Satellite ground monitoring for Paris Metro works 31 May 2018

Over the coming years to 2030, the Grand Paris Express project in France will gradually bring into service 68 new underground stations on 200km of new metro tunnel lines to expand the current metro system of the French capital. As an unprecedented programme of urban development, the project is a fundamental rethink and redesign of the public transport network to provide greater Paris with a more integrated multimodal transport solution.

Fig 1. InSAR base line data for the Line 15 western section

From the earliest stages of the project, the Société du Grand Paris, the public agency set up by the French Government to deliver the Grand Paris Express vision, gave priority to a close surveillance of the construction works in order to minimise any potential impact. The design and the construction progress of the project is driven by risk management and the reduction of disturbances to urban neighbourhoods.

Among the monitoring methods implemented, TRE Altamira, as part of the CLS Group of Spain, was commissioned in 2015 by the Sociéte du Grand Paris to monitor from space the impact of the underground excavations operations along its route to the nearest millimeter. The satellite-based solution measures ground movement by processing satellite radar images acquired over the same area in different points in time.

The monitoring work started with a retrospective study of the ground movement over the 200km route of the Grand Paris Express network. Historical ground movement in the area was mapped and an extensive inventory of the ground surface behavior, from April 1992 to March 2015, was provided, offering complementary information for the identification of vulnerable structures before the start of any work. This mapping was generated from the processing of archived images acquired since the early 1990s to 2010 by medium-resolution satellites operated by the European Space Agency, and by high resolution images captured by the Italian and German Space Agency satellites for the more recent period of 2011 to 2015.

The outputs of this satellite historical study are data that can be combined with historical maps and conventional geotechnical surveys to detect ground movements due to the evolution of quarry operations and possible slope instability. In zones where clays are found, as is the case for different Grand Paris Express project lines, it would allow for the detection of soil movements caused by the swelling of the clays.

TRE Altimera engineers on the project in Paris are currently working in cooperation with Setec of France, the design and construction manager of the western section of Line 15, the first section of the network to be built, and with French company Gauss Monitoring, which has the responsibility for specific surveys of sensitive buildings along the Line 15 western section route (Fig 1).

The analysis of the historical and real time current data, focuses on the amplitude of the movement accumulated over time. For each measured point, it is possible to see at what period the displacements have been initiated, and whether these displacements are stabilised or still active. These observations are crossed with existing knowledge in order to understand or better manage the cause of the movement, and to qualify their potential impact on the project at the time of tunnelling.

Monitoring activities will continue during the whole project to detect and measure any ground displacements in the area with the ultimate aim of keeping the prime contractors fully informed of the impact of tunnelling and underground excavation activity on the surface and for operations to be adapt accordingly.

An overview of the importance of the satellite monitoring operation and the work achieved to date on the western section of the Line 15 project was presented during a technical session of the World Tunnel Congress (WTC) in Dubai in April this year (2018).