So, Good morning again.
And today is the second
By the time, you know
 the second month has already started
Second half of the year 
has already started
And today we are in the second day 
of the second half of the year
Whether we like it or not,
 time still flies
So, let us, let us see
what best we can, sorry
let's see what best we can do
In this time which we have together
So, 2nd
Okay, yesterday and for 
the past few days
we were seeing some properties of the earth
[Topic: Properties of the Earth]
Okay, we saw that Geology helps us
[Topic: Geology]
to study the surface of the earth
some of the properties of the earth
like - how is soil formed?
Okay, and may be even 
going a little beyond
we can say that you know it tells 
us how historically earth was
how earth has been
evolving with time
how earth has probably
being doing a lot of good things
And a lot of bad things
Why some things happen ?
like for example
Why do we have a volcanic eruption?
Why do we have a earthquake?
 and probably one of the things
which we have not done is
try to see what are the ways
we will which earthquake
is probably measured
What are the ways by which
earthquakes probably been assist and
some of the things like that
now let us, let us for a minute
I mean like you know 
think about what
what all we have already seen
so we saw that soil is a
[Topic: Soil]
mixture
soil is a mixture of organic substances
and inorganic substances
and if we look at soil
we call it as granular
okay, and the other thing is actually
it will become 
amorphous or powder
 like and this is what we call
as the clay basically
and each of this have
 different properties
So, soil is actually a 
mixture of substances
which are cohesive and non-cohesive
and now this is the new word 
which I am using
I will be probably coming back
 to this in some time
 in a little later point in time
now let us take for example
one of the favourite
 toys of children
Okay I am sure all of you have 
atleast one time in your life
travelled on a slippery slide
so typically what is a 
concurrence of a slide
you have a ladder
so children climb on the ladder
and then they slide on the ladder, okay
now what are the 
things which determine
the way by which you can slide, okay
then each of you say one thing
I think Rithwik,
you have not given me 
any answer till now
so what makes you go down a slide
Sorry sir
how what are things that
 makes you go down
I mean like you know, in a slide
 you can go very fast
or you can go very slow
okay what are the 
things that happens
that help you enjoy the slide
what are the actions
or what are the things which
so, action is probably the speed
at which you slide down
sir, if you hold the really then you can 
go down on your own speed
slide hold the side, okay
when you are holding the side
what is happening
you are increasing friction, okay guys
[Topic: Friction]
increasing friction
yeah, you are increasing friction
much like going the speed
 at which we go down
yes, much like how we
what you call as hold 
a rope and come down
 so, or friction or bicycle brake
 or whatever you want to call,
friction is increased or decreased
Okay, so when you increase the friction
that happens, now let me
ask you one more thing, okay
what kind of clothes 
makes you go down faster
is it a jean, or is 
it something where
you are having when 
your skin is against the slide
which when do you go faster
Sir, what's the first option
you are having a nice tight jean
you are having a
you are just having a bare skin
more like a bare skin
if you see small children, 
usually go in shorts, right
so more of bare skin
which is going to 
help you go faster
sir, jeans
yeah, again, again it is a thing
which friction comes in, right
So, friction is one of 
the very important things
that help you move
Okay, Aditya, Point number 2
What helps you go down a slide?
 Sir, slope.
Slope, Very good
So if the slope is more
 or if the slope is steeper
what happens you go
 faster or slower
faster, sir, faster
you go faster
In other words, to resist 
your or to
try to go at constant speed 
at a comfortable speed
 you will have to apply more friction
Am I right?
Yes, sir. Okay
Now let us say that I am
applying some oil on the slide
 I am reducing the friction
this is another thing another case
which can happen
so these, probably are the things
that happen
with the slide and this is
what happen in a slope also
Now I am going to go back again
to 10th standard, okay
because, I am not going to teach
anything from engineering
because I know many of you 
have not done
many of the papers in
 the engineering, okay
so have you heard of the
parallelogram law of forces
okay, first and foremost
 we will do friction
let us say, friction, suppose
you are having, okay, let us say
What is friction?
What is friction?
Have you heard of 
the word "friction"
The force that opposes the movement
 yes, very good
force that opposes the movement
and it is proportional to the
 downward force
okay, or what we call 
as the normal force
In other words, 
suppose, your weight is say
F kilo newton
that means the amount of lateral force
 which is generated
 is proportional to the
 friction constant, okay
so this is a common rule, okay
so now let us say that the
soil has some mass
So, this mass helps to
generate the lateral force
and you will get an angle
which is normally what we will call as
the "angle of internal resistance"
angle of internal resistance, okay
so, if suppose
 this becomes more
automatically you find that
this slope becomes much steeper
slope can keep on
increasing or decreasing
now, one of the biggest problems
which we are facing is
when we are increasing this slope
many times, you will find that
that is whenever increasing the
downward value of force
the required amount of friction
might not get developed
the required amount of friction
might not get developed
to keep the slope in
 a stable configuration
so typically, what happens
this value 'μ' is a constant, okay
[**Imp]
this value of 'μ' is a constant
so when the ,when the value
of the force keeps on increasing
it the soil tends to I mean like you know
the resultant force tries to
go in a steeper angle
however the stable configuration
could be here
so, it means that as long as the
slope formed is lower than this
equilibrium, the one we are you know
like you can get equilibrium,
or the critical force
the force, the slope at which
the soil just as to slide
so as long as is lesser than that
there is no problem
our problem comes when this
 increases, like for example
you need told that
there is a cutting for a road
so typically the mountain slope
 would have been like this
so this portion of the mountain
 has been cut off
it has been removed
and this soil is most slightly
taken and filled over here
now the road is formed
now the road is formed
that means I need to
provide a slope protection here
and here, okay so, I think
till we talked about this in may be
slightly different terms earlier, right
Now, when this slope
protection mechanism is not sufficient
when the slope protection
mechanism is not sufficient
'landslides' takes place
Okay, so when the, so this
is one of the what we call
it as the reasons I mean like you know
things which we have to consider
So the following are
some of the what we call
as the typical ways by which we do it
 so, you can construct
 'retaining structures'
using probably concrete walls, okay
or you can go for "gabion walls"
you can go for 
reinforced earth works (RE)
and so on and so forth
another thing we also
do is they try to "stabilize the soil"
in other words,
they know that the soil is weak
so what they do is they
 try to change the 
composition of the soil
because the internal friction
and other things
are properties of the soil
so you can try to
change the properties of the soil
and this is called as 
'stabilizing the soil'
when you talk about
 stabilizing the soil
typically you will inject something
like line or some other
 grouting material
especially it is a 
pumplime or cement
or some cementitious
 material into the soil
making it more strong
than what it was before, okay
now, at till this point, I think you should
 not be having any problem, okay
so basically, when you
 have cuts and fills
and when you have for example,
 some embanking structure
you typically have the 
what you call as
the chance of a earthquake of a
landslide happening, okay
now I told that
soil is a mixture of
cohesive and non-cohesive materials
soil is a mixture of
cohesive and non-cohesive materials
"Cohesive materials" are sticky
okay
Cohesive materials are sticky
like clay, so if suppose
you take some red sand
or some clay and 
rub on your hands
you will find that
it leaves a residual
leave some mark on your skin
so this is cohesive soil
and the other one is
 non-cohesive soil
cohesive is sticky
so the other one is
 non-cohesive
 typically, this is sand
what you see in river,
 what you see in beach
are all sands
usually there is a mixture
of the sandy component
 and the clayey component
so, soil is very funny
when you add water
 to some extent
it will be beneficial
see for example
when you add water what happens
 first thing is it displaces air
because "it displaces air,
 the weight increases"
the weight increases
now when the weight increases
the lateral force also increases
but it cannot extend
 increase infinitely
it cannot increase infinitely
at one point, the equilibrium
 point is reached
and when the water goes beyond that
the lateral force or the resisting
force becomes negligible
see when you are standing
okay let us say you are standing
you're standing on
 a parabola skates
okay, now you will be standing
like this until somebody 
give you a push
when a person gives you a push
the wheels if suppose
 they are rusty
they will not let you 
go immediately
you may displace slightly
but still you are able
to be in equilibrium
now when somebody
 oils this parts
that means the resistance decreases
immediately you are starting to move
this is the case for every thing
suppose you are sitting,
 the downward force you exceed
you are exert on the chair
balance may be upward force
if this balance is not there
typically your chair is going to break
and you are going to fall down
so, anyway you would
 have studied for a
body to be in equilibrium
all the forces should
 be equal to zero
that is all the horizontal force
all the vertical force
and one more thing
the turning effect could
produce by a force
 which is moment
all of these three should be zero
same thing happens here also
 okay
Now one of the things
you would have seen
is you are again 
talking about the
what you call as the
when you go to the beach and
sand, stand on the sand
the sand looks level
however, the reason 
why you go down
when the sea water rushes in it
the water fills all the force
 and does two things
one, when more lots and
 lots of water comes
 there is a thing called "buoyancy"
okay, so what is buoyancy?
[Topic:**Buoyancy]
yeah I think who joined late
Ishaan
Rithwik, I think you
 have joined in late
okay, what is buoyancy?
what is buoyancy?
okay, forget it
'it's an apparent loss of
 weight experienced by the body'
 now two things happened
when water is much much more
it makes the soil 
behave like water itself
so the soil when the 
person is pushing down
he displaces the sand
next to the person
so the soil tries to go up like this
so, because there is nothing 
to keep it from going upward
there is nothing to keep it 
from going upward
so the man typically sinks
 infact this was a lesson which 
I was learnt much after my engineering
my professor joined also 
L&T as working there
Dr Murugesan
Late. Dr Murugesan
he was the one who
 pointed out
We would have learnt it, 
but you know
when you are experiencing
 a certain thing
you will never forget it
so there was a problem 
with one of the foundations
we were discussing
 I was thinking why not keep the
 foundation at a higher level
so that time he told
because the soil was good
John, unless you keep the foundation
 at a slightly lower level
the soil may take the load
but then what will happen
the adjacent soil will raise up
increasing the amount of the person
the foundation sinking
same thing as what 
you experience in the beach
so on the other hand, if suppose
lot of people were 
standing on the both sides
you will not be able to
 feel the sinking action
mainly because the upheaval
of the soil is arrested, okay
now the same thing actually happens
also when you look at 
what you call as
this is probably little,
 little out of context
I am only saying little
its not totally out of context
it is a little out of context
but when you look at landslides
the presence of water
 does two things
initially it increases weight, okay
then at one point, it touches
the equilibrium level
after the equilibrium level
it actually starts to cross the 
soil to move along with it
so other time which starts 
to cause to soil to flow
and this is a dangerous condition
typically we call this
 as "seepage of water" okay
seepage of water
so when these condition happen
it is very likely that
 the soil is likely to collapse
Any questions?
Any questions till this point
 so, in conclusion when shear stress
sir.., yes please
yes, Aditya
Sir, while building a building
yes
Sir, If we keep the foundation,
 yes
lower means, Sir, make
 the building strong,
we have to keep the 
foundation more, yes.
correct
see, it is not to make
 the foundation stronger
it is to prevent settlement
basically settlement, okay
so like I was telling 
you when you keep the soil
will be strong enough
 to take the foundation
instead of plan the 
foundation is there
but what is going to happen
when the load increases
the soil around it starts to move
so when the soil moves 
a little, more and more
the soil starts to flow out and 
this causes the 'sinking of the foundation'
so initially the foundation would
 have been here
the footing the level
then it goes lower all 
foundations will sink
but when it sinks excessively,
 there interprets
so, that is one of the reasons
why we take the soil 
for the foundations lower also
okay, many times what will happens is
the soil at a lower level is stronger
usually the soil and 
the top is just you know rubbish
you just throw it is all
 loosely packed
and you know so these are
 all and also the material
which makes it up may be
 more of organic content
which doesn't have 
high compressistance
of course I am not saying always
 it is the case because if clay layer
 is found at the slightly lower level
you will still be finding that the 
foundation is not strong
so, typically when we design a structure
we take what is 
called as a bore lock
and we do and geotically investigations
so one of the things is a bore lock
so we see what is the type 
of soil at different levels
we do the tests on the
 engineering properties
and then we will get
what is called as a 
"bearing capacity of the soil"
okay how much load it can take
and typically when the
 foundation is designed
does one more very important thing
 doesn't always mean that
 a bigger foundation is safer
now this may sound 
counter-intuitive
but take it from me that 
the shape of the foundation
the founding depth did the
what you call as the nature
 of the soil at that level
and the water table 
presence of the water table
will all determine the load 
carrying capacity of the foundation
are you able to I mean you know
 just get the key points
just keep these points in mind okay
so actually the strength of
 foundation is based on strength
of footing or what you call 
as the "bearing capacity"
is based on size of the footing, size
 shape, as also important part
so rectangle or square or circle
 or you know some 
other thing like octagon
each of them will have
 different bearing capacity
so we have size, we have shape
we also have the depth
 at which it is founded
so depth of founding
 or foundation depth
and presence of water table
so when I say depth of foundation
please remember, that is 
also the type of material
which is there in that level ,okay
moving on
so, Shear stress is caused by the load
[Topic:**Shear Stress]
so, basically soil, the one
 of the points told was
overburden, okay and so 
actually the soil weight
 I will just put it 
as a soil weight
so when what are the step
weight is automatically 
going to be more
when overburden is there
again that is going to
 be making a difference
then, if suppose you are 
having the right amount of water
you will find that for example
if you go to the beach,
 and you dig a hole
or you build a castle
okay many of you would have done 
considering sand castles
I have also done, sand castles
so what do you do
you keep the sand castle 
walls, sufficiently wet
if you pour too much
 of water, the soil will flow out
if you don't have water, 
the sand gets blown away, right
and it or you just slips and falls
nothing but a slope failure
now what happens 
you add just enough water
and then you will 
be able to see that
the wall is actually 
able to stand straight
So I am putting the word,
 presence of water
okay, and lastly the slope
so please do not think
that just because you have add water
soil is going to become weak
or just because you remove water,
soil is goint to become weak
so it says quite a complicated
what you call as a equilibrium
state of equilibrium
in which the soil is present
and that is why when any one of these 
things get changed, the soil slips
okay, so this is the
this is the
what you call as the
actually what I should say is
 these two are things
which resist and this is the thing
which is causes slip, okay
so when you are having a soil 
with it when you are having
what you call as the overburden
they cause the shearing stress
the presence of water, the slope 
at which the soil is heaped
cause the what you call as the 
resistance to movement of moisture, okay
so guys any questions on this point
moving on
so one more thing which happens is
so when you find that the stress
what is the action
is greater that the resistance
there is a slip
so immediately you will say
okay let me do some plantation
when the roots of the plant 
be sufficient to hole the soil mass
 if it is not so, then you go 
for some engineer solution
so you will go for like 
I already told when we construct a wall
may be construct a
may be stabilize the soil
may be do both, okay
so, this is probably going
 to be the way
by which you prevent a landslide
 from occuring, in other words
these are the mitigating techniques
so first you know what is 
the cause of the landslide
then you are seeing 
the mitigating techniques
so it means that doing a study
[Topic: Causes of Landslides]
you are in a position to say
these are the places
where the landslides lightly it occur
so if you do this, this, this
you are not going to have 
a problem with the landslides
friends
as I told you earlier there are  few things
like gabien wall, RE wall 
so on and so forth
I will request you to please go and have 
a look at the pictures, okay
and I will conclude this session with
 just adding one thing, okay
the need for weep holes
need for weep holes
[Topic:**Need for Weep Holes]
this is a very very important thing
now when you go through a bridge
and you see a wall
it is includes your subway walls
you will find that there are some
 rectangular holes
 or circular holes present
These holes are called as 
weep holes, okay
now these weep holes
so, when the soil is weeped
 on the other side
and there is water
now what we do is we 
keep this weep holes
so that the water will 
flow out of these holes
thereby reducing the
lateral force, okay
essentially it reduces the 
lateral force, okay
so whenever there is a surface
this is the normal force
and this is the 'lateral force'
or 'shearing force'
okay so we will reduce 
the water pressure
and thereby you know like
ensuring that the wall 
is kept safe, okay
we've had many 
what you call as the
we have done some construction work
and also seen how when 
the water is you know
like more
the wall or the slope
 protection mechanism
is likely to collapse
so many times you know
when we have not 
made provision for the
what you call as the weep hole
we will artificially have to do that
okay but once you experience it
you will never make 
a mistake, okay
any questions please, from your side
right
I hope you found this 
interesting and useful here
yes sir
