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Sector
Highways
Location
Tewkesbury
Services
Bridge and Tunnel Structural Inspections
Specialist Access Support
Our partnership with Atkins
Project overview
XEIAD undertook investigation survey works on Mythe Bridge as part of a structural assessment.
Comprising of a main segmental iron arch with smaller masonry arch approach spans, Mythe Bridge carries the A438 over the River Severn on the edge of Tewkesbury (works carried out to iron arch span only).
The challenge
Investigation works were split into the three key stages as detailed below:
Stage 1 – Point Cloud Survey
A 3D point cloud model that can be manipulated to obtain a 2D plan, elevation and sectional views, which can be used as a reference for the structural analysis; See figure 1.
UAV / mobile survey to enhance the base point cloud survey and used to visually identify the structural defects.
Stage 2 – Roped Access Dimensional Survey
To measure the structural members of the bridge to confirm unknown dimensions from the as-built drawings and to also confirm the 3D scan.
Stage 3 – Roped Access Condition Survey
Following the UAV Survey, a roped access Condition Survey was carried out to evaluate the current condition of the structure.
As part of Stage 1 works, XEIAD needed to devise a safe and innovative method of capturing point cloud data, whilst reducing the number of shadows caused by the large quantity of deck elements. Two additional constraints included the use of local land to access the banking of the River Severn and the inability to close the road during daytime hours due to foreseen commuter disruptions.
The solution
During the planning phase, XEIAD decided to utilise one of its main surveying subcontractors to assist with the survey.
Upon exchanging ideas with the subcontractor surveyor, a method of works was devised that incorporated rope access and remote surveying methods, all whilst preventing the need for traffic management.
The approach utilised the Trimble X7 laser scanner due to its remote capabilities (30m WiFi range) and external carrying handle; the latter allowing a lanyard to be attached and secured to either one of XEIAD’s rope access operatives or the structure.
It was established beforehand that the scanner, when in operation, would be manoeuvred and placed onto the horizontal plates (found in the spandrel) using the durable tripod as a footing with the lanyard system as a backup. See figures 2 & 3.
For the crown (where there was no horizontal plates) the scanner was attached to the vertical plates found between rib elements, utilising a bespoke ‘T’ shaped timber bracket fabricated specifically for the project. The bracket was able to provide a safe and stable platform when used in conjunction with the lanyard system and proved an efficient and safe method of works.
During the works, the XEIAD operative manoeuvring the scanner would communicate via Radio with the XEIAD subcontractor, who would be positioned in the safety boat below. To ensure scanning shadows were reduced in the data, real time image feedback was analysed via the remote handheld device after each scan and the next scan location was chosen.
Throughout all stages of works, XEIAD used temporary signage that was placed up and downstream of the structure, informing River users of the works being carried out in the vicinity. All equipment that could potentially fall from height was attached to either the XEIAD rope access operatives or the structure via a lanyard system.
Weather was closely monitored during the works, including the flow velocity of the River Severn. Fortunately, no delays were caused due to adverse environment conditions, but XEIAD are particularly aware of this constraint and try to plan work outside of the winter period.
XEIAD staff and the external surveyors working on or over the River Severn were provided by the Lead Inspector certified (in-date) lifejackets and work did not begin until the safety boat was deployed in line with the method statement protocols.
The results
Upon completion of works, XEIAD created an Investigation Report and were able to provide Atkins with a full 3D digital model of the structure that was created by the subcontractor surveyor.
Key outcomes
Safely captured point cloud data using rope access and remote surveying, minimising disruption.
Utilised rope access and remote surveying to prevent traffic disruption.
Used Trimble X7 laser scanner for easy manoeuvrability and safety.
Provided Atkins with a full 3D digital model of the structure
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