The Basque Y High Speed Rail (HSR) network aims to connect three regions of northern Spain: Vitoria, Bilbao and San Sebastián. The Basque Y includes 74 tunnels with a total length of 105km, and 88 viaducts of 25km, comprising 63% and 18% of the line respectively.

The project involves the construction of 74 new tunnels and the enlargement of three existing tunnels, which are part of the upgrade work to the conventional track connecting the new HSR with the city of San Sebastián. Due to their large number and the great variation in their lengths, there is also a variation in the depth of the tunnels; for example Udalaitz West reaches almost 550m in its deepest part.

The new rail network is one of the 14 infrastructure projects chosen as a priority by the European Union at the Essen Summit in December 1999 and will help the Basque Country to form a new railway link with Europe.

ADIF, the Spanish state-owned company, is responsible for the construction, although the Bergara-San Sebastián Branch is the responsibility of the the regional Railway Administration (ETS).

The project is being financed by the Government of Spain and the European Union, by means of the Trans-European Network (TEN) and Connecting Europe Facility (CEF) programmes.

The estimated cost for the complete scheme is about EUR 3.6bn (USD 4.04bn). The number of tunnels that have been completed as Tunnels and Tunnelling goes to press comprise 78% of the total length of tunnels along the line.

Two sections near Bilbao and other two near San Sebastián have been delayed due to administrative issues, but the completion of civil works is expected to be by the end of 2022.

The main track gauges used in Spain are the “UIC gauge” (1,435 mm) and the “Iberian gauge” (1,669mm). The HSR will be equipped with the first of them and in the Astigarraga-Irún stretch a three rails track will be mounted, allowing both HS and commuter traffic.

The restrictions for the alignment of the tunnels are the same as those for the design of the whole line, both in plan and in elevation. The main requirements for tunnels involve housing the required gauge as well as a design aim to minimise the aerodynamic disturbance of the train at full speed.

The HSR Vitoria Bilbao San Sebastián will connect three basque capitals by the construction of a double UIC gauge track laying over a 14m wide trackbed. It is designed for both passengers and freight traffic and a line speed of 250km/h, although there are some specific restrictions as shown in Table 1.

The construction of works has been divided in 36 sections, making 139km, and joined in two different sections:

  • Vitoria-Bilbao Branch and Mondragn-Elorrio Bergara Knot (80km length, with construction managed by ADIF)
  • Bergara-Astigarraga Branch (59km length, with civil works managed by ETS although the overall project’s construction is managed by ADIF)

The line will have new stations in the three capitals, as well as another one at Ezkio/Itsaso (Bergara-Astigarraga branch). A track-gauge changing device will be constructed near to Vitoria. Three passing-loop and stabling sites have been designed, two in the Vitoria-Bilbao Branch (Aramaio and Amorebieta) and another in the Bergara-Astigarraga Branch (Ezkio, which coincides with the station before mentioned).

There will be three electrical substations, near the city of Vitoria and the towns of Amorebieta and Astigarraga. The separate data for each section are shown in Table 2. The majority are double-track tunnels, with an 85sqm cross section, and only three (Albertia, Udalaitz II and Zumárraga) are twin-tube, with a 2 x 52sqm cross section.

Due to the mountainous nature of the terrain the line goes through (a rapid succession of small valleys and mountains) the project includes a lot of consecutive tunnels and viaducts of relative short or medium length.

Therefore, although TBMs were considered at the beginning of the project, the idea was rejected. Drill and blast has been chosen, following the principles of the NATM, with excavation proceeding in two phases, heading and bench.

The majority of tunnels have been excavated (others will be excavated) with drill and blast, except for the Astigarraga-Irún tunnels, which need to be widened.

According to a spokeperson of the Subdirectorate of Construction II of Adif AV, some enlargement operations are important to maintain, at least partially, the rail service. “As the line between Irún and Astigarraga has a very high occupancy rate, it is important to keep as much capacity as possible,” says a spokesperson for Adif AV.

“The TBM has been chosen by the contractor to allow the trains to work at the same time.”

The Herrenknecht Tunnel Enlargement System

The TBM has been developed by Herrenknecht, which adapted its prototype to the needs of the Astigarraga-Irún branch, served by electric locomotives. That’s why a specific isolating system has been designed.

The Herrenknecht Tunnel Enlargement System machine (TES), which consists of four modules that are assembled together.

An excavation and shotcreting unit has three multifunctional arms with fast interchangeable tools to facilitate both operations. Two rails on each sidewall allow the movement of the TES along the tunnel.

The TES is a protection gantry for railway operations and an equipment carrier for excavation and support works.

By using the machine technology, contractors can enlarge and re-line railway tunnels in one step. The TES is designed as a self-propelled system. By means of a walking mechanism it pushes itself forward step by step on temporary auxiliary foundations placed in the tunnel. It has a total length of 34m.

The TES is divided in four functional parts: pre-support unit for supporting the existing (old) tunnels; an excavation and support unit; a drilling unit for ground consolidation with a pipe umbrella; and an equipment carrier.

The TES machine started to enlarge the Gauntxurizketa tunnel, but it isn’t operational as Tunnels and Tunnelling goes to press due to administrative reasons. During the first 200m of excavation, the advance rate was 11m per week.

Ground Conditions

The main geological formation of tunnels includes sedimentary materials that belong to the Urgonian Supraurgonian complex (Cretacic period). Mudstones, sandstones and limestones are the main rocks in the area. There are several geological risks associated to this type of materials such as squeezing; ravelling and the presence of karsts have a very high likelihood of appearing.

All the geological investigations have already completed, except for the tunnels in the central area of the Line, called “Bergara’s Knot”. The construction of these tunnels has not begun. According to the Rock Mass Rating (RMR) developed by Bieniawsky in 1989, six parameters are used to classify a rock: uniaxial compression strength, rock quality designation (RQD), spacing, condition and orientation of discontinuities and groundwater conditions.

This index can vary from 0 to 100 points. Depending on the punctuation, we’ll have the rock quality and the support requirements.

There is a wide range of rock strengths depending on the lithology; mudstones can reach 10MPa, the limestones of the Udalaitz tunnel reach 80MPa (see Table 3).

“The higher the index is, the less support requirement you have,” explains the spokeperson of Subdirectorate of Construction II of Adif AV. “Gunite, bolts and metallic trusses are the main elements used for supporting the tunnels.

“A lining reinforcement is sometimes needed when the support elements are not enough for guaranteeing the tunnel stability.”

There have been necessary different ground treatments during the construction works; such as:

  • Resin injection for segmentation between watertight sections, in order to prevent drainage in tunnels excavated in karst limestones;
  • Waterproofing injections, and the maximum water flow registered during the construction of the Udalaitz 1 tunnel was 486cbm/h.
  • Consolidation grouting, so as to allow passing under existing Dump
  • Filling of holes in karstic ground
  • Heavy micropile umbrellas both at the portals and inside karstic terrain
  • Temporary inverts in tunnels with squeezing phenomena;
  • Grouting stabilisation treatments in subsidence affected areas;
  • Two-component resin filling in sinkhole affected areas The deformation suffered from the materials excavated in a tunnel need to be monitored in order to ensure the stability of the excavation and the workers’ safety.

Monitoring instruments like portable extensometers and topographic equipment have to be placed regularly every 15-25m once the excavation has begun. This monitoring is based on the tunnel convergence measurements and topographic control of crown descents.

Except those controls, in critical areas where the geotechnical features are complex (faults, squeezing or swelling presence, etc.) a specific instrumentation is required, like:

  • Extensometers to check the ground deformation at several distances of the tunnel
  • Pressure cells to monitor the stress between ground and Lining
  • Inclinometers so as to check the stability of the slopes at the pre-portal excavation.

The main challenge is the large number of tunnels and the different conditions related with its excavation (karst terrains, squeezing phenomena in mudstone formations under great elevations, digging under water table) and, in the other, the restrictive environmental conditions in regard to the landfills management.

Depending on the type of construction method chosen for the tunnels, the spoil is transferred directly from them or from slight intermediate stockpiles to the landfills.

These landfills have been chosen taking into account the environmental restrictions determined by the Environmental Impact Assessment and minimising the transport distance from the portals of the tunnels. There is a wide range of capacities, reaching 1.5M cbm, with the average volume about 750,000cbm.

The main restrictions come not only from the orographic conditions that the tunnels have to be constructed through but from other environmental restraints, such as:

  • Atmospheric quality protection (limiting dust, noise and vibrations).
  • Soil protection (picking up and maintaining topsoil for further use).
  • Hydrological protection (treating process waste water before discharging into the river).
  • Fauna protection (this s a very relevant issue due to the important restrictions that must be considered, both in several design aspects and in the construction timeline, in order to prevent damage to some endangered species).

All the tunnels on the line have a waterproofing system that consists of a PVC membrane and a protective geotextile layer between it and the shotcrete lining; a drainage pipe enclosed by this geotextile collects the water, discharging it into the main side drain.

The shotcrete lining varies depending on the rock classification assigned by means of the RMR; typical thickness range between 250mm and 450mm. The standard cast concrete lining is 300mm thick.

A ventilation system is required during blasting and digging phases in order to release fresh air into the tunnel and to clean out the harmful gases.

This system isn’t necessary once the excavation has finished and the lining starts. A regular measurement of air quality and dangerous gases absence takes part of the Health and Safety team work. The health and safety of the workers and of all the people involved in the project has the highest priority during the construction of the HSR. Each of the projects of the different sections includes an in-depth Health and Safety Study, where the activities to be developed during the construction phase are analysed, the possible risks involved in the construction are identified and, finally, a series of preventive measures are established.

Furthermore, each contractor must develop, prior to the start of the works, a detailed study based upon their own methods to identify any improvements or possible faults in process. In this way, the project hoped to get the best possible start.