Geoscience Reference
In-Depth Information
Table 5.1 Calculation of threshold angle
Melnikov and Chenso-
kov (
1969
)
Borga et al. (
1998
)
Carson (
1977a
,
b
,
c
)
z cos
2
(W cos
ʸ
tan
u
)/W
tan
)
(Introducing pore water pressure
in saturated condition)
ʱ
ʱ
= (1-u/
γ
ʱ
) tan
u
P
s
P
w
1
h
z
¼
tan
h
1
tan
u
sin
ʸ
>1
(Dry condition)
h
or,
u h
(Dry condition)
tan
—
Thickness of total soil
= Threshold angle of failure
u = Pore water pressure on potential
sliding surface
z = Depth of potential shear plane
γ
—
z
Thickness of saturated
φ
= Co-ef
cient
soil
Ps
of friction
u
—
Wet soil density
= Angle of repose
W = Weight of soil
ʸ
—
Pw
density of water
= Bulk unit weight of the sliding materials
u=
u—
Angle of repose
γ
w
z cos
2
= Slope on scar face
ʱ
)(
γ
w
= Unit weight
ʸ—
Slope on scar face
of soil water)
Cohesion is ignored in the long term due to
weathering and unloading processes
u = 133.75 g/cm
2
5.2.3 Determination of Threshold Rainfall to Initiate Slide
Campbell (
1975
), Caine (
1980
), Larsen and Simon (
1993
) established that the
empirical threshold condition to initiate landslide refers to relational value based on
statistical analysis of the relationship between rainfall and landslide occurrences
where as the physical thresholds are usually determined with the help of hydrologic
and stability models that take into consideration of various attributes such as slope
(b), upslope contributing area (a), transmissivity (T), wet soil density (p
s
), density
of water (p
w
), slope angle (
), relation between rainfall
and pore-water pressure etc. In the absolutely unstable condition the role of rain-
water to initiate the threshold for sliding could be determined. If the hydrological
factors like rain fall and seepage flow are considered the threshold condition for
absolute instability can be predicted. The critical rainfall was derived (cr) after
Borga et al. (
1998
) (Eq.
5.10
).
ʸ
), angle of internal friction (
u
b
p
s
ap
w
:
tan
h
r
cr
¼
T
:
sin
h
1
ð
5
:
10
Þ
tan
u
5.2.3.1 Estimation of Average Catastrophic Rainfall to Obtain Return
Period and Probabilistic Recurrence Interval of the Critical
Rain
The Selim Hill Tea Estate situated 250 m North West of Tindharia registered 52 days
having more than the critical rain fall to initiate threshold condition during the years
2005
2010 (Table E.1, Appendix E). Average day wise rainfall for catastrophic days
in 2005, 2006, 2007, 2008, 2009, and 2010, are 120.7, 127, 128.5, 161.12, 141.53
-
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