Lecture – 9 Origin And Types of Soils


Hello everyone and welcome to this new lesson
on engineering geology. Today we are going to
discuss about types and origins of different types of, different soils that are found across
the world and before we take up the subject matter
of this lesson, we are going to look back at the
questions that I gave when we were making the last presentation and the questions that
are asked are on the slide here. . The first question that I gave you was how
in general would you expect the percentage of oxygen
to change in a silicate mineral with weathering? So, what happens typically is that if you
consider, if you consider the Bowen’s reaction series, then you will find that the silicate
minerals actually will have higher percentage of oxygen
in comparison with silica near the top of the
reaction series and as you come down, the oxygen percentage becomes smaller in comparison
with silica, in comparison with the silica content. .Now, to give you an example actually, the
feldspars typically will have a radical of this type;
feldspars if you recall, you will have let us consider potassium or orthoclase actually
which occur little bit up on the Bowen’s reaction series.
The formula of potassium feldspar or orthoclase is
this; it is essentially potassium alumino silicate whereas as you come down, actually
this is becoming a little bit fuzzy, let me open up
a fresh page for doing this. So, what we are saying here is that orthoclase
mineral has got a chemical composition as given
by this formula here. It is essentially potassium alumino silicate and it occurs high on Bowen’s
reaction series in comparison with quartz, quartz has got a chemical formula of simply
SiO2. You can see that you can, actually this clearly
shows how the oxygen as a percentage of silica changes as you climb down the Bowen’s reaction
series. . So, this is the direction, let us say if we
can coin the word weatherability, then that increases, that
actually decreases sorry not increases, decreases as we climb down Bowen’s reaction series.
So, you can see how percentage of oxygen changes
as you climb down Bowen’s reaction series. Getting back to the question set, the second
question was which of the two minerals abundant on
earth’s crust would be more resistant to weathering? I gave you the two minerals – quartz
and .feldspar and quartz, if you recall the Bowen’s
reaction series, quartz actually occurs at the
bottom of the reaction series and feldspar occur a little bit high above. . So, the susceptibility of quartz weathering
is going to be much smaller in comparison with
feldspar and actually feldspar really is different; it is not a single type of mineral, there
are different types of feldspars. It could be
containing potassium or it could contain sodium or it
could contain aluminum. So, these different types of feldspars are also susceptible to
weathering, to a chemical weathering actually to a different
two different extents. Now, sodium feldspar is
typically more resistant to weathering than the other types of feldspars that I was just
now mentioning. Now, the third question that was asked was
what is meant by the term diagenesis. Diagenesis is
really chemical change form one type of mineral to another type of mineral. I will give you
an example; let us consider again the weathering
of feldspar and also weathering of orthoclase mineral. I mentioned that orthoclase is simply
potassium alumino silicate and the weathering, a
major weathering, chemical weathering goes like this; it occurs in a little bit acidic
environment in the presence of ground water and what you
get out of this thing is a mineral called kaolinite
given by this formula that I am just writing and that releases
silicic acid and this is in solution .actually, this remains in solution and it
also releases potassium ion; so this one, these two things,
species – chemical species are in solution and this one is kaolinite, as I mentioned,
this one here. . So, that is a reaction which could be classified
as diagenesis. So, you can see that the chemical characteristic of the original parent material
actually gets altered into a different type of material.
So, that sorts out the second problem, second question that I asked – getting back once
again – or the third question rather. The fourth one, the fourth question that I
asked that was how magma can remain in a partially molten state? So, magma can be a composition
of several different mineral species as I indicated
in the last presentation and all these minerals have got different boiling points and as a
result the mixture can remain in a partially molten state
under different types of environmental values which are pressure, the chemical environment
and temperature. So, basically the magma, the magma essentially
could remain in molten state because of the difference in boiling point, difference in
melting points rather of different types of mineral
species that is contained within the body. So, that is actually wraps up the question
set and then we get into the objectives of this particular
lesson. .. What we are going to discuss in this lesson
is essentially different types of soil forming processes, we are also going to discuss about
what are the different types of different types of
soils that develop because of these processes from parent rock and also we are also going
to look at the compositional characteristics of different
types of soils and how do we, what are the principles really of classifying these soils
and finally, we are going to look at the inventory of
different soil types that we come across the nation. So, these things are listed here on the slide.
So, you can have a look at it again; we are going to
discuss the soil forming process, we are going to classify as I indicated and different types
of soils based on genesis, different formation
processes really and composition and then we are
going to describe compositional and weathering behavior of different types of minerals that
composes these different types of soils and we are going to describe the different soil
types that one would encounter, is likely to encounter
rather in different parts of the country. Now, origin of soils; how soils originate
basically? The major processes involved there include
weathering of parent material which is essentially soil, preexisting soil or rock mass and these
things actually give rise to a class of soils called mineral soils, then another process
that can give rise to soil deposits is volcanism. As a result
of volcanism, directly soil deposits can form and .example of this thing is volcanic ash and
mummy sands, then soils would also develop because
of human action like tailings dams, tailing deposits where mining activities, because
of mining activities waste materials get dumped. . So, tailings are essentially mine wastes or
soils can form also because of biogenic activity, activity of other types of biological organisms.
For instance, there could be excreta from birds or
bats or there could be deposits really because of biogenic activity and example of this thing
is phosphate deposits that are found at different
places like some islands of the coast of Peru and
other places and actually these are economic minerals really, they are called guano deposits
and they are essentially a very large source of
phosphate minerals and they are commercially exploited as well. So, the biogenic activity
also can give rise to very large volume of soil like
deposits. Now, this actually tells you the varied nature
of soil because of the fact that soils can form from
processes as varied as those listed on the slide, the characteristics of different types
of soils can also be expected to vary by a wide margin. .. Now, we look at on this slide; we look at
typical composition of soil mass. Soils in a sense are
composed of solid grains, then they would, the soil mass will also include void spaces
and some of this void spaces can be filled with pore
fluid or and some of it is going to be filled with air. Now, pore fluid can be of any, can be of very
wide, a large number of different types of chemical
characteristics really and mineral, rather the solid grains can also be of a very different
or a great variety of solid grains can also be included
in the soil mass. Now, the soil grains are typically
formed of minerals which are weathering products of preexisting rocks or they could be from
the deposits, they could be essentially deposits
formed because of biological activities or they could
be even partially decomposed organic material like wood fibers and other things. Now, as
a result, the composition of soils can also
range over a wide; can range over a wide variety. Now, a typical composition of soil is shown
on this particular slide here and we have considered
two examples here; one is a mineral soil. Mineral soil would be roughly 25% water, 25%
air voids and 25% solids and as I indicated, the
solids could include minerals – inorganic minerals or
organics like wood debris and other different types of organic inclusions and then on the
bottom part what I have shown is a typical composition
of organic soil and here, these types of soils .would contain a very large proportion of
the total volume as water and this could be more than
50% of the total volume really and rest of it would be composed of organics and minerals. . We consider saturated soil in this case and
if the soil is saturated, then of course the air voids, the
volume of air voids is going to be 0 as indicated on the bottom pie chart. So, the one on the
top there, this one here, this pie chart represents
mineral soils and the pie chart on the bottom here,
bottom right represents a typical composition of organic soils. Now, these are typical compositions you should
realize and the composition of a particular soil
that you might actually encountered during an assignment of engineering geology can in
fact vary over a very wide margin and the details
that is presented on this slide should be considered
only an approximate indicator of different types of constituents of a volume of a certain
volume of soil. .. To summarize, we look at the constituents
and structure of soils and as I indicated when I was
showing the previous slide was the soils have got solids in it and the solids are, the solids
could be minerals or they could be in fact organics,
they could also be organics and the minerals; so
this part here is solid, soil solid and the minerals could be clay or non-clay minerals.
What we mean by clay or non-clay minerals will become
a little bit more apparent when we proceed through this particular presentation and they
originate from, typically they originate from weathering of rock or soil or they originate
from biological activity or they are really, they are
really waste material from decomposition of biological inclusions within the mass of soil. Now, the liquid phase of a soil mass is typically
composed of water but there could be other types of liquid also within the soil mass
like there could be other polar fluids like in polluted
areas, polluted environments there could be pore fluid composed largely of petroleum waste.
Now this thing, the liquid phase depends on the chemical weathering process, the chemical
characteristics of the liquid phase that depends on chemical weathering process and that actually
strongly influences the engineering behavior of a type of soil. .. Now, in addition to it which I did not mention
here, you have got air voids as well, air void and
the way these different constituents mineral or soil solids, pore fluid and air void, they
are distributed within the volume of soil that
is given the name of soil structure and the structure that
develops also strongly depends on the state of weathering, cementation and the characteristic
of pore fluid in addition to the mineral grains
or organics or soil solids that the soil mass is
composed of. To give you an example, what is meant by a
structure; now you think about soil grains, soil
solids; the soil solids could be oriented in this manner and the soil solids could also
be oriented in a card house like structure open worked,
card house like structure because of several different
things and this thing is called a non-sensitive structure and this is called a sensitive or
collapsible structure. .. Now, the question arises; how a collapsible
structure could actually develop? A collapsible structure could develop because of weak cementation
at the inter-granular context, at the intergranular context or because of other chemical influences.
For instance, the clay masses that originate in marine environment, they sometimes
are uplifted and they form a part of the subaerial environment and in that case what happens,
ground water infiltration leaches out the brackish water that was occupying the interstitial
void space when the clay was in marine environment and eventually, the water that
occupies the inter-granular void space that becomes
fresh water, that becomes non-brackish and as a result what happens actually, that process
also gives rise to a sensitive structure. So, what I mean by that? Let us say, you have
got a clay that was originally deposited in marine
environment that is called a marine clay and then that gets subjected to an uplift and
becomes a terrestrial deposit, terrestrial clay and
if you consider this type of transition, then this has got
fresh pore fluid or non-brackish actually, non- brackish pore fluid, let us call it non-brackish
pore fluid, non-brackish pore fluid and this one
here has got a brackish for saline pore fluid. .. This particular clay, this particular clay
could be non-sensitive to begin with but because of the
leaching of the saline pore water by fresh ground water that infiltrates the clay mass,
a sensitive structure could develop within the soil. . So, this one here is non-sensitive clay, this
is a non-sensitive clay, sensitive structure whereas the
one at the bottom may have a sensitive structure. So, that is another example of development
of .sensitive structure because of chemical changes.
Now, what are the mineral constituents of the
soil? . As I indicated earlier, the mineral constituents
could be non-clay minerals and mineral constituents could also be clay minerals.
Now, non-clay minerals actually will have a non-plate
like, non-plate like structure or non-plate like particle, non-plate like; what I mean
by that? .. So, these are essentially roundish grains
whereas the clay minerals that developed, they are
typically plate like. So, clay minerals will have grains which are likely to be much longer
in one direction in comparison with the dimension
that is there in perpendicular to those long directions.
So, this is going to be a very short dimension in comparison with other direction whereas
nonclay minerals have got other types of shapes and the shapes could be of very different
geometry but they are not going to be plate like, they
are typically not going to be plate like. .. And these things, the clay minerals typically
develop negative surface charges because of isomorphic, because of chemical substitution,
substitution of cations and that is why they develop negative charges; whereas, the charge
density on non-clay minerals are much smaller. Examples of non-clay minerals are quartz and
feldspar – the other types – these are most abundant actually, quartz and feldspar, sodium
feldspar, they are most abundant because of their
resistant to weathering. There could be other minerals found in special environments which
are not especially exposed to chemical weathering. Now clay minerals, they would typically include
kaolinite. They are the major, these minerals are the major types of clay-minerals. They
include kaolinite, Illite and Smectite and here you can
see, I indicate that kaolinite is a 1:1 clay mineral, whereas Illite and Smectite are 2:1
clay mineral. What do I mean by that is these clay
minerals typically are combinations of tetrahedral silica sheets, tetrahedral sheets
of silica and octahedral sheets of Gibbsite or Brucite. So, this is
actually Gibbsite or Brucite; G is Gibbsite and the symbol B I used here is for Brucite
whereas this one here is silica. .. So these things, the top one, the top layer
is actually octahedral, it could be bi octahedral or other
types of octahedral species whereas the silica sheet, they are tetrahedral in composition
in geometry and this is an example really of
1:1 clay mineral where you have got basically one
tetrahedral sheet and one octahedral sheet. Now, if we have got two tetrahedral sheets,
two tetrahedral sheets and one octahedral sheet,
then what we are going to end up with is basically 2:1 clay mineral. So, that tells you actually what is the structure
or what is the structure of a grain of clay mineral
and now these basic units, they are going to actually, they are going to be interlinked
to other grains by covalent or other types of bonding
which we will examine in details when we talk of
clay minerals later on in one of the future presentations of this particular course. So, then the mineral constituents of soil
could be clay minerals or non-clay minerals and most
abundant non-clay minerals include quartz and sodium feldspar whereas most abundant
clay minerals include Kaolinite, Illite and Smectite.
Now, what are the characteristics, what are the
characteristics that one needs to consider while looking at clay or a soil deposits? .Soil deposits typically show a very high
degree of variability in terms of composition, grain size,
behavior and spacial distribution though the composition of different types of soils as
I mentioned in the previous slides there, would
vary by a very wide margin. For example, one could have an organic deposit which is composed
of organic, partially decomposed organic material and a large volume of it is comprised
of water voids whereas you could have a relatively compact mineral soil which has got a relatively
smaller proportion of void space and a noncollapsible structure. Whereas, an organic soil could
have an open work, open worked or card house like structure in many situations. . Then you could have grain size distribution;
the grain sizes that is composed that the soil is
composed of can also vary over a very wide margin. You could have soils which are composed
largely of very fine clay sized particles which are typically I mean, their dimensions
could be on the order of a few microns; whereas, you could
have soils that could include boulders and these
boulders could a meter across in size. Now, the behaviour of the soil could also
vary because of the reasons that, because of the reasons
some of which I mentioned earlier and the thing that we consider here was the open worked .structure and non-open worked structure or
collapsible or non-collapsible structure that develops
because of different types of orientation of soil solids and wide space within the soil
mass. And, facial distribution of soils can also
vary by a wide range and what you could get is a soil of
very different characteristics right next to a side occupied, right next to a side where
the soil type is totally different. So, that actually gives
you a very challenging environment, it poses a great
problem in designing different types of structures or different types of projects because of
the soil variability, particularly in certain
environment as we will see later on in the course of this
and other presentations. Now, soil genesis; how soil actually forms
out of rock or chemical or biological processes or
volcanism is indicated on this slide here. . As I mentioned, from weathering of rock, a
class of soil develops which is called essentially residual soil. In – situ, from in – situ
weathering, from weathering without any transportation and
then residual soils can be transported from the place where they originally formed by
different agents of transportation like water, wind,
glacier ice or even submarine current and they give rise
to another class of soils that is called transported soils. .You also could have volcanic soils and chemical
or biological deposits; all of these things can be
subjected to different types of processes, physical and chemical processes that could
be compaction, weathering and lithification and
that may end up in formation of sedimentary rocks. So, this is basically, this basically tells
you how different types of soil deposits are formed and
how they evolve with time. Now, we consider soil types based on the formation process.
As I indicated, residual soils formed out of weathering
of bedrock and example of that is decomposed granite; they are also called in some places,
they are also given the name in some places Gruss or
Saprolite. . Then you could have transported soils and
transportation could be by gravity and if you find a
soil deposit that is basically because of gravitational transport and that is called
Colluvium. For example, slope failure debris Talus slopes,
transportation could be by air and these things are
called Aeolian deposits. Loess is an example of Aeolian deposit, example of Colluvium is
Talus. Then you could have water deposited soils
and they include fluvial if they are worked by flowing
river water or there could Lacustrine which are lake bed deposits or glacio-fluvial, basically
they .are melt waters that form in different parts
of glacier, glaciers or transported soils could be
directly deposited from ice. They are called glacial deposits. Then wave action can also lead to development
of transported soils or submarine currents can
also transport, can also lead to the development of transported soils. Examples of all these
soils include alluvium, alluvium are essentially
fluvial deposits or till they are deposited by ice or
actually till or moraine is used for essentially the same kind of deposits, similar type of
deposits. . You could also have chemical or biological
soils as I mentioned earlier. Chemical soils, chemical
soils and biological soils include evaporites; evaporites are solid precipitates because
of evaporation of different types of water that
have dissolved colloids in it and we could have one
of the examples of evaporite is basically the salt deposits that develop in dessert
environment. We could have phosphorites, they could be
droppings of different organisms, they are phosphate
rocks. You could have carbonates, they also get deposited from solutions of carbonate
minerals or it could be hydrogenous deposits that develop
in submarine environment, environments where near hydrothermal vents which we will consider
later on in one of the future presentations. .Examples of these deposits include calcareous
sand, they are basically composed of grains broken grains of calcite and these calcite
grains could have biological or inorganic origin. Many
of the continental shelves are underlain by calcareous deposits of this type and finally
you could have volcanic soils and volcanic soils as
I mentioned directly originates, volcanic soils directly
originates from volcanic activity. Examples of it include volcanic ash and sands composed
of pumice grains. So, these are the different types of classification
of soils that you might get depending on the process of origination, how the soils actually
originate. Engineering classification of soils is
slightly different and here what we actually are interested about are the engineering behavior
of soil when we classify soils in this manner
and as we have seen from one of the previous slides is
that soils, the soil behavior is going to be very different depending on whether the
soil, I mean the soil solid is composed of clay minerals
or non-clay minerals. Why is that it is because in case of clay
minerals, the individual grains are comparatively less
heavy in comparison with the inter-granular forces that develop because of the charge
density as I indicated on the surface of the clay particles . .Whereas in case of non-clay minerals, the
charge density as a percentage of the weight of the
particle is comparatively much less and as a result, the behavior of soils composed of
clay minerals is going to be greatly different
from soils composed of non-clay minerals. While clay
minerals are going to be driven by the stickiness or plasticity that develops because of the
interaction of electrical nature between individual soil grains while non-clay minerals, they
will derive the strengths based on the packing
or interlocking of individual soil particles. So, the behavior of soils composed of clay
particles or clay minerals will be different from the
soils composed of non-clay minerals. So here, what we have got is basically the classification
looks at the grain size distribution mainly and to some extent it looks at the compressibility
of the structure and also the organic content
of the soil. How the individual classifications, individual classifications of soil will be
considered later on in one of the future presentations of
this course. Then we get into weathering; weathering as
I indicated is mainly driven by mechanical and
chemical action. Rate of weathering depends on susceptibility of parent material; actually
this is quite intuitive, it depends on the susceptibility
of parent material to weathering that includes mineralogy and structure, it also depends
on environmental factors that include temperature, rainfall, biological activities and topography
and the rate will depend on time; the more is the
time of exposure, greater will be the chance of weathering. .. Now, you can imagine, let us consider the
susceptibility of weathering of soils depending on the
mineralogy of the parent material. As I indicated, quartz is very resistant to weathering, quartz
and sodium feldspar in comparison with other, in comparison to other basic mafic minerals.
Temperature actually is another factor that increases the weathering rate. You can imagine
that if there is a 10 degree Celsius rising average
temperature, then the rate of chemical reactions that
occur that drives the weathering processes; the rate of those equations, the rate of those
chemical reactions can increase two fourth. Rainfall also increases the weathering rate,
biological activity, root penetration, burrowing animals; they actually gives access of oxygen
to different layers underground, underneath the
surface of the soil, gives oxygen, actually gives access to oxygen to different layers
and this actually gives rise to different weathering
processes. Topography, actually also triggers weathering
because the weathering products, they quickly get
carried away because of cope failure or because of flowing water in steep terrain and as a
result that may accentuate actually the rate of weathering. .Mechanical weathering breaks down the parent
material, it actually reduces the size of the
original mass of parent material and because of reduction of size, the surface area that
is exposed to the environment increases and that actually
increases the rate of weathering. . The main processes involved in mechanical
weathering are listed there, listed on the slide. They
include uplift and burial because of uplift and burial, rock masses can crack and that
gives access to the environmental agents, weathering agents
to the different parts of the rock mass and that
drives the weathering process, erosion would also expose different parts of rock mass to
weathering, temperature driven expansion and contraction can lead to development of cracks
and that may in turn lead to mechanical weathering. Crystal growth and frost action actually can
split a rock mass because of the volume of the
secondary crystal that develops in that process is different from the original solid and that
splitting of rock mass actually can also give rise to mechanical weathering. Colloid plucking
or shrinkage of colloids as they harden can also
crack rock mass and give rise to mechanical weathering. Biological processes can also
cause mechanical weathering. So, these actually are
the major agents that drive mechanical weathering. .. Chemical weathering; chemical weathering as
the name suggests it alters the chemistry of parent
material and here the main processes that we look at include dissolution and precipitation,
hydrolysis, carbonation, oxidation and reduction, ion exchange, chelation and leaching. . Now, we have to actually consider the different
types of, different classes of reactions that give
rise to chemical weathering. Let us consider hydrolysis. We are going to consider whether
this .one is already shown to you when we were
considering the weathering of feldspar, weathering of
potassium feldspar or orthoclase. Then let us say hydration;
anhydrite, a mineral called anhydrite upon absorbing moisture, this actually hydrates
the crystal and that gives rise to gypsum. Then
we have got carbonation, combination of carbon dioxide in atmosphere with water forms a mild
acid which is called carbonic acid and that reacts with limestone or marbles giving rise,
releasing calcium iron in the atmosphere and dissolved
by bicarbonates. So, that gives you or that take care of the
first three classes of chemical weathering processes.
Then we have got oxidation; let us consider an oxidation process. . Let us consider weathering of olivine mineral.
What you have got here is basically ferrous silicate and that reacts with carbonic acid
and in presence of water giving rise to ferrous ion in
solution, hydroxyl ion in solution, silicic acid
and dissolved bicarbonates. Ion exchange; the
Illite mineral actually converts to ((55:05)) because
of this transformation and what it leads or what it
originates from is loss of potassium and update of magnesium and then chelation, chelation
is essentially taking on mineral ions in hydrocarbon
chains that is what is meant by chelation and
let us take an example of leaching because leaching is a very important weathering process
in .tropical and sub-tropical environment like
those encountered in many parts of India and we are
going to consider the leaching of kaolinite soil. . So here, what you get, as I indicated kaolinite
soil has got this kind of chemical composition and
leaching takes place mainly in presence of water and what you will end up with is
hydrated alumina or Gibbsite and silicic acid in solution.
So, this one is gibbsite and this is in solution, this
silicic acid is in solution. So, that is an example of the leaching process. We look at the weathering products of different
felsic minerals and what we consider here are the
felsic minerals of which are the main rock forming minerals. You may recall that we considered
9 rock forming minerals that composes most of the igneous rocks and we considered the
felsic minerals of those 9 main rock forming minerals
and this table here shows the rock forming, the
weathering process that actually weathers 3 of these minerals. .. Muscovite actually re-crystallizes to form
Illite or in the presence of acidic ground water, it gives
rise to kaolinite and releases potassium ion in the solution. Orthoclase, in case of shallow
burial carbonation and hydrolysis, it actually gives
rise to kaolinite and that eventually leaches to form
bauxite. It also gives rise to the formation of quartz and potassium ((Video Problem. Refer
Slide Time: 58:31)) And, finally we end this presentation
with the question set. . .What I asked here in the first question was
that how quartz percentage in mineral soils and rocks
changes with weathering? Which clay mineral forms from chemical weathering of feldspar?
Then I asked you to explain the terms; chelation, cementation and lateralization. You try to
answer these questions and I will try to give my take on these questions when I meet you
the next time with the next presentation. .

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