Landsliding in Great Britain
Abstract
Britain experiences a large number of landslides each year. The causes of the landslides vary, but
typically include the interaction of geological control and marine or climatic input. Increasingly, human
activities are affecting the occurrence of landslides and their distribution. The human response to
landslides varies too, from the ‘do nothing’ approach to the environmental-modification approach.
Landslide types
There are four broad categories of contemporary landslides in Great Britain, and these are:
1. Coastal landslides – these are largely the result of wave attack influenced by rising sea-levels
(currently varying between 1.1mm and 6.0mm per year), which have caused cliff retreat rates
averaging 0.1–1.5m per year along most of the unprotected ‘soft rock’ coastline of eastern and southern
England. The cliff retreat rate rises to 2m per year on Holderness (Humberside), in north Norfolk,
along parts of the Suffolk coastline, near Herne Bay (Kent) and in Middleton (Sussex), with even
higher values for Warden Point, Sheppey (5m per year) and Selsey Bill, Sussex (8m per year).
2. Inland landslides on natural slopes (natural causes) – these are for the most part unaffected by human
activity, and are in many cases concentrated at sites where the base of the slope is being undercut by
rivers (e.g. the Ironbridge Gorge in Shropshire, and Lower Teesdale).
3. Inland landslides on natural slopes (human causes) – these are largely produced by the disturbance of
pre-existing ancient landslides as a consequence of human intervention (e.g. Sevenoaks Bypass, 1966).
4. Landslides in cuttings, fills and waste dumps – these are produced wholly as a consequence of
human activity (e.g. Aberfan, 1966).
The sites of the landslides mentioned above are located on Figure 1.
Figure 1 – Locations of selected landslides in the UK.
The location of the 31 ‘best-known’ landslides in Great Britain (Figure 2) shows a clustering in the
southern half of Britain; however, this may just reflect the fact that research and site investigations
have been strongly focused on this area. Ground materials are undoubtedly the dominant control on
landsliding, influencing the type of movement and therefore the character of slipped ground. Figure 2a
shows the effects of landsliding at Barton, Hampshire.
Figure 2 – The location of the 31 ‘best-known’ landslides in Great Britain.
Figure 2a – Rotational slide at Barton, Hampshire.
A distinction should be drawn between ‘first-time’ failures, on slopes which show no signs of having
been affected by landslides in the past, and ‘reactivated’ failures. The majority of failures in groups 1, 2
and 4 above are essentially ‘first-time’ failures. This distinction can be widened to include ‘repeated
failure’ in which the destabilising forces become repeatedly dominant, either due to seasonal changes
or where erosion removes the support provided by previously slipped material (e.g. coastal cliff). Once
materials have been involved in slope failure they become much more prone to further movements.
Coastal landslides
In Britain, the most dramatic landslides tend to be on the coast, for example the event at Holbeck Hall
Hotel near Scarborough in June 1993, which involved an overnight cliff retreat of 135m.
Coastal landslides are categorised by the ground-forming material in which they occur, and the four
main categories of ground-forming material in which landslides occur are:

Weak superficial deposits

Stiff clays

Stiff clay with an overlying layer of hard rock

Hard rock.
Weak superficial deposits
The east coast of England, from Flamborough Head to Essex, is largely developed in thick sequences
of glacial till, interbedded with sands and gravels. Coastal landslides are ubiquitous except where the
cliffs are protected by sea defences (see the case study ‘Coastal defences in Norfolk’). The undefended
Holderness coastline has retreated at rates of 1–6m every year since 1852, and it is known that 200km
of land has been taken by the sea over the last thousand years, including at least 26 villages listed in the
Domesday survey of 1086. At those locations of the Norfolk coast where the ground is sufficiently
elevated to form high cliffs, dramatic failures have developed, with particularly impressive examples
recorded near Trimingham and Cromer.
Coastal landslides play an important role in supplying sediment to beaches, sand dunes and mudflats on
neighbouring stretches of coastline. A study of the East Anglian coastline revealed that sediment
supplied to the shoreline by cliff failures between Great Yarmouth and Cromer was essential for beach
nourishment in adjacent areas. However, due to coastal defences along this stretch, and therefore the
reduced nourishment of beaches, the potential for erosion elsewhere (i.e. in areas with no coastal
defences) will increase and lead to further spending on coastal defence works. The real lesson of such
studies is that there is no point in attempting to eradicate all slope instability through some ill-founded
belief in the power of engineering technology, but rather that society should learn to co-exist with slope
failure.
Stiff clays
Stiff clays are particularly prone to landsliding, with classic examples occurring along the southern
shore of the Thames estuary. Average retreat rates are up to 2m p.a. At Warden Point on the Isle of
Sheppey huge failures have repeatedly occurred in response to rapid basal erosion, while at nearby
Herne Bay urban developments have been threatened by major failures involving the whole height of
the cliff.
To the north of Folkestone on the Channel coast, the 130–150m-high cliffs of soft white limestone
(chalk) have foundered on the underlying Gault Clay for a distance of 3km to create Folkestone Warren
(Figure 3). Although originally a purely natural feature, the creation of Folkestone Harbour between
1810 and 1905 seriously disrupted the north-eastward movement of beach shingle along the shore,
thereby leaving the base of the cliffs exposed to erosion and exacerbating the frequency and magnitude
of landslide movements. Twelve major slips have been recorded since 1765, culminating in huge
movements throughout the Warren in 1915, including one slipped block which moved the Folkestone–
Dover railway seaward by 50m and derailed a train. The railway has been closed 30 times due to
landsliding since it was constructed in 1844.
Figure 3 – Folkestone Warren before and after the 1915 landslide.
Black Ven in Dorset is probably the most active and photogenic large landslide in Great Britain
(Figures 4 and 5). The backscar has retreated at an average rate of 0.71m p.a. this century, although
cliff collapse can cause local retreat of up to 30m in a single year, with the majority of the material
being removed by mudslides. Since the 1950s, movements of material within Black Ven have been
dominated by two huge mudslides that fed enormous lobes of debris which sometimes extended nearly
100m into the sea. There was especially marked activity in the winters of 1957–58, 1969–73 and 1995–
97. The winter of 1986–87 saw the reactivation of old degraded slides in the western part of the
complex.
Figure 4 – The geology and debris flows of Black Ven, Dorset.
Figure 5 – The distribution of landslides along the coast of Lyme Bay.
Stiff clay with an overlying layer of hard rock
Large failures in the Upper Greensand of the Weald create landslide blocks on the benched area which
are gradually broken up by further sliding and slumping until the material reaches the cliff edge where
it falls onto the beach to be redistributed by waves and tides. The average backscar retreat this century
has been estimated at 0.4m p.a., giving rise to about 160,000 tonnes of beach material each year, over
half through cliff falls.
Hard rock
Some landslides are developed in hard rock. Coastal cliffs developed in hard rocks are continually
suffering minor collapse due to basal undermining by the sea. Seven Sisters and Beachy Head are chalk
cliffs in East Sussex, which are currently retreating at an average rate of 0.97m p.a. (Figures 6 and 7).
In January 1999 a large section of Beachy Head fell into the sea. It was the biggest single loss of
coastline in Britain in living memory. Hundreds of thousands of tonnes of chalk crashed 160m into the
sea, lengthening the beach by more than 30m. The collapse was so large that a lifeboat spotted it some
7km out at sea.
Figure 6 – Formation of the Seven Sisters.
Figure 7 – Beachy Head, East Sussex.
The fall may have been caused by water entering the chalk, freezing, expanding and causing the cliff to
crumble. Another possibility is that waves along the south coast are getting stronger. Wave height in
the Atlantic has increased by 10% during the 1990s.
Another theory is that following a series of dry years, during which time the chalk dried out, 1998–99
was very wet. Thus the chalk became increasingly wet and unstable. If a severe frost occurred when the
chalk was moist, a collapse would be possible.
Other landslides are related to abandoned cliffs. In the southern half of Great Britain, sea-level has
varied by less than 5m over the past 5000 years. This follows a period of rapid rise from 121m at
18,000BP (Before Present) to +5m above sea level 5000 years ago. As a consequence, some stretches of
rapidly evolving coastal cliff-line have become abandoned by the sea. At Hadleigh in Essex, the former
cliff-top is considered to have retreated 40m over the last 6500 years. It now threatens the remains of
Hadleigh Castle built in the thirteenth century. A similar situation exists along the northern margin of
Romney Marsh, Kent, where the former cliff-line has suffered extensive failure since it was abandoned
about 6000 years ago.
Inland landslides in Britain
Away from the coast, the majority of landslides are ancient features, inherited from the late
Pleistocene. Widespread landsliding occurred on glacial valley sides, the flanks of hills and along
escarpments. In consequence, a legacy of ancient landslides exists, often concealed under ground that
has become degraded and vegetated or superficially remodelled by later events. In inland areas, the
greatest hazard is associated with the often unexpected reactivation of these features during prolonged
heavy rainfall or as a result of human interference.
Britain does not suffer from landslides of the magnitude experienced in actively growing mountainous
regions such as the South American Andes. However, we do have many landslides in Britain (more
than 10,000 are known), and some are very large and spectacular such as those at Mam Tor. Ancient
landslides account for almost 20% of inland landslides. There are strong concentrations on the Lower
Greensand escarpment of the Weald, the rocks of the Devon–Dorset–Somerset borders, Avon, the
Cotswolds, the Northamptonshire–Leicestershire borders and the western margins of the North
Yorkshire Moors.
Mam Tor
The most dramatic area of inland landsliding is without doubt Mam Tor in Northern Derbyshire
(Figures 8–11). This is the largest active inland landslide in Britain. Ever since the turnpike road was
constructed over the landslide debris in 1802 there has been a record of intermittent movements.
Further displacements in the 1950s, 1960s and 1970s resulted in the eventual abandonment of the route
in January 1979.
Figure 8 – Main features of a landslip.
Figure 9 – Features of the landslip at Mam Tor.
Figure 10 – Landslide at Mam Tor.
Figure 11 – Landslide at Mam Tor.
The road at Mam Tor was one of the most notorious sites for landslides and mass movements. The
A635 was one of only three cross-Pennine routes in Derbyshire and therefore functioned as an
important link between the cities of Manchester and Sheffield. The road had been built in 1802 as an
alternative route to the narrow Winnats Pass. However, the new road was inadvertently constructed
across one of the largest landslides in the High Peak with the result that it was repeatedly affected by
cracking and settlement (Figure 12).
Figure 12 – Summary of recorded landslide damage to the A625 at Mam Tor, Derbyshire.
1909
1912
1915
1918
1919
1920
1929
1930
1931
1937
1939
1942
1946
1948
1949
1950
Road cracked and repaired.
Road badly cracked and sinking, later repaired.
Fractures and settlements of up to 30cm.
Subsidence to road.
Continued movement.
Fractures and holes, later repaired.
Road affected by movement, alternative routes suggested.
Continuous movement.
60m-long crack on road.
Considerable settlement.
100m-long crack, 25cm subsistence.
Settlement of up to 20cm.
Major roadworks needed, including realignment of the road, costing £8000.
Renewed subsistence.
Slip recorded (no details).
Road damaged by movement, major roadworks carried out costing £13,500.
1955
1965
1977
1978
1979
Large-scale movement requiring £20,000 of repairs.
Road closed after movement, later repaired and reopened.
Road closed due to movement.
Road reopened for single-lane traffic.
Final closure of road, traffic diverted.
The road was closed permanently in January 1979 when a section slumped down by 2m. Heavy traffic
was diverted via the B6049 and B623, passing through Bradwell, Peak Forest and Sparrow Pit, causing
considerable congestion on the narrow roads.
Ironbridge Gorge
A second area of well-known inland movements is the Ironbridge Gorge of Shropshire. Here, several
landslides are present on both the northern and southern valley sides. The original Iron Bridge, built in
1779, was reported as showing signs of distress as soon as 1784 and was squeezed by about a metre
over the period 1795–1905.
Aberfan, South Wales
Human interventions were largely responsible for the loss of life at Aberfan, South Wales. On 21
October 1966 a landslide involving a coal tip slag heap at Aberfan killed 146 people, 116 of whom
were children at the Pantglas Junior School. Aberfan lies on the banks of the River Taff and was
overlooked by the tips of the Merthyr Vale Colliery. The landslide involved over 100,000 cubic metres
of colliery waste travelling at speeds of up to 30 km/hr. Earlier landslides had occurred in the vicinity.
The National Coal Board believed that the speed of movement was slow enough to allow a warning to
be given. Spoil heap number 7 was located above a spring. Water seeping through the sandstone
emerged as a spring in the lower part of the tip. As the water passed through, it removed fine clay from
the ‘toe’ of the tip, thereby increasing its steepness. See Figure 13 for a full list of the contributing
factors.
Figure 13 – Factors contributing to landsliding in the Rhondda Valleys.
Intrinsic factors
Slope aspect
Slope angle
Lithostratigraphic units
Superficial deposit thickness
Dip in relation to ground slope
Faulting
Groundwater potential
Joint direction/density
Ground height
Extrinsic factors
Erosion potential
Ground strains due to mining
Ground tilt due to mining
Excavations and filling
Vegetation changes
Seismicity
Rainfall
Figure 13a – Sign highlighting the dangers caused by quarrying.
Causes of landslides
The descriptions below allow us to recognise two sets of factors on the basis of their role in promoting
slope failure. They show that there is a build-up of pressure (or a reduction of strength) over time, and
then eventually the land gives way:
1. Preparatory factors – these work to make the slope increasingly susceptible to failure without
actually initiating it, i.e. cause the slope to move from a stable state to a marginally stable state,
eventually resulting in a relatively unstable state.
2. Triggering factors – these actually initiate movement, i.e. shift the slope from a marginally unstable
state to an actively unstable state.
Essentially, the cause of a landslide can be put down to the balance between shear strength and shear
stress (Figures 14 and 15).
Figure 14 – A simple sub-division of the causes of landslides.
Figure 15 – The causes of landslides in Great Britain.
Causal feature
(Process)
Weathering
Number
Number
126
73
Causal feature
(resultant change)
Change to physical
properties
Geometric change
Unloading
Progressive failure –
creep
Undermining
Loss of cements
Natural erosion
Artificial erosion
Ground subsidence –
removal of support
Deposition
Shocks and vibrations
– seismic
Shocks and vibrations
– man-induced
Water regime change
1297
276
103
37
Loading
105
1204
Ground movement
Physical effects
Water status
89
4
2627
647
293
1471
336
808
58
36
First-time landslide activity occurs from time to time through natural causes, such as unusually heavy
rainfall and the weakening of rock as it weathers. More often, movement is a reactivation of a dormant
slide that may have moved originally in the wetter conditions at the end of the last ice age. Landslides
may also be triggered artificially by ill-advised land use, such as excavations at the foot of slopes,
saturating slopes by the ill-considered disposal of surface water and loading slopes by dumping
material on them.
For example, following the flooding of Boscastle in Cornwall, in August 2004, geologists issued
warnings of increased landslide activity in the region. Where slopes are covered with permeable soil
over a layer of rubble, they could become suddenly saturated in the kind of downpour that caused
havoc at Boscastle.
Similarly, in the Trossachs region of Scotland, landslips near Lochearnhead were triggered by the
heavy rain in 2004. A huge operation to clear and stabilise one of Scotland’s main tourist roads took
place after the series of landslides, which trapped almost 60 people in their vehicles. Thousands of
tonnes of mud and boulders were swept down the steep slopes of Glen Ogle on to the A85, which links
Perth in the east with the western Highlands. Small streams had turned into cascades of water,
loosening huge sheets of earth, rocks and vegetation. Fifty-seven people trapped on the road had to be
airlifted to safety by helicopter. In all, more than 150 people were stranded by the flooding caused by a
day of torrential rainstorms.
Landslides as hazards
A landslide defined is a hazard diminished. Most landslides in the UK are usually small, infrequent and
soon forgotten. Thus, in the past they were not thought to be a significant hazard by most people and,
because the threat to life and property was not seen to be great, little research or survey was done.
However, the cumulative effects of slow movements pose a very real threat to construction and
development, causing damage to buildings and infrastructure. Figure 16 describes the features of active
and inactive landslides.
Figure 16 – Features indicating active and inactive landslides.
Active
Scarps, terraces and crevices with sharp edges.
Crevices and depressions without secondary infill.
Secondary slides on scarps.
Fresh slickensides.
Fractured block surfaces.
Disrupted drainage.
Pressure ridges.
Tilted trees, mainly fast growing vegetation
species.
Inactive
Scarps, terraces and crevices with rounded edges.
Crevices and depressions with secondary infill.
No secondary slides on scarps.
No fresh slickensides.
Weathered block surfaces.
Integrated drainage.
Marginal fissures, abandoned levees.
Trees tilted but with new vertical growth,
vegetation cover predominately slow growing
species.
There are a number of costs associated with landslides (Figure 17).
Figure 17 – A range of costs commonly associated with landslide problems in Great Britain.
Personal costs
Immediate costs
Fatal accidents
Injuries
Psychiatric problems
Evacuation and provision of temporary or replacement housing
Mobilisation of relief workers and emergency services
Transport delays
Costs of investigation
Cost of repair
Indirect costs
Cost of prevention
Compensation
Increased insurance premiums
Depreciated property or land values
Costs of legal actions
Costs of public inquiries into causes and responsibilities
Research into the nature and extent of landslide problems at universities
Formation of planning policies related to development on unstable land
Coastal protection schemes
Design and construction of preventative measures including drainage and
regarding
Costs of monitoring potentially unstable slopes
There are a number of ways of reducing the risks associated with landsliding (Figure 18). These
include:

‘Excavation and fill’ to level the slope’s surface and make it more secure

Draining the build-up of water in slopes and thereby making them less likely to fail

Using restraining structures such as gabions and stone walls to keep the failed material behind
the structure

Erosion control such as rock armour and revetments minimise the forces acting at the base of
cliffs

Other methods, such as the diversion of roads away from active areas, or over them in the form
of bridges, are also important.
Figure 18 – Principal methods of slope stabilisation.
Approach
Excavation and filling
Drainage
Restraining structures
Erosion control
Miscellaneous methods
Methods
Remove and replace slipped material.
Excavate to unload the slope.
Fill to load the slope.
Lead away surface water.
Prevent build up of water in tension cracks.
Blanket the slope with free draining material.
Installation of narrow trench drains aligned directly downslope,
often supplemented by shallow drains laid in a herring-bone
pattern.
Installation of interceptor drains above the crest of the side
slope to intercept groundwater.
Drilling of horizontal drains into a slope, on a slightly inclined
gradient.
Construction of drainage galleries or adits, from which
supplementary borings can be made.
Installation of vertical drains which drain by gravity through
horizontal drains and adits, by siphoning or pumping.
Retaining walls founded beneath unstable ground.
Installation of continuous or closely spaced piles, anchored
sheet or bored pile walls.
Soil and rock anchors, generally prestressed.
Control of toe erosion by crib walls, rip-rap, rock armour,
revetments, groynes.
Control of surface erosion.
Control of seepage erosion by placing inverted filters over the
area of discharge or intercepting the seepage.
Grouting to reduce ingress of groundwater into a slide.
Chemical stabilisation by liming at the shear surface, by means
of lime wells.
Blasting to disrupt the shear surface improve drainage.
Bridging to carry a road over an active site.
Rock traps to protect against falling debris.
Figure 19 shows an example of a restraining structure used at Barton, in Hampshire. Steel piles are
used to reduce the chance of slumping.
Figure 19 – Steel piles used to reduce slumping at Barton, Hampshire.
Conclusion
Landslide activity is widespread in Britain. The causes are variable and there is a range of impacts. It is
likely that the nature of the landslide hazard will change over time as climate changes and as human
activities affect more former landslide sites. In addition, the management of landslides in one area
(notably along the coast) will have an impact on the landslide hazard elsewhere. Human intervention
may be for the right intentions, but it does not always have the anticipated outcomes.