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Polhill Tunnel
Badger's Mount, Kent, UK
associated engineer
Peter Ashcroft
date  1864 - 1868
era  Victorian  |  category  Tunnel  |  reference  TQ494619
ICE reference number  HEW 1610
Polhill Tunnel was constructed to carry the South Eastern Railway under the summit of the North Downs, between Orpington and Sevenoaks in Kent. It has remained in continual use under ownership by a series of rail companies, and is now part of Southeastern Railway’s main line to London.
The South Eastern Railway was formed in 1836. When its main line between Charing Cross and Tonbridge in Kent opened in 1868, it shortened the previous train journey from London to Dover by some 21km. The route includes three tunnels — Knockholt, Polhill and Sevenoaks.
Peter Ashcroft (c.1809-70), engineer to the South Eastern Railway, designed the 2.4km long Polhill Tunnel. It is located 29km south east of Charing Cross Station and slopes down towards Sevenoaks at a gradient of 1 in 143. The tunnel passes through a 1.2km long deep chalk cutting and commences (TQ493623) north of Badger’s Rise, passes under the summit of the North Downs and ends (TQ506602) at Old Polhill, east of the M25.
The 6.7m high twin-track tunnel is cut through sound chalk throughout. Its side walls are of exposed chalk and only the roof has a lining — a semicircular brick arch 460mm thick with a diameter of 8.7m at the springing, which is 2.5m above rail level. The north and south entrance portals are horseshoe-shaped and constructed in 685mm thick brickwork.
Five working shafts were sunk from the surface during construction, varying in depth along the tunnel — 36m, 49m, 57.6m, 64m and 66.75m respectively. A total of 145,300 cubic metres of chalk was taken out via the shafts. Most of the excavated material was used to construct a railway embankment, up to 12m high, south of Dunton Green to Riverhead, north of Sevenoaks.
In 1907, the permanent way of the tracks was relaid, with routine replacement scheduled for 1928 on the above ground line. Inside Polhill Tunnel, atmospheric conditions dictated that the permanent way was relaid every five years.
At 5.30pm on 24th August 1927, the K-class tank engine locomotive River Cray (No. 800) travelling south on the embanked line experienced an increasing rolling motion. It began to derail 3.8km south of Polhill Tunnel resulting in the deaths of 13 people and serious injuries to 20 more. Derailment was complete within a further 500m. Heavy rainfall that day "may have caused the track to go down rapidly under traffic" (subsidence) and the accident report concluded "there was an insufficiency of hard and clean ballast foundation, a lack of proper drainage, and irregularities in the level and gauge of the rails".
In February and March 1948, tunnel maintenance work was carried out, including renewal of ballast and tracks as necessary.
Over the years, in various places water seepage has caused the tunnel’s chalk arch supports to disintegrate. The affected areas of chalk have been cut out and repaired with brickwork.
Between 1994 and 2003, Eurostar trains used the Polhill line for travel to and from the Channel Tunnel. Since 23rd September 2003, the Eurostar service has used the High Speed 1 route through Kent.
Contractor: John Jay (contract no. 2)
Research: ECPK
bibliography
"Railway Accidents" report by Colonel Sir John W. Pringle C.B., Ministry of Transport, London, 1928
"Renewal of Ballast and Track by Mechanical Means in Polhill Tunnel" by A.H. Cantrell, in Journal of the ICE, Vol.31, pp.331-350 and discussion pp.364-376, London, February 1949
www.ice.org.uk
www.thisiskent.co.uk
reference sources   CEH South
Location

Polhill Tunnel