Strood Tunnel, photographed on 2nd February 2008. photograph by Gregory Beecroft

Higham Tunnel, photographed on 10th April 2009. photograph by Gregory Beecroft

Strood and Higham Tunnels were originally built for the Thames & Medway Canal, which ran from Gravesend to Strood, the engineer being William Clark. The canal reduced the distance by lighter between London and Rochester by about thirty miles. Construction was a lengthy task, taking from 1800 to 1824, mainly because of the time taken to excavate the tunnels. Strood Tunnel is 1 mile 569 yards long and Higham Tunnel is 1,531 yards long. The two are separated by gap of 50 yards, which assisted construction and also served as a passing place for boats. The tunnels pass through chalk and, as built, were mostly unlined. Unlike most canal tunnels in England, those on the Thames & Medway Canal were quite large in cross-section. That was to allow passage by river-going ships.

In 1844 the canal company built a single track railway from Milton, near Gravesend, to Strood, engaging James Urpeth Rastrick as engineer. For most of its length the line ran along the tow path. That was only 5 feet wide through the tunnels, so the track was partly supported by a timber structure built on piles in the canal. This was on the southwest side of the canal. The Inspector General of Railways was not satisfied with the structure as built and the strengthening work that he required delayed the opening of the railway by four months. Rail traffic commenced in February 1845.

The canal company was purchased by the South Eastern Railway in 1846. The canal was closed and the tunnel section filled in. The railway re-opened as double track between Gravesend and Strood in August 1847. It followed the original route, parallel with the canal, save in the vicinity of what is now Hoo Junction, where a more direct alignment was adopted. The railway was completely isolated from any other until the North Kent line to Gravesend via Woolwich and Dartford opened in 1849.

The canal was formally abandoned between Gravesend and Higham in 1934, but survives largely intact, though heavily overgrown. A swing bridge over the canal, mid-way between Gravesend and Hoo Junction, was rebuilt as recently as 1992 by the Network SouthEast civil engineer. Although the timber deck and parapet are new, the original metal components were re-used. The bridge no longer opens and it carries only pedestrians and cyclists.

Brick lining was added to parts of Strood and Higham tunnels, eventually extending to about half their length. Some steel sheet lining was also installed. Despite this, during the latter part of the 20th century falling chalk became an increasing problem. The chalk had become wetter and softer. The tunnel was also prone to flooding, a contributory factor being the impermeable layer of clay comprising the former canal bed. A speed restriction of 20mph became necessary in case trains hit chalk boulders on the track. The tunnels were closed for a year from January 2004 for modernisation. They were lined with cast in-situ, high-strength, self-compacting concrete and a new drainage system was installed. It is unfortunate that such early structures had to be so drastically altered, but this was essential if they were to remain functional.

This plaque at Strood station marks the renovation of the tunnels in 2004. photograph by Gregory Beecroft