Alsina was involved in the full construction of the underground station, including the station’s 100-meter cavern, the 150 meters of pedestrian access shafts, and other structural components needed for this type of building (ventilation stacks, junctions, walls, columns, etc.).

Station cavern

The station cavern is an oval-shaped section measuring nearly 16 meters wide by 12 meters tall, with a total volume of 14,500 m³ requiring 3,200 m² of formwork. The concrete was poured in three stages: inverted vault, side walls, and vault. The construction process and the length of the sections to be concreted were key aspects in the final decision: a tunnel formwork system in a rented mine.

The construction technicians explained that “this solution is designed to withstand concrete overpressure in mining tunnels; in other words, the pumped concrete is confined between the excavated rock and the temporary structure, well exceeding conventional loads. Alsina’s combination of standard products for civil works (Multiform, Circular One-sided Wall, and high-bearing-capacity scaffolding) and custom pieces was what convinced the client, meeting the requirements for performance, finishes, and price.”

To build the arch of the inverted vault, Alsina supplied climbing equipment that could adapt to any position with circular formwork, pouring 30 m² sections each day. The side walls used One-sided Wall units over 6 meters in height, allowing for weekly pours. Finally, for the keystone, the company designed two 6-meter moving trolleys to be able to pour up to twice a week.

Pedestrian access shafts

Passengers can access the Bentaberri station either from horizontal pedestrian galleries or from two sloped galleries at an angle of nearly 60%. The solution for both was two 5-meter trolleys and 150 m³ of scaffolding, installing and removing the formwork with hydraulic jacks, and including a system for moving to the next pouring position.

Ventilation stacks and elevators:

The 70-meter pits excavated using the raise boring technique were clad in concrete using custom-designed metal formwork. The interior climbing system and the special circular mold would be raised in a single movement, allowing for daily pouring.

Description of the Alsina systems:

As well as some special custom-designed pieces, Alsina also used its One-sided wall, Multiform, Alsiply Circular and AR-80 Scaffolding systems for the support and formwork of the different sections of the project.

The Alisply Circular system for vaults allows for the perfect adjustment to the vault’s geometry. The circular wall system, adapted for vault formwork is resistant, versatile and is compatible with the Multiform product family, therefore capitalizing on the use of the equipment on site without the need for any special pieces. The formwork surface of the system is phenolic and therefore offers multiple advantages compared to metal surfaces, such as a lower weight, greater quality finish of the concrete, increased performance, and greater resistance to inclement weather.

On the other hand, Alsina’s AR-80 Scaffolding system is very versatile and is easy to assemble for building slab formwork. It consists of vertical support elements and a system of multidirectional interconnections, allowing it to support both mesh scaffolding and independent towers and affix auxiliary elements such as Access ladders and Safety platforms. It is a highly resistant multi-directional system, due to its high bearing capacity (up to 8 Tn/ft) that allows for any structural element in civil works to be scaffolded.

Finally, the Multiform System, formed by double UPN metal beams, wooden beams, gibs and aligners is one of Alsina’s most versatile systems as it allows for the “ad hoc” design of the necessary recoverable formwork, therefore meaning that it can be adapted to any geometry, whether straight or curved walls, one-sided walls, lintels or bridge decks.