SOIL AND ITS ORIGINSOILDefined as the accumulation of
unconsolidated sediments and deposits of soil particles as a result
of integration of rocks.ROCKDefined as the accumulation of
unconsolidated sediments and deposits of soil particles as a result
of integration of rocks.Rock types Igneous Rock Sedimentary Rock
Metamorphic RockIgneous RockIgneous rock resulted from cooling and
hardening of molten rock called magma that has originated within
the earth.Sedimentary RocksSedimentary Rocks are formed from
accumulated deposits of soil particles or remains of certain
organism that have hardedned by pressure or cemented by
materials.
Metamorphic RocksMetamorphic Rocks are rocks resulted from
metamorphism due to changes in temperature, pressure and plastic
flow changing the original rock structure and mineral composition
of rock.
Categories of SoilResidual Soil or Sedimentary SoilFormed from
weathering of rocks or accumulation of organic materials remained
at the location of origin.Mechanical WeatheringPhysical
disintegration due to effects of wind, rain, running water or
tectonic forces (earthquake).Chemical and Solution
WeatheringDecomposition of rock due to chemical reactions that
occur as a result of exposure to atmosphere, temperature changes,
reaction with water or other materials.Transported SoilThose
materials transported from their place of origin.SOIL TYPES Boulder
Cobble Gravel Sand Silt Clay Colloid Loam Loess Peat Silty Sand
Silty ClayCHARACTERISTICS OF SOIL1. Grain size2. Grain shape3.
Surface texture and electrical surface charges.General
Characteristics and classification of soil particles1. Gravel2.
Coarse sand3. Fine Sand4. Silt grain5. Clay6. Colloidal Clay7.
GravelConsist of rock fragments more or less rounded by water
action or abrasion.Quartz, well rounded pebbles and boulders,
slightly worm gravel.Fine SandHas particles that are more angular
than coarse and fine sand particles.Coarse SandUsually rounded like
gravel with which it is found and generally contains the same
materials.Silt SandSimilar to fine sand with the same mineral
composition. They are found in rock flour in glacier
moraines.Contains:1. Pumice2. Loess3. Materials foreign to
associated sand.ClayPlate like, scale like, or rod like in shape as
a result of chemical weathering. Performance is influenced by
moisture and surface chemistry.Colloidal ClayA finer clay particle
that remains suspended in water and does not settle by
gravity.Important Grain Shape Characteristics1. Rounded particles
are considered strong materials.2. Flat and flaky particles are
weak and variable.3. Angular or roughly cubical shape particles
increases resistance of soil mass to deformation when subjected to
load.4. Rounded particle has tendency to roll over each other when
subjected to load.
SOIL CLASSIFICATIONSWhy soil classification?The particles above
gravels are identified as rocks and are generally not considered as
a soil. Soil classification is primarily based on the size of
particles. The purpose of soil classification is to arrange soil
into groups and label them according to certain characteristics and
engineering behavior. The name of the group gives useful
information about the behavior of soil as a construction material
and foundation support.Although soil classification apparently
gives very useful information about soil behavior and about some of
its properties but it doesnt eliminate the requirement of in-detail
study of soil behavior and study of properties like shear strength,
unit weight, compaction characteristics, degree of saturation,
susceptibility to frost action etc.Soil Classification
SystemsDifferent organizations from around the world have made
systems to classify the soil according to their requirements like
some agricultural department, highway department, building
department, aviation department etc. There are two soil
classification systems in common use for engineering purposes. The
Unified Soil Classification System is used for virtually all
geotechnical engineering work except highway and road construction,
where the AASHTO soil classification system is used. Both systems
use the results of grain size analysis and determinations of
Atterberg limits to determine soils classification. Soil components
may be described as gravel, sand, silt, or clay. A soil comprising
one or more of these components is given a descriptive name and a
designation consisting of letters or letters and numbers which
depend on the relative proportions of the components and the
plasticity characteristics of the soil.Two of the most commonly
used soil classification systems are(a) USCS Unified Soil
Classification system which is mostly used for buildings and for
structures where soil will be used to support foundations or
footings(b) AASHTO (American Association for State Highway and
Transportation officials) soil classification systemTest Required
for Classification of SoilTo classify the soil in the laboratory
thus some tests results were used to classify the soil;Tests
include;(a) Liquid limit and Plastic limit tests Atterberg limit
results(b) Plastic size analysis testUnified soil classification
system (USCS):The Unified Soil Classification System is based on
the airfield classification system developed by Casa Grande during
World War II. With some modification it was jointly adopted by
several U.S. government agencies in 1952. Additional refinements
were made and it is currently standardized as ASTM D 2487-93. It is
used in the U.S. and much of the world for geotechnical work other
than roads and highways. In the unified system soils are designated
by a two-letter symbol: the first identifies the primary component
of the soil, and the second describes its grain size or plasticity
characteristics. For example, poorly graded sand is designated SP
and low plasticity clay is CL. Five first-letter symbols are
used:
G for gravelS for sandM for siltC for clayO for organic soil
Clean sands and gravels (having less than 5% passing the No. 200
sieve) are given a second letter P if poorly graded or W if well
graded. Sands and gravels with more than 12% by weight passing the
No. 200 sieve are given a second letter M if the fines are silty or
C if fines are clayey. Sands and gravels having between 5 and 12%
are given dual classifications such as SP-SM. Silts, clays, and
organic soils are given the second letter H or L to designate high
or low plasticity. The specific rules for classification are
summarized as follows and described in detail in ASTM D 2487.
Organic soils are distinguished by a dark-brown to black color,
an organic odor, and visible fibrous matter.For soils that are not
notably organic the first step in classification is to consider the
percentage passing the No. 200 sieve.If less than 50% of the soil
passes the No. 200 sieve, the soil is coarse grained, and the first
letter will be G or S;if more than 50% passes the No. 200 sieve,
the soil is fine grained and the first letter will be M or C.For
coarse-grained soils, the proportions of sand and gravel in the
coarse fraction (not the total sample) determine the first letter
of the classification symbol. The coarse fraction is that portion
of the total sample retained on a No. 200 sieve. If more than half
of the coarse fraction is gravel (retained on the No. 4 sieve), the
soil is gravel and the first letter symbol is G. If more than half
of the coarse fraction is sand, the soil is sand and the first
letter symbol is S. For sands and gravels the second letter of the
classification is based on gradation for clean sands and gravels
and plasticity of the fines for sands and gravels with fines.
For clean sands (less than 5% passing the No. 200 sieve), the
classification is well-graded sand (SW) if C 6 and 1 Cc 3. Both of
these criteria must be met for the soil to be SW, otherwise the
classification is poorly graded sand (SP). Clean gravels (less than
5% passing the No. 200 sieve) are classified as well-graded gravel
(GW) if Cu 4 and 1 Cc 3. If both criteria are not met, the soil is
poorly graded gravel (GP). For sands and gravels where more than
12% of the total sample passes the No. 200 sieve, the soil is a
clayey sand (SC), clayey gravel (GC), silty sand (SM), or silty
gravel (GM).
The second letter is assigned based on whether the fines
classify as clay (C) or silt (M) as described for fine-grained
soils below. For sands and gravels having between 5 and 12% of the
total sample passing the No. 200 sieve, both the gradation and
plasticity characteristics must be evaluated and the soil is given
a dual classification such as SP-SM, SP-SC, GW-GC, etc. The first
symbol is always based on gradation, whereas the second is always
based on plasticity. For fine-grained soils and organic soils,
classification in the unified system is based on Atterberg limits
determined by the fraction passing the No. 40 sieve. The liquid
limit and plasticity index are determined and plotted on the
plasticity chart. The vertical line at LL = 50 separates
high-plasticity soils from low-plasticity soils. The A-line
separates clay from silt. The equation of the A-line is
PI = 0.73 (LL 20)The U-line is not used in classification but is
an upper boundary of expected results for natural soils. Values
plotting above the U-line should be checked for errors. Inorganic
soils with liquid limits below 50 that plot above the A-line and
have PI values greater than 7 are lean clays and are designated CL;
those with liquid limits above 50 that plot above the A-line are
fat clays and are designated CH. Inorganic soils with liquid limits
below 50 that plot below the A-line are silt and are designated ML;
those with liquid limits above 50 that plot below the A-line are
elastic silts and are designated MH.
The plasticity chart has a shaded area; soils that plot in this
area (above the A-line with PI values between 4 and 7) are silty
clay and are given the dual symbol CL-ML. If the soil under
consideration is the fines component of a dually classified sand or
gravel, the soil is classified as SM-SC or GM-GC. Soils with
sufficient organic contents to influence properties that have
liquid limits below 50 are classified as OL; those with liquid
limits above 50 are classified as OH. Soils that are predominantly
organic, with visible vegetable tissue, are termed peat and given
the designation Pt.
Explanation of GI the Group IndexGI = (F - 35)[ 0.2 + 0.005
(LL-40) ] + 0.01 (F - 15) (PI - 10) F = Percent of soil passing the
0.075 mm sieve LL = Liquid Limit PI = Plastic Index
a) Soils which are non-plastic and for which the Liquid Limit
cannot be found have GI = 0.b) A-2-6 and A-2-7 soils use only the
second term of GI equation.c) If the final GI has a negative
number, it is reported as zero (0).d) Although not explicitly
expressed in either the AASHTO or ASTM specifications, GI for A-2-4
and A-2-5 are zero.Introduction to AASHTO Classification
System:This system of classification is based upon the observed
field performance of soils under highway pavements (flexible and
rigid) and is widely used around the world for highway and airfield
construction.Soils which have same general load carrying capacity
i.e. bearing capacity and serviceability characteristics are
grouped together and classified into eight groups from A1 to A8A-1,
A-2 and A-3 soils are coarse-grained soils While A-4, A-5, A-6 and
A-7 represent the fine grained soils. A-8 includes Peat, muck, and
other highly organic soils.A-1 soil is best to use as sub-grade
then a-2 and so on. And poorest is A-7 soil.Group IndexTo check the
quality of the soil, a term group index is defined. Higher the G.I
weaker will be the soil, a soil having G.I of 20 is considered as
poorest soil overall.Following is the formula for Calculating G.IGI
= (F200 35) [0.2 + 0.005(LL-40)] + 0.01 (F200 15)(PI-10)WhereF200 =
%age passing for sieve no. 200 (0.075 mm) expressed as whole
numberPI is plasticity Index where PI = LL PLAnd LL is liquid limit
and PL is plastic limitSome rules:a. If Group Index for your soil
comes out to be negative take it as zerob. Group index is expressed
as rounded whole numberc. No upper limit can be any integerd. For
soils A-2-6 and A-2-7 following Group Index formula should be
used;G.I = 0.01(F200-15)(PI-10)Soil is represented as A-4(5) where
A-4 is soil type and Group Index is 5A group index of (0) indicates
the best construction material for sub-base, or subgrade.A group
index of more than (20) indicates a very poor material for sub-base
or subgrade.ExampleClassify the following soil by the AASHTO
classification system:Percent passing no. 4 sieve = 82Percent
passing no. 10 sieve = 71Percent passing no. 40 sieve = 64Percent
passing no. 200 sieve = 41Liquid limit = 31Plasticity index =
12SolutionRefer to table 8. More than 35% passes through a no. 200
sieve, so it is a silt-clay material. It could be A-4, A-5, A-6, or
A-7. Because LL = 31 (that is, less than 40) and PI = 12 (that is,
greater than 11) this soil falls in group A-6. From equation
(23),GI = (F35) [0.02+0.005(LL40)] + 0.01(F15) (PI 10)\soGI =
(4135) [0.02+0.005(3140)] + 0.01(4115)(1210)=0.370Thus the soil is
A-6(0).