Karraskain Tunnel

    Karraskain Tunnel

    • Project – High velocity train tunnels in the Basque Country 
    • Location – Mondragón, Guipúzcoa, the Basque Country
    • Installation dates – September-October 2020
    • Owner – ADIF
    • Constructor – UTE Campezo-Comsa-Cycasa-Nortunel
    • Installation company – Aquaprotección
    • Type of works – Waterproofing of the vaults of the two 2,800 m tunnels
    • Product - RENOLIT ALKORPLAN Tunnel C 35034      - 2.00 mm thick covering a total of 76,000 m2

    With the aim of improving the connections between the Basque Country and the rest of Spain, it was decided to create a rail network to efficiently join the Basque capitals with the rest of the peninsula, resulting not only in the transporting of passengers, but also of merchandise by opening up to the Cantabrian Sea at the port of Bilbao. With this infrastructure being put into operation, the travel times between the three capitals will be reduced: Vitoria-San Sebastián by 60%, and between Bilbao-San Sebastián and Bilbao-Vitoria by 80%.

    To date, due to the successive changes in legislation and environmental regulations, the project has undergone various modifications. The current route will cross areas of high ecological value, such as the Aizkorri-Aratz and Aralar Natural Parks, thus being of particular importance in the reduction of the environmental impact. The final plan includes the construction of 44 viaducts and 23 tunnels.

    The most interesting one are those located in sector two. (Mondragón-Elorrio-Bergara). At this point the single route divides into two, to divert towards Bilbao and San Sebastián, creating a “Y-shaped” route, as the project is popularly known.

    The challenge

    The challenge

    The two tunnels are constructed using the New Austrian Tunnelling Method (NATM). This procedure offers economic advantages by making the most of the geological force, achieving tensional relaxation and decompression of the ground, with the objective of taking advantage of the self-supporting capacity. In this way, the support requirements necessary to achieve excavation stability are reduced. This work philosophy is made up of various stages:

    • Top heading excavation, in this case through perforation drilling and blasting. The blasting phase involves ventilation with the subsequent debris removal and scaling of the excavation face.
    Image 1. Industrial drill carrying out perforation tasks
    Image 2. Scaling of the excavation face using mechanical methods
    • Initial supports with a layer of shotcrete with metal fibers.
    Image 3. Shotcreting with metal fibers.
    • Placement of a mixed support system using trusses and bolts. Shoring with this type of reinforcement makes it possible to incorporate and unify the strain.
    Image 4. Bolting process
    Image 5. Trusses as support elements
    • Second layer of shotcrete with nylon fibers. These fibers reduce cracks caused by the shrinkage of the concrete.
    Image 6. Second layer of shotcrete

    Once the vault support has been completed, the bench excavation will be carried out, attacking mainly one of the sides, continuing with the rest of the support. Subsequently, this work must extend moving alternatively from one side to the other. One of the side walls has to be excavated before the other, with the aim of ensuring there is complete support on one of the sides. Once this phase has finished, the excavation of the inverted vault, the bottom drainage installation and the concreting of the supporting structure and the bearing blocks is started.

    The solution

    The solution

    The success of the tunnels’ usability would not be possible without them being perfectly waterproofed. The drainage and waterproofing offer major technical and economic advantages. The tunnels isolated from the water coming from the saturation of the surrounding or phreatic ground, have higher quality finishes and offer a reduction in maintenance costs. This barrier protects against corrosion, preventing damage to the cladding structure and to existing installations.

    In the case of Sector Two, the proposed waterproofing system has to divert the water, so it does not generate pressure on the structure. This will be directed to a longitudinal drain in the tunnel that will facilitate its drainage. The parts which make up this waterproofing method are as follows:

    • Placement of the geotextile. The geotextile will be held in place using PVC pegs and disks. This has two functions:
      • To drain the water from the ground over the vault, facilitating its evacuation and preventing it from creating pressure and acting on the PVC geomembrane.
      • To protect the geomembrane against punctures.
    Image 7. Installation of the geotextile with fastenings in the concavities.
    • The selected fastenings will be applied every 25 to 35 cm depending on the support. As it drops down from the vault to the side walls the pegs can be separated more, being placed 50 cm apart. These fastenings fulfil a dual function: on the one hand the hold the felt in place and on the other, to enable the sheet to be welded and thus fasten it in place too.
    Image 8. PVC disks.
    • Longitudinal installation of the PVC RENOLIT ALKORPLAN Tunnel C 35034    on the surface of the side walls and then around the drainage pipe. A homogeneous geomembrane has to be installed to allow it to be attached to the vault’s sheet. Firstly, it will be attached to the support above the level of the bearing blocks. The sheet strip will be wrapped around the lateral drainage pipe, which will be embedded into the bearing block.
    Image 9. Drainage pipe with a longitudinal strip
    • Waterproofing of the vault. The RENOLIT ALKORPLAN Tunnel C 35034    geomembrane must be fastened in place using flat PVC disks pinned into the support concrete, which were previously placed to hold up the geotextile. These disks will allow the geomembrane to be thermal welded onto the disks, across the entire surface of the support and leaving spaces that allow it to adapt to the geometry of the excavation.
    Image 10. Welding works on the mobile tubular frame scaffolding system.

    Elongation at break values of the RENOLIT ALKORPLAN Tunnel C 35034 of around 300-350%, will be vital to achieve a perfect adaptability to the support. Overburden excavation is usually common in hard rock tunnels; therefore, the elongation of the geomembrane plays an important part in the concrete cladding phase. During this phase, the concrete will put pressure on the geomembrane, causing it to come into contact with the support, thus making it important that the sheet can stretch until it reaches it.

    Images 11 and 12. Examples of the overburden excavation and the adaptation of the RENOLIT ALKORPLAN Tunnel C 35034 geomembrane

    The thermal welding is carried out using a hot-air welding machine with a test channel. There are three factors that influence its proper fusion: temperature, velocity and pressure. The correct calibration of the equipment will enable the teams to ensure that the join has been carried out correctly. 

    Image 13. Automatic welding machine
    • Welding Test. The welded seam will have a minimum overlap of 8 cm. The control check for each welded seam will be subsequently carried out by means of a compressed air test, under 2 atm pressure for a duration of 15 minutes, allowing for a loss of 10% due to the flexibility of the RENOLIT ALKORPLAN Tunnel C 35034    geomembrane, as set forth in the UNE 104 481/3 3 regulation.
    Images 13 and 14. Quality control, carried out by means of a needle and pressure grips.
    Image 15. Completely executed waterproofing
    The result

    The result

    • Cladding, formwork and subsequent concreting: the lining of a tunnel constitutes a structure in itself, in direct contact with the cavity or with the previously placed support. This structure is put in place to ensure resistance and the waterproofing, as well as for esthetic and functional reasons.
    Image 16. Formwork carriage
    Information Enquiry

    After years of waiting for the high-velocity railway connection between the Basque Country and Madrid, the works are under way and the light at the end of the tunnel is in sight. The investments in infrastructures have implied effort made by public institutions and by extension, their taxpayers. Extending its life cycle represents an economical and environmental achievement.