Lifeboat station construction 101
In West Sussex, Dean & Dyball is busy on a project that could help save mariners lives. By Paul Thompson
Scheme:RNLI Shoreham Harbour Lifeboat Station
Value: £4.5 million
Main contractor:Dean & Dyball Civil Engineering
Piling sub-contractor:Rock & Alluvium
Contract: NEC3 Option A
On a warm, calm, early spring day there is little to indicate that the harbour and port town of Shoreham-by-Sea in West Sussex is ever anything but a peaceful haven for weather beaten mariners.
Ships slip in and out of the commercial port through the harbour entrance and the air is noisy with the clang of ropes chattering against masts in the light northerly wind.
Closer inspection of the scenes though indicates that this is not the case all of the time and occasionally this seemingly benign stretch of coastline has a habit of biting the unwary. A small lighthouse sits on Kingston Beach – a stretch of shingly sand opposite the harbour entrance and the mouth of the River Ardur - and just a few yards away, facing directly out to the open ocean is RNLI Shoreham Harbour.
The Royal National Lifeboat Institution has maintained a lifeboat service at Shoreham since 1865 when it took over duties from the harbour commissioner and until earlier this year its current 14 m Tyne class all weather lifeboat Hermione Lady Colwyn, and the more recently added inshore lifeboat Barry Lazell were housed in a 1930’s built facility built on the shingle of Kingston Beach.
But that aged station has been razed to the ground by Burgess Hill based demolition contractor Dorton Group and now contractor Dean & Dyball Civil Engineering has the job of building the new station with up-to-date facilities, ready to accommodate the latest Tamar class lifeboat when the crew’s current boat retires.
Project manager Nigel Lorraine is the man charged with helping to bring facilities at RNLI Shoreham Harbour up to a reasonable level so that the crew – all of whom are volunteers bar the boat’s mechanic – should be able to at least shower and change in a warm and dry building when they return from a call out.
“The existing building was very dated in terms of capacity and function;” says Mr Lorraine, “Although emotionally there were quite a few ties, it really didn’t work particularly well as a building. The new facilities will be miles away from what the crew have been used to. They will finally get the building they deserve.”
That new facility will be a three storey, engineered timber framed building featuring storage halls for both the Hermione Lady Colwyn or its Tamar class replacement and the Barry Lazell, office and mess accommodation, training facilities, kitchen and showers as well as slipways for the two boats. It is due to be delivered in spring 2010.
Before that handover date though the site team is busy piling for the foundations of the building as well as the ramps, and casting the reinforced concrete structure that sits inside the outer Glulam engineered timber building.
Piling specialist Rock & Alluvium has installed the 34 end bearing, 600 mm diameter continuous flight auger piles across the site, some of which reach 23 m down through the 7 m or so of accumulated beach material and into the underlying chalk bedrock. The sub-contractor also used its Soilmec CM50 rig to install a secant wall to form a wave pit at the front of the lifeboat house. This wave pit helps dissipate the energy of storm waves as they race across through the open harbour mouth and crash against the slipway, a problem particularly associated with spring tides characterised by higher than average high water.
“South easterly storms push waves up and straight through the harbour mouth. The site and old boathouse used to get flooded occasionally on spring tides so the wave pit helps stop this,” explains Colin Rattray, project engineer for design engineer Haskoning UK.
The 1 in 5 gradient of the slipway tends to accentuate the size of waves hitting it and the wave pit - or wave dissipation void to give it its rather grand title - ensures that the waves break before they hit the boat house door. The resulting sea water will drain away back into the harbour through a channel running parallel to the slipway, to be designed into the final scheme at a later date.
Site workers have cast 40 structural hard male piles comprising C45 concrete and steel reinforcement to a 6 m depth, through a ring of softer C10 unreinforced female piles, to produce the 5 m by 12 m wave pit. This will feature a steel framed extension span for the slipway to carry the lifeboat over it during launch.
And it is the launch and retrieval operations that will see the most stress placed on the structure itself. Within the engineered timber frame there is a reinforced concrete skeleton which carries the bulk of the loading during these operations and also allows the reinforced concrete floor slabs to be cast around it.
Three large plinths will carry the boat tipping mechanism allowing the boat to be launched. In essence at rest, the 28 tonne boat is strapped to a steel cradle which is inclined at a gradient of 1 in 12. During an alarm or ‘shout’, holding straps and the cable at the rear of the boat are released causing it to slip down the gradient, gaining momentum.
Once it passes the centre point or the pivot of the cradle, the whole structure tilts to the increased 1 in 5 gradient, increasing the launch speed of the vessel as it slides down the slipway. This ensures the lifeboat hits the water with enough pace to get it away from the boat house before the engines fire, sending the boat speeding off to its shout location.
The loading on the pivot and the front bearing plinths, as well as rear beam and winch plinth, means that the whole structure is heavily reinforced with 20 mm diameter rebar at 200 mm centres.
“The pivots, rams and winches are all of a pre-existing design, but the design of the plinths is bespoke for this particular scheme,” says Mr Rattray.
Incorporated within the whole structure is a 10,000 litre fuel tank which sits on the middle floor and will enable the crew to refuel the boat inside the station. When fully laden this fuel tank alone weighs in at 14 tonnes, but this loading will be borne by the engineered timber frame that acts as a super-structure to the boathouse.
Manufactured in Austria these laminated timber beams measure 140 mm by 500 mm and will span more than 9 m across the boat house. At that point the columns measure 140 mm by 700 mm either side of the main span, but the bulk of the columns feature a scaled down cross-section of 140 mm by 300mm.
Despite the main frame members being sourced in mainland Europe, the delivery and lead-in time is not an issue for the project team. “We just couldn’t get them in this country,” says Mr Lorraine, “they are manufactured over in Austria and the lead-in is something we have to factor in. They are becoming an increasingly poplar choice for the RNLI,” he adds.
The reasoning behind its choice of timber is simple - longevity. It stands up well to the harsh marine environment and means a reduced maintenance regime once the station has been brought into service. The bulk of the cladding around the frame will similarly be timber material, this time larch to the upper two storeys with masonry cladding at ground floor level. A Kalzip aluminium topsheet roof completes the structure.
All of the work to the 6.5 m cast in situ concrete slipway is being carried out within a steel sheet piled cofferdam that will protect the work from the advancing tide in the later stages of construction. The slipway is being cast in sections using a formwork system adapted to enable easier use by Dean & Dyball’s on-site staff. The move will help complete the scheme, partly funded by direct donations and money raising campaigns by local well wishers, ready for the RNLI crew’s busiest period in summer 2010.
Demolition of tradition
Cramped, too small and well passed its sell by date it might have been but the old lifeboat house, built on the sand and shingle at Kingston Beach in 1933, was well loved by locals and crew alike.
Unfortunately, lifeboats have become larger over the years. After being built initially to house an 8 tonne boat, the 28 tonne Hermione Lady Colwyn had exacerbated the boathouse and slipway’s deterioration, causing the slipway to sink and collapse. “It was a well known and well liked landmark but it just wasn’t up to the job anymore,” says Dean & Dyball Civil Engineering project manager Nigel Lorraine, “It needed to go.”
Burgess Hill, West Sussex based demolition contractor Dorton Group won the deal to take down the old reinforced concrete lifeboat station under the watchful glare of the local media. “It caused quite a stir around the area,” says Dorton director Terry Quarmby, “there was a lot of local interest but in terms of actual demolition the project was reasonably straightforward.”
The Dorton team used a 30 tonne excavator base unit with a concrete cracker attachment to power through the thinly reinforced walls and slabs of the old building. All this material was crushed and recycled on site. “We initially priced on a two week job and in the end I think it took a little longer than we anticipated. It was all reinforced concrete and masonry but a little bit of asbestos in there too,” says Mr Quarmby.
The lifeboat crew were on site to watch the Dorton team start to drag the station down but by a turn of fate were called out to a job just as the machines were poised, ready to pull through the old building.
“The crew had to go out on the shout, they never got to witness the final moments,” says Mr Lorraine.
Perhaps that was a blessing.
Sea Kale serves up an environmental hot pot
Engineers and contractors are become more adept in dealing with difficult site conditions and environmental concerns over the years as knowledge and understanding of these issues increase.
At Shoreham-by-Sea the project team has had its fair share of obstacles put in the way of delivering the new lifeboat house to the RNLI. Not only is the project within a Site of Nature Conservation Importance thanks to the presence of sea kale and Kingston Beach being a protected vegetated shingle beach, but the piling had to be completed by the end of March. This was so that the work did not interfere with the migratory Sea Trout returning to spawn in the River Ardur.
The initial plan had been to use percussive techniques to bore the piles, however it was deemed that the vibrations given off by the hammering would prove disruptive to the fish. The sea kale, found growing in a colony on the shingle beach has also had to be protected against any impact the work may have on the rare species. The project team has saved all the plants in a tank on the beach and will reintroduce it fully once the scheme is completed.