454 lecture 4 Mass Movements and Hillslopes Erosion (or lack of) results from balance between internal resistance of materials & magnitude of external forces acting on them Evolution of landscapes depends largely on regional slope development Mechanics of slope erosion are related to processes of physical weathering – the forces disintegrating rocks also lower the internal strength of the unconsolidated cover
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454 lecture 4
Mass Movements and Hillslopes
Erosion (or lack of) results from balance between internal
resistance of materials & magnitude of external forces
acting on them
Evolution of landscapes depends largely on regional slope
development
Mechanics of slope erosion are related to processes of
physical weathering – the forces disintegrating rocks also
lower the internal strength of the unconsolidated cover
454 lecture 4
Resisting forces (the properties of matter that resist the stresses
generated by gravitational force)
Shear strength
1) overall frictional characteristic, expressed as
angle of internal friction, Φ
a) plane friction: grains sliding past one another on planar surfaces;
varies with moisture, smoothness of plane surface,
mineralogy
b) interlocking friction: particles move upward and over one another
(greater resistance than plane friction); varies
with moisture, mineralogy, density of packing
454 lecture 4
2) effective normal stress, δ’, acts to hold material together and to
increase internal resistance to shear
total normal stress: δ = δ’ + μeffective pore
normal stress pressure
pore pressure can increase or decrease δ
in unsaturated zone, water molecules attached to surface
particles by tension increase weight of soil (eg. wet sand)
in saturated zone, water exerts hydrostatic pressure upward
& supports soil
3) cohesion, c, causes increase in shear strength when grains are packed
or cemented together (eg. clay)
454 lecture 4
Properties of material change with increasing or decreasing
moisture:
water added to dry soil – voids fill – plastic behavior
more water decreases cohesion – all pores filled
liquid behavior
“Plastic” refers to the way the material responds to stress
(force per unit area), in terms of strain (deformation) resulting
from applied force
stress
strain
y B
y: yield stress (permanent
deformation begins)
B: breaking strength (rupture occurs)
plastic
failure
454 lecture 4
Atterberg Limits: indicate transition from solid to plastic state, &
from plastic to liquid state
liquid limit expressed as moisture contents
plastic limit (wt. of contained water/wt. of dry soil)
Range of water contents between two limits is plasticity index
Atterberg limits function of
• types of clay minerals (eg. limits higher for montmorillonite
than kaolinite)
• size of particles (limits increase with smaller particles)
• history of wetting and drying
454 lecture 4
debris flow along
I-70 corridor near
Georgetown, triggered
by rainfall
454 lecture 4
Soil slips along Rt. 287 triggered by rainfall, 8/97
landslide above
Horsetooth Reservoir,
8/97
454 lecture 4
Factors influencing shear stress & resistance in slope materials