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Fighting on the beaches

GEOSYNTHETICS : A geotextile filter layer is playing a key role in a massive rock revetment built to protect three historic villages on the Fife coast in Scotland.

The North Sea regularly batters the south coast of Fife in south-east Scotland, with little respite from the prevailing weather from the south west and the east.

Villages such as East Wemyss, West Wemyss and Dysart are particularly badly affected, subjected to long-term erosion from short period waves generated within the Firth of Forth estuary and longer period waves travelling across the North Sea. This erosion is punctuated by severe losses of beach material during storms.

The Dysart to East Wemyss coastal protection scheme, which is nearing completion, aims to protect these historic villages, using 1. 5km of rock revetment placed along a 7km stretch of the coast.

Work, which began in April this year, is being carried out by contractor RJ McLeod (Contractors) for Fife council. Geotextile supplied by UK manufacturer Geofabrics forms a key part of the design, forming a filter layer within the revetment to prevent loss of fines and to prevent the build-up of pore water pressure, ensuring the scheme's effectiveness.

Coal mining was the dominant industry in Fife over the last century and its legacy is one of severe environmental damage. As mining became increasingly mechanised, it adopted the practice of long walling, where the full coal seam is removed by cutters to conveyors for transfer to the surface.

The technique allows the seam roof to collapse as mining proceeds, producing large amounts of waste and causing widespread subsidence. In some areas, where coal was particularly abundant and shafts were driven to great depths, there has been up to 4. 5m of settlement.

It was common practice to dump at sea waste from those mines near the coast. This helped to reduce the rate of coastal erosion, with waste reworked by the sea and transported eastwards by longshore drift. However, when waste dumping stopped, the balance between deposition and removal was lost, leaving the villages of East Wemyss, West Wemyss and Dysart at great risk of erosion.

A major project was needed to protect the villages, but severe constraints on UK local authorities' finances in recent years meant Fife council did not have enough money to carry out the work, which was estimated to cost several million pounds.

A bid for money to the Scottish Office Challenge Fund in 1997 was unsuccessful because of uncertainties in the cost of the scheme, given the many consents and permissions applicable to coastal protection schemes.

Following the failed bid, Fife council appointed consultant Posford Duvivier to develop a shoreline management plan for the whole Fife coast and supervise any future site works, while consultant Halcrow developed a design for the three villages.

Halcrow had to consider a number of factors in the design. Any measures had to be cost-effective in relation to the assets they were protecting and had to fit in with the picturesque historic villages.

Design also had to consider the possibility of future subsidence.

The Firth of Forth is a proposed special protection area because of its wintering bird population, so work had to be completed between April and September to satisfy Scottish Natural Heritage.

Once all the design criteria had been met, the Scottish Executive gave Fife council extra funding in 2000 for work to begin in April this year.

After considering various design options, including vertical sea walls, groynes and breakwaters, beach replenishment and concrete armour units, it was decided to use rock armour revetment for the scheme.

The method was chosen because it is durable enough to cope with the harsh environment, breaks up wave energy, is aesthetically pleasing and is also flexible enough to cope with any further subsidence, says Geofabrics' Doug Langton.

Because natural materials are used (in this case locally occurring igneous rock Dolerite) the scheme will also encourage a natural habitat to develop, he adds.

There was concern about the impact on the villages of the lorry movements needed to import large volumes of rock to site. So, while delivering the rock by sea was 9%more expensive, the reduction in traffic, noise, vibration and dust meant that it was chosen over road delivery.

Work involved regrading the beach before placing two layers of two sizes of rock armour (1t to 4t blocks overlying 10kg to 1000kg blocks) directly on to a geotextile filter separator.

A filter medium was needed below the rock armour to prevent fine material being washed away, which would render the revetment ineffective. The filter layer also has to allow free draining of water to prevent build-up of pore water pressure.

'If the subgrade became charged with pore water it would become vulnerable to erosion at the toe from suction, as the water inevitably tried to escape, ' explains Langton. Geology along the coast comprises Carboniferous coal measures and raised beach deposits.

There were two options: the filter could have been built using layers of aggregate or by using a layer of geotextile. The latter was chosen because it was cheaper and easier to lay in the difficult coastal environment, with work having to be carried out between tides.

Geofabrics MP1200 was used as the filter separator. Langton says the material's durability meant RJ McLeod could place the rock armour directly on the geotextile without causing damage, hence saving valuable time between tides.

'A further benefit of MP1200 is that it is produced in 6m wide rolls, reducing the frequency of joints, which offered further time savings, ' he adds.

The fabric is manufactured in the UK and was sent to site in batches throughout construction to save space on site.

In all, more than 100,000t of rock armour and 15,000m 2of geotextile were placed in the 1. 5km long revetment, costing an estimated £4. 5M.

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