Mobile Mapping – Usk Viaduct Survey Summary:

Severn Partnership have been involved with the Great Western Electrification Project (now Route Modernisation) since 2009. In November 2015, as part of Route Section 8 enabling works, a full 3D Revit Model of the Usk Viaduct was required to assess construction and strength for overhead line gantry positions.

Project Overview:

Whilst the top deck was surveyed using traditional static laser scanning techniques, the underside posed significant access challenges. The Usk River has the second largest tidal range in the world, second only to the Bay of Fundy in Canada, and is the deepest river in the British Isles at its mouth.

Various Surveying Solutions:

  • Inverted scanning rigs from deck level
  • Photogrammetry from a boat
  • Brackets mounted to the bridge piers to scan from

However all these options either required several members of staff to be in hazardous locations for extended periods of time, or had accuracy limitations.

Severn Partnership utilised their Leica Pegasus:Two Mobile Mapping system, completing the work in a single daytime shift, from the safe confines of a boat and without any additional track access. Using existing static scan as control, both datasets were merged to an overall accuracy of +/-10mm ready for modelling in Revit.

Technical Services:

  • Complete merged 3D Laser Scan Point Cloud
  • Post processing to project grid – GWPBS09 Snakegrid
  • Traceable survey accuracy linked to trackside control
  • Revit model deliverable – modelled from Point Cloud
  • Georeferenced site imagery with 360º view

Benefits:

  • Significant reduction in staff site time compared to alternative methods
  • Excellent coverage of underside bridge structure in order to model complex non-uniform beam arrangements
  • Allowed bridge engineers to examine the structure during same site visit, reducing footfall and safety paperwork
  • All work completed from safe confines of a boat, with no need to lean over the side
  • A survey process allowed traceable accuracies back to existing trackside control network