Cornwall-based ScaffFloat has completed what it calls ‘another milestone project’ in North Wales, replacing an old railway bridge across a tidal estuary in North Wales.
Pennys Demolition and Alun Griffiths hired the firm for the project, for which ScaffFloat invented a set of durable, 100% recyclable plastic floats that integrate with standard scaffold components to make pontoons, rafts, and workboats.
These were used on the project to transport the old 25t bridge sections from the bridge to the beach, and then again to carry out the new deck sections for the new bridge.
Mobilisation and demolition
First the pontoons were assembled by hand at the top of the beach. The tracked excavators used for the demolition then lifted and carried them down the beach at low tide, where they were moored before floating off on the flood tide.
Once the ScaffFloat team had mobilised, the demolition of the old bridge started. To support the weight of the old bridge sections after they were cut, a gantry on the new bridge was used to take their weight.
As soon as the bridge sections were hanging free, a ScaffFloat pontoon was push towed into place under the bridge an hour before slack water.
A mooring line was run from the pontoon bow to one of the pre-laid anchors and then tension was taken up on the windlass capstan, which held the pontoon against the flow.
Two lines were run out sideways from the deck winch on the pontoon to the bridge piers, which allowed the crew onboard to fine tune the position of the pontoon to the 100mm of tolerance needed to get the bridge section onto the pre laid deck chocks.
Once the weight was taken by the pontoon, the lifting strops were removed, the deck winches released and the pontoon’s capstan pulled the pontoon and bridge section into clear water before the workboat behind took control released the anchor line.
As all moves had to be done at high tide, there was hardly any space to get the old bridge sections out into clear water, often less than 100mm.
Once in clear water the ScaffFloat skippers had the challenge of pushing the laden pontoons across the estuary, through the moorings and onto the beach.
Delays in the bridge lowering meant this had to be done after the ebb tide had started to flow. In this situation, the tide would be flowing out of the estuary mouth with some power, which made it imperative that the critical turn round the starboard channel marker into the beach was done without error.
Over the course of several weeks this process was repeated as the weather and tides allowed, with the team managing three pontoon moves on a single high tide to help make up for lost time.
New bridge decks
Once the last bridge section was on the beach and cut up, the heavy-lift pontoons were dried out on the beach moorings and Attridge scaffolding pre-installed the suspended scaffold on the decks of the pontoons.
The new bridge deck sections, complete with rail track, were lifted on top of this scaffold and connected together. One by one these were taken back out to the bridge and the process was run in reverse.
Once the new deck sections were in place the scaffold would hang underneath, which saved a lot of work having to install it in the air.
Hoisted for access
At various stages during the project, access was needed to the underside of the bridge when the scaffold had been removed, or not yet installed.
The ScaffFloat had been designed so that as well as floating, they could be hoisted up under the bridge on a four-point lift with the gantry to be secured to provide an access platform.
Weather & Tides
One of the biggest challenges to the marine element of the project was working around the tide and weather.
Given the large volume of water held by the estuary upstream of the bridge, there was only a very short period of slack water, often under half an hour, when all the marine works had to be completed.
The project was also completed during the autumn, with very changeable weather and during several named storms passing through the region.
Design & Certification
The team at ScaffFloat provided a full set of RAMS as well as designs for all the ScaffFloat pontoons and moorings.
Richter provided a full Cat 3 check for the heavy-lift pontoons and hoisting, which included full structural, stability and buoyancy analyses.
“On a calm day at slack water you would let your kids swim under the bridge, it looked so benign, said MD Toby Budd. “But a few hours later you could have 1m standing waves with 5 knots of flow against a full gale.
“It was a great project delivered by a top team. We worked well with the team from Pennys and Alun Griffiths, everyone had a ‘let’s get it done’ attitude, tempered with a sensible level of safety and risk management.
“Being such an innovative engineering project, problem-solving was a daily practice, and we loved being part of the team that overcame a long list of challenges to get the job done. It was great to see the line reopen and a train cross the bridge.”
