Introduction

In the early 19th century Thomas Telford was engaged to design extensive improvements to the Coast Road including the now historic suspension bridge crossings of the Conwy estuary and the Menai Straits. Robert Stephenson's Chester-Holyhead railway followed the coastal route and rapidly took over the mail, passenger and goods traffic to and from Ireland.

However, it was not until after the establishment of the Welsh Office that the A55 finally became accepted as the primary route into North Wales and, following the approaching completion of the M4 in South Wales, work began on the overall improvement of the route from Chester to Bangor.

Topography and geology along the North Wales Coast Road are as dramatic as any found in the UK. Finding a route for a high speed dual carriageway has challenged engineers for a quarter of a century.

Traffic problems in Colwyn Bay, Conwy and Llandudno had sparked the Welsh Office to set up a study team in 1966 which reported in 1968. The report - dubbed Collcon - recommended the construction of an east-west expressway bypassing the urban centres. The plan for dualling the road from Chester to Bangor had been formally backed by the Welsh Secretary in July 1967, and supported by his successors ever since. Prior to that a number of local bypasses and improvements had been built.

The A55 is a showcase of the best of engineering. It is rare to find so many innovative solutions in such a compact area - the run from Chester to Bangor is under 100km but includes three tunnels, a huge variety of ground engineering techniques and novel structures.

There would have been challenge enough in finding a route for the road along the narrow coastal plane, close to areas of outstanding natural beauty, across the natural obstacle of the Conwy Estuary and through rock headlands. But added to this was complex geology, and a legacy of man's activities in the area. Lead mining dating as far back as Roman times had left shafts and fissures. Telford's road and Stephenson's railway had taken the best corridor in many sections of the coast.

The A55 is to North Wales what the M4 is to the South - though it cannot be a motorway as in some locations there would have been no route for non-motorway vehicles. However, use was made of the powers contained in the Special Roads Act in negotiating with affected bodies.

The old roadThere was little doubt that an expressway was needed. Legendary traffic jams faced summer holiday makers heading from England to the coast resorts. Improving the crossing of the Conwy Estuary was the key - travellers often had to sit patiently in queues even when there was no apparent problem. Traffic had to pass through town centres on totally unsuitable streets, especially in Conwy where lorries often scraped their paintwork as they passed through 12th century arches in the town's walls.

Pragmatism played a major part in determining the sequencing of the work. By the time the decision was taken to go ahead, preparatory work was well advanced on some of the contracts towards the eastern end of the road.

This work continued while preliminary work started on more complex and demanding schemes in western Clwyd and Gwynedd. This covers the coastal sections through Colwyn Bay, Conwy, Penmaenbach, Penmaenmawr, Pen-y-Clip and Llanfairfechan. The coastal section was the subject of a lengthy public inquiry.

Despite the major benefits and time savings achieved, some of the schemes could not be justified in economic terms on a standalone basis, because of the very high construction costs. Pen-y-Clip for example came out as having a negative cost benefit ratio, but there was a positive cost benefit for the whole route. This was thanks in large part to the Conwy Crossing.

Generally, financing did not dictate the programming, though A55 progress was sometimes dependent on the phasing of the M4 to the south. The total investment has been some £700M, at the prices of the time, for the 100km stretch, reflecting the high costs of building in an area where the topography is extreme.

Brief details of sections (see also the Cheshire schemes)

The Hawarden By-pass has the only concrete carriageways along the length of the A55. Extensive excavation was needed, much of it in rock cuttings to a maximum depth of 16m. Construction of the bypass was immediately preceded by opencast mining to remove coal that would otherwise be locked in by the construction. It was a massive operation, leaving a 30m deep hole to be backfilled. A zone of the backfilling was carried out to a higher specification than the rest of the hole, in effect creating a "buried embankment" to carry the road. The concrete carriageway used the latest techniques. It was one of the first jobs to use the new high strength 40N/mm² mix. Newly designed sawn sealed joints were also used - a notch was sawn as soon as the concrete was hard enough. Wet formed joints formed by inserts had tended to disturb the wet concrete and break it up.

The Northop By-pass relieved the village, and by-passed a tortuous series of bends. Road bridges on the 5km stretch were of an innovative design - later used at Rhuallt Hill - to reduce maintenance by creation of a continuous deck.

The major engineering challenge during Stage 2 of the Holywell By-pass was to treat the shafts and veins of the extensive lead mines along the route. A total of 5000m³ of concrete was needed to cap 34 shafts and provide rafts over the veins.

The Travellers Inn Improvement route generally follows the line of the old A55 but none of the old surface has been incorporated. There were extensive swallow holes in six places along the 3km route which were excavated and filled. At other locations, geotextiles were used to avoid catastrophic collapse of the carriageway in case cavities remained undetected.

Formation of the two major cuttings on the Rhuallt Hill Improvement scheme - the deepest being 27m - required excavation of over 900,000m³, over half in rock. Much of the excavated material was used for embankments and landscaping, and there was great emphasis on environmental solutions to minimise visual impact along the 4.3km stretch. As at Travellers' Inn, geotextiles were used where cavitation was suspected.

Sympathetic use of stone walling on the Bodelwyddan By-pass at the boundary to Bodelwyddan's Marble Church led to a commendation from the Campaign for the Protection of Rural Wales. This section is mainly of new construction, with some of the westbound carriageway formed from the old road.

The Improvement East of Abergele includes four bridges, two of which carry the dual carriageway over the existing terminal roundabout at Abergele.

Contract A of the Colwyn Bay By-pass included sea defence works extending 1.5km along the shore, a main feature of, which included a 270m ship jetty. Contract B continues west, heading Inland, and a major challenge was to put the road through the town centre. Work included the diversion of 3km of railway, with a piled raft needed to cross soft alluvium.

The biggest and most challenging contract on the scheme, was the Conwy Crossing which was opened by the Queen on 25th October 1991. The scheme relieved the bottleneck in the walled town of Conwy. Centrepiece of the scheme is the crossing of the Conwy Estuary by Britain's first immersed tube road tunnel, approached by cut and cover sections to east and west.

This represents solid proof that Britain's road inquiry system can bow to objections and discard a "preferred option" in favour of a more expensive but environmentally more acceptable alternative. The initial plan had been to bridge the Conwy adjacent, to the three existing structures and run the new highway tight up against the 13th century fortress town. This would also have involved a costly rock tunnel to the south of the town. Following the then longest running public inquiry ever - from May 1975 until February 1976 - this plan was abandoned.

Conwy tunnel portalsNew designs involving various options for an immersed tube crossing were worked on. There were revisions and a Parliamentary Bill before the main contract was let in September 1986. After five years of challenging construction, Britain's first, immersed tube road tunnel was opened by the Queen in October 1991.

When tenders were finally called in for the project in September 1986 it went to Costain -Tarmac (now Carrilion Construction) Joint Venture for just over £102M with a contract period of four years three months. Excluding inflation the bill was finally settled in 1993 at a cost of £144M, reflecting the unforeseen circumstances encountered on the route.

The immersed tube tunnel elements make up just 710m of the 6km project. Cut and cover sections of some 260m to the east and 120m to the west, lead into long lengths of exceptionally difficult open cut approaches which had to be excavated within the confines of the estuary and equipped with retaining structures that would neither flood, nor float away due to the very considerable uplift forces. There are 14 bridges including a 10 span viaduct, on 40m deep piles where the road emerges from the estuary at the east.

The immersed tube sections were prefabricated as six 118m long elements each containing both carriageways in a single monolithic structure with a central dividing wall. They are plated on the sides and bottom with 6mm thick steel as a waterproof membrane in the manner of early Continental immersed tubes and acting compositely with the concrete. Corrosion protection of the plating involved blast cleaning and coating with coal tar epoxy. Tunnel roofs are waterproofed with bitumen impregnated polyester fibre fabric protected from abrasion by a reinforced concrete slab.

Conwy is designed to an overall factor against flotation of 1.1 and close attention was needed to ensure dimensional control of the structure and that the required concrete density was being achieved. Coarsely ground Portland cement and pulverised fuel ash in the mix along with chilled mixing water and insulated shuttering were among the main temperature control measures.

Dredging the tunnel trenchesInstallation of the units was done by HBM. Following float out from the flooded casting yard they were ballasted down on to concrete pads in the dredged trench and mated together at their gasket jointed faces. Sand jetted beneath the units provided a permanent foundation then graded backfill went around the sides and on top.

At the eastern end sands and gravels had to be stripped by dredging and stockpiled to enable a pontoon mounted backhoe to excavate the soft alluvial clay below so that bunds could be built up with hydraulically placed granular fill. Then a cut off wall was inserted in the 5ha bund using the Vibwall grout lance technique. Meanwhile an ejector dewatering system was installed so that, material within the boundary of the Vibwall could be excavated in the dry. Structural diaphragm walls up to 45m deep retain the cut, as the road runs down into the eastern cofferdam.

On the western side of the estuary a similar sequence was followed for the larger twin cofferdams at the other portal where excavation for the cut and cover approach had to go down 17m below Ordnance Datum.

Other structures include a 230m viaduct with 10 spans and a complex combination structure where the new road goes between two diaphragm walls, with four bridges spanning the gap.

Penmaenbach tunnelsThe main feature of the Penmaenbach section is a 660m tunnel, driven through a volcanic headland parallel and close to the existing rail tunnel, providing a new carriageway for eastbound traffic while westbound travellers use the old road. Tunnelling could not start until some 40,000m³ of scree was removed from above the western portal. After removing the scree, the whole face had to be pressure washed - which needed a plentiful supply of water, taken from and recirculated back to a nearby glacial pond. Pressure washing removed debris, and allowed the geologists to see where work was needed to stabilise the hillside.

Blasting established the shape of the tunnel's mouth, ready for the drive through hard rock from the western portal to start. The tunnel was formed by drill and blasting, supported with rock bolts and shotcrete and lined with in situ pumped concrete arches. Rock at Penmaenbach was hard and abrasive, but was evenly jointed and easily worked - though wearing on drill heads.

But the 90m of alluvial drift at the eastern end of the tunnel was anything but easy and took almost the same time as the 550m of hard rock. It was sidelong into the headland, at a very oblique angle to the rockface. There was great concern about stability and excavation had to he done by hand in segments, with ribs installed progressively to prevent collapse.

The concrete lining has no structural role to play and is there to provide a smooth surface to assist airflow for ventilation and a high reflective surface for lighting purposes.

Proximity to the sea and railway of the Penmaenmawr By-pass lead to this section being packed with major engineering work, including land reclamation, major sea defences and the construction of the UK's first promenade since Victorian times. The scheme also features two viaducts and an unusual bridge over the railway, cast in situ against corrugated steel arches.

Pen-y-clip TunnelThe main feature of the scheme at Pen y Clip is the 1.0km hard rock tunnel. The existing headland road has been upgraded to become the eastbound carriageway and Telford's old road has been re-established for non tunnel traffic. Where the dual carriageway splits to two levels to the west of the Pen-y-Clip tunnel, an intricate arrangement of massive retaining walls was needed.

The retaining wall between the two carriageways was anchored into the hillside, and props the upper wall. The lower wall retains the fill which carries the eastbound carriageway, and its deep foundations intercept a potential slip plane.

The first of the walls was the central one. Its completion allowed contractor Laing Civil Engineering to work below to build the 12m high wall at the bottom of the slope.

Construction of the lower wall required a novel approach. Space was limited, and with the railway only a metre away, it was impractical to excavate massive foundations.

Instead of having a continuous base, the wall is supported on underground posts, or barrettes, and hung from counterforts which are extensions to these. The barettes, by Bachy UK, are 6m by lm in plan, and up to 25m deep. They are at 5m centres, so there are two beneath each l0m length of wall. Excavations were made using a grab, and supported by bentonite slurry. Huge reinforcement cages were lowered in, ready for the barrettes to be concreted.

Tunnelling subcontractor Trafalgar House Construction Tunnelling found poor quality rock until it reached the main outcrop of Microdiorite. The ground in the outer thirds of the tunnel was described as "rubbish" - wide open joints, infilled with clay and what looked alarmingly like top soil. Support in the ,jointed zones is by steel ribs, with a concrete lining in front. Drainage systems are concealed behind ceramic coated steel cladding.

Most of the Llanfairfechan By-pass is built on embankment up to 7m high. The by-pass crosses alluvial deposits of silt, clay and peat overlying glacial deposits ranging from sands and gravels to boulder clay. On areas of particularly soft peat a rolled concrete raft on piles was constructed whilst elsewhere, embankments were surcharged for several months to accelerate consolidation prior to road pavement construction. In the transitions between surcharged and piled raft embankments piles were continued but ICI's geotextile Paragrid was used instead of the concrete raft.

On one of the smaller bridges, surcharging proved the best solution to ensure stability over the soft ground. The bridge is a box structure, which gives access under the A55 to the seafront. Piled foundations would have been much more expensive. Instead the base and walls were built and backfilled, with the roof only constructed when settlement had finished.

The Bangor By-pass was originally opened in late 1983, but was extensively reconstructed in 1993. An unusual feature of this reconstruction was the extensive use of slate waste in pavement construction, for haul roads, to face bridge abutments and wing walls and as a drainage medium. The largest structure is the 140m Ogwen Viaduct.

A55 across the Island of Anglesey

In early 1999, UK Highways A55 Ltd, a subsidiary of UK Highways Ltd, in which Laing holds the majority stake, won the concession to build, operate and maintain the A55 trunk road across the Island of Anglesey, off North Wales. The new road is a dual carriageway 32 km in length. The project was valued at £101m.