Unexpectedly soft seabed material was a pleasant surprise for marine contractor Seacore when it began constructing a caisson off the Norfolk coast as a key part of the cooling water outfall system for the new Great Yarmouth Power Station.
Seacore's original plan was to use its specially designed drill bit to form a 7m diameter excavation in a single pass, while lowering the 8m diameter,15.75m high caisson 15m into the seabed.
Instead, working from its jack-up platform Deep Diver, the firm used a simple airlift pumping system to vacuum and excavate the hole.
Seacore project manager Sam Whitaker explains: 'Judging by the site investigation borehole logs, we expected to have to drill and lower the caisson into the seabed after initially toeing it in [about 2.5m] with the airlift.
The firm specially designed and built the bit, which works like an upside-down, spinning umbrella. Its point would have been placed into the seabed and its cutting wings splayed out. Drilling would have been carried out inside and in advance of the caisson, suspended from the jack-up.
'But the seabed proved much softer than anticipated so we just continued vacuuming with the airlift, 'Whitaker explains. Fortunately we were able to position very accurately and lower the caisson to full penetration without the drill.'
The 540t precast reinforced concrete caisson was cast with a soft concrete eye that was later punched out by a 2.4m diameter tunnel boring machine, forming a 770m long tunnel from the onshore power station.
Seacore's £1.7M subcontract for Costain Tunnelling also involved removing most of the TBM from the caisson (its shell formed part of the final structure), placing a vertical riser pipe inside the shaft, backfilling and fixing a water diffuser cap on top of the caisson.
Deep Diver brought the reinforced concrete caisson sections to site from Great Yarmouth harbour. The first 9m high section included the 600mm high, 8.1m diameter steel cutting shoe and the soft tunnel eye. The rest of the caisson is made up of three 1m high, and five 750mm high rings.
Seacore was concerned about the amount and effects of caisson oscillation caused by strong currents and wave height and frequency during the lowering sequence. It carried out tests of the lowering sequence in Southampton University's wave tank.
'The current was not so much of a problem, but wave action oscillated the caisson up to 600mm around its aiming point, ' says Whitaker. 'Waves above 500mm or increasing the wave frequency caused too much motion, so we modified the lowering sequence and lifting points and cut the oscillation.'
Once on site, the lower section was lifted into position and two of the 1m high rings fixed to it, each sealed with rubber gaskets, epoxy glued and pretensioned with 16 anchor bolts cast into the tops of each ring.
Now 11m tall, the caisson was lowered on PSC Freyssinet strand jacks until the cutting shoe penetrated the seabed. The remaining 1m high ring section was then added. Lubricating mud was injected into the 50mm wide annulus between the outside of the caisson and the cutting shoe and compressed air piped down inside the cutting edge.
At the same time, the main airlift mucking pipe was placed inside the caisson to start vacuuming, with the caisson simultaneously lowered into the seabed.When the excavation had reached 2.5m depth, the clay and sandy clay seafloor material was softer and easier to vacuum than anticipated, so airlifting continued without the need to swap over to the drill.
Excavations continued with the giant vacuum cleaner, with the remaining five 750mm high concrete ring segments added as work progressed.These were followed by a temporary steel cone and a pair of 4m high, 5.5m diameter steel casings. The 15.75m caisson was then lowered until its top was just proud of the sea bed.
A 400t concrete plug was tremied into the flooded shaft to the tunnel invert level and grout injected in the annulus between caisson and cutting shoe excavation.
Seacore pumped out the shaft and, despite the plug, the caisson still had 200t positive buoyancy.To prevent it rising out of the seabed, Seacore used the jack-up to provide additional weight to the caisson through a pair of temporary struts.
A thickened concrete wall was cast behind the soft eye in the shaft, backfilled with a cement and sand mixture for the TBM to drive into, followed by a concrete cap.The shaft was then reflooded to await the TBM's arrival.
Once the TBM had arrived, the shaft was pumped out again and the capping broken through to allow it to be removed. A 90 degree glass reinforced plastic bend with a vertical riser was lowered in and fixed to the tunnel opening, with mass concrete infill around the pipe up to seabed level.
Costain installed a temporary bulkhead in the tunnel near the shaft, which was then reflooded.Seacore removed the top two steel casing sections, leaving the cone behind to stop any spoil falling into the riser.The jack-up took the TBM to Great Yarmouth and returned with the 170t diffuser cap, which was fitted after the steel cone was removed.
Costain, working for the power station main contractor Bechtel, will complete intake works, channelling cooling water from the River Yare to the 400MW gas fired combined cycle power station, which was due to go on stream early this year.