, the Madrid method, the German method and box jacking. This involved 1.2Mm2 of diaphragm walling and 15M.m3 of muckshifting. Co-ordinating the settlement data for this work was SECOIM, the body responsible for security and control for municipal infrastructure.
"We have done all the monitoring for the ring road in the south," says SECOIM manager of field operations Jesus Sobrino.
Spanish geotechnical contractor Geocisa was heavily involved with the work. Its manager Diego Del Saz López explains: "The subcontractors send monitoring information to the main contractors and to SECOIM and its geotechnical advisers.
"Geocisa worked with SECOIM to install Trivec and
He says that although less accurate and only able to measure in two dimensions, the electrolevels have a big advantage in being automated and giving readings in real time.
"Permissible movement where there are no buildings could be 50mm, but where there is infrastructure it could be just 5mm," says SECOIM manager CarmenGonzalez.
Using the electrolevels meant data was sent to an internet server. In an emergency, it could also be texted to mobile phones.
Geocisa geotechnical unit manager Ivan Henriquez describes the electrolevelling system, which comprises a sensor in a bar: "As the bar moves we can calculate settlement from bar length and change of position. We also used this system to control compensation grouting."
The technique relies on absolute settlement rather than relative. This means a continuous line of bars must start from a reference point that has zero movement, outside the influence zone from tunnelling work.
Ground conditions in the Spanish capital's south are generally fill over sand, over hard clay, over harder clay, over gypsum. Henriquez says: "Madrid has good ground for this type of work so we have not had many problems and registered very little settlement, less than 10mm in most cases."
López adds: "We installed 1246 linear metres of the bars and 419 sensors. The bars are normally 3m long and have one sensor each with an accuracy of about ±0.1mm per metre of bar."
"Where the 15.2m diameter tunnel boring machines crossed metro Line 6 while it was in service the new tunnel passed within 5m, but we only recorded a maximum 5mm settlement."
However, this proximity was far from the closest. He says tunnelling passed within 0.5m of Line 1 at Tetuan Station. The first tunnel (of two in parallel) caused 20mm movement. But the second caused 44mm because of the influence from the creation of the first.
The construction sequence at this point, where new tunnels crossed beneath the station, was consolidation grouting to prepare the ground, micropiles and then excavation. But settlement, however minor, still meant correction and rails had to be raised in places to compensate.