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M4 in Wales. Baglan to Lon Las (J41 to J44)

Introduction

The London to South Wales M4 motorway was conceived in the 1950s, and by the late 1960s it was largely complete between London and Newport. Through South Wales, the motorway was to supersede the A48, which was the main east-west link in this industrially active area. Traffic pressures had already required the construction of the A48 Neath Bypass as all-purpose dual carriageways by 1957. The Port Talbot Bypass, built to motorway standards and designated A48(M) and the dualling of the section between Briton Ferry and Baglan, followed in 1966.

The 1970s saw the filling in of the motorway between Newport and Port Talbot, and westwards between Lon Las and Pont Abraham. But the 10km gap - the "Missing Link" as it became known locally - survived as a very busy trunk road until the completion of the M4. Plans for its completion presented formidable problems, with extensive industrial and commercial development, estuarine floodlands and the navigable River Neath all lying in the path of possible routes.

Owen Williams were first commissioned to carry out a feasibility study for "improving or adapting or converting the existing A48 west of Baglan, or providing a new road between the Port Talbot and Morriston Bypasses" in 1971. This resulted in four possible routes for the "Missing Link" being presented for public consultation in 1974, and a route was subsequently chosen which crossed the River Neath approximately 1km south of the existing A48 crossing. A year or so later, it became apparent that the Dupont Steelworks - to the east of the Briton Ferry Dock - was going to close by 1980, so the Welsh Office asked Sir Owen Williams and Partners (now Owen Williams Consulting Engineers) to review the routes.

In the new circumstances, the review found the route which has now been built to be optimal, providing a direct connection with the A465 Heads of the Valleys road and making maximum use of the existing trunk road corridor.

In October 1977, the Secretary of State for Wales announced this as the new Preferred Route, and the scheme progressed to publication of the Draft Line and Side Road Orders in 1979 and to a Public Inquiry in 1980.

Extensive ground investigations were carried out which confirmed that the soft peat and loose sands along much of the route were unsuitable for structural foundations and would not support embankments without large settlements. As a result, therefore, support for most of the structures and for much of the road itself relies on piles reaching down to the underlying strata of middle and lower coal measures. At Earlswood, a major cutting was required through the strong rock of the upper coal measures.

Being in an industrial area, the scheme affected a large number of services. Some of these were very significant, and it soon became clear that many diversions should be carried out as advance works. A programme of service diversions was carried out during the early 1980s, including moving a major electricity transmission anchor tower near Briton Ferry, numerous underground high voltage (33kV) electricity cables, large water and gas mains, telecom cables, large sewers and more than 2km of oxygen and nitrogen supply lines.
New marina

New Monkhouse Cruising and Sailing Club marinaOther advance works involving replacing or moving two railway sidings, the marina for the Monkstone Cruising & Sailing Club, golf course tees, fairways and greens on the Jersey Marine Golf Course, and plant and buildings for Wales Gas and BP Chemicals were also carried out. Advance contracts were also let to fill in old mine workings and to form the embankment at Pentyla to permit settlement to take place before the main scheme was started.

The scheme was designed by Owen Williams except for the most westerly Lon Las to Llandarcy section, where design was undertaken by West Glamorgan County Council. But for construction purposes, the scheme was rationalised into three contracts, namely:

Contract 1 - M4: Baglan to Briton Ferry

Contract 2 - M4: Briton Ferry to Earlswood

Contract 3 - M4: Earlswood to Lon Las

all supervised by Owen Williams.

Baglan to Briton Ferry (J41 to J42)

Mowlem Civil Engineering began work on the £20 million, 3.1km Baglan to Briton Ferry section in August 1992. The route took it over the main South Wales railway network and across the original sedimentary deposits laid down by the River Neath as it entered the Bristol Channel.

Work on the main carriageway fell into three sections.

At the western end, the route had to pass through the Raglan Lagoon - a shallow man-made lake which forms part of the local area flood control system. Here, it was necessary to exclude water during construction, and to achieve this over such a large area, a 15m wide, 6m deep trench across the lagoon was sequentially excavated and backfilled with crushed rock. The tipped rock formed a bund along which the excavator moved, and once the 350m long rock bund was complete, one side was sealed with a layer of clay allowing the motorway section to be pumped dry. Bulk excavation and backfill were then completed using conventional earthworks plant.

For the middle 2.1km, the existing ground was too soft to support the new carriageway and the solution was to support a 0.5m thick fully reinforced structural concrete deck on 2,100 600mm diameter piles varying in length between 10m and 25m.

Piles founded on rock were constructed using two teams of tracked cranes fitted with augers which removed the soil from within an open ended temporary steel casing. Piles founded in sands and gravels were installed by up to five teams of tracked cranes fitted with 5 tonne drop hammers which drove closed ended steel tubes into the ground, thereby displacing the ground to form a void. The steel tubes, which act as temporary casing, were then filled with reinforced concrete and withdrawn for reuse while the concrete was still wet.

At the eastern end of the main carriageway, furthest from the River Neath, the underlying ground begins to improve. Consequently as the motorway rose to cross over the main South Wales railway, it could be constructed on embankments built of lightweight material. This work was carried out by Sir Alfred McAlpine under a separate contract in 1991 at a cost of £1.25M. The lightweight material is slag, supplied from the local steelworks at Margam, which was screened to produce a material with a bulk density of 1.5 tonnes per cubic metre. This material gave an embankment which was one third lighter than one constructed of conventional earthworks material.

The new route crosses the South Wales main railway line near St Joseph's School. The angle of intersection at the crossing is very small, requiring a structure 160m long. This required great care and planning to ensure the continued safe operation of the rail network. The first job was to install more than 600 piles bedded into the underlying rock using small tripod rigs rather than the conventional crane mounted type.

The reinforced concrete side walls were then constructed insitu. To minimise disruption to the rail system, the top half of the structure was formed in precast concrete portal units each weighing 38 tonnes manufactured in Taunton. These were placed over the railway during weekly possession periods - midnight Saturday to 6.00 am Sunday - using a 200 tonne mobile crane.

Once in place, the portal units and their foundation walls formed a tunnel which allowed all other construction activities adjacent to the railway to progress safely.

The westbound exit slip road at Pentyla required the construction of a new 11 span viaduct crossing over the main railway, descending to join the existing M4 westbound carriageway to Sunnycroft roundabout, Baglan.

Apart from the span crossing the railway, the foundations, piers and decks were cast insitu using reinforced concrete founded on piles up to 25m deep.

Due to the poor ground, the scaffold support system itself had to be supported on temporary steel column piles driven to depths of 20m to 25m.

For the span over the railway, three large precast beams were fabricated on site adjacent to their point of placement. Each beam, weighing approximately 140 tonnes, was curved both vertically and horizontally to suit the profile of the new road and was lifted into its final position using two of the Iargest mobile cranes available in the UK. The operation went very smoothly and was completed within two hours during one Saturday night possession.

Clearly the contract required the use of a large quantity of concrete, and the decision was made at an early stage to erect a batch plant on the site.

In all, 47,500m of concrete piles, 3,000m of temporary piles, 13,500 tonnes of steel reinforcement, 75,000m³ of concrete and 300,000 tonnes of slag fill have been constructed or used in the completion of the Baglan section, much of which is underground and not now visible to the road user!

Briton ferry to Earlswood (J42 to J43)

In 1991, an Anglo-German joint venture was formed between Kier Construction Ltd and Hochtief AG to build this section of motorway.

Elevated over its entire length, it is formed by three separate and different structures. Long reinforced concrete approach viaducts mounted on slender circular columns run in from each side of the Neath Estuary to join the imposing steel bridge crossing the river. The western approach viaduct - carrying the motorway across the A483 Fabian Way, and its associated westbound slip road viaduct conveying traffic towards the eastern side of Swansea - complete the scheme.

Work started on site in spring 1992 with the mobilisation of equipment to install more than 1800 piles required for the viaduct foundations. These varied in diameter from 480mm to 1500mm, with a number of the largest having to be constructed in the bed of the River Neath itself. A temporary sheet piled cofferdam was driven into the river bed for this purpose, designed to retain tides of up to 6 metres. Access for the heavy piling equipment which had to bore down more than 40 metres to the underlying rock was provided by an earth embankment and steel jetty from the west bank. On completion of piling here, the bed of the river within the cofferdam was then excavated to a depth of 4m to construct the pier base. Piling work on the west bank proved to be complex, with variations in ground conditions calling for two types of construction. Large bored piles were used where the underlying rock could be reached, but where this was not possible, smaller diameter piles were driven into the gravel beds.

With 766 prestressed precast beams needed in the approach viaducts, the contractor elected to set up a precasting works on site. Two beds, each more than 100m long and capable of applying prestressing forces of 1000 tonnes, produced up to sixteen beams per week and worked year round with the aid of a hot water heating system to accelerate curing of the concrete. By manufacturing the beams on site, the need for almost 800 abnormal loads on the roads was avoided.

M4. Neath viauct

M4. Neath viaduct. Central span is closed.The focal point of the works was the large central viaduct crossing the River Neath. Founded on a sandstone outcrop at its western end, the structure crosses the river almost 30m above high tide level to join the eastern approach viaducts. Its deck is supported by eight rows of steel plate girders varying in depth between 2.5m and 4.5m. These girders, weighing almost 4000 tonnes in total, were fabricated off site and delivered in 27m lengths for assembly prior to erection. By autumn 1993, the heavy columns and crossheads had been built and girders erected to the two sections on either side of the river. Erection of the central section was carried out by an 800 tonne mobile crane working systematically first the west and then the east riverbank. The final sections closing the gap over the waterway were 60m long and 130 tonnes in weight. To lilt them into their final position - more than 60m from the crane - required 400 tonnes of counterweight.

Completion of the deck over the river opened up almost two kilometres of roadworks, leaving the installation of street lighting, traffic signs and motorway communication before the adjacent contracts 1 and 3 could be finally linked.

Earlswood to Lon Las. (J43 to J44)

This contract was started by Costain in March 1990, ahead of contracts 1 and 2, in order to realise the benefits of improvement to the existing A48 as quickly as possible. The project, valued at £31 million, called for the upgrading of the existing A48 trunk road between Earlswood junction - adjacent to the existing crossing of the River Neath - and the M4 motorway at junction 44, on the Morriston Bypass. The three years taken to construct the 4.6km length indicates the complex nature of the scheme. Construction involved up to ninety sub-contractors and more than 700 suppliers to the main contractor alone.

To carry out the works much of the expertise and labour was from local sources. Materials were purchased from throughout Britain, including very local suppliers such as Cambrian Stone providing slay waste from Port Talbot for rise as sub base. Bridge expansion joints manufactured in Spain were supplied and fitted by Universal Sealants Limited of Sunderland.

The scheme had a large structural works content. Two existing structures were demolished and one refurbished. One of the two structures demolished involved working over the main South Wales railway line and the other over the existing A48 trunk road, with both operations needing rail possessions or road closures to complete the demolition process.

Four new bridges were required, the first, a bridge spanning the main London to Swansea railway line. An innovative design used an inverted U-beam for the top sections of walls and soffit, and this coupled with the contractor's use of a travelling cantilever gantry for abutment shutters, eased construction of the 135m long structure and minimised disturbance to passing trains.

Crymlyn Road Bridge forms the second major structure. The existing bridge carried this busy local road over the A48, and the first task was to divert the Crymlyn Road traffic over an adjacent temporary Bailey-type bridge. This allowed demolition of the old bridge, which required total closure of the A48 passing underneath. This was carried out during a single weekend with traffic being diverted via Skewer. Having demolished the original structure, the new composite widened structure was constructed in its place.
Tennant canal viaduct

M4. Tennant canal viaductTennant Canal Viaduct was the existing 217m long structure carrying the A48 over the canal and two live railway lines. It needed to be widened and strengthened. Existing columns were retained and refurbished, the existing deck demolished and re-built wider on new columns to accommodate the auxiliary lanes on the new carriageway and an adjacent cycletrack and footway.

The need to maintain two-way traffic throughout demolition and construction entailed extensive and detailed planning and programming, and many disciplines and sub-contractors were involved in each phase of the works.

The final major structure was a bridge to carry the new alignment over the westbound link road and the tidal Red Jacket Pill into Earlswood Cutting. The 210 metre long, eight span, curved in-situ reinforced viaduct deck was built in four main stages, each pour of approximately 1,000m³ of concrete taking 19 hours to complete.

Overall construction of these new structures involved driving and boring a total of 10 miles of piles, pouring 26,500m³ of concrete, and fixing more than 5,000 tonnes of reinforcing bar - sufficient, if placed end to end, to reach from Inverness to Penzance and back!

Due to the lack of alternative routes, conversion of the A48 into motorway had to be carried out while maintaining existing traffic flows and converging alignments of existing and new carriageway. It involved twenty eight phases of traffic management, and many more minor adjustments, contraflows and off peak lane closures. To assist traffic flow, vehicles were monitored throughout the site area and approaches by a closed circuit television system linked to a control room manned 24 hours per day. Recovery vehicles removed broken down and accident damaged vehicles on a full time basis.

Earthworks operations involved the formation of two rock cuttings at Crymlyn Road and Earlswood. The Crymlyn Road cutting was in hard sandstone, and was situated in an urban area. The use of explosives was thus very limited and the majority of excavation had to be carried out by impact hammers on hydraulic backacters. At one point, twelve machines were involved in the excavation to meet a traffic control deadline. Material from the cutting was used as fill to embankments on other parts of the contract.
earlswood cutting

M4. Earlswood slip road cuttingAt the eastern end of the scheme, the Earlswood cutting which is 450m long and up to 30m deep, required the removal of 700,000m³ of hard rock, This was excavated in the same way as a quarry, with material blasted and removed in 10m high benches. Coal measures found within the cutting were treated by drilling more than 2 miles of probe holes and grouting the voids. Nearly all of the material excavated from the cutting was used as fill, but many of the large boulders were used on river and sea defence contracts nearby.

A very unusual feature for such a scheme was the construction of a separate footway and cycle track along most of its length. This replaced the footways and cycle tracks flanking the old trunk road and its links with similar facilities on the A483 Fabian Way and on the A48 through Briton Ferry.

It is gratifying to all concerned that throughout the period of the contract no major accidents occurred nor were there any fatalities. The contract was successfully completed within the contract period and opened as a motorway on 26 February 1993.