Soil Properties
(SOIL SURVEY
OF HENDERSON COUNTY, NORTH CAROLINA)
When
he makes soil borings during field mapping, the soil scientist can identify
several important soil properties. He notes the seasonal soil moisture
condition, or the presence of free water and its depth in the profile. For each
horizon, he notes the thickness of the soil and its color; the texture, or the
amount of clay, silt, sand, and gravel or other coarse fragments; the
structure, or natural pattern of cracks and pores in the undisturbed soil; and
the consistence of soil in-place under the existing soil moisture conditions.
He records the root depth of existing plants, determines soil pH or reaction,
and identifies any free carbonates.
Samples
of soil material are analyzed in the laboratory to verify the field estimates
of soil properties and to characterize key soils, especially properties that cannot
be estimated accurately by field observation. Laboratory analyses are not
conducted for all soil series in the survey area, but laboratory data for many
of the soil series are available from nearby areas.
Based
on summaries of available field and laboratory data, and listed in tables in
this section, are estimated ranges in engineering properties and
classifications and in physical and chemical properties for each major horizon
of each soil in the survey area. Also, pertinent soil and water features and
engineering test data are presented.
Most
soils have, within the upper 5 or 6 feet, horizons of contrasting properties.
Information is presented for each of these contrasting horizons. Depth to the
upper and lower boundaries of each horizon in a typical profile of each soil is
indicated. More information about the range in depth and in properties of each
horizon is given for each soil series in "Soil Series and
Morphology."
Texture
is described in Table 12 in standard terms used by
the United States Department of Agriculture. These terms are defined according
to percentages of sand, silt, and clay in soil material that is less than 2
millimeters in diameter. "Loam," for example, is soil material that
is 7 to 27 percent clay, 28 to 50 percent silt, and less than 52 percent sand.
If a soil contains gravel or other particles coarser than sand, an appropriate
modifier is added, for example, "gravelly loam." Other texture terms used
by USDA are defined in the Glossary.
The
two systems commonly used in classifying soils for engineering use are the
Unified soil classification system (2) and the American Association of
State Highway and Transportation Officials soil classification system (AASHTO)
(1). In Table 12 soils in the survey area are
classified according to both systems.
The
Unified system classifies soils according to properties that affect their use
as construction material. Soils are classified according to grain-size
distribution of the fraction less than 3 inches in diameter, plasticity index,
liquid limit, and organic matter content. Soils are grouped into 15
classes-eight classes of coarse-grained soils, identified as GW, GP, GM, GC,
SW, SP, SM, and SC; six classes of fine-grained soils, identified as ML, CL,
OL, MH, CH, and OH; and one class of highly organic soils, identified as Pt.
Soils on the borderline between two classes have a dual classification symbol,
for example CL-ML.
The
AASHTO system classifies soils according to those properties that affect their
use in highway construction and maintenance. In this system, a mineral soil is
classified as one of seven basic groups ranging from A-1 through A-7 on the
basis of grain-size distribution, liquid limit, and plasticity index. Soils in
group A-1 are coarse grained and low in content of fines. At the other extreme,
in group A-7 are fine-grained soils. Highly organic soils are classified as A-8
on the basis of visual inspection.
When
laboratory data are available, the A-l, A-2, an A-7 groups are further
classified as follows: A-l-a, A-l-b, A-2-4, A-2-5, A-2-6, A-2-7, A-7-5, and
A-7-6. As an additional refinement, the desirability of soils as subgrade
material can be indicated by a group index number. These numbers range from 0
for the best subgrade material to 20 or more for the poorest. The AASHTO
classification for soils tested in the survey area, with group index numbers in
parentheses, is given in Table 15. The estimate
classification, without group index numbers, is given in Table
12.
Also
in Table 12 the percentage, by weight, of cobbles, or
the rock fragments more than 3 inches in diameter, are estimated for each major
horizon. These estimates are determined largely by observing volume percentage
in the field and then converting it, by formula, to weight percentage.
Percentage
of the soil material less than 3 inches in diameter that passes each of four
standard sieves is estimated for each major horizon. The estimates are based on
tests of soils that were sampled in the survey area and in nearby areas and on
field estimates from many boring made during the survey.
Liquid
limit and plasticity index indicate the effect of water on the strength and
consistency of soil. These indexes are used in both the Unified and the AASHTO
soil classification systems. They are also used as indicators in making general
predictions of soil behavior.
Range
in liquid limit and plasticity index are estimated on the basis of test data
from the survey area or from nearby areas and on observations of the many soil
borings made during the survey.
Permeability
is estimated on the basis of known relationships between the soil
characteristics observed in the field-particularly soil structure, porosity,
and gradation or texture-that influence the downward movement of water in the
soil. The estimates are for water movement in a vertical direction when the
soil is saturated.
Not considered in the estimates are
lateral seepage or such transient soil features as plowpans and surface
crusts.Permeability of the soil is an important factor to be
considered in the planning and design of
drainage systems, in evaluating the potential of soils for septic tank systems and
other waste disposal systems, and in many other aspects of land use and
management.
Available
water capacity is rated on the basis of soil characteristics that influence
the ability of the soil to hold water and make it available to plants. Important
characteristics are content of organic matter, soil texture, and soil
structure. Shallow-rooted plants are not likely to use the available water from
the deeper soil horizons. Available water capacity is an important factor in
the choice of plants or crops to be grown and in the design of irrigation
systems.
Soil
reaction is expressed as range in pH values. The range in pH of each major
horizon is based on many field checks. For many soils, the values have been
verified by laboratory analyses. Soil reaction is important in selecting the
crops and ornamental or other plants to be grown, in evaluating soil amendments
for fertility and stabilization, and in evaluating the corrosivity of soils.
Shrink-swell
potential depends mainly on the amount and kind of clay in the soil.
Laboratory measurements of the swelling of undisturbed clods were made for many
soils. For others it was estimated on the basis of the kind of clay and on
measurements of similar soils. Size of imposed loadings and the magnitude of
changes in soil moisture content are also important factors that influence the
swelling of soils. Shrinking and swelling of some soils can cause damage to
building foundations, basement walls, roads, and other structures unless
special designs are used. A high shrink-swell potential indicates that
special design and added expense may be required if the planned use of the soil
will not tolerate large volume changes.
Risk
of corrosion, as used in Table 13, pertains to
potential soil-induced chemical action that dissolves or weakens uncoated steel
or concrete. The rate of corrosion of uncoated steel is related to soil
moisture, particle-size distribution, total acidity, and electrical
conductivity of the soil material. The rating of soils for corrosivity to
concrete is based mainly on the sulfate content, soil texture, and acidity.
Protective measures for steel or more resistant concrete, help to avoid or
minimize damage resulting from the corrosion. Installations of steel that intersect
soil boundaries or soil horizons are more susceptible to corrosion than
installations entirely within one kind of soil or within one soil horizon.
Flooding
is rated in general terms that describe the frequency, duration, and period of
the year when flooding is most likely. The ratings are based on evidences in
the soil profile of the effects of flooding, namely thin strata of gravel,
sand, silt, or in places, clay deposited by floodwater; irregular decrease in
organic-matter content with increasing depth; absence of distinctive soil
horizons that form in soils of the area that are not subject to flooding local
information about floodwater heights and the extent of flooding; and local
knowledge that relates the unique landscape position of each soil to historic
floods.Most soils in low positions on the landscape where flooding is likely to
occur are classified as fluvents at the suborder level or as fluventic
subgroups. See the section "Classification of the Soils."
The
generalized description of flood hazards is of value in land use planning and
provides a valid basis for land use restrictions. The soil data are less
specific, however, than those provided by detailed engineering surveys that
delineate flood-prone areas at specific flood frequency levels.
A
seasonal high water table is the highest level of a saturated zone more
than 6 inches thick in soils for a continuous period of more than 2 weeks
during most years. The depth to a seasonal high water table applies to
un-drained soils. Estimates are based mainly on the relationship between
grayish colors or mottles in the soil and the depth to free water observed
during the course of the soil survey. Indicated are the depth to the seasonal
high water table; the kind of water table, whether perched,
artesian, or the upper part of the ground
water table; and the months of the year that the high water commonly is
present. Only those saturated zones above a depth of 5 or 6 feet are indicated.
Information
about the seasonal high water table helps in assessing the need for specially
designed foundations, the need for specific kinds of drainage systems, and the
need for footing drains to insure dry basements. Such information is also
needed to decide whether or not to construct basements and to determine how
septic tank
absorption fields and other underground
installations will function. Also, a seasonal high water table affects ease of
excavation.
Depth
to bedrock is shown for all soils that are underlain by bedrock at depths of
5 to 6 feet or less. For many soils, limited ranges in depth to bedrock is a
part of the definition of the soil series. The depths shown are based on
measurements made in many soil borings and other observations during the soil
mapping. The kind of bedrock and its relative hardness as related to ease of
excavation is also shown. Rippable bedrock can be excavated with a single-tooth
ripping attachment on a 200 horsepower tractor, but hard bedrock generally
requires blasting.
characteristics of the soil at the
specified location. The physical characteristics of similar soils at other
locations may vary from those of the soil sampled. All samples were obtained at
a depth of less than 7 feet.
The
engineering classifications in Table 15 are based on
data obtained by mechanical analyses and by tests made to determine liquid
limits and plastic limits. Mechanical analyses were made by combined sieve and
hydrometer methods.
Moisture
density data are obtained by compacting soil material at a successively higher
moisture content. Assuming that the compactive effort remains constant, the
density of the compacted material increases until the optimum moisture content
is reached. After that, the density decreases with increase in moisture
content. The highest density obtained in the compaction test is termed
"maximum dry density." Optimum stability is obtained if the soil is
compacted to about the maximum dry density when it is at approximately the
optimum moisture content.
The
tests to determine plastic limit and liquid limit measure the effect of water
on the consistency of the soil material. As the moisture content of a clayey
soil
increases from a dry state, the material
changes from a semisolid to a plastic state. As the moisture content is further
increased, the material changes from a plastic to a liquid state. The plastic
limit is the moisture content at which the material passes from a semisolid to
a plastic state. The liquid limit is the moisture content at which the material
passes from a plastic to a liquid state. The plasticity index is the numerical
difference between the liquid limit and the plastic limit. The plasticity index
indicates the range of moisture content within which soil material is in a
plastic condition.