Carnon viaduct carries the railway from Truro

Truro

Truro is a city and civil parish in Cornwall, England, United Kingdom. The city is the centre for administration, leisure and retail in Cornwall, with a population recorded in the 2001 census of 17,431. Truro urban statistical area, which includes parts of surrounding parishes, has a 2001 census...

 to Falmouth

Falmouth, Cornwall

Falmouth is a town, civil parish and port on the River Fal on the south coast of Cornwall, England, United Kingdom. It has a total resident population of 21,635.Falmouth is the terminus of the A39, which begins some 200 miles away in Bath, Somerset....

 (now branded the 'Maritime Line

Maritime Line

The Maritime Line is a railway line that runs in the valley of the River Fal from Truro to Falmouth on the south coast of Cornwall, United Kingdom.-History:...

') over the valley of the Carnon River in west Cornwall

Cornwall

Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...

, United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

. The viaduct is situated half-a-mile (800 metres) northeast of Perranwell station

Perranwell railway station

Perranwell station is on the Maritime Line from Truro to Falmouth Docks, the services are operated by First Great Western.-History:The station was opened with the Cornwall Railway Falmouth extension on 24 August 1863...

 which is five miles (8 km) from the line's terminus at Falmouth and three miles (5 km) from its junction with the Cornish main line

Cornish Main Line

The Cornish Main Line is a railway line in the United Kingdom, which forms the backbone for rail services in Cornwall, as well as providing a direct line to London.- History :...

 at Truro.

The present nine-arch masonry

Masonry

Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...

 viaduct replaces an earlier structure designed by Isambard Kingdom Brunel

Isambard Kingdom Brunel

Isambard Kingdom Brunel, FRS , was a British civil engineer who built bridges and dockyards including the construction of the first major British railway, the Great Western Railway; a series of steamships, including the first propeller-driven transatlantic steamship; and numerous important bridges...

 for the Cornwall Railway

Cornwall Railway

The Cornwall Railway was a broad gauge railway from Plymouth in Devon to Falmouth in Cornwall, United Kingdom. The section from Plymouth to Truro opened in 1859, the extension to Falmouth in 1863...

. The original viaduct was opened to traffic when the line was extended from Truro to Falmouth in 1863 and had a timber deck supported by timber trestles springing from eleven masonry piers. It was 756 feet (230.4 m) long and 96 feet (29.3 m) high.

The present viaduct is of roughly the same dimensions as the original. It was built by the Great Western Railway

Great Western Railway

The Great Western Railway was a British railway company that linked London with the south-west and west of England and most of Wales. It was founded in 1833, received its enabling Act of Parliament in 1835 and ran its first trains in 1838...

 as an entirely new structure immediately south of its predecessor and it cost £40,000. It has nine arches and opened to traffic in June 1933. The timberwork of original structure was dismantled and removed but the masonry piers were not demolished and still stand beside the replacement viaduct (see photo).

Construction of the original structure posed specific problems not encountered at the sites of other viaducts in Cornwall. The tidal limit of Restronguet Creek

Restronguet Creek

Restronguet Creek is a tidal ria in south Cornwall, United Kingdom. It is a tributary of Carrick Roads, the estuary of the River Fal, and is situated approximately four miles south of Truro and three miles north of Falmouth....

 extended further up the Carnon River valley than it does today and at the site of the viaduct the valley floor then consisted of intertidal mudflats and a great quantity of silt

Silt

Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...

 washed down from the numerous mines

Mining in Cornwall

Mining in Cornwall and Devon began in the early Bronze Age approximately 2,150 BC and ended with the South Crofty tin mine in Cornwall closing in 1998...

 upstream. This soft layer was over 20 feet (6.1 m) thick and "...not an ideal foundation for a 96ft high viaduct."

After exploratory drillings, the engineering contractors sunk cast iron caissons

Caisson (engineering)

In geotechnical engineering, a caisson is a retaining, watertight structure used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam, or for the repair of ships. These are constructed such that the water can be pumped out, keeping the working...

through the silt to the bedrock at each pier location. The cylindrical caissons, 16 feet (4.9 m) in diameter, were then emptied of silt so that masonry footings could be built from the rock up to surface level, pumps being employed to keep the workings dry. This added to the cost of construction but proved entirely satisfactory as Carnon was among the last of the original Cornwall railway viaducts to be replaced.