An essential and significant infrastructure asset in the very well-used southern railway coastal route between Brighton and all points westward is the 16-span Shoreham viaduct, which carries the railway across the tidal estuary of the River Adur.
The present viaduct, which was completed in 1892, has a superstructure of early steel simply supported on concrete-filled cast-iron caissons. With its marine situation, it has needed much maintenance attention over the years, the most recent being in 2004. In addition, during World War II, Shoreham viaduct was damaged by bombs on at least three occasions.
Now, however, Network Rail has commissioned a very comprehensive refurbishment to give it a new extended lease of life. BAM Nuttall is the principal contractor, managing the project which is to repair and replace areas of the steelwork suffering loss of section through corrosion, along with comprehensive grit blasting and repainting of the entire structure.
The main works commenced in April 2016 and will be completed by December 2017. However, prior to the actual refurbishment work, an ancillary major task was carried out, between August and October 2015, to remove a redundant gas main attached to the south face of the superstructure. This enabled approximately 100 tonnes of dead load to be eliminated from the structure – prior to the addition of alternative, but essential, dead load in the form of the extensive steel plating repairs to cross and main girders throughout!
The removal of the gas main was accomplished by working in midstream from a barge provided by Jenkins Marine and over the soft, silty shore areas by using a load-spreading paving of 100mm thick Durabase mattresses.
Assessing its condition
Mott MacDonald, the consulting civil engineer for the work, prepared outline design proposals based on the available preliminary data concerning corrosion and section loss.
Two previous heavy maintenance packages have been carried out on the viaduct in recent years, firstly over one half of the structure during the 1990s and secondly on the remainder of the structure in 2003/4. Records of girder conditions from these works were used to estimate the scale and scope of works necessary for the current project.
However, once the structure was fully scaffolded and access became available, Mott MacDonald was able to conduct a comprehensive and up-to-date corrosion survey and use the results to refine the exact specification required for every individual cross girder, main girder and component throughout the viaduct.
There was a considerable variation in what was required where. Therefore, to simplify the specification of the work as far as possible, a suite of repair categories was devised and drawn up in detail. For example, in the design schedule for repairs to the cross girders, there were eight different specifications, dependent upon the location and extent of corrosion loss. So, although all 69 cross girders throughout the structure are essentially identical, each end of each individual cross girder has been designated with its own specific repair type, from 1 to 8, depending on what is required.
This approach has targeted the work required to the structure, which has required collaboration between all parties to assist with the planning of the work, the ordering of materials and the pre-fabrication of the steelwork repair plates.
Access and protection
Underslung scaffolding across the entire structure was designed and installed by Hadley Scaffolding and provides easy access to every part of the superstructure being repaired and painted. Optima Scaffold Designs checked the safe loading and structural performance of the scaffolding. Mott MacDonald assessed and checked the viaduct’s structural capacity to take the additional loading from the scaffolding.
The entire scaffolding is covered by shrinkwrap cladding to minimise the effects of wind and wet weather and to contain the grit and scalings from blasting, essential as the Adur estuary is a Site of Special Scientific Interest (SSSI). Approval for the method of work to safeguard the SSSI was sought from several authorities, including the Environment Agency, Natural England, West Sussex County Council and the Marine Management Organisation.
One innovation at Shoreham viaduct is the use of a biometric security system. Everyone who works on the site has their details, which includes a copy of their index fingerprint and a scan of their CSCS (Construction Skills Certificate Scheme) card, logged into the system. Entry to the site is via a turnstile, which only responds to a biometrically recognised fingerprint.
Another sophisticated feature that BAM Nuttall has introduced is a wireless emergency fire and first aid point. If the fire alarm is triggered, the turnstiles revert to free wheel, allowing safe and quick egress. The system prints out a list of all who have entered the site, thereby enabling a roll call.
Possessions not required
It is striking that almost all the repairs are being carried out without the need for track possessions, although these formed part of the original design. However, an innovative approach to the repair of those cross girders suffering bottom flange corrosion resulted in a reduced requirement.
Because of the extensive nature of corrosion to some of the cross girder bottom flanges, and the consequent difficulty of achieving a good seating surface for the addition of conventional strengthening plates, an alternative solution has been devised by Mott MacDonald.
eavy angles are being attached to the cross girder web, slightly above the bottom flange, running horizontally in those locations where there is a need for strengthening.
In order to fit these angles, it is necessary to curtail some of the stiffeners running between the top and bottom flanges. This alteration to the stiffeners cannot be done whilst the structure is carrying live load. After the new horizontal angles are installed, along with replacement stiffeners and web repair plates where required, the cross girder can once again go back into service.
This article was written by Mark Phillips