Causes
of Landslides
Conditions and processes that cause landslides
The basic causes of slope instability and landsliding are fairly
well known. They include the composition or structure of rock or
soil types, the inclination of the slope, groundwater levels, seismic
activity and human activity. These can be classified into groups
reflecting the basic conditions that affect slope stability, that
include geology, geomorphology, hydrology and vegetation, and the
factors that produce unfavourable conditions that may cause slope
failure.
Geology
Simple landslide hazard zonation maps show geological formations
with annotations about the relative stability of these rock units.
More useful maps give more importance to grouping of the rock units
by lithology rather than stratigraphy and these are then ranked
according to their relative slope stability. Even more useful maps
subdivide the underlying bedrock and overlying surficial deposits
for hazard zonation, which will then map existing landslide deposits.
In producing these maps particular attention must be given to establishing
processes of development of the rock and surficial deposits and
how they may have been altered (Varnes, 1984, p. 11-13).
- Lithology:
physical or chemical properties of rock and surficial deposits
determine the shear strength, permeability, susceptibility to
chemical and physical weathering etc. which affect slope stability.
- Structure:
stratigraphic sequence, attitude of layering, gross changes in
lithology, bedding planes, joints, faults and folds create weaknesses
which lead to land instability
Geomorphology
Figure 6.
 Geomorphological
maps tend to identify slope elements such as previous landslides
and their degree of activity, and the features associated with these
landslides, including scarps, reversal of slopes and bulging toes
and these are illustrated in Figure 6. Geological formations and
their engineering properties may also be included in these maps
(Varnes, 1984, p. 13).
- Evidence
of previous landslides is the most important geomorphological
characteristic considered in landslide zonation; landslides are
most likely to occur in geological, geomorphological and hydrological
situations that have resulted in landslides in the past.
- Steepness
of slope: steeper slopes are more likely to be unstable than shallow
ones although in some situations steep slopes of strong rock may
be more stable than shallower slopes in weak material.
- Slope
curvature (e.g. concave or convex)
Figure 7.
Figure 7 shows examples of the relationship between maximum stable
angle and height of slope for some New Zealand rock types.
Hydrology and climate
Water is one of the most important factors in slope instability.
Landsliding activity varies between regions largely as a function
of variations in rainfall and temperature. Heavy rainfall causes
widespread debris sliding in regions predisposed towards instability
by geological, geomorphological and groundwater conditions. Seasonal
variations in groundwater flow and pressures in temperate regions
with moderate rainfall may be sufficient to trigger movement in
old deep seated landslides. Freeze-thaw and frost action can trigger
rockfalls and melting of frozen ground can cause debris slides and
flows (Varnes, 1984, p. 13-14).
Water in the soil effectively makes the soil much heavier and also
increases the water pressure (Costa and Baker, 1981, p. 266). Soil
is a effectively closely packed particles which are loosely bonded
together. Small spaces between the particles are filled with air
and varying amounts of water. During heavy rainfall, or infiltration
from leaking pipes or irrigation channels, these spaces, or pores,
are filled which then raises the pore water pressure in the soil.
These pressures may be raised sufficiently to cause the soil particles
to "float"and when this happens the soil collapses (Costa
and Baker, 1981, p. 266).
Establishing long term precipitation records correlated with reports
of past landslide activity, determining the locality of springs
and identifying variations in groundwater conditions all provide
useful information for landslide zonation. Variations in groundwater
conditions are usually reflected in changes in vegetation and can
also be established from observations of water levels in wells (Varnes,
1984, p. 14).
Vegetation
Vegetation cover can promote slope stability by (1) protecting
the ground surface and underlying rock and surficial deposits from
the action of rain, wind and sunshine, (2) absorbing rainfall and
then eliminating it through evapotranspiration, (3) immobilising
rainfall in the fallen vegetation on the forest floor thus reducing
runoff and erosion and (4) increasing the strength of the rock or
soil through the network of plant root (Varnes, 1984, p. 14).
However, increased loading of slopes by plants may trigger instability.
As trees away in high winds or during earthquakes the stresses they
bear may be transmitted to the slope material causing failures.
Large tree roots may penetrate the rock creating fractures and promote
infiltration of water.
Vegetation can be a useful indicator of slope movements or zones
where slope instability may occur. Variations in the vigour of vegetation
growth may indicate moisture differences in the groundwater which
may reveal cracks in areas uphill of landslide headwalls. Groups
of backward leaning trees are indicators of rotational ground movement.
As they continue growing the upper parts of the trees will grow
vertically and the then bent tree trunk is a record of the landslide.
Tree ring damage will also record damage from tilting that can also
be used to date the timing of the landslides.
Unfavourable changes in ground conditions
- Changing
stress conditions: Slopes are subject to stresses which vary
in response to (1) natural processes such as uplift, erosion,
seasonal groundwater variations and earthquakes and (2) human
activities including construction, cuttings, reservoirs and changes
in landuse practises.
Figure
8 shows a few examples of the types of ways that stable slopes can
become unstable through changing stress conditions.
- Changing
strength of rocks and soils: Physical and chemical weathering
can decrease the strength of rock
and soils. Water plays a leading role in these processes.
Increasing water content acts by (1) further reducing the strength
of rock or soil that is already weak, (2) increasing the pore
fluid pressure within the material reducing internal frictional
resistance and thus decreasing its strength.
Figure
8.
 Long
term changes in groundwater levels may result from landuse changes,
e.g. conversion of agricultural land to urban development or forest
clearing. These often lead to increased runoff and erosion leading
to debris slides and flows. Landslides themselves may also weaken
adjacent slopes by creating cracks. Large scale subsurface movements
which are less obvious to the naked eye can occur within clays or
shales and these will act to orient the particles within the mass
thus creating weak zones (Varnes, 1984, p. 14-15).
|
