Soil

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Soil

• Soil is a combination of mineral and organic
  mater, water, and air
• That portion of the regolith (rock and mineral
  fragments produced by weathering) that supports
  the growth of plants
• Depending on who you are you may have varying
  definitions of soil
• Just dont call it dirt!

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Typical components in a soil that yields good
plant growth-soil in Indiana
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Soil

• Factors controlling soil formation
• Parent material
• Residual soil parent material is the underlying
  bedrock
• Transported soil forms in place on parent
  material that has been carried from elsewhere and
  deposited

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Soil

• Factors controlling soil formation
• Time
• Important in all geologic processes
• Amount of time for soil formation varies for
  different soils depending on geologic and
  climatic conditions
• Climate
• Most influential control of soil formation
• Key factors are temperature and precipitation

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Soil

• Factors controlling soil formation
• Plants and animals
• Organisms influence the soils physical and
  chemical properties
• Also furnish organic matter to the soil
• Slope
• Steep slopes often have poorly developed soils
• Optimum terrain is a flat-to-undulating upland
  surface

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Soil

• The soil profile
• O horizon organic matter
• A horizon organic and mineral matter
• High biological activity
• Together the O and A horizons make up the topsoil
• E horizon little organic matter
• Zone of eluviation and leaching

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Soil

• The soil profile
• B horizon zone of accumulation
• C horizon partially altered parent material
• The O, A, E, and B horizons together are called
  the solum, or true soil

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Soil

• Soil erosion
• Recycling of Earth materials
• Natural rates of soil erosion depend on
• Soil characteristics
• Climate
• Slope
• Type of vegetation
• Impact by humans

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Soils vary across the landscape, as shown by this
example of three soil profiles from forest,
grassland, and desert regions. Differences are
explainable in terms of regolith composition,
slope steepness, vegetation cover, soil biota,
climate, and the time required to develop the
profile.
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Soil

• Soil erosion
• In many regions the rate of soil erosion is
  significantly greater than the rate of soil
  formation
• Sedimentation and chemical pollution
• Related to excessive soil erosion
• Occasionally soil particles are contaminated with
  pesticides

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Mass Wasting

• Mass Wasting the rapid or slow movements (creep,
  slow flows) of material down slope.

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A Classification of Mass Wasting

• Nature of material
• Rock
• Unconsolidated material (debris, mud)
• Velocity
• Nature of movement

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Controls on Mass Wasting Rates

• Topography/ steep slope
• Is water present water serves as a lubricant
  high rainfall?
• How consolidated is the slope?
• Slope modificationhumans
• Vegetation? Slows mass wasting
• Shocksearthquakes, volcanic eruptions?

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A Classification of Mass Wasting
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Hill Creep
Slow mass wasting process
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Slope Failures
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A Classification of Mass Wasting
Rock Debris
(rare)
Fall
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A Classification of Mass Wasting
Rock Debris
Avalanche
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A Classification of Mass Wasting
Rock Debris
Slide
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A Classification of Mass Wasting
Earth Debris Mud
Flow
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A Classification of Mass Wasting
Creep
Slump
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So what makes a slope more or less stable?

• Angle of repose higher with coarser and more
  angular particles

• Slope stability
• Driving forces
• Mass
• Gravity
• Resisting forces
• Inertia
• Friction
• Cohesion

Think of a sandcastle or a sand-Shrek
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Just add Water

• Dry unconsolidated grains will form a pile with a
  slope angle determined by the angle of repose
• Slightly wet unconsolidated materials exhibit a
  very high angle of repose because surface tension
  between the water
• Saturation ? the angle of repose is reduced to
  very small values and the material tends to flow
  like a fluid.
• Lots of water reduced the cohesion between grains

http//www.tulane.edu/sanelson/geol111/masswastin
g.htm
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The component of gravity
http//www.tulane.edu/sanelson/geol111/masswastin
g.htm

• perpendicular component of gravity, gp, helps to
  hold the object in place on the slope.
• The tangential component of gravity, gt, causes a
  shear stress parallel to the slope and helps to
  move the object in the down-slope direction.
• On a steeper slope, the shear stress or
  tangential component of gravity, gt, increases,
  and the perpendicular component of gravity, gp,
  decreases.
• shear strength - frictional resistance and
  cohesion among the particles that make up the
  object.down-slope movement - steeper slope
  angles (increasing the shear stress) anything
  that reduces the shear strength (e.g. water)

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• Slope stability scenario what are the problems
  here?

Note potential slip planes dip parallel slope
Bad situation especially with a clay
layer Road-cut steepens immediate slope such
that increase DF decr RF (closer to
vertical) Upper slope also has less length for
friction to operate after cut especially after a
few slumps or flows at cut scarp
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Role of climate and vegetation

• Climate controls water and vegetation
• Vegetation absorbs rain impact and roots increase
  cohesion
• Vegetation adds weight
• Logging (clearcut) for example
• Less transpiration ? more water to soil
• Increased snowpack in clearings
• Roots rot and lose cohesion
• Long vertical run for rill development
• Logging roads are perhaps worse than the cuts
  themselves

President Bush wants to push for more logging
roads in areas that are prone to forest
fireshmmmm. Will this introduce even more
natural hazards to the people living there????
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• Role of water

• Adds weight (increase driving forces)
• Initially increases cohesion, but decreases after
  a point (decrease in resisting forces)
• Often saturated zone at surface if above
  impermeable layer (frozen, clay, rock)
• Promotes weathering
• Erodes gullies and slope bases

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• Role of vibration
• Earthquakes, traffic, sonic booms
• Can ? immediate loss of cohesion
• Liquefaction shock water saturation

The road into Turnagain
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Susceptibility to landslides throughout the US

• http//landslides.usgs.gov/html_files/landslides/s
  lides/slide1.htm

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Mt. St. Helens Lahar
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Human Use and Landslides

• Landslides are a growing problem.
• 1. Increasing population in landslide-prone
  areas
• 2. Human activity that affects magnitude and
  frequency of landslides

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Logging

• Swanson Dryness (1975) Cascade study
• Found logging and roads increase landslide
  erosion by several times if performed on weak
  unstable slopes
• Roads interrupt surface drainage (culverts
  blocked), alter subsurface movement, redistribute
  mass

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Urbanization
La Conchita, CA 1995
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Urbanization

• Seattle scenario
• Esperance Sand over Lawton Clay
• Steep slopes where stream valleys and the Sound
  cut into glacial deposits
• Springs and slides
• Excavations for homes, freeway, etc.

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Minimizing the Hazard

• If suspect, hire a consulting geologist to
  evaluate
• Grade slope
• Cut-and-fill remove top and fill in at base
• Must do with care, since this often causes
  problems
• Benching series of cut-and-fills with diversion
  drains
• Slope supports
• Retaining walls (permeable and/or drained)
• Rock bolts
• Landslide warning systems
• Periodic inspection
• Tilt meters
• Well monitoring