BRIDGE STRUCTURE BTW/67

Central Alliance was instructed to undertake a topographical survey of a historic railway structure which now serves as a section of a National Cycle Route, where the Castleman Trailway spans the River Avon.

The specified requirement was to capture accurate and detailed information of the bridge structure and all landform within a 30 m radius, including approaches to both the cycle route at deck level and River Avon below.

Terrestrial 3D laser scanning was specified from the deck and underside levels, to allow the capture of all structural elements, to produce 2D general arrangement drawings and 3D model, which was to be integrated with high resolution imagery.

Although the survey of at deck level was relatively straight forward, access to the underside of the structure of the River Avon was problematic. There was very limited access to the riverbank due to steep slopes and heavy vegetation, such that there were few suitable set-up locations for the capture of 3D scan data, from below.

This severely restricted lines of sight from the riverbank to large parts of the bridge underside, meaning that an alternative approach was required for full point cloud coverage and delivery of specified outputs.

Along the river channel itself there was sufficient safe access to deploy an inflatable dinghy, for the survey of channel detail including hard and soft bed levels, by conventional total station observations. This included the survey of cross section profiles at 10 m intervals.

Several data capture options were considered for the underside scanning, including unmanned aerial surveys (UAS), mobile mapping from a small boat, and terrestrial scanning from a pontoon, however none of these would provide the required accuracy to deliver in accordance with the specification.

It was therefore decided to utilise inverted scanning techniques, to lower a lightweight 3D laser scanner from the bridge deck. Although challenging, we were eventually able to design and fabricate a davit arm type system, which incorporated inverted tripod components and counterweight, such that the laser scanner could be safely setup on the bridge deck and swung over the parapet fence, before being secured to the structure and lowered beneath the bridge underside level.

Through multiple carefully positioned (and gently lowered) scans, we were able to capture a comprehensive 3D point cloud of the bridge underside structure, which was integrated with the deck cloud via referencing common survey control.

Following registration, georeferencing, cleaning and colourisation of the 3D point cloud, the survey outputs were produced, which included a detailed topographical survey layout drawing, 3D model of the structure, sectional elevations, and watercourse channel cross sections.