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The Moy Viaduct is a five span timber viaduct carrying the single track Highland Line over the Altnaslannach watercourse. The bridge was constructed in 1897, and listed in 1976 as a Grade A Historic Structure.
Each bridge support consists of a timber trestle founded on timber piles. Two individual timbers spliced together make up each support pile. Each trestle comprises four vertical columns supported on either side by a raking A- Frame. The A-Frames have horizontal steel ties connecting back at low level, at approximately mid height, and at upper level. The crossheads at upper level support the six main longitudinal timbers forming the deck. Each length of longitudinal beam is built up from two individual timbers bolted together.
Transverse timbers run between the longitudinals, with each transverse beam containing a centrally placed tie rod. A system of diagonal and longitudinal bracing stiffens each support trestle longitudinally at low level. The timber deck supports transverse trough steel decking. which in tum supports the permanent way.
The steel bridge parapets and kicker plates are suffering from corrosion. The exposed trough decking forms a walkway of sorts on both sides of the track. The softwood (southern pine) timbers are suffering to varying degrees from fungal decay with both Antrodia Zantha and Gkloeophyllum spp fruiting bodies present on a range of timbers. The penetrations of the decay into the inner sections of timbers has been facilitated by the presence of unsealed holes drilled into timbers as part of an earlier testing regime.
The resulting loss of section has led to the imposition of speed restrictions of 40mph for passenger trains and 20mph for freight trains. The load carrying capacity of the bridge at these speeds is restricted to RA8.
It is proposed to replace the central track supporting section of the deck but retain as much of the existing structure as possible. The new deck will be a three span continuous, steel structure with twin main girders built within the existing framework of the bridge and supported on new concrete precast bankseats and piers. In cross section the two new girders will be located between the second and third, and the fourth and fifth columns of the existing bridge. The existing timbers will be treated for the fungal decay. While the new deck will take all the live loads from the track, the existing structure will be used to carry the walkway plus its own self-weight only.
Work started during April with it seemed dozens of lorries carrying tons of metal for new embankments to facilitate pile driving. This has now been completed and as you will see when passing the metal has been spread around the site creating a sound foundation for the many cabins required. Carillion, the main contractor, will give a full report for our next newsletter but to date it has taken 7.00 tons of metal to make the ramps, most of which will be removed from the site and recycled. The firm are carrying out remedial works to reinforce the Dalmagarry Bridge and a bridge at Dalnaspittal.