Constructed between 1902 and 1906, the Chalfont viaduct is of a standard design commonly seen on Great Western rail lines and is constructed of blue/black engineering brick with decorative brickwork including projecting brick imposts on the piers and parapet wall and bullnose brickwork defining the semi-elliptical archways. The brickwork has several areas of mortar repair and occasional loose or missing bricks but is generally of good condition. The viaduct remains largely as constructed, despite the addition of the M25 motorway beneath its central arches in the mid 1980s. These works included the raising of the ground level for the carriageway and the re-routing of the River Misbourne through concrete culverts beneath ground level. A low brick retaining wall was added to the westernmost pier to provide stability for a farm track. Further concrete supports are used above the track to support the embankment. Current works to expand the M25 at this point will include the replacement of the crash barriers and removal of the hard shoulder, thus allowing the motorway to have four lanes in each direction running beneath the viaduct. New crash barriers fixed to the base of the piers adjacent to the motorway will be narrower and provide the viaduct with protection. The viaduct is one of many surviving viaducts of similar construction from this date, however it is unique in being the only brick built bridge on this stretch of the M25 and is a local landmark for road users. This report serves as a record of the viaduct condition, appearance and function prior to these works.
