The Cornwall Railway company constructed a railway line between Plymouth and Truro in the United Kingdom, opening in 1859, and extended it to Falmouth in 1863. The topography of Cornwall is such that the route, which is generally east–west, cuts across numerous deep river valleys that generally run north–south. At the time of construction of the line, money was in short supply due to the collapse in confidence following the railway mania, and the company sought ways of reducing expenditure.
On the advice of the Victorian railway engineer Isambard Kingdom Brunel, they constructed the river crossings in the form of wooden viaducts, 42 in total, consisting of timber deck spans supported by fans of timber bracing built on masonry piers. This unusual method of construction substantially reduced the first cost of construction compared to an all-masonry structure, but at the cost of more expensive maintenance.
Replacement of the timber viaducts by all-masonry structures began in the 1870s but a few remained in service until the 1930s.
History The Cornwall Railway linked Plymouth with Falmouth. The section from Plymouth to was opened on 4 May 1859, and the remainder to Falmouth on 24 August 1863. Although the line had been designed by Brunel, this was after his death and the construction under the supervision of R P Brereton. It was built as a single-track broad gauge line.
The of railway crossed 45 rivers and deep valleys. Of these 43 were spanned by viaducts of various types built partly or entirely from timber. Workshops were established at where timber could arrive on barges to be preserved and cut to size. The offcuts from the timbers used for the viaducts and track were then used for the construction of the railway's buildings.
The choice of timber was made to keep initial costs down, but Brunel had warned that this meant more expensive maintenance—running to £10,000 annually. Replacement of the viaducts started in 1875 but led to a dispute in 1884 between the Cornwall Railway and the Great Western Railway which was leasing the line. The lease precluded the conversion of the line to standard gauge, and the Cornwall Railway refused to pay for the widening of the viaducts during rebuilding sufficient to accommodate a double line of standard gauge tracks. Following the amalgamation of the two companies on 1 July 1889 all the remaining viaducts on the main line to Truro were replaced. Most were either rebuilt in situ or by a replacement viaduct built immediately alongside, and in the latter case many of the original piers still remain today.
Between Saltash and St Germans, a deviation line was built in 1908, eliminating the wooden viaducts on by-passed section of line. Those on the Falmouth branch were all replaced between 1923 and 1934.
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, a process patented by William Burnett) 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 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 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 manila ropes. It was possible to work on the viaduct without closing the viaduct to trains, but a temporary 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.
Viaducts from Plymouth to St Germans
Stonehouse Pool
Milepost 247.25 on original Millbay to Devonport line between Five Fields Lane (now North Road West) and Stuart Road, south of station. ()
The only double track viaduct on the line, it was a Class A viaduct but with five fans of struts on each of the dwarf piers. It was high and long on 5 dwarf piers. Rebuilt with iron girders on brick piers in 1908.
The land below the viaduct was the head of a tidal creek but is now drained and forms a park. In 1876 a new Cornwall Loop between Plymouth North Road station and the Cornwall Railway was opened, built on a viaduct alongside the Stonehouse Pool Viaduct. Millbay station and its connecting lines (on which Stonehouse Pool Viaduct was located) were closed in 1964; the girders have since been removed and a steel work of art has been erected in its place.
Keyham Milepost 248.75, north of Devonport. ()
A Class A viaduct high and long on 6 piers. It was rebuilt using iron girders in 1900 which were again replaced with steel in 1937.R J Woodfin, The Cornwall Railway to its Centenary in 1959, Bradford Barton, Truro, 1972
Weston Mill
Milepost 249.5, north of Devonport, above Weston Mill Creek. station was opened at the southern end in 1900. ()
A Class C viaduct high and long on 29 trestles. It was replaced by a steel structure in 1903.
Royal Albert Bridge
Milepost 252, immediately east of station. ()
This was the largest metal bridge on the route when it opened. A wrought iron bridge (including two main spans of ); it stands clear of high water.
Coombe by Saltash Milepost 251.5, west of Saltash station. ()
A Class D viaduct high and long on 9 trestles. It was replaced by a stone viaduct on 19 October 1894. Because it crossed a deep, muddy tidal inlet, Brunel constructed this viaduct on timber piles and used timber trestles instead of stone piers. These were made from four groups of four timber baulks, each group raking inwards towards the top of the trestle.
Forder Milepost 252.25, west of Saltash. ()
A Class C viaduct high and long on 16 trestles. It was demolished after the line was diverted to a more inland alignment on 19 May 1908, and replaced by a stone viaduct on the new alignment.
Wivelscombe Milepost 253.5, west of Saltash. ()
A Class C viaduct high and long on 4 trestles consisting solely of two uprights each plus a cross brace. It was demolished after the line was diverted to a more inland alignment on 19 May 1908.
Grove Milepost 255.0, east of . ()
A Class E viaduct high and long on two dwarf piers. The fans consisted of two raking struts on either side of the track, tied beneath the track by cross braces, and a central pair of struts which were joined at the top as an inverted V; from the side this gave a \|/ fan. It was demolished after the line was diverted to a more inland alignment on 19 May 1908.
A fatal accident occurred here just two days after the opening of the railway. On 6 May 1859 the engine of the 7.25 p.m. train from Plymouth was approaching St Germans when it left the rails, hit the parapet of the viaduct and fell into the mud below, landing upside down. Two of the coaches also ended up in the creek. The driver, fireman, and one guard were killed. A second guard, Richard Paddon, was given a reward of five pounds for his part in keeping the remainder of the train on the viaduct and helping to rescue the survivors. At the inquest held on 10 May 1859, the Permanent Way Inspector, the Traffic Superintendent, and Mr Brereton, Brunel's Chief Engineer, were all unable to account for the derailment, and the jury verdict was accidental death.The Times newspaper, London, 11 May 1859, The Cornwall Railway Accident
Nottar Milepost 255.25, across the River Lynher east of . ()
A Class C viaduct high and long on 27 trestles. It was demolished after the line was diverted to a more inland alignment on 19 May 1908.
St Germans Milepost 256.0, across the River Tiddy east of St Germans. ()
This timber viaduct was not included in Margary's classification system as it was not a fan viaduct. Instead it was a timber truss on 16 timber trestles, creating a viaduct high and long. Piles were driven into the mud and the trestles built on top from four groups of four timber baulks, each group raking inwards towards the top of the trestle. Where the piers were on the river bank the trestles rested on low masonry plinths. It was not possible to remove individual timbers from the trestles, unlike the fan viaducts which were designed with piecemeal maintenance in mind. It was demolished after the line was diverted to a new alignment on 19 May 1908.
Viaducts from St Germans to Liskeard
Tresulgan Milepost 261.0, east of . ()
A Class A viaduct high and long on 8 piers. It was replaced by a new stone viaduct on 26 March 1899.
Coldrennick Milepost 261.5, immediately east of Menheniot station. ()
A Class A viaduct high and long on 16 piers. The piers were raised in brick and new iron girders replaced the timber, the work being completed on 23 January 1898. The piers were further strengthened in 1933 by encasing them in stone.
The Times newspaper reported that it "is stated to be 134ft. high and is the highest of the whole series of bridges by which the [Cornwall] railway is carried".The Times Newspaper, London, 10 February 1897, Serious Viaduct Accident
Note: the original typography is maintained in the above quotation and in the quotations below.