Slippage of the hillside on which Lyme Regis is built has saved the Dorset town from intensive development over the years, but also threatens its future.
Marcus Brierley reports on efforts to halt Lyme's slide into the sea.
In the centuries since Lyme Regis was granted its Royal Charter by Edward I in 1284, the charming bay, fishing harbour and town have remained comparatively secluded and underdeveloped for one compelling reason - weak ground.
'Lyme Regis loses a house to slippage every five years or so, ' says Geoff Davis, who is West Dorset District Council (WDDC) project leader for the coastal protection scheme that is now under way.
'If you hang around and do nothing, it is certain disasters will continue to happen.' He taps the wall-sized aerial photo of the bay in his office.
'A large span of geological time is exposed in these cliffs - 185 million years of major scientific interest focused in 95 miles of coastline.' Davis and his colleagues have spent 10 years developing strategies to protect Lyme Regis for the next 50-60 years. WDDC and consultant High-Point Rendel (HPR) drew up a £16M programme of marine and land works, funded by the Department for the Environment, Food & Rural Affairs. Contractor Mowlem be an work in February.
HPR geotechnical engineer Ben Highfield says: 'There are two main triggering mechanisms which the design of the scheme is addressing. When you remove the toe of the landslide, you remove the support to it, so erosion and lowering of the foreshore weakens the lower slopes and permits them to slip more easily.' The second major trigger is groundwater, which lubricates the slip plane.
New defences consist of forti'cations to Lyme Regis' famed 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 the shingle against longshore drift.
On the hillside, improved drainage is being laid and the slope is being extensively pinned with stabilisation piles; soil nails will be installed at the tail of the landslide. New retaining walls are being built where the ground falls steeply away.
Mowlem worked closely with piling subcontractor Systems Geotechnique to select equipment and develop the working method - there is a 4t weight restriction on all of the piling plant.
There are clear signs of the project all over Lyme Regis. Piling is taking place in parks, people's back yards, driveways, gardens and garages.
Excavators move shingle and sand on the foreshore. Rock armour is delivered to the Cobb by barge and placed by 80t excavators.
Before work stopped for the summer tourist season on which much of the town's prosperity depends, Systems Geotechnique had installed 300 of a total 1150 piles. Most are 300mm diameter augured piles, ranging from 5.5m to 12m in depth and reinforced with reclaimed steel tube which is transported through the narrow roads and alleys of Lyme Regis on a purpose-built trailer.
Now back on site, Systems Geotechnique project manger Richard Hayman-Joyce expects activity to peak in November with work taking place on several fronts. In addition to the stabilisation piles, the company is installing anchors for the jetties.
A total of 220, 6-8m long Grip-Bar stainless steel anchors are needed for the two structures.
They will be installed using excavator-mounted down the hole-hammer rigs working off platforms built alongside the jetties. Elsewhere, Systems Geotechnique is installing temporary Grundomat piles for the work platforms needed at a number of points because of access dif'culties. Another task is the drilling of horizontal drains to draw water from the bedrock.
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 out to the seafront. These will be drilled using a down-the-holehammer drilling system. Bores will be lined with plastic pipe. A number of 450mm diameter retaining wall piles will be installed along the seafront.
At the peak Hayman-Joyce expects to have seven to eight gangs working simultaneously on various aspects of the contract. The company is using lightweight Klemm and Technodrill rigs, weighing between 3.5t and 4t, because of the poor load-bearing qualities of the ground.
Reaction from residents has mainly been positive, he says. The arrival of a drill rig and crew may disrupt normal life, but it means their property is secured from the threat of landslip and its value will certainly increase.
Construction of the two new precast concrete jetties is no mean feat - they measure 70m and 50m long.
Mowlem site agent Paddy Rosborough says: 'They're built within the surf zone. They are precast and a lot of thought has gone into their construction so that there's a robust logistical chain to ensure that all the [differently sized] blocks will arrive in the right sequence.' Mowlem is also underpinning and extending with concrete blocks Cart Road, the promenade joining the eastern and western limbs of Lyme, to protect the toe of the slope.
As part of its work this season Systems Geotechnique will install a piled retaining wall alongside the precipitous Cobb Road. This forms part of a road-widening scheme which is co-funded by Dorset County Council.
The 350mm diameter piles will be topped by a capping beam. Because the wall will be only 1m away from the road and bordered by a steep fall, work will be done from a scaffold platform supported by temporary piles.
The contractor will have finished most of its work by Christmas, including soil nailing at the tail of the landslide, but there will be at least two more visits to Lyme to install some final groups of piles before the scheme is completed next year.