# Lecture – 7 Soil Mechanics

We have studied about index properties particularly
soil grain properties and their Grain Size Distribution. In the Grain Size Distribution we have introduced
two methods; one is coarse-grained soils based on the sieve analysis and other one is for
fine-grained soils where we have introduced a method called hydrometer analysis. In this class we try to learn about the measures
of gradation and we will try to classify the soil based on the gradation. If this procedure is not adequate for classifying
the fine-grained soils then we will introduce a method for classifying the fine-grained
soils based on its consistency. This lecture is with title Index Properties
and Soil Classification part 2. The measures of gradation as we discussed
in the previous class, the percentage finer is plotted on the y axis and particle size
is plotted on the x axis on the logarithmic scale. Basically different sizes are used for analyzing
the Grain Size Distribution curve. This is a typical Grain Size Distribution
curve shown here and different particle sizes are represented here D10, D30, and D60. D50 is known as the average particle size
and D10 is termed as the effective particle size it means that 10 percent of the particles
are finer and 90 percent of the particles are coarser than that particular particle
size D10. Similarly, D60 means diameter of the soil
particles for which 60 percent of the particles are finer and 40 percent
of the particles are coarser than D60. So, typically we use D10, D60 and D30 in arriving
at measures of gradation. Some commonly used measures are the uniformity
coefficient. The uniformity coefficient Cu is defined as
the ratio of D60 by D10. So when Cu is greater than 4 to 6, it is understood
as a well graded soil and when the Cu is less than 4, they are considered to
be poorly graded or uniformly graded. Uniformly graded in the sense, the soils have
got identical size of the particles. For example for desert sands Cu will be approximately
is equal to 1. Another coefficient to measure gradation is:
Cc is equal to (D30 square) by (D60 into D10) where coefficient of gradation or coefficient
of curvature is (D30 square) by (D60 into D10). For the soil to be well graded the value of
coefficient of uniformity Cu has to be greater than 4 and Cc should be in the range of 1
to 3. So higher the value of Cu the larger the range
of the particle sizes in the soil. So if the Cu value is high it indicates that
the soil mass consists of different ranges of particle sizes. Engineers frequently like to use a variety
of coefficients to describe the uniformity versus well-gradedness of the soils. Let us consider a Grain Size Distribution
curve which is shown in this slide. Percentage finer by weight on y axis and particle
size D represented on the logarithmic scale on x axis. This is a typical curve shown here. These different points are obtained by performing
sieve analysis. Here as we said D60, D10, and D30 can be worked
out like this. For example D60 is read from the curve as
0.7mm. Similarly D10 is read from this curve as 0.12mm
and D60 is equal to 0.7mm. From here we can find out that D30 as around
0.3mm. If you find out coefficient of uniformity
here that is the ratio of (D60 by D10) works out to be 5.8 and coefficient of curvature
(D30 square) by (D60 into D10) gives around 1.07. So this particular Grain Size Distribution
curve represents a well graded soil. In that case, it represents the particles
with different ranges. Particularly the Grain Size Distribution curves
are typically plotted on the semi logarithmic scale by keeping in view the different ranges
of the soil particles. The soil particle size can range from 20mm
down to less than 0.002mm. Let us look at the particle size classification. Based on the grain size, once we get the different
gradation of different particles then we will be able to classify the soil based on their
particle size. This is the table which shows the particle
size classification as per ASTM D2487. This is the code in ASTM which talks about
the particle size classification. So here in this column a sieve size is given
and particle diameter and the soil classification are also shown in this table. Different types of the sieves are indicated,
for example here it is retained in the 12 inch sieve that means the particle diameter
is greater than 300mm it represents boulder which passes 12 inch sieve and retained in
3 inch sieve which indicates the particle diameter ranges from 75 to 300mm which represents
cobble. The sieve size which passes 3 inch and retained
in 0.75 inch represents that particle diameter ranges from 19 to 75mm which is a Coarse Gravel
type. Similarly, when you come down you can see
it which is passing through number 40 sieve and retained in 200 sieves that means that
the particle size ranges from 0.075mm to 0.42mm which represents the fine sand. Below this, passing to number 200 sieves which
means less than 0.075mm particle size represents the fines that is silt and clay fraction. If you look into this here, the sizes from
19 down to fine sand, they are treated as coarse-grained soils. These soils which are fines are treated as
the fine-grained soil that means that from soil particle size at the size ranging from
0.075mm to 975mm diameter. Basically it is consider up to 19mm treated
as coarse-grained soils. Fines are passing 75 micron sieve or number
200 sieve, they represent fine-grained soils which consists of silt and clay fractions. Next we are going to see the typical characteristics
of the GSD curves. If you look into it, suppose it is possible
to get different types of the curves with different steep with inclinations or slopes
of the Grain Size Distribution curves. So if the curve is steep that is steep Grain
Size Distribution curve it indicates that the value of the Cu is low and it is a poorly
graded soil. It indicates that the soil mass consists of
identical sizes of the particles. As we discussed earlier it is very difficult
to compact identical sizes of the particles. So the steep curves indicate low coefficient
of uniformity or uniformity coefficient values. Poorly graded soil, which is also known as
uniformly, graded soil. Cu is less than 5 for uniform graded soils. Flat curves in the sense which is extending
from the larger particle size to finer most particle size indicates that higher Cu values
which indicates that it is a well graded soil. So flat curves generally indicate well graded
soils. Most gap graded soils have a coefficient of
curvature outside the range. If we have introduced like a gap graded soil,
some times an absence of intermediate particle sizes exists. So with this what happens is that, you know
that particular type of soil is called gap graded soil or bimodal soil. Most gap graded soils such as a Cc outside
the range. Let us look at a problem now in terms of GSD
of soils. In the previous class we have introduced a
problem, where in we took a weight of dry soil for the sieve analysis as 500g. It sieved through different sieves of sizes
4.75 to 75 micron sieve and it represents the grain diameter here in this column. Weight retained in the each sieve and this
is the cumulative weight retained in the each sieve is calculated. The cumulative weight passing is given by
490g is passed through this sieve only 10g is retained here. So (490 by 500) into 100, it gives you 98
percent. That is the percentage finer by weight which
is actually the first point on the Grain Size Distribution curve with ordinate y equivalent
to 98 percent and x in the terms of particle size has 4.75mm on the logarithmic scale. Similarly once we get, this we will get as
a percentage finer by weight. Once we plot percentage finer by weight on
y axis on arithmetic scale and the particle diameter on x axis on a logarithmic scale
we will get a typical Grain Size Distribution curve. In this case, once we plot then we will get
the typical Grain Size Distribution curve. Let us look now; we noticed that only 20g
of soil passed through 75 micron sieve. To calculate the percentage gravel the gravel
size particles which are greater than 4.75mm, the percentage gravel is calculated in a soil
mass. Let us take the particle size greater than
4.75mm, so 100 minus 98 gives you 2 percent. It indicates that 2 percent of the soil mass
has got a gravel size particle that is greater than 4.75mm. Similarly percentage coarse sand range from
2mm to 4.75, percentage medium sand 0.425 to 2mm, percentage fine sand range from 0.075mm
to 0.425mm which gives us percentage coarse sand as 33 percent, percentage medium sand
as 37 percent and percentage fine sand as 24 percent. So in a percentage silt clay fraction, a particle
having size less than 0.075mm is 4 percent and it is less than 12 percent. As we discussed the hydrometric analysis is
not generally recommended if the percentage of the silt clay fraction is less than 12
percent. If it is more then it is required to be determined
and then incorporated in the Grain Size Distribution curve. So having obtained this percentage gravel
and percentage sand and percentage silt and clay fraction we can determine different coefficients
which are used for measuring the gradation. Coefficient of uniformity can be obtained
by D60 by D10. So here the D10 is determined from the Grain
Size Distribution curve as 0.13mm, D30 as 0.5mm and D60 as 1.8mm. So by determining now, Cu works out to be
13.8 and Cc which is (D30 square) by (D60 into D10) works out to be 1.1. As Cu is greater than 6 and Cc in the range
of 1 to 3, the soil is said to be a well graded soil. These particle sizes are obtained from the
graph and this soil is well graded. Once we complete this analysis we say that
soil is well graded with percentage gravel 2 percent, percentage sand 94 percent and
percentage silt clay as 4 percent. Revise once again, D60 is nothing but 60 percent
of the particles are finer and 40 percent of the particles are coarser than D60. So this is required to be remembering while
determining these particle sizes from the Grain Size Distribution curve. So this is how we solve typical problems or
analyze the Grain Size Distribution curves. Now let us look into another example, something
that we have not seen so far. This is another example in which a typical
Grain Size Distribution curve is shown where the particles ranging from sand, silt and
clay is plotted with effective particle diameter on x axis and percentage finer weight on y
axis. If you determine here, the percentage gravel
is 100 minus 100 is 0, the percentage sand is 100 minus 60 that is around 40 percent,
silt is around 60 minus 12 which is 48 percent and clay is around 12 percent. Basically here the major amount of the soil
is with fines and then some sandy soil that is something like a silty sand type. So sandy silt where the sand is less compared
to composition of the silt and clay. Time to time these Grain Size Distribution
curves also can provide indication about soil’s history. As we have studied from the origin of soils,
soil can be a residual soil or a transported soil. In the case of a residual soil let us consider
typical Grain Size Distributions of residual soils collected at different ages of the soil. So here, this particular particle size distribution
curve represents for the young residual soil and this one is a intermediate maturing soil
and this curve represents the fully maturing soil. As you can see from the young residual soil
type to intermediate maturing to fully maturing, gradual changes in the particle sizes can
be noted. A residual deposit has its particle sizes
constantly changing with time as the particles continue to breakdown because of certain process
of weathering. So a residual deposit has its particle sizes
constantly changing with time as the particles continue to breakdown. So these are the typical Grain Size Distribution
curves for the residual soils where we have seen as young residual soil deposit, intermediately
maturing soil deposit and fully maturing soil deposit. As we run down we see that a residual deposit
has its particle sizes constantly changing with time as the particle continuous to breakdown. So we can say that the GSD can provide an
indication of soils history. This can give a conclusion about the soil’s
history in the past. Let consider a typical Grain Size Distribution
curve for transported soils. Here, a Grain Size Distribution curve for
a glacial type soil and glacial-alluvial soil can be seen. River deposits may be well-graded, uniform
or gap-graded depending upon the water velocity, the velocity with which the particles are
being drifted and the volume of the suspended solids, and the river area where the deposition
occurred. So as we can see with glacial to glacial-alluvial,
the particle size distribution of the particular soil is getting changed. This is also a typical example of the Grain
Size Distribution curve for the transported soils. This particular slide shows typical Grain
Size Distributions of different soils. Let us see different soils have been represented
here present as finer by weight on y axis and particle diameter on the x axis. You can see here, this curve indicates a typical
Grain Size Distribution for gravelly sand. If you see here, the large amount of the particle
size which is more than 4.75mm is gravel and then followed by sand. So we can say that, a composition of soil
mass is gravel in certain percentage, and predominantly sand, then we call it as gravelly
sand. This particular Grain Size Distribution curve
is shown for silty fine sand. We can see here there are fine sand particles
and the composition also has some silt particles in it so this particular curve represents
silty fine sand. This particular curve represents clayey sandy
silt because silt is predominant over the clayey and sandy soils. So, major portion of the soil here is silt
and these two curves s how a different Grain Size Distribution of the flocculent and dispersed
kaolinite. So dispersed kaolinite is finer and flocculated
kaolinite is in the silt size. This is the typical Grain Size Distribution
curve of sodium bentonite that is Montmorillonite. The Montmorillonite has got the finest particle
size, that is what we have learnt while introducing the structures of the soil minerals. We said that the bentonite has got the finest
of the finest particles with highest specific area and highest cation exchange capacity. So this slide represents typical grain size
curves for the different soils. In this slide, particle size distribution
of the bentonite, illite and kaolinite are shown. So as we learnt in the previous lectures,
kaolinite is supposed to have large particle sizes compared to illite and bentonite. The similar thing can be seen here which is
represented after Koch 2002. This is the kaolinite particle size distribution
curve and this represents illite clay and this is the sodium bentonite. Sodium bentonite is observed to be the finest
among bentonite, illite and kaolinite family. So having learnt about the measures of gradation
let us try to understand or see the significance of this Grain Size Distribution curve. That is, the practical significance and why
we it is required to determine the gradation of soils. This is the practical significance of GSD
that is Grain Size Distribution. So GSD of soils smaller than 75 micron that
is passing 200 number sieves is of little importance in the solution of engineering
problems. GSDs larger than 0.075mm (75 micron sieve)
have several important uses. So GSD affects the void ratio of soils and
provides useful information for use in cement and asphaltic concretes. So the Grain Size Distribution curve affects
the void ratio of soils. We have seen that, if the soil is well-graded,
the void ratio is supposed to be less and if the soil is uniformly-graded, the void
ratio is supposed to be very high. Well graded aggregates require less cement
per unit of volume of concrete to produce denser concrete and less permeable and more
resistant to weathering. So this is one of the practical significance
with the Grain Size Distribution analysis. The second significance is knowledge about
the amount of the percentage fines and the gradation of coarse particles is useful in
making a choice of material for base materials under highways, runways and rail tracks. Basically in highways and runways, it is utmost
requirement to give a mechanically stable foundation to support the loads which are
coming on to the pavement. So for that we are required to have the matrix
with certain amount of the fines. So, knowledge of the amount of the percentage
fines and the gradation of the coarse particles is useful in making a choice of material for
base courses under highways, runways and rail tracks. Another practical significance which we will
be discussing later is to determine the activity of the clay based on the percentage clay fraction. That means based on the percentage clay fraction
we can say whether that particular clay soil with particles finer than 2 microns is active
or inactive or in what extent it is active. All these things can be said by determining
the activity which is possible with percentage clay fraction. To design filters, basically number of filters
are required either to provide drainage in case of parking lots or road sub bases, etc. In case of retaining walls basically the filters
are designed or along the canal banks the filters are designed. Basically the criteria for designing the filter
is to see that to control the seepage and the pores must be small of to prevent the
particles from being carried from the soil. Let us look into this particular requirement
for the practical significance of the Grain Size Distribution curve with reference to
retaining wall. That is, a filter material extends to a retaining
wall. Basically a filter is required to be provided
behind the retaining wall to get rid of the water pressure. Once the wall is getting the effect of the
water pressure then the lateral pressure exerted on the wall increases. To get rid of the pressure from the water
which is collected behind the wall it is required to provide different ranges of filters such
as a coarse filter or a fine filter and a backfill soil. Then to drain the water from here either collection
pipes can be kept here or it can be kept with poles which are called as dipoles. There are certain criteria for selecting the
filter material basically for retaining wall: the ratio of D15 of the filter to D15 of soil
should be with in 4 and 20. Another criteria which is required to be satisfied
is D15 of filter to D50 of soil should be less than or equal to 25 and D15 of filter
to D85 of soil shall be less than or equal to 5. By fulfilling these criteria, one can design
different types of filter materials. The filters can be designed basically to see
that the backfill material, the fine particles in the back fill materials are not required
to be washed out. Only the water which is there within the back
fill soil is required to be drained. So that the wall is free from the lateral
pressure exerted from the water. Similarly this criteria for selecting the
drainage material is followed in Indian Roads Congress 37-2001. The criteria works out as follows: For the
pavement structural section, as the functional performance of the pavement depends upon the
drainage of the water the life of the pavement depends upon the drainage which is there at
the sub grade level. For that it is required to provide the drainage
material. So to design the drainage material it should
be seen that ratio of D15 of drainage material to D15 of sub grade should be greater than
or equal to 5 and the ratio of D15 of sub grade to D85 of sub grade should less than
or equal to 5. One more criteria is there that the ratio
of D50 of drainage to D50 of sub grade shall be less than or equal to 5. So these two criteria are basically
selected or required to be fulfilled to prevent entry of soil particles into the drainage
layer. For good drainage materials, basically it
is required to be seen that D85 of a drainage material is less than 4 times the D15 of a
drainage material and D2 of a drainage material shall be greater than or equal
to 2.5mm. Then we can select this particular material
as a qualified drainage material in the case of a pavement structural section. Another practical significance is that, to
estimate the coefficient of permeability of the coarse-gained soils. Hazen has tried to find out coefficient permeability
of the coarse-grained soils basically using the effective particle size D10, number of
collisions has been found out and it has been documented that for determining the coarse-grained
soils, basically sandy soils which has the particle size D10 that is effective particle
size is used to determine the coefficient of permeability of soils. That is the permeability is nothing but ease
with which the water can pass through the soil matrix. Also, the other significance is to assess
the frost susceptibility in soils based on the percentage clay fraction. Basically this frost susceptibility is required
to be high for the fine-grained soils that are particles passing 75 microns sieve also
or particles having less than 2 micron size. So to determine that, the reason for this
particular phenomena is that whenever the water in the voids of a saturated clean sand
or the gravel freezes then what happens is that the structure remains unchanged because
there will not be much expansion as freezing nearly increases the volume of each void by
9 percent in coarse-grained soil (because the expansion of water is contained with in
the void itself). So because of that they are not more susceptible,
whereas in case of a fine-graded soils the water which is prevalent in the void increases
and it causes a sieve called as frost sieve in case of a clay soils which is because of
this particular nature. To assess the frost susceptibility of given
fine-graded soils we are required to determine the percentage clay fraction. By assessing that we can find out how much
frost susceptible the soil is. Some applications of the GSA Grain Size Analysis
in Geotechnology and construction is the selection of fill material. So we wanted to select a fill material for
constructing embankment or constructing earth dams. In the case of earth dam, different gradations
are required in the different zones. So the selection of fill material is important
where the gradation will come into the picture because for certain type of constructions
we are required to have well graded materials to contribute to the strength. Road sub base material: As we said, to provide
a mechanically stable foundation we are required to have good knowledge about the grain size
analysis of the particular material under consideration. Next one is drainage filters, basically it
is required to know the drainage characteristics of the filter material with the edges and
soil characteristics and then ground water drainage and grouting and chemical injection. This is a phenomenon to strengthen the ground
or to fill the voids in the ground or to fill the voids or cavities in the ground. This grouting is a phenomenon which is used
by a chemical injection or cement sand grouts are inserted into the ground. To workout these, it is required to know the
knowledge about the grain size analysis. The concreting materials we have seen that,
in order to produce a denser concrete unity we need to have well graded materials so that
less amount of cement or asphalt can be consumed. Denser the concrete more resistant it is to
weathering. Dynamic compaction is another phenomenon to
compact the soils. For this also the knowledge of the grain size
analysis is required. Now let us look into the summary of grain
size analysis. We have seen that coarse-grained soils which
is greater than 75 micron is analyzed by using the sieve analysis. Then it is subjected to the soil mass which
is placed in the series of seals and subjected to shaking. While measuring the percentage retain and
calculating the percentage stimulate weight of the soil retain in each sieve, percentage
finer weight can be calculated which is plotted here. Based on that we can say that the different
coefficients of soils can be found out and then we can say that it is well graded or
a uniformly graded. Uniformly graded looks like the particles
are of equivalent size and you can see that this curve is a steeper curve that represents
uniformly graded or a uniform fully graded soil. This represents a well-graded soil because
the finer particles are filling the voids within the large particle sizes. So this is a well-graded purely sorted soil
tank. So coefficient of uniformity here we said
is that Cu is equal to D60 by D10 and coefficient of curvature as (D30 square) by (D60 into
D10). We also said that this effective particle
size is indicated as D10 which indicates that 10 percent of the particles are finer and
90 percent of the particles are coarser than this size. We also said that the D50 is the average particle
size. So, for the particles which are finer than
75 micron, it is required to find the particles by using the hydrometer analysis. Depending upon the sizes of the particles,
they settle and then hydrometer is required to be worked out with different corrections
to get the fine particle size percentages. Once it is corrected it can be joined with
the original Grain Size Distribution curve to get the complete curve. So this hydrometer analysis is required to
be carried out if the percentage fines are more than 12 percent. Now, basically having determined the particle
size of a coarse-grained soil, to some extend we are able to classify the soil. But we also have the soils with major amount
of soils or major portions of soils with passing 75 micron sieve that is very very fine soils. In that case, we need further information
to classify these fine-graded soils. So for that we consider the physical states
of the soil that means if you look into this slide different types of soils are shown. A gravelly soil with different particle sizes
and a sandy type soil is shown here And a clay type soil is shown here. As you can see here, this particular shape
can be attained with less amount of water. By adding more amount of water, this particular
lump of soil may lose its shape. So different physical shapes are possible
with fine-grained soils and that is what leads to consistency of a given soil. Before classifying the fine-grained soils
or before introducing the classification of fine-grained soils let us introduce a term
called consistency. The consistency of the fine-grained soil is
generally defined as a property of a material which is manifested by its resistance to flow. It represents the relative ease with which
the soil may be deformed. So degree of the firmness of the soil and
is often directly related to strength. Generally it is conveniently represented like
this: We say that fine-grained soils of soft consistency or medium consistency or medium
stiff consistency or medium firm consistency all are alike, stiff or firm or very stiff. These terms are unfortunately relative, higher
and also have different meaning to different observers. But this is a relative explanation of the
different physical states with different consistencies like soft that is said to be soil and it may
be having very low strength and medium stiff, stiff and very stiff soils. In soil mechanics, it is a need to determine
the range of the potential behavior of the given soil based on very few simple tests. Typical concerns are the following: You know
what will happen when the soil is having less amount of water or more amount of water, that
is, basically we are referring either coarse-grained soils or fine-grained soils. So in case of different states, typical concerns
for the geotechnical engineers are that soil might shrink or expand excessively in an uncontrollable
manner after they have been placed in geotechnical structures (roadway sub grades, dams, levees,
foundation materials). Soil might lose their strength and the ability
to carry loads safely. Test used to detect the potential problems
for coarse-grained soils basically gravel and sands are different from those used to
detect the potential problems for the fine-grained soils basically silts and clays. So if you see the coarse-grained soils, basically
water content is generally not a major factor; Major factor leading to shrinkage is the structure
of the soil skeleton. That means that the arrangement of the soil
particles or soil fabric can change from the loose state to the dense state. That is only in the case of a coarse-grained
soils is possible. Major factor leading to the shrinkage in the
sense of coarse-grained soil which indicates that a loose fabric can change to a dense
fabric or very dense fabric. In the fine-grained soils the water content
is a major factor. If you see here as water content increases
from left to right soil expand and loose in strength. As water content decreases from right to left
soils shrink and gain in strength. So if you look into this, fine- grained soils
are proved to get affected with the presence of water. So that is an interesting phenomena and A.
Atterberg a Swedish scientist basically for agriculture purposes has come out with certain
limits of water content at which the soil changes its physical states. So that is being currently used to determine
these limiting water contents at which the soil undergoes the physical states. These limits are used for determining or classifying
the fine grained soils. As we have studied earlier it was discussed
that fine-grained soils have higher SSA, because finer is the particle size whereas higher
is the specific surface area and electrical charges on their particles. They are supposed to have higher electrical
charges around the surfaces because of these fine-graded soils and clays in particular
can change their consistency quite dramatically with changes in water content. With the availability of water, the consistency
of the soil can be changed from the firm to soft state or very stiff to soft state. So each soil type will generally have different
water content at which it behaves like a solid, semi-solid, plastic and liquid. Different states have been introduced like
solid, a solid state which is like a brick tile and semisolid, plastic and liquid state. For a given soil, the water contents that
mark the boundaries between the soil consistencies are called as Atterberg limits [After Swedish
Soil scientist A. Atterberg (1902)]. Here onwards we will see about how the Atterberg
limits are defined, how they are deduced and how they are determined. So that and all we will be looking into the
further course of the lecture. The consistency of the fine graded soils is
mentioned here. The soil with the solid state is referred
as the limiting water content. The
limiting water content, in between solid state and semisolid state is indicated or termed
as shrinkage limit. Then the limiting water content between semi-solid
state and plastic state is indicated as a plastic limit and limiting water content between
plastic state and liquid state is indicated as a liquid limit. So these liquid limit, plastic limit, shrinkage
limit are known as Atterberg limits. So Atterberg limits are nothing but water
contents where the soil behavior changes. So here, there is a changing behavior in solid
to semisolid state, semisolid to plastic state and the plastic to liquid state. Mostly the natural soil deposits occur in
this range very close to plastic limits. Let us consider this particular curve which
is plotted with water content on the x axis and volume of sample on the y axis. This shows a different transition stages from
liquid to solid state. Let us consider a different transition point
like A, B, C, D, E and F. Here V0 is the initial volume of the soil mass, Vs is the volume
of the solids and Vw is the water. That means, here the volume of water is many
times more than the volume of the solids. V0 is the initial volume of the soil mass
that is above the liquid state. If the soil is allowed to dry, then it undergoes
changes from liquid state to plastic state, plastic state to semi-solid state and semi-solid
state to solid state. You can see here, the limiting water content
between the liquid state to plastic state is indicated as a liquid limit which is indicated
as WL (liquid limit). If you are indicating the liquid limit in
percentage, it is indicated as WL or if you writing the value of the liquid limit, then
it is indicated as LL (liquid limit). Similarly at point C, interface between the
semisolid state and plastic state is known as the plastic limit. So the difference between liquid limit and
plastic limit gives the range of the plasticity of the soil. That is liquid limit minus plastic limit will
give you the plasticity index. Once you allow the soil to dry up to point
D, then you can see that upon further drying, soil mass will not experience any change in
the volume. From point D to F, the curve undergoes that
is this particular zone is said to be a transition zone. When it is changing from semi-solid state
to a solid state the curve is not linear, it is in the curvy linear shape. So it is said that one of the possible reasons
for this phenomena is because air starts entering from this point. So this is the point where entry starts, then
it comes up to this point and then here by this time, mostly the water is replaced by
air. The soil grains are pressed so close that
there will not be further reduction in the volume. Even up on further drying if you keep it in
the oven, the soil mass will not further reduce in volume because the particles have been
pulled close in such a way that there will not be any further reduction in the void ratio. So at different levels, for example at the
initial volume is represented here, here it represents the volume of the soil mass at
point B and this corresponding point represents the water content corresponding to this state. Here this curve is supposed to be inclined
at 45 degrees because if the gamma W is around 10 kilometer per meter cube. The volume change of the soil at any point
during this process will allow you to drain from A to D. The volume change of the soil
is equivalent to the volume of the moisture lost. So this point B is the limiting water content
between the liquid state and the plastic state and it is said to be a liquid limit. At point c, the limiting water content between
plastic state and semisolid state is said to be a plastic limit. The difference between liquid limit and plastic
limit is indicated as plastic index and limiting water content at point E is indicated as shrinkage
limit. Shrinkage limit is the minimum water content
at which still the soil is completely saturated. Afterwards the soil transforms or undergoes
change in colour. It indicates that drying is taking place or
the change of the colour indicates that the water is being replaced by air. Here the evaporation of the soil is where
air starts entering. This is the entry point and here completely
it enters and it replaces all the water in the voids. If you extend this curve further down, here
this point gives the volume of the solids. The volume of the solids is constant throughout
in this process of drying. This slide shows the different states of the
physical states of the fine grade soil. That is interface between liquid state and
plastic state is defined as a liquid limit and interface between semisolid state and
solid state is defined as shrinkage limit. Interface between plastic state and semisolid
state is defined as a plastic limit. So as we defined, let us look into the definitions
of Atterberg limits. Liquid limit is a water content at which a
soil is practically in a liquid state, but it has infinitesimal resistance against flow
which can measure. So at liquid limit the soil has practically
a small shear strength which can be measured. So most of the soils have a shear strength
at liquid limit is around 2.7 kN by m square. Above liquid limit the soil state
changes into stationary state. Plastic limit is the water content at which
the soil would just begin to crumble when rolled into the thread of approximately 3mm
diameter. The plastic limit is thus with the same definition
or the same phenomena we determine the plastic limit of a given soil. The plastic limit is the water content at
which the soil would just begin to crumble when rolled into thread of approximately 3mm
diameter. Shrinkage limit is the water content at which
a decrease in water content does not cause any decrease in the volume of the soil mass. Even at the shrinkage limit, the degree of
saturation is said to be 100 percent. If you wanted a domain void ratio at the shrinkage
limit we can say e shrinkage limit. The void ratio shrinkage limit is equal to
water content of shrinkage limit which is nothing but the shrinkage limit times specific
gravity to the solids. Similarly for void ratio of a given soil at
liquid limit El is equal to Wl into Gs. The volume void ratio of the soil at the plastic
limit is indicated as Ep is equal to Wp into Gs. So Atterberg limits provide a good deal of
information on the range of the potential behavior of the given soil might show in the
field and variations in the water content. This particular behavior is shown here. When the soil is solid that is stiff or very
stiff, semisolid, plastic and liquid. This interface limiting water contents here
is given as shrinkage limit, plastic limit and liquid limit. Here it shows that a solid soil is hard and
brittle in the stress strain behavior. Suppose if you take a sample which is pressed
with a certain stress sigma and with a strain axial strain epsilon, the soil experience
or exhibits a brittle behavior when it is hard. During the semisolid state, the soil has combined
brittle and ductile behavior like a stiff cease material. And when the soil is in the plastic state,
soil e has very ductile and malleable behavior. If the liquid state that is soil behavior
is like a thick or a thin viscous fluid, almost the soil is having a juristic. So here liquid limit is the point where the
soil still possesses an infinitesimal shear strength which can be measured. Normally this shear strength of the soil at
liquid limit is said to be around 22.7 kN by m square. We also define the plasticity index is the
range of the moisture content over which soil exhibits plasticity. Plasticity is defined as the property of a
material which allows it to be deformed rapidly without rupture. So plasticity is very important for the fine
graded soils which are defined as the property of material and which allows it to be deform
rapidly without rupture or without breaking. Ip the plasticity index, if it is indicated
in percentage or if it is indicated in values, it is indicated as PI the liquid limit minus
plastic limit. The greater the difference between Wl and
Wp, greater is the plasticity of the soil. This is an important parameter while selecting
different materials for construction. In this lecture, we have studied about the
measures of the gradation. Also, we have been introduced to the consistency
of fine-graded soils and different physical states of fine-grained soils. We introduced the Atterberg limits and we
defined them as liquid limit, plastic limit and shrinkage limit. These Atterberg limits are used further to
classify fine-grained soils. In the next class, we will try to look into
different classifications based on the plasticity index that is Atterberg limits and their integrated
details pertinent to Atterberg limits and their determination.

## 4 thoughts on “Lecture – 7 Soil Mechanics”

1. Vishnu Savant says:

really very useful to the civil engg students thanks a lot

2. ankit sharma says:

superb way of explaning…thanx sir

3. saikiran akula says:

nice

4. KORI BIJENDERA KUMAR says:

bast method of explaining every topic.