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Caught in the slips

Coastal - Hillside slippage has saved Lyme Regis from intensive development, but also threatens its future.Marcus Brierley reports.

Lyme Regis in Dorset is famed for its fossils - revealed by the slow but steady collapse into the sea of the steep hillside on which the town sits.

'Lyme Regis has also had a fair share of disasters in its time - it loses a house to slippage every five or so years - and if you hang around and do nothing, it is certain that disasters will continue to happen, ' says Geoff Davis, West Dorset District Council (WDDC) project leader for a coastal protection scheme now under way. Many millions of pounds worth of homes and infrastructure are at risk.

Lyme was granted its Royal Charter by Edward I in 1284. The bay, fishing harbour and town have maintained comparative seclusion and underdevelopment for hundreds of years, despite their charm, for one compelling reason - weak ground.

Davis and his colleagues have spent the last decade developing strategies to protect the town for the next 50 to 60 years.

WDDC and consultant High-Point Rendell have drawn up a £16M programme of marine and land works, funded by the Department of the Environment, Food & Rural Affairs. Contractor Mowlem started work in February.

'Two main triggering mechanisms are being addressed by the design of the scheme - when you remove the toe of the landslide, you remove the support to it, ' explains HPR geotechnical engineer Ben Highfield. 'So erosion and lowering of the foreshore weakens the lower slopes and lets them slip more easily.' The second major trigger is groundwater, which lubricates the slip plane.

Defences being built consist of fortifications to Lyme Regis' 13th century sea defence, the Cobb, as well as the protective Beacon Rocks and Rockery, to reduce wave impact. The town's beach is being replenished with shingle and sand so that it will act as a proper buttress to waves at the toe of the landslip - new jetties at either end of the beach will help contain shingle against long-shore drift.

On the hillside, improved drainage is being laid and the slope is being extensively pinned with stabilisation piles; soil nails are being installed at the tail of the landslide. And new retaining walls are being built where the ground falls steeply away.

Piling is taking place in parks, people's back yards, driveways, gardens and garages. The foreshore is plied by excavators moving shingle and sand, and rock armour is being delivered to the Cobb by barge. At the end of the Cobb 80t excavators move lumps of granite as effortlessly as sugar cubes.

'We are removing and replacing existing rock armour structures which were built to protect the coastline jetty, ' says Mowlem site agent Paddy Rosborough. 'About 36,000t of material is coming from Norway by sea. Blocks are placed with the aid of GPS to form a random-pattern mass structure.

Placement is directed by GPS.

'Beach replenishment involves importing 60,000t of sand and shingle, ' he adds.

Construction of the two new precast concrete jetties is no mean challenge - they measure 70m and 50m long. 'They're built within the surf zone. They are precast and there's a robust logistical chain to ensure that all the [differently sized] blocks arrive in the right sequence, ' says Rosborough. Mowlem is nearing completion of the jetties' insitu concrete footings.

Mowlem is using concrete blocks to underpin and extend Cart Road, the promenade joining the eastern and western limbs of Lyme, to protect the slope toe.

Although working at sea presents hazards in the form of tides and foul weather, Rosborough points out the potential dangers of the landslip.

'We have ground unsuited to the loads imposed by conventionally sized construction machines.

Plant and equipment weights are restricted, so small, light machines are essential.' Mowlem worked closely with piling subcontractor Systems Geotechnique to select equipment and develop a working method - there is a 4t weight restriction on all piling plant. Some outbuildings will have to be taken down for piling access and be reinstated afterwards, notes HPR project engineer Ben Highfield.

Systems Geotechnique has just finished installing 300 of a total 1,150 piles. It has stopped for Lyme's summer tourist season. Most are 300mm diameter augured piles, ranging from 5.5m to 12m in depth. Piles are reinforced with reclaimed steel tube. Tube is transported to the piling point through the narrow roads and alleys of the town using a purpose built trailer.

Further pile installation involves a number of 450mm diameter retaining wall piles along the sea front.

Groundwater drainage is provided by a combination of surface level low capacity excavated trench drains and 150mm diameter horizontally drilled drains up to 50m in length, which will underdrain the area and take water to the sea front. The scheme is due to be completed next year.

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