Unit 3: Landslides
Explain how the impact of landslides depends on 1) population density, 2) economic
infrastructure, and 3) population preparedness.
o As population density increases, increasing damage.
o Most fatalities from a slides are under-reported as they accompany major
Japan has the most deaths from landslides per year, but highest pop
density as well
o Largest landslides in Canada happen on West coast (here), between BC and
Alberta and in Quebec + Ontario
Explain why British Columbia has the highest frequency of landslides in Canada and
what we should expect as our population expands into the mountains.
o Mountainous terrain, lots of rain, complex geology (unconsolidated glacial
sediments) and many possible triggers
South BC: return period is 25 – 70 years for large slides
Distinguish between the 3 main failure modes (falls, flows, and slides) and how they
are influenced by geology.
o Falls: very fast.
Occur on very steep slopes (usually rock)
Material detaches because of weakness – falls very fast due to gravity
o Slides – cohesive block moves on a surface
Vary from slow to fast. Usually soil, rock or debris
Curved/bowl shaped surface: Rotational slide (slump)
Usually weak material (sediment). Characterized by
scarp above slide.
Flat/planar surface: Translational slide
Usually strong material. Cohesive block slides down.
o Flows – fluid motion (Chaotic)
Very fast – mudflows can be up to 80 km/h
Soil, mud, wet debris, rock. Water usually involved
o Complex movements (combination of above)
Categorize, identify, and name a variety of different landslides
o Landslides classified mainly by type of material and type of movement. Rate of
movement also important.
Materials: Rock, Soil/Earth, Mud, Debris
Define Angle of repose
o Steepest angle a slope can maintain without collapsing. Depends on material.
Assess the balance between the strength of the slope and the destabilizing forces
acting on it (Factor of Safety)
o Stability of a slope depends on
Driving forces: Gravity
Gravity is manifested as Shear stress “T”
Gravity force is parallel to the slope, moving downwards
Resisting forces (Shear strength) – “Tf
Friction – resistance to sliding Cohesion – material holding together
Normal stress – perpendicular to slope
o Factor of Safety (Fs) = (Tf/T) = (Shear strength / Shear Stress) = (Strength /
FS >> 1, stable slope
FS < 1, unstable – failure may occur
At angle of repose, FoS = 1 or just above.
Compare and contrast landslide causes and landslide triggers.
o Causes – usually long term factors that lead to instability of slope
Reduce shear strength of a slope
o Triggers – translate instability into motion. Usually short term events.
o There can be many causes, but only one trigger.
List and describe several external causes of landslides
o Factors outside of slope that affect stability
o High slope angle
o Undercutting of a slope (removal of bottom) by roads, rivers, buildings
o Overloading a slope with excess weight
o Vegetation is weird…