EG4 Mass Wasting Notes

8/9/2019 EG4 Mass Wasting Notes

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SLOPE FAILURESLOPE FAILURE

Landslides, Mudflows, Earthflows, and otherLandslides, Mudflows, Earthflows, and other

Mass Wasting ProcessesMass Wasting Processes

Read Chapter 5 in your textbook (Keller, 2000)Read Chapter 5 in your textbook (Keller, 2000)

GEOL g406 Environmental Geology

S. Hughes, 2003Gros Ventre landslide, Wyoming

There are many types of slope failure.

Slope failure, also referred to as mass wasting, is thedownslope movement of rock debris and soil in responseto gravitational stresses. Three major types of masswasting are classified by the type of downslopemovement: falls, slides, and flows.

S. Hughes, 2003

In addition,another typeof groundfailure:subsidence,is importantto humanexistence.

Halemaumau Pit Crater, Kilauea

Material is constantly moving downslope in response togravity. Movement can be very slow, barely perceptibleover many years.

Or, movement can be devastatingly rapid, apparent withinminutes. Whether or not slope movement occurs depends

on slope steepness and slope stability.

SLOPE PROFILE

Some slopes are gently rounded, while others areextremely steep. Profiles of naturally-eroded slopes areprimarily dependent on climate and rock type.

SLOPES

GEOL g406 Environmental Geology S. Hughes, 2003


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Common Slope Elements

Slopes common in semiaridregions or on rocks resistantto weathering and erosion.

Convex-concave slopescommon in semihumid

regions or in areas withrelatively soft rocks.


Figure from Keller (2000)

MASS WASTING PROCESSES

Flowage, or flow = downslopemovement of unconsolidated material.Particles move around and mix withthe mass.

Sliding = downslope movement of acoherent block of earth material.

Falling = free fall of earth material, asfrom a cliff, the free face of a slope.

Subsidence = sinking of a mass ofearth material below the surroundingground level; can occur on slopes oron flat ground.




Common type oflandslide consistingof an upper slumpmotion and a lowerflow.

Upper slump

Lower flow

MASS WASTING PROCESSES



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Read Table 6.1 in Keller (2000)

Type of Movement Material InvolvedROCK SOIL

Falls rockfall soilfall

Slides

Rotational rock slump block slump blockTranslational rock slide debris slide

Slow rock creep soil creepearthflow

Flows mudflowdebris flow

Fast debris avalanche

Complex combinations of slides and flowsGEOL g406 Environmental Geology S. Hughes, 2003

When is a slope not stable?

Slope stability is based on the interplay between twotypes of forces:

driving forces and resisting forces. Driving forces promote downslope movement of

material.

Resisting forces deter movement.

When driving forces overcome resisting forces, the slope

is unstable and results in mass wasting.

The main driving force in most land movements isgravity.

The main resisting force is the material's shear strength.

SLOPE STABILITY


Gravity: Does gravity act alone? NO!! Slope angle,climate, slope material, and water contribute to the effect ofgravity. Mass movement occurs much more frequently onsteep slopes than on shallow slopes.

Water plays a key role in producing slope failure. In theform of rivers and wave action, water erodes the base of

slopes, removing support, which increases driving forces.Water can also increase the driving force by loading, i.e.,adding to the total mass that is subjected to the force ofgravity. The weight (load) on the slope increases whenwater fills previously empty pore spaces and fractures.

An increase in water contributes to driving forces thatresult in slope failure.

DRIVING FORCES



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Resisting forces act oppositely of driving forces.

The resistance to downslope movement is dependent on

the shear strength of the slope material. And shearstrength is a function of cohesion (ability of particles toattract and hold each other together) and internal friction(friction between grains within a material).

Chemical Weathering (interaction of water with surfacerock and soil) slowly weakens slope material (primarilyrock), reducing its shear strength, therefore reducing

resisting forces.

IMPORTANT: The shear strength of the slope material is

decreased by increasing the pore water pressure(pressure that develops in pore spaces due to the

increased amount of water).

RESISTING FORCES



SLOPE STABILITY

Safety Factor (SF) = The ratio of resisting forcesto driving forces:

Resisting ForcesSF =

Driving Forces

If SF > 1 then SAFE

If SF < 1 then UNSAFE

NOTE: A safety factor of ~1.25 or somewhat higher is

acceptable for slope stability. A safety factor of ~10 isoften used in building design to accommodate slightvariances in materials and construction practices.

SLOPE STABILITY

W = Weight of total mass of earth material (at center of mass).D = Vector component of W parallel to potential movement.N = Vector component of W normal to slip plane.



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SLOPE STABILITY


SLOPE STABILITY

CD

WN

Potential slip plane(clay).

ROCK

A

Calculate the safety factorusing D to obtain driving forceand N to obtain resisting force.This is a simplified example, sothe clay layer is assumed to

have constant internal friction,i.e., the shear strength is thesame everywhere, when wet.

D = W sin A = driving force the downslope component of gravity.

N = W cos A = the normal component of W contributes to the shear strength along the slip plane contributes to the resisting force.


The safety factor involving a clay layer may be calculated

by the unit thickness method using the followingequation:

SF = SLT/W sin AEXAMPLE

S = shear strength of the clay layer 9x104 N/m3

L = length of the slip plane 50 mT = unit thickness (assume 1) 1 mW = area (500 m2) x thickness (1 m) x

unit weight (1.6x104 N/m3) 8x106 NA = 30, sin A = 0.5 0.5

SF = 1.125 (conditionally stable)

Can you think of examples where this can be applied?

SLOPE STABILITY



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Ground material affects the pattern of

slope failure:Type # 1 Homogeneous material leads torotational failure.


Ground material affects the pattern of

slope failure:Type # 2 Material with planes of weaknessleads to translational failure.


Ground material

affects thepattern of slope

failure:Type # 3 Rockand colluvium slopeleads to soil slipfailure.

NOTE: There are actually only two types of failurepatterns, rotational and translational. Shallow soil slip is

also a type of translational movement.




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FLOWS

Flows are the downslope movement of unconsolidated

material in which the material behaves like a viscous fluid.

Flows can be very slow or can be exceedingly fast.

Creep a type of flowExample: trees on a slope

where the base of each treebows outward in thedownslope direction

What other examples canyou see in daily life?


EFFECT OF WATERPerched water table decreases slope stability by causingtemporary increase in pore water pressure which reduces

shear strength in the earth material.

Colluvial soil;relatively

permeable.

Bedrock;low permeability.



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1SafetyFactor

Time

3

2

1RateofCreep

Time

FAILURE

Influence of TIME on the development of a landslide:progressive creep (left) and progressive wetting (right).

3

2

1SafetyFactor

Time

Wetting events

1 2 3 4

FAILURE

WaterRoad fill

Culvert

Wetting events

1

23

4

FAILURE

Soil

Rock



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SUBSIDENCE

Depression the result of subsidence. By definition,subsidence is the very slow to rapid sinking or settling ofthe land surface.

Subsidence can be the result of natural causes.

Some type of carbonate rock underlies topography

containing numerous natural depressions, known assinkholes. The topography is known as karst topography.

Limestone and dolomite, both carbonate rocks, are solubleand susceptible to chemical weathering. Chemicalweathering produces void spaces (very very small tocavernously large). Sinkholes result when enough"support" has been removed from the carbonate layer. Thesurface then collapses into the void space, producing asinkhole.


Retaining Wall

Used to help stabilize a roadcut



Landslide near Dam




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Landslide on Road


Landslide on Hillside Development


Avalanche


Figures from Keller (2000)


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Sinkhole in Karst Topography


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EG4 Mass Wasting Notes