Satellite monitoring for London 30 Nov 2017

Ground motion monitoring using satellite technology is often used when planning large scale tunnelling projects like those associated with the Crossrail train line in London, allowing engineers to monitor ground deformation before, during and after construction.

Tunnelling projects can face geotechnical and geological hazards in the form of subsidence, clay shrink-swell, voids, faults, shallow groundwater, and pre-existing infrastructure, all of which have the potential to halt or delay the project during its planning, construction and operational phases.

Fig 1. PSI displacement across central London (each image 15km across)

In London, underground space is at a premium, and the importance of understanding historical ground deformation, the status of current deformation hazards, and the long-term impact of engineering works has never been greater.

During the last 35 years London has seen many large infrastructure projects, such as development for the 2012 Olympics, and more recently Crossrail. Monitoring of these works has been critical to ensuring the integrity of existing underground infrastructure as well as buildings of national importance in the vicinity.

Interferometric synthetic aperture radar (InSAR) satellite monitoring technology is increasingly being used for tunnelling projects due to its ability to map past and present ground stability with high precision across tunnel alignments and the surrounding area. InSAR is useful throughout the lifecycle of a tunnelling project, from planning and route selection, through to construction and operation as it can provide historical and contemporary measurements at all stages.

For urban tunnelling projects, persistent scatterer InSAR (PSI) is used to provide high-precision measurements over a number of years. It identifies places that reliably and persistently reflect the radar signal back to the satellite during each orbit. Referred to as persistent scatterers, these locations typically correlate to fixed man-made structures such as buildings or bridges.

Since the 1980s there have been close to 28 major tunnelling projects in London, five of which are depicted in Fig 1.

• Pimlico and Wandsworth to Wimbledon cable tunnel (1992-1995),
• Jubilee Line Extension (1993 to 1999),
• Channel Tunnel Rail Link (also known as High Speed 1 – 2001 to 2005),
• National Grid power tunnels (2011 to present) and
• Crossrail (started in 2012 and due for phased completion in 2018/2019).

Satellite data has been processed to highlight key changes during 1992 to 2001 (A), 2005 to 2010 (B) and 2010 to 2015 (C) respectively. Fig 1 shows the mean displacement rate maps for each era, where green indicates stability of ±1.5mm/year; red indicates ground motion away from the satellite (i.e. subsidence) of >5mm/year; blue indicates ground motion towards the satellite (i.e. uplift) of >5mm/year.

Fig 2. A&B PSI ground settlement measurements 2010 to 2015 (A-Bond Street Station B-Whitechapel Station C-Stable location in Lambeth close to Waterloo Station)

Fig 2 shows the Crossrail tunnel alignment and the mean displacement rate map for the period spanned by the Radarsat-2 data (2010 to 2015), using the same displacement thresholds as the data in Fig 1.

• Label A and its associated time-series graph relates to tunnelling at Bond Street station. The red section of the graph shows the period of tunnelling and associated ground settlement. Approximately 12mm of settlement occurred during tunnelling, with settlement continuing after construction.
• Label B and its associated time-series graph relates to tunnelling at Whitechapel station. Approximately 20mm of settlement occurred with a slowing in the rate towards the end of construction, however unlike Bond Street, there is a clear levelling off of the displacement rate towards the end of the construction.
• Label C and its associated time-series graph relates to a stable reference location in Lambeth, close to Waterloo station. During the same period of time as that shown for Label A and B this location remained approximately stable.

InSAR is invaluable for mapping and monitoring ground deformation before, during and after tunnelling activities across London. The unique time series of results that span 1992 to 2015 highlight the benefit of using PSI data throughout the lifecycle of a tunnelling project, from wide area, synoptic surveying along the entire alignment, through to discrete measurement monitoring at higher risk locations.

The recent availability of public domain Sentinel-1 satellite imagery, which is currently being collected every six days across London and other major cities, will greatly enhance our ability to use InSAR during the lifecycle of tunnelling and other infrastructure projects.