Region: South West
The Twyning Green (J8) to Edithmead (J22) section of M5
INDEX TO SECTIONS M5 Twyning to Tewkesbury (J8 to J9) Contract 1B extended 4.8 miles from the River Avon flood plain to the A438 road at Ashchurch where a two level roundabout interchange was constructed. The scheme, which provided a partial bypass to Tewkesbury, was let to Richard Costain & Co Ltd. at a tender sum of £3.39m and commenced in August 1968. Ten bridges and two culverts were built and earthworks amounted to about 1,242,000 cu.yds. M5 Gloucester - Cheltenham (J9 to J12) Contract 2/3 was the longest contract on the M5 covering a distance of 12.5 miles. It was let to a consortium of Cementation and Fairclough at a tender price of £10.2 m commencing in May 1969. The interchange on the A438 road on the more northerly Costain Contract only enabled a partial Bypass to Tewkesbury so there was a requirement in the contract for Cementation-Fairclough to complete the section of Motorway from the A438 to the next interchange at Piffs Elm by May 1970. In addition to the Piffs Elm interchange, which only provided connections in the north facing direction, there was a major three level interchange with the A40 Golden Valley Bypass between Gloucester and Cheltenham. This required five bridges with the Motorway at the lowest level, a roundabout at the middle level and the A40 crossing over the top on a high level structure. In order to facilitate the main contract advance contracts were let for the construction of a railway bridge at Churchdown and for a service culvert at Piffs Elm. The contract required the construction of 23 bridges and 11 culverts. Earth works necessitated 3,795,000 cu.yds. of excavation and importation of 552,000 cu.yds. of filling material. This section of the Motorway received substantial adverse press publicity which led to a Parliamentary Select Committee Inquiry over allegations that the Contractor had been overpaid for the earthworks. However, after Corduroy and Partners had been employed to undertake an analysis, it was accepted by the Ministry of Transport that the Contractor was not paid more than he was due overall, though early payments were higher than they should have been. Mr.R Bridle was the Chief Highway Engineer at the Ministry during this period and a letter from him on the subject is included in the archive. M5 Moreton Valance Section (J12 to J13) This contract commenced in June 1969 and was completed at the same time as the more northerly Cheltenham section in March 1971. The length was 6.3 miles and the contract was awarded to Costain- Green Consortium for a tender price of £5.5 million. A railway bridge at Haresfield was constructed as an advance contract by Turriff. There were two interchanges on the section, an elevated roundabout at Stroudwater, and with the A419, where South facing slip roads were provided. The disused Stroudwater canal was filled in adjacent to the interchange. Where the main carriageway crossed the disused Gloucester Aviation Airfield it made use of the main runway which had been designed to cope with the Javelin Jet Planes; the taxi ways were broken out and restored to farm land and the material used in backfill to structures. The bridges at the Stroudwater Interchange had decks requiring 1,380 cu.yds of concrete. These decks were constructed in a continuous concreting operation over 24 hours (unusual at that time) by making use of mobile concrete pumps. The earthworks required approximately 1,794,000 cu.yds. of excavation and the importation of about 372,600 cu.yds. of filling material. In addition to the railway bridge there were twelve other bridges. M5 Michael Wood Section (J12 to J13) This section was 8.2 Miles long and crossed open farm land, skirting the edge of the Cotswold escarpment. In the area of Michael Wood the geology was very complex. There were twelve bridges including two over line railway bridges. The Michael Wood service area site was within this length. The earthworks involved 2,415,000 cu.yds. of excavation and 317,400 cu.yds. of imported filling material. The Contract, which started in November 1969, was let to a Consortium of Wimpey and Kier for a tender sum of £5.6m. It was opened to traffic in December 1971. M5 Alveston Section (J14 to J15) This 8 mile section was also let to the Wimpy-Keir Consortium for a tender price of £5.85m. The works, which commenced in October 1969, included the construction of the north facing slip roads at the Almondsbury Interchange. When it was opened to traffic in December 1971, at the same time as the previous contract, the M5 was completed between Strensham and the M4. The contract included a "diamond" or single bridge interchange with the B4509 at Falfield which provided connection to Chipping Sodbury also to the A38 and the upper Severnside area including Thornbury. There were twelve bridges on this length a variety of other minor structures, and the earthworks included 1,794,000 cu.yds. of excavation together with the importation of 317,400 cu.yds. of filling material. A geological fault near the Little Avon river has caused a sharp change in the geology of the area, to the north lies the Keuper Marls and to the south the
mudstones and limestones of the Falfield beds. At Milbury Heath the route required a cutting thirty feet deep in old red sandstone. M5 Almondsbury Section (J15 to J16) This section of the Motorway was built as part of the M4/Almondsbury Interchange construction and unfortunately no details are currently available. M5 Filton By Pass to Avonmouth Section (J16 to J18) This section of the Motorway commenced at the southern end of the Filton Bypass, which was completed by Gloucester County Council in 1962, and ended at the Portway between Bristol and Avonmouth. A link road, also subject to motorway regulations, three quarters of a mile long, connected a roundabout at the Portway with the Motorway. The length of the contract was 4.2 miles and it was let to A.E.Farr Ltd. in March 1967 for a tender sum of £3.67m. At Hallen a mile long cutting was excavated, with a maximum depth of about 60 feet, which generated 1,587,000 cu.yds. of filling material. In addition there was a requirement to import 745,200 cu.yds. of materials for embankment construction which included selected granular material, common fill and fly ash. The rate at which these materials were required placed considerable pressure on the contractor. The railway bridge over the motorway at Hallen was constructed in advance of the main contract. West of Hallen the motorway is on low embankment until it rises at the approach to the Avonmouth Bridge. The side roads and the railway cross over the motorway on high embankments. These embankments rest on a plain composed of alluvium with some peat layers. During the design stage it was decided, from preliminary soil surveys, the alluvium required special earthworks treatment as calculations showed that if common fill were used on the high embankments there would be failures of the underlying ground before the full height could be constructed . Furthermore settlement of the underlying material would be very slow and it was estimated that 80% would occur after the Motorway was completed. Due to the lower weight pulverised power station fly ash could be used to form embankments of the required heights; so it was decided to undertake an embankment trial to verify the assumptions and to obtain experimental data. As a result, a system of temporary surcharging of the embankments to accelerate settlement was determined and included in the contract. These ground conditions also posed difficulties for drainage culverts and service pipelines under the embankments which also required special measures in construction.The design measures and research which were undertaken is the subject of a paper (and subsequent discussion) in the Proceedings of the Institution of Civil Engineers by N.J.Dallard F.I.C.E. of Freeman Fox and Partners in June 1971 Volume 49 and April 1972 Volume 51 entitled "Design and construction of embankments on an alluvial plain". This paper gives a detailed account of the experimental work and the measurements when the earthworks were undertaken. Granular layers were placed beneath the embankments which were temporarily surcharged to accelerate settlement. The settlement period written into the Contract was 630 days but results of on site monitoring of settlement and pore- water pressures showed that the surcharge could be removed some 130 days earlier and thus the section of motorway was opened three and a half months before the contract date. The experience gained from the work at Avonmouth were employed in work on later contracts in further Southwest on the Somerset levels. The research by the Road Research Laboratory is reported in document LR419 Avonmouth Bridge (J18 to J19) As a result of routing the M5 to the West of Bristol it was necessary to cross over the tidal estuary of the River Avon, which has a tide range of some 40 feet, and the adjacent roads and a railway. The Consulting Engineers, Freeman Fox and Partners, who had already designed box girder structures elsewhere in the World, prepared the design using continuously welded twin box girders to achieve an overall width of 132.5 feet. The width catered for dual three lane carriageways and hard shoulders, a central reservation, a cycle track and footway. The design allowed for the conversion of the hard shoulders, at a later date, into carriageways and the cycle track and footpaths into new hard shoulders. The main span over the estuary is 570 feet long with a clearance of 100 feet over high water. The anchor spans are 370 feet long. Due to low bearing value of the ground on the flood plain adjacent to the bridge it was necessary to construct approach viaducts of ten spans on the north side of the bridge and seven on the south. The overall length of the total structure was 4550 feet. The contract for the bridge was awarded to Fairfield-Mabey of Chepstow for a tender sum of £4.2 million. and work commenced in 1969. Fairfield-Mabey placed a domestic contract with Tarmac Civil Engineering (now Carrilion Construction) for the foundations, piers and deck concreting works. Work on the foundations began immediately and were substantially complete in early 1972. This was in line with the planned opening date in 1972, itself part of the overall programme of extending the M5 southwest. Prior to the letting of the contract for the bridge the Department of the Environment had become concerned about the failure of some continuously welded steel box bridges and appointed a committee chaired by Professor Merrison to examine the criteria for the design of such structures. In May 1971 the Department distributed the first interim rules prepared by the Committee. As a result it was necessary to re-appraise the whole Structure and make considerable changes to the design. Considerable stiffening and/or thickening of plates was required and the combined effect of the design and workmanship requirements of the rules doubled the work at site joints. The redesign led to a considerable delay to the Bridge completion and this was further aggravated by industrial action by the site welders who, because of the extra strengthening, recognised their strong bargaining position and demanded more money. The combined effect of these problems led to the granting of a two year extension of contract with the opening of the bridge in May 1974. As stated earlier the M5 north of Avonmouth became a continuous dual carriageway Motorway to Birmingham in December 1971. Hence for a while traffic from the motorway heading on towards the Southwest had to travel through Bristol by way of the Portway and the Cumberland Basin complex and then into Somerset on A38. Meanwhile construction of the motorway further southwest between Avonmouth Bridge and Highbridge had commenced in a series of contracts before May 1971 and the completion was January 1973. The consequences, in Bristol and North Somerset, of the delay of nearly eighteen months in the opening of the Avonmouth Bridge resulted in Motorway traffic flows discharging on to the local road network. The local County, City and District Councilors became very concerned about the potential effects and a conference chaired by a Minister, Mr. Keith Speed, was held to discuss how the situation could be alleviated. At this meeting the County Surveyor of Somerset's representative put forward proposals for a one way signing system from the interchange at Gordano, south of Avonmouth. The scheme, which needed the construction of minor highway improvements, required the signposting of northbound traffic from junction 19 via the A369 to the Cumberland basin and then the A4 Portway to Junction 18 at Avonmouth: the southbound traffic was signed onto the Portway at Avonmouth and then onto the Cumberland Basin leaving on the B3128 through Failand and then via a minor road through Portbury to junction 19. The Councils involved approved the proposal and despite the need for a Public Inquiry the works were completed and the signing installed in time for the opening of the motorway to Edithmead. Thus the worst of the congestion effects were contained for the period until the completion of the Avonmouth Bridge in May 1974. M5 Gordano Valley This contract, which commenced in April 1970, was let to Cementation Ltd. for a tender sum £5.7m. It started at the south abutment of the Avonmouth Bridge and extended to Clapton Wick in the Clevedon Hills. It included the construction of the elevated roundabout interchange with the A369 at Portbury and the adjacent Service Area. On sidelong ground in the hills the works were mainly in rock cutting in massive limestone.
In order to accommodate the sidelong ground the two carriageways were constructed at separate levels which varied up to 30 feet.
In order to support the lower northbound carriageway a crib wall comprised of interlocking precast
concrete units infilled with rock was built and to support the higher carriageway a reinforced concrete retaining wall, tied back with rock anchors, was used. To cross over two adjacent valleys at Wynol Farm twin parallel prestressed viaducts, some 1600 ft long and up to 100 ft high were built and are a feature of this length of Motorway. Access to the site of the viaduct was extremely difficult. In all nine new bridges and 1.5 m cu.yds. of excavation, of which 60% was rock, were required on this 5 miles of M5. M5 Clevedon Hills Section (J19 to J21) There are many outstanding Civil Engineering works associated with the M5 but none were more demanding than the section through the Clevedon Hills. Fortunately Mr. Walter Eyre, the Project Manager for Freeman Fox and Partners, has donated a number of important personal papers to the Archive: these illustrate clearly the difficulties encountered by both the designers and the contractors in successfully completing the construction. Amongst the papers are explanations of treatments to stabilize different conditions encountered in the rock faces which are an excellent example of Civil Engineering application. Also included is a copy of a paper presented by Mr. M.G. Lewis at a technical symposium in Columbia in 1974. The Contract for this 8.7 mile section was awarded to John Laing for a tender sum of £10.73m. the commencement date was December 1969 and it was opened in January 1973. The route rises along the north face of the hills and passes through a saddle before descending to the Congresbury Yeo plain of the Somerset levels. Two interchanges were constructed at Clevedon and St. Georges at Weston-super-Mare. The greater part of the rock cutting through the hills was in the limestone mass which contains a thrust fault, in places the rock was fissured and required extensive grouting and over 4000 rock bolts were installed. The maximum depth of cutting was about 33 ft. Based on further development of the work on alluviums at Avonmouth, trial embankments were erected and extensive testing carried out on the soil conditions on the Somerset levels. These trials and calculations showed that the maximum safe height for embankments was about eighteen feet, but as side road and interchange embankments were about 25 feet in height, light weight pulverised fuel ash (PFA) had to be used. The importation of large quantities of PFA was organised by rail to a specially constructed railway siding. In order to speed up settlement of the under lying silts the motorway embankments were surcharged at a carefully controlled rate and left to settle for a period of twelve months. Drainage below the banks was achieved by placing a 3 foot layer of rock onto the unstripped vegetation. In total some 1,794,000 cu.yds. of filling materials were imported including 897,000 cu.yds. of PFA. 14 bridges were constructed mainly on piled foundations: foundations for bridges on the levels could not be piled until settlement surcharge material was removed. When this section of motorway was designed Somerset County Council were planning a new route to Western-super-Mare from Bristol. The first section, from the Cumberland Basin complex in Bristol, by-passing Long Ashton, was already open and the line was being protected to the M5 Clevedon interchange. Accordingly provision was made in the width of the central reservation of the section of the M5 from the Clevedon interchange to St. Georges interchange with the A370 for a future widening of each carriageway to four lanes. However after the formation of the Avon County Council in 1974 the County Scheme was Abandoned. Mendip Hills Section (J21 to J22) This section passed though a saddle near the western end of the Mendip Hills and crossed a further part of the Somerset levels ending at the interchange with the A38 at Edithmead. The contract, which was awarded to A.E.Farr Ltd (later to become Bovis) for the sum of £8.05m, commenced in December 1969 and was completed in January 1973. The length was 8.05 miles and the work included the construction of a "trumpet" interchange at Edithmead. The earthworks required 2,070,000 cu.yds. of common excavation, mainly in marl, some 97,000 cu.yds. of excavation in rock and nearly 1,.100,000 cu.yds. of imported material including 207,000 cu.yds. of PFA. On this section of motorway the alluvium extended to a maximum depth of some 90 ft and trials of embankment loading and failure were undertaken at East Brent in advance of completing the design. The criteria derived from these trials established the practice for embankment construction and piling work at bridges. In the latter case the problem of the settlement of the alluvium under load introducing negative friction on piles causing over load or drag down, was of real concern but the trials demonstrated that coating the piles overcame any difficulties. As on the previous contract, arrangements were made for the importation of PFA from South Wales by rail and a handling centre was set up to transfer the material into lorries at a temporary siding in the disused Highbridge Locomotive Works. The material was used to form the embankments above the height of 12 ft.. In order to accelerate settlement of the alluvium the motorway formation was surcharged with a rock blanket for a period of a year. Twelve bridges were constructed on this section, nine of them were on piled foundations. Additionally the contract included provision for the embankment surcharge material to be hauled southward onto the next length of the Motorway to form an embankment between Edithmead and the River Brue; It also included the associated advance drainage works.
Twyning Green Section (J8 to J9) -M5 Twyning to Tewkesbury (J8 to J9) - M5 Gloucester - Cheltenham (J9 to J12) -
M5 Moreton Valance Section (J12 to J13) -M5 Michael Wood Section (J13 to J14) -M5 Alveston Section (J14 to J15) -
M5 Almondsbury Section (J15 to J16) -M5 Filton By Pass to Avonmouth Section (J16 to J18) -Avonmouth Bridge (J18 to J19) -
M5 Gordano Valley -M5 Clevedon Hills Section (J19 to J21) -Mendip Hills Section (J21 to J22)
