Isle of Wight Council has declined to undertake or fund remedial works to improve stability of a slope in Shanklin.
Ground instability at the site is affecting a former railway cutting on land owned by the council. The instability at the site also affects adjacent land and a property development known as Rush Close.
Signs of instability were first reported on the south side of the cutting in 2007. Heavy rain last winter led to further ground movement problems.
As a result of the most recent ground movement, Mott MacDonald was appointed to undertake a technical assessment of the causes of the failure and to suggest potential remedial options.
Mott MacDonald mapped the geology of the cutting, which comprises sand and clay, interspersed with layers of grit and pebbles. The team also mapped the geomorphology of the slope, which consists of a number of benches and steep scarp slope, both convex and concave in shape.
The results of the ground investigation did not indicate that ground movement is caused by any one factor. The instability has been attributed to a combination of issues, including construction of the property development, drainage, wet weather and presence of large trees in the cutting.
The Mott MacDonald report, published in April 2014, outlined two potential engineering solutions to resolve the identified failure mechanisms. The options included a retaining structure built from rock-filled gabion baskets or bored piles installed at the top of the cutting with the slope below re-graded.
As the council owns the top of the cutting and the former railway which includes the sides of the cutting, legal advice was sought regarding the council’s liability to its neighbours. The council was advised that it was not responsible for the instability, but it should make a “modest contribution” towards the cost of remedial works.
According to a committee report from the council: “As the adjacent landowners would be the major beneficiaries of any works, the council has previously, in 2008, offered to contribute 10% of the overall costs.
“The landowner was not prepared to contribute anything to the works and so the works were not undertaken.”
The potential solution proposed in 2008 was for a smaller area and did not address the need for drainage. Therefore, the works were considerably less expensive than the options outlined in the Mott MacDonald report.
The report estimated that implementing geotechnical ground investigation would cost £75,000, with a further £20,000 for a technical interpretive report. It is anticipated that permanent stabilisation work could lead to a total scheme cost approaching £1M.
The council has agreed to continue to monitor the slope and carry out tree works when necessary.