The partner interview

What is the role of the Société des grands projets (Major Projects Agency - SGP)?

Jean-François Monteils — Established in 2010, SGP is developing the Grand Paris Express project: 200 kilometres of underground railway – the same length as the existing network – 90% of which is underground; 68 stations to be built, some of which will be more than 50 metres below ground level. From the very beginning, a major concern for SGP engineers has been the precise identification of the geological formations through which the project with its tunnel-boring machines passes. This identification has involved extensive geotechnical reconnaissance surveys since 2012, with nearly 12,000 boreholes drilled to date. No other project has achieved as much. SGP is a company with extensive expertise and experience in the construction of complex underground structures.

Why did you enter into a partnership with BRGM?

J.-F. M. — The Paris Basin, known to experts for its sedimentary and hence diverse geological composition, held some surprises for us despite this survey work, involving significant changes to our construction methods: the presence of pockets or lenses of sand in the Ypresian formations, steeper stratigraphic dips and the flooding of a station when a tunnelling machine bored into an aquifer. We therefore felt that a partnership with BRGM was crucial in order to benefit both from its database and from its researchers, PhD students and undergraduates who can provide an independent analysis of the project’s geological context using a 3D model, with which we can better understand the vast amount of information collected and hence the geological and geotechnical risks involved and the behaviour of the aquifers.

What can be gained from the data collected during the construction of the Grand Paris Express?

J.-F. M. — Apart from helping us carry out the project, the use of the samples is the second objective of our collaboration. The vast amount of data accumulated by the SGP is a legacy of the Grand Paris Express project which will provide valuable insights for many planners, developers and project sponsors. It is in the public interest to gradually share these data with BRGM’s France Géotechnique database, which is intended to be publicly accessible, and to use it for various research programmes with a view to working with academic partners to build a 3D geological model of the subsurface of Paris. This is what drives the work of our two institutions.