Much of the construction of Barcelona’s cross-city high speed tunnel, between Sants and La Sagrera, is entirely underground (85% of the 5,644 m tunnel), so the effect on the city life during construction will be minimal for an infrastructure of this nature. The only surface works will be the construction of the entry and exit sections of the tunnel, the ventilation shafts, the emergency exits (each approximately 500 m) and the connection with Sants station. The shafts will also provide an access for changing the TBM’s cutting tools as they wear down and for compensation grouting into the ground.
Intermediate shafts, located at street intersections, have been planned during construction to allow road traffic to flow with just one lane restriction. In the Sagrada Familia district, where a protection wall of concrete piles has to be built, traffic will be maintained down Mallorca Street with a lane restriction. Visits to the unfinished Sagrada Familia basilica will be allowed at all times.
The construction of the concrete wall between the tunnel and the cathedral has been decided to reduce the maximum expected settlement of the Gloria facade to 2 mm when the tunnel is constructed and the tunnel settlement to 9 mm when construction of the cathedral is completed. Interestingly, the Gloria facade will settle by around 22 mm from its own weight.
The foreseen protection wall of concrete piles and a concrete block will stop the earth from moving laterally thereby preventing the ground under the cathedral from moving. The only residual movement that could occur (in millimetres) would be less than the movements that the weight of the parts of the cathedral that remain to be constructed would cause on its foundations and the ground supporting them. The tunnel poses no risks other than those already existing due to the construction of the basilica.
The first excavation works between Sants and La Sagrera are slated to begin in June or July but the earth pressure balance TBM ordered by Sacyr will not start excavating the 11.4 m-diameter tunnel until 2009. Click es/20.
Geological and geotechnical surveys
In April, a study by the Catalonia society of geologists about the construction of the tunnel in the Eixample district of Barcelona reckoned the project was "viable" provided three conditions would be fulfilled: an exhaustive knowledge of the ground, an adecuate tunnelling method and a “perfect construction of the project that does not compromise safety”. The report also recommends more investigation of the subsoil to determine the risks for the Sagrada Familia cathedral, a Unesco world heritage site, before tunnelling next to the holy monument starts. Visit www.colgeocat.org
The report recommended to carry out further ground investigation to check the efficiency of the wall of concrete piles planned to be constructed alongside the cathedral to protect its foundations from any impact by the TBM when it drives nearby. It also stressed that the knowledge so far available on the other sections of the tunnel route could not be deemed optimal. A total of 111 ground samples totalling 4,257 lineal metres have been taken. Nevertheless, the report said that some of these ground exploration campaigns were performed to analyse different tunnel routes, so that "many" of the core drills do not concern the tunnel route finally selected. An important aspect to map the subsoil is the distance at which the samples are taken. The closest they are, more accurate will be the knowledge of the ground for engineers.
To find out more about the geology and geotechnics of the tunnel, five ground probe studies were carried out with extraction of continuous samples at depths of 25 to 60 metres, with complementary geotechnical studies to define the geotechnical conditions of the excavations. Also, complementary geotechnical studies were carried out to define the geotechnical conditions of the excavations, foundations and design of the tunnel, and to ascertain the geotechnical characterisation of the terrain and materials (granulometry, plasticity, density, humidity, resistance and deformability) and the geological and lithographic information on the structure and tectonics of the geological formations. In-situ and laboratory tests and studies have been conducted on the materials. This has given a complete geological and geotechnical profile of the entire route, which has been essential for determining the most suitable construction procedure (tunnelling machine) and the type of tunnelling machine.
Further to the publication of the report by the Catalonia society of geologists, complementary ground samples were taken along Mallorca Street (one every 36 metres) between Sardenya and Marina streets to find out more about the geology and geotechnics of the tunnel.
The results of the borehole tests confirmed the safety of the tunnel works in the area around the Sagrada Familia, in agreement with the previous geological reports for the project. These latest underground investigations were commissioned by the city council as requested by the basilica’s management of constructors and as part of the commitment to guarantee maximum security as well as transparency in the construction of the high speed train tunnel in Barcelona. The central court rejected a petition against the report for the project, which, according to the councillor for urban planning, further confirmed that the works should go ahead.
The raising of the groundwater level has been confirmed because of the possible barrier effect caused by the tunnel’s construction. Hence, a model of the entire city has been made, which includes the two Barcelona rivers (Llobregat and Besos), the Mediterranean sea and the mountain. The Catalan water agency set down the criteria and subsequently approved the hydrogeological report for the entire route. In the most difficult area, the difference between the maximum ascent and minimum descent is less than one metre, thus meeting the most restrictive possible criterion. This area is not near the Sagrada Familia, where the difference is less than 0.5 m.
Another report prepared by British engineers Guy Lance, of the London university, and John Anderson, of the Glasgow university, reviews 2,017 tunnels (8,750 kilometres) built in soft ground by TBMs or traditional methods in urban environment in about 100 countries since 1970. The data comes from the ITA. It reveals that 19 tunnels suffered serious problems (less than 1%). In 52% of these 19 incidents, “unknown and unpredicted ground” is to blame.
The report states that during the studied period, Spain leads the ranking in terms of projects, with 424 tunnels built (1,475 km), followed by Italy (205 projects, 928 km), China (170, 650), the US (105, 646) and Switzerland (61, 330).
The report also underlines that there are five main factors "that could become potential threats" derived from the characteristics of the subsoil: cohesionless ground above the tunnel crown, ground of variable stiffness at the face, heterogeneous geology along the tunnel path, aquifers and existence of cavities.
Auscultation
Because the route in the urban stretch of the high speed line in Barcelona follows the central axis of streets with 20 metres between facade and facade, the tunnel will mainly run under streets rather than passing beneath buildings. Therefore, the influence on the built-up areas will be very minor. The effect is expected to be imperceptible. This means that if cracks occur, they would be less than 0.1 mm. Under normal conditions, slight downwards subsidence movements of the terrain can be limited to a few millimetres by injecting cement mortar into the terrain. This will be done, without affecting the surface, from the lower part of the shafts built in the area that the tunnel will cross to create a mass of concrete.
The TBM is able to pass under buildings but because of the so-called El Carmel syndrome (a collapse on Line 5 of the metro in January 2005 although a tunnelling boring machine was not the method used), officials preferred to avoid doing so as much as possible.
Much of the tunnel will be built below the groundwater table. It will be constructed on very homogeneous terrain with good support characteristics. This means that the theoretical deformations of the project can be compared to the real values obtained at the start of construction and the calculation model can be adjusted to the reality in order to determine future situations, such as when it passes near the Sagrada Familia. It also allows cutting tools specific for this type of terrain to be used, which should not require frequent changes.
The route passes close to the Torre del Fang, the Sagrada Familia, Hospital Clinic, a number of listed buildings and La Pedrera, another of Gaudi's creations and the second most visited museum in Barcelona after the Sagrada Familia. The Catalan regional government and the city council have requested that the behaviour of the structures of La Pedrera and the other listed buildings be studied in detail and independently before works begin, despite the minimal expected effect.
Because of its unique load-bearing structure, the Sagrada Familia is the only special building along the route from the point of view of its physical interaction with the tunnel, due mainly to its height and the loads that the ground supports and will support. Hence special precautions have been taken to prevent any effect on the building.
As regards topography, 52 spots - with known coordinates and used as reference for all other works - have been established along the route. The tachymetric survey has been carried out in the area affected by the project on a scale of 1:500 and 1:200 to determine the current heights and gradients of the streets on the route. Also, 38 tachymetric measurements have been conducted in areas where construction works are expected to affect street surfaces (emergency exits, ventilation shafts, etc.) and information on junctions and special spots has also been gathered. The project also includes an inventory of all buildings near the route, which will be complemented by the prior inspection of the adjacent buildings.
Until 2011, the 5.6 km section along the Provença, Diagonal and Mallorca streets between Sants and La Sagrera will be closely monitored by a dense network of 5,072 sensors installed by Intemac. The entry shaft is situated in Mallorca Street, between Espronceda bridge and Navas de Tolosa. The exit shaft will be in Provença Street. In the shafts areas, 139 properties will have to be inspected. Visit www.intemac.es
This instrumentation will control the possible impact of the tunnel on the 533 buildings built along the route. The aim is to detect ground settlements and act immediately in case this happens. There are 10,000 homes in the area. To refine very accurately the possible effect of the project on the surrounding constructions, it is necessary to register all the data well in advance before tunnelling starts.
The monitoring system will be implemented at four places: the tunnel walls, into the surrounding ground between the tunnel and the surface, in the streets, and on the facades and roofs. A total of 39 main auscultation sections will be created, one each 150 metres and up to a depth of 40 metres. Also measuring instruments will be placed on the buildings every 15 metres and on the chamfers at the intersections with perpendicular streets, where two measuring spots (one on the hillside and one on the seaside) will be installed at 50 metres from the intersections.
The auscultation (measurement of movements) of the buildings and the Sagrada Familia already commenced, almost a year before tunnelling starts. Precious information on the movements of the buildings during each season of the year will be collected to detect possible movements caused by the change in seasons. This allows to differentiate between the usual movements of a building (due to temperature changes, for example) and those actually caused by the works. Factors like the cold, the heat or humidity have an incidence on buildings.
An auscultation protocol is set up to indicate the most relevant values of the measurements of the tunnel, terrain and buildings at any given time. By comparing them with the forecasted values, it is possible to instantly assess the safety level and keep the situation under control with a team of technicians working exclusively in this area. This protocol is accompanied by a contingency plan with pre-defined technical or communication actions covering any abnormal situation.
The instrumentation network consists of Soldata’s CYCLOPS (CYCLic OPtical Surveyor) system using
1,800 target prisms placed on the facades and walls of the buildings on which dozens of theodolites will make distance and volume measurements by means of laser beams. Technicians ensure that the CYCLOPS system can detect a movement of one millimetre at a distance of 100 metres. Visit www.soldatagroup.com
In the subsoil, 550 extensometers, 78 inclinometers, 86 piezometers and 261 pressure cells will monitor lateral and vertical movements, changes in density, variations of the groundwater pressure as the TBM progresses, and radial and tangential stress in the tunnel lining. 25/08.
