Bridges on M25 have generally been designed to Departmental standards. Since 1973 all bridges would have gone through technical approval procedures. The Merrison rules for box girder bridges - first as appraisal rules and then as Interim Design rules - based on partial load factor format were introduced soon after 1971. This format was extended to plate girder design in 1976.
The Department had adopted all parts of BS5400 for bridges by 1983, but by that time the designs had been completed. However, some steel designs which were submitted as alternatives by contractors, and accepted, were based on BS5400.
As with other motorways, M25 follows the inherent principle of sole use by certain prescribed categories of motor vehicles and complete control of access. This has required the provision of a great number of bridges - a total of over 260 over or under M25 itself or on other nearby motorways or all-purpose roads affected, an average of over two per mile length of M25.
Each bridge has been tailor-made to suit the circumstances of its location and its design has had to pay due regard to engineering, economic and aesthetic factors. A few of the more unusual bridges are described.
Lyne Railway Bridge
The Lyne railway bridge at Chertsey carries the twin track Chertsey - Virginia Water line over M25. Because it crosses the motorway on a skew of some 60° the overall span length required is 110m. This led to the adoption of a two-span prestressed concrete cable-stayed bridge supported from two towers rising 30m above the level of the motorway. It is the first concrete cable-stayed railway bridge in Great Britain and was designed by the Southern Region of British Rail and Stressed Concrete Design Limited.
New Haw Viaduct
The New Haw Viaduct at Byfleet, Surrey, carries the motorway over the London - Portsmouth/Southampton railway line and the River Wey Navigation Canal which is owned by the National Trust. The viaduct, which is 285m long, was designed by Gifford and Partners and is of an elegant, yet simple, design consisting of seven spans of post-tensioned in-situ concrete construction with an eighth span of pre-cast concrete beams to facilitate the construction over the busy railway line.
The design of the new bridge to carry M25 across the River Thames alongside the crossing designed by Sir Edwin Lutyens at Runnymede presented particular aesthetic and engineering problems. This crossing was built in 1961 as part of A30 Staines Bypass and the original bridge is a low and wide arch bridge with brick spandrels. For M25, it was decided at an early stage, that merely to provide an even wider replica alongside would have been wrong
visually as well as being difficult and expensive to construct. The solution chosen appears as an open spandrel arch of the same basic shape, but the construction is in fact a series of parallel concrete frames, each one made up of two trapezoidal portal rings which ensure that loads are transferred vertically to the ground to avoid disturbing the foundations of the older bridge. The open spandrels allow light to penetrate and thus avoid an undesirable tunnel effect. The design, by Ove Arup and Partners, has been much praised for producing a pleasing new bridge which preserves and enhances the integrity of the original.
The alignment involved the use of the Chalfont Viaduct through which to thread M25. The Viaduct is a five-span blue-brick railway viaduct built at the turn of the century.
The M25 between M40 and Maple Cross was fitted between two of the exisiting arches of the viaduct with 1.1m lopped off the hardshoulder and the former occupant of the centre span, the River Misbourne, has been diverted into one of the adjacent spans. Careful research was carried out into the original details of construction and special protective measures were taken to provide a slip surface between the seven metres of filling supporting M25 and the pier surface to prevent imposing any downdrag on the pier foundations.