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Lecture 15 - Types of Landslides (Mass Movement Classification (Flows -…
Lecture 15 - Types of Landslides
Mass movements
Mass movement
= movement of a large volume of material (e.g. rock, mud, soil, snow, ice, lava, ash) downslope under the influence of
gravity
Landslides
= when the material is composed of rock, mud, or soil
Other types of mass movements - snow avalanches and lava flows, pyroclastic flows, and lahars
Landslides killed fewer than earthquakes/tsunamis, however there's many more deadly landslides
2004-2010, ~2,600 individual landslides killed ~32,000 people globally (excluding those triggered by earthquakes)
Examples
Wenchuan earthquake landslides (2008): 20,000 killed
Vargas (Venezuela) debris flows (1999): ~30,000 killed
Hurricane Mitch landslides (1998): ~10,000 killed
Still major hazard to Canada, even though last major disaster occurred in 1970s
Western provinces (BC, AB, Yukon) are most prone to mass movements
subduction zone, earthquake zone, topography
Brazeau Lake landslide, AB. Topography steepened by combination of tectonics & glaciation
Conditions of slope instability
Gravitational forces
acting on material on or within sloping topography can be separated into…
Component parallel to slope
- acts to drag/pull material downslope
Component perpendicular to slope
- acts to keep material in place
If the force parallel to slope
exceeds
frictional resistance, then the material will move downhill
Slope instability/Lansliding is increased/promoted by:
(1) Increasing the slope angle, increasing forces
parallel
to slope & reducing forces
perpendicular
to slope
(2) Adding to the mass of material, e.g. saturating it with water, increases forces
parallel
to slope
(3) Reducing
coefficient of friction
on the plane of slip, or reducing
internal strength
(cohesiveness) of the rock
Precondition vs. Triggers
Preconditions
= bring a slope to the brink of failure
Triggers
= push it over the edge
Examples
E.g. 2008 Wenchuan earthquake, China
o Preconditions? Tectonic uplift & erosion by rivers that steepened the slopes
o Triggers? Shaking of the earthquake
E.g. 2007 Pe Ell landslide, WA
o Preconditions? Deforestation/logging activities, rivers
• River incision steepened slopes, geology (glacial till) & logging activity weakened them
o Triggers? Heavy rainfall preceded landslide
E.g. 2017 Montecito landslide, CA
o Preconditions? Mountain lifted by tectonics, forest fire
• Tectonics steepened slopes, wildfires weakened them
o Triggers? Heavy rainfall preceded landslide
The role of geology
It determine slope instability, since different rock types have different strengths
Hillslope determined by
pre-existing geological conditions
Sedimentary rocks contain bedding planes along which sliding may preferentially occur
The orientation of bedding planes within a hillside may create a strong stable condition, where weakness are angled into the slope, or a weak unstable condition, where weaknesses parallel the slope
Tectonics generates topography & steepens slopes, & ground shaking in large earthquakes that dislodges materials from hillslopes
The role of water
(1)
Clays
are platy "sheet silicate" minerals composed of layers of “sheet silicates” that are common in near-surface rocks
expand when wet, as water molecules are absorbed between mineral layers, causing clay expand
increase separation & lubrication allow layers to slide past each other more easily, greatly reduce rock strength
Clays are by-products of grinding action of ice in bedrock;
glacial till
= sediments left behind by glaciers
(2)
Soils & sediments rocks
contain pore spaces between grains
The porosity (volume of pores + total volume) is 10-30% of the volume of most sedimentary rocks, & up to 50% of the volume in soils
If replace air with water in pore spaces (making it water-saturated), it greatly increasing its weight
reduce stability of sediments and internal strength (as water is incompressible)
(3)
Groundwater
(water flowing through rocks) can greatly reduce rock strength by dissolving minerals that normally bind the rock together (
cement
) or by physically eroding material, forming caves
Cement help glue grains together, water can dissolve the cement chemically & reduce rock strength
(4) Water is unusual in expanding when it freezes (which is why ice floats on water - it is less dense); Repeating melting & freezing cycle leads to
freeze-thaw weathering
or
"frost shattering"
When water gets into cracks in a rock, & freezes, this expansion exerts pressure on the rock & forces the crack apart
Mass Movement Classification
General classification
(1)
Material being moved
: rock, mud, earth, debris, snow, volcanic materials
(2)
Type of movement
: fall, avalanche (composite of mass movement: start with fall, then flows), slide, creep (moving slowly)
Further classification
water content
&
geology
which determines the
speed
of the mass movement
Flows
- materials becomes
fluidized
, flow over landscape, move as very viscous fluids, turbulence within moving mass
Earth flow
(soil)
Debris flow
(fragmented rock)
Mud flows
(wet mud)
Lahars
(wet volcano ash)
Slide
(movement is downward & outward along a failure plane)
Blockslide
- a block of intact rock - moves coherently (together tightly)
Rock slides
- rock breaks up as it slides
The material above the failure surface is divided into
driving
mass
&
resisting
mass
Many have a distinctive geomorphology with a
head scarp
&
toe
Rotational slides
- above curved (concave upward) failure planes
• Movement is rotating around axis parallel to the slope
• Move short distances: any motion reduces driving mass & increases resisting mass
• Less dangerous
Translational slides
- above planar failure surfaces
• Have no such relations & can travel much further
Falls
(movement is mostly downward)
Rock fall
- free fall of a block from a free face, following opening of a fracture
Free-fall, dominantly vertical downward movement, move as separate blocks
Soil Creep
- the slowest but most common slope failure, caused by cyclic expansion & contraction of the soil due to
freezing/thawing
,
wetting/drying
of clay minerals, or heating/cooling
Expansion acts in the direction perpendicular to the hill slope, whereas contraction under gravity is vertical. This results in a slow, downward creep
Sturzstrom
comprises debris which has a great horizontal runout compare to it vertical drop (ratio can be 20-30)
means "fall stream" in German
Rocks breakup & travelled in compressed air, lubricate & allow to slide further
Avalanches
Rock avalanches
- start as rock falls then morph into debris slides, when they hit steep scree slopes below & the intact rock disintegrates
Composite landslide - start with one landslide then becomes the other