Estimation of Preconsolidation Stress (Casagrande Method) Video Lecture | Soil Mechanics - Civil Engineering (CE)

hey guys welcome back right here
commands math and engineering and we're
going to do a quick video for you on pre
consolidation pressure using the virgin
compression curve okay so we have the
virgin compression curve here don't
worry if you don't know what that is
right away I'll give you a quick
explanation we're asked to find the pre
consolidation pressure Sigma C okay so I
know in some books in geotechnical
engineering the notation is different
for a lot of these questions especially
when you do foundations engineering like
there's like four different formulas for
everything so we're using Sigma C here
for the pre consolidation pressure but
you know in some books that might be
Sigma PE
ask your professor make sure okay so
it's good before we start to have a
slight understanding of what
consolidation is so consolidation
happens in clays very fine grained soils
not granular soils like sand okay wet in
which the clay after it's loaded okay
the water within the saturated clay
cannot leave immediately okay and
because it can't leave immediately as
for water pressure is increased as a
result of that and then slowly over time
the clay will drain into nearby sand
layers and that that drainage over time
causes a a settlement essentially that
that is what consolidation is is it's a
gradual increase in the effective stress
of the clay layer and it causes
settlement over a period of time just
kind of drew a little diagram down here
for you
what's that exactly looks like okay is
you have a loading for example you could
build like a foundation a concrete
foundation and on top of a break below
it you may have a sand here's a water
table here so we have a saturated clay
layer okay that's being that's being
exposed to some stress as a result of a
external loading okay and the water is
slowly being squeezed out of that that
clay layer into the end it's draining
into the sand layer essentially and endi
as a result the the effective stress in
the clay layer is is gradually
increasing so let's take a look at this
virgin compression curve so what does
this curve mean well so at the end of
consolidation so you'll probably in your
labs and geotechnical engineering you'll
you'll do the consolidation test
essentially what it is it's a it's a
period of of 24 hours your periodically
taking settlement readings over time and
you're slowly slowly increasing need the
the load in order to kind of have an
exaggerated idea of what the
consolidation would look like on a
claisen
well at the end of the consolidation
this is what the void ratio relationship
to gulag of the stress of the vertical
effective stress this is a semi
logarithmic graph because as you can see
in the y-axis we don't have a log value
and in the x-axis we do have a log value
of the vertical effective stress you may
also see it written as log P okay we're
using log Sigma right now so essentially
what's happening here okay as we're
loading and near the end of the loading
what's going to happen all right is as
we can see the void ratio is decreasing
okay and as we get to the unloading
phase this is called the rebound curve
and as we unload slowly we measure that
that unloading and how it affects the
void ratio and the you know and the
effective stress relationship okay so
with that being said okay and actually
this is Sigma prime which is the
effective stress how do we find the
pre-consolidation pressure okay so it's
actually a graphical method and it's
called the casa grande method and how
we're going to solve that is pretty
simple so you'll probably be given this
and I'll show you the steps so there's
five steps the first thing is that we're
going to do is we're going to find by I
what you think to be the sharpest
curvature of this portion of the curve
okay so I think it looks to be about
here okay so whatever you think has the
sharpest curvature okay it's essentially
the smallest radius of curvature okay
that is what you're going to select as
your point
oh okay so we're going to select that as
point O and what we're going to do is
we're going to draw a horizontal line
okay so we're going to a horizontal line
okay you can call it o a okay call that
line o a next next step we're going to
draw a tangent line okay that is tangent
to the the curve at point O so at point
O we're going to draw a tangent okay to
this curve here that intersects point oh
okay so that's going to be referred to
as our o B line okay and next what we're
going to do okay is we're going to draw
line OC and why don't see bisects line o
a and line OB so we just draw what that
means is we're going to draw a line as
best as we can do you want to use the
protractor just to make it perfect
that's good too okay so we're going to
draw a line that
so that's in the middle of a B I know
that's not exactly perfect but just try
to make it as good as you can a lot of
the times they just want to see that you
know how to do this okay
so where this is going to be Oh see okay
so the next step the straight line
portion of this curve here this
straightens out so this is straight and
we call that actually a C sub C so the
coefficient of consolidation and what
we're going to do is we're just going to
extend that line where the straight
portion is and we're just gonna extend
it straight up like this and where we
extend that line okay and we're
intersects line OC okay we drop a line
down to the log effective stress axis
okay and where that intersects the
x-axis okay is the pre-consolidation
pressure Sigma prime C alright so there
you go one thing to note is actually the
value of this we can determine whether
or not the the soil is normally
consolidated or over consolidated or pre
consolidated so that's that's that's for
another video though that's how to find
the pre-consolidation pressure using the
virgin compression curve I hope that I
explained the consolidation lately and
as always like and subscribes in the
video
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