why does BC have so many landslides?
return period of 25-70 years
heavy rain
types of landslide material
rock
soil/earth
mud
debris
types of motion
falls: steep slopes, very fast
slides: cohesive block moves on failure surface, v-slow to fast, soil rock or debris
flows: fluid motion, v-slow to v-fast, soil mud or wet debris
combo movements
types of slides
rotational slide: curved failure surface
translational slide: flat failure surface
rates of movement
slow -> creep, soil flow, earth flow
angle of repose definition
when shear stress is exactly balanced with shear strength
when factor of safety is exactly one
factor of safety
shear strength/shear stress
shear stress driving force
gravity
shear strength resisting forces
friction & cohesion
shear strength of different materials
igneous or metamorphic: high
sedimentary: moderate
sediment: low
external causes of landslides
- slope angle
- undercutting
- overloading
- vegetation (too much can overburden slope, removing roots can make slope unstable)
- high amount of rainfall
internal causes
- water (adds weight & increases weathering, too much decreases cohesion, some water is good)
- inherently weak materials (volcanic rock, quick clay ~ house of cards structure)
- bad geologic structures
triggers
- earthquakes
- heavy rainfall
- snow melt
- volcanic eruptions
- skiing
- vehicles
oso landslide case study
causes: 45 days of heavy rain, overburdening
trigger: earthquake
type: rotational rockslide
avoidance
very expensive, involves moving whole towns and homes
prevention
- removing material, simple but expensive
- undercut slope -> build a wall
- anchor cables in stable rocks
- too much water -> drain slope
protection
- rock barrier
- rock net
- rock fences
indications of slope failure
- cracking of house elements
- cracks on slope
- bulges on slope
- leaning trees & utility posts
