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Storm in a port

Sheet piling - Piling in a small working harbour has involved some close encounters with shipping, Jon Young discovers.

On 5 October an aggregate laden dredger crashed into piles being installed in Whitstable Harbour, North Kent, setting back a £1.2M project to replace the east quay by a couple of weeks.

Main contractor Dean & Dyball's working area required a 20m wide restriction zone, which effectively halved the width of the harbour entrance.

The bash happened as the dredger misjudged a manoeuvre into the harbour and headed for the solid wall of the west quay.

In a desperate effort to avoid a collision the skipper made a hard turn toward the east quay, slewing the vessel's stern into the construction works.

'The harbour's east quay wall is being completely rebuilt using 165m of anchored sheet piling installed in front of the existing raked wall, ' explains John Davison, resident engineer for client and scheme designer Canterbury City Council. 'The repair is needed to keep the harbour working and open to tourists.' Both are important to the local economy.

Reconstruction is the only realistic option as the existing wall is suffering from accelerated low water corrosion (ALWC) over its inter-tidal section. The wall has been further weakened by the bumping and scouring of ships moored alongside the quay as the tide falls and rises.

Over the years Whitstable harbour's use has changed, requiring alterations to its layout.

'Whitstable is known for its oyster industry, but, although the harbour is still used by the fishermen, its main use is to supply an asphalt works, ' says Davison. As a result the wall now has to sustain much greater horizontal loads imposed by the mounds of aggregate piled on the quay and by plant.

The construction process is relatively simple, but in a marine environment wind and water can make even a simple task challenging. Dean & Dyball is using a temporary piling gate that stands out from the quay wall, 6m above low water level, on steel spud legs. A crane operating on the quay is used to lift individual 13m long interlocking U-section sheet piles and lower them into the gate.

Piles are aligned by two men, and driven by a vibrating hammer. Ground consists of soft, sandy soils over clay beds.

When the acceptance limit has been reached the vibrating hammer is moved aside and a 7.5t hydraulic impact hammer used to force the pile into the clay.

Once piles are driven they are tied back to a reinforced concrete beam cast well back from the quay edge and running parallel with the wall. Tie bars have a working load of 420kN.

When all the piles are in place and tied, the existing concrete surface of the harbour will be broken up and used as fi ll. A new reinforced surface will be cast.

Work was due for completion by Christmas, but is now expected to finish in January.

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