Ground investigations to help stabilise Southend's soft cliff coastline

Ground investigations were required at Southend-on-Sea, where the soft cliff coastline has suffered ongoing ground movement and cracking.

The cliff frontages of Southend-on-Sea in Essex are predominantly formed of London Clay and have a long history of slope instability.

This instability derives from historical erosion processes causing the clay slopes to become over-steepened. Ongoing erosion and climate change contribute to further instability in the slopes.

Rotary rig at Belton Hills East

In early 2021, concerns were raised about the ongoing cracking and settlement of the pavement and the slope next to Belton Way East, despite superficial repairs being undertaken regularly and the installation of piles on the downslope side of the road.

This road is an important transport link between Leigh-on-Sea station in the south and Marine Parade in the north just above Belton Hills. Southend-on-Sea City Council's highways department identified that these areas required a more substantial investigation and a remedial solution to provide long term stability. It secured government funding to pursue a more permanent stabilisation scheme for the highway.

Campbell Reith was appointed as Southend-on-Sea City Council's geotechnical consultant in 2019. As part of the framework agreement, it provided consultancy services to help manage 5.3km of the borough's soft cliff frontages, which reach heights of 30m with a maximum slope angle of 24°.

"Our role was to review historical data and assess slope stability through quarterly inspections," says Campbell Reith associate Katharine Barker.

After an initial visual inspection of the cliff sites it developed a bespoke risk categorisation system in which areas of the slopes were designated Category A, B or C, with A the lowest and C the highest risk category.

The inspection programme was influenced by the risk categorisation, with category C areas inspected more frequently.

Four category C areas were identified: Belton Hills East, Belton Hills West, Cliff Parade and Southend Cliff Gardens – Flagpole.

After the council received the highways funding for Belton Hills, the ground investigation programme was expanded to cover all of the category C areas. This meant Campbell Reith was put in charge of the scope of a £1M ground investigation programme across 30ha of soft cliff frontage in the borough.

Through an open tender process, Concept Consultants was chosen as the main contractor for the ground investigation works. Concept employed drilling subcontractors CC Ground Investigations and Wheeler Site Investigation. The team began work on site in August 2022.

Due to the presence of badgers on the Cliff Parade site, the council contracted ecological consultant SES to identify the badger setts. Any work that was carried out had to take place at least 20m from any of the setts, meaning some of the ground investigation boreholes had to be relocated to avoid them.

When Campbell Reith was originally appointed to the council's framework, it set out to review the historical geotechnical information relating to the sites, but this was scarce, Barker notes.

"There were some historical reports and information of localised, spotty investigations," she continues.

"On Belton Way East and West, piles had been installed to retain the slope, so investigations and design would have been done for these. But there was no information available. On Cliff Parade, a line of piles with 1m to 2m space in between them was installed to support the road, but the badgers found their way in between the piles. A mid-slope row of piles topped with a pile cap was also present and ground settlement on the downslope side was clearly visible.

"The Flagpole site had also suffered a landslide in 2003 and had been fenced off from the public ever since. While there were other mitigation measures, such as two medium term risk management catch fences at the toe of the slope in Cliff Parade, there was just an absence of information, so it was like picking apart a puzzle."

Campbell Reith has overseen various ground investigation methods used across the sites to gather data to carry out a stability analysis (see table).

The investigations looked at the existing slope angles, where they were too steep to remain stable, and where remediation structures will be required.

The rotary drilling was undertaken to depths between 10m and 30m. Based on the state of the historical shear surfaces and slip planes, the failure planes were not expected to be particularly deep, Barker explains. In areas where access was difficult, dynamic sampling was employed instead of bringing in the rotary rig.

A large claystone encountered during investigations

Cable percussive companion holes were drilled adjacent to selected rotary holes to allow targeted sampling at critical locations, identified from the rotary core logging.

"We also conducted trial pits to help identify some of the historical slip surfaces, particularly at the toe of the slope," she says.

Barker adds that for most of the investigations, the team encountered London Clay at shallow depth, with some weathered material and reworked material on top.

The ground investigation teams demobilised in January. The AGS data and borehole logs from Concept have been delivered to Campbell Reith, and laboratory testing of samples is nearing completion.

The modelling team will update the initial ground models with additional data to create a 3D model showing the sites’ topography and ground conditions.

The scope of the remediation works will be huge, and it might not all take place at once, Barker predicts.

Campbell Reith is in the process of evaluating the ground investigation data, carrying out slope stability modelling, risk assessment and analysis to provide a comprehensive appraisal of the slopes for the council.

The results of the stability analysis are expected to be ready by mid-year, so that the project can move on to remediation design and construction works.

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