Meldon Viaduct

Meldon Viaduct is a disused railway viaduct crossing the West Okement River at Meldon, 2.5 miles (4.0 km) south-west of Okehampton, on the edge of Dartmoor in Devon, South West England. This truss bridge was constructed from wrought iron, instead of stone or brick arches. It opened in 1874 for a single track; in 1879 its width was doubled for a second track. Although regular services were withdrawn in 1968, the bridge was used for shunting by a local quarry. In the 1990s the remaining single track was removed.

Meldon Viaduct
Meldon Viaduct in 2010
Coordinates	50°42′47″N 4°02′03″W
Carries	The Granite Way cycle & walkway
Crosses	West Okement River
Locale	Dartmoor
Owner	Meldon Viaduct Company Ltd
Heritage status	Scheduled monument
Characteristics
Design	Warren trusses on lattice trestles
Material	Wrought iron
Total length	535 ft (163 m)
Width	Double-track standard gauge rail
Height	151 ft (46 m)
No. of spans	6
History
Engineering design by	W. R. Galbraith
Opened	12 October 1874
Closed	1990
Location

The crossing is now used by the Granite Way, a cycle track skirting Dartmoor. It is a scheduled monument, and is one of only two surviving railway bridges in the United Kingdom that use wrought iron lattice piers to support wrought iron trusses.

History

v t e Dartmoor Railway
Legend
miles Tarka Line to Exeter St Davids 0¾ Crediton Salmon Pool level crossing 3¾ Yeoford Coleford Junction Tarka Line to Barnstaple 8½ Bow 11¾ North Tawton 14½ Sampford Courtenay Okehampton Interchange (planned) 18¼ Okehampton 20¼ Meldon Viaduct Meldon Viaduct London & South Western Rly and Exeter–Plymouth line to Padstow and Bude to Plymouth via Bere Alston

In the 19th century, the London and South Western Railway (LSWR) was in competition with the Great Western Railway (GWR) to provide passenger trains from London to Devon and Cornwall. But the South Devon Railway, an associate of the GWR, had already built a line from Exeter St Davids to Plymouth along the south coast and the south side of Dartmoor, under the direction of Isambard Kingdom Brunel. Thus the LSWR, and its engineer, William Galbraith, were left with a route from Exeter that would have to skirt around the north and west sides of Dartmoor.^[1]

Meldon Viaduct was begun in 1871, and opened on 12 October 1874, linking Okehampton and Lydford (Lidford). It was part of the LSWR's routes to Plymouth (from 1876) and Bude (from 1898) via Okehampton. Originally it carried a single track, but it was doubled in 1879 when a second viaduct of similar design was constructed next to it and the two were joined.^[1][2] This was done while the original track remained open. To obviate high scaffolding, the trestles were erected by derricks mounted on the original structure. The 12 trusses, each weighing 21 long tons, were constructed nearby and lifted by two heavily-ballasted, rail-mounted cranes, which were then pulled onto the original bridge, and the truss lowered into position.^[a] Each truss was installed in about 2 hours 30 minutes, thus the work could be arranged around the railway timetable without closing the line. The method, devised by engineer T. Wrightson and supervised on site by W. Jacomb, worked well, and the entire ironwork was erected in 16 weeks.^[4] On 5 May 1931 a carpenter, F. Rook, was killed in an accident while replacing timber decking on the down line.^[5]

The viaduct is 535 ft (163 m) long, standing 151 ft (46 m) above the bottom of the valley. It has a slight curve^[b] which, combined with its exposed position, resulted in speed and weight restrictions being applied to trains crossing it. The speed limit was set at 20 mph (32 km/h) in 1927.^[7]

In 1938 braces were added between the lower ends of the older trestles. In 1944, to allow heavy wartime traffic, the outer trestle legs were weighted with additional concrete to resist uplift. The viaduct was further strengthened from 1959–1960, with the inner trestle legs being weighted, and the bracing of the original trestles being replaced with stronger members.^[1]

Passenger services on the line were withdrawn in 1968, after which the line reverted to a single track and the viaduct was used predominantly for shunting freight trains serving the ballast quarry at Meldon. In 1970, a road was built across the viaduct to permit lorries access to the Meldon Dam construction site. The bridge was assessed in 1990 as being too weak to carry trains and the rails were removed. Six years later, the viaduct was refurbished and re-opened as part of the 11 mi (18 km) Granite Way, a combined cycle and foot path^[1][8] (part of NCR27). It is now a scheduled monument,^[2] and is owned by Meldon Viaduct Company Ltd,^[9] a charitable company^[10] set up in 1998 to safeguard it.

At various times since the closure of the line, local people and enthusiasts have campaigned for its re-opening, a campaign which gained added prominence in 2014 after severe winter storms severed the Great Western Main Line at Dawlish in February. According to Network Rail, the condition of Meldon Viaduct was a significant obstacle to the re-opening of the line, and it would need to be replaced before trains could run on the line.^[11][12]

Design

The viaduct is constructed mainly of wrought iron. It is one of two surviving wrought iron truss and trestle railway bridges in the United Kingdom, ^[1][6] the other being the Bennerley Viaduct,^[13] between Nottinghamshire and Derbyshire, which differs by having lattice trusses (meaning the diagonals are criss-crossed).^[14][c]

The structure is actually two viaducts side-by-side. Each viaduct consists of six spans of 90 ft (27 m), bridged by a pair of Warren trusses at 5 ft (1.5 m) centres, which are supported by five lattice trestles.^[1][6] When the original viaduct was widened to allow double-track operation, a second viaduct^[d] of almost identical construction was erected on its south-east side, 7 ft 6 in (2.29 m) from the original one, with the gap spanned by bracing and timber decking.^[1][6] The new viaduct then carried the down line (from London). The trusses are 9 ft 4 in (2.84 m) high.^[6] The bottom (tension) members of each newer truss include a continuous plate underneath for extra strength. The trestles vary in height from 48 ft (15 m) to 120 ft (37 m); each is constructed of four columns connected by horizontal and diagonal bracing. Each column is made of a varying number of sections, end-to-end, each consisting of six flanged curved pieces riveted together side-by-side.^[6] The trestles taper inwards from the bottom, and rest on 24 ft (7.3 m) wide masonry bases,^[6] the newer trestles straddling the older ones at the bottom.

At each end of the viaduct, the trusses are supported on cast-iron sliding bearings on four cast-iron cylinders filled with concrete, which are deeply embedded in the ground.^[16][18]

The Victorian ironwork's gentle curve spanning the gorge, a masonry base, and abutment cylinders

Trestle detail, showing added collars and adjustable braces on the originals

Underside of trusses (originals on the right), and cantilevered deck

Notes

1. A photo of the viaduct being widened in 1879 is shown in this book (facing page 1)^[3]
2. The horizontal radius is 1,980 ft (30.0 chains; 600 m).^[6]
3. The only road bridge in the UK supported by metal lattice trestles is Ovingham Road Bridge.^[15]
4. Some sources claim this is wrought iron,^[6][16] others welded steel^[1][17] (despite being clearly riveted and bolted).

References

1. "Meldon Case Study" (PDF). Devon County Council and Dartmoor National Park Authority. Retrieved 15 January 2023.
2. Historic England (11 June 1976). "Meldon viaduct (1002631)". National Heritage List for England. Retrieved 21 April 2014.
3. Hall, Cyril (c. 1913). Conquests of Engineering (PDF). Blackie & Son Ltd. Facing page 1. Photo of the viaduct being widened, 1879.
4. Wrightson, T. "Correspondence: The different modes of erecting iron bridges". Minutes of the Proceedings of the Institution of Civil Engineers. 63 (1881): 213. doi:10.1680/imotp.1881.22142. eISSN 1753-7843.
5. Ministry of Transport (1931). "Three Months ending 30th June, 1931, Part B". Railway Accidents: Reports by the Inspecting Officers of Railways of Inquiries Into Accidents. p. 30.
6. Otter, R. A. (1994). Civil Engineering Heritage: Southern England. London: Thomas Telford Ltd. p. 61. ISBN 0727719718.
7. "Meldon Viaduct". Legendary Dartmoor. 23 December 2016.
8. "The Granite Way" (PDF). Devon County Council. 2008. Retrieved 15 January 2023.
9. "Meldon Viaduct Company Limited". Charity Commission for England and Wales.
10. "Charitable companies limited by guarantee". Community Companies.
11. Roden, Andrew (20 August 2014). "Dawlish Diversion: the main contenders". Rail. No. 755. Peterborough: Bauer Media Group. pp. 72–74. ISSN 0953-4563.
12. Modern Railways, September 2014, p. 67.
13. "History of the use of iron in foundations". Information Resources Library. Institution of Civil Engineers. p. 7. Archived from the original on 27 April 2014. Retrieved 26 April 2014.
14. Biddle, Gordon (2011). Britain's Historic Railway Buildings: A Gazetteer of Structures (Second ed.). Hersham, Surrey: Ian Allan Publishing. pp. 188–189. ISBN 9780711034914.
15. Rennison, Robert William (1 January 1996). Civil Engineering Heritage: Northern England. London: Thomas Telford Ltd. p. 31. ISBN 0727725181.
16. "Meldon Viaduct". Grace's Guide To British Industrial History. Retrieved 22 January 2023.
17. "Meldon Viaduct". Dartmoor Railway Association. Retrieved 22 January 2023.
18. "Meldon Viaduct". National Transport Trust. Click "Description" tab.

External links

• Video footage of Meldon Viaduct.
• Photo of the single-track viaduct in the mid-1870s.
• "The Okehampton Line: Restoring the second route from Exeter to Plymouth around the north side of Dartmoor" by Railfuture.