Introduction - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 25

Biological force is being generated when plant root system push materials side ward
and if this occurs on a slope a down slope movement may take place. Gray ( 1970 ),
Brown and Sheu ( 1975 ), Greenway (1987) and Yim et al. ( 1988 ) studied the rela-
tionship between slope stability and vegetation.
Earth materials are always subject to stress and strain where a stress is a force
that tends to move materials down slope. All the mentioned forces produce stress
within a body. The stress of a body of soil on slope depends upon the mass of the
soil body, m and the slope angle,

. Slope
materials possess an inherent resistance against downslope movement of the
materials. Friction force acts against gravity and resist movement. The downslope
movement of a soil body can happen when the applied stress is large enough to
overcome the maximum frictional resistance. Friction is expressed as co-ef

(theta), expressed as, Stress = m sin

Ѳ Ѳ

cient,

μ
which is equivalent to the angle at which sliding starts and called as angle of sliding
friction (Hugget 2007 ). Frictional resistance is a function of both the inherent
frictional properties of slope materials and the normal stress acting upon them. As
the shear plane angle becomes steeper, the shear stress becomes larger and the
normal stress smaller. The slope angle attained after the slope failure is known as
the threshold angle of stability.
Cohesion is one of the most important factors of mass movement that affects the
internal strength of the slope materials. It includes the chemical bonding of rocks and
soil particles. The chemical bonding of rocks and soils occur with the presence of
cements composed of silica, carbonates or iron oxides. Cohesion also rises through
capillary suction of water in pores, compaction, and plant root systems. Soil particles
affect cohesion of a soil body and generate friction between one another, called as the
internal friction or shearing resistance which is determined by particle size and
shape and the degree to particles touch to each other. Mohr-Coulomb equation
de

nes the shear stress that a body of soil on a slope can withstand before it moves.
This equation shows the shear strength as effective normal stress and cohesion,
referred to as the Coulomb-Terzaghi shear strength equation (Summer eld 1991 ).

The total shear strength of slope materials (s) is given as,

s ¼ c þ r tan u; ð 1 : 1 Þ

where c is cohesion, ˃ is the effective normal stress and tan ˆ represents the

coef cient of plane sliding friction.

The slope stability of the hill slope can be expressed in terms of the relationship
between the forces tending to drive the slope materials and the forces tending to
resist driving stresses. It is clear that the movement will starts when driving force
exceeds the resisting force and this relationship is represented as the safety factor
(FS). The slope can exist in three states: i. where shear strength is larger than shear
stress, the slope is described as stable slope (FS > 1.3), ii. Where shear stress
exceeds the shear strength (FS < 1.00), the slope is described as actively unstable
slope. Sometimes the shear strength can vary over time with the interaction of
climatic phenomena and where the third stability category becomes prominent such