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SHORING UP THE SEASHORE

PILING - Site workers had to check the tide before installing vibro stone columns as part of a new sea defence scheme at a coastal town in north west England.

Piling contractor Pennine could only gain access to the beach during low tide while working on a 150m stretch of sea wall in Lancashire at the end of 2006.

The £200,000 (2300,000) contract to strengthen the ground along a section of sea wall at Cleveleys is part of a major sea defence project being carried out between Blackpool and Fleetwood by main contractor Birse Coastal. The Department for Environment, Food and Rural Affairs (DEFRA) is fi nancing the 227M project for Wyre Borough Council.

Designers considered a new coastal defence strategy that involved the maintenance of high beach levels.

But following public consultation and model testing, a stepped revetment with a raised rear wall emerged as the best option.

While much of the ground along a 1.1km stretch of the sea wall is solid enough for construction of the new defence system, the mid section at Cleveleys has an old river channel where soft peat and clay could cause settlement problems.

'Initially we thought we would have to install pre-cast piles to strengthen the ground along this section, at a cost of more than 21.1M, ' says the main contractor's regional engineering manager, Brian Farrington. 'But Pennine came up with an alternative vibro stone column solution that will get the job done for less than a third of the price.' 'Sometimes we get a clear eight hours in which to manoeuvre our equipment onto the beach, install a number of columns and then move everything out again, ' says the subcontractor's technical director, Marc Evans. 'But at other times there is only a four hour gap between the tide going out and coming back in, which makes things very tight - especially when you consider that it takes up to 30 minutes to move our equipment between the promenade and the beach.' Evans explains how the technique works in soft ground: 'The difficulty with soft peat and clay is that the borehole will not stay open, ' says Evans. 'By using a bottom feed method we are able to vibrate the fl ot [vibrator] down to the desired depth, in this case 9m, then introduce stone to the bore through a tube fixed to the flot. The stone is compacted in stages as the flot is withdrawn.' He says that bottom feed relies on an air fl ush to keep the borehole from collapsing and is normally a dry technique. But site workers on the Cleveleys project used a water flush technique to insert stone in the soft soils, which he says also allowed Pennine to build bigger columns.

Nearly 1000 columns were installed once work began in November last year. The seven week subcontract also involved building access ramps between the promenade and the beach to aid mobilisation and demobilisation of the Terrafi rma rig and a standby hydraulic power pack.

'With such short working windows we do not have the luxury of being able to do repairs in situ, ' said Evans. 'It's critical that we have the standby power pack on hand since it allows us to operate the rig tracks independently and move the rig off the beach in the event of a major breakdown.' Three months of settlement monitoring has recently started in the soft ground area and levels are taken each day at low tide. Once this period is over, the main contractor will start work on the stepped revetment and rear wall.

At the north end of the site, where founding material is a firm to stiff boulder clay, settlement is not an issue and work on the sea defence is already under way.

The entire scheme, including promenade lighting, architectural details and landscaping, is due to be completed by November.

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