Slope Stability Model and Landslide Susceptibility Using Geo-technical Properties of Soil - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 172

be calculated on the basis of available rainfall data (De Smedt 2005 ). On the basis
of this assumption, wetness index equation can easily be derived and it is possible
to see the effect of few days

consecutive rainfall in 1 day, if the soil is half
saturated. In the present study wetness index (m) value of 0.00, 0.50 and 1.00 under
dry, half-saturated and full-saturated conditions were taken into account.

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6.2.5 Depth of Failure Surface/Depth (z) of Soil Below
the Terrain Surface

Repeated and continued field studies for long duration were made for the proper

cognition of the processes and their interaction. The depth of the failure surface was
measured by holding a measuring tape at both the margins of scar and the other tape
was allowed to hang, the reading was then taken from the base of the hanging tape.
The margin of the scars was surveyed by prismatic compass. The intensive survey
of the sliding scar for 40 different landslide locations was carried on by Abney s

level at 0.5 m interval along radial lines originating from lower most part of the
scar. The altitude of the points at 0.5 m interval along the radial lines was then
estimated using Sine rule in reference to the central base point of known altitude
determined by GPS (Basu and Maiti 2001 ). The total thickness of soil and that of
saturated soil for 10 sites during monsoon were measured from slope cutting. After
estimating the approximate depth of all known points, a soil depth map (z/D) was
prepared using Arc GIS tool (see Chap. 2 , Fig. 2.13).

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6.2.6 Soil Density ( ʳ s ) and Density of Water ( ʳ w )

Speci c unit weight of water and unit weight of the soil were estimated by

examining the soil samples collected from 50 landslide locations during field

investigation. The density of soil and water varies from place to place due to in situ
geo-hydrologic condition (Fig. 2.16 ). The saturated soil density was also consulted
and adopted from the field experiences done by Deoja ( 1991 ).

6.2.7 Identi cation of Major Landslide Location/Landslide

Inventory Map

To determine the frequency ratio (FR) and to assess the overall classi cation accuracy,

themajor landslide locationof theShivkholaWatershed(Chap. 2 , Fig. 2.1)wasdetected
by intensive

field investigation with GPS, clinometers, and Abney ' s level. Besides,