Introduction - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 24

geotectonic factors like angle of repose, static and dynamic friction of particles,
interlocking and sorting of grains (Brudsen 1979 ; Jumikis 1967; Carson 1977 ),
thickness of total soil and that of saturated soil (Borga et al. 1998 ) are considered as
important in the analysis of slope failure.
The hydrologic factors like daily rainfall threshold in connection with slope
angle and regolith thickness (Gabet et al. 2004 ), rainfall intensity, in ltration etc.

are practiced in the analysis of slope instability. The geographical factors including
the anthropogenic actions for slope instability in Himalayan slopes are widely
studied by Basu and Sarkar (1985, 1988), Basu and Ghatowar ( 1988 ). The slope
failure in the upper catchments and its subsequent effects on fluvial dynamics are
discussed considering hill and foot hill as an interactive whole and the operating
geomorphic processes being interconnected (Basu and Ghatowar 1986, 1988 , 1990;
Basu 1989 ; Basu and Ghosh 1993). The anthropogenic processes like unscienti

c
use of slope for agriculture, mining (Basu and Ghatowar 1988 ; Basu and Maity
2001 ; Bhattacharya 1999 ) and deforestation (Bhattacharya 1996 ) etc. are widely
studied in relation to the slope instability. Numerous models in connection to the
slope stability, shallow and deep seated landslides are introduced and veri ed by

Hollingworth and Kovacs (1981), Burton and Bathrust (1994), Bradinoni and
Church ( 2004 ), Young ( 1963 ) and Montgomery et al. ( 1994 ). The stability of
natural slopes are examined and analyzed by Skempton and Hutchinson ( 1969 ).
The interaction among those processes are not always possible to express in
mathematical language and so an attempt is made for the establishment of empirical
and verbal models both to express the possible interactions which partially support
the governing processes of hill slope evolution following Ahnert ( 1970 ), Kirkby
( 1980 ), Poesen (1985), and Deploey (1982). The understanding of probable inter-
action among the major factors helps in the assessment of processes and in their
management for the restoration of slope to ensure optimum utility from land and
mineral resources.
A body will not move unless a force is applied. In geomorphic processes, gravi-
tational force, water pressure force, expansion force and biological force play a
signi

cant role in changing the shearing strength of slope materials and in moving the
earth materials down slope. We can say, the forces that drive sediments largely derive
from the gravity, the climatic effect and the action of plants and animals. Gravita-
tional force acts directly on rock bodies, sediments, water and ice and tending to make
them move. It acts world over at a nearly uniform magnitude of 9.81 m/s 2 , with slight
variation resulting from distance from the Earth s centre and latitude. The presence of

water within rock-soil debris produces various forces that can drive them downward.
The forces developed by the presence of water are called as water-pressure force.
Attewell and Farmer ( 1976 ) pointed out that when the pore spaces are

'

filled up, a
pore-water pressure is generated. This situation can reduce the pressure of contact of
the grain. The pore water pressure

υ

at depth h below the water table is given by

υ ʳ ʳ

is the density of water. The pore water pressure acts in all direction and
it exerts uplift or buoyancy effect and that can produce slope instability. Sometimes
expansion forces expand and contract sediment, soil and rock body by changing
physical and chemical properties of the minerals and cause slope movement.

= h where