Major ground stabilisation that will blend in with the environment sounds too good to be true, doesn’t it? But that is exactly what a scheme in the Dorset town of Lyme Regis is aiming to deliver.
Most clients would expect to be able to see the results of investing £19.5M but the main benefit of work to stabilise a landslide in Lyme Regis will be its ability to blend in with its environment. According to Balfour Beatty Construction Services project manager Chris Hill, the aim at the end of the scheme is for the only difference to be that the landslide risk to have been removed.
The wet weather in 2012 caused eight shallow landslips during the early stages of the work – the changing geomorphology meant the design has had to be changed and checked throughout
The scheme is the fourth stage of ground stabilisation to be completed in Lyme in the bid to halt the historic landslide which the town is built on and will conclude work that first started in the early 1990s.
This current phase focuses on the eastern side of the town - East Cliff and Church Cliff - and replaces a 70-year-old seawall that is in a poor state of repair and where the landslip is at risk of moving over the top of it.
“The work will protect around 240 houses and a 900m section of the Charmouth Road - the main access route into Lyme - which would be at risk over the next 50 years,” says Hill.
The client is West Dorset District Council, but only £600,000 of the cost is being met by the local council. Dorset County Council is also contributing £4.27M but the remaining £14.6M of the funding is coming from Defra and was secured when the Environment Agency approved the scheme in March 2012.
Work on site started in 2012 and the main civils work is expected to be completed this spring.
According to Hill, Dean & Dyball, a subsidiary of Balfour Beatty, won the design and construct scheme on the basis of quality rather than cost. “This project is an environmental challenge as much as an engineering one - the site forms part of the World Heritage Jurassic Coast and there is a Site of Special Scientific Interest too,” says Hill. “We won the contract through our demonstration of our understanding of the challenges and local knowledge.”
Hill, as well as a number of the team, have all worked on previous phases of the stabilisation work at Lyme so the project has the benefit of local knowledge. Nonetheless, this phase is more rural than previous ones and the landslide is more active, so the challenges are different.
“The wet weather in 2012 caused eight shallow landslips during the early stages of the work,” says Hill. “The changing geomorphology meant that the design has had to be changed and checked throughout the work.”
Much of the early work on site focused on protecting the ecology. “The vegetation was mapped and during the clearance measures were taken to move the native species, such as reptiles and dormice, from west to east.”
More ground investigation was carried out during these early stages by CC Ground Investigation. “We already had lots of information but we needed some more detail in some areas,” says Hill.
The construction is all within the tidal zone so the work has to be timed around the tides
During this stage Datum was called in to install new monitoring instrumentation with inclinometers and piezometers to supplement the existing instrumentation. “All the new equipment is automated and uses solar power,” says Hill.
“The system reports to a web-based system in real time with trigger levels set for raising alerts with the project team.
“The system helped prove that the movements that have occurred since we moved onto site are a natural part of the landslip movement and not as a result of the work itself.”
The first main construction work was installation of a 4m deep drain below the car park on Charmouth Road and Spittles Lane, where the site office is located, to create a ground water cut off.
Another early stage in the project saw a haul road installed in front of the existing seawall to provide access and to protect the rock ledges that form a distinctive part of the World Heritage site. The haul road material will be used later in the project as backfill material.
“The sea wall is a conventional design but has a 2.3m deep toe rather than using a rock apron to protect it as that would obscure the rock ledges,” explains Hill. “The deep toe also caters for future foreshore lowering.”
The sea wall has been cast insitu in 8m bays using bespoke formwork with a four-day turnaround for each section. “The construction is all within the tidal zone so the work has to be timed around the tides,” says Hill.
“The formation level of the wall is not fixed but is based on excavating to one of three specific geological horizons and the cage design allows for the varying depth,” he explains.
“The geology at the foreshore is weak mudstones interbedded with harder ones, hence the erosion problems. We have a geologist on site who is looking for beds of the Third Quick, Top Tape or Second Tape, which vary in thickness from 100 to 300mm.”
Fossils have been found on the site during the work. “We have had a specialist on site to check for fossils throughout the work,” says Hill. “The large ammonites we have found will be used as features in the sea wall, but we have also found shark teeth and an Ichthyosaur vertebrae.”
The wall will be finished with 370t of rock armour formed with stone from the Mendips at the eastern end of the sea wall.
While the sea wall will protect the toe of the landslip, the upper slopes of the system are being stabilised using 2,500 soil nails installed by Can Geotechnical using rope access techniques and A-frame rigs, as well as excavator-mounted drill rigs. “The nailing area protects Church and East Cliff and then wrap around the back scar,” says Hill.
The stabilisation is designed to be zero maintenance
The aim is to create a transition zone between the stabilised area and the still active landslides to the east, as well as to protect the natural habitats of the wildlife that used to thrive in the area.
“The nails we are using are a hybrid system - open holes with flush and a hollow bar - to allow the nail to be measured before grouting.”
The nails are up to 19m long but some are as short as 8m and are being installed at 25° to the horizontal. “The rock dips at 2.5° towards the sea and the angle of the nails is the best one to ensure we can drill through the harder beds,” says Hill.
The nails are being installed on a grid pattern of 2m horizontal and 1m vertical in staggered rows, although the locations in some areas have been changed. “We found a lot of old rubble-filled drains in the cliffs and we have relocated nails to prevent the work grouting up these drains,” says Hill.
Once installed, the nails will be covered with Geobrugg Ultra Coat Tecco mesh, which has a high corrosion resistance with a 60 to 120-year design life. The nail heads will be recessed using a bespoke design developed by Can and URS to ensure the stabilised slopes look natural at the end of the work.
“The stabilisation is designed to be zero maintenance,” says Hill.
So far 1,500 nails have been installed and mesh installation started in January. Hill expects that element of the work to be completed by the end of March.
The work also includes installation of gravity drains that follow the natural dip of the rock and are designed to drain the slip plane.
The nailing work has concentrated on maintaining the micro topography of the slopes. “It would have been easier to regrade all the slopes but the aim is for the end result to look much as it did before, but without the risk of landslips,” says Hill. “The need to maintain the look of the area is partly the reason why the soil nailing doesn’t extend across the whole site but focuses on protecting the at risk properties and road.”
The soil nailing work will be further supported by two banks of small diameter dowel piles above East Cliff. The 8m to 12m long, 300mm diameter piles will be filled with grout and reinforced with a steel tube and are designed to protect the new sea wall and prevent shallow landslips from progressing inland.
The final part of the stabilisation involves piling around the top edge of the work area to create a cut off through the slip surface. Work on installing the 86 900mm diameter bored piles started in January and extends a piled wall installed by Balfour Beatty as emergency works in 2004. The new single row of heavily reinforced 27m deep piles will extend the wall westwards and will be topped by a capping beam and tied back using multi-strand ground anchors.
Once all the stabilisation work is complete, the whole area will be reseeded with native species using seeds from more than 700 plant species that were collected from the site before the start of the work. If Hill’s team is successful in delivering their aim, the only evidence of the work will be the new sea wall.
“It will take two years for the vegetation to re-establish itself but the use of seeds gathered from the site will mean that the final appearance will be natural,” says Hill.
Demonstrating an understanding of the ecological, as well as env ironmental, challenges may have been a key part of Dean & Dyball’s securing of the Lyme Regis contract, but communication with the local community also played a key role.
“West Dorset District Council established the Coastal Forum in the early stages of the stabilisation work in the 1990s and it has continued to meet every quarter since then,” says Balfour Beatty’s Chris Hill.
“The open public format has been a good way for us to get feedback on our work. We have been fortunate to have almost 100% support for what we’re doing.”
Despite the support, Hill’s team have made efforts to ensure that the work does not impact on the town - especially during the busy summer months. “Deliveries are carefully planned around the tides and we hold trucks at our site office to prevent congestion in the actual town centre,” explains Hill.
The site has also hosted a number of visits for local schools, business group and also one for members of the ICE.