Cremorne Bridge, known as Chelsea Bridge in rail circles, was constructed in 1863 and today carries around 18 trains per hour over the River Thames – that’s passenger services operated by TfL and Southern as well as freight. The bridge is believed to be the second oldest of its type in the world and is one of the earliest railway bridges crossing the Thames surviving in its original form.

Chelsea Bridge is a Grade 2* listed structure comprising five spans, each approximately 45 metres long. The spans are slightly skewed and constructed from three pairs of wrought iron lattice girders with lateral and spandrel bracing. Four mass-filled stone piers support the bridge spans across the river bearing on firm London Clay below the river bed.

Masonry arch viaducts carry approaching rail traffic and these would have been regularly flooded by the Thames before the construction of Chelsea Embankment. However, they are now used as storage units and were also the site team’s office and welfare facilities.

Planned refurbishment

150 years of rail traffic and the harsh riverine environment had taken its toll on the wrought iron structure and, in 2012, Network Rail awarded the scheme to Kier to repair, strengthen and apply a protective coating system to the bridge, to extend its life by a further 25 years.

The nature of the work was not expected to be unusual, as the repair and strengthening of wrought iron and lattice structures, as well as working on bridges over water, are all fairly standard in the rail industry.

However, what was unusual was the sheer volume of components making up the bridge’s superstructure. Kier’s team had to undertake a detailed condition survey of its 9,000 structural elements and their connections to accurately identify and locate any defects. This information would then be used to produce detailed, timely and practical repair solutions while maintaining an assurance process throughout. Project manager Colin Barnes realised that managing all that data could well be as challenging as the management role of nearby José Mourinho.

Colin had used 4D visualisation techniques in the roof and facades project at Kings Cross and, at an internal BIMXtra presentation, became enthusiastic about the possibilities of using the technology to simplify data management on Chelsea Bridge. BIMXtra is a collaborative workspace where consolidated project information can be viewed, edited and compiled into intelligent revision controlled information.

His project team collaborated with Clearbox, a former Kier company that was spun off in 2013, to develop the BIMXtra system to carry out the recording of defects, detail the associated repairs and record the completed works in one elegant, integrated solution. Clearbox tailored the application to the project’s specific requirements and further streamlined workflows as the project progressed.

The system delivered significant time, cost and programme savings when compared to traditional paper- based methods, and ensured that the site team could maintain a consistent high level of quality assurance.

Benefits of new technology

Whilst the BIMXtra system was developed to establish a benchmark, spans 1 and 2 were surveyed using traditional paper-based methods. Working from the scaffolded structure, two engineers took three weeks to record data and enter this manually into the database. Span 3 used both methods for comparison, with the new BIMXtra method taking two engineers just four days. Thereafter this became the sole methodology. Over the whole bridge, the system achieved savings of £864,000 and 21 programme days, providing greatly increased assurance and resulting in further savings in document management.

Chelsea Bridge

In addition to the survey cost and time savings, the use of BIMXtra also resulted in a faster turnaround on design solutions and technical clarifications. Assurance was much improved with clear, consistent repair details reducing the risk of non-conformance while human error was virtually eliminated with no re-work required.

The availability of centralised, accurate and robust data, shared across the two organisations, allowed the team to conduct real-time desktop reviews and audits of progress and assurance. This was especially valuable given the challenging logistics for managers visiting every part of the structure.

So what did BIMXtra add to this project? Colin described it as “a 3D model of the bridge, enabling every one of its 9,000 components, its condition, designs of any repairs, progress and assurance, all to be simply and accurately identified, recorded and shared by our engineers using iPads”.

At the start of the contract, the team had available the original but unreliable construction drawings and a 2D survey carried out by rope-access engineers back in 2000. From this, the 3D model known as the ‘Backbone’ was constructed, a data rich 3D CAD model that provided locational reference for later survey and repair activities. Details were recorded for every element, ranging from whole girders to small brackets.

The engineers had real-time access to the model from the bridge, using Wi-Fi or 3G-linked iPads. They took photographs of every element, pinned to the relevant location on their screen. The engineer then selected, from a standard list, the details of any observed defects such as cracks, reduced sections or corrosion, failed or missing bolts or rivets, and added text to further describe specific problems.

The data entered by the engineer on site was instantly synchronised back to the BIMXtra database and used to automatically generate a survey sheet, allowing the designer to promptly produce a work sheet including repair instructions, standard repair details and sketches if necessary.

Each defect repair proposal was colour coded to aid the authorisers’ understanding. Red – critical to stability and structural integrity, requiring individual designs, Amber – standard minor repairs such as missing bolts, knife-edging and rust-jacking of members, and Green – no affect for 25 years life, ‘nice to have’ repairs. Typically there were 1,700 Red/Amber and 500 Green for each span.

Documentation was electronically signed off by the surveyor and designer, authorised by Kier’s managers and finally by Network Rail’s project manager. The system’s electronic workflow and designated authority levels ensured that this governance was both timely and accurate. Every stage and every change was date- stamped and could be accessed by Network Rail and Kier managers using their iPads or PCs.

As works were completed, engineers used their iPad to verify each task’s completion against its whole history – its survey, location, defects and repair details. Photographs of the completed repairs were added to the record together with any relevant notes. As tasks were signed off on the structure, the progress records were instantly updated. The status of each task was available for progress review using colour coded images, complementing formal reporting.

Chelsea Rail Bridge Navis Image 5 Oct

The repairs

Whilst the use of BIMXtra simplified the challenge of managing the huge volumes of data, the physical works to refurbish Chelsea Bridge were equally challenging for Colin’s team, given its location.

A system access deck was designed and installed to the existing external bridge beams. This provided a safe working area without compromising the strength of the structure and allowed normal train services to continue. Installation of this was carried out from barges and pontoons on the tidal River Thames.

It was at this stage the engineers’ scaffold-based inspection found that the anticipated 100 defects per span was a significant underestimate as around 2,000 per span were identified. As a result, the cost of the works increased, although Network Rail now has an iconic structure in excellent condition, ready to stand up to the next 25 years of intensive use.

As well as developing large scale applications, the project team also developed bespoke small-scale applications to manage the works packages. One of the key structural concerns identified whilst developing the design was the insertion of additional shear bolt capacity in connection between the top flange and the deck troughing. The design stipulated a strict minimum number of bolts which had to be achieved in each zone and this was reflected in defect repair proposals.

The challenging programme, and the difficulties of installing the bolts in the existing structure, required a thorough approach to the monitoring, inspection and compliance of this aspect. The BIMXtra process included a tracker on the number of bolts installed against the design specification – where the design was achieved, and where additional bolts would be required to meet the design.

Managing a winning team

Summing up the successful completion of the project in September 2015, Colin told Rail Engineer that the high quality, transparent and detailed BIMXtra surveys had been of mutual benefit to both Kier and its subcontractors. They had also provided Network Rail with the most detailed condition survey of the structure that has ever existed.

He added that the Chelsea Bridge project has been the pinnacle of his long career. In common with José down the road, Colin believes that he found the winning formula for success.

Written by Bob Wright