Engineering Geology ECIV 2204

Chapter (8) Mass Wasting

Instructor : Dr. Jehad Hamad

2017-2016

Landslides & Mass WastingChapter 8

Earth’s Surface is shaped by external processes…

Earth’s Surface is shaped by external processes…

In sculpting the Earth’s surface, the two most

important agents

of erosion are : 1) Mass wasting

2) Running water

Mass Wasting

In the evolution of most landforms, mass wasting is the step that follows

weathering.

The combined effects of mass wasting and erosion by running water produce

stream valleys.

Gravity is the controlling force of mass wasting.

Other factors that influence or trigger downslope movements are saturation of

the material with water,

oversteepening of slopes beyond the angle of repose,

removal of anchoring vegetation, and

ground vibrations from earthquakes.

The various processes included under the name of mass wasting are classified and

described on the basis of

1) the type of material involved (debris, mud, earth, or rock),

(2) the kind of motion (fall, slide, or flow), and

(3) the rate of movement(fast, slow).

The various kinds of mass wasting include the more rapid forms called:

slump, rockslide, debris flow, and earthflow, as well as the slow

movements referred to as creepand solifluction.

Mass Wasting: Downslope, mass movement of Earth materials

Driven by: The extensive background force of …GRAVITY…

Contributing factors:

Saturation of sediments by water

Over-steepened slopes

Removal of vegetation

Earthquakes

Water fills pore spaces between sediment grains,reduces internal resistance, adds weight.

Plants add slope stability byprotection against erosion.

Strong ground vibrations.

Slopes become unstable once they reach the angle of repose = The steepest angle a slope can attain without slumping.

There are a wide variety of manifestations of the downslope movement of materials by gravity, some faster and some slower.

All of these processes have destructive effects…

Stability againstgravity depends on the strength of a material, which can berepresented

by its angle ofrepose…

In sediments, thisangle depends on grain and sorting.

Fig. 16.20 c

W. W. Norton

In sediments, the angle of repose depends on grain size and sorting of materials …

Mass Wasting

Types of materials:

Types of movement:

Rates of movement:

Soil/regolith -or- Rock/bedrock

Rock Falls - Free-fall of materialRock/Debris Slides - Coherent slabs slide along fracture surfacesMudflows - Soil and rock mixes with water and becomes fluidized.Earth or Debris Flows - Materials move as a viscous mass.

Fastest - Rock falls & avalanches.Avalanches “float” on entrapped air.

Slowest - Creep (cm/year).

Talus slopes

Fig. 16.12

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Types of mass wasting processes arrayed by typical velocity of movement….

Fig. 16.12

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Rock Fall/Debris Fall

Rock/Debris Falls MASS WASTING

Blocks of bedrock break free, and fall from a steep cliff face.

Contributing factors:

- Steep slopes.- Rocks loosened along joint fractures…

…by expansion of water on freezing,…by thermal expansion/contraction,…by biological activity (e.g. root growth)

- Ground shaking during earthquakes.

Fig. 16.14

Stephen Marshak

Fig. 16.15

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Rock Falls…

Fig. 16.22

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Steps to mediate…

Fig. 16.27 g

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Mediation…

Fig. 16.27 e

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Mediation…

Rock falls and avalanches produce talus slopes…

Talus

Fig. 16.08

Stephen Marshak

Mediation by terracing

Fig. 16.12

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Avalanches

Fig. 16.12

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Rock/debris slides

Rock Slides…

Beds dip downslope.

Rock SlideMASSWASTING

Blocks of bedrock break free, and slide down slope along a fracture surface.Often occurs where strata are inclined, with slip occurring along bedding planes of weak units,like shales.Other important contributing factors:

- Slopes become undercut by stream or wave erosion.- Rain or melting snow seeps into deposits andlubricates a slip surface.

Often deadly!If materials are unconsolidated called a “debris slide”.

Fig. 16.18

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Common triggering mechanism: Saturation (water) of a weak,expansive, clay-rich shale unit.

Fig. 16.20 a

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Common triggering mechanism…undercutting of slopes by streams or waves…

Rock slides candevelop in any type of rock

where there is are preferredplanes of weakness

dipping downslope…

SedimentaryMetamorphic Igneous

Jointing may facilitate process.

Mediation…

Fig. 16.12

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Mudflow/Debris Flow

Mudflow / Debris Flow:

Common in high rainfall areas

where fine materials mobilized

by abundant water…

Mud Flow / Debris FlowMASSWASTING

Flow of water saturated soil/regolith

Called lahars, if volcanic)Common in semi-arid regions and activevolcanic areas (e.g. Mt. Saint Helens)

Move fast!Extremelyhazardous!

Fig. 16.12

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Slumps

Slumps: Rotational TypeMASS WASTING

Mass of material slides downward alonga curved surface (slump surface)

Speed is usually intermediateand material doesn’t travel very far.

Slumping often involves several massesthat move separately (along diferentslump planes).

Common in weak, water saturated sediments that are over-steepened.

Common in coastal areas where sea cliffs are constantly removed by wave erosion.

Fig. 16.04 b

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Morphology of a Rotational Slump

Rotational Slump: Headwall shows evidence of backward rotation.

Destructive effects of rotational slumping…

Fig. 16.26

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Rotational Slumps Along Shorelines

Slumping triggered by coastal wave erosion…

Earthquake-triggered slumps, Alaska EQ 1964

Earth/Debris Flow

Slumps: Earth/Debris Flow TypeMASSWASTING

Common in high rainfallareas.

Occur on hillsides.Develop in rock units

rich in clay/silt. Slow rate of movement. Stabilized by “toe” and

by “dewatering”. Destructive!

Fig. 16.open

Robert L. Schuster/U.S. Geological Survey

Fig. 16.27 a

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Stabilization of slumpswith plant cover…

Fig. 16.27 d

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Stabilization by terracing…

Fig. 16.20 b

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Which of these situations is most favorable foir stability?

Fig. 16.27 c

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Stabilization by lowering water table…

Fig. 16.27 b

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Reduce slope angle…

Fig. 16.12

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Creep

Soil/Regolith CreepMASSWASTING

Creep

- Slow (cm/year) downhill movement of material.- Driven by alternate expansion/contraction of material during freeze/thaw or cycles of wetting/drying.

Gravitational force acts onrocks/soil to move them downslope…

Fig. 16.02 b

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Fig. 16.02 a

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Effect of cycles of freeze-thawon soil/regolith creep…

Soil/regolith creep…

Soil/Regolith Creep

Slow!

Assisted by:

“Frost heaving”(expansion of iceupon freezing).

Soil

Regolith

Fig. 16.02 c

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Tell-tell signs of soil/regolith creep…

Signs of soil/regolith creep…

Fig. 16.03 a

(c) Martin Miller

Solifluction: Soil creep in high latitude, cold climate areas where freeze-thaw is active…

Reducing soil creep…