Transcript Historical Geology - Wharton County Junior College

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 material’s 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 repose=the 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
Severity of landslides in US. Areas of greatest severity occur
along coastal mountain ranges.
Chp14: Mass Wasting
Shear strength of a slope depends upon: a. strength and cohesion
of material underlying the slope, amount of internal friction between
grains, any external support of the slope. Force of gravity acts vertically
but also has a component acting along slope….
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
Undercutting by stream erosion:
a. removes a slope’s base, which
increases the slope angle.
B. Which leads to slope failurenotice the curved weakness plane.
C. example from Michigan
Sea Cliff north of Bodega Bay, CA undercut by waves during
winter of 1997-98. as result, part of land slid into ocean, damaging homes
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 categoriesa. rapid mass movements-most dangerous
b. slow mass movements-move a much greater volume of earth materials
a. Falls
1. Rockfalls- rocks of any size fall through the air; occur along highways, cliffs and
roadcuts
talus- buildup of loose rocks and fragmentsat 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
Rockfalls result from failures
along cracks, fractures or
bedding planes in bedrockcommon in areas with steep
cliffs.
Highway excavations disrupt equilibrium of slope-removing support and
over-steepening it at point of removal. Results in landslides
Huge rockfall closed lanes on Hwy 70 near Rogers Flat, CA
on July 25, 2003. Note dip of beds on right….
Chp14: Mass Wasting:
a. rocks dipping in same
direction as hill’s slope are
very susceptible to mass
wasting. Note stream at
base of slope….
B. fractures dipping in
same direction as slope can
be enlarged by chemical
weathering, which can
weaken the rocks and
cause mass wasting.
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
Slump: material moves downward along curved surface of rupture,
causing slump block to rotate backward.
Massive slump in Palisades, CA caused by wave undercutting. Cranes…
Chp14: Mass Wasting:
heavy winter rains caused this
large landslide in March 1995
in La Conchita, CA. 9 homes
were destroyed.
Rock Slide- occurs when blocks of material move downslope along
planar surface.
Rock slide in Laguna
Beach, CA caused by1. 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..
Turtle Mountain Rock
Slide that killed 70
people…..
April 29, 1903 caused
By a combination of
Factors:
1. 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.
2 meter high mudslide that buried car in CA
A mudflow near Estes Park, CO
Debris flow …note large boulders that are part of flow
Earthflows form tongue-shaped masses of wet soil that move slowly
downslope.
Earthflow
Near
Baraga, MI
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.
An earthquake 65km away
triggered a landslide in Peru that
destroyed 2 towns and killed
25,000 people.
a. Distribution of permafrost areas
in Northern Hemisphere.
b. Solifluction flows in Alaska-typical
lobate topography characteristic of these conditions
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
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
Complex movement that began with slump at the head, followed by
earthflow….
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 map
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
Minimizing damage from rockfalls:
a. Wire mesh covers rock face.
b. Wire mesh fence along roadside
Mitigation proceduresa. Driving drainpipes that are perforated
into a hillside.
Stabilize hillside: Cut and Fill Method-remove material from top of
slope, add it to base of slope-lessens slope angle and provides support.
Slope stabilization…Benching…make several cuts along slope, reduce
overall slope. Any failures are now small in size, material collects on
benches.
a. Retaining wall
Rock bolts…with wire mesh…
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 material’s 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 repose=the 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
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 categoriesa. 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)
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
Point Fermin, CA landslides….wave action along base undercuts slope
Chp14: Mass Wasting