Geoscience Reference
In-Depth Information
interlocking or bridging effects. For sand, however, the mineralogical content
seems to make little difference except if the sand contains mica. In that case the
void ratio is usually larger, thereby resulting in loose interlocking and lower
friction angle.
Particle size distribution of the soil: The soil having the well-graded size will
have higher frictional angle than the soil having poorly graded soil.
Cohesion (C) is the attraction of particles to each other which is not directly
governed by a friction law but does provide a measure of strength of a material.
Thus sands do not exhibit cohesion, while soil which contains clay show cohesion.
It can be measured, as in soil mechanics, by the Mohr-Coulomb Equation
(Eq.
6.10
).
6.2.3 Surface Inclination/Slope (
ʲ
)
Slope gradients are sometimes considered as an index of slope instability, and
because of the availability of a digital elevation model (DEM), slope can be
numerically evaluated and depicted spatially (O
'
Neill and Mark
1987
; Gao
1993
).
Firstly, the contour map at 20 m interval was prepared and digitized from the
topographical map 73B/8 (1987) at the scale of 1:50,000 and subsequently used for
generating Digital Elevation Model on ARC GIS platform. Then slope gradient
map (Fig.
2.6
) was extracted from DEM and it was the classi
ed after Anbalagan
(
1992
) and Dhakal et al. (
2000
) into ten equal classes.
6.2.4 Soil Saturation Index/Wetness Index (m)
Simple models have been developed for estimating the soil saturation of the
mountainous region as the wetness index is de
ned in TOPMODEL by Beven and
Kirkby.
a
tan
h
m
¼
1n
ð
6
:
14
Þ
where a, is the contributing area per unit contour length and
⊝
is the slope of the
pixel.
More acceptable soil saturation model was applied by Montgomery and Dietrich
(
1994
), Borga et al. (
1998
) and Pack et al. (
1998
). The model envisages that the soil
saturation index can be determined with the help of topography, soil type, and
rainfall intensity of the area under study. But in practical sense, the soil is not
completely dry or fully saturated in the area, therefore it can be imagined that the
soil is half saturated. The soil saturation index is either
fixed for stationary scenarios
i.e. dry, semi-saturated and full saturated soils, given by m = 0, 0.5 and 1.00 or can
Search WWH ::
Custom Search