Landslides can be
triggered by both natural and man-made changes in the environment conditions.
The geologic history of an area,
as well as activities associated with human occupation, directly determines, or
contributes to the conditions that lead to slope failure. The causes of
landslide can be inherent, such as weaknesses in the composition or structure
of the rock or soil; variable, such as heavy rain, snowmelt, and changes in
ground-water level; transient, such as seismic or volcanic activities; or due
to new environmental conditions, such as those imposed by construction
activities (Varnes and the IAEG, 1984). Among these factors, rainfall,
earthquake and human activities are important trigger factors.
1.1 Monsoon Rainstorm
The Himalayas are affected by the monsoon, as are other parts
of South Asia in general. Due to the recurring
of the Summer Monsoon near the Bay of Bengal
towards northwest, there is a general decrease in rainfall from East to West.
Thus while Eastern Himalayas (Assam) have about eight months of rainy season
(March-October), the Central Himalayas (Bhutan, Sikkim, Nepal and Kumaon) have
only four months of rainy season (June-September) and in the Western Himalayas
(Kashmir), the Summer monsoon is active only for two months (July-August)
(Chalise, 1994).
Monsoon
rainstorms initiate many landslide each year in the Himalaya
region. During heavy rainstorm, loose/unconsolidated deposits, and strongly
weathered and fractured sedimentary and metamorphic rocks become saturated and
with an increase of precipitation and raise of ground water level. As a result,
these materials are especially prone to sliding when slopes are steep.
Rainstorms, therefore, are recognized as important landslide triggers in the Hindu
Kush-Himalayan region.
The relationship
between rainfall and incidence of landslide has been studied by many scientists
in China India Nepal China
If cumulative
precipitation amounts to 50 mm to 100 mm in one day, and daily precipitation is
more than 50 mm, somewhat small-scale and shallow debris-landslide will occur;
When cumulative
precipitation, within two days amounts to 150 to 200 mm, and daily precipitation
is about 100 mm, the number of landslides has a tendency to increase with
precipitation; and
When cumulative
precipitation exceeds 250 mm in two days, and has an average intensity of more
than 8 mm per hour in one day, the number of large and vast landslides
increases abruptly.
Studying the
relation between rainfall and landslides in China
The principal
geological factors impacting on landslide process are the type of bedrock,
crack and structure, soft band, the thickness of weathering zone, the thickness
and the grain composition of the soil (the surface deposits). The impact of
topographical conditions are represented in two respects. On the one hand,
there are the regional cut depth, cut density and the erosion basis plane. On
the other hand, there are impacts of the gradient and the form of slope and
water convergent area on the upper part of the slope.
1.2 Earthquakes
The Himalaya mountain belt represents a type example of an
orogen formed due to collision of two continents viz., the Asia
and the India Himalaya
bounded by latitude 22°N to 38°N and longitude 72°E to 98°E, over 600 earthquakes of magnitude 5 and
above have occurred during the period of 1950 to 1990. Till date four very
major (great) earthquakes of magnitude more than 8 have been recorded in the Himalaya or adjacent regions. These are the Great Assam
earthquake of 1897, Kangra earthquake of 1905, Bihar-Nepal earthquake of 1933
and the Assam
Earthquakes not
only trigger landslides, but over time, the tectonic activity causing them, can
create steep and potentially unstable slopes. It is recognized that significant
numbers of landslides occur only when earthquake magnitudes are greater than 6.
In the mountain areas, large-scale landslide triggered by earthquakes can block
rivers and form lakes.
Apart from the
characteristics of earthquakes themselves (i.e., seismic accelerations,
continuous time of shock, focal depths, and angle and direction of the approach
of seismic waves etc.), environmental factors, such as geology, landform and
drainage, play an important role in the formation of landslide induced by
earthquakes.
The influence of
geology is reflected in both geologic structure and lithologic character. The
landslides triggered by the Songpan earthquake (Aug. 16, 1976, M = 7.2), in
northwestern Sichuan
Province
1.3 Surface water
Erosion, or
soaking of surface to cause shallow sliding. Effects of water infiltrating from
surface. Causes shallow failures.
Various surface treatments, according to material type.
Grass planting with or without the combination of jute
netting and mulch for soils. Revetments for steep toe slopes in soil and soft
rock. Surface renderings for rock slopes without noticeable ground water presence.
1.4 Groundwater
Ground water
causes increased pore water pressure at depth. Failure plane is deeper than in
surface water failure. Ideally, remove ground water by drainage.
1.5 Weathering
Rock shear
strength is reduced by weathering. Rock strength is reduced as constituent
minerals are broken down into weathering products and clay minerals. Physical
bonds between rock constituents are weakened or broken. The rock can fail along
weakened fracture planes or through its body. It is progressive process where
cyclic failure possible and difficult to stabilize.
1.6 Undercutting
Slope is undercut
by a flowing stream or by the opening up of a road cutting. Incision (down
cutting) or lateral scour by streams is a major cause of slope failure. The
initial failure can work rapidly up slope. Stream bank and stream bed
protection required. May be too late to save slope from progressive failure of up
slope.
1.7 Addition of weight
Weight added usually
by the dumping of spoil or landslide debris. Remove extra material and
re-vegetate slope.
2.1 Erosion
Removal of
particles from the surface by flowing water is called erosion. An arbitrary
depth limit of 25 mm has been adopted for erosion. This depth refers only to
the initial removal of particles and is used to distinguish erosion from mass
movements. If particles are continually washed away, the surface will be
progressively lowered, giving rise to the forms of erosion described in 'a' to
'c'3 below. For example, a gully 2 m deep can be developed by the steady
removal of particles from its sides to a depth of no more than 25 mm at a time.
The process, which causes this, is still erosion.
2.2 Sheet erosion
Water flows over surface in an even film, not in channels.
Vegetation stabilisation should
be adequate.
2.3 Rill erosion and gully erosion to less than 2 m depth
Scour by water flow in channels.
Gullies begin as
very shallow, narrow incisions in the slope (rills). An arbitrary depth limit
of 2 m has been set for gullies as erosion features. If a gully is deeper than
2 m, its sides fail in ways similar to a normal hill slope. Hill slope
protection measures are then appropriate.
Check dams to
stabilise gully floor. Vegetation to stabilise gully head.
2.4 Piping
Removal of fines along an underground
channel.
Percolating
ground water in permeable fine soils of low plasticity can remove fines along
fissure to a point where an underground stream is formed. The roof of this
stream cavern can enlarge upwards towards the surface and eventually collapse
to create an open, elongated chasm or pit.
Difficult to
stabilise unless underground waterways are exposed and treated as gullies. Even
this will not stop piping in lateral channels. A deep interceptor drain can be considered.
Any mass movement of soil or debris
down slope.
Includes
translational slides of soils or debris, rotational slumps, and flows. The
plane of failure can be:
-
within a soil
or debris mass;
-
along the
interface between soil and weathered rock;
-
the
uppermost layer of weathered rock itself (in which case the failure plane would
be in rock;
-
between
soil and a rock plane in unweathered rock.
Translational slides are the most common form of slide in Nepal
A slump is a rotational movement of material, forming a spoon-shaped scar on the hillside, which is roughly
circular in plan. The debris forms a bulge near the toe.
Slumps are
commonly caused by high ground water pore pressures deep in the hillside, and
the slip circle usually goes several metres deep.
In practice in Nepal
Deciding which
mode is dominant is useful because rotational failures indicate a deep failure
plane and may therefore be more difficult to stabilise than a translational
slide.
Flows are caused
by liquefaction of material, usually by the action of heavy rainfall upon a
permeable soil surface. The soil literally flows down the slope. The failure plane
is usually shallow, sometimes only a few centimetres deep. However, the fluid mass is
very difficult to control or stop. Deep flows, which can travel a long way, are
very destructive and potentially pose a high risk to life and property.
For slides less than 100 mm deep, vegetation and/or bolsters should hold
slope. Fences may become undercut by liquefaction.
For slides 100 ‑ 250 mm deep, diagonal vegetation may be sufficient to
preserve rill system, provided maturity is reached. Support slope at base with
gabion wall.
2.6 Plane failure in rock
Any mass movement whose failure plane or planes is controlled
principally by fracture planes in rock, and whose debris consists chiefly of
rock fragments.
The weathering grade of the rock is 1 ‑ 4 (the rock rings when struck
with a hammer). Failure types commonly include plane failure, wedge failure,
and toppling (rockfall).
Standard rock mechanics procedure are the solutions.
2.7 Disintegration
A special type of rock failure, found in massive or sparsely‑jointed permeable,
weatherable rocks, e.g. porous sandstones, and in dense soils and
unconsolidated materials that stand in a vertical or near‑vertical face. Upon
landing, the material breaks up into a pile of loose debris, consisting mostly
of loose rock mineral particles e.g. sand containing a few boulders of
weathering grade 4 or 5. All traces of rock structure or stratification are
destroyed in the fall.
For this reason the mechanism is distinguished from a fall of hard rock,
which is considered a plane failure. Cause is weathering. Saturation and
weathering cause the rock to fail by planar or arc-like shearing throughout the
mass. Sometimes this is partially controlled by weakly developed joint planes.
Strictly, the mechanism is a 'fall', but the form of failure is
distinctive. The mechanism is typical of thick beds of soft Siwaliks sandstone
and terrace deposits. It is very difficult to cure.
It is very difficult to stabilize. Cut back to a stable angle, which is
determined by shear strength of, saturated and weathered material.
2.8 Differential Weathering
Weathering of rock layers whose susceptibility to weathering is strongly
contrasting. This failure occurs typically in alternating thin beds of hard and
soft rock e.g. sandstone and mudstone or siltstone. These formations are
characteristic of the Middle Siwalik rocks of Nepal
The cause is a combination of weathering of the soft rock layers and plane
failure of the hard rock layers. The soft rocks weather back from the face to
leave the hard rocks sticking out.
Eventually the hard rocks overhang so far that they break off along
vertical fractures. The process then starts again and the whole face retreats.
This mechanism is very common in Nepal
Debris is not rock, it
includes soil and terrace material. Soft rock may be rock that is naturally
soft e.g. mudstone, or hard rock that has become soft through weathering. Hard
rock has internal strength, which is much greater than the frictional strength
along its fracture planes.
Type of material
|
Cause of failure
|
Mechanism of failure
|
Debris
|
Erosion
|
Shear failure
|
Soft rock
|
Weathering
|
Plane or shear failure or
disintegration
|
Hard rock
|
Weathering
|
Plane failure
|
Alternating hard and soft rock
|
Weathering plus plane failure
|
Differential weathering
|
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