Focus on Saint-Martin-La-Porte
This service gallery is the first structure on the tunnel’s trajectory travelling from west to east. It descends to the level of the future base tunnel 80 m below. With a length of 2400 m, it lies at an altitude of 695 m and has a gradient of 8%.
Built between 2003 and 2009, this gallery serves a double purpose: to survey and describe the different types of geology encountered by test bores during excavation, and to provide access to the base tunnel of the future trans-frontier section.
In 2015, a tunnel boring machine began excavation of a 9 km stretch of tunnel along the definitive trajectory of the tunnel at the required diameter of the future base tunnel.
Difficult geology
The formations traversed in the Houiller Productif formation were mainly sandstones but presented local faults of considerable extent, filled with loose, highly-deformable material.
The operations at Saint-Martin-La-Porte therefore represented a new phase in the construction of the main tunnel of the trans-frontier section, where the geotechnical feasibility of crossing the Houiller Productif formation remained to be proved.
A tunnel boring machine excavated a 9 km section of the south tube of the base tunnel to link the existing service galleries of Saint-Martin-la-Porte and La Praz, plus a 1.3 km section from the core of the daunting Houiller Productif formation. The objective of these tasks was to verify hypotheses on the geological formation and acquire the experience necessary for the excavation of the base tunnel in the Houiller Productif zone.
Egis in charge!
As main contractor, Egis is in charge of design and construction supervision in this stage of the project. We are responsible for incorporating the design modifications imposed by the geological and geotechnical complexity of the project, plus aspects relating to environment, quality, safety, budget and completion schedule.
Our expertise has contributed not only to the success of stages SMP1, 2 and 4, but also to the confirmation of the feasibility of the TELT project as a whole: excavations have been the most complex ever undertaken, not merely in France but anywhere in Europe.
Measures for the environment
Several important environmental measures have been taken in this project stage: relocation of protected species, regular air quality measurements, reduction of dust generated by work, water preservation with a plant for the treatment of waste water from site activities, landscaping of depot sites, valorization of excavated earth under a circular economy model for the disposal of excavation debris with sorting directly performed by an online analyser, and continuous communication with local authorities and residents.
Given the significant volumes of earth to be excavated (over 1 million cubic metres), one of the biggest challenges was to minimize the pollution caused by the rotation of trucks carrying earth from the excavation sites to storage sites. To address this problem, an original solution was put in place: the earth was directly carried to the storage sites via a conveyor belt.
