Cornwall Railway Viaducts - Constructional Features

Constructional Features

The distinctive timber viaducts were constructed using yellow pine which was preserved by Kyanising (using chloride of mercury), or sometimes by Burnettising (using chloride of zinc) in the workshops at Lostwithiel. The various timbers were seated in cast iron chairs, and wrought iron was used for parts of the bridge which would be in tension, such as tie rods between spans. Five distinct types of viaduct were built to suit local conditions at each site. Peter John Margary, the Cornwall Railway engineer from 1868 to 1891, classified them as classes A to E.

• Class A: The majority of viaducts were constructed on stone piers that rose to about 34 feet (10 m) below track level. From the tops of these radiated three fans of timber struts to support the beams beneath the track; one fan on each side and the third beneath the centre of the track. The struts radiated at around 55, 75, 105 and 125 degrees from horizontal (resulting in a \\// form), although there were some slight variations. This gave piers spaced at about 65-foot (20 m) centres.

• Class B: A stronger support was given to St Pinnock, Largin, and Ponsanooth viaducts by replacing the central set of fan struts with a pair. These joined the outer fans at the top of the pier and met each other at the top, giving a W form when viewed across the width of the piers.

• Class C: The soft ground of the tidal valleys at Weston Mill, Forder, Wivelscombe, and Nottar required a lighter structure. This was achieved by dispensing with the stone piers and replacing them with vertical timber trestles built on top of timber piles driven into the mud. There were no fans supporting the track, instead timber trusses were built directly on top of the timber trestles.

• Class D: Coombe (by Saltash) and Moorswater viaducts had only two fans on each pier - one on each side - but the bottoms of these were joined by laminated timber beams between each set of fans, which created a continuous beam along the length of the viaduct above the top of the masonry piers.

• Class E: The shallowest valleys at Grove, Draw Wood, and Probus were crossed by simple trestles of three parallel fans with two struts each in a V shape.

Brunel's designs allowed any defective timber to be withdrawn and replaced. The first decay usually occurred at the bottom of legs where they were seated in the cast iron chairs, and around bolt holes. The other area with significant decay was generally underneath the decking that carried the track and ballast. After World War I it became difficult to obtain the preferred yellow pine and other timber with shorter life was used instead, although by this time only the viaducts on the Falmouth branch remained in use.

Special gangs of men worked together on the viaduct maintenance. To reach timbers below the deck they worked from 2.5 inches (64 mm) manila ropes. It was possible to work on the viaduct without closing the viaduct to trains, but a temporary 10 miles per hour (16 km/h) speed restriction was enforced until the replacement was properly in place. The actual movement of the larger timbers was carried out on a Sunday when there were few trains operating.