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8/4/2019 Slope Processes Landslides
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CHAPTER 9
SLOPE PROCESSES,
LANDSLIDES, AND
SUBSIDENCE
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La Conchita slide
January 10, 2005 Triggered by heavyrainfall, reactivationalong an older
landslide surface(35,000 years ago,6000 years ago, and1995)
Potential solution:relocate people andbetter land use
regulation
R.L. Schuster, U.S. Geological Survey
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Introduction
Landslide and other ground failures causesubstantial damage and loss of life
In U.S., average 2550 deaths; damage morethan $3.5 billion annually
For convenience, definition oflandslideincludes all forms ofmass-wastingmovements
Landslide and subsidence: naturallyoccurred and affected by human activities
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Slope Processes
Slopes: The most common landforms Consists of cliff face (free face) and talus
slope or upper convex slope, a straight
slope, and a lower concave slope Dynamic evolving feature, depending upon
topography, rock types, climate, vegetation,
water, and time Materials constantly moving down the slope
at varied rates
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Slopes
Figure 9.3
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Figure 9.4
Types of Landslides
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Slope Stability
Safety Factor: = Resisting/Driving ForcesIf SF >1, then safe or stable slopeIf SF
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Human Land Use and Landslide
Urbanization, irrigation Timber harvesting in weak,relatively unstable areas
Artificial fillings of loosematerials
Artificial modificationof landscape
Dam construction
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Mitigating the Landslide Hazard (1)
Identify potential landslidesPhotographic analysisTopographic map and detailed field check
Historic data
Landslide hazard inventory mapGrading code from the least stable to the most
stable
Application of geologic and engineeringknowledge before any hillside development
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Mitigating the Landslide Hazard (2)
Preventing landslidesDrainage control: Reducing infiltration and
surface runoff
Slope grading: Reducing the overall slopeSlope supports: Retaining walls or deep
supporting piles
Avoid landslide hazardsLandslide warning for critical evacuationsCorrecting landslides
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Warning of Impending Landslides
Monitoring changesHuman surveillance
Instrumental survey: Tilt meter and geophones
Landslide warning system
Info for public awareness and education
Enough time for public evacuation
Stop or reroute traffic flow
Emergency services
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Snow Avalanche [GNFAvCtr]
Mountainous regionsRapid downslope
movement of snow andice
Location, location
Preventive measures:
Well-designedexplosives
Engineering structuresto retain, divert, or
retard avalanches
http://www.avalanche.org/~gnfac/current/http://www.avalanche.org/~gnfac/current/8/4/2019 Slope Processes Landslides
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Subsidence Subsurface groundfailure
Natural or human-induced
Slow settling orrapid collapse
Causes:Withdrawal of fluids
(water, oil and gas,
steam)Removal of solid
materials(dissolution, mining)
Fi 9 25
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Process of
Subsidence
SettlingLong Beach harbor
Houston
Mexico City
Mississippi delta
Figure 9.25
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Sinkholes Dissolution of carbonate rocks, limestone,
and dolomite
Affecting most of the conterminous states Natural or artificial fluctuations in watertable increasing the problem
Triggering other problems: Sinkholes as
waste dumping sites
Removal of Solid Materials (1)
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Salt and coal mining Salt dissolution and pumping
Active coal mines and abandoned coal
mines Ground failure due to depleted subsurfacepressure
More than 8000 km2 of land subsidence due
to underground coal mining
Removal of Solid Materials (2)
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Landslide hazard maps not preventingdevelopment
Common perception: It could happen on otherhillsides, but never on this one.
Infrequency and unpredictability of large slidesreducing awareness of the hazards
Often people taking chances and unknown risks
Perception of the Landslide Hazard
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National landslide hazard map
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Professional geologic evaluation for a propertyon a slope
Avoid building at the mouth of a canyon,regardless of its size
Consult local agencies for historical records
Watch signs of little slidesoften precursor forlarger ones
What Can You Do? (1)
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Look for signs of structure cracks or damageprior to purchase
Be wary of pool leaking, tilt of trees and utilitypoles
Look for linear cracks, subsurface watermovement
Put observations into perspective, one aspect
may not tell the whole story
What Can You Do? (2)
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Discuss the reasons why our society could notprevent slope development.
Assume you have been hired by a communityto make the citizens more aware of thelandslide hazard in a very steep topographicarea. Outline a plan of action and defend it.
Compare and contrast landslide hazards and
impact risks in the east coast vs. west coast,and tropical vs. polar regions.
Applied and Critical Thinking Topics