Work has continued today on planing the South cantilever lane and completing the preparations for the start of planing on the North cantilever which is due to begin tomorrow.
Resurfacing a steel deck like the one on the Tamar Bridge involves a series of highly complex processes. Each individual process needs to be completed in sequence before work can begin on the next one. Once the existing surface has been removed from a lane, it is not possible to re-open it to traffic until the new surface has been laid as the steel deck is not safe for people to drive on.
Planing – the first stage in the resurfacing process – removes the majority of the existing surfacing material using a specialist road planer machine. A thin 8 – 10 mm layer of the surfacing is left bonded to the steel deck so that the deck is not damaged by the aggressive teeth on the planer. This remaining layer is then removed either by hand using mechanical hand tools or using a large flat blade on a suitable digger/dozer machine.
After the planing has been completed, the steel deck will be blasted with a very high pressure water jetting system to remove any final remains of the existing surfacing and deck waterproofing material. This provides a clean deck, enabling engineers to thoroughly inspect the steel deck and welds for cracks or damage.
Depending on the condition of the deck a further blast could be required using an enclosed short-blasting machine to ensure that the steel deck surface is suitable to accept the waterproofing system.
Following repairs to any damage on the deck, paint ‘primer’ will be applied to the bare steel deck to protect the steel from corrosion. This will be followed by the application of a two-layer waterproofing system, providing additional corrosion protection. A ‘tack-coat’ is then applied to help the surfacing material bond to the waterproofing material. This creates a composite surfacing system.
The surfacing material is laid in two thin layers using a special surfacing machine that runs on rails. A specialist, asphalt material has to be used to resurface the steel deck. At just 45mm thick, this material is much thinner and lighter than the materials used in standard road construction.
The rails are set to deliver the correct material thickness while also providing a smooth running surface. Once this has been completed the road markings will be applied, and a replacement illuminated road stud system installed.
Work on both cantilevers is currently due to be completed by the end of July when the North cantilever will be returned as a general traffic lane for the remainder of the project. At the same time the South cantilever will be opened to westbound traffic. This will enable three lanes to remain open for traffic when the works move onto the main deck.
A number of people have asked why the resurfacing was not carried out during lockdown at the same time as the kerb replacement works.
The simple answer is that the kerb replacement and waterproofing works had to be completed first so that the new surfacing material could be laid right up to the kerbs with minimal construction joints in the new surfacing. This not only ensures a good seal between the kerb and new surfacing, but also provides the best waterproof protection to the steel bridge deck.
Detailed preparations for the resurfacing programme began in 2019, with work originally due to start on site in Spring 2020 following the completion of the kerb replacement works.
When the UK went into the first national lockdown in March 2020, we were part way through the kerb replacement project. We were able to continue working on that contract by adapting our working methods, reducing resources and adjusting our work sequencing to ensure that our contractor adhered to Government guidance at the time.
Additionally, as experienced across the whole of the UK, material supply issues also had an impact on the duration of the project. This meant that the contract took longer than expected and the work was not completed until December 2020. As the resurfacing works have to be carried out in warm, dry weather, it was not possible to immediately begin work on resurfacing the bridge.
Both the contracts also require very different expertise, plant, equipment and materials and trying to integrate them together would have been complex and inefficient.
Some people have also asked why the work cannot be done at night and lanes re-opened to traffic during the day.
There are a number of reasons why we cannot carry out the work solely at night.
Safety – carrying out construction work at night is generally less safe than during the daytime because it is difficult to fully illuminate the work sites sufficiently and workers and drivers are generally less alert and more tired.
Noise – we need to consider the impact on our neighbours. Many of the site operations require large pieces of plant and equipment that generate a lot of noise and vibration. The road planer, hydro-blasting and shot-blasting equipment used to remove the existing surfacing material are particularly noisy and would disturb people living adjacent to or near the bridge. There are legal noise limits for night time working which we need to comply with. The noise generated by this machinery could breach this legislation.
Work sequencing – The various processes involved in the resurfacing works have to be carried out in order. This means that, as explained above, once we have removed the existing surfacing material, we cannot run traffic on that lane until the new surface has been laid.
Also, the products required for the waterproofing system cannot be laid if it is too cold or wet. Steel is particularly susceptible to condensation. The cooler overnight temperatures and high humidity caused by the riverside location of the bridge means that condensation regularly forms on the steel structure. This could affect the waterproofing works from being carried out.
Further information about the resurfacing project, together with additional FAQ’s are available on the Tamar Crossings website Resurfacing Frequently Asked Questions/