GEOL105 Chapter Notes - Chapter 8: Soil Survey, Soil Texture, Groundwater Pollution
31 May 2018
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CH 8: Subsidence and Soils
8.1 Soil and Hazards
● Soil: a generally loose accumulation of surface earth material that has been altered by
chemical weathering in the place that it accumulated and can serve as a medium for
plant growth (geology def)
● Study of soils helps evaluate natural hazards since floodplain soils differ from upland
soils
● Consideration of soil properties helps delineate natural floodplains
● Determining relative ages of soils on landslide deposits may provide an estimate of the
freq of slides and thus assist in minimizing their impacts
● Weathering: the physical and chemical breakdown of rocks into smaller pieces and is
the first step in soil development
● Soil can either be residual or transported
○ Depends on where and when it has been modified
○ The more insoluble weathered material may remain in place and be modified to
form a residual soil
○ If weathered material is transported by water, wind, or glaciers and then is
modified in its new location, it forms transported soil
● Characteristics of a particular soil are a function of:
○ Climate
○ Topography
○ Parent material (the rock or alluvium from which the soil is formed)
○ Time (age of soil)
○ Organic processes (activity of soil organisms)
● Many of the differences we see in soil are the results of the climate and topography
where the soil forms
● Soil profile: created by vertical and horizontal movements of the materials in a soil
system
○ The layers are called zones or soil horizons
○ Soil horizons: collection of distinct soil layers parallel to the surface each
produced by vertical and horizontal movements of the materials in a soil that
creates the soil profile
Soil horizon
Horizon description
O
Composed mostly of organic materials, the color is often dark brown or black
A
Composed of both mineral and organic materials, the color is often light black to
brown, leaching occurs in this horizon and moves clay and other material such
as iron and calcium to the B horizon
E
Composed of light-colored materials resulting from leaching of clay, calcium,
magnesium, and iron to lower horizons (A and E horizons create the zone of
leaching)
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B
Enriched in clay, iron oxides, silica, carbonate, or other material leached from
overlying horizones. This horizon is known as the zone of accumulation
C
Composed of partially altered (weathered) parent material, rock material may
also be alluvial in nature, the horizon may be stained red with iron oxides
R
Unweathered (unaltered) parent material
● Leaching: the process of dissolving, washing or draining earth materials by percolation
of groundwater or other liquids
● Caliche: irregular accumulation of layers of calcium carbonate in soils
● Hardpan: a hard (compacted) soil horizon and is often composed of compacted and/or
cemented clay with calcium carbonate, iron oxide, or silica
○ Are nearly impermeable, restricting the downward movement of soil water
● B Horizon
○ Bt horizon (argillic B): enriched with clay minerals that have been translocated
downward by soil forming processes, probably the most important type of the B
horizon
○ Bk horizon: accumulation of calcium carbonate
● Soil Color
○ O and A horizons- tend to be dark bc of their abundant organic material
○ E horizon (if present)- almost white due to leaching of iron and aluminum oxides
○ B horizon-vary from yellow brown to light red brown to dark red, depending on
the presence of clay minerals and iron oxides
■ Bk horizons- may be light colored due to their carbonates, but are
sometimes reddish as a result of iron oxide accumulation
○ K horizon (if has developed)- almost white bc of accumulation of calcium
carbonate
○ Original parent material- if rich in iron may produce a very red soil
● Soil color can be an indicator of how well drained a soil is
○ Well drained soils are well aerated and iron oxidizes to a red color
○ Poorly drained soils are wet and iron in it is reduced rather than oxidized, color is
often yellow
■ Associated with environmental problems
■ Poor choice for siting waste disposal functions
● Soil texture is defined by the relative proportions of sand, silt, and clay sized partciles
○ Clay particles- diameter of less than 0.004 mm (is cohesive)
○ Silt particles- diameters ranging from 0.005 to 0.074 mm (feels like baking flour)
○ Sand particles- diameters from 0.074 to 2.0 mm (gritty)
Soil structure type
Typical size range
Horizon found in
Comments
Granular
1-10 mm
A
Can also be found in B and C
horizons
Blocky
5-50 mm
Bt
Are usually designated by angular
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or subangular
Prismatic
10-100 mm
Bt
If columns have rounded tops,
structure is called columnar
Platy
1-10 mm
E
May also occur in some B
horizons
● Relative soil profile development
○ Weakly developed soil profile: generally characterized by an A horizon directly
over a C horizon, C horizon may be oxidized, such soils tend to be only a few
hundred years old in most areas, but they may be several thousand years old
○ Moderately developed soil profile: may consists of an A horizon overlying an
argillic Bt horizon that overlies the C horizon, these soils have a B horizon with
translocated changes, a better developed texture, and redder colors than those
that are weakly developed, often date from at least the Pleistocene (more than
10,000 years old)
○ Well-developed soil profile: characterized by redder colors in the Bt horizon,
more translocation of clay to the Bt horizon, and strong structure, a K horizon
may be present, vary widely in age, typical age ranges between 40,000 and
several hundred thousand years and older
● Soil chronosequence: a series of soils arranged from youngest to oldest on the basis
of their relative profile development
○ Allows us to evaluate site stability when locating such critical facilities
● Water in soils
○ Saturated condition: if all pore spaces in a block of soil are completely filled
with water
○ Unsaturated condition: opposite of saturated condition
● The amt of water in a soil can be important in determining engineering properties such
as the strength of the soil and its potential to shrink and swell
● Classifying soils
○ Two separate systems of soil classification- engineering classification and soil
taxonomy
● Soil taxonomy
○ Emphasizes the physical and chemical properties of the soil profile
○ Sixfold hierarchy that groups soils into several levels- orders, suborders, great
groups, subgroups, families, and series
○ The eleven orders are mostly based on gross soil morphology, nutrient status,
organic content, color, and general climatic considerations
○ Especially useful for agricultural and related land use purposes
● Unified soil classification system
○ Widely used in engineering practice along with the evaluation of hazards
○ The major divisions of this system are coarse-grained soils, fine-grained soils,
and organic soils
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Document Summary
Soil: a generally loose accumulation of surface earth material that has been altered by chemical weathering in the place that it accumulated and can serve as a medium for plant growth (geology def) Study of soils helps evaluate natural hazards since floodplain soils differ from upland soils. Consideration of soil properties helps delineate natural floodplains. Determining relative ages of soils on landslide deposits may provide an estimate of the freq of slides and thus assist in minimizing their impacts. Weathering: the physical and chemical breakdown of rocks into smaller pieces and is the first step in soil development. Soil can either be residual or transported. Depends on where and when it has been modified. The more insoluble weathered material may remain in place and be modified to form a residual soil. If weathered material is transported by water, wind, or glaciers and then is modified in its new location, it forms transported soil.
