In Dorset Dean & Dyball is working on a complex scheme to stabilise cliffs and protect rich geology at the coastal town of Lyme Regis.
Project: Lyme Regis Environmental Improvements Phase 4
Client: West Dorset District Council
Contract value: £9.4m
Project value: £19.5m
Defra element: £14.6m
Dorset County Council element: £4.3m
West Dorset District Council funding: £0.6m
Contract type: NEC Option A – Design and build
Main contractor: Dean & Dyball
Lead designer: URS
Project manager: Halcrow
The pretty seaside town of Lyme Regis is well known as a holiday destination rich in natural history and literary connections.
It is located in the middle of the Jurassic Coast – a 95-mile section of coastline stretching between east Devon and the entrance to Poole Harbour designated England’s first natural World Heritage site – and boasts startling geology that gives the coast its name.
This makes the town a mecca for fossil hunters and literary enthusiasts researching the setting of John Fowles novel The French Lieutenant’s Woman.
Every year thousands of dinosaur hunters come to pour over the rocks fallen from the cliffs of Jurassic limestone and mudstone deposits that lie in dipping bands in the area.
Unusually rich in fossils these strata may be, but for the residents of Lyme Regis the very rock that is the foundation of the area’s tourist industry is proving to be rather less stable when it comes to propping up their houses.
In a bid to protect the town from coastal erosion, contractor Dean & Dyball – part of the Balfour Beatty stable – has taken on the latest phase of a project that will stop the sea undermining swathes of the town over the next 50 years.
Phase 4 of the Lyme Regis Environmental Improvements Scheme is part of an ongoing coastal protection project, the first stage of which started in the 1990s.
“There is also evidence of typical ‘block slide’ activity in the land above the beach. It’s quite a dynamic area”
Chris Hill, Dean & Dyball
This latest section of work includes the construction of a 400 m section of sea wall along the foreshore to the east of the town as well as installing soil nails and piles to stabilise the cliffs above.
“The area is subject to undermining of the existing coastal protection and the existing seawall is being outflanked,” explains Dean & Dyball project manager Chris Hill. “There is also evidence of typical ‘block slide’ activity in the land above the beach. It’s quite a dynamic area.”
He is charged with managing the team to complete the work, which is being carried out under a £9.4m NEC3 Option 3 design-and-construct contract with client West Dorset District Council.
The seawall itself is a nominal 5 m high, cast in-situ reinforced concrete construction with a curved profile.
Pressure and erosion protection
Set in front of the existing seawall, the void between the new and existing wall is filled with a reclaimed granular fill. This allows groundwater seepage to find its way through into the back of wall drainage system being installed, preventing a build-up of water pressure.
A 200 mm-thick reinforced concrete slab stretches between the new and existing wall, providing a walkway and emergency vehicular access with a 200 mm-thick splash wall defending the toe of the cliff against any impact from splashing.
More drainage behind that splash wall wicks surface water away from the base of the cliff.
“We cored along the length of the wall to pinpoint exactly where the better, thicker layers of limestone are”
Chris Hill, Dean & Dyball
Initially the team had wanted to install rock armour at the base of the wall to help prevent any chance of scour undermining it, but that design was deemed too ugly.
Following a rethink, the team has developed a deep toe design which varies in depth between 0.6 m and 2.3 m.
This ties onto the layers of harder limestone deposits on the foreshore.
“We cored along the length of the wall to pinpoint exactly where the better, thicker layers of limestone are so that we can cast the toe of the wall onto it,” Mr Hill says. “We used a rock cutter attachment to cut the trench along the toe.”
By using the rock cutter trenching tool to cut through the layers of rock on the foreshore, the team was able to accurately plot the layers of mudstone and limestone.
But its use also ensured that overbreak on the seaward side was minimised, helping reduce the effect of wave action and possible maintenance issues in the future.
Casting the curved wall
The curved wall itself is cast using a specially designed formwork system, which allows profiled inserts to be inserted which tweak its curvature.
This ensures the team could reuse the same 2 m-long formwork sections when casting all of the 8 m bay lengths for the wall.
In all, some 700 cu m of concrete has been poured to construct the sea wall, with access along a 5 m-wide temporary haul road installed along the beach.
Despite the massive storms over recent weeks, the route has remained in reasonable condition, with the main post-storm task being to clear the seaweed, rocks and pebbles thrown up by the huge waves from the rebar mesh before casting the remaining sections of wall.
The rock armour used in the construction of the haul road will be placed at the eastern end of the sea wall to help protect a section of the cliff.
“We have installed piezometers and inclinometers which give real-time information about the state of the slope above the seawall and the likelihood of it slipping”
Chris Hill, Dean & Dyball
But the sea wall is just part of the overall protection scheme. The team has been carrying out slope stability work by inserting soil nails (see box) to help prevent slippage across a number of block-slip planes as well as installing drainage systems.
Major piling work at the top of the cliff (see box) should also help prevent further downslope slips.
“We have installed lots of instrumentation – piezometers and inclinometers – which give us and the client real-time information about the state of the slope above the seawall and the likelihood of it slipping,” Mr Hill says. “We needed to understand the instability before we could do anything about it.”
A build-up of water seeping through the ground, resulting in a loss of friction between layers of deposits and causing ground slips, has been managed by the installation of huge 4 m-deep and 1 m-wide cut-off drains.
With an impermeable membrane on the down-slope side, these drains take the troublesome groundwater and feed it into a new drainage system designed to manage it more effectively.
These drains are the first line of defence against Lyme Regis becoming a victim of the very cliffs that give the town its fossil hunting fame.
Coupled with the rest of the work being carried out, they should help alleviate any fears of its imminent collapse.
CAN nails stabilise the slope
A major section of the work at Lyme Regis is the stabilisation of the land immediately above the sea wall and the east beach.
Here, specialist access subcontractor CAN Geotechnical is installing 2,500 soil nails to between 9 m and 18 m depths to help retain that land and prevent further slips.
The nails are placed using roped access equipment and rig teams working down the slope, while a drill rig mounted on a long-reach excavator arm works from the base of the slope where access permits.
The nails are generally placed at 2 m centres horizontally and 1 m centre vertically, but this arrangement can alter where required. They were installed in 2 m sections.
Specially configured 300 mm-wide by 500 mm-deep diamond-shaped head plates are being used to retain the slope and the highly protected steel mesh that holds it in position.
“We found the diamond-shaped head was the best solution,” Mr Hill says. “If we had used square heads then they would be too close together.
“We went through a long engineering process and the diamond shape and size was by far the best.”
The slope will also benefit from a series of drains drilled at its base to help manage groundwater flow and a series of 8 to 12 m-long 300 mm-diameter dowel piles that have been installed by ground engineering specialist Van Elle to supplement the nailing at the crest of the slope in the central section of the sea wall.
Big kit moves in to stabilise sea cliffs
At the top of the cliff near the Charmouth Road car park that looks out over Lyme Bay, a team from Balfour Beatty Ground Engineering is mobilising to install a contiguous pile wall designed to retain the land from edging further down the coastal slope toward the east beach.
Fault and slip lines are already visible on these coastal slopes, but the huge buried wall being installed by BBGE should provide enough resistance to safeguard these slopes and their important flora and fauna.
In all, 86 bored piles with steel cage reinforcement and anchored back into the upslope with 40 m-long anchors will be installed.
The 900 mm-diameter piles will be placed using a 120-tonne rig at 2 m centres to depths of 27 m, while the anchor ties will be installed at 30 and 35 degrees from the horizontal into 5 m of the hardest layers of limestone.