Good marine access to the location of all pier foundations was important. The better ground conditions and accessibility are found on the east side of the river and so the consortium decided to concentrate activities on that bank. To ensure access across the English Stones at low tide, a 2 km long causeway was constructed from the Avon shore to the east pylon of the main bridge. Over a 500 m section, the bed was 2.5 metres lower than elsewhere and it was necessary to build it up to the same height as other parts of the causeway so as to maximise usable time. Precast concrete culverts were installed under this part of the causeway to allow the water to ebb and flow without overtopping the roadway prematurely.
A protective earth bank was used to assist work on foundations on the west side of the estuary. Large diameter bored piles, suitable for use in soft ground (alluvium and peat), were used for the first eight foundations on this side. SRC decided to use pre-cast concrete caissons in areas where the rock was sound, or where problems were caused by the strength, or the rise and fall, of the tides. Caissons could be relied upon to spread the load, resist overturning, and, particularly, to resist horizontal sliding caused by ship impact. They also provided good protection against strong currents and exceptionally high tides. All caissons were built and launched from the main construction yard on the Avon side, with its special launching ramps and towing equipment.
On the Gwent side, a protective earth bund was constructed out from the shore, over the alluvium, and large 2 metre (6.5 ft) diameter bored piles sunk to support the viaduct pier bases. Near the Avon bank, large diameter bored and de-bonded piles were used to support the caisson that would form part of the special foundation in the vicinity of the Severn Bridge Tunnel, see “Design of Viaduct Foundations”. The piles bear onto rock below, well clear of the tunnel. The caisson is, in effect, cantilevered out from the pile cap. One end of it is located close to the tunnel but all the loads pass into the ground at a safe distance from the tunnel.
The hollow caissons were all specially braced to resist stress during transit. They were transported in holding frames on huge crawler tractors, and delivered down the approach ramp. From there, they were loaded onto a special transport barge that was controlled by 4 separate engines, one on each corner, to ensure that the vessel would be able to hold its position, at high tide, to allow the caisson to be lifted off the barge and lowered into its final position.
Each caisson was placed onto a prepared bed of bags filled with grout, where they would be held by the crane until the grout had gained sufficient strength. In some cases, breaks would be created in this supporting arrangement to ensure that river water could flow freely through the base of the caisson. This is because of the possibility that the huge structure might otherwise sink into the underlying formation and effectively seal its base. Large and heavy, it may be, but a caisson with a sealed base would be treated by the rising tide as a de-facto vessel and so it might be caused to lift momentarily and so be dislodged from its desired location. To avoid this, the tide was therefore allowed to enter until a sufficient window of opportunity was available for the base of the structure to be properly sealed with concrete, with no further ballast needed to prevent any possibility of floating.
Sequence of operations involved in moving a Caisson from the casting yard to its location in the Estuary.
Copyright; Neil Thomas of Photographic Engineering Services.
The pier units were precast in the Avon compound, in purpose-made steel moulds, using vertical match-casting to ensure an accurate, tight and secure fit between adjacent units in the completed pier. High density polystyrene (HDPE) ducts for the pre-stressing tendons were cast into the walls. The steel U-tubes, to link between the two HDPE ducts, had previously been incorporated into the steel reinforcement cage for the last pour of in-situ concrete infill at the top of the caisson. Great precision was required to ensure that the tendon ducts were properly positioned right up through the pier. Epoxy glue was applied by gloved hand to the upper surfaces of the precast pier units before the first unit was placed in position. This process was repeated to build the pier to full height, using additional thicknesses of epoxy glue to maintain the alignment.
A temporary platform was provided across the top of each pair of ducts, to assist with the stressing of the tendons within the piers. The tendons, each comprising 19 strands, were made upon on this platform and then winched through the ducts. They were stressed from both ends simultaneously and hot wax was pumped up from the bottom of each duct to protect against corrosion. On completion, the profiled chapeaux were cast on top of the caissons to protect against ship impact. The more vulnerable piers were further protected by the pouring of mass concrete into lower parts of the internal void within the caisson.
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