Introduction - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 42

controlled by the sliding mass resulting from large potentially weak surface around
the boundary of the sliding mass, eventually leading to rapid movement on the
plane with loss of cohesion and friction. This kind of landslide is associated with
long-term relief fracture, weathering disintegration, erosion and washout. Zaruba
et al. ( 1969 ) identi

ed three main controlling factors of slope instability: (i) slope
gradient, (ii) slope consolidation and (iii) presence of water. There are four different
stages of landslides (Coppola et al. 2006 ; Chowdhury et al. 2000 ):

(a) Pre-failure stage: the soil mass is continuous and is mostly controlled by
progressive failure and creep.
(b) Failure stage: characterized by the formation of shear surface through the
entire rock-soil mass.
(c) Post-failure stage: includes the movement of the rock-soil mass involved in
landsliding.
(d) Reactivation stage: occurs when soil-rock mass slides along one or several
pre-existing shear surface.

The landslide in the mountainous slope is being governed by resisting force or
shear strength and driving force or shear stress which is explained by means of
ratio, i.e. resisting forces/driving forces, a dimensionless value, known as Safety
Factor (FS). If the safety factor value is less than or equal to 1, the slope will fail
because driving forces will equal or exceed the resisting force. During rainy reason,
the driving force becomes maximum and resisting force becomes minimum and so
landslides are quite common in this season in the concerned study area as a result of
reduction of soil cohesion in response to soil wetting, increase of soil weight
resulting from water absorption and decrease in effective stress derived from pore-
water pressure.
The character and mechanism of slope failure and the processes responsible for it
along with their control mechanism were studied in details by Terzaghi ( 1950 ),
Skempton and Hutchinson ( 1969 ), Eden (1970), Brudsen ( 1979 ), Zaruba and Mencl
( 1982 ) etc. The mechanism of slope failure and related soil loss as a result of rain as
well as gradient was studied by Ghosh (1950), Dutta (1966), Starkel ( 1972 ),
Onodera et al. (1974), Weichmier and Smith (1978), Morgan (1986), Borga et al.
( 1998 ) etc. Triggering mechanism of slope instability is caused due to rise of
ground water level that saturates the soil and increase the pore water pressure.
Simple models have been developed for estimating the soil saturation of the
mountainous region as the wetness index was de

ned in TOPMODEL by Beven
and Kirkby ( 1979 ). More acceptable soil saturation model was applied by Mont-
gomery and Dietrich ( 1994 ), Borga et al. ( 1998 ) and Pack and Tarboton ( 1998 ).
Such model considers the ground water condition and its flow and rainfall intensity.
In the present study, threshold rainfall and threshold slope angle have been
considered as triggering mechanism to illustrate site speci c slope instability that is

described in detail in Chap. 6 . An One Dimensional (1-D) slope stability model has
been summarized in Chap. 7 to explain the distribution of instability by estimating
the safety factor value (FS) from 50 locations considering cohesion, angle of
internal friction, slope angle, saturation index, soil-water density and depth of the