Geomorphic Threshold and Landslide - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 145

geomorphic threshold and the shear stress to increase over shear strength. Some-
times, moderate levels of rainfall for few days in the basin become more critical
because of weak lithological composition. Moreover, the concentration of human
settlement in Tindharia, Gayabari and Shiviter generates enormous pressure on
slope materials and favours threshold values by reducing the shearing strength. The
study shows that only 105.88 and 88.928 mm daily rainfall is the critical rain for
initiation of slide at Tindharia and Lower Paglajhora respectively. So there is every
possibility for the generation of geomorphic threshold and initiation of slide due to
hydrologic factor. In this way, there is a frequent occurrence of debris slide which
reduces the slope angle on landslide scar face to that of repose angle to attain
temporary stability through internal feed back in a process of homeostatic

adjustment.
Study involves the measurement of upslope contributing area, contour length,
slope angle, transmissivity, depth of soil, depth of the saturated soil, density of water,
wet soil density, unit weight of the materials, pore-water pressure, cohesion, and
angle of repose for the determination of

(Schumm 1977a , b )
condition such as critical slope angle, rainfall and slope height beyond which slope
materials may undergo chemical decomposition and thereby lose its former strength
and the slope may collapse without an extrinsic type of threshold being crossed. The
response to threshold crossing may induce dramatic erosion and striking changes of
the concerned landforms which is shaped primarily by the disturbances rather than by
normal events. The recoveries of such disturbances are often a long and slow process
which is mainly accomplished through Self-organized Feed Back Mechanism in the
geomorphic system.

' intrinsic threshold '

5.2 Materials and Methods

5.2.1 Angle of Internal Friction/Angle of Repose
and Cohesion

The angle of repose is the angle at which the debris attains temporary stability on
slope is very essential to estimate as the debris slope attains instability. The
geo-technical factor like angle of repose of the debris is measured after Bloom
( 1991 ). The tangent of angle of repose of dry granular materials is slightly greater
than, but approximately equals to the co-ef

cient of sliding friction of the material

or its mass friction ( ) (Van Burkalow 1945 ; Bloom 1991 ). The cohesion and angle

of internal friction is measured by tri-axial compression test (Fig. 5.1 ) following
Mohr Stress Diagram (Fig. 5.2 ).
The saturated conductivity of the sail varies from 10 − 2 ms − 1 for the soil depth
less than 0.5 m

u

10 − 5 ms − 1 for soil depth between 1 and 2 m (Fenti 1992 ). Based on
these and other data (Metteotti 1996 ) estimated the transmissivity (T) of saturated
soil to lie between 5 and 30 m − 2 day − 1 , with a mean value of 15 m − 2 day − 1 .

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