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Chapter 10

Slope Processes, Landslides,

and Subsidence

Dr. Joao Santos

Chapter 10

Slope Processes, Landslides,

and Subsidence

Dr. Joao Santos

Essentials of Geology, 3rd edition, by Stephen MarshakEssentials of Geology, 3rd edition, by Stephen Marshak Chapter 18: Amazing Ice: Glaciers and Ice AgesChapter 18: Amazing Ice: Glaciers and Ice Ages

Introduction to Mass Movements

� Most humans consider Earth to be “terra firma.”

� Earth’s surface, however, is mostly unstable ground.

� This is a consequence of weathering and erosion.

� We may be reminded of this instability without warning.

Edited by Joao Santos

Mass Movements � Downslope motion of earth materials by gravity.

� Mass movements are a type of natural hazard.

� Natural feature of the environment.

� Can cause damage to living things and buildings.

� These hazards can produce catastrophic losses.

Edited by Joao Santos

Mass Movements � Mass movements are important to the rock cycle.

� The initial step in sediment transportation.

� A significant agent of landscape change.

� All slopes are unstable; they change continuously.

� Mass movement is often aided by human activity.

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Types of Mass Wasting � Classification based upon four 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).

� Can cause damage to living things and buildings.

� These hazards can produce catastrophic losses.

� All slopes are unstable; they change continuously.

� Mass movement is often aided by human activity.

Edited by Joao Santos

Types of Mass Wasting

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Types of Mass Wasting � Creep – 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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Types of Mass Wasting � Creep initiates tilt of trees, gravestones, and walls.

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Types of Mass Wasting � Solifluction – Slow downhill movement of tundra.

� Melted permafrost slowly flows over deeper frozen soil.

� This process generates hillsides with solifluction lobes.

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Types of Mass Wasting � Slumping – 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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Types of Mass Wasting � Mudflows and debris flows – H2O-rich movement.

� Mudflow – A slurry of water and fine sediment.

� Debris flow – A mudflow with many large rocks.

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Types of Mass Wasting � 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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Types of Mass Wasting � 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 – 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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Types of Mass Wasting � 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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Types of Mass Wasting � Rockfalls and debris falls – 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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Types of Mass Wasting � 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 Movement? � 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 � 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 forces = Gravity.

� The weight of earth materials.

� The weight of added water.

� The weight of added structures.

� Resisting forces = Material strength.

� Cohesion.

�Chemical bonds.

�Electrical charges.

�Surface tension.

� Friction.

� Steeper slopes = larger forces.

Slope Stability

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Slope Stability � 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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Failure Surfaces � Weak subsurface layers can initiate motion.

� 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 � A triggering event is not necessary for movement.

� 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.

� Ground vibrations decrease material friction.

� On an unstable slope, the downslope force takes over.

� Vibrations are common.

�Motion of heavy machinery or trains.

�Earthquakes.

Edited by Joao Santos

Failure Triggers � 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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Failure Triggers � Changes in characteristics can destabilize a slope.

� Removing support – Undercutting a slope leads to failure.

�Natural – River eroding the base of a slope.

�Human-induced – Excavating the base of a slope.

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Failure Triggers � Changes in slope strength.

� 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 vegetation.

Edited by Joao Santos

Failure Triggers � Changes in slope strength.

� 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.

Edited by Joao Santos

End of Chapter 10