Introduction - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 34

(c) Gelifluction of clays
(d) Other periglacial processes
(e) Rebound
(f) Non-uniform swelling.

B. Pore-water Pressure [effects on µ ]

1. Short-term (undrained)- no equalization of excess pore water pressure set up
by the change in total stress.
2. Intermediate- partial equalization of excess pore-water pressures. Delayed
failures of cutting in stiff clays fall in this category.
3. Long-term (drained)- complete equalization of pore-water pressures to
steady seepage values.

The several types of landslides are being observed in the Darjiling Himalaya.
The fall of boulders from the steep slopes is the simplest form of landslide. Such
rock body is separated from the bedrock. Second type is the sliding of rock masses
which is quite frequent in the Tista Valley between Sivok and Kali Jhora, where the
hills consist of interbedded sandstones and shales inclined at high angles in the
same direction as the hill slopes. The scouring of underlying bands of soft shales by
rain-water causes the overlying sandstones to slip and slide down the hillsides. The
third type is the soil slip, caused by slow downward movement of soil or uncon-
solidated materials along unprotected hill slopes. This happen frequently in the
vicinity of the Hill Cart Road, between Mahanadi and Rangdong, where sections of
the road sink from a few inches to several feet. In the fourth type, slow downward
movements of soil sometimes causes violent landslip. In Darjiling Himalaya, the
soil slips are of small magnitude in regard to

. But the
debris slips are of greater magnitude and are devastating in nature in Darjiling
Himalaya. Major rock slips were not observed in Darjiling, where true soil slip and
debris slips are common. The shearing resistance in slopes is made up of a com-
bination of internal friction and cohesion. In clay type soil, the angle of internal
friction is not more than 2

' length ' and ' affected height '

and with the internal friction alone, soil cannot be
stable. But due to cohesion, for some heights a soil slope is stable at very steep
angles. The angle of internal friction in rock debris which is sometimes 30

° or 3 °

is
known as angle of repose. A slope will safely stand to a great height with an
inclination slightly flatter than the angle of repose. In slopes formed of rock, slip
occurs along certain planes of weakness like joints, bedding planes etc.

° - 40 °

1.7 Objectives of the Present Work

The main objective of the study is to estimate, quantify, and analyze various
landslide inducing parameters of land, water and soil and to prepare landslide
susceptibility map on a river basin scale which will be very much helpful in
planning, designing and implementing the development programmes. Without a