What are the driving and resisting forces acting on this

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What are the driving and resisting forces acting
on this Body of rock or soil on a hillside?
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What are the driving and resisting forces acting
on this Body of soil or rock on a hillside?
Force of gravity (Fg)
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What are the driving and resisting forces acting
on this Body of soil or rock on a hillside?
Shear force (Fs)
Force of gravity (Fg)
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What are the driving and resisting forces acting
on this Body of rock or soil on a hillside?
Shear force (Fs)
Normal force (Fn)
Force of gravity (Fg)
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Effect of slope
Decreased normal force (Fn)
Increased shear force (Fs)
Force of gravity (Fg)
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Shear force (Fs) sin q Fs / Fg Fs Fg sin q
(q)
Normal force (Fn) cos q Fn / Fg Fn Fg cos q
(q)
Force of gravity (Fg) Weightmassgravity
Slope angle (q)
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Shear force (Fs) sin q Fs / Fg Fs Fg sin q
(q)
Normal force (Fn) cos q Fn / Fg Fn Fg cos q
(q)
Force of gravity (Fg) Weightmassgravity
Slope angle (q)
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Force vs. stress?
Stress Force/area (N/m2) In the abstract, we are
dealing with force Specific situations dealing
with stress
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Stresses acting on a planar surface
unconsolidated material
Since this is not a discrete mass (boulder) the
weight of the material is determined by W
(weight) specific weight(g) (kg/m3) h (m)
Ground surface
h
Buried plane
(q)
x
W
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Stresses acting on a planar surface
unconsolidated material
Normal force cosq x/h, xcos q h In terms of
pressure ( s) (stress) s ghcos2 q
Ground surface
W specific weight (g) (kg/m3) h
h
Buried plane
(q)
x
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Stresses acting on a planar surface
unconsolidated material

• Shear stress (t)
• W sinq
• t ghcosqsinq

Normal force cosq x/h, xcos q h In terms of
pressure ( s) (stress) s ghcos2 q
Ground surface
h
Buried plane
(q)
x
Wmassgravity
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Stresses acting on a planar surface
unconsolidated material

• Shear stress (t)
• W sinq
• t ghcosqsinq

Normal force cosq x/h, xcos q h In terms of
pressure ( s) (stress) s ghcos2 q
Ground surface
W specific weight (g) (kg/m3) x
h
Buried plane
(q)
x
h
(q)
x
Wmassgravity
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Driving Forces
Resisting Forces

• Surface normal force (but force/area so normal
  stress)
• Normal stress s
• normal stresss
• s ghcos2 q

Surface parallel (shear) force.

• Shear stress (t)
• W sinq
• t ghcosqsinq

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What else adds to shear strength? (or what
reduces shear stress?)
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What else adds to shear strength? (or what
reduces shear stress?)
Friction Sliding the friction of sliding
along a plane surface. Example a flat block
surface against a flat block all sliding
friction Internal interlocking friction of
irregularities in the material surfaces. Example
irregular grains in a soil mass
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Plane friction
Interlocking friction
f
Overall frictional characteristics are usually
expressed as the angle of internal
friction. Angle of internal frictiontan f
angle of repose
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Driving Forces
Resisting Forces

• Surface normal force (but force/area so normal
  stress)
• Normal stress s
• normal stresss
• s ghcos2 q
• 2. Friction (internal and sliding)
• Angle of internal frictiontanf

Surface parallel (shear) force.

• Shear stress (t)
• t ghcosqsinq

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What else adds to shear strength?
Friction Sliding the friction of sliding
along a plane surface. Example a flat block
surface against a flat block all sliding
friction Internal interlocking friction of
irregularities in the material surfaces. Example
irregular grains in a soil mass Cohesion
stickiness or attraction between individual soil
particles. Sources of cohesion
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What else adds to shear strength?
Friction Sliding the friction of sliding
along a plane surface. Example a flat block
surface against a flat block all sliding
friction Internal interlocking friction of
irregularities in the material surfaces. Example
irregular grains in a soil mass Cohesion
stickiness or attraction between individual soil
particles. Sources of cohesion 1. Natural
chemical deposits (CaCo3, iron oxides. .) 2.
Electrostatic charge on soil particles (esp. clay
particles and humic material have and
charges) 3. Microscopic water films between
soil grains.
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Driving Forces
Resisting Forces

• Surface normal force (but force/area so normal
  stress)
• Normal stress s
• normal stresss
• s ghcos2 q
• 2. Friction (internal and sliding)tanf
• 3. Cohesionc

Surface parallel (shear) force.

• Shear stress (t)
• t ghcosqsinq

Coulomb Equationsum of resisting forceSHEAR
STRENGH (S) S c s tanf
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Shear strength

• S stanf c
• Sshear strength
• s normal force
• tanf angle of internal friction
• Ccohesion

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Shear strength

• as you increase normal force, strength increases
• As you increase the angle of internal friction,
  strength increases
• As you increase cohesion, strength increases

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What can act to REDUCE the normal force?? And
so castles made of sand, fall in the sea,
eventually Jimi Hendrix
Shear force (Fs)
Normal force (Fn)
Force of gravity (Fg)
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Effective normal stress s
In saturated soil fluid pressure exerted on soil
grains by water in pore spaces counteracts normal
stress.
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Effective normal stress s
In saturated soil, fluid pressure exerted on soil
grains by water in pore spaces counteracts normal
stress.

• Other effects of saturation
• Water in pore spaces adds to the weight of the
  soil (increases both normal and shear stress
• Water acts as a lubricant on grain surfaces and
  rock surfaces

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Effective normal stress s
In UNSATURATED soil, pore pressure is NEGATIVE.
Pore pressure acts to INCREASE the normal force.
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Effective normal stress s
Effective normal stress (s) normal stress(s
)-pore pressure (m) What is pore pressure? m
portion of the normal stress supported by water
(or air) in interstitial spaces
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Effective normal stress s
Effective normal stress (s) normal stress(s
)-pore pressure (m) What is pore pressure? m
portion of the normal stress supported by water
in interstitial spaces Dry soil s s-0 Below
water table (saturated soil) s s-m Above water
table s s-(-m) S c s tanf positive
porewater pressure is a buoyant force, that is,
is supports part of the weight of the soil and
therefore wet sediment has very low shear
strength
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Driving Forces
Resisting Forces

• Surface normal force (but force/area so normal
  stress)
• Normal stress s
• normal stresss
• s ghcos2 q
• But water reduces normal stress.
• s (s-m)
• 2. Cohesionc
• 3.Friction (internal and sliding)tanf

Surface parallel (shear) force.

• Shear stress (t)
• W sinq
• t ghcosqsinq

Coulomb Equationsum of resisting forceSHEAR
STRENGH (S) S c s tanf
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Mass Movements Failure Criterion
Think of driving and resisting forces Failure
criterion (Fc) resisting forces (shear
strength) driving forces (shear stress)
Fc c s tanf t
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Tuesday February 21
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Processes moving material downslope
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Processes moving material downslope
Fall Slide Flow Wash
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Processes moving material downslope
Fall Very steep slopes, material is out of
contact w/ slope much of the way down, may break
on contact debris accumulationTALUS Slide No
internal deformation. Translational slide plane
of movement is straight, material does not
change orientation Rotational slide (slump)
plane of movement is curved, material rotates as
it moves. Flow More fluid motion does not
move as an intact mass, mixes as it moves.
Difference in velocity from the base of the slow
to the top, moving faster at the upper surface
than the baselaminar flow, streamflowturbulent
flow. Wash overland flow, common on bedrock,
sparse vegetation
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Types of materials involved in mass movements
Rock Mass movement starts in solid bedrock, but
usually breaks up as it moves the resulting
deposits may include large solid masses of rock
as well as broken fragments. Debris Combination
of soil material, fine particles, and rock, with
a fairly high proportion of boulder, cobble and
gravel size rock material. Earth Dominated by
sand, silt and clay-sized particles, may have
some stones supported in a fine matrix. Mud wet
earth
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Processes moving material downslope
Fall Very steep slopes, material is out of
contact w/ slope much of the way down, may break
on contact debris accumulationTALUS Slide No
internal deformation. Translational slide plane
of movement is straight, material does not
change orientation Rotational slide (slump)
plane of movement is curved, material rotates as
it moves. Flow More fluid motion does not
move as an intact mass, mixes as it moves.
Difference in velocity from the base of the slow
to the top, moving faster at the upper surface
than the baselaminar flow, streamflowturbulent
flow. Wash overland flow, common on bedrock,
sparse vegetation
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Features of Slides (see chapter 4, figs 4.36,
4.37)
Equidimensional, or only slightly longer than
they are wide.
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Creep very slow plastic movement, widespread on
hillslopes.
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Wash
1 volume, 1 runoff rate
2 volume, 2 runoff rate
3 volume, 3 runoff rate
Concave slope profile
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Creep
1 volume
2 volume
3 volume
Concave slope profile Creep is a function of
gravity To keep creep moving, you need to
increase the slope angle as you move downslope
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Creep
Concave slope profile Creep is a function of
gravity To keep creep moving, you need to
increase the slope angle as you move downslope
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Convex creep-dominated slopes, De-na-zhin
badlands, NM (also eroded by runoff!)
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Creep very slow plastic movement, widespread on
hillslopes.
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Creep very slow plastic movement, widespread on
hillslopes. Mechanism some process displaces
or lifts up a particle or clod of soil. When it
settles down, the influence of gravity causes it
to settle slightly farther downslope.
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Creep very slow plastic movement, widespread on
hillslopes. Solifluction (water-logged soil,
often underlain by frozen ground) Gelifluction
(same as solifluction, but always underlain by
frozen ground) Caused in areas where soils
freezes, and particles are moved downslope though
freeze-thaw processes. Faster than creep.
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As you keep adding sediment, you give the fluid a
yield strength and cohesion.
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As you keep adding sediment, you will give the
fluid a yield strength and cohesion. In a
debris flow, water and sediment are mixed
together to form a slurry. Debris flows contain
between 40-80 sediment/volume.
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Debris Flows
Initiation of debris flows Saturation of
debris (like in Lowman) en mass failure
mixes with waterproduces debris flow in
channel Dilution of unsaturated debrisaddition
of sediment from hillslopes to flood-flow in
stream eventually becomes sediment-charged
debris flow. Common after volcanic eruptions and
fires). Lahars volcanic mass movements
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Geomorphic Response to Fire
Saturation-induced failures
Runoff generated events
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Characteristics of Debris Flows
levees
High velocity
Low velocity
Boulders supported by grain support, buoyancy in
high density fluid with high strength
Snout of boulders
Precursor surge loss of water and fines
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How do debris flows move such big rocks?
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Stress and strain

• Stress is a force/area
• Strain is deformation resulting from stress
• change in length
• Newtonian fluids are fluids where the stress is
  proportional to the strain

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Newtonian fluids strain is recoverable-no
permanent deformation
stress
strain
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Newtonian fluids strain is recoverable-no
permanent deformation
High viscosity
stress
low viscosity
Strain
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Newtonian fluids strain is recoverable-no
permanent deformation
. . For a given stress
strain
time
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Stress and strain

• Stress is a force/area
• Strain is deformation resulting from stress
• change in length
• Newtonian fluids are fluids where the stress is
  proportional to the strain
• A non-Newtonian fluid is a fluid in which the
  viscosity changes with the applied shear force.

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Non-Newtonian fluids strain results in plastic
deformation.
Non-Newtonian fluids have a yield strength
stress
Yield strength
strain
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Newtonian fluidsstrain rate (change in
length)/(change in time)
High viscosity
stress
low viscosity
Strain rate
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strain rate (change in length)/(change in time)
Water (Newtonian)
Debris flow
Ice (above a certain stress, moves rapidly
stress
Yield strength
Strain rate
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Newtonian fluids strain is recoverable-no
permanent deformation
. . For a given stress
strain
time
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Slumgullian Debris Flow San Juan Mountains,
Colorado. (lower velocity, high of clays)
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Earthflow more a SLIDE than a flow. Better to
call it a mudslide.
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Granular flows are unsaturated air in pore
spaces, where the weight is borne particle to
particle. There is a major component of SLIDING
along the base.
Earthflow more a SLIDE than a flow. Better to
call it a mudslide.
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Debris Avalanche Rapid, high velocity debris
slide, sometimes with a component of flow at
base. Includes snow avalanches.
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Vallée de la Sionne, Switzerland 
http//www.cs.umd.edu/class
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Rock Avalanche - (Sturzstrom) - Relatively dry,
extremely rapid masses of broken debris En masse
movement, may or may not flow at end of
depositional sequence. Can travel enormous
distances with little elevation difference rocks
possibly travel on a cushion of airAccoustical
fluidization (Melosh) Transferring vibrational
energy (sound) at base of slideenergy
transferred from grain to grain like pool balls.
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Hebgen Lake Montana

• Magnitude 7.5 earthquake triggered an enormous
  landslide that buried a campground, causing 28
  deaths and dammed the Madison River, forming
  Quake Lake.

http//neic.usgs.gov/neis/eq_depot/usa/1959_08_18_
pics_2.html
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Hebgen Lake Montana

• Overal view of the Madison River slide from
  Earthquake Lake side. The main mass of the slide
  covered by trees and soil is in the center of the
  photograph. The dolomite debris that acted as the
  leading edge of the slide is in the right center.
  The source area of the slide is in the upper left
  corner, with the steeply dipping schist beds
  showing along the ridge line. Madison County,
  Montana. August 1959.

http//neic.usgs.gov/neis/eq_depot/usa/1959_08_18_
pics_2.html
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Hebgen Lake Montana

• Oblique aerial view of Madison Canyon landslide
  from over Earthquake Lake somewhat east of
  drowned toe of slide. Photograph was taken after
  preparation of spillway had begun. Madison
  County, Montana. 1959.

http//neic.usgs.gov/neis/eq_depot/usa/1959_08_18_
pics_2.html
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Hebgen Lake Montana
http//neic.usgs.gov/neis/eq_depot/usa/1959_08_18_
pics_2.html
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Hyperconcentrated flow Intermediate between
debris flow and stream flow.
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Hyperconcentrated flow Intermediate between
debris flow and stream flow. Enough sediment
to produce to a measurable, but low, yield
strength.
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Hyperconcentrated flow Intermediate between
debris flow and stream flow. Enough sediment
to produce to a measurable, but low, yield
strength. Deposits have particles in contact
with each other (clast supported), show some
sorting and gradation, some weak horizontal
stratification, and lower silt and clay contents
than debris-flow deposits
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Normal Streamflow!
Sediment load does not affect flow behavior,
imparts no yield strength to the flow.
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Normal Streamflow!
Sediment load does not affect flow behavior,
imparts no yield strength to the flow. Sediment
concentrations up to 50 by volume for mixtures
of coarse particles of uniform size and up to 35
by volume for more poorly sorted mixtures impart
no yield strength to flowing water
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Normal Streamflow!
Sediment load does not affect flow behavior,
imparts no yield strength to the flow. Sediment
concentrations up to 50 by volume for mixtures
of coarse particles of uniform size and up to 35
by volume for more poorly sorted mixtures impart
no yield strength to flowing water Turbulence
is the primary mechanism for sediment transport.