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Groynes take the strain

WORK IS ALMOST complete on a coastal protection scheme at a Dorset beach widely acclaimed as one of the best in Britain.

Aarsleff Piling has supplied and installed driven precast concrete piles through a series of rock armoured groynes on the beach at Sandbanks, Poole.

The piles are an integral part of works that will not only protect the beach but support inspection walkways on the groynes, built from Portland Limestone by main contractor Dean & Dyball.

In the 1890s 13 timber groynes were built to prevent the sea breaking through the peninsula and forming a second entrance to Poole Harbour. The groynes retained the sand and the beach steadily widened, but by the 1950s the groynes had decayed and were a danger to the public.They were removed and since then the beach has continued to erode.

In 1995 four rock groynes were built at the western end of the beach where they have reversed erosion and encouraged a build-up of sand.Work is almost completed on the £1.3M second phase of the scheme, which involves building five groynes at the eastern end of the beach.

Project design is by the client, Borough of Poole Construction Related Services. The work is funded by the Ministry of Agriculture, Fisheries & Food.

The Borough of Poole looked at various options to produce a self-sustaining natural build-up of the beach and long-term protection of the coastline and adjacent properties. Pumping sand on to the beach was considered, but thought to be too expensive and, with a design life of less than 50 years, only a temporary solution.

After discussions with independent consultants, the Environment Agency and English Nature, the borough opted for longer lasting groynes. It was then a choice between timber and rock.

Timber groynes were ruled out because they are made from imported hardwood, which takes decades to grow in tropical rain forests.And while they have a long lifespan, it is nonetheless limited and as the timber decays the groynes are difficult to repair.

Although rock groynes take slightly more beach space, they are more effective at retaining sand and much more environmentally friendly. They are more robust at resisting storms and should last longer, needing only the occasional top-up of fresh rock.

Rock was chosen, and Dean & Dyball began work in November 2000.The groynes, between 70m and 90m long and between 110m and 160m apart, are about 2m high and 4.5m wide at the crest.

Dean & Dyball first placed a series of 1m diameter concrete manhole rings at 6m centres along the centre line of each groyne.The vertical concrete tubes were backfilled with sand and surrounded by 3-6t Portland limestone rock armouring, placed by hydraulic excavators.

Subcontractor Aarsleff Piling followed on with its Banut piling rig.This worked around the tides to pitch and drive 64, up to 15.5m long, precast concrete piles in only six days.The 250mm square section piles, jointed up to 13m, were made with self-compacting concrete at the firm's precast pile factory at Newark, Nottinghamshire.

The piles were driven through the sand-filled manhole rings and into the beach and sandy silt to toe into the underlying dense gravel. Between 9 and 15 piles were installed in each groyne, with working loads of 20t.

Dean & Dyball then placed and fixed 2m wide, 1m long and 450mm deep precast hammerhead pile caps.

Precast concrete slabs, 5.5m long by 2m wide, were put between the pile caps to complete the walkway along each groyne.Work is due to finish by Easter.

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