Further work is requred to complete the interal links and popup references (98 broken links out of 384)
back to home page Based on part of the GeotechniCAL reference package
by Tony Price, Warwick University

Slope stability

Soil or rock masses with sloping surfaces, either natural or constructed, are subject to forces associated with gravity and seepage which cause instability. Resistance to failure is derived mainly from a combination of slope geometry and the shear strength of the soil or rock itself.

The different types of instability can be characterised by spatial considerations, particle size and speed of movement. One of the simplest methods of classification is that proposed by Varnes in 1978:

  1. Falls
  2. Topples
  3. Slides rotational and translational
  4. Lateral spreads
  5. Flows in Bedrock and in Soils
  6. Complex

In which the mass in motion travels most of the distance through the air. Falls include: free fall, movement by leaps and bounds, and rolling of fragments of bedrock or soil.

Toppling occurs as movement due to forces that cause an over-turning moment about a pivot point below the centre of gravity of the unit. If unchecked it will result in a fall or slide.
The potential for toppling can be identified using the graphical construction on a stereonet. The stereonet allows the spatial distribution of discontinuities to be presented alongside the slope surface. On a stereoplot toppling is indicated by a concentration of poles "in front" of the slope's great circle and within 30 of the direction of true dip.

Lateral Spreads
Lateral spreads are disturbed lateral extension movements in a fractured mass. Two subgroups are identified:

  1. Where the spread is without a well-defined controlling basal shear surface or zone of plastic flow.
  2. In which extension of rock or soil results from liquefaction or plastic flow of subjacent material.

Two subtypes are identified:

A. In Bedrock
Where flows include spatially continuous deformation and superficial as well as deep creep. They also involve extremely slow deep creep and extremely slow and generally non-accelerating differential movements among relatively intact units. Movements may:

  • be along shear surfaces that are apparently not connected,
  • result in folding, bending or bulging, or
  • roughly simulate those of viscous fluids in distribution of velocities.

    B. In Soils
    In which the movement within the displaced mass is such that the form taken by moving material, or the apparent distribution of velocities and displacements, resemble those of viscous fluids. The slip surfaces within the moving material are usually not visible or are short-lived. The boundary between the moving mass and material may be a sharp surface or differential movement or a zone of distributed shear. Movement ranges from extremely rapid to extremely slow.

    Complex movement is by a combination of one or more of the five other principal types of movement described by Varnes' Classification. Many landslides are complex, although one type of movement generally dominates over the others at certain areas within a slide or at a particular time.


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    Causes of instability

    Starting from the general definition for the factor of safety,
    Fs = tf / t = (shear stress at failure) / (shear stress)

    Terzaghi divided landslide causes into external causes which result in an increase in shearing stress and internal causes which result in a decrease of the shearing resistance.

    Varnes pointed out that there are a number of external or internal causes which may be operating either to reduce the shearing resistance or increase the shearing stress. There are also causes affecting simultaneously both terms of the factor of safety ratio.

    The influence of different contributory factors on the factor of safety of a slope varies in time due to a variety of factors.


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    Classification of causes of instability

    It must be appreciated that the causes of instability are often complex and any attempt at classification will be approximate and incomplete. The Working party on World Landslide Inventory have proposed a list of causal factors grouped under four main headings:

    Ground conditions

    Geomorphological processes

    Physical factors

    Man-made processes


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    Influence of different factors on instability

    It can be seen that short-term variations in factor of safety may occur due to seasonal variations in groundwater levels while longer term trends may reflect the influences of weathering or longer term changes in groundwater conditions. This approach is useful in emphasising that landslides (and slope instability in general) may not be attributable to a single causal factor.

    From the physical point of view it may be useful to visualise slopes as existing in one of the following three stages:
    Stable: the margin of stability is sufficiently high to withstand all destabilising forces.
    Marginally stable: likely to fail at some time in response to destabilising forces reaching a certain level of activity.
    Actively unstable: slopes where destabilising forces produce continuous or intermittent movements.

    These three stability stages provide a useful framework for understanding the causal factors of instability and classifying them into two groups on the basis of their function:

    Preparatory causal factors - which make the slope susceptible to movement without actually initiating it and thereby tending to place the slope in a marginally stable state.

    Triggering causal factors - which initiate movement. These causal factors shift the slope from a marginally stable state to an actively unstable state.


    Examples of External causes Resulting in Increased Shearing Stress

    Examples of Internal Causes Resulting in Decreased Shearing Resistance


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    Further Reading

    Brunsden D., "Mass movement, in Processes in Geomorphology", ed. C. Embleton and J.
    Thornes, Edward Arnold, London, 1979, pp. 130-186.

    Brunsden, D., "Landslides and the International Decade for Natural Disaster Reduction: do we have anything to offer. Landslides Hazard Mitigation", Royal Academy of Engineering, June 1993, London, 1995, pp 8-18.

    Crozier, M.J., "Landslides - Causes, consequences and environment", Croom Helm, London, 1986, pp 252.

    Cruden, D.M., "A simple definition of a landslide", Bulletin IAEG, No. 43, 1991, pp 27-29.

    McRoberts E.C., Morgenstern, N.R., "The stability of thawing slopes", Canadian Geotechnical Journal, Vol. 11, 447-469.

    O'Shea B.E., Ruapehu and the Tangiwai disaster, New Zealand Journal of Science and Technology, B, 36, 1974, pp 174-189.

    Popescu, M., "Landslides in overconsolidated clays as encountered in Eastern Europe", Proceedings 4th International Symposium on Landslides, Toronto, Vol. 1, 1984, pp 83-106.

    Popescu, M., "A suggested method for reporting landslide causes." Bull IAEG, No. 50, Oct. 1994, pp 71-74.

    Terzaghi, K., "Mechanisms of landslides", Geological Society of America, Berkely Volume, 1950, pp 83-123.

    Varnes, D.J. "Slope movements and types and processes." In: Landslides Analysis and Control, Transportation Research Board Special Report 176, 1978, pp 11-33.

    Working Party on World Landslide Inventory, "A suggested method for reporting a landslide", Bulletin EEG, No. 41, 1990, pp 5-12.

    Working Party on World Landslide Inventory, "A suggested method for a landslide summary", Bulletin IAEG, No. 43, 1991, pp 101-110.