Region: South East
M20. London to Folkestone and A20 to Dover
A20 from Folkestone to Dover is the only non-motorway section of trunk road in the South East Archive since it forms an essential part of the link to the Channel port of Dover. Having taken the decision that the main motorway link from London to Folkestone and Dover should be via M20 rather than M2, it was necessary to provide a high standard link for the final section. However necessary this final link may have been to meet the traffic needs, it was also politically expedient to satisfy the recurring demands, from the port of Dover and from others, that with the arrival of the Channel Tunnel all cross-channel transport modes should be equally well served with good quality approaches.
The motorway, like so many others, was built in stages. The first section was the Maidstone By-pass which was one of the earliest motorways in the country, the contracts having been let in 1958 and 1959 and the two sections opened in 1960 and 1961. The standard was dual 2-lane and this may well have been the case where Ernest Marples, at the opening ceremony, said it would be the last dual 2-lane we would build in the country. (He certainly said it somewhere). Not surprisingly, with the build-up of traffic over the years, especially cross-channel traffic, and the particular problems of conflicting traffic flows into Maidstone at peak periods, a widening scheme became a necessity; and a scheme was commissioned, in about 1986 or 87. The studies soon showed that the straightforward addition of more lanes would not be enough to deal with the conflicting flows and a design incorporating additional lanes on the through motorway together with feeder-distributors for the heavy flows of turning traffic was adopted. Construction took place after 1988.
Swanley to Kingsdown
The 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:
(i) 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.
(ii) 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.
(iii) 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.
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.
Three 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.
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.
The various sections between Swanley and Maidstone and between Ashford and Folkestone went ahead in stages between 1973 and 1981. A number of problems arose, for example the need, due to ground conditions, to adopt side slopes flatter than the norm and the decision therefore to make them flat enough to be farmed right up to the motorway fence which was then located immediately behind the hard shoulder. There were also problems with unforeseen ground conditions on the West Kingsdown to Addington section and difficulty in agreeing a Final Account for a contract which had taken twice the contract period to achieve as well as the usual difficult decisions with landowners and sympathy with some of those affected.
Perhaps one of the more extraordinary events was the decision by Ministers, following a review of the road programme in 1980 or thereabouts, to suspend work on the section of the motorway between the eastern end of the Maidstone By-pass and Ashford. It was restored to the programme in the mid 1980s after a period of heavy lobbying. The Engineer to the scheme was W S Atkins & Partners. The contract was let to Costains in January 1989 and the road opened to traffic in May 1991. The missing link in the motorway had the effect of deterring traffic, with drivers preferring to use A2/M2 to Dover: and this in turn meant that for many years the dual 3-lane motorway between Ashford and Folkestone was one road in England where you probably didnt need to look both ways before crossing! When the motorway was finally opened all the way from Swanley to Folkestone the expected 80/20 traffic split between it and A2/M2 did not, it is understood, materialise but is nearer to 50/50. We shall never know to what extent this arises from drivers having become accustomed to using the latter when the link was not there.
The section of A20 between Folkestone and Dover is a dual 2-lane all purpose trunk road which continues seamlessly from the end of M20. It was interesting and unusual in that part of the scheme was included in the Parliamentary Bill for the Channel Tunnel. This was done partly to satisfy the Dover Harbour Board that the road beyond the Tunnel terminal was being taken seriously and partly to avoid potential problems of interaction between the two projects: at first the idea was to include the whole length to Dover in the Bill, but this would have been difficult to justify and was dropped. The section from Court Wood to Dover was taken forward through the normal highway procedures.
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|M20. Western Section. M25 to Maidstone||M20. Eastern Section. Ashford to Folkestone|