Chapter 16
Unsafe Ground: Landslides and Other
Mass Movements
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Unsafe Ground:
Landslides and Other Mass Movements
Prepared by
Ronald Parker
Earlham College Department of Geosciences
Richmond, Indiana
Introduction
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.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Mass Movement
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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
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.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Types of Mass Wasting

Classified 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).

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Types of Mass Wasting
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Types of Mass Wasting

Creep initiates tilt of trees, gravestones, and walls.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Types of Mass Wasting

Solifluction – Slow downhill movement of tundra.
Melted permafrost slowly flows over deep-frozen soil.
 This process generates hillsides with solifluction lobes.


Rock glaciers – Mixtures of rock fragments and ice.
Rocks are added faster than ice can accumulate.
 These behave like glacial ice, flowing downhill slowly.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Types of Mass Wasting

Slumping - Sliding of regolith as coherent blocks.
Slippage occurs along a spoon-shaped “failure surface.”
 Variety of sizes and rates of motion.
 Distinctive…

Head scarp.
Bulging toe.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
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.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Types of Mass Wasting

Lahar – A special volcanic mud or debris flow.
Volcanic ash from recent or ongoing eruptions.
 Water from heavy rains or melted glacial ice.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Lahar Case History
Nevada del Ruiz volcano in the Colombian Andes.
 The volcano erupted the night of 11/13/85.

Eruption melted some of the mountain’s snowcap.
 Melt water mixed with ash and raced down river valleys.
 Armero was buried, killing 20,000 residents in their sleep.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Types of Mass Wasting

Landslides – Movement down a nonvertical slope.
Rock slide – Slide consisting of rock only.
 Debris slide – Slide mostly made of regolith.
 Movement down failure surface is sudden and deadly.
 Slide debris can move 300 km/hr on a cushion of air.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Landslide Case Study

The Vaiont Dam disaster – Illustrates the need to evaluate
geology when siting 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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
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.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Types of Mass Wasting

Rockfalls and debris falls – Vertical freefall.
Bedrock or regolith that falls rapidly down.
 When blocks impact, they fragment and continue moving.
 Talus blocks pile up at the base of the slope.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Types of Mass Wasting
Submarine mass movements.
 3 types – Based on degree of disintegration.

Submarine slumps – Coherent blocks break and slip.
 Submarine debris flows – Moving material breaks apart.
 Turbidity currents – Sediment
moves as a turbulent cloud.


Usually well-preserved.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Types of Mass Wasting

Submarine mass movements.

Gigantic submarine slope failures are widespread.
Much larger than land-based slope failures.
Important process sculpting adjacent land.
Create catastrophic tsunamis.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Weakening the Surface

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 that solid crustal rock.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Weakening the Surface
Slopes may be stable or unstable.
 Slope stability is a trade-off between two forces.

Downslope force – Gravitational pull.
 Resisting force – Material properties that repel motion.


Movement occurs
when downslope
forces prevail.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Slope Stability

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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Slope Stability
Loose granular material assumes a slope angle.
 “Angle of repose” is a material property due to...



Particle size, shape, and surface roughness.
Typical angles of repose.
Fine sand
 Coarse sand
 Angular pebbles

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
35o
40o
45o
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Failure Surfaces
Weak subsurface layers can initiate motion.
 Types of “failure surfaces” include…

Saturated sand or clay layers.
 Joints parallel to the surface.
 Weak sedimentary bedding (shale, evaporites).
 Metamorphic foliation.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Failure Triggers
A destabilizing event usually triggers slope failure.
 Triggers are both natural and anthropogenic.

Shocks and vibrations.
 Changes in slope characteristics.
 Changes in slope strength.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
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.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Failure Triggers

Shocks and vibrations.
Ground vibrations decrease material friction.
 On an unstable slope, the downslope force takes over.


Many sources of vibration
are common.
Motion of heavy machinery,
vehicles, or trains.
 Earthquakes.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Failure Triggers

Vibrations cause saturated sediments to liquefy.
Quick clay – Pore water slurries clay flakes when shaken.
 Saturated sand – Fluidized by increase in pore pressure.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Failure Triggers

Changes in characteristics can destabilize a slope.

Removing support – Undercutting a slope leads to failure.
Natural – River eroding base of slope.
Human-induced – Excavating the base of a slope.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Failure Triggers

Changes in slope strength.
Weathering – Creates weaker regolith.
 Vegetation – Stabilizes slopes. Removing it…

Greatly slows removal of excess water.
Destroys an effective stapling mechanism (roots).
Failures common after forest fires.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
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.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Case Study
1925 Gros Ventre slide, near Jackson Hole, Wy.
 Gros Ventre River cut dipping beds on Sheep Mtn.
 Lubricated by rain, the slope slid down a shale unit.


40 million m3 slid 600 m, creating Slumgullion Lake.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Tectonic Linkage

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.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Identifying Slope Hazards

Geologic mapping can identify regions at risk.
Can identify past failures (scarps, hummocky land, etc.)
 Can identify currently unstable slopes.

Cracked and bulging ground.
Measurable changes in surveyed land features.

GPS can detect
slow movements.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Mitigating Slope Hazards

How can humans mitigate the slope hazards?

Develop strategies for minimizing disaster potential.
Institutional controls on development.
Well-designed evacuation plans.
Slope-monitoring programs.
An educated populace.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Prevention

Action can reduce mass-movement hazards.

Revegetation – Adding plants has two positive effects.
It removes water by evapotranspiration.
Roots help to bind and anchor regolith.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Prevention

Action can reduce mass-movement hazards.
Regrading – Reshaping slopes below the angle of repose.
 Drainage – Dewatering reduces weight, increases
strength.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Prevention

Action can reduce mass-movement hazards.

Slowing or eliminating undercutting – Increases stability.
Removing agent of erosion at the base of a slope.
Reducing the effect of the agent of erosion (i.e., riprap).
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Prevention

Action can reduce mass-movement hazards.

Engineered structures – Safety structures can be built to
improve slope stability or to reduce movement hazards.
Retaining walls – Barriers that pin the base and trap rock.
Covers – A fence or coating that drapes over the outcrop.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Prevention

Engineered structures.
Rock staples – Rods drilled into rock to hold loose facing.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
Prevention

Engineered structures.
Avalanche sheds – Structures that shunt avalanche snow.

Controlled blasting – Surgical removal of dangerous rock.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements
This concludes the
Chapter 16
Unsafe Ground: Landslides and Other
Mass Movements
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 16: Unsafe Ground: Landslides and Other Mass Movements