Building the Severn Bridge

 

The austerity of the later 1940s and early 1950s meant that little happened on the ground during that period. At the end of 1960, the Minister of Transport made a public statement about the M4 and road needs in Wales.  And work started on the foundations for the Severn Crossing in 1961.

Severn Bridge Foundations and Anchorages

The construction of a suspension bridge starts with the foundations and towers. The main suspension cables are firmly anchored to abutments at both ends of the bridge and are held up by the towers, to provide the bridge with its iconic shape. The bridge deck is supported by hangers suspended from these cables.

The two main towers were each built on a boat-shaped pier 40 m (130 ft) long by 11.5m (38 ft) wide, with cut-water ends. Each tower would consist of two full height vertical steel legs 23.5 m (77 ft) apart, centre to centre, joined by three steel portal beams. Both piers were constructed from temporary jetties that were built out into the estuary from the respective shore.

The Aust pier is founded on a hard limestone outcrop that is only exposed at the lowest spring tides. It is formed of solid concrete and built high enough to protect the steelwork of the tower legs from splashing by salt water.

The Beachley pier is located at a point, as far offshore as possible, where the top of the limestone is at a depth of 20 m (65 ft) below a river bed level that is exposed at low tide. When further borings were taken at the start of construction, it was found that there was not a good limestone bedrock layer available at this point. This pier has therefore had to be supported by two massive cylindrical concrete bases, one under each of the twin legs of the tower, each 18 m (60 ft) in diameter and founded about 10 m (35ft) below river bed level, through the marls and on the top of steeply inclined hard carboniferous mud-stones. The pier has been constructed across the top of these two bases, with extensions at both ends to provide the cut-waters.

Cables were set into the mass concrete of both piers. Later, they would be used to fasten down the bases of the steel towers as they were being erected on the piers in a free-standing mode.

The anchorages, which secure and hold firm the ends of the main cables, are massive reinforced concrete blocks, each weighing about 100,000 tonnes. The one on the Aust side was constructed on a further outcrop of limestone that is exposed at low tide, about 105 metres from the Aust cliff. The Beachley anchorage was constructed in open excavation in the soil and it was keyed into the underlying limestone.

The short element of construction that is required to complete the structural element of the Crossing beyond the eastern abutment of the Severn Bridge at Aust, is known as the Aust Viaduct. It is very different, in structural content, from any other item of viaduct, comprising just three short lengths of twin box girder supported on concrete columns and joined with steel cross girders, and with a composite, cast in-situ, concrete deck slab. The adjacent image shows one of the box girder units being taken across the estuary from Beachley to Aust, travelling on a barge.

For more information on the Construction of the Severn Bridge Foundations and Anchorages, Click Here

Severn Bridge Towers

The twin columns of each tower were built as hollow steel boxes standing on the concrete foundations. The many individual steel plates in each face of the towers, up to 1 inch (2.5 cm) thick, were stiffened, trial assembled and fitted with machined flanges at both ends so that they could be bolted and stressed together on site. Three portal beams were inserted between the legs and above the road deck level of each tower, to produce a strong and resilient structure. The steelwork for the towers was fabricated in sections in Glasgow and taken by road to a stock yard in Chepstow. Transporting the sections for the Aust tower across the river, with its difficult tides and water levels, required careful planning.

A special climbing structure was devised to assist in the construction of the towers. It included a platform that could climb up the 20 metre high tower sections, as they were being built. It carried its own crane to lift the sections of the tower. It was designed to operate in high winds and it weighed 160 tonnes. As the towers got taller, the effects of wind on the lifting operation became more severe. On two or three occasions, no lifting could be carried out for two days.

Part of Aust Viaduct loaded on barge

 

Construction of Tower, Climbing Tower Crane in place

Completed Towers with hanging Cat Walks attached

For more information on the construction of the Severn Bridge Towers, Click here

Spinning the Main Cables

Each main cable consists of 8,322 individual galvanised wires, each the thickness of a pencil. The total length of these wires is approximately 29,000 km.

The spinning of the main cables began by stretching two wire ropes across the river, by boat. The ropes were then lifted to the tops of the towers, one on each side of the bridge, and their ends were made firm in the anchorages. They were used to haul out further wire ropes to support the access catwalks and the overhead supports for the cable spinning gear. Winches were set up on the Beachley anchorage, one to each side, to operate the two 3.5 km long continuous haulage ropes that would pull the 16,644 wires across the estuary to make up the two main cables.

View from one of the Cat Walks of a pulley taking two bights (four wires) across the estuary. The picture shows quite a few strands of wire already across, beneath the spinning wheel.

The process of building up the main catenary Cables continues; each cable consists of 8322 individual wires

 

The completed sheaf of wires is tightly bound to provide the finished cable

 

 

 

 

 

 

 

 

 

 

 

 

As the spinning wheel passed back and forth, high up on the towers and cat walks, 24 hours a day and in all weathers, workers placed the new wires in combs to prevent them becoming tangled. Each new set of wires was fixed to the anchorage blocks and adjusted to exactly the correct length. After checking the completed 500 mm diameter cables, the individual wires were compacted into a single cable that was tightly wrapped with galvanised wire before painting. Finally, the hangers were suspended from the cables ready to attach the deck sections.

The engineering feat achieved so far was already most impressive but the most risky task was about to start.

For more information on spinning the main cables, Click here

Suspending the Severn Bridge Deck

Imaginative engineering had produced a revolutionary design for the girder that would act as the stiffening girder and the road deck. It was fabricated off-site, in nearly 2000 stiffened panels and then assembled and matched end-to-end on slipways on the River Wye in Chepstow into 88 sections, each 18 metres long. Each section was constructed to be capable of floating on the river, like a pontoon. Section by section, they were launched into the water, towed to the bridge site, and then lifted into position. The work was heavily dependent on the tides and winds.

To handle the sections in the swiftly flowing river, a special craft – the Severn Knave – was built to push them one by one into the correct place. It could manoeuvre itself in all directions and hold the sections, floating on the river, in the position required.

Preparatory trials, using models of the deck sections in a water tank, concluded that erection could only take place fortnightly, as the tides would only allow time for 3 to 6 sections to be erected each fortnight. And, of course, bad weather could further restrict progress in these difficult conditions.

The skill of the skipper of the Severn Knave proved to be paramount in the positioning and the lifting of the deck sections. The process involved collecting a deck section from its moorings in the Wye, pushing it out to the centre of the river, adjusting the position by varying the engine thrust direction, lowering lifting tackles and attaching them to the deck section, winching clear of the water, washing down to remove mud and salt from the underside, and then final lifting to the required height and attaching it to its hangers and the previous section of deck.

The sequence for lifting and incorporating individual sections into the emerging deck started at the centre of the main span and continued in either direction, towards the towers. The centre span deck was completed before any sections were lifted up to the side spans. Completion of the deck heralded the final stages of this striking engineering story. It was an awe-inspiring sight.

For more information on suspending the severn bridge deck, Click here

Building the Wye Bridge and Viaducts

Introduction

At about the time that the foundations of the Severn Bridge were being completed, work began on this important series of structures that are located immediately adjacent to the western end of the bridge. From there, a viaduct has been built across the Beachley peninsular to connect to the Wye Bridge, with a further section of viaduct, beyond, terminating on an abutment on the Welsh shore.

Both sections of viaduct were built with steel box girder decks resting on steel box trestles a modest 64 metres (210 ft) apart. The Wye Bridge itself has a main span of 235 metres (770 ft) and side spans of 87 metres (285 ft). The cable-stayed box girder deck of the bridge was supported by a simple steel tower, or pylon, in the middle of each main bridge pier, with inclined cables stretching down from the top of each pylon, to be anchored into the appropriate deck boxes.

Foundations for the Wye Bridge and Viaducts

The foundations for the viaducts are simple twin reinforced concrete shafts (one under each of the splayed legs of the trestles) that were sunk through softer soils, down to limestone beneath. The foundations of the main piers of the Wye Bridge each comprise twin hollow shells (caissons) that were sunk through the very soft mud of the river bank, to limestone some 15 metres below and then filled with concrete. Each pair of twin foundations for the viaducts were joined together with a concrete tie beam below ground level that linked across their tops, to resist the outward forces in the splayed trestle legs. At the main pier a crosshead beam was placed on top of the caissons and it remains visible above high water level.

Construction of the Viaducts

A single cross section was adopted, for both the bridge deck of the Wye crossing and the linking viaducts, to provide continuity from the Beachley anchorage of the Severn Bridge to the Gwent abutment in Wales. Splayed steel, hinged trestles carry the viaduct across the Beachley Peninsula in ten spans, varying from 182 feet (55 m) to 210 feet (64 m). There are a further two viaduct spans of 210 feet (64 m) between the western side of the Wye Bridge and the Gwent abutment, where the new structure crosses the main railway line into South Wales.

Construction of the Beachley viaduct commenced at a fixed point in the middle of the peninsula and proceeded simultaneously in both directions from there – up to the back of the Beachley anchorage of the Severn Bridge, and down to the eastern bank of the Wye. Construction on the Gwent side commenced at the western end where the viaduct was fixed to the fairly massive concrete abutment located on a limestone outcrop.

The viaduct deck was constructed, incrementally, by adding new units to the end of a developing cantilever. Starting from a pier or trestle on which a deck unit had been made secure, each new section of deck would be welded to the exposed edge of its predecessor. This sequence would be repeated until the end of the developing cantilever reached the next trestle. There, it would be raised up sufficiently to allow the next unit to be welded in sequence, this time on top of the latest trestle – and then the whole cycle could be repeated.

Construction of the Wye Bridge

The procedure that had been used to erect the viaduct deck (see previous paragraph) was also applicable to much of the work on the Wye Bridge. Never the less, a girder made up solely of deck units similar to those employed on the viaduct, would not have been stiff enough, on its own, to cope with being cantilevered out for half the main span of the bridge (i.e., 117.5m, compared with 64m for each viaduct span). Considerable additional stiffening would be needed. The solution was to erect a single pylon, or tower, on the centre of each of the main span piers. A long sheaf of high tensile steel cables was then lifted to the top of each of the pylons so that individual strands could, when needed, be isolated, positioned and stressed, and then firmly anchored within the bridge deck at a distance of 78 m (255ft), in both directions, from the particular pylon involved.

Without the use of the pylons and cables, the front end of the emerging box girder deck would have drooped alarmingly before reaching mid span, and the structure would undoubtedly have suffered the same fate as four other box girder bridges across the world that collapsed while under construction during the following decades. Bridges of this type, that rely upon the use of cantilevers in their construction, are always particularly vulnerable while under construction.

The two pylons used in the construction of the Wye Bridge were both 30 m (100 ft) high, constructed of a steel box section and pivoted at its base. Each cable, when completed, was made up of twenty 2.5 inch (63 mm) diameter strands of galvanised wires, in a configuration similar to that used for the hangers on the Severn Bridge. The strands passed though the upper surface of the deck and were anchored to beams in special chambers within the appropriate box girders.

This simple and elegant solution allows the Wye Bridge to be classified as a cable stayed structure, albeit a very simple example. A much more complex member of the genre would grace the Severn Estuary itself, just 5 kilometres downstream, before the end of the Twentieth Century.

For more information on Building the Wye Bridge and Viaducts, Click Here

Finishing Operations

Before the crossing could be opened, further operations had still to be completed, including surfacing the roadways, erecting the safety fences, fixing the parapets, installing lighting, and the final painting

To prevent corrosion in the harsh environment, all of the steelwork was protected by the application of special high quality paint. To ensure that this did not peel off, all of the external steel had to be thoroughly blast cleaned and spray-coated with metallic zinc before the paint was applied.

Safety

Attention to the safety of staff associated with the construction and to the safety of the public was of prime importance – construction on such a difficult site is, by its very nature, an activity fraught with risk. The accident rate during construction was low but sadly four men lost their lives in the period, albeit in two different workboat accidents rather than in the construction works. Two further fatalities occurred at the very end of the project, following finishing works inside the towers which resulted in the emission of toxic fumes.

The completed Severn Bridge from the Beachley side

British Engineering leads the World

The Severn Bridge was opened by her Majesty the Queen on 8 September 1966. It was the lightest suspension bridge in the world for its span and loading, and the main span was the seventh longest in the world. The Wye Crossing was also technically advanced in its design and construction. Altogether a highly significant example of British civil engineering and a remarkable achievement on the part of all those involved.

 

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