Digswell Viaduct

The Digswell Viaduct,^[2][3] also known as the Welwyn Viaduct^[4][5] and officially the Welwyn Railway Viaduct,^[6][7] is a railway viaduct that carries the two tracks of the East Coast Main Line over the Mimram Valley in the East of England. A prominent local landmark, it is located between Welwyn Garden City and Welwyn North railway stations, and is located above the village of Digswell and the River Mimram. It has been Grade II* listed since 1980.

Digswell Viaduct
The viaduct as viewed by drone
Coordinates	51°49′06″N 0°11′41″W
Carries	East Coast Main Line
Crosses	River Mimram
Official name	Welwyn Railway Viaduct
Other name(s)	Welwyn Viaduct
Characteristics
Height	100 ft (30 m)
No. of spans	40
Rail characteristics
No. of tracks	2
Electrified	25kV AC OHLE[1]
History
Designer	William Cubitt Joseph Cubitt
Constructed by	Thomas Brassey
Opened	7 August 1850
Location

The viaduct has forty arches and was originally built from red facing bricks, with blue facing bricks also added in the 1930s. Metal gantries were added to the side of the viaduct in 1980 as part of the electrification of the Great Northern route. At 1,560 feet (475 m) in length and up to 100 ft (30 m) in height, it was the one of the largest works on the Great Northern Railway when it opened in 1850.

Design

The viaduct carries the East Coast Main Line across the Mimram Valley.^[5][8] Due to the width of the viaduct, the railway narrows from four tracks to two tracks, making it a bottleneck which is the limiting factor for capacity on the congested and strategically important route.^[5] This problem is exacerbated by Welwyn North railway station situated at the northern end of the viaduct, which blocks the line while trains are stationary, and by two tunnels to the north. Several ideas to overcome the limitations of the viaduct and station without damaging the viaduct's essential historic character and rhythmic design are periodically discussed.^[9]

The viaduct was styled to be similar to roman aqueducts,^[2] and was one of the largest works on the Great Northern Railway when it was constructed.^[3] It is around 1,560 feet (475 m) in length and comprises forty arches with a maximum height of 100 ft (30 m). It is built from red brick, and the entire viaduct took thirteen million of these bricks to build,^[6][10] which were fired from clay mined from the same site at the time the viaduct was being built.^[2] Each pier was built with 65 piles driven into the ground by steam-fuelled piling engine.^[3]

The original waterproofing originally consisted of a bitumen membrane, which the brickwork was covered with so that the water would drain into cast iron pipes at the centre of each pier. The drainage system had become clogged over time but this could not be fixed as the pipes were inaccessibly positioned inside the viaduct. Furthermore, the membrane's efficacy had deteriorated to a point that water was leaking through and running down inside the brickwork.^[11] These systems were subsequently replaced in 1986.^[2]

Due to the nature of the ground in the Mimram Valley, eleven of the forty piers were reinforced with timber, for which 19,844 cubic feet (561.9 m³) was needed.^[3] The viaduct joins embankments at both ends, which used approximated one million tons of earth to build; this was all moved by manpower and horsepower.^[2] The construction of the viaduct has been described as a "great engineering feat" and the viaduct has become a prominent part of local history in Welwyn.^[2][8]

The viaduct has been Grade II* listed since 4 November 1980,^[6] meaning that it is a "particularly important [structure] of special interest".^[12] Historic England's official list description for the viaduct describes it as "An impressively monumental and elegant railway viaduct of 40 arches for the Great Northern Railway by eminent engineers Sir William and Joseph Cubitt".^[6]

The viaduct as viewed from Digswell Park Road in 2021

History

1848–1850: Construction and opening

The viaduct was built for the Great Northern Railway between 1848–1850,^[6][10] and was built to carry their line between Kings Cross and Welwyn (now Welwyn North), although they planned for this to extend to York.^[8][13] The viaduct was designed by railway engineers William Cubitt,^[6] who at the time was the Chief Engineer of the Great Northern Railway,^[3] and his son Joseph Cubitt. Prominent railway engineer George Hudson opposed the viaduct's construction, saying that the viaduct would collapse into the valley due to the nature of the ground, but also because he wanted to protect his monopoly on the Midlands' railways.^[8]

The Digswell Viaduct in October 1864

The construction of the viaduct came at a cost of £69,397,^[6] which is equivalent to £9,374,000 in 2023.^[14] The construction of the viaduct was delivered by Thomas Brassey,^[6] who stayed in the Cowpers Arms while his workmen lived in temporary huts in Digswell. The behaviour of these workmen led to complaints from locals, and they became known locally as the 'tray bong' (i.e. an anglicised très bon) gang from their poor French.^[8]

There is an urban myth that the viaduct was officially opened by Queen Victoria on 6 August 1850, but she was reportedly so frightened of its height that she refused to travel across it and left the train, using a horse-drawn carriage to travel the length of the bridge on the ground. However, this is unlikely because her diaries show that she was staying at Osborne House on the Isle of Wight on that day.^[2]

The actual opening of the line, including the viaduct, was on 7 August 1850.^[6] The inaugural trip left at 9:00, and involved 400 people travelling from Kings Cross to Peterborough to view the newly-built line. The train stopped at Welwyn to allow people to disembark and "view the magnificent viaduct by which the railway there crossed the valley". Although Queen Victoria was not present at this opening, she did use the viaduct in 1851 to travel to Balmoral, and her journey received significant press coverage at the time; there is no record of her refusing to cross the viaduct during this journey.^[10]

1851–present: operation and refurbishment

On 7 October 1864, a Midland Railway train collided with a freight train belonging to the Great Northern Railway on the viaduct, which was being shunted at the time. The points were set incorrectly such that the train was shunted onto the main line rather than the sidings, where the collision then occurred. The Great Northern Railway immediately took responsibility for the accident, however a court case, Drakeford vs. the Great Northern, arose as to the damages owed to the Midland Railway. A jury eventually settled these damages at £500,^[15] which is equivalent to £62,000 in 2023.^[14]

The viaduct in 2009, showing a closer view of the added electrification gantries.

In 1858, the tie rods were strengthened to further support the viaduct.^[2] In response to signs of deterioration appearing by the 1930s, an extra layer of 9.1 inches (230 mm) blue facing bricks were also added to the viaduct in a project that was completed in 1935.^[2][11] Tie bars were then added to reinforce both layers of bricks in 1965.^[2] As part of the electrification of the Great Northern route by British Rail in the 1970s,^[16] overhead lines were added to the viaduct with metal gantries suspended on the side.^[6] Extra drainage and waterproofing work was completed on the viaduct in 1986.^[2]

In his 1889 book The Railways of England, William Mitchell Acworth reports that the Great Northern Railway traditionally considered the viaduct to be more important than the roof of Kings Cross, and yet somehow "not perfectly safe". According to Acworth, the company's engineers used to get regular letters and telegrams from worried passengers about its safety; however, engineers never found any issues with the structure.^[17]

Capacity issues have been raised since as early as the 1890s, when the Hertford Loop line was extended to Stevenage in order to prevent having to widen the viaduct from two tracks to four. The project was completed in 1898,^[18] and the Hertford Loop line is still used as a diversionary route for trains today when there is disruption on the East Coast Main Line.^[19] Capacity issues were raised again in the Department for Transport's 2006 study on railway capacity.^[5]

A new station opened in the new town of Welwyn Garden City in September 1926, replacing the halt that had been built shortly after the town was incorporated in 1920.^[20][21] To coincide with this, Welwyn railway station was renamed to Welwyn North.^[13] This arrangement is still the case today.^[1]

In August 2013, BBC News listed the quadrupling of the viaduct and adjacent Welwyn Tunnels as one of the top five infrastructure projects that could improve life for people in the United Kingdom. It estimated the cost of such a project at £440 million and considered its likelihood to be "probable, at some point".^[22] In June 2020, locals raised concerns at proposals to build developments on land adjacent to the railway that was used to graze animals. Locals requested that Welwyn Hatfield Borough Council restrict development in the area due to its value to the landscape of the Mimram Valley.^[23]

See also

• Ouse Valley Viaduct on the Brighton Main Line
• Ribblehead Viaduct on the Settle–Carlisle line
• Glenfinnan Viaduct on the West Highland Line