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SLOPE PROCESSES,

LANDSLIDES, AND

SUBSIDENCE

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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 of landslideincludes all forms of mass-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 orupper 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 slope

If 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)

Identifypotential 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)

PreventinglandslidesDrainage control: Reducing infiltration and

surface runoff

Slope grading: Reducing the overall slopeSlope supports: Retaining walls or deep

supporting piles

Avoidlandslide hazardsLandslide warning for critical evacuationsCorrecting landslides

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Warningof Impending Landslides

Monitoring changes

Human surveillance(is the monitoring of the behavior, activities,or other changing information, usually of people for the purpose of

influencing, managing, directing, or protecting them.)

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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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 statesNatural or artificial fluctuations in watertable increasing the problem

Triggering other problems: Sinkholes as

waste dumping sites

Removal of Solid Materials

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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

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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?

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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