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Introduction
Mass wasting - The downslope movement of
material resulting from the force of gravity.
Chapter 11
Mass wasting results when the force of gravity
acting on a slope exceeds the strength of that slope.
Mass Wasting
Introduction
Deadly Landslides
Mass wasting often results in the loss of life and
property.
Landslides are the most well-known examples of
mass wasting.
Factors That Influence Mass Wasting
Shear strength - Forces that help maintain
slope stability
January 2011: Flooding and landslides in
southeast Brazil kill more than 800 people and
leave at least 14,000 homeless.
Landslides kill about 25-50 Americans per year
and cause more than $2 billion damage in the US
annually.
 Forces
The material’s strength and cohesion
 Internal friction between grains
 Any external support of the slope
Factors That Influence Mass Wasting
Opposing a slope’s shear strength is the
force of gravity.
Factors That Influence Mass Wasting
Gravity
Acts vertically
Steeper slopes are generally less stable.
Angle of repose is the steepest angle that a
slope can maintain without collapsing; dry sand
is ~30o
Whenever the gravitational force acting on a slope
exceeds the slope’s shear strength, slope failure
and thus mass movement occur.
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Factors That Influence Mass Wasting
Factors That Influence Mass Wasting
What causes mass wasting?
Slopes are in dynamic equilibrium.
Slopes adjust to changing conditions.
If a highway is built on a slope, the equilibrium of
the slope is affected.
Slope may undergo mass wasting to reestablish
equilibrium.
Factors That Influence Mass Wasting
 What causes mass wasting?
 Usually more than one factor is involved in the failure
of a slope.
 Usually more than one factor is involved in the failure of a
slope.
 Factors affecting mass wasting include:
 Slope Angle
 Slope angle is probably the major cause of slope failure.
 Generally the steeper the slope, the less stable it is and the
more susceptible it is to failure.
Factors That Influence Mass Wasting
What causes mass wasting?
 Usually more than one factor is involved in the failure of
a slope.
Weathering and Climate
Highway excavation
disturbs the equilibrium
of a slope and can
result in mass wasting.
Mass wasting is more likely to occur in
loose ore poorly consolidated slope material
than in bedrock.
Water Content
Large quantities of water from melting snow
or heavy storms increase the likelihood of
slope failure.
Factors That Influence Mass Wasting
Factors That Influence Mass Wasting
 What causes mass wasting?
What causes mass wasting?
Water content is an important factor in slope
stability at Point Fermin, California.
Vegetation
 Usually more than one factor is involved in the failure
of a slope.
When fine-grained sediments become wet, they
become slippery and slopes fail.
 Usually more than one factor is involved in the failure of a
slope.
Vegetation can help absorb water from the
rains, and its root network can help stabilize a
slope.
Overloading
Overloading, almost always from human activity
such as dumping, filling, or piling up of material,
can increase water pressure within the slope
material, which decreases its shear strength
and thus weakens the slope material.
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Factors That Influence Mass Wasting
Factors That Influence Mass Wasting
Triggering Mechanisms
Geology and Slope Stability
Generally, rocks that
are horizontal or
dipping in the
opposite direction of
a hillside’s slope are
more stable than the
rocks that dip in the
same direction as the
slope.
The most common triggering
mechanisms are earthquakes
and excessive amounts of
water, although anything that
disturbs the slope’s
equilibrium will result in mass
wasting.
Fig. 11.5, p. 278
Types of Mass Wasting
 Mass movements are classified according to
Summary of Types of Mass Movement
Classification of Types of Mass Movements
1. Rate of movement (rapid vs. slow)
2. Type of movement (Rockfalls, Slides or Flows)
3. Type of material (rock, soil, or debris).
Rapid – sudden, visible movement
Slow – recognized by their effects
Table 11.1, p. 266
Types of Mass Wasting
Rockfalls
Falls
Rockfalls, the free-fall of rocks, are a common
type of mass movement.
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Slump on the coast of California
Types of Mass Wasting
Slides
Slumps and rock slides are the two types of slides.
Slumps are rotational slides in which material
moves along a curved surface, and usually involve
poorly consolidated or unconsolidated materials.
Types of Mass Wasting
Slides
Laguna Beach,
California
Slumps and rock slides are the two types of slides.
Rock (or block) slides take place on a planar
surface and usually involve solid pieces of rock.
Types of Mass Wasting
Flows
Turtle Mountain
Rock Slide
Alberta, Canada
Mudflows, debris flows and earth flows are the
three main types of flows.
 The different types of flows are recognized by their
1. Rates of movement (rapid or slow)
2. Water content
3. Material composition (rock, sediment, or soil).
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Types of Mass Wasting
Types of Mass Wasting
Flows
Flows
Mudflows
Debris Flows
 Mudflows consist largely of
clay- and silt-sized particles,
contain more than 30% water,
and generally follow preexisting channels.
 Debris flows contain less water than mudflows and are
composed of larger particles. They are more viscous than
mud flows and move more slowly.
 Mudflows are most common in
arid and semiarid climates.
Types of Mass Wasting
Types of Mass Wasting
Flows
Flows
Earthflows
Quick-Clays
 Earthflows move downslope as thick, viscous
masses of wet regolith that move more slowly
than debris flows or mudflows.
Clays that
spontaneously liquefy
and flow when
disturbed are known as
quick-clays.
Quick-Clay Slide Quebec, Canada
Quick-clay activated
by ground shaking
Anchorage, Alaska
1964 Earthquake
Types of Mass Wasting
Flows
Solifluction
The slow downslope movement of water-saturated
surface sediment known as solifluction is most
common in areas of permafrost.
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Types of Mass Wasting
Types of Mass Wasting
Flows
Flows
Creep – Most expensive and most difficult to stop
Creep – Most expensive and most difficult to stop
The slowest and most widespread type of mass
wasting is called creep. This involves the
imperceptible downslope movement of soil and rock.
Creep produces tilted trees and power poles, broken
streets and sidewalks, cracked foundations, and
bent rock layers.
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Minimizing the Effects of
Mass Wasting
Types of Mass Wasting
 A thorough geologic investigation of an area is the
most important way to reduce or eliminate hazards.
Complex movements
Combinations of different types of mass
movements, most involving sliding and flowing,
are considered complex movements.
Recognizing and Minimizing the
Effects of Mass Movements
Making slope stability
maps
Recognizing former
landslides by observing
scarps, open fissures,
tilted objects.
Collecting bedrock and
soil samples.
Combine with
topographic/slope info
Regrading slopes
 How to eliminating or minimize the effects of
mass wasting
 Retaining walls, draining excess water, regrading
slopes, and planting vegetation can all be used to
help stabilize slopes.
Draining
Excess
Water
Cut-and-Fill
Benching
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Retaining walls
Rock Bolts and
Netting
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