Soil is one of the most important earth materials we encounter each day, but the definition of soil is difficult.

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Soils and the Environment

• Soil is one of the most important earth materials
  we encounter each day, but the definition of soil
  is difficult.
• Soil Scientists (and most ordinary people)
• fine-grained, well-weathered earth material that
  is able to
• support plant growth
• focus on the physical and chemical properties
• Engineers
• any earth material that can be removed without
  blasting
• focus on particle size and the amount of organic
  material
• engineering applications

GEOL g406 Environmental Geology
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Soils and the Environment

• Environmental Geologists
• must understand soil from many perspectives
• characteristics affect agriculture, engineering,
  hydrology,
• natural hazards and other aspects of land
  use
• soil development and soil character is crucial
  to good land
• use planning.

• Read Table 3.1 (Soil Taxonomy)
• Understand the meaning of soil types, but do not
  memorize
• all of them.
• Read Table 3.2 (Unified Soil Classification)
• Learn the definition of each constituent that
  makes up soil.

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Soil Development Soil is an important part of the
geologic cycle and soil characteristics are
influenced by parent material, climate,
topography, weathering, and the amount of time a
particular soil has had to develop.
Unsurprisingly, variations in climate, parent
material, type of weathering and amount of time
produce distinct soils that express these
variations.
As soil develops, weathering creates distinct
layers in soil. We call these layers soil
horizons, and each soil horizon has distinctive
characteristics. Every soil has a soil profile, a
list of the horizons that describe a particular
soil.
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Soil Horizons
Materials in a Soil System Vertical and
horizontal movements create a soil profile made
up of distinct layers parallel to the surface,
which are called soil horizons.
Organic top layer (O) Zone of leaching (A and
E) Zone of accumulation (B) Weathered rock (C
and R)
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Soil Development
A soils profile depends on its age and its
conditions of formation. Soil profile is the
primary criteria for soil classification. Soils
can be compared in terms of their relative
development. Weakly developed soil profiles are
generally younger and may have fewer horizons
well-developed soils are generally older and have
more horizons. Chronosequences Relative
development of a series of soils allows their
arrangement in a soil chronosequence. A soil
chronosequence gives information about the
history of the landscape. The relative
development of the soils in a chronosequence
tells the investigator about the climate and
depositional history of the area.
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Soil Taxonomy Entisols - soils with little or no
morphological development Vertisols - clayey
soils with high shrink/swell capacity Inceptisols
- soils with weakly developed subsurface
horizons Aridisols - CaCO3-containing soils of
arid environments with moderate to strong
development Mollisols - grassland soils with
high base status Andisols - soils formed in
volcanic ash Spodosols - acid soils with a
subsurface accumulation of metal-humus
complexes Alfisols - soils with a subsurface zone
of silicate clay accumulation and gt35 base
saturation Ultisols - soils with a subsurface
zone of silicate clay accumulation and lt35
base saturation Oxisols - intensely weathered
soils, tropical and subtropical Histosols -
organic soils (peak, bog, muck) Gelisols - soils
with permafrost within 2 m of the surface
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Soil Texture Texture relative proportion of
sand, silt and clay. Texture classes
Coarse sands, loamy sand and sandy loams with
less than 18 clay, and more than 65 sand.
Medium sandy loams, loams, sandy clay loams,
silt loams with less than 35 clay and less than
65 sand the sand fractions may be as high as
82 if a minimum of 18 clay is present.
Fine clays, silty clays, sandy clays, clay loams
and silty clay loams with more than 35 clay.
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Soil Classification
Soils are often referred to as being sandy or
clayey, or sometimes silty. Different countries
use different standards to define sand particle
and silt particle sizes. Particle sizes Gravel,
Cobbles, and Boulders particles greater than 2
mm diameter Coarse and medium sand particles
from 2 mm to 0.2 mm diameter Fine and very fine
sand particles from 0.2 mm to 0.074 mm
diameter Silt particles from 0.074 mm to 0.004
mm diameter Clay particles less than 0.004 mm
diameter
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Soil Classification
WELL SORTED
WELL GRADED
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Unified Soil Classification System
GW well-graded gravel GP poorly graded
gravel GM silty gravel GC clayey gravel SW
well-graded sand SP poorly graded sand SM
silty sand SC clayey sand ML silt MH
micaceous silt OL organic silt CL silty
clay CH high plastic clay OH organic clay PT
peat and muck
Clean (lt5 fines) Dirty (gt12 fines)
Clean (lt5 fines) Dirty (gt12 fines)
Non-plastic Plastic
gt50 larger than 0.074 mm
FINE-GRAINED COARSE-GRAINED
Clays Silts Sands Gravels
gt50 smaller than 0.074 mm
Mostly Organics
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Water in Soils

• Types of water
• Water on Earth is known by different terms,
  depending on where it is and where it came from.
  
• Meteoric water water in circulation.
• Connate water "fossil" water, often saline.
• Juvenile water water from the interior of the
  earth.
• Surface water water in rivers, lakes, oceans
  and so on.
• Subsurface water groundwater, connate water,
  soil, capillary water.
• Groundwater exists in the zone of saturation,
  and may be fresh or saline.

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Water in Soils
Moisture Content of soil is calculated as
follows W weight, so that (Wwet -
Wdry)/Wdry x 100 H2O content () Moisture
content affects the engineering properties and
stability of soils. A soil that is stable in dry
conditions may become unable to support the
structures built on it when saturated with
water. Be sure to read the sections of your text
describing the engineering properties of soil.
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Adhesion and Cohesion
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Engineering Properties of Soils

• Plasticity
• related to the water content
• Liquid Limit (LL)
• water content above which the soil behaves like
  a liquid
• Plastic Limit (PL)
• water content below which the soil is no longer
  plastic
• Plasticity Index (PI), PI LL - PL
• range of water contents that make the soil
  behave as a
• plastic material
• Low PI (5 ) small change in water content, soil
  changes from solid to liquid
• High PI (gt 35) potential to expand and contract
  on wetting and drying

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Engineering Properties of Soils

• Expansive Soils
• high content of swelling clay (montmorillonite)
• soils swell when water is incorporated between
  clay plates
• shrinking occurs when soil is dried
• damage to building and road foundations
• Study Table 3.3 in textbook to understand more
  about soil descriptions and their significant
  properties.
• Study the Universal Soil Loss Equation (erosion)
  
• A RKLSCP

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Universal Soil Loss Equation
A RKLSCP A long-term average annual soil loss
for the site R long-term rainfall runoff
erosion factor K soil erodibility index L
hillslope/length factor S hillslope/gradient
factor C soil cover factor P erosion-control
practice factor Used to predict the impact of
sediment loss on local streams and other
resources and to develop management strategies
for minimizing impact.
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Effect of Land Use on Sediment Yield, eastern
U.S. Piedmont Region.
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Soils and the Environment

• Key Terms to Review
• weathering
• soil horizons
• soil profile development
• soil chronosequence
• soil fertility
• unified soil classification
• soil strength
• soil sensitivity
• liquefaction

• compressibility
• erodibility
• permeability
• corrosion potential
• shrink-swell potential
• expansive soils
• soil pollution
• desertification
• water table
• soil plasticity index

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