Geoscience Reference
In-Depth Information
LISS-III Satellite Image (2010), SRTMdata (2008) andGoogle earth image (2010) had
been incorporatedwith the surveyed landslide locations by thorough recti
cation and it
was then modi
ed and mapped accordingly (Fig.
2.1
).
6.3 Application of 1 Dimension Slope Stability Model
and Stability Analysis
With the help of derived geo-technical parameters i.e. cohesion, friction angle,
slope angle, unit weight of the soil, unit weight of water, soil depth, and saturation
index value from 50 landslide location points of the Shivkhola watershed the safety
factor values (FS) for dry, semi-saturated and saturated condition were being
estimated by applying the 1D slope Stability model (Eq.
6.4
). The safety factor
values were transformed into raster value domain on ARC GIS Platform. Finally,
the landslide susceptibility maps/safety factor distribution maps were prepared by
'
operation and then stability classes for each condition (dry, semi and
saturated condition) had been performed by studying the cumulative frequency and
their abrupt change points of the safety factor values (the instability threshold
boundaries). A 3
slicing
'
×
'
'
technique was also applied to all the prepared
safety factor distribution maps as a post-classi
3
majority
lter
cation
filter to reduce the high
frequency variation. Higher the value of
, greater is the propensity of slope
stability and vice versa. To assess the chances/probability of landslide phenomena
in each class to all the prepared maps under various conditions frequency ratio (FR)
was extracted by means of a ratio between landslide frequency/landslide events (%)
and landslide susceptibility area (%). FR value is approaching to 1 indicates equal
chances of landslide events, 0 indicates lesser chances and more than 1 shows
greater probability. Finally, an accuracy assessment was made after Congalton
(
1991
).
'
FS
'
6.4 Shear Stress, Shear Strength and Safety Factor
and Stability Analysis
Safety factor (FS) refers to the ratio between the shearing forces (simply known as
stress,
).
Increase in shearing stresses produces rapture within the underlying rock beds and
decrease the cohesive strength as well as the shearing strength due to which the slope
materials start to move downward and make the slope most vulnerable to land slip.
˄
) and resistance of the materials to shearing forces (shearing strength,
λ
¼
ðs
¼
r
1
r
3
aÞ
ðÞ
¼½
r
n
tan
u þ
sin
2
2
FS
ð
6
:
15
Þ
c
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