It is considered that the development of a large public infrastructure, the high-speed train tunnel, could threaten the structural stability of the Church of Sagrada Familia. The World Heritage Committee at its 32nd session (Quebec City, 2008) requested that the State Party halt the construction of the tunnel section in the vicinity of the Sagrada Familia and consider changing the route of the proposed high-speed train tunnel to avoid any potential adverse impact on the structural stability of the Church. The World Heritage Committee also requested the State Party to submita detailed report on the measures taken to ensure the protection of the property.
On 22 January 2009, the State Party provided a detailed report describing the technical solution for the segment of the Sants-La Sagrera Tunnel in proximity to the Sagrada Familia Cathedral. The purpose of the report was to describe and demonstrate in detail the measures that have been adopted for the tunnel construction project with the purpose of safeguarding the integrity of the Cathedral. As background material, the report presents an administrative record of the project, details of the technical analysis carried out to date, contact details of four experts who were consulted, and the results of detailed contacts with the representatives of the Building Committee of the Cathedral.
The technical analysis was undertaken to guarantee the technical feasibility of the proposed solutions to allow the construction of the tunnel to occur in the immediate vicinity of the Sagrada Familia Cathedral. It presents the evidence of the geological-geotechnical studies that were carried out; details behind the selection of the intended construction procedure using an earth pressure balance boring machine; details on the construction of the intended tunnel lining; the methodology and study of the potential for ground settlement; a study of the effects produced by vibration, and what is intended to be adopted to ameliorate these effects; details of a hydro-geological study in order to assess any possible casual effects on the level of the water table; and details of the ground survey plan which defines the type of measurement instrumentation required, and the placing of these, to monitor a variety of real measurements as work progresses in order to ensure that the movements are within predetermined quantitative limits.
The report also states that all of the consulted renowned experts confirmed the technical feasibility of the proposed solution. In addition, contact with the representatives of the Cathedral Building Committee was made to obtain the best possible background information for the project, and with the purpose of determining a solution based on consensus of all parties involved.
In offering a description of the solution which is to be adopted to minimise the impact of expected surface settlement in the vicinity of the Cathedral, the following characteristics have been determined: A tunnel boring machine is to be used for the excavation work, the railhead depth is located at 33 metres, while the crown of the tunnel is located at a depth of 25 metres, a retaining wall of casing piles, with an overall length of 230 metres, will be created on the side of the tunnel in closest proximity to the Cathedral, and the construction of an emergency exit is planned at the intersection of calle Mallorca and calle Sardenya diagonally across from the Cathedral.
The report emphasises that, due to the unusual loads transmitted by the Cathedral to the surrounding ground, during the preparation of the project special attention will paid to the tunnel segment passing in near proximity to the Cathedral. In addition, particular attention was paid to the analysis of the surface movement and a number of options were studied to minimise, as much as possible, the potential of surface settlement at this point on the tunnel route.
As a result of analysing four different scenarios, the construction of screening piles on the side of the tunnel in closest proximity to the Cathedral was considered to be the best solution. It is intended that the piles will have a diameter of 1.5 metres and have a 2 metre separation between their centres. The piles are joined at the head by a beam with a thickness of 2 metres which, in turn, rests on a concrete blocks measuring 3 metres by 3 metres, resting on ground treated with an injected consolidating material. Noting the success of this solution in similar circumstances elsewhere, it is argued that this approach will lessen movement at the head of the piles (and in the immediate vicinity of the Cathedral).
From computerised theoretical and technical studies, it is expected that the level of vibrations from passing trains is within the restrictive levels required by legislation. In addition further mitigating measures, involving the installation of elastomers along the immediate length of the tunnel between the slab and the tracks, will further reduce the propagation of vibration.
The report concludes that all necessary administrative steps and technical analysis were implemented for the tunnel project. It also indicates that the study findings were satisfactory in all respects with special attention being paid to the results in the proximity of the Sagrada Familia Cathedral.
In addition, in order to ensure the integrity of the structures surrounding the work site an inspection of buildings in the proximity of the tunnel is currently being undertaken. Combined with proper monitoring protocols, in addition to the supplementary measures aimed at minimising the effect of the tunnel boring machine, it is concluded that the project is technically feasible. The submitted report concentrates on technically proving that the proposed alignment of the tunnel, its construction phases, the tunnel boring technology that will be used, and its eventual train running operations will not cause any significant damage to the structure of the Sagrada Familia. No mention is made in the report of the possibility of halting the construction of the tunnel section in the vicinity of the Cathedral, or of the possibility of re-routing the tunnel further away from it.
Two staff of the World Heritage Centre had the occasion to visit the Cathedral jointly with the Chief Architect and the staff responsible of cultural heritage of the UNESCO Catalunya Centre during their mission to the World Conservation Congress (Barcelona, October 2008) and were informed of great concerns by the Chief Architect of the Cathedral about potential impacts due to vibrations and the instability of the ground.
The World Heritage Centre and the Advisory Bodies consider that while the computerized studies may show a potential to bore a tunnel at this location, it should be a precautionary principle to avoid any potential threats to the World Heritage property. They encourage the State Party to consider alternatives and discussions with the responsible authorities of the project to avoid irreversible damage to the monument and any impacts on the Outstanding Universal Value and integrity of the property.
The World Heritage Centre and the Advisory Bodies further note that no overall planning schedule or overview maps of the entire tunnel crossing of the city of Barcelona has been provided.
ICOMOS experts reviewed the technical material provided in the State Party report as a desk analysis. The construction techniques used by Gaudi for the Sagrada Familia reflect technology almost forgotten at the time: The Hangig Model which uses cords. It is known that such structures are very sensitive in case of differential settlement because of the change of geometry and thus the reserves of the structure are very limited. The Sagrada Familia, being a unique structure, should not be expected to be safe when vibrations are held within tolerances established for contemporary structures. Again such tolerances should not be left to computer simulations alone: in situ tests are also necessary. Detailed conclusions of the desk analysis have been sent to the State Party.
The only way to safeguard the Sagrada Familia with any degree of certainty is to move the location of the tunnel. Assuming however that such a recommendation is not feasible due to the planning and capital that has thus far been expended, it is essential that the proposed Tunnel Boring Method in connection with the Earth Pressure Balancing method should be described in more precise detail, and, as installation of the proposed retaining wall in itself may use damaging vibrations, this part of the project should be reviewed and monitored with the same detail as the tunnel installation.
Overall, it would be desirable for tunnel experts to carry out an independent check of the proposed solution.