welcome to the course uh geology and soil
mechanics so basically uh today we will be
talking about the description of soil engineering
geology of soils and their formation so uh
first uh i will talk about the objective of
this particular course the course prepares
the students to be able to make effective
learning of formation of soil and basic soil
mechanics and the grading policy is like this
uh so basically the home assignments will
be having 40% weightage whereas the examination
will be having 60% weightage
now this is the total course plan so in the
week 1 the first week basically we will be
concentrating on description of soil engineering
geology of soils and their formation second
week we will be talking about the index properties
of soil third week we will be talking about
classification of soils soil structure and
clay mineralogy and fourth week soil compaction
will be covered
then permeability in soil seepage in soil
and flow net construction so that will take
2 weeks uh we will be covering this and then
in situ stresses and criteria for filter design
and then in week 8 effective stress principle
and soil water systems capillarity then in
the 9th week fundamental of consolidation
week 10 we will consider fundamental of consolidation
week 11 also we will be talking about stress
in soil and finally earth pressure theories
however so we will we will be little bit flexible
uh in this distribution suppose i mean week
1 and week 2 may be clubbed together and something
like that but overall at the end of the course
you will be learning these topics so however
the i mean this distribution among the weeks
will be little bit flexible now these are
the reference books so i am not talking about
a particular text book
so rather uh you should have different reference
books apart from that there are other books
available in the market so you can refer any
classical soil mechanics book which will cover
these topics so the first one principles of
geotechnical engineering by b m das then soil
mechanics lambe whitman and basic and applied
soil mechanics by gopal ranjan and a s r rao
now coming to the actual topic whatever we
thought about uh initially first we should
know the geology of the soil i how the soil
is getting formed in the in the atmosphere
or in the uh actual situation so first we
should understand what are the different cycles
for a rock to get disintegrated and forming
the soil now first we will discuss about the
rock cycle
the mineral grains that form the solid phase
of a soil aggregate are the product of rock
weathering so we will be talking about weathering
later on what actually weathering means so
on the basis of their mode of origin rocks
can be divided into 3 basic types as most
of you know about this that is igneous rock
sedimentary rock and metamorphic rock so these
are 3 broad classification of rocks which
are generally found in the practical situation
now coming to the igneous rock so they are
formed by the solidification of molten magma
ejected from deep within the earths mantle
so as i mean as you know from some volcanic
eruption or something for something like that
if you if some molten magma comes out and
ultimately if it solidifies then it will form
the rock which will be known as igneous rock
for example granite gabbro and basalt are
some of the common types of igneous rock
now coming to the sedimentary rock what are
the different types of rock which come under
the classification of sedimentary rock as
well as how do you know this is your sedimentary
rock the deposits of gravel sand silt and
clay formed by weathering may become compacted
by overburden pressure and cemented by different
agents
basically as you see in the river flow or
maybe in the lake deposit or uh marine marine
deposit basically the gravel silt clay these
particles or these materials will be basically
uh sedimented uh layer wise and finally uh
over the years it will be lying like that
and finally it will be forming the sedimentary
rock
they fill the spaces between particles and
form detrital sedimentary rocks so sedimentary
rocks can be uh classified in 2 categories
one is detrital sedimentary rocks and another
one we will talking about later on so basically
the detrital sedimentary rocks are formed
by filling the spaces between particles uh
whatever i just talked about means gravel
sand silt like that
then conglomerate breccia sandstone mudstone
and shale are some examples of detrital type
of sedimentary rocks it can also be formed
by chemical process known as chemical sedimentary
rocks so once we have seen the detrital sedimentary
rock now we are talking about the chemical
sedimentary rock
so these are 2 broad classification under
the sedimentary rock uh head so it can also
be formed by chemical process known as chemical
sedimentary rock such as limestone chalk dolomite
gypsum and anhydrite so these are also your
sedimentary rock but that comes under second
category which is chemical sedimentary rock
Then coming to the third group of rock that
is metamorphic rock now what is metamorphism
metamorphism is the process of changing the
composition and texture of rocks without melting
by heat and pressure so basically whatever
rocks you have seen so far igneous rock as
well as sedimentary rock so any kind of rock
can be uh transformed to the metamorphic rock
by the process of metamorphism where your
texture of rocks will be changing or uh under
the heat and pressure but there will be no
melting happening in the process
during metamorphism new minerals are formed
and minerals grains are sheared to give a
foliated texture of metamorphic rock so basically
during metamorphism you will be having some
chemical reaction and you will be you may
get some new materials or new minerals okay
in the rock formation
now granite diorite and gabbro become gneisses
so basically this is the metamorphism process
so granite diorite gabbro these were the igneous
rock initially and after the process of metamorphism
basically you have got the new kind of rock
that is metamorphic rock that is gneisses
similarly shales and mudstone which were actually
your sedimentary rock so now they had become
slates and phyllites
now coming to the weathering process so basically
one thing is very clear to you that whether
you are dealing with igneous rock or whether
you are dealing with uh sedimentary rock and
if you talk about the metamorphism process
that means if you are talking about the metamorphic
rock basically you have to come across some
weathering process
now what is that weathering process so weathering
is the process of breaking down rocks by mechanical
or chemical process into smaller pieces and
this is the process by which basically you
will be getting soil from formed from the
rockso rock can be transformed under definite
weathering process and ultimately it will
be disintegrated and forming the soil
so weathering is the process of breaking down
rocks that means you are going to break down
the rocks or you are going to disintegrate
the rocks by mechanical process or chemical
process either one of them and ultimately
you will be getting very small pieces from
a chunk of rock so mechanical weathering may
be caused by the expansion and contraction
of rocks from the continuous gain and loss
of heat which results in ultimate disintegration
so what is the mechanical weathering process
so mechanical weathering process i mean it
is caused by the expansion or contraction
of rocks so rock can be expanded or it can
be contracted from the continuous gain and
loss so basically your temperature is uh going
to vary or the i mean you either the rock
is gaining some heat or losing some heat based
on that basically the ultimate disintegration
is happening and which is known as mechanical
weathering process
now what is chemical weathering then in chemical
weathering the original rock minerals are
transformed into new mineral by chemical reaction
so basically you have some rock and you are
allowing the rock to go for some chemical
reaction and some transformation in the minerals
will be happening and ultimately you will
be getting some new mineral which will be
known as and this process will be known as
chemical weathering
like water and carbon dioxide from the atmosphere
form carbonic acid as we all know from the
chemistry which reacts with rock minerals
so whatever rocks are lying in the in the
atmosphere or in the in the field right so
those rocks will be going under some chemical
reaction in presence of water and carbon dioxide
which will be forming some carbonic acid and
this this rock minerals will form some new
minerals and soluble salts so this is this
is purely uh some chemical reaction and based
on that reaction you will be getting some
some different mineral as well as soluble
salts
so these photos will show you the evidence
of the mechanical weathering process as you
can see on the left side photo basically this
is the evidence of your mechanical weathering
process by the ocean waves and you can see
the rocks are getting disintegrated by the
ocean waves and on the right side basically
you will be observing the weathering mechanical
weathering process caused by the wind
so as i told you the mechanical weathering
process will disintegrate disintegrate the
rock and this disintegration may happen due
to ocean waves wind rainfall or any kind of
mechanical agents
now coming to the actual rock cycle so after
understanding what are the different types
of rocks how they are getting formed and how
they are getting disintegrated by some weathering
process and finally they are forming some
new minerals so after understanding all those
things lets look at this rock cycle
so basically if you look at this this is a
magma i mean uh below the earth i mean crust
or this is happening or this is lying in the
mantle so basically this magma when it comes
out and after solidification basically it
becomes igneous rock then this igneous rock
after transportation erosion weathering after
all this activities it i mean creates some
sediments then those sediments are getting
compacted cemented or crystallised and then
becomes sedimentary rock and then some sedimentary
rock after metamorphism it may become the
metamorphic rock and again metamorphic rock
after melting if you melt it again it you
may get the magma
similarly from igneous rock you can directly
go to the metamorphism process and come to
the metamorphic rocksimilarly after sedimentation
again you get this loop that is again transportation
erosion and weathering is happening and again
further disintegration is happening and sedimentation
is happening so this is the cycle
then even after sedimentary rock it may come
to the transportation erosion or weathering
process and again sedimentation and goes to
goes back to the sedimentary rock so this
is a kind of cycle that means you are starting
from one you are coming back to the original
position so this is known as rock cycle and
you need to understand this rock cycle properly
to understand the behavior of uh different
rocks as well as the formation of different
rocks how they are getting formed and how
they are getting disintegrated to form our
actual uh material of interest that is soil
now coming to the soil formation and soil
types on the basis of geological origin of
their constituent sediments soil can be divided
into 2 main groups now what are those groups
those which owe their origin to the mechanical
and chemical weathering of the parent rocks
now when i am talking about the mechanical
weathering then it retains the minerals that
are present in the parent rocks example gravels
and sands that means you have some rock
now how the soil has soil is getting formed
so you have the rock now you are applying
some mechanical weathering process in the
rock or chemical weathering process in the
rock so if you apply the mechanical weathering
process in the rock so basically it retain
retains its minerals that were present basically
in the parent rock like i mean if you consider
gravel sand and all those things they are
basically the disintegrated part of the parent
rock
whereas if you if you allow the chemical weathering
to this rock basically the decomposition of
rocks caused mainly by chemical process though
you may get some change in the mineral composition
in the rock material or the disintegrated
rock material so like your clay to some extent
silt so we will be talking these materials
in more detail in the subsequent lectures
so basically what are the different types
of soil now you have got after disintegration
of the rock one is gravel another one is sand
so these are basically are getting formed
uh by your mechanical weathering process whereas
we can get clay as well as silt which are
basically coming from your uh chemical weathering
process of your rock
now those which are chiefly of organic origin
now basically there are 2 groups as i told
you the first group is getting formed by the
weathering process another one is chiefly
organic origin that means you have the organic
composition in the soil like if you talk about
the forest soil and all so they will be having
more of organic origin so that will not be
really coming from the disintegration of the
rock rather some other organic or the vegetable
materials are uh lying or representing the
soil
now if the products of rock weathering are
still located at the place where they originated
they are called residual soils that means
you are considering some rock or you you you
see some rock lying over there and after weathering
process it could be mechanical it could be
chemical but after the weathering process
if the disintegrated rock that means soil
is located still located at the same place
then that is known as residual soil
so you must know i mean if i say if you tell
me that this is the residual soil then i will
immediately understand the soil has been formed
at that location itself by the disintegration
of the rock then coming to the another type
of soil that is transported soil now what
is transported soil any soil that has been
transported from its place of origin that
means where they have formed now it has been
transported by wind water ice or any other
agency and has been redeposited in somewhere
else then it is called transported soil
so that means it is the origin or the formation
of the soil is happening somewhere else and
then it is getting transported by some agencies
or by some agent and ultimately it is getting
(dece) redeposited at somewhere somewhere
else and that is known as transported soil
transported soil is further classified as
per transporting agency and method of deposition
first one is the alluvial deposit soil that
has been deposited from suspension in running
water suppose if you consider the soil getting
formed somewhere else and then it is getting
transported by the river water and it is getting
deposited somewhere else so it is of course
it is a transported soil but the soils that
had been which have been transported and uh
by water fresh water and deposited uh in the
riverbed that will be known as alluvial soil
similarly lacustrine deposit that is i mean
it is known as uh happening in the fresh water
that is in the lake site or something like
that that is these are the soils that have
been deposited from suspension in still fresh
water of lakesthat means below the reservoir
or near the reservoir whatever deposition
you are seeing whatever soil deposition is
happening so that is known as lacustrine deposit
then marine deposit soils that have been deposited
from suspension in sea water so you will be
having uh too much of salt uh presence uh
in the in the soil deposit and that is known
as marine deposit
then aeolian deposit that is soils that have
been transported by wind so it is getting
formed somewhere else and then it is getting
transported by the agency that is known as
wind so that is known as aeolian deposit then
glacial depositsoils that have been transported
by ice simple ice so that is known as glacial
deposit
so depending on different types of deposition
you will be getting different properties so
those things you must remember i mean otherwise
why should we know all those things because
based on the transporting agency okay you
will be getting different properties of engineering
properties of the soil
the soils of india can be broadly divided
into the following groups based on the climatic
conditions topography and geology of their
formation first one is marine deposits as
i told you they are very soft i mean uh if
you if you see the coastal region of our country
so there basically you will be getting the
marine deposit and obviously you have you
you you appreciate or you agree that they
are really soft material and may contain organic
matters of course
they possess low shear strength as of now
the shear strength and compressibility those
things have not been coveredbut shear strength
means some kind of strength which will be
provided by the soil to take care of the external
load or external force coming on uh the particular
soil deposit
so it (po) it possesses low shear strength
which is not good for any kind of construction
and high compressibility that means it is
a kind of i mean you have seen some spongy
material so it will be having high compressibility
like so if the soil material is like that
which is having very high compressibility
then it is it should not be very good material
for the construction and hence pose problems
as a foundation material or as a material
of construction
that is very true because if you try to place
the foundation on top of this kind of uh say
marine deposit then what will happen because
of its softness very soft uh soil is there
because of its softness the foundation may
settle down excessively or due to its compressibility
i mean due to its compressibility it will
settle down and due to its low shear strength
basically you will be getting the failure
that means the foundation if you put the foundation
and if you apply the load on top of that it
will collapse so this material is not at all
good for any kind of civil engineering construction
then coming to the laterites or lateritic
soils they are formed by the decomposition
of rock removal of the bases and silica and
formation of oxides of iron and aluminium
at the top of the soil profile generally lateritic
laterites pose no difficulties as foundation
material and retain their slopes well so lateritic
material is a kind of say uh rocky kind of
thing very hard very tough material or tough
soil so there will be no problem for construction
then coming to the black cotton soil that
is the most problematic soil in our country
so these soils have been formed from basalt
or trap and contain the clay mineral montmorillonite
at this moment uh i have not talked about
the clay minerals so it may not be clear to
you what is montmorillonite of course after
subsequent lectures it will be clear to youso
clay basically is formed by different clay
minerals
so one of the mineral is montmorillonite and
that mineral is very very problematic particularly
in any type of clay so it contains the clay
mineral montmorillonite which is responsible
for excessive swelling so if you add water
so montmorillonite is having some property
if you add water it will swell excessively
and if you take out water it will shrink excessively
so that means swelling and shrinkage both
the things are excessive in this kind of soil
so i mean that is why it is also a very problematic
soil suppose you are placing some foundation
on this kind of deposit and if some water
is getting added to this kind of soil then
immediately it will swell and it will give
some swell pressure on the foundation and
if you are not designing the foundation based
on that then you are gone similarly if during
dry season if the water is coming out from
the soil it will shrink that means the foundation
will try to collapse or try to settle so both
the things or both the times during addition
or removal of water this soil will create
hell lot of problems
then coming to the alluvial soils the deposits
have alternating layers of sand silt and clay
as it is quite obvious because this kind of
soil you are getting uh under the running
water so there is a great deal of variation
in the thickness of these layers and their
horizontal development then coming to desert
soils so these soils are wind blown deposits
of sand
the sand dunes have an average height of about
15 meter but can at times be considerably
higher they are formed under highly arid conditions
so suppose any any desert area like rajasthan
or ahmedabad or this kind of location if you
see you may get this kind of soil desert soil
now boulder deposits rivers flowing in hilly
terrains and near foot hills carry large boulders
downstream and the deposits thus formed may
contain large quantities of boulders so that
kind of say generally it is happening or it
is getting formed in the hilly region or the
region where you have lot of mountains like
Himalayas and other things
now this is the regional soil deposits of
india uh generally so as you look at this
part so the northern part is completely alluvial
soil because lot of rivers are there so and
you know the alluvial soil deposit is generally
happening uh on the bed of river so and then
coming to the desert soil as i told you you
will get this kind of desert soil maybe in
the in the rajasthan uh or in punjab or somewhere
else and then laterites and lateritic soil
you will be getting this part uh the western
part of west bengal and uh jharkhand and bihar
area and little bit of maharashtra and then
black cotton soil
so you can see the black cotton soil is completely
covered in mp and maharashtra and then part
of andhra pradesh so this is a real problematic
soil as i told you and this black cotton soil
if it is happening for any kind of geotechnical
engineer or the civil engineer then you are
not really lucky let me tell you so this black
cotton soil should be avoided for any kind
of construction but we are really unfortunate
that we we have lot of black cotton soil in
the country
then you will get the marine deposit along
the coastal area as i told you so this is
this is along the coastal area and you will
be getting the marine deposit as well as eastern
part of your uh west bengal then boulder deposits
which is generally happening along the mountaineering
region or the hilly region so okay so thank
you very much for your patience hearing so
we have seen the description of soil engineering
geology of soils and their formation thank
you
