On Christmas Eve 2015 a 250m section of sea wall and rail track between Dover Priory and Folkestone Central collapsed, putting the railway out of action. Costain was drafted in to replace the retaining wall supporting the embankment for the line, as part of the major South East Multi-Functional Framework rail upgrade and maintenance project.
Sending in surveyors on foot to record the extent of the damage was considered too hazardous, due to the threat of collapse, so a drone was mobilised to survey the entire area.
The UAV used stereo photogrammetry techniques to capture around 200 high definition photos and topographic survey data, including the position of ground control points placed at intervals along the beachfront. This delivered location data accurate to 10-15mm tolerance.
All images and data were uploaded to the cloud and stitched together using Agisoft photoscan software. Meshlab software was then used to convert the model into 3D dxf file format for use in AutoCAD, mapping the entire area to enable engineers to make design decisions in the office.
A 3D point cloud model was created in Autodesk Recap 360 3D scanning software using points exported from Agisoft photoscan software.
“This form of survey was around 30% cheaper and involved 70% fewer emissions than traditional methods,” says Peter Slater, aerial solutions UAV manager at Costain Group. “The team was just 1.5 days on site with the drone, whereas a topographical survey would have taken two people at least three days, probably more. In addition, the process was more accurate than traditional survey due to increased point collection.”
Repairs to the sea wall involved installing very large rock armour sections, weighing several tonnes each and shipped from Scandinavia, to shore up the base of the wall along the length of the beach.
The client required proof that the elements in the revetment were being placed to the correct profile in compliance with the design. The irregular geometry of the armour again made conventional surveying on foot dangerous, so the drone was mobilised to map the entire rock armour installation in 3D.
A 3D point cloud model was created in Autodesk Recap 360 3D scanning software using points exported from Agisoft. This was laid over the scheme’s original BIM design model, in Trimble Business Centre, and comparison checks made at five metre intervals along the length of the beach front. Remedial changes to armour alignment were then made by the on site team, where necessary.
In future the model will provide the client with a permanent record to detect any rock movement over time. The project served as a valuable trial for Costain’s implementation of drones, adds Slater, and the benefits of giving sites the autonomy to decide when and where they should be deployed.
