GEOG 216 Lecture Notes - Lecture 7: Mass Wasting, Slope Stability, Landform
18 May 2018
School
Department
Course
Professor
Goal:&to&explain&various&mass-wasting&processes
Slopes&and&slope&stability
•
Mass&wasting
•
Mass&wasting&classification
•
Creep,&slides,&and&flows
•
Marine&clays
•
Slopes
The&term&slope&has&2&very&different&uses:&
1st:&measurement&of&inclination&of&a&surface/slope&angle&in°rees,&a&
slope&gradient&usually&expressed&as&the&ration&of&rise&over&run&1:7&or&as&a&
%&which&is&rise/run&x100
○
2nd:&to&denote&land&form&elements&and&usually&involves&a&description&of&
slope&shape&(straight,&concave,&convex),&character&(steep,&gentle),&and&
terms&like&"break&in&slope"&to&denote&a&change&in&slope&shapes.&Also,&since&
all&landforms&are&composed&of&sloping&surfaces,&the&description&of&slope&
characteristics&is&an&important&part&of&slope&studies&–this&is&why&we&do&
slope&profiles.
Straight
§
Concave
§
Convex
§
○
Slope&profiles&provide&one&of&the&most&useful&tools&to&analyze&slope&
properties.&Slope&processes&include&gravity-driven&changes&(i.e.&mass&
wasting)&and&the&role&of&slopes&in&other&processes&–like&hydrology&(i.e.&hill&
slope&hydrology).
○
•
Slope&profiles:&quantitative&analysis&in&geomorphology;&e.g.&from&convex&to&
straight&to&concave
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Slope&evolution&popular&in&the&1960s&and&1970s&(Davis):
Slope&decline: steep&upper&slopes&erode&more&rapidly
○
Slope&replacement: steeper&slope&replaced&by&gentler&slope
○
Maintain&constant&angles
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Parallel&retreat: controlling&variables&are&geology,&climate,&and&time
○
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Slope&stability&and&mass&wasting
An&exposed&ground&surface&that&stands&at&an&angle&with&the&horizontal&is&called&
an&unrestrained&slope
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The&slope&can&be&natural&and&artificial•
If&the&ground&surface&is¬&horizontal,&a&component&of&gravity&will&tend&to&pull&
the&soil&down&slope
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If&the&component&of&gravity&is&large&enough&and&the&soil's&internal&shear&strength&
is&small&enough,&a&slope&failure can&occur
•
When&geomorphologists&check&a&slope&against&potential&failure,&they&compare&
the&shear&stress&developed&along&the&most&likely&rupture&(failure)&surface&with&
the&shear&strength&of&the&soil&(usually&along&the&same&failure&plane)
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When&failure&occurs,&this&is&mass&wasting•
Gravity
Acceleration&due&to&gravity&at&sea&level:&9.8&m/s•
The&main&force&responsible&for&mass&wasting&is&gravity,&in&fact&"mass&wasting&is&
defined&as&the&down&slope&movement&of&material&under&the&influence&of&
gravity"
•
Gravity&is&the&force&that&acts&everywhere&on&the&Earth's&surface,&pulling&objects&
in&a&direction&towards&the¢er&of&the&Earth
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Slopes&and&gravity
On&a&flat&surface,&gravitational&force&acts&to&hold&objects&in&place&by&pulling&on&
them&in&a&direction&perpendicular&to&the&surface.&So&long&as&the&material&
remains&on&the&flat&surface,&it&will¬&move&under&the&force&of&gravity
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On&a&slope,&the&tangential&component&of&gravity&acts&down&a&slope:&it&causes&
objects&to&move&downhill
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Shear&stress&is&the&downslope&component&of&the&total&stress&involved&(tau)•
Normal&stress&is&the&perpendicular&component&(sigma)•
On&a&slope,&the&force&of&gravity&can&be&resolved&into&two&components&that&are&
closely&related&to&slope&angle:
A&component&acting&perpendicular&to&the&slope&(normal&stress)
○
A&component&acting&tangential&to&the&slope&(shear&stress)
○
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Assessing&a&slope's&potential&for&failure: safety&factor&analysis•
Factors&affecting&slope&stability
Increases&in&shear&stress&by:
Removal&of&lateral&support&(rivers,&road&cuts),&steeping
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Addition&of&mass
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Earthquakes
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Regional&tilting&(tectonics)
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Removal&of&underlying&support&(solution,&mining)
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Lateral&pressure&(swelling,&freezing)
○
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Decrease&shear&strength&by:
Weathering
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Increased&pore&water&pressure
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Structural&changes
○
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Mass&wasting
When&gravity-induced&shear&stresses&become&greater&than&the&combined&forces&
holding&rock/soil&on&a&slope,&it&will&move&down-slope&&–slope&failure
•
Mass&wasting&is&the&downslope&movement&of&material&under&the&influence&of&
gravity
•
Three&modes&of&movement:
Slides:&rapid&and&accelerating,&dry&failure&along&a&well-defined&plane,&constant&
velocity&profile&moves&as&a&cohesive&mass
1.
Flows:&rapid&and&accelerating,& wet&or&dry,&velocity&profile&decreases&through&the&
deforming&mass,&moves&by&inter-particle&shearing&–like&a&fluid
2.
Heaves&and&creep:&slow&imperceptible,&involves&up&and&down&pattern&of&
movement&of&individual&particles,&up&normal&to&surface,&down&is&vertical,&
velocity&decreases&with&depth
3.
Mass&wasting&classification
The&downslope&movement&of&surface&material&under&the&direct&influence&of&
gravity&is&called& mass&wasting.
•
2&approaches:
The&first&classifies&different&types&of&mass&wasting&in&terms&of&movement&
processes.&Carson&and&Kirkby&1972&use&3&criteria,&mode&of&movement&
(slide,&flow,&heave),&rate&(fast-slow)&and&water&content&(wet-dry)&-WILL&BE&
ON&THE&FINAL
○
The&second&approach&(Varnes&1958)&focuses&more&on&specific&types&of&
landforms&and&then&organizes&them&based&on&failure&type&(slide&and&flow)&
material&(bedrock,&soil)&and&rates&of&movement
○
The&first&is&a&more&functional&approach&and&puts&less&emphasis&on&the&
specific&type&of&landform&and&more&on&process
○
•
Marine&clays&–a&special&case:
As&water&infiltrates&into&the&pore&spaces,&it&can&both&be&absorbed&into&the&clay&
minerals,&and&can&dissolve&away&the&salts&holding&the&"house&of&cards"&together
•
Compaction&of&the&soil&or&shaking&of&the&soil&can&thus&cause&a&rapid&change&in&
the&structure&of&the&material.&The&clay&minerals&will&then&line&up&with&one&
another&and&the&open&space&will&be&reduced.
•
But&this&may&cause&a&loss&in&shear&strength&of&the&soil&and&result&in&slippage&
down&slope&or&liquefaction.&This&is&referred&to&as&remolding.&Clays&that&are&
subject&to&remolding&are&called&quick&clays.&Thixotropic&clays&will&strengthen&if&
left&undisturbed,&but&when&disturbed&they&will&lose&their&shear&strength.
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7.1:%Slopes%and%mass%wasting
Tuesday,& February& 20,&2018
10:11&AM
