More information about the construction of the Wye Bridge and Viaducts
One year after the contract was let for the superstructure of the Severn Bridge, the Cleveland Bridge and Engineering Company won the tender competition for constructing the crossing of the Wye River and the Beachley Peninsula.
The contract was awarded to Cleveland in early 1963, with the Fairfield Shipbuilding and Engineering Company of Chepstow acting as their sub-contractors for fabrication and welding of the steel superstructure. In this instance, Fairfield fabricated the stiffened panels in their workshops in Chepstow, delivered them by road to Beachley and Newhouse where Cleveland’s erectors assembled them into boxes. Fairfield provided the platers and welders for work in the assembly yards
Construction of the Viaduct foundations
The foundations of the Beachley viaduct are conventional twin square shafts of reinforced concrete formed in open excavation through the loose sediments of gravel and sand on the peninsula to limestone strata some 20 – 30 feet (6 – 9m) below ground level. The tops of the shafts are linked by a pre-cast, pre-stressed concrete, tie-beam cast into the tops of the shafts just below ground level. These ties resist the horizontal forces from the splayed legs of the steel trestles supporting the deck.
Construction of the Wye Bridge foundations
The main pier foundations were constructed using a pair of caissons on either side of the Wye. They were sunk through the soft mud of the river banks down to the underlying limestone about 50 ft (15 metres) below ground level. Arrangements were in place to use compressed air if needed to keep the river water out during excavation but the mud formed an effective seal around the caissons as they were sunk into the ground. After founding on good rock the hollow caissons were filled with mass concrete and the tops were joined by a pierhead of reinforced concrete to create the two boat-shaped piers, the tops of which are above high tide level.
Construction of the Viaduct Deck
Erection of the viaduct decks was carried out by cantilevering the developing deck, from trestle to trestle, a distance of 64 m (213 ft), using a specially designed gantry that was slung under the completed deck and moved progressively forward as each new box was welded to the end. When each span was nearing completion, the next trestle was erected from above and the front end of the cantilever was lifted up onto the trestle, because each long length of cantilever would droop under its own weight due to the elastic nature of the steel box girders. The trestles had spherical bearings at the concrete pier level and line bearings at the top on which the deck unit rested. This arrangement allowed the growing deck to expand and contract longitudinally from fixed points in the middle of the Beachley peninsular and the Gwent abutment.
Construction of the Wye Bridge Deck
The side span deck units for the bridge were erected in exactly the same way as on the viaduct although, at 87 m, the span was significantly greater than in the case of the viaduct (64 m). The extent of the drooping that occurred as the leading edge of the side span cantilevers approached the main span piers, though not unsafe, was quite apparent. A temporary trestle was used just in front of the permanent trestle, to jack up the end of the cantilever.
A number of steps were taken, in advance, to prepare for the installation of the cable-stayed elements on the bridge. Special anchorages for the cables that would later be erected, were installed in the deck units that would be positioned over each of the back span piers. When these particular units were in place, they were pinned down to the piers on which they rested. This arrangement provided resistance to the uplift that would occur on the back span piers, as the two halves of the long main span of the bridge were being cantilevered out towards each other.
Erection of the deck units continued, as on the viaducts, past the main piers of the bridge and for three more units into the main span, on each side. At that stage, it was necessary to erect the two pylons on the centre-line of the deck, one over each of the main piers. Once this was done, further units were erected in two longitudinal halves to enable them to pass the pylons. Having passed the pylons, the two halves were brought together and welded longitudinally to make a typical box unit. The outermost box of each of these particular cantilevers would contain one of the main span cable anchorages of the bridge. At this stage, three of the eventual 20 strands in the completed cable stay system were hoisted to the top of the tower and draped loosely back towards the side span anchorage and also loosely connected into the main span anchorage box.
The next stage of erection was the most daring and critical operation of all and had to be completed in a continuous 3-day operation. The erection rig was moved forward and readied to receive one of the special deck units into which a main span cable anchorage had been installed. As the unit was moved onto the rig and lowered, ready for welding to the end of the completed steelwork, the rear ends of the three cables were pulled back, keeping in balance the longitudinal force on the top of the tower. The unit was then mated to the end of the cantilever and welding continued until sufficient strength was achieved to start tensioning the three strands which would begin to raise the end of the cantilever and relieve the maximum moment at the root of the cantilever that had been experienced earlier in the erection process. Welding of the box-to-box joint and tensioning of the three strands continued without a break until a pre-determined level of tension was achieved.
The final stages of the construction of the bridge included the erection and tensioning of the remaining strands and the erection of two more boxes beyond the outer cable anchorage to achieve the half main span on each side of the river. When both sides were at this stage, a small gap had been allowed for thermal movements so that the two half spans had to be pulled together before the final joint could be welded.
Closure came in spring 1966 when the last two deck sections were rolled over the completed part of the Wye Bridge and lowered into place in the centre of the span. The two half spans were pulled together and the joint, between them, welded. A continuous ribbon of steel was then ready to carry the then M4 across the Severn and Wye Rivers, finally fulfilling the aspiration first envisioned by Telford 143 years earlier.
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