Lyon-Turin Rail Line, Avrieux ventilation shafts

Context

The Lyon-Turin rail line is part of the programme to improve passenger traffic and develop European economic exchanges, for a sustainable and responsible transition in transport, by shifting long-distance road transport to rail.

The aim of the Lyon-Turin line is to relieve the Alpine roads of one million heavy goods vehicles by 2030, thereby reducing greenhouse gas emissions by around 3 million tonnes of CO₂ equivalent. More broadly, the aim is to improve European links via the Alps.

The work package for the ventilation shafts and safety caverns at Avrieux is a major challenge for completion of the line. It is part of the essential preparatory work for the construction of the tunnel, which should significantly reduce the pollution caused by road traffic in the valleys, by allowing the base tunnel to be ventilated throughout the digging phase.

Meeting the delivery dates for the construction of the ventilation shaft at Avrieux is essential for work to continue on the base tunnel, which is expected to operate for 150 years.

Technical overview

The owner, TELT, has awarded the contract for preparatory work on the future safety site at Modane to the consortium led by Dodin Campenon Bernard. The 4 shafts at Avrieux, each 500 m deep, were dug using the raise-boring method.

This raise-boring technique optimises the control of excavation costs and timescales; it considerably reduces the impact of transporting excavated materials and provides a high level of safety for the teams, particularly compared with conventional blasting methods used for large caverns and galleries.

It involves digging a pilot hole from top to bottom, 40 cm in diameter, using a method tried and tested by tricone and RVDS (rotary vertical drilling system), with a remarkable deviation accuracy of less than 5 cm over 500 metres of drilling. The drilling tool is replaced by a reamer that moves upwards from the bottom. The cutting wheel of the tool has a diameter of 5.20 m.

The material torn from the rock is then recovered at the base of the borehole and loaded onto earth-moving machinery for removal. A lift is used to descend and install all the materials and systems required for the support: shotcrete, bolts, reinforcement, etc. A sliding formwork extends from the base of the shaft, allowing the entire structure to be sealed with reinforced concrete. This method makes it possible to dig tailor-made caverns: the Avrieux caverns are up to 22 metre-high and 23 metre-wide.

The project draws on the expertise in underground work gained by the consortium partners on public transport projects in multiple countries. Water management is an issue that has been shared with the stakeholders, and the municipality of Avrieux has undertaken to provide the necessary water resources for the project via the construction of reservoirs and UV treatment units.

Impact

The first impact is environmental, with a drastic reduction in the pollution generated by cross-border traffic. The objective improvement in the quality of life of those living in the valleys will be real, with the elimination of a million vehicles passing through the region every year.

The second impact relates to the overall progress of the Lyon-Turin line project, which also depends on the quality of ventilation during work on the base tunnel. The shafts and caverns at Avrieux ensure that the tunnel boring machines can operate safely at a depth of 500 metres.

Lastly, the impact on water resources is controlled, as are 135 other parameters (mineral dust, presence of gases such as radon, air quality, etc.) which are constantly monitored by measuring stations and sensors spread throughout the site.

This excavation project, which is exemplary in the way it systematically measures its environmental impact, will bring significant short and medium-term economic benefits to the region and its residents through the jobs created.

It will have a major commercial impact, as it will enable VINCI Construction Grands Projets to make the most of its vertical digging skills through raise-boring.