The latest tranche of work on Madrid's ring road (Street 30) needed close scrutiny to ensure it did not adversely impact on existing infrastructure.
And this is in a city where work is often done in spurts of activity to strict deadlines. This is because big projects in Madrid tend to be part of electoral pledges from mayoral candidates and often need to finish within four-year terms of office.
However, a major reason for careful tunnelling on the project, which involved undergrounding the ring road's south section, was a strata already containing an expanding metro network. In 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, 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 all of 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.
Big Spanish geotechnical contractor Geocisa (about 1200 employees) was heavily involved with the work. Its manager Diego Del Saz López explains: "The subcontractors send monitoring information to the main contractors, for example [Geocisa parent company Dragados], and to SECOIM and its geotechnical advisers.
"Geocisa worked with SECOIM to install Trivec and electrolevel monitoring. Trivec is very accurate in three dimensions and used for academic research, but it is not automated."
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.
"We can only get access from 3am until 6am [for the metro], but the new tunnelling work is continuous," says SECOIM manager Carmen Gonzalez. "Permissible movement where there are no buildings could be 50mm, but where there is infrastructure it could be just 5mm.
"Also, if the tunnel is near the surface, for example, where its diameter is greater than the height from its crown to the surface, the allowable movement is far less than for a deeper tunnel." This means manually collecting the data would be difficult.
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."
He continues: "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 it 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.
So the work is now complete and Madrid's frenetic infrastructure activity has the chance to pause for breath. before picking up again to implement whatever the regional government has in store next.