THE BASINGSTOKE CANAL

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Engineering Aspects The Greywell Tunnel

ENGINEERING Greywell Tunnel Ash Aqueduct Frimley Aqueduct Water Supplies

The Greywell Tunnel is a monument to the ability of 18th century engineers and builders, using basic tools, to construct a navigation channel for 1230 yards (1125m), 131ft (40m) below Greywell Hill. It is the 12th longest canal tunnel in the United Kingdom and the 2nd longest in southern England. Built throughout using local bricks, it has the traditional three-brick ring which became the standard specification for nearly all transport tunnels built during the canal era. Its unique history has prompted many requests to arrange inspections of the bore from engineers interested in this type of construction. In 1932 a section of the tunnel roof collapsed, caused by water pressure in a pond above, and a tree fell through, which was left standing upright in the bore. The tunnel remained passable until the early 1950s when two canoeists finally passed through. The final and total blockage of the bore by spoil occurred and now extends to the roof. The eastern portal of the tunnel was given a facelift by Hampshire County Council in 1975 to mark European Architectural Heritage Year.

INSTANT FACTS Tunnel length: 1230yds (1125m) (12th longest in UK) Depth below hill (max): 131ft (40m) Construction (approx): 1792 Roof collapse: 1932

The tunnel has no towpath, barge horses being walked over Greywell Hill while the boats were legged through by the crew. The tunnel lining appears to be generally in good condition, although there are some wet patches. The bore is clear and navigable for 800 yards (731m) to the extruded fall. However in 1985 it was discovered that a number of bats hibernated in the tunnel, and it was declared a Site of Special Scientific Interest (SSSI). Following this discovery, a locked gate was installed at the eastern portal to protect the colony of bats by preventing unauthorised access. The blockage, although believed not to be solid throughout, extends for 210 yards (192m).

ENGINEERING Greywell Tunnel Ash Aqueduct Frimley Aqueduct Water Supplies

There are five ponds situated above the western end of the tunnel, some of which were formed as a result of earth subsiding into the tunnel. Subsidence has caused the western portal to collapse. All the brickwork has been swept away, and the tunnel bore is partially blocked. Inside, the tunnel is navigable and the lining appears to be sound for a distance of 220 yards (201m) to the extruded fall. At present the canal is navigable to within a few hundred yards of the tunnel. The eastern part of the tunnel is in chalk for about 800 yards (731m) and the part beyond runs through clay. The collapsed areas are within the clay section. At its deepest the brick-lined bore is 131ft (40m) below Greywell Hill. On one day each year during summer the eastern part of tunnel is entered to check the condition of the interior brickwork. In 1990 a diver inspected the floor of the tunnel and discovered that, contrary to popular belief, no brick floor had been built. In addition the depth of water in the tunnel was found to be only 5ft (150cm) instead of the intended 6ft (180cm). This reveals a probable reason for building Lock 30, the remains of which can be seen not far from the eastern end of the tunnel, after the canal had been opened. The lock had a fall of only 1ft (30cm) but allowed an increased level of water inside the tunnel to help navigation through it. NEXT - more on the tunnel Contents Engineering Index

Last updated April 2000