detail of sand

Transcriptor : P
Soil Mechanics
Prof. B.V.S. Viswanadham
Department of Civil Engineering
Indian Institute of Technology, Bombay
Lecture - 2

In the previous lecture, we have seen the origin of soils and different types of soils based on their
deposition like residual soils and transported soils.

(Refer Slide Time 00.58)



In this lecture we will be studying about soil aggregate basic relationships. Before looking into
this soil aggregate basic relationships, let us look into different types of soils basically.

















(Refer Slide Time: 01.20)



If you see, this is a coarse-grained soil, coarse-grained sand with coarse particles having different
shapes and sizes. So this particular type of soil is originated from the disintegration of parent
ock.

(Refer Slide Time: 01.44)



This is the similar type of sand which is originated from the fine weathering of the rock into
forming something like fine particles. This is called some medium coarse to fine sand. So
typically we have seen different types of coarse and fine sands.


(Refer Slide Time: 02.22)



Let us look at another type of the soil which is basically silty in nature. The particles are very
fine but not finer than clay particles. This particular type of soil has predominantly silt particles
which are followed by clay particles, that is why this particular soil is called clayey silt. And this
particular soil is also known as black cotton soil. This is basically an expansive soil which is
prone for shrink swell characteristics. This is due to the prevalent mineral in the particular soil.
The black color of the soil can be noted.

(Refer Slide Time 00.58)



This particular soil is known as red soil which is predominantly having silt particles as well as
some fine clay particles and also coarse sand to fine sand particles. It is known as red soil
ecause of the color. But it is generally collected from southern India in a place called
Malaprabha in Karnataka. As we discussed, one of the examples of the transported soil is
BENTONITE which is a fine grained volcanic ash. You can see the fine grained nature of this
particular soil which has got very high shrink swell properties. This particular soil has also got
varied applications in the soil mechanics particularly in borehole investigations, in ba
ier
construction, etc. So having seen different types of soils, let us try to establish soil aggregate
asic relationships.

(Refer Slide Time 4.25)



If you look into it, last time we have said that soil is basically a three phase material having a
solid phase, liquid phase and gaseous phase and we call that state as a partially saturated soil. So
soil is inherently a multiphase material. Soil can also be in two phase, if it is having only solids
and air then it is called a dry state a two phase material and soil is also called as a saturated state
if it has got solids and liquid in the form of water. Now with this background let us try to
establish relationship and try to obtain different properties of soil mass.














(Refer Slide Time 5.12)



If you look into it, this particular diagram is an idealized 3-phase system for a partially saturated
soil. What you see in this particular block diagram is the green particles which are soil solids, the
own ones are organic matter, other particles which are in white color are actually voids filled
with air, the rest of the mass which is represented with
oken line indicates the voids that is
filled with water. This is a soil which has got solids, liquids and air in it. So this particular type
of soil is called as a partially saturated soil.

Generally this can occur above the ground water table that is in the water zone. This particular
soil can be idealized as follows: If you assume that if you are having a block of unit cross
sectional area as you have seen here then the solids representing volume of solids which is
nothing but the heights equivalent to the volumes because the area is having unit area, water
epresenting volume of water which is nothing but height equivalent to volume of water and air
epresenting volume of air.

On the left hand side you will see a gradation or a scaling with volume and on the right hand side
you will see with weights. If you see on the right hand side here Ws is nothing but the weight of
the solids, Ww is nothing but the weight of water and Wa is assumed to be 0 that is the weight of
air is assumed to be 0. So, total volume of the soil mass is equal to volume of solids plus volume
of water plus volume of air. If you see here volume of voids is a combination of volume of air
plus volume of water. And weight of soil mass is weight of solids plus weight of water. So this is
an idealized block for a partially saturated soil.







(Refer Slide Time 7.25)



Now as we said the soil also can be a 2-phase material in dry state as well as in saturated state. If
you see here, this is a dry state where solids and air are prevalent. Then what you see here
volume of solids and volume of voids is nothing but equivalent to volume of air because all the
voids are filled with air here so total volume here is nothing but volume of air plus volume of
solids. And the weight of soil mass is nothing but weight of solids because the weight of air is
assumed to be 0. In this case here if you see a saturated soil mass, the weight of solids and the
weight of water forms the weight of soil mass where the volume of solids and the volume of
water forms the total volume. We have seen this block, now let us try to establish relationships
y using volumetric ratios or weight ratios and the combination of these two to obtain different
parameters within the soil mass.


















(Refer Slide Time 8.35)



If you look into this, there are different types of volumetric ratios. The volumetric ratios
commonly used in soil mechanics are Void ratio e, Porosity n, Degree of saturation Sr, Air
content ac and Air void ratio or Percentage air voids is na. These are the universal rotations which
are used everywhere. So Void ratio is indicated with e, Porosity n, Degree of saturation Sr, Air
content ac and Air void ratio or Percentage air voids na. Now, let us try to define these parameters
and try to establish relationship among them.

(Refer Slide Time 9.25)



Now as we said the first volumetric ratio is void ratio e. Void ratio e is defined as the ratio of
volume of voids to the volume of solids. That is volume of voids to the volume of the solid mass
is nothing but a void ratio. Volume of voids refers to that portion of the volume of the soil not
occupied by the solid grains. Since the relationship between the Volume of air and the Volume
of water usually changes with ground water conditions as well as imposed loads it is convenient
to designate all the volumes not occupied by solid grains as void space that is volume of voids
Vv. In this slide we have tried to define the void ratio as ratio of volume of voids to volume of
solids.

(Refer Slide Time 10.21)



Now let us look into the different ranges of void ratios. If you see, e is equal to 0 indicates that
the material is having no voids that is absence of voids means that the material is called a solid
material. If the void ratio is greater than 1 which means that volume of voids are much greater
than the volume of solids and that can be possible in the soil mass. If you take the different soil
types and voids ratios e: For a Uniformly graded sand in a loose packing that is uniform sand in
loose state, the void ratio can be 0.85.

A Mixed-grain sand in a dense packing state means that mixed grain in the sense the soil has got
different ranges of particles and which are packed as close as possible that indicates the dense
packing, in that case we can get a void ratio which is as low as 0.43. Soft glacial clay which is
obtained through glaciation can have a void ratio of about 1.2. Soft highly organic clay can have
a void ratio 3. The Soft BENTONITE that is from the volcanic ash which is very fine grained in
nature can have a void ratio as high as 5.2. So we have seen different types of sands and clays
and their typical range of void ratios. These are just to give us an idea about the ranges of void
atio. Void ratio is expressed generally as a fraction. If you look into it, if e is equal to 0 it is
absence of voids that is solid material. If e can be greater than 1 that is volume of voids is greater
than volume of solids that can be possible in a soil mass.




(Refer Slide Time 12.17)



In nature, even though the individual void spaces are larger in coarse-grained soils, the void
atios of the fine-grained soils are generally higher than those of coarse-grained soils. The reason
is that the fine-grained soil has got a very very finely divided particle which has got some
chemical repulsions taking place. These chemical repulsions will not allow the particles to come
closer that make the void ratio of the fine-grained soils to be on the higher side compared to
coarse-grained soils. So the ratio of the volume of voids to the total volume is another ratio,
which is called as Porosity n which is given as n is equal to volume of voids to total volume.
That is here Porosity another volumetric ratio can be defined as n is equal to volume of voids to
total volume V. But V is equal to Vv plus VS that is volume of voids plus volume of solids. This
can be rea
anged as by taking the common VS out we get (1 plus Vv by VS) VS that is (1 plus e)
VS. So if you see we have established a relationship between Porosity and Void ratio that is n is
equal to e by (1 plus e). So if you know the Void ratio of a given soil mass you can calculate the
Porosity of the soil mass and if you know the Porosity of the soil mass you can estimate the void
atio by using e is equal to n by (1 minus n). In this slide what we have seen is that, in nature the
fine-grained soils are supposed to have a very high void ratio because of the high repulsions
which are taking place between the particles than the coarse-grained soils and also we have
defined the Porosity n is equal to volume of voids to total volume.











(Refer Slide Time 14.20)



If you look into the range of the Porosity values of a soil, the Porosity n of soil cannot exceed
100 percent. That is it has got a value ranges from 0 to 100. So, Porosity n of a natural deposit is
a function of shape of the grains, uniformity of grain size and the conditions of sedimentation,
how it is being deposited and things like that. Generally Porosity is expressed as a percentage.
For natural sands the porosity of a soil mass can range from 25 to 50 percent that is the porosity
of a given soil mass can have a values ranging from 25 to 50 percent for natural sands. And n is
equal to 30 to 60 percent can be there even for soft natural clays.

For some of the very soft clays we can have a value even as high as 90 percent. So the range of
the upper limit of the porosity value can be 100 percent and the range is from 0 to 100. The
Porosity n of a natural deposit is a function of the shape of the grain which means that the grain
can be angular, sub-rounded or rounded in shape or uniformity of the grain size that means that
whether it is uniformly graded that is all the particles are having same size or particles having
different ranges. In that case there is a possibility of decrease in the Porosity and the conditions
of sedimentation. This means that different types of sedimentation may be possible when the soil
is getting sediment, it can have a loose packing as well as dense packing.












(Refer Slide Time 14.10min)



If you look into it, we have defined Void ratio and Porosity. Out of Void ratio and Porosity, the
Void ratio is used frequently in soil engineering. The reason is basically if anybody has got the
values for Void ratio and Porosity one prefers to use Void ratio because any change in the
volume is a direct consequence of similar changes in volume of voids while volume of solids
emains the same. For example, if you look at the definitions of Void ratio, Void ratio is equal to
volume of voids to volume...