The bleak flat landscape of Sheerness on the Isle of Sheppey does not look its best on a grey, wet and windy March morning, but if you are an importer of cars or fresh fruit, then this is the place to be.
This remote town on the north coast of Kent is one of the biggest fresh fruit and vehicle ports in the UK. Owned and run by Mersey Docks & Harbour, the port handles cargo ships from around the world. Vast car parks surround the dock area, with seemingly every square centimetre of available space filled by thousands of new cars ready to be delivered all over the country.
And the port is continuing to expand. Contractor Christiani & Nielsen Special Projects is currently building a massive fresh fruit storage facility on the harbour edge for MDH. When the £30M facility takes delivery of its first shipment from South Africa in January next year, it will be able to hold 30,000t of fruit in 12 temperature controlled chillers, doubling the port's storage capacity. It will mean that even the largest ships handled at Sheerness (up to 38,000t) can be offloaded in under 18 hours.
'The floor slab is equivalent in size to eight football pitches and the building six,' says C&N Special Projects operations manager John Henderson, who is obviously not at all daunted by the size of the structure and the relatively short 15 month contract time. He joined C&N about two years ago, around the time that the Special Projects team was set up, and generally deals with technically difficult and often high profile work in the UK and throughout the world, usually marine and major civil engineering projects. One of the current jobs the team is handling is the Preveza immersed tube road tunnel being built on the west coast of Greece.
Foundations for the massive 42,000m2 slab and the 30,000m2 hangar-like building at Sheerness are essentially two elements. Around one third of the building sits on the reclaimed Lappel Bank that forms the existing dockside. Keller Ground Engineering was subcontracted to carry out driven cast in place piling for this area. Some 918, 380mm diameter concrete piles between 16m and 20m long were installed through the sand and alluvium, 3m to 6m into the London Clay below.
The remaining two thirds and the quayside are being built out into the harbour and will sit on a regular grid of 846 hollow steel tube piles. The marine piling is being carried out directly by C&N. Its two piling rigs operate from platforms that shuffle around on temporary metal beams placed directly on the already installed piles, driving an average of 10 to 12 piles a shift between them.
'The 'Traveller System' was used because the water is too shallow to use floating plant,' explains Henderson. The front of each platform has a row of 'gates' through which the piles are pitched and driven, ensuring they are installed in the correct positions. Once a row of piles is installed, the rig picks up a section of the beams from behind and places them in front to allow the platform to move. The platform is then pushed forward on a hydraulic jack and ratchet system to the next position.
At the moment, one rig is installing four piles at a time, while the other installs five. Their movements are closely co-ordinated and were planned well in advance of work. Once one of the rigs has finished on the main section it will move on to the new car jetty also being built as part of the contract. This will run between the front edge of the new platform in line with an existing jetty to the west.
A number of different piling hammers are used on site, but mostly a Junttan X12/14 and IHC S150 are worked, with the others in reserve in case of breakdowns. It takes between 40 and 105 minutes to install each pile, which vary in length (32m to 51m) and diameter (813mm, 914mm and 1067mm). They are driven up to 20m into the underlying London Clay and will carry loads of 50kN/m2 inside the building and 30kN/m2 on the exposed dock. Pile wall thickness also varies from 19mm in the splash zone to 14mm.
'We aim for each rig to drive five piles every shift,' says Henderson. Driving work must be done during the day, because of local residents' complaints about vibrations. Work at night is restricted to delivery and pitching of the piles ready for the next day. The piles are delivered to site by sea from further along the coast. Plugged at each end, they are dropped on to the beach, floated at high tide then towed to site in batches of five.
The deck structure is made up of precast reinforced concrete beams and 450mm thick slabs fabricated in sheds next to the site offices to the south of the town and transported by road to the dock. In all, 783 beam and 2,014 slab sections will be used for the offshore section, with the ground slab for the land section cast insitu. The final surface of 100mm thick concrete will also be cast insitu. At the seaward edge, the slab has been beefed up to cope with the additional battering it will receive as ships dock.
When NCE visited the site, around 40% of the marine slab was finished, with 470 marine piles already installed. Work on the building itself on the land section had been going for a couple of weeks, with the first structural steel elements erected.
C&N is wholly responsible for the design and construction of the contract, comprising 37 weeks of civils work and 44 for building, with some overlap. Henderson says the job is largely self-policing with minimal client supervision and even though the time and cost is fixed, there is allowance for some flexibility. This meant that C&N was able to adjust piling operations to keep to programme when work was restricted at night. It also meant some pile lengths could be reduced because of the high strength of the London Clay.
This way of running the contract appeals both to contractor and client.
As Henderson says, 'All the client should have to worry about is that the facility is up and running on time.'