Swanley to Kingsdown

Darent River bridge under constructionThe Swanley/West Kingsdown contract was part of a section of the M20 from Swanley to Addington which was published in 1971. The order for the major length being made in 1974.

Some 19km of the M20 motorway were already in use between Addington and Hollingbourne. On completion of this contract and the adjacent length from West Kings down to Addington there was a continuous length of motorway 34km long between Swanley and Hollingbourne.

The scheme, 8.4km long, involved the crossing of two deep valleys in an area of undulating, mainly arable and pasture land. The two valleys were at Farningham, where the River Darent was crossed, and at Brands Hatch which was a dry valley.

A temporary junction with the A20 involving traffic lights was constructed at the South Eastern end of the scheme. This was removed when the West Kingsdown to Addington section of the M20 was opened to traffic in 1980.

The work comprised:

  • The construction of approximately 8.2 km of dual three lane motorway and 0.6 km of connecting roads, both in flexible pavement, together with the installation of Traffic Signs, Lighting and Power and Communications Cabling.
  • Associated bridgeworks consisting of three overbridges (including a gas pipeline bridge) four underbridges (including a 75 m three span steel concrete composite bridge over the River Darent) and one farm access underbridge.
  • Alterations to approximately 5.5 km of adjoining side roads.

The total cut volume was approximately 2.4 million cu m of which approximately 60% was chalk. There was a surplus of cut within the scheme.

Chalk earthworks were generally restricted by the specification to the months of April to October inclusive unless a specific relaxation was given. The specification required chalk excavation to be carried out by a face shovel or similar shovel loading equipment and the haulage of material over chalk was restricted to equipment with a struck capacity not exceeding 15 cu metres. Compaction of chalk was again to specific requirements. The specification incorporated methods of operation for chalk cut to fill designed to reduce the degratation of the material to a minimum.

The other major earthworks material was clay with flints overlying the chalk. Granular material was required for flood protection at the base of the embankment where the River Darent was crossed and as backfill to the Bridge abutments.

The side slopes in cut areas varied with the material encountered, from a normal 1 in 2 to a slope of 12 in 1 at the base of the chalk cutting. Fill slopes were 1 in 2 and 1 in 22, depending on the material of which the embankment was formed and its height. Maximum depth of both cutting and embankment was 18 metres.

Carriageway drainage was to standard DOE requirements using French drains in cuttings and central reserve and a positive system on the outside of embankments. All outfalls were to lagoons for initial storage in times of storm and thence to soakaways or to the River Darent.

All roads were designed to Road Note 29 (Third Edition) and were of flexible construction. Alternatives for flexible or rigid pavements for the main carriageway were included in the tender documents. A capping layer of chalk was being placed on embankments and a minimum construction depth of 450 mm was being used due to frost susceptibility. The Contractor's chosen alternative construction for the motorway carriageways was for composite construction of 510 mm depth.

The Contractor elected to provide in-situ cast concrete margins as a permanent datum for the motorway carriageway construction above subbase level. This also facilitated the early completion of work in the verge areas.

Statutory Undertaker Services were mainly encountered at Dartford Road and in the Brands Hatch area. In both instances, programmed diversions were carried out as final service lines passed through the new underbridges.

With no interchanges to be constructed, the total number of bridges in this contract was eight. Three of the underbridges took side roads under the motorway, the fourth took the Darent River under it and the fifth was a farm accommodation underpass. Two overbridges carried side roads and the third carried a gas main over the motorway.
M20 River Darent

The motorway line crossed two valleys and cut through a hill in between, with maximum cut height of 25 feet and maximum fill height of the order of 50 feet. The two major underbridges, Darent River Underbridge and Brands Hatch Underbridge, were situated at the lowest points of the valleys and accordingly have about twice the minimum headroom. Foundations for all but these two bridges were spread footings bearing on block chalk, but the poor quality of the chalk in the Darent Valley and Brands Hatch areas made it necessary to pile both these bridges. Driven cast-in-situ shell piles were used.

The underbridges were in all other respects different in character. At the West end of the contract, the underbridge carrying the A225 under the motorway, was a single span high skew structure with a post-tensioned beam and slab deck. The bridge adjacent to it, carrying the motorway over the River Darent was, however, a three span structure with a minimum clearance of the order of 35 feet. The piers for this structure were piled, but in order to avoid the expense of piling abutments, chalk embankments were formed in advance of construction of the bridge and abutments were subsequently founded on these. The super-structure was composed of three span continuous high yield steel plate girders, acting compositely with an in-situ reinforced concrete deck.

Eglantine Lane Underbridge was a single span low skew underbridge with a deck composed-of precast pre-tensioned inverted T-beams and solid infill concrete. Brands Hatch Underbridge which was again an excess headroom structure, was also single span and had a precast pre-tensioned M beam deck. A feature of this structure was the massive wing walls which were cantilevered from the backs of the abutments. Barnshaw Accommodation Underpass, the final underbridge in the contract, was a straight forward in-situ reinforced concrete box.

Gas pipeline exposed on temporary cradlesThree Gates Road Overbridge and Crowhurst Lane Overbridge were similar structures although their geometry was somewhat different. Three Gates Road was a two span low skew structure, whereas Crowhurst Lane was a multi-span high skew structure. Typical cross sections of the decks were similar, as both decks were of in-situ reinforced concrete with circular voids.
M20 gas pipeline bridge

Perhaps the most interesting structure in the contract from the point of view of construction, was the Gas Pipeline Bridge. This was required to carry a 30" diameter high pressure main carrying North Sea Gas to the South East of England and it had to be constructed without disrupting gas supplies.

The pipeline was thus in continuous use during construction operations and the bridge had to be constructed around it. The structure itself was a single span cast-in-situ post-tensioned deck spanning between bank seat abutments. Very close liaison has been maintained both during the design stage and during the construction process with the British Gas Corporation who were the authority responsible for the pipeline. Because of the unusual circumstances, it was found necessary at the design stage to specify a construction method for the structure. This involved first of all excavating pits at intervals along the pipeline constructing temporary supports, completing the excavation and subsequently forming a deck round about the pipeline. Stressing of the bridge lifted the pipeline off the temporary supports, transferring its load to the permanent supports contained within the structure. The temporary supports were then removed and earthworks operations under the bridge continued to finished level.