Sapperton Railway Tunnel

Sapperton tunnel

Railway tunnels with cutting
Overview
Line	Golden Valley Line
Location	Sapperton, Gloucestershire
Status	operational
Technical
Length	1 mile 104 yards
Track gauge	4 ft 8 1⁄2 in (1,435 mm)

The Sapperton Railway Tunnel, named after the nearby village of Sapperton (near Cirencester, Gloucestershire) carries the Golden Valley Line from Stroud to Swindon through the Cotswold escarpment. It was begun by the Cheltenham and Great Western Union railway in 1839 and taken over by the Great Western Railway in 1843, being completed in 1845. There are actually two tunnels, the main one at 1 mile 104 yards (1704m) in length, and separated by a short gap, a second at just 353 yards (323m).

The initial plans for the tunnel, dating from 1835, were unusual in that it was proposed to construct the tunnel on a curve, but this seems to have been abandoned before any construction was done; some works remain which are thought to relate to the approach route for the original line, but no excavations were made on that line for the tunnel itself. In 1836 "Mr Brunel" was appointed as engineer for the project; this refers to Isambard, but the involvement of "Mark (sic) Brunel" is also recorded. Brunel promised to get rid of the "objectionable" curve, and plans of the revised straight alignment were deposited in 1838. Preliminary shafts were dug, the work beginning in 1837, to ascertain the geological conditions, on the same straight alignment on which the tunnel was eventually built. In 1841 work began on four additional shafts of a larger diameter, quoted as 3 or 6 metres by different sources, plus a trial heading along the tunnel alignment. The line was opened in 1845.

It was found that the intended route passed through a layer of fuller's earth, which was not sufficiently stable to allow construction of a tunnel. The plans were therefore revised to situate the tunnel in more stable strata at a shallower depth, at the expense of steeper gradients on the approaches. This also reduced the length of the tunnel and so reduced construction costs; it has been asserted that this was the actual reason for the change to the shallower depth, rather than simply an effect of it, but the geological cross-section shows clearly that the header, on the originally-proposed deeper level, passes through a much greater length of fuller's earth than the tunnel as built, and also makes it clear that information gained from digging the shafts would have made it apparent that this would be the case.