Introduction - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 40

Landslide often occurs when there is heavy rainfall for a long period. Siddle and
Ochiai (2006) identi ed four common unstable bedding sequences: (1) alternating

bedding of hard and soft rocks (2) highly altered and permeable regolith overlaying
relatively low permeable sub-strata, (3) thin soils overlaying bedrock or till, and (4)
hard caprock overlaying deeply weathered rocks. Besides, faults, lineaments or both
are usually recognized as the most important triggering factors of slope instability.
The relative strength of the regolith is strongly influenced by past tectonic setting as
well as contemporary weathering (Julian and Anthony 1996; El Khattabi and
Carlier 2004). According to Ibetsberger (1996), and Pachauri et al. (1998) the
neotectonics contribute to slope instability by fracturing, faulting, jointing and
deforming foliation structures.
Engineering, chemical and mineralogical properties of soil determine shear
stress and shear strength of the soil. Shear strength of the soil is an important
engineering property which governs the stability of natural and constructed hill
slope. The shear strength is described as the function of normal stress, cohesion and
angle of internal friction. The relationship within these properties to other attributes
of the soil has been given by Terzaghi and Peck ( 1976 ), Wu and Sangrey ( 1978 ),
and Fredlund and Rahardjo (1993). Clay minerals are another important chemical
weathering product of the soil and regolith. Yatsu ( 1966 ), Duzgoren-Aydin et al.
(2002) studied soil chemical properties and concluded that landslide susceptibility
and slide type are closely associated with specific clay minerals. Clay accumulation
within relict joints is also associated with landslides. Matsuura (1985), Shuzui
(2001), Zheng et al. (2002) and Wen et al. (2004) mentioned that clay mineralogy
and chemistry can also provide indicators of potential sliding plane conditions.
Geomorphic Factors of Slope Instability include slope angle, slope aspect,
height of slope, slope curvature etc. Slope gradient is sometime treated as an index
of slope instability and due to availability of Digital Elevation Model (DEM) it can
be numerically evaluated and depicted spatially (O

Neill and Mark 1987; Gao
1993). Siddle and Ochiai (2006) suggested that slope aspect strongly affects the
hydrologic processes by influencing the evapotranspiration process and thus affects
weathering and vegetation and root development especially in drier environments.
Churchill (1982), Gao (1993), Hylland and Lowe (1997) and Lan et al. (2004)
studied the increase in slope failure in relation to slope aspect characteristics.
Altitude and landslide are intimately related by virtue of other factors such as slope,
lithology, weathering, rainfall and land use. The strong statistical relationship
between elevation and landslide has been studied by Pachuri and Pant (1992),
Linebak Gritzner et al. (2001) and Die and Lee ( 2002 ).
Spatial and temporal distribution of rainfall, water recharge into soil, lateral and
vertical movement of water within the regolith, evapotranspiration and interception
are the important hydrologic attributes for the initiation of landslide in hilly area.
Spatial distribution of rainfall are closely associated with landslide initiation by
means of their influence for the generation of pore water pressure in unstable hill
slope and it was studied by Campbell (1966), So (1971), Starkel (1976), Siddle and
Swanston (1982), Siddle (1984), Iverson and Major (1987). Generally, researchers
consider

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total amount of rainfall, short-term intensity, antecedent storm