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Instrumentation and monitoring: Millimetre precision potential

Satellite control has revolutionised earthmoving operations and work is now underway to enable a similar step-change for remote deformation monitoring of civil engineering structures.

Placement of instrumentation and monitoring equipment around major civil engineering projects could become a thing of the past as a result of some new research currently underway. The work could see satellite technology harnessed to give true remote monitoring information for the structure under construction, as well as neighbouring structures, with millimetre accuracy.

The research focuses on the use of satellite interferometric synthetic aperture radar (Insar) to remotely assess small ground movements in geotechnical or structural infrastructure. The project is being led by Queen’s University Belfast together with the British Geological Survey (BGS) and the Geological Survey of Northern Ireland (GSNI).

Maidenmount Salt Mine Collapse UAV 10 5 16

Images taken on a drone flight over one of the salt mine collapses at Carrickfergus enabled 160 images to be combined as a composite using specialist photogrammetry software.

According to the project team, satellite radar data is available from 1992 onwards and can be used to either assess historical ground movements or alternatively monitor current movements or ground subsidence.

The research is being carried out in conjunction with Transport NI (TNI), Northern Ireland Rail (NIR), and the Department of Trade and Industry (DETI). According to the project team, each of these partners have one common need which this project addresses: they are required to monitor ground deformations across their geotechnical assets using the most efficient, cost effective methods, with a view to minimising and managing the geotechnical risk to their businesses and the road/rail users.

Arup is also working with the project team to help embed Insar monitoring techniques in its organisation to improve methods of managing geotechnical risk.

Queen’s University Belfast senior lecturer David Hughes has said that the intention is that this project will demonstrate the many benefits of using Insar. “Such benefits include coverage of large and remote areas not easily accessible on foot and consistent accuracy of the measurement of small movements over long periods of time,” he says.

Site evaluation

During the work, which is funded by the NERC under the Environmental Risks to Infrastructure Innovation Programme, the project team will examine landslide instability and subsidence at four major sites.

The objective of the work is to apply the methodologies that the BGS has already developed through past research projects of assessing the deformation of geotechnical infrastructure, such as slope movement or ground subsidence, using Insar. The project will validate this methodology through ground truthing, using geotechnical monitoring and high resolution photogrammetry developed by the university.

One of the sites to come under scrutiny is in North Belfast along the edge of the basalt escarpment. Historically this area has been subject to shallow translation landslides and evidence of movement can be seen in Ligoneil Park and along the Antrim Road.

The second study site is on the railway line between Belfast and Bangor where sections of the track are in steep sided cuttings prone to instability, particularly after periods of heavy rainfall. NIR hopes that the analysis will enable it to correlate the slope instability against rainfall data for the rail route.

The third site is the abandoned salt mine workings around Carrickfergus, which have caused huge crown holes to appear at various locations around the town as a result of collapses. This area is currently monitored by DETI using ground based survey techniques.

DETI anticipates that the project will validate new methods of monitoring and provide baseline data of ground motion to form the basis of future strategic decisions in regards to geohazards. At Carrickfergus, the Insar project could potentially provide greater knowledge of extent of subsidence boundaries and provide indicators to potential catastrophic collapse by analysing the Insar data against periods of rapid ground collapse.

The fourth site is at Straidkilly on the Antrim Coast Road where the road cuts through soft Jurassic clays and debris from the slide area has frequently reached the carriageway leading to road closures.

TNI anticipates that the use of Insar data will help form their strategies for monitoring their geotechnical assets and will feed into the existing GIS based risk assessment methods for their infrastructure assets. The site at Straidkilly is only one of many sections along the A2 coast road that is unstable and it is hoped that this project will give a much greater insight into the behaviour of a variety of geohazards that impact on the road and will inform their maintenance strategies and lead to more cost effective better targeted maintenance.

TNI is also committed to developing a better understanding of the mechanisms of failure on the slow moving failures on the Throne Bend in Belfast and the Insar data will allow a much better correlation between slope movement and rainfall intensity and duration to be undertaken. Insar data will also allow better mapping of the extent and magnitude of the instability.

Huge potential

Fig london

Ground subsidence following the excavation of the Jubilee line extension in London between 1992 and 2000 shows the power of Insar analysis

According to Hughes, one recent example of the power of this technique is illustrated by the ground subsidence following the excavation of the Jubilee line extension in London between 1992 and 2000. Analysis of satellite data shows up to 15mm per year of surface subsidence which coincides with the line of the Jubilee line tunnel. The findings of this analysis was reported on in the Journal of Pure and Applied Geophysics last year by BGS senior remote sensing geoscientist Francesca Cigna (and others) who is part of the current research team.

Through this project the team hopes that the stakeholders will be able to monitor ground deformations in a more cost effective, efficient, more thorough and more robust way. The team believe that embedding the use of this methodology across an organisation will make a step-change in the assessment and management approach to resilience of geotechnical infrastructure.

Hughes believes that there are many applications of the technique beyond those the team is working on with the project partners and, with work set to conclude on the 18 month-long project in July next year, the benefits should soon be clear. “It may not be too long before we can measure road surface deformation (rutting) or embankment subsidence from these satellite data,” he adds.

What is Insar?

In sar big

Put simply, satellite interferometric synthetic aperture radar (Insar) works by scanning the Earth’s surface from various satellites using radar frequencies. The scans are taken at regular intervals and the Earth’s motion or deformation can be inferred by the examining the difference between successive scans. Rates of movements of sub millimetres per year can be achieved provided strong radar reflections are achieved from the Earth’s surface.

 

 

The project team

  • Queen’s University Belfast: David Hughes, senior lecturer; Jenny McKinley, senior GIS lecturer; Conor Graham, GIS officer; and Shane Donohue, lecturer
  • British Geological Survey: Francesca Cigna, senior remote sensing geoscientist; and Vanessa Banks, project manager and hydrogeologist
  • Geological Survey of Northern Ireland: Kieran Parker, geologist: environmental and hazards; and Alex Donald, information delivery and licensing

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