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Sevenoaks Tunnel
Southeastern Railway mainline, Sevenoaks, Kent, UK
Sevenoaks Tunnel
associated engineer
Peter Ashcroft
date  1864 - 1st May 1868
era  Victorian  |  category  Tunnel  |  reference  TQ525545
ICE reference number  HEW 1394
photo  © Marathon and licensed for reuse under this Creative Commons Licence
Sevenoaks in Kent has the longest mainline railway tunnel in the south of England. It was built at the same time as Polhill Tunnel, under the North Downs, for the twin tracks of the South Eastern Railway. Despite ongoing problems with water ingress, it remains in continual use as part of the modern Southeastern Railway’s main line to London.
Around 1855, John Hawkshaw (1811-91, knighted 1873) surveyed the route of a railway from Sevenoaks to London. The South Eastern Railway (formed 1836) rejected his proposals owing to the cost of constructing the tunnels required. However, on 30th June 1862 an Act was passed for a line some 40km long, including tunnels at Knockholt, Polhill and Sevenoaks.
Peter Ashcroft (c.1809-70), the South Eastern Railway’s engineer, designed the tunnel at Sevenoaks and the one to the north at Polhill (TQ493623 to TQ506602). They shortened the journey between Dover and London by some 21km, abandoning the previous route via Redhill.
Sevenoaks Tunnel is situated 35km from Charing Cross Station in London. It is 3.2km long and slopes gently downhill towards Tonbridge at 1 in 144. Its route runs generally south south east from near Oakhill Road in Kippington to south of the A21 in Sevenoaks Weald.
On 16th May 1863, the first sod was cut in a ceremony to mark the commencement of the tunnel’s construction. By February 1864, work was underway day and night.
The geology of the ridge through which the tunnel passes, where blue and brown clays meet greensand, is subject to ground movement and water springs. Unfortunately, trial borings carried out by the railway’s surveyor Edward Ryde (1822-92) failed to identify the problem. Water ingress was an issue throughout construction and constant pumping was required to prevent inundation.
The tunnel was dug with hand tools, using explosives where necessary to penetrate hard rock. Its construction required the sinking of 15 working shafts to depths of between 35m and 120m. Some 211,000 cu m of material was excavated. Spoil was transported from the tunnel headings to the shafts where it was hauled to the surface in buckets, with power provided by both horses and steam engines.
The horseshoe-shaped tunnel is 7.6m high from soffit to invert and 7.5m wide. Its arch is fully lined with brickwork, using up to five courses of bricks, and varying in thickness from 570mm to 915mm. The twin rail tracks are about 1.4m above the 460mm thick brick-lined invert and were laid originally on a bed of shingle taken from Dungeness beach.
The northernmost and southernmost (pictured) working shafts became the tunnel portals, and the 13 intermediate shafts were retained to provide air flow for the finished tunnel. The shafts, 3m to 3.65m in diameter, are lined with brickwork and have brick ventilation towers above ground level.
Shaft 13 (TQ527541), at the northern end of the tunnel near Oak Lane, was affected particularly by water seepage. The inflow became so great that the contractor John Jay (1805-72) used it to supply the town. He formed the Sevenoaks Water Works Company and constructed underground storage in Oak Lane, from which the water was pumped to a reservoir at Bayley’s Hill.
In April 1864, the workforce went on strike. The workers and their families were housed in settlements of huts built by Jay, who provided a tin church at Tub’s Hill that also served as a school. However, the overcrowded and unhygienic living conditions resulted in outbreaks of disease, including smallpox, and rioting among the inhabitants.
By August 1864, some 2.1km of tunnel had been excavated but only 365m of brickwork completed. In October 1864, the Sevenoaks Nuisances Removal Committee was prosecuting Jay for the lack of adequate sanitation in the accommodation at Shangden. It’s thought likely that he had already left the works having run out of money.
Ashcroft took over the works, presumably using the railway company’s direct labour to complete the tunnel. Nevertheless the contract remained beset by strikes and water ingress, compounded by the stock market crash of 11th May 1866.
In February 1868, the government’s rail inspector, engineer Captain Henry Whatley Tyler (1827-1908), approved the railway to Sevenoaks and on 2nd March that year the line carried its first passengers. On 1st May 1868, the tunnel opened without ceremony for freight and passenger traffic.
The tunnel’s soffit was lined with galvanised iron sheeting to deflect incoming water. The damp conditions were said to be responsible for the rails and sleepers lasting only half the time of those in the open air. The shingle track bed was later replaced.
From 1994 until 2003, Eurostar trains ran along this line using the Sevenoaks Tunnel as part of the route to and from the Channel Tunnel. On 23rd September 2003, the Eurostar service switched to the newly completed High Speed 1 route through Kent.
Resident engineer: Francis Brady
Contractor: John Jay
Research: ECPK
"The Construction of the Sevenoaks Railway Tunnel 1863-1868" by Tessa Leeds, Archaeologia Cantiana, Vol.120, pp.187-204, Kent Archaeological Society, 2000
reference sources   CEH South

Sevenoaks Tunnel