Geology 101 - Mass Wasting

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General Info

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downslope motion of earth materials by gravity
-mass movements are a type of natural hazard
-hazards can produce catastrophic losses
important to rock cycle
-all slopes are unstable
-often aided by human activity

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classification

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based on 4 factors:
-Type of material (rock, regolith, snow, or ice).
-Rate of movement (fast, intermediate, or slow).
-Nature of moving mass (cloud, slurry, or distinct blocks).
-Surroundings (subaerial or submarine).

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Creep

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– Slow downhill movement of regolith.
Due to expansion and contraction.
Wetting and drying.
Freezing and thawing.
Grains are moved…
Perpendicular to slope upon expansion.
Vertically by gravity upon contraction.

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Solifluction

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– Slow downhill movement of tundra.
Melted permafrost slowly flows over deeper frozen soil.
This process generates hillsides with solifluction lobes.

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slumping

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– Sliding of regolith as coherent blocks.
Slippage occurs along a spoon-shaped “failure surface.”
Display a variety of sizes and rates of motion.
Have distinctive features…
Head scarp.
Bulging toe.

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mudflows and debris flow

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– H2O-rich movement.
Mudflow – A slurry of water and fine sediment.
Debris flow – A mudflow with many large rocks.

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lahar

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Lahar – A special volcanic mud or debris flow.
Volcanic ash (recent or ongoing eruptions) mixes with...
Water from heavy rains or melted glacial ice.

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landslides

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Landslides – Movement down a non-vertical slope.
Rock slide – A slide consisting of rock only.
Debris slide – A slide comprised mostly of regolith.
Movement down the failure surface is sudden and deadly.
Slide debris can move at 300 km/hour on a cushion of air.

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landslide case study
the vaiont dam disaster

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Illustrates the need to evaluate underlying geology when developing critical structures.
Built in 1960 in a deep synclinal gorge in the Italian Alps.
Limestone over shale dipped toward the valley center.
On 10/9/63, 600 million tons of limestone fell into the reservoir.
A wave crested the dam, destroyed villages, and killed 2,600.

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avalanches

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Avalanches – Turbulent clouds of debris and air.
Snow avalanche – Oversteepened snow that detaches.
Debris avalanche – Rock and dust fragments.
Move up to 250 km/hr on a cushioning layer of air.
Reoccur in defined chutes; destroy stationary objects.

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rockfalls and debris falls

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– Vertical freefall of rock mass.
Bedrock or regolith falls rapidly downward.
When blocks impact, they fragment and continue moving.
Talus blocks pile up at the base of the slope.

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submarine mass movements

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Submarine mass movements – Slides under ocean water.
Enormous volumes of material are moved downslope.
Large slides alter the sea floor bathymetry.
These movements trigger gigantic tsunami waves.

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why mass movements?

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Mass movements require that earth materials...
Be subjected to topographic (slope) forces.
Be weakened or loosened from their attachments.
Fragmentation and weathering.
The upper crust is broken by jointing and faulting.
Chemical and physical
weathering produces regolith.
Surface material is much
weaker than solid crustal rock.

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weakening the surface

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Slopes may be stable or unstable.
Slope stability is a dynamic balance between two forces.
Downslope force – Gravitational pull.
Resisting force – Material properties that repel motion.
Movement occurs
when downslope
forces prevail.

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downslope = ?

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Downslope forces = Gravity.
The weight of earth materials.
The weight of added water.
The weight of added structures.

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resisting forces = ?

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Resisting forces = Material strength.
Cohesion.
Chemical bonds.
Electrical charges.
Surface tension.
Friction.

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steeper slopes = ?

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Steeper slopes = larger forces.

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slope stability

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Loose granular material assumes a slope angle.
“Angle of repose” is a material property due to...
Particle size and shape and the surface roughness.
Typical angles of repose.
Fine Sand 35o
Coarse Sand 40o
Angular Pebbles 45o

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types of failure surfaces

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Types of “failure surfaces” include…
Saturated sand or clay layers.
Joints parallel to the land surface.
Weak sedimentary bedding (shale, evaporites).
Metamorphic foliation.

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failure triggers

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A destabilizing event usually triggers slope failure.
Triggers are both natural and anthropogenic.
Shocks, vibrations, and liquefaction.
Changes in slope angles, loads, and support.
Changes in slope strength.
Tectonic effects.

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failure triggers are not always necessary because...

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Slope materials weaken over time.
Gravity continues to operate.
Mass movements are often random and unpredictable.

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Failure Triggers - Shocks, vibrations, and liquefaction.

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Ground vibrations decrease material friction.
On an unstable slope, the downslope force takes over.
Vibrations are common.
Motion of heavy machinery or trains.
Vibrations cause saturated sediments to liquefy.
Quick clay – Pore water slurries clay flakes when shaken.
Saturated sand – Fluidized by increase in pore pressure.

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failure triggers - changes in charactersistics can destablize a slope

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Changes in characteristics can destabilize a slope.
Angle – Steepening a slope beyond the angle of repose.
Loading – Adding weight to the top of a slope.
Water – As rain or via humans (lawns, septic systems).
Waste materials and fill.
Buildings.

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changes in slope strength

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-Weathering – Creates weaker regolith.
-Vegetation – Stabilizes slopes. Removing vegetation…
Greatly slows removal of excess water.
Destroys an effective stapling mechanism (roots).
Slope failures common after forest fires destroy
- Water – Reduces slope strength in several ways.
Adds a great deal of weight .
Water in pores pushes grains apart, easing disintegration.
Water lubricates grain contacts.
Removing water, thereby, strengthens a failure surface.

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tectonic linkage

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Tectonic processes influence mass movements.
Create uplift – Topography is directly linked to gravity.
Fragment crust – Joints and faults ease disintegration.
Generate seismicity – Earthquakes trigger motion.