"Troubled viaduct over water"
A 264m long Victorian stone viaduct in Northumberland has been restored and repointed with 3000m2 of lime pointing.
Lambley viaduct crosses the South Tyne river in the North Pennines, Northumberland, as a series of elegant stone arches. More than 260m long, it once carried the Halt- whistle to Alston railway which was opened in 1852 to haul coal from the Alston mines.
After the line was closed in 1976, the viaduct decayed. Trees began to sprout from the stone arches, vandals pushed the stone copings into the river, and spandrel masonry began to fall off. In 1991, spurred to action by the Northern Viaduct Trust which had already acquired and repaired Smardale Gill Viaduct in Cumbria, the British Rail Property Board agreed to repair the viaduct and hand it over to the North Pennine Heritage Trust, who would maintain it in the future. The restoration was carried out by Charles Blackett-Ord, a structural engineer specialising in conservation, who restored Smardale Gill viaduct in 1991.
The viaduct may have been designed by George Barclay Bruce, an eminent Victorian engineer who was involved in the Alston line before leaving for India to pioneer rail- way construction there. It is a particularly elegant example of Victorian engineering: the river is crossed by nine 17m-wide arches which support a deck 32m above the river but, as it carried a single rail track, only 3.5m wide. The piers to the arches are built of massive rough-faced stones each weighing up to 500kg, with similar-sized stones in ashlar to the main arch voussoirs. The spandrels and piers to the 6m-wide approach arches are built of coursed rubble masonry.
Apart from the rampant vegetation, one of the main causes of spalling masonry was rainwater discharged down the inside of the arches by the original deck drains, saturating the structure. A new drainage system set outlets at the crowns of the arches, which also does away with the need for conventional downpipes which would have been impossible to maintain. The deck was water- proofed with a DPM topped with concrete, asphalt, then gravel on bitumen emulsion. Stainless-steel dowels were cast into the concrete and drilled into the spandrel walls on each side to tie the walls together.
The stonework was repaired and in some cases replaced. The elevation of arch 18 is an example of the repair schedule.
The source of the Nattras Gill Hazle stone, a nearby quarry, had long been closed and a Derbyshire stone, Stanton Moor Buff, was found to be a close match. It has been used for parapets, dressed and tooled arch imposts, voussoirs and quoins. It will appear new for some years but cleaning the original stone was not considered in Blackett-Ord's view: 'Why clean off the patina of age? We prefer to let the new stone weather to the appearance of the old.'
Repointing in lime
The viaduct was built using lime mortar, which is flexible and porous, allowing moisture which enters the structure to evaporate through the joints. Previous repairs had used cement mortar, which sealed water in the structure, saturating it and leading to damage.
The whole structure has now been repointed with lime mortar comprising pure lime putty hydrated St Astia natural moderately hydraulic lime from France and sharp sand. Traditional techniques for using lime were taught on the scaffold by Blackett-Ord Consulting Engineers. The viaduct used 3000m' of lime mortar, surely one of the largest lime pointing contracts this century. The arch barrels have been left unpointed to provide nest sites for bats and birds.
Client: Rail Properties Ltd
Elaine Rigby designed new balustrading, gates and an external access stair for the public to use when crossing the viaduct. The design captures the spirit of nineteenth-century railway architecture without pastiche.
The original stone parapet, designed for trains rather than people, was only 800mm above deck level. To make it safe, a single 48.4mm-diameter balustrade rail on cast-iron stanchions has been installed 1100mm above the deck.
Lambley Station was sold off by BR and is now a private house. To stop people straying into its garden, one end of the viaduct has been fenced off with gates of oak boarding framed with cast-iron posts to match the stanchions.
Architect: Elaine Rigby Architects
Quantity Surveyor: Wardlaw Surveyors
Contractor: Laing Stonemasonry