Introduction - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 2

slope movement types. The continuous downward movement of weathered rock
materials involving large blocks, earthen materials from steep slopes or cliffed valley
sides of streams under the impact of gravity, termed as fall. According to Selby
( 1982 ) rocks falls are relatively small landslide caused by the removal of individual
and super

cial blocks from cliff base and are promoted by hydrofracturing, stress
release, the wedging action of tree roots and other weathering process. Bloom ( 1978 )
treated fall as one of “ the distinct landslide process but it is rarely independent of

subsequent events . This type of landslide is common in humid region. Slide is one

of the most common forms of failure and can be subdivided into translational and
rotational slides. Rotational slides are sometimes called slumps because they move
with rotation. Translational slides have a planar, or two dimensional surface of
rupture. Slides are most common when the toe of the slope is undercut. They have a
moderate rate of movement and the coherence of material is retained, moving largely
intact or in broken pieces. The accounts of various translational slides in the
Appalachians are cited by Jacobson et al. ( 1989 ), in Puerto Rico by Simon et al.
( 1990 ), in New Zealand by Salter et al. ( 1981 ), and in California by Ellen and
Wieczorek ( 1988 ). Flow is the most destructive and turbulent form of landslide.
Flows have a high water content which causes the slope material to lose cohesion,
turning it into slurry. They are channeled by the landscape and move rapidly.
Hutchinson and Bhandari ( 1971 ) studied this type of landslide and suggested that
flows may be promoted by the collapse of soil from the surrounding cliffs, steep
slopes on to the upper part of the concave moving mass and raising pore-water
pressure in the debris. Spread is characterized by the gradual lateral displacement of
large volumes of distributed material over very gentle or flat terrain. Failure is caused
by liquefaction which is the process when saturated loose sediment with little or no
cohesion such as sands or silts are transformed into a liquid-like state. This process is
triggered by rapid ground motion most commonly during earthquakes. Nearly all
known examples come from southern Norway, the St. Lawrence lowlands of eastern
Canada and the Alaska Coast. Topple is characterized by the tilting of rock without
collapse, or by the forward rotation of rocks about a pivot point. Topples have a rapid
rate of movement and failure is generally influenced by the fracture pattern in rock.
Material descends by abrupt falling, sliding, bouncing and rolling. Varnes ( 1978 )
observed various states of activities of landslide with time such as active, dormant,
reactivated, suspended, abandoned, stabilized and relict. All these characteristics of
landslide vary from one place to other with varying geological, morphological and
physical attribute of topography. A landslide should have some nomenclatures such
as crown, main scarp, top, head, main body, foot, toe, failure surface, toe of failure
surface, surface of separation, displaced mass, zone of depletion, zone of accumu-
lation, depletion, accumulation, flank and pre-failure topography. But the entire
landslide does not possess all of these nomenclatures. The failure of the slope
happens when gravity exceeds the strength of the earth materials. Although the
action of gravity is the primary driving force for a landslide to occur, there are other
contributing factors affecting the original slope stability. Typically, pre-conditional
factors build up speci

”

c sub-surface conditions that make the area/slope prone to
failure, whereas the actual landslide often requires a trigger before being released.