Castleton to Coryton (J29 to J32)

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M4 in Wales. Coryton to Baglan (J32 to J41)

Line Orders

map

The former Glamorgan County Council had established and protected a development line for the "South Wales Motor Road" in the early post war years. This development line became the starting point for detailed investigations.

The route restrained the development of Cardiff in the vicinity of the Llanishen Reservoir, and for some years the protected corridor had deterred the northwards push of the city. However a more northerly route became the preferred option, and finally became the established route. Its intersection with Route A470 at Coryton, now junction 32, was fixed by the extensive development from the south. The topography of the area and the presence of the Wenallt Reservoir were also major factors for both the Castleton to Coryton and Coryton to Miskin lengths.

At the Public Inquiry stage there was considerable opposition to the motorway northern boundary to Cardiff, duplicating much of the argument already presented on the Castleton to Coryton section.

Two well known citizens with strong views on transportation matters, advocated the extension westwards of the then established route of the Eastern Avenue (A48), which linked into the Gabalfa area of Cardiff. The route suggested would then continue alongside the River Taff (see location map), to connect to the protected development line route to the north west of the city. This became known as The Llantrisant Radial route. It was intrusive and would have required a dramatic change of direction after it had crossed the River Taff. The adoption of an "helical spiral" to overcome the problems of the three dimensional changes to the alignment in the vicinity of the River Taff was suggested!

One of the public Inquiries spent considerable time dealing with an objection raised by a veterinary surgeon advocating a route that made considerable use of dismantled railway lines. Although not accepted finally, David Pugh suggests that the economic arguments against the option may have been swamped in the present climate, by what he perceives might have been presented as the environmental benefits of revitalising a "spoiled corridor". However that is speculation on his part, and objectivity has not been abandoned entirely yet.

The lateral restraint to the line of the motorway at Coryton has already been noted, and the vertical alignment at this point was also restricted by the need to cross over the dual carriageway A470 Trunk Road. Exceptionally, for a trunk road, it carried in its verges the main water supply to Cardiff and was crossed by a single high pressure gas main. The presence of these services effectively predetermined the start of the highway profile across the Taff valley.

There was also strong opposition from a substantial land owner at Miskin, to the line of the road in that area, and general concern about the route of the northern link from the Miskin Interchange providing access to Llantrisant and the Rhondda valley. The prospect of a Llantrisant New Town and also a major sewage works in the vicinity was still an issue at the time, and the position of the junction and the northern link was seen to have an impact on the development opportunities in the area.

As explained previously, opposition to motorways generally was gaining momentum in the nation at large, at the time.

The generally recurring basis of objection to new highway alignments in the early 1970s, was to the impact of the new road on householders, since no mechanism for establishing compensation existed unless a property was physically affected by a scheme. An understanding of the impact of noise was only just developing, and science began to make a contribution. The Land Compensation Act of 1973, and its associated Regulations, went some way to addressing this problem and became applicable to these schemes. Threshold levels of noise, and changes resulting from a highway development, became the basis for compensation payments or measures to alleviate noise nuisance. The word 'decibels' entered debates.

The main aim of a witness at any Inquiry is not to put forward obscure evidence or do other than aim at the plain truth. The fact that the decibel measure was logarithmic and further weighted to allow for the impact it made, provided for some difficulty in explanation. "It was so quiet you could hear a pin drop'' is a familiar phrase, and yet the change in noise level caused by the pin dropping may be a difference of 5dB(A). On the other hand "It was so noisy I couldn't hear myself speak" and the difference made by shouting may still be 5dB(A). People tend to think of the noise volume scale as linear and expect 5dB(A) to be the same change whatever the ambient noise level happened to be. However an increase of 5dB(A) over 65dB(A) is a much greater change than a 5dB(A) change from 40dB(A). It was a good witness who could make this understood and present a convincing example.

Further to the west the line of the Stormy Down to Groes (Pyle Bypass) impacted upon the route options for the intervening length between Pencoed and Pyle. Ewart Wheeler who was project engineer for the Pyle Bypass in the early 1970s, had seen the bypass scheme grow from a single carriageway to a dual three lane motorway. He also had the unusual experience, as project manager, of giving evidence at the Public Inquiry promoting the alignment on behalf of the Welsh Office, whilst at the same time objecting to certain aspects of the route, on behalf of Glamorgan County Council.

A considerable number of rights of way crossed the route of the motorway, They were stopped up and diverted parallel to the motorway to cross at a suitable bridge or subway serving a side road or cattle creep.

The Design and Contract stages

The line and level of the motorway at Coryton Interchange, and the need to cross over the Taff river and the Radyr to Morganstown road to the west, determined the level of the motorway across the Taff valley. A significant earthworks imbalance resulted, but the Radyr to Morganstown road was lowered, helping to achieve a better earthwork balance, and also to reduce the impact on the

The nature of the fill available for the Taff Valley crossing was such that drainage layers and a surcharge were used to ensure that sufficient settlement took place before the carriageway was constructed.

A dozen or so layouts were considered for the Coryton Interchange. The eventual design allowed for the motorway to cross over the trunk road and roundabout with a circumference of 1 mile, fitted into a plane under the motorway and the A470 north, and over the A470 south. The junction continues to function well but development adjacent to the interchange and the general increase in traffic flows have necessitated the use of traffic control on the roundabout, and lane widening on the eastbound off slip and approach.

Due to the presence of the substantial public utilities apparatus alongside and across Route A470, special provisions were needed to accommodate them. The western verge of the motorway overbridge was made wider than normal to create a corridor for the apparatus. No costs were recovered from the utilities to meet this non-standard expenditure. A non-contractual plan indicating a suggested construction sequence in the Coryton area was included in the contract documents. This sequence was adopted by the Contractor, and trunk road traffic and utilities apparatus experienced minimal disruption as construction work was carried out.

In the Groesfaen area, on the Miskin to Pencoed length, drainage measures proved necessary in the cuttings as a result of what was thought to be a perched dew pond. Both the cutting slope and the formation in the cutting proved to be very wet. It was thought finally that there was artesian pressure in the area. At a number of locations along the Miskin-Pencoed length the stability of cutting slopes was such that flattening from the customary 1 in 2 was required.

The hilly topography of the area was such that up to 7 metres proved necessary to accommodate drainage ditches at the highway boundaries, particularly between the bottom of embankments and fence lines. The engineers from the Welsh Office were convinced by energetic debate that the hilly country justified such departures from central standards applicable to less hilly and drier sites, a point emphasising that 'rules are for the guidance of the wise'.

The impact of the motorway on Miskin Manor adjacent to the Miskin Interchange (junction 34) was such that traffic noise attenuation measures were needed to comply with the Land Compensation Act 1973. The nature of the windows at Miskin Manor precluded double glazing however, and a high and costly planted bund was provided in its place.

Further to the west, in the Pencoed area, limestone had been shown by the geological and soils study, and swallow holes, a common phenomena in limestone, was expected and found. "Dr North's" solution of infilling with a clean single sized non soluble rock and then concreting over, has proved to be effective. It has also been used to deal with swallow holes that have developed since the motorway was opened to traffic.

The eastern section of the Pyle Bypass (stormy Down to Groes - Stage 1) length featured a substantial cutting in marl. The cutting was excavated as an advance contract to provide fill for the adjoining western section of motorway in the vicinity of Kenfig Pool. Shortly after the cutting was excavated however a large hole, large enough to swallow a tractor, developed in a field adjoining the cutting to the south. It remains unexplained but it did not appear to be a result of the motorway activities.

This concludes the record of any differences on these lengths from the general experiences in building large road schemes. There were no serious problems in the building any of the structures. All the schemes were let in separate contracts, but the Miskin Pencoed and Coryton Miskin lengths went to tender at the same time, and tenderers were given the option of offering a discount if offered both contracts. The offer was not taken up by any of those bidding.

Coryton to Pencoed (J32 to J35) and Stormy Down to Groes (J37 to J39)

The line of the M4 between Castleton in the east and Groes in the west lay totally within the Glamorgan County Council area. Except for the Pencoed to Stormy Down section prepared and constructed by Freeman Fox and Partners, the length between Coryton and Pencoed, and Stormy Down to Groes (Pyle Bypass), were therefore initially prepared by the County Council. The same principles and standards were set out as for the adjacent Castleton to Coryton section and others.

Glamorgan County Council were asked to prepare a scheme for the Coryton to Pencoed length, split into two sections. Initially this was at Capel Llanilltern, a legacy of the abandoned Llantrisant Radial Road.

The early years were spent establishing the need, and defining the preferred route. The schemes were prepared in parallel with the Castleton to Coryton scheme, and were progressed to the Public Inquiry stage to deal with the published recommended routes and the extent of land required for construction.

From April 1974 Local Government was split into new entities however, and three new County Councils appeared viz. Mid, South, and West Glamorgan. A joint Mid and South Glamorgan team was therefore established to project manage the contract preparation and tendering procedures.for the Coryton to Miskin section, which traversed both areas, and David was seconded to this team. Mid Glamorgan was to manage the Miskin to Pencoed and Stormy Down to Groes - Stage 1 as these lay totally within their area, whilst West Glamorgan took over the Stormy Down to Groes - Stage 2 lying within their boundaries.

Pencoed to Stormy Down (J35 to J37)

Pencoed to Stormy Down section was the longest length of dual 3 lane carriageway on the M4 in South Wales. The route, a significant length of which crosses common land, lies to the north of Bridgend. It has a maximum gradient of 4%, includes three grade separated interchanges, two crossings of the main London to Swansea railway, and a dual carriageway link to Aberkenfig Bypass.

A service area was built close to the central interchange initially providing limited services only. In addition to the care needed to deal with affected landowners, the section passed alongside a reservoir and close to an open psychiatric hospital having some patients with suicidal tendencies.

The section and its structures were designed by Freeman Fox and Partners, for whom Bill Austin, well known and admired in the Industry, was Managing Partner. Walter Eyre was the Resident Engineer.

An advance earthworks contract was let in order to construct a high embankment and to monitor stability over a section of the route where ground conditions were uncertain. The site investigations had identified an area of major solution cavities in fractured limestone, as at Pencoed, but at this location treatment had to take account of underground watercourses essential to field irrigation and, in some cases, providing drinking water for the surrounding rural area. Some cavities were cleaned out and filled with graded rock, whilst others were capped at rock head level with reinforced concrete slabs. Prior to construction of the embankment, and to ensure that there were no cavities that might cause differential settlement, the area was subjected to dynamic loading with 10 tonne weights at 2 metre centres, falling 10 metres.

ogmore viaduct

One of the major structures was the Ogmore Viaduct, carrying the motorway at a high level sag curve across a valley that contained the Ogmore River, the A4063 and several side roads. The 373 metre viaduct, shown at B10 - 1, consists of twin prestressed insitu concrete boxes at 22metre centres, transversly connected by prestressed precast concrete beams every 4.5 metres. The boxes were each 4,8 metres wide and 3,5 metres deep, while the transverse beams were 17.25 metres long and 2.25 metres deep. The boxes and beams were stressed together to form the supporting framework for a 230mm thick reinforced concrete deck slab, and cantilevers of varying depth. The viaduct is straight in plan with square abutments, but due to the topography of the site and the viaduct crossing both river and road at an oblique angle, the single columns supporting each box are staggered with spans of 54 metres and 81 metres.

The design was very sophisticated. The beams were continuous and the profile of the prestressing accurately located so that there were no parasitic moments induced because of the redundant nature of the structure ie a concordant profile. Loads at the piers were optimally adjusted by transformation of the concordant profile.

Inevitably, a rigorous construction sequence and order of prestressing was specified. The BBRV prestressing system used 12 tendons in each box, with each tendon comprising 181 no.7mm dia. Wires stressed to 7900Kn lock off. These were, at the time, the heaviest installed in the UK, and required jacks that weighed 6 tonne and could exert a pull of 850 tonne. An incident considered to be a major problem, causing a delay of about 3 months, was the failure of a prestressing coupler within the web of one of the boxes. The web had to be cut open and several cables destressed while the coupler was replaced. The strands connected to the coupler had to be reconnected and the excision made good, after which the rigorously specific stressing order, modified for the interruption, had to be started again. Overall progress was good however, and work which started in June 1978 was completed in April 1981.

Tenders for the roadworks were invited on the basis of two administratively separate contracts, Eastern and Western, with an opportunity for tenderers to indicate a tender reduction if awarded both contracts. A combined tender was awarded, once again underlining the possible advantages of serial tendering, as advocated by the Lofthouse Report. The Contractor established a single base for his operations, although the Ogmore Viaduct contract prevented continuous access throughout the site.

The earthworks were extensive, and included excavation of some 3.1 million cubic metres of material. A large percentage was rock, mainly sandstones and shales with some carboniferous limestone. No particular problems were encountered.

The carriageways were constructed with a fully flexible pavement, with 40mm of hot rolled asphalt wearing course, 60mm of dense bitumen basecourse, 230mm of dense bitumen road base, and a varying sub base thickness.

The remaining structures included 4 side road bridges, 3 side road over bridges, 4 bridges over railways, 2 footbridges and a farm underpass. These were generally of standard design and offered no particular problems in design or construction. Work started in November 1978 and was completed in 1981.

Objection to motorway schemes had been growing, and although the proceedures were not disrupted in the way many were elsewhere, there were some instances of vandalism by road protesters and others. This delayed the opening of the road, and at the opening ceremony there was a demonstration from Welsh Language enthusiasts. As a consequence the police presence was more visible, and secure ("riot control") vehicles were seen for the first time in Wales.

Groes to Baglan (J39 to J41)

This was a very early scheme to bypass Port Talbot which became part of the M4. It was commissioned as early as 1953, but it was not until 1962 that the statutory processes were completed and contracts let. No particular problems in scheme preparation are recorded, but none were likely at that time since it was before the period of strong opposition to road building.

This dual 2 lane section was the first length of motorway in Wales, opened to traffic on 22nd July 1966. The client, at that time, was the then Ministry of Transport, and the Engineer was Sir Owen Williams and Partners. The length was 4.25 miles (6.8 Kms) and its cost 5.06m. At the time this was a substantial scheme.

Little can now be recalled or records found. Material has been lost due to the reorganisation of local government., the casual attitude of the Industry to Archiving, and an inability to contact people who were involved, in some cases due to age.

Such inferences as can be made shows that the scheme was an early example of construction through a densely built up area. Over long stretches the only solutions were retained embankment or viaduct construction. For the most part the latter was chosen presumably for reasons of economy, to free land beneath the road for future access or development, and to avoid large scale demolition. It is very unlikely that the route would have been accepted today.

Structures generally comprise a flat slab reinforced concrete road deck 14in (0.36m) deep, continuous over columns at 18 to 20 ft centres in both directions. It is evident that a great deal of effort had been made to standardise both design and construction, since the depth of slab exposed on all elevations is constant, even where spans are larger than normal.

pentyla interchange

Where these larger spans are needed, generally at interchanges, columns were omitted and replaced by open concrete structures with the top flanges cast integrally with the deck. At river crossings the piers are solid vertical slabs, but below ground the piers are hollow in order to reduce dead weight on the foundations, and to ensure the economy and certainty of the spread footings.

It is a pity that so little is known of this scheme, and in particular that there seem to be no contemporary photographs to be found, describing and illuminating the construction processes in those early days for the motorway programme in Wales. The design and construction lessons to be learnt might have broadened the overall grasp and experience of those wishing to continue their professional development. It does however, illustrate the value of striving to ensure that archives are properly kept.


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Baglan to Lon Las (J41 to J44)


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