Introduction - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 26

as conditionally stable slope (FS = 1.00 1.30). Basically the triggering factors

transform the slope from a conditionally stable to an actively unstable slope.
The safety factor (FS) is de

-

ned as:

s
s

FS ¼ ð 1 : 2 Þ

where s is the total shear strength along a speci c shear plane, and ˄ is the total

amount of shear stress developed along this plane.
For the shallow translational slide FS is described as:

þ ðc

2

b Þ tan u

z cos u

FS ¼ c ð 1 : 3 Þ

c z sin b cos b

where c is cohesion, ʳ is the unit weight of regolith, z is the vertical depth to the

shear plane, ʲ is the angle of the shear plane, u is pore-water pressure and ˆ is the

angle of internal friction.
The stability condition of the hillslope varies from place to place as a result of
the variation in prevalence of landslide triggering factors. It is inferred that some
factors are responsible for increasing shearing stress and some factors for reducing
shearing strength. The changes of both shear stress and shear strength are major
contributing factors against slope instability. Varnes ( 1978 ), introduced several
factors contributing the increase in shear stress and the reduction of shear strength
that is stated below.

A. Factors contributing the increase in shear stress:

1. Removal of lateral support: erosion by rivers and glaciers, wave action,
faulting, previous rock fall or slide.
2. Removal of underlying support: undercutting by rivers, waves, sub-surface
solution, and loss of strength by extrusion of underlying sediments.
3. Loading of slope: weight of water, vegetation and accumulation of debris.
4. Lateral pressure: water in cracks, freezing in cracks, swelling and pressure
release.
5. Transient stress: earthquake and movement of tress.

B. Factors contributing the reduction of shear strength:

1. Weathering: disintegration and decomposition of granular rocks, hydration
of clay minerals, dissolution of cementing materials in rocks or soils.
2. Changes in pore-water pressure: saturation and softening of weathered
materials.
3. Changes of structure: creation of

fissures in shales and clays, remoulding of

sands and sensitive clays.
4. Organic impact: burrowing of animals and decay of tree roots.