M48 Severn Bridge Main Cable Inspection

In October 2015, American Bridge International (ABI) started work on the M48 Severn Bridge in the United Kingdom. This $3M contract involved providing access and project management to facilitate the client’s inspection of nine main cable panels on the 1960’s suspension bridge crossing the Severn Estuary between England and Wales. The work at each panel involved the removal of Cableguard and wire wrap, cleaning of lead paste residue, wedging open of the wires, wire sampling and repairs, re-compaction and replacing wire wrapping, Cableguard and re-enabling of the dehumidification system.

Low Level Access Platforms
The contract involved the design and execution of low (LLP) and high level access platforms (HLP) to facilitate the main cable inspection. The LLP was a tube and fitting scaffold had to satisfy various constraints including: providing suitable access to the main cable, be of suitable height to allow inspection around the full circumference of the main cable panels, permit vehicle access past the scaffold once erected and allow for full containment of lead dust contained on and within the main cable parallel wires. It also had to withstand the specified wind loading. ABI worked with our sub-contract scaffolder to ensure the scaffold roof was designed to withstand the highest levels of wind speed permissible in the existing design codes. The design was further revised at the request of Highways England and Severn River Crossings PLC to try and avoid having to remove the scaffold roof during moderate winds and risk delaying the works.

High Level Access Platforms
The HLP was a bespoke design and fabricated gantry system. The HLP allowed for the inspection of 2 high level main cable panels at different locations on the bridge. The design had to allow for wind loading at maximum levels, permanently sheeted roof and retractable side curtains for the containment of lead dust and for the handling of materials and equipment to and from the man riding access cradle. ABI adopted the Institute of Structural Engineers Purple Book as a guide to the successful training of operatives, operation, inspection and maintenance of the gantries. This system used in conjunction with our designer’s O&M manual ensured there were bespoke daily and weekly examination check sheets for the gantry which were used successfully throughout the works.

On this contract both the high level and low level access platforms were designed with full sheeted containment to protect the environment on the bridge, travelling public, work force and the wider environment from lead dust generated when removing the external wrapping wire from the bridge’s main cables.

Traffic Management
ABI worked to innovate and expedite the works where it was safe to manage traffic. Manual activities such as loading and unloading materials from the low and high level platforms was replaced with much quicker truck mounted crane handling. Training was provided for all operatives, even those with years of experience on bridge main cable works, on the assembly, operation and dismantling of the complex compaction and wire wrapping equipment so that the learning curve duration was dramatically reduced.

The low-level platforms were linked together with a scaffold “walkway” to enable the rapid deployment of personnel, equipment, and materials between cable panels. This resulted in a much quicker delivery of the low-level works.

These measures expedited the overall project by 2 months, creating a significant reduction in traffic management and benefitting the local community who use the bridge daily.