Chp14: Mass Wasting

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Chp14 Mass Wasting
Mass Wasting (mass movement) -defined as the
downslope movement of material under the
influence of gravity -aided most by
weathering -usually involve surficial
material -move at different rates 1.
creep-slow 2. rockfall or slide-
fast Factors that influence mass wasting when
gravitational force acting on a slope exceeds
the resisting force-failure occurs 1. Shear
Strength of the slope a function of the slope
materials strength and cohesion, amount of
internal friction between grains, and external
support of the slope 2. gravity opposing
force acting along slope and in some cases
perpendicular to slope angle of reposethe
steepest angle that a slope can maintain without
collapsing a. Slope angle- THE
major cause of mass wasting in general, the
steeper the slope, the less stable that slope
is.. slopes are over steepened by-
-undercutting by stream or waves
-excavations for roads, houses, etc b.
Weathering and Climate Mass Wasting is more
likely to occur in a loose or
unconsolidated rock than in bedrock
-climatic effects- depth of weathering zones
varies with climate-deeper in tropics
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Chp14 Mass Wasting
Severity of landslides in US. Areas of greatest
severity occur along coastal mountain ranges.
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Shear strength of a slope depends upon a.
strength and cohesionof material underlying the
slope, amount of internal friction
betweengrains, any external support of the
slope. Force of gravity acts vertically but also
has a component acting along slope.
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Chp14 Mass Wasting

• c. Water Content-the more water contained in a
  rock or soil, the more likely it is to fail
• -water percolating through soil decreases
  the friction between grains. clay can contain
• lots of water (made up of platy
  particles) which makes clay susceptible to
  movement
• down slope
• d. Vegetation- vegetation absorbs water,
  decreases the water saturation in soils
• -root systems stabilize slope by binding soil
  particles and holding soil to bedrock
• -removal of vegetation adversely effects
  slope stability (e.g. fires followed by heavy
• rains...)
• e. Overloading- almost always result of human
  activity
• dumping,
• filling or piling up material....
• additional weight increases the water
  pressure in the soil
• f. Geology and Slope Stability relationship
  between topography and geology
• if rocks underlying slope dip in same
  direction as the slope, mass wasting is more
• likely to occur
• - water can percolate along fractures and
  bedding planes
• - joints can act as failure planes or water
  pathways

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Undercutting by stream erosiona. removes a
slopes base, whichincreases the slope
angle.B. Which leads to slope failure-notice
the curved weakness plane.C. example from
Michigan
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Sea Cliff north of Bodega Bay, CA undercut by
waves duringwinter of 1997-98. as result, part
of land slid into ocean, damaging homes
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Chp14 Mass Wasting

• g. Triggering Mechanisms
• -vibrations from earthquakes
• -excessive amount of water winter snow melt,
  or heavy rain
• -explosions or sounds avalanches in
  mountains
• What are Different types of Mass Wasting?
• geologists classify them into two general
  categories-
• a. rapid mass movements-most dangerous
• b. slow mass movements-move a much greater
  volume of earth materials
• Falls
• 1. Rockfalls- rocks of any size fall through
  the air occur along highways, cliffs and
• roadcuts
• talus- buildup of loose rocks and fragments at
  base of slope due to rockfall
• -result from failures along joints or
  bedding planes in bedrock
• -triggered by undercutting (human activity)
  or earthquakes
• -chemical weathering caused by water
  percolating along cracks in carbonate rocks
• responsible for rockfalls
• -common in mountainous areas

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Rockfalls result from failures along cracks,
fractures or bedding planes in bedrock- common
in areas with steep cliffs.
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Highway excavations disrupt equilibrium of
slope-removing support and over-steepening it at
point of removal. Results in landslides
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Huge rockfall closed lanes on Hwy 70 near Rogers
Flat, CAon July 25, 2003. Note dip of beds on
right.
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Chp14 Mass Wastinga. rocks dipping in
samedirection as hills slope are very
susceptible to mass wasting. Note stream at
base of slope.B. fractures dipping insame
direction as slope can be enlarged by chemical
weathering, which canweaken the rocks and cause
mass wasting.
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Chp14 Mass Wasting
b. Slides -involve movement of material along one
or more surfaces of failure. 2 types are
recognized 1. slumps or rotational
slides- movement occurs along curved surface
downward movement of material along a
curved surface of rupture, characterized by
backward rotation of slump block -
occurs in unconsolidated material -size
small to large -causes erosion along
base of slope along highways, wet climate and
tectonic activity 2. rock or block slides
occur when rocks move downslope along a planar
surface (e.g. bedding planes) c. Quick
clays-clays that spontaneously liquefy and flow
like water when disturbed -occur in deposits
of fine silt and clay particles ground up by
glacier action d. Solifluction slow downslope
movement of water saturated surface sediment
-most common in permafrost areas (high latitudes-
cold) -during warm weather the upper
layer of permafrost melts, making surface
sediment and water into a soggy mixture
-this can flow down hill, produces a lobate
topography causes problems for construction
in high latitudes
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Slump material moves downward along curved
surface of rupture, causing slump block to
rotate backward.
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Massive slump in Palisades, CA caused by wave
undercutting. Cranes
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Chp14 Mass Wastingheavy winter rains caused
this large landslide in March 1995 in La
Conchita, CA. 9 homes were destroyed.
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Rock Slide- occurs when blocks of material move
downslope along planar surface.
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• Rock slide in Laguna
• Beach, CA caused by-
• Rocks dipping in same
• direction as slope of sea
• cliffs.
• 2. Undercutting of sea
• cliffs by wave action.
• 3. Clay beds that became
• slippery when wet..

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• Turtle Mountain Rock
• Slide that killed 70
• people..
• April 29, 1903 caused
• By a combination of
• Factors
• Joints that dipped in
• same direction as slope
• of Turtle Mountain.
• 2. A fault part way down
• the mountain.
• 3. Weak shale and
• siltstone beds underlying
• base of mountain, and
• 4. Mined out coal seams.

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2 meter high mudslide that buried car in CA
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A mudflow near Estes Park, CO
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Debris flow note large boulders that are part of
flow
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Earthflows form tongue-shaped masses of wet soil
that move slowly downslope.
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Earthflow Near Baraga, MI
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Ground shaking during 1964 Alaska earthquake
turned parts of Bootlegger Cove Clay into a Quick
clay Note the small scarps as the snow covered
ground moved down slope.
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An earthquake 65km away triggered a landslide in
Peru that destroyed 2 towns and killed 25,000
people.
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• Distribution of permafrost areas
• in Northern Hemisphere.
• b. Solifluction flows in Alaska-typical
• lobate topography characteristic of these
  conditions

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Chp14 Mass Wasting
e. Creep - slowest type of flow, most widespread,
responsible for moving largest volumes of
material down slope - most effective in
humid regions, can occur everywhere How to
recognize? -tilted trees and power
poles -broken streets and sidewalks
-cracked retaining walls and foundations .very
difficult to control !! f. Complex movements-
when several types of mass wasting occur together
or in succession -debris avalanche
occur in steep mountain ranges begin as rock
falls, then slides or flows down slope,
picking up more material as it goes downhill
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Evidence of Creep a. curved tree trunks,
displaced monuments, tilted power poles,
displaced and tilted fences, roadways not
in alignment, hummocky surface. Bent rock layers,
tilted stone wall
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Complex movement that began with slump at the
head, followed by earthflow.
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Chp14 Mass Wasting
How do we recognize and minimize effects of Mass
Wasting? - need a thorough geological
investigation of the region to identify areas
of high potential for failure. this report may
produce slope stability maps..... Ways to
mitigate impact of Mass wasting -add
drainage to hillsides place drainage pipes in
hillsides -divert surface waters culverts,
ditches, etc -plant vegetation on steep
hillsides -reduce the slope a. cut and
fill method remove material at upper slope
and fill in the base. - benching cut
a series of benches or steps -build
retaining walls provide support at base of
slope -rock bolts fasten loose blocks to
bedrock
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• Minimizing damage from rockfalls
• Wire mesh covers rock face.
• Wire mesh fence along roadside

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• Mitigation procedures-
• Driving drainpipes that are perforated
• into a hillside.

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Stabilize hillside Cut and Fill Method-remove
material from top of slope, add it to base of
slope-lessens slope angle and provides support.
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Slope stabilizationBenchingmake several cuts
along slope, reduce overall slope. Any failures
are now small in size, material collects
on benches.
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a. Retaining wall
Rock boltswith wire mesh
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Chp14 Mass Wasting- SUMMARY
Mass Wasting (mass movement) -defined as the
downslope movement of material under the
influence of gravity -aided most by
weathering -usually involve surficial
material -move at different rates Factors
that influence mass wasting 1. Shear Strength
of the slope a function of the slope materials
strength and cohesion, amount of internal
friction between grains, and external support of
the slope 2. gravity opposing force acting
along slope and in some cases perpendicular to
slope angle of reposethe steepest angle that a
slope can maintain without collapsing
a. Slope angle- THE major cause of mass
wasting in general, the steeper the slope, the
less stable that slope is slopes are
over-steepened by - -undercutting by
stream or waves -excavations for roads,
houses, etc b. Weathering and Climate Mass
Wasting is more likely to occur in a loose or
unconsolidated rock than in bedrock
-climatic effects- depth of weathering zones
varies with climate-deeper in tropics c. Water
Content-the more water contained in a rock or
soil, the more likely it is to fail d.
Vegetation-vegetation absorbs water, decreases
the the water saturation in soils e.
Overloading-almost always result of human
activity dumping, f. Geology and Slope
Stability relationship between topography and
geology dip of beds
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Chp14 Mass Wasting- SUMMARY
g. Triggering Mechanisms-vibrations from
earthquakes -excessive amount of water
winter snow melt, or heavy rain -explosions
or sounds avalanches in mountains What are
Different types of Mass Wasting? geologists
classify them into two general categories-
a. rapid mass movements-most dangerous b.
slow mass movements-move a much greater volume of
earth materials a. Falls Rock falls-
rocks of any size fall through the air occur
along highways, cliffs and road cuts b.
Slides-involve movement of material along one or
more surfaces of failure. 2 types are
recognized 1. slumps or rotational
slides- movement occurs along curved surface
downward movement of material along a
curved surface of rupture, characterized by
backward rotation of slump block 2.
rock or block slides occur when rocks move down
slope along a planar surface (e.g. bedding
planes) c. Quick clays-clays that spontaneously
liquefy and flow like water when disturbed
-occur in deposits of fine silt and clay
particles ground up by glacier action d.
Solifluction slow down slope movement of water
saturated surface sediment -most common in
permafrost areas (high latitudes- cold)

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Chp14 Mass Wasting- SUMMARY
How do we recognize and minimize effects of Mass
Wasting? - need a thorough geological
investigation of the region to identify areas
of high potential for failure. this report may
produce slope stability maps..... Ways to
mitigate impact of Mass wasting -add
drainage to hillsides place drainage pipes in
hillsides -divert surface waters culverts,
ditches, etc -plant vegetation on steep
hillsides -reduce the slope a. cut and
fill method remove material at upper slope
and fill in the base. - benching cut
a series of benches or steps -build
retaining walls provide support at base of
slope -rock bolts fasten loose blocks to
bedrock
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Point Fermin, CA landslides.wave action along
base undercuts slope
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Chp14 Mass Wasting