Landslides that occur in populated areas can damage property and infrastructure and trigger
secondary disasters that are difficult to deal with.
Valleys in mountainous regions have experienced accelerated economic development in
response to general population growth and associated demands for increased mining,
forestry, and agricultural activities.
Vulnerability to landslides is increased by:
● increasing population density
● use of marginal land
● rapid landuse change
● global warming
Elm Event, Switzerland, 1881
Became famous in landslide studies because the vents leading up to and accompanying failure
were well documented one eyewitness used a stopwatch to time the initial failure.
La Conchita, California, 1995 and 2005
Two massive landslides have occurred. Both of these landslides were caused by heavy rainfall
during the rainy season in the area.
1995 and 2005 disasters could have easily been prevented had the necessary and proper
geologic engineering work were done prior to the development of the area.
Haiyuan, Ningxia (Gansu), China, Earthquake and Landslides, 1920
Earthquake and landslides in China were responsible for 200,000 deaths due to the destruction
of homes (direct damage) and exposure to the harsh winter climate (indirect damage).
Other Mass Movement Events
Rockfall and Slide Events
Involve a smaller rockmass that disintegrates into numerous blocks that fall, bounce, and roll on
steep slopes after detachment.
Rockfalls are a major hazard for highways and railways through rocky terrain in Canada and,
though small, have resulted in numerous deaths in this century. However, rockfalls do not pose
the same level of economic risk as large scale failures which can and do close major
transpiration routes for days at a time.
Mudslide occurred in Vancouver in 2005: we live in a mountainous region with steep slopes,
areas where landslides occur more frequently than in others. Heavy rain and snowfall add to
the hazardous conditions.
Effects of gravity
Mass movement (mass wasting) refers to the downslope motion of soil and/or rock materials
under the influence of gravity.
Landslide is a general term for mass movements. Landslides occur when the gravitational force pulling the slope material down exceeds the slope
material's internal resistance to shear or failure. Materials move downslope by sliding or shearing
along a weak failure plane or a number of failure planes.
Gravity and Shear stress
Gravity is the main force responsible for landslides.
Gravity force: object pulling everything toward the centre of the Earth.
Gp: the component of gravity that acts perpendicular to the slope. It helps to hold an object or
slope materials in place on the slope.
Gt: the component of gravity that acts tangential to the slope surface or parallel to and down the
slope shear stress.
When the slope is steeper, the shear stress or tangential component of gravity, Gt increases,
and the perpendicular component of gravity, Gp decreases.
As shear stress pulls slope material downslope, forces resisting this movement will be induced.
These forces included frictional resistance and cohesion among the particles of the slope
material and are grouped under the term shear strength. We define shear strength as the
internal resistance of a body to shear stress or a material's internal shearing resistance.
By increasing friction and cohesion, shear strength increases.
Shear strength by a number of different factors that affect friction and cohesion:
● type of soil/rock
● condition of geologic materials
● presence of weak surfaces
● pore water pressure within the slope
The Factor of Safety, Fs
Slope failures occurs when the force of gravity parallel to the slope. Landslides occur when
shear stress at the time of failure is greater than the shear strength of the geologic material.
Fs = shear strength / shear stress
if) Fs > 1 : shear strength is greater than or equal to shear stress, the slope is stable
Fs 4.0 trigger landslides.
Where the Factor of Safety decreased sharply due to both an increase in the shear stress and a
decrease in the shear strength due to the effects of the earthquake.
Slope stability and anthropogenic activity
Humans can increase he likelihood of a landslide:
1. Excavation of a slope at its toe
2. Loading of a slope at its crest
3. Deforestation for construction and development, including clearcut logging on slopes
6. Water leakage from utilities
7. Artificial vibration
These (1 and 2) activities are commonly done during construction.
In residential construction, loading of the slope at its crest is one way to increase the size of a lot
by extending its backyard.
The Principles of Landslide Classification
TYPE OF MATERIAL
T B D E
y e e a p d b rt
e r ri h
o o s (
f c ( S
M k u o
o n rt
v s e
e o d
m rt S
e e o
n d il
t s )
F R D E
a o e a
ll c b rt
s k ri h
F s F
a F a
ll a ll
T R E
o o a
p c rt
p k h
l B B
e l l
s o o
S R D E
li o e a
d c b rt
e k ri h
s S s S
l S l
u li u
m d m
p e p