The key to the successful erection of the viaduct was the acquisition of a bespoke launching gantry. It was 234 m long, designed to carry a maximum load of 200 t, and it was used to build the viaduct deck. It consisted of a pair of parallel steel trusses that stretched its entire length and two transverse primary supports. It also had secondary supports at the front and rear of the trusses, and a pair of crabs. It operated above the deck and was able to move itself forward. Also, for a reason that is described below, it was able to move itself from one side of the deck to the other, as the work progressed.

The gantry was used to build the viaduct deck using the balanced cantilever method. In outline, this involves building out two sections of deck from each pier, in the form of cantilevers, one to the front of the pier, the other behind, so that the new length of deck remains in balance over the pier. There are 27 segments in each span, excluding those fixed to the pier, and when 13 have been erected on each side of the pier, the gantry would be moved forward to the next pier and the whole process would be repeated. After the first and subsequent repetitions, a further complete span would have been constructed, except for a final segment in the centre, and that segment would be placed before the next repetition commenced.

In this particular case, there was a further complexity because the decision had been taken to construct the deck using twin box girders, one under either side of the deck, in order to halve the weight of the box segments. Both box girders had to be built at the same pace in order to keep the growing length of deck in balance over the pier, laterally as well as longitudinally. The effect of this was to require the gantry to be moved from one side of the bridge to the other, after placing just four deck segments (one in front and one behind the pier to catch up with the second pair on the previous cycle, and then one more of each, to surge ahead again). In anticipation of all this, the gantry had been constructed to deal with just half the deck at any one time.

All deck segments were carried from the construction yard to the viaduct by a transporter that would travel to, and along, the completed section of deck to deliver the segments. Construction started at the Avon side, with the placing of the two special segments that would sit, side by side, on the first pier. Pier segments were more robust and complicated than others and they exceeded the weight limit of 200t imposed by the use of the gantry. SRC’s solution to this was to construct these segments in two parts, the first in the usual manner, the second by adding a further 180 t of in-situ concrete to each, after the segments had been placed on the piers.

The gantry was initially positioned to one side of the viaduct location, with its front primary support on the Avon abutment, the rear support behind the abutment and the front leg on a temporary bracket which cantilevered forward from the front face of the pier. It was then able to place the first special deck segment in the correct position on the pier but on temporary supports. The gantry was then winched backwards a short distance, before being moved laterally to deal in similar fashion with the special segment for the other side of the pier. With the second pier segment in place, the 180t of additional concrete, which had been omitted from the casting of the segments because of the weight restriction mentioned above, was added.

The primary supports of the gantry were then moved forward until the forward support was above the pier that had just received both its deck segments, and the rear support was on the abutment. The gantry was then fixed, temporally, before other deck segments were added to those on the first pier, as described above, using the principle of balanced cantilevers. Working out from the pier, new segments were placed, the first to the front of the pier and then one behind it, to keep the growing sections of deck in balance, longitudinally. As soon as a segment was lowered into position, it would be stressed tightly against its predecessor, using temporary tendons. After delivering a single pair of ‘balanced’ units, the gantry would be moved across to the other side of the viaduct, to repeat the process, so that the balanced cantilevers on both sides of the viaduct could be brought forward at the same pace, to minimise the lateral loading on the pier.

It is important to appreciate that a single move of the gantry did not produce two whole spans of box girders but four half spans, centred on the pier, i.e. two pairs of “balanced cantilevers”. Not counting the special pier units, 27 segments of equal length went into the construction of each completed box girder span, a total of 54 per span.

The segments themselves consisted of reinforced concrete box shells, open at both ends. They were manufactured on both sides of the estuary, approximately half each side, using five casting bays on both sides. The segments were constructed with a single pour of concrete into a mould that already contained the necessary cage of steel reinforcing bars and ducts for the tendons. Each segment would be match-cast against its future neighbour. It was possible to manufacture 20 units from five moulds in a six-day week, in all but the worst of weather conditions.

In order to ensure the correct and secure positioning of the tendons, complicated internal diaphragms were installed in all the deck segments that were located over piers. Two other, more simple, diaphragms were added to each span, one in each of the deck segments located at each end of the low-height central section of the span. The purpose of the additional diaphragms was again to accommodate locking devices and deflectors in order to prevent any slippage of tendons (in compliance with the requirement that the free length of a tendon must not exceed 40% of the span) and to effect a change in the direction of some tendons. The locking devices and deviators were part of purpose-made steel assemblies, bolted into the appropriate segments after they had been removed from the casting shed and were no longer on the critical path. When tendons had been stressed, they would be wedged in place to prevent any longitudinal movement or loss of tension.

As each deck segment was delivered and lowered into position using the gantry crab, epoxy glue was applied by gloved hand to the joint surface, and the new segment was temporarily pre-stressed to its predecessor. Once a balanced pair of segments had been erected, the permanent cantilever tendons at the top of the segments were stressed so that the temporary tendons could be de-stressed and released, allowing these tendons to be reused.

When all 27 segments had been erected on both carriageways, the permanent bearings were fixed in position and the vertical load transferred from the temporary jacks on the pier segments to the bearings. The nominal 2 m stitch in the centre of the deck between the twin box girders was concreted and the permanent continuity tendons above the bottom flange were stressed to enable the gantry to be moved forward again to start the next cycle. Further continuity stressing was installed at a later stage in the erection process.

The permanent post-tensioned tendons within the box girders were all enclosed in high density polyethylene ducts that were temporarily supported on frames suspended from the soffit of each unit. The ducts were made continuous between the anchorages and they were passed through holes in the deviator plates into which pre-bent galvanised steel ducting had been fitted. The ducts were fixed both at the anchorages and at the deviator plates. Strands were introduced into the ducts and all tendons, even the 250m long continuity tendons, were stressed from one end, using a multi-strand jack, and were then secured. Finally, the ducts were injected with hot wax to complete the protection system.

A special construction cycle had to be devised for the final half spans connecting to both the Avon and Gwent abutments where the balanced cantilever method was not available. The launching gantry was located behind each abutment in turn and used to deliver the deck units, placing them on to a pair of beams supported from the ground by a military trestle. When they were in position, the units were glued and temporarily stressed together before being connected to the completed balanced cantilever span that was perched on the first pier. Tendons were then introduced, stretching between the abutment and the first pier, and stressed to form a coherent beam out of the 26 separate units. Eventually, the first span was stressed to the rest of the viaduct as far as the first movement joint.

Precast concrete beams at the Avon end span. Copyright; Neil Thomas of Photographic Engineering Services.

The ends of the slip roads on the west side of the M 49 junction extended 240 m past the Avon abutment, causing an additional complication. The two box girders had to deviate from each other to accommodate the road geometry so that the concrete stitch between them had to be made progressively wider. Precast concrete beams were constructed in the yard, to fit transversely across the gap between the box girders. Then assisted by a false-work frame from the ground below, the beams were used to support construction of the concrete stitch.

Road surface.

To complete the roadway surface, the top face of each pair of viaduct units and the in-situ concrete stitch between, was sprayed with a polyurethane waterproof membrane. Then a 20 mm thick sand asphalt carpet was laid to protect the waterproof membrane and this was followed by 100 mm of hot-rolled asphalt.

Mechanical and Electrical Works.

The following features were among those added to the completed viaduct as part of the provision for its future maintenance:-
• A monorail access train, and stations, suspended beneath the viaduct and bridge decks.
• An access gantry for the main span of the Shoots Bridge.
• An access gantry for each stretch of viaduct (plus the adjacent back-span of the Shoots Bridge).
• Access lifts in each of the four pylon legs of the main bridge.
• Lighting in all the accessible internal parts of the bridge deck, pylons, viaduct decks and piers.

The monorail train operates over the whole length of the crossing between the east and west abutments and may be accessed from both ends. It is designed to carry personnel, with a trailer for carrying equipment and materials. Intermediate steel platforms, that were suspended from the bridge deck or fixed to the pylons, allow access to the cable stayed bridge.

Access to the main span of the bridge was provided by a conventional under-slung steel gantry supported on rails beneath the deck and at the edges of the deck. However, the under-slung access gantries which service the viaduct spans and the back-spans of the bridge are of a novel design, comprising a central rigid platform from which a rotating steel truss access beam is suspended. This is to allow the gantry to pass through the space between the twin legs of the viaduct piers by turning the beam through 90 degrees. Hydraulically operated telescopic platforms permit access to a number of different levels beneath the deck.

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