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Landslides. GEOL 4093 Risk Assessment. Resources. landslides.usgs.gov Murck, Skinner, and Porter, 1997. Dangerous Earth: An Introduction to Geologic Hazards , John Wiley & Sons, 300 p. Bloom, 1998. Geomorphology: A Systematic Analysis of Late Cenozoic Landforms , Prentice Hall,482 p.

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landslides

Landslides

GEOL 4093 Risk Assessment

resources
Resources
  • landslides.usgs.gov
  • Murck, Skinner, and Porter, 1997. Dangerous Earth: An Introduction to Geologic Hazards, John Wiley & Sons, 300 p.
  • Bloom, 1998. Geomorphology: A Systematic Analysis of Late Cenozoic Landforms, Prentice Hall,482 p.
mass wasting
Mass Wasting
  • Collective term for all gravitational or downslope movements of weathered rock debris
  • The term “landslide” is not a real geologic term
  • Flowing water not involved, but water is very important in mass wasting
role of water in mass wasting
Role of Water in Mass Wasting
  • Adds weight to rock mass
  • Generates seepage pressure by groundwater flow
slide5

The empty can on the left slides on a dry surface when the angle reaches about 17 degrees. With a little water dripping thgough a small hole in the bottom of the can on the right, sliding will occur at a much lower angle

slide7

Angle of repose is the natural steepnees of a pile of unconsolidated material. It varies depending on grain size, grain shape, and water content.

types of mass movements
Types of Mass Movements
  • Slides—blocks
  • Flows—internal shear
  • Heaves—vertical
  • Subsidence—vertical but no free surface
modifiers
Modifiers
  • Composition
    • rock, soil, mud, mixture
  • Speed of movement
    • Fast, moderate, slow
basic types of mass movements
Basic Types of Mass Movements
  • Slides
  • Flows
  • Creep
  • Subsidence
  • There is no single perfect way to classify all of the types of mass movements
slides

surface

slide plane

Slides
  • Cohesive blocks of material
  • Moven on well-defined surface
  • No internal shearing
  • Velocity profile of pure slide:
flows

surface

base of flow

Flows
  • Differential shearing within mass
  • No clear plane of movement at base
  • Velocity of flow decreases with depth in flow
  • Velocity profile of pure flow:
heave

surface

base of heave

Heave
  • Forces act perpendicular to ground surface by expansion and contractin of material
  • Slow, downslope movement by gravity
  • Velocity profile of pure heave:
subsidence
Subsidence
  • No free surface
  • Downward settling of material
  • Little or no horizontal motion
  • Main cause is slow removal of material below subsiding mass
speed of movement
Speed of Movement
  • Creep—very slow
  • Slides—fast
  • Avalanche—really fast
slide16

One way to classify mass movements is by speed of movement and by composition—slurry flows versus granular flows.

geologic controls on mass movements
Passive Controls

Nature of material

Degree of lithification

Degree of weathering

Interstitial water

Stratigraphy

Structure

Topogoraphy

Climate

Organic (vegetation/critters)

Active Controls

Support removal

Slope oversteepening

Saturation with water

Earthquakes

Geologic Controls on Mass Movements
examples of passive controls
Examples of Passive Controls
  • Nature of material
    • Degree of lithification (sed rocks)
    • Degree of weathering
    • Insterstitial water
    • All pretty self explanatory
passive control structure
Passive Control—Structure

Dipping beds, jointed rock

slide45

Mudflows from 1980 eruption of Mount St. Helens reached speeds as high as 40 m/s and traveled as far as the Columbia river, ovee 90 km away.

slide47

The 1920 Grand Banks earthquake caused a submarine landslide that broke underwater telegraph cables and disrupted communications between North America and Eurpoe. It also proved the existence of turbidity currents.

slide51

Landslide susceptibility map and recommended land use for Congress Springs area near San Francisco

human intervention
Human Intervention
  • Slope modification for construction of roads, bridges, homesites can lead to destabilization of hillsides, and increased hazard of mass wasting.
mitigation
Mitigation
  • Engineering techniques are used to stabilize slopes
  • Awareness and avoidance may be the best approach in some cases