Cliff Stabilisation

Coastal cliffs can be unstable due to the combined effect of several factors. However the basic cause of cliff instability is normally the marine erosion of the foot of the cliff. Therefore methods of presenting or reducing cliff erosion concentrate on either reducing wave energy using breakwaters or by increasing beach sediment thickness, or on increasing the cliff face strength by constructing some form of revetment at the toe.

This will exclude further erosion of the foot, but at that stage the slope of the cliff may very well be so steep that weathering and sliding may still occur. In some cases this can be achieved by artificial smoothing of the slope, if there is enough space at the foot as well as at top of the cliff for this.

This will counteract future uncontrolled weathering and sliding. Once this has been achieved the establishment of a vegetation cover on the cliff will help protect against weathering and groundwater seepage, and thereby to some extent against sliding.

		Where a cliff suffers from sliding due to high groundwater pressure and poor drainage conditions, horizontal
and vertical drains can be used as well as the regulation of the surface runoff.

This has been an issue at Durlston in Dorset, where in the winter of 2000/01 a landslide took away approximately 10m of cliff including the coastal footpath and part of the gardens to the development on Belle Vue Road.

Following the landslide, a stream draining the valley to the west was left discharging into the landslide zone .Collapses have been partly attributed to storm runoff permeating through the ground (as well as to cliff erosion); and an embargo on new soakaways being used for drainage of roofs and paved areas within 400m of the cliff edge has been put in place

		These gabions form the northern wall of the cliff stabilisation scheme below the precarious flats on Belle Vue Road, in the middle of Durlston Bay. The stone is Portland Stone, which is a much lighter colour than the underlying cliffs. Plants have started to establish themselves on the edges of the structure, but there is little soil available in the gaps and the greening of this slope will take a long time.
		Gabion wall of cliff reinforcement, Durlston Bay © Copyright Jim Champion and licensed for reuse under this Creative Commons Licence.

In some cliffs mass failure planes develop, in these cases pinning the material through the failure planes using pilling procedures can prevent slumping. These techniques are usually used in conjunction with some form of toe erosion protection so that the recession of the cliff face is prevented

At Lyme Regis extensive cliff stabilisation works took place in the 1990s to protect the town.

Without hard engineering schemes Lyme Regis would have gradually be eaten away by the sea and afflicted by destructive landslides - threatening homes, businesses and the lives of local people and visitors alike.

This diagram shows a simplified cross-section of the land at Lyme Regis and the processes that make it unstable.

The cliff stabilisation work included the stabilisation of the ground behind the foreshore using soil nailing and 1,150 bored piles to pin the unstable ground to the firmer ground below. A new drainage system including boreholes and trenches to disperse water - a major factor in causing land instability and slope regrading, removal of landslide debris and building of soil buttresses at the back of the gardens to support the hill behind.

The diagram below shows this pictorially