Linking Devon with Cornwall across the river Tamar, the single track Royal Albert Bridge is of strategic importance, being the only remaining mainline rail link into Cornwall. If it were to be out of action, then the Royal Duchy rail network would be cut off from the rest of the UK.
The bridge opened in 1859, the last great civil engineering project of Isambard Kingdom Brunel, and was constructed mainly of wrought iron. Modifications and strengthening was carried out by the GWR in the 1930s, primarily to replace or enhance the cross girders upon which the track is positioned using steel components. These had rendered some of the original Brunel girders redundant but they were kept in place to improve the rigidity of the structure.
Surveys had shown that, while the bridge was in generally good condition, some corrosion was present. A major refurbishment was scheduled, as reported last year by the rail engineer (issue 82, August 2011). This covers the central spans of the structure, including the removal of certain lower diagonals which were installed in the early seventies. One year on, it was time to visit the bridge again and see how work was progressing.
Preparation and access
Various consultations had to be carried out before work could start. This was not only with English Heritage (the bridge is a Grade 1 listed structure) but also with the local communities since noise and potential damage from falling debris were perceived as real risks. With everyone onside, Network Rail and its principal contractor, Taziker Industrial (TI), could move forward with confidence.
Despite preconceptions, the bridge had been built to a restricted budget and the original plan for a double track railway had to be dropped on grounds of cost. The resultant single track bridge is therefore quite narrow which makes it more susceptible to side wind movement. Great care was needed to design an access system that did not worsen wind loading. The two main centre sections are linked to each shore by a number of approach spans, the ones on the Cornish side being on a sharp curve. The access system has thus to include these spans, much of which is over land rather than water.
An underslung scaffold system is employed with the downward poles being fixed either side of the track level platform and with a walkway constructed between these poles. Conventional scaffolding is used on the approach spans and around the three central towers but under the two main central bowstring spans, a lightweight scaffold system is employed. This HAKI Lite system is manufactured in Sweden from a steel alloy and is 30% lighter than conventional tube and fitting systems. It is designed to slot together without the need for conventional clamps. Modular components of varying sizes allow the scaffold to be made to fit the structure, including complete staircases which are built up around the central towers to access the very top of the main tubes, keeping the weight to a minimum.
Measurement of likely wind speeds indicated that a maximum of 700 square metres of scaffold cap could be allowed on each of the two central spans. Conventional heat shrink capping is used around the central towers but, in mid span, the HAKI system includes a series of vertical runners into which a “curtain” of durable fabric is rolled up or down so as to complete the encapsulation. If the wind speed exceeds 70mph, then work stops and the curtain is rolled up so as to decrease the side wind pressure. So far these conditions have not occurred but regular checks with the weather forecasting offices are made so as to be prepared. A safety boat is always on duty in the Tamar below should anyone fall into the river
Once the work area is completely encapsulated, that section of the bridge is fully blast cleaned to remove all the existing coatings and expose the original surface. This needs compressed air and electricity. Two large compressors (one on each shore side compound) have been provided with pipes running alongside the scaffold walkway. A 415V 3-phase electricity supply is run out on to the bridge with transformer units provided by each central tower from which machinery can be powered.
The upgrade work
Whilst the work has been well planned, the extent of the repairs needed cannot be fully established until all the dirt and paint from past decades has been removed. Grit blasting is used to achieve this, itself a noisy and messy process. When blasting is taking place, additional screening is erected in the specific area so as to fully encapsulate and contain both the debris being removed and the spent grit. An industrial vacuum system sucks out all of this so that a clean work area results.
Once the base metal is revealed, the necessary remedial work can then be planned. Firstly a primer coat of paint is sprayed on to the exposed surface so as to prevent any further corrosion in the salt air. The H section cross girders will typically have part of the web and flanges corroded away and to repair this damage, the web will be ‘sandwiched’ between two new steel sections bolted together. This new steel will itself have been grit blasted and primed at the main depot site to ensure a surface free from imperfections.
Several of these steel “sandwich” platings have already been installed and more will be needed as the work progresses. Each one has to be made to order so as to fit the exact repair shape needed. The new steelwork is being supplied by Cutlass Engineering with preparation and installation being carried out by Taziker Industrial. An excellent relationship is in place between TI and Cutlass Engineering.
With the repair completed, the whole section will be sprayed – firstly with a base coat of epoxy glass-flake paint to give long life protection, and then a top coat of acrylic based paint to provide the required grey colour and to protect against sunlight degradation. This is designed to have a service life of 25 years.
Moving upwards to the two central bowstring spans, these were originally constructed of 10’ x 2’ wrought iron plates, slightly curved to take up the shape of the bow, and then overlapped and riveted to adjacent plates so as to form the complete arch. Inspection inside the 10 foot diameter tubes show them to be in generally good condition. Where corrosion has taken place on the external surface, then the area will be cut out and new plating installed by bolting through to the inner surface. The same protective paint coatings are applied.
Lastly, there are the chain suspensions that provide the intermediate support to the track level deck. This 14-section link structure is generally in good condition but cleaning and painting between the links is something of a challenge. Manual methods are the only effective solution and painting is done by something akin to a thin roller for getting behind radiators in the home. Where the flexible pins that allow movement in the chain are corroded, new ones are being manufactured and installed.
It was expected that the worst corrosion would have taken place on the seaward side of the bridge as this takes the full force of the prevailing wind, rain and spray. The reverse has proved to be true, the reason being that the leeside tends to retain any dampness without the effect of a drying wind.
Phases and completion
To ensure compliance with the wind loading and noise restrictions, the work is being carried out in five main phases, four of these having four stages and the last one having two stages. This enables work to take place at a number of locations simultaneously. As the blasting, priming, repair and painting work is completed so the scaffolded sections are moved along to the next phase. For the casual visitor, this enables the before, during and after work to be seen at the same time.
Most work does not affect train movements, but remedial activity does impact on the track deck from time to time. It is possible to get short one hour possessions between trains during the day and a phone link with Plymouth Power Box enables these to be planned and implemented at relatively short notice. Longer possession requirements mean work having to be done at night but care is taken to minimise these so as not to disturb the local residents.
Completion is expected to be in November 2013. The overall budget is £11.5 million of which £1.5 million is for the purchase and erection of the scaffolding. Around 50 people are working on the bridge at any one time, these mainly being TI employees, many of which have been recruited locally. TI have just won the Network Rail Star Award for Site Environment and Safety, the first such winner in the Western Territory, and a suitable accolade for the innovative processes that are being used.
Thanks to Peter Cook, TI site manager and John Womack of Network Rail for their help in preparing this article.
