Hello I'm Professor Kitch and today we're going to do this really simple tabletop
direct shear test to give you a good feel for how soils behave in shear I'm
gonna just do a quick review of the direct shear test I know you've probably
all read the book but this is our direct shear apparatus what we have looking
down from on top I've got a ring like this so this is a top view and I've got
soil inside the ring there so that's what it looks like from the top this is
an elevation view it's just an elevation view so I'm looking at it sideways and
here's my ring but my rings actually split in two there's a top part to it in
the bottom part to it so I'm kind of looking at a cross-section through it
here and then my soil today we're using green soil my green soil is all inside
there and then I'm going to take my ring of soil I'm gonna put a loading cap on
the top of it it's just a just a metal thing that I can use to make sure I have
a nice even load on it and then what I'm going to do is I'm going to put some
normal force here on the top I'm going to push down with a normal force and
then I'm gonna pull in this direction with some shear force so what's going to
happen is this top part of the ring is going to slide this direction this part
is fixed to the table and the soil is going to fail right along a line here
and so I'm going to put a certain normal force on here I'm gonna measure the
shear force that's required for this to move or shear or fail along this line
and then I can develop a table of the normal force and the shear force and I'm
gonna do this for FN is equal to FN one and I'll get an F s is equal to FS
one so I'll have a value here for the first one so this will be f n1 and FS 1
and then all changes I'll increase it I'll eat to a number that's f n 2 and
then I'll pull here again so I have F n 2 here now and I'll get a new force it's
required for it to shear along here that'll be FN - sorry that's F s 2 and
so here's FS 2 and I'll just keep doing that for a few of these so F in 3 FS 3
etc and then what I can do is I can do a plot of the shear force required to fail
as a function of the normal force that was applied and I'll have these points
on here and then I can do like a best-fit line through them and then the
angle of that with the horizontal is going to be my friction angle and the
intercept down here is going to be my cohesion so we're gonna do a tabletop
version of that so let's go do that now
so here's my super simple tabletop demonstration of a direct shear test so
this is how I'm going to do this I've got a brass ring here that's attached to
this pipe cleaner and that's attached to this spring scale and that's gonna
measure the horizontal force applied to the specimen it's resting on this piece
of sandpaper and I've got a bunch of sand spilled on it so I get a good sand
to sand contact on the shear surface so I'm gonna place my ring here on the sand
and I'll pour a little bit of sand in there fill it up some maybe about half
way or so and I'm gonna put this piece of cardboard on top so I have a good
surface arrested on and I can place weights on the top to be my normal force
pressing down and then of what I'm going to do is pull this piece of sandpaper to
create a direct shear plane right across the bottom of the specimens so I'm just
gonna pull a piece of sandpaper it's going to slide on there
and I'll measure the shear force here so I'm putting a normal force on here I
know what that is it's failing on a horizontal plane here
that's Santis and that's why I'm using the piece of sandpaper and I'll measure
the shear force there and knowing the normal force and the shear force I can
calculate the friction angle so what I'm going to do now is reset the camera up
so we can look down on the top of this and you guys can see the measurements
I'm going to take a bunch of different direction or measurements ready okay
we're ready to do a little bit of simple the direct shear testing here the very
first test I'm gonna put some sand in here but I'm not gonna put any normal
force on it so you just put a little bit of sand in there now I'm gonna gently
I'm gonna gently pull the sand paper out from under there and I want you to watch
the measurement on the spring scale I'm gonna try and come to like a equilibrium
point here where it were it's not static friction but dynamic friction so I'm
pulling along there there's no normal freshman you notice that we're basically
reading to zero okay so that was our first test and we had no normal force
and notice we were reading basically zero shear force there all right so now
let's do our second test I'm gonna put a little more so put some sand back in
there and for our second test I'm going to set this up and put this piece of
paper so I can load it I'm gonna put 500 grams half the kilogram as my normal
force and now let's watch the gauge here the spring scale and I'm gonna pull out
when I get it smooth in there and it looks like it's just a little over 200
yeah I had 500 grams here and we had about 200 grams there all right so
remember that the normal force was 500 the shear force was 200 all right I'm
gonna go out here again put some sand back in there make sure it's right put
my piece of paper on the top okay now I'm gonna put a thousand grams on the
top of this and I'm gonna pull
a paper I'm gonna see what happens they've got about a thousand grams and
it looks like it's about right at 500 it was bouncing between 400 and 600 so
we're gonna call that a thousand grams normal force one kilogram and 500 grams
of shear force and the very last test we're gonna do that's more sand back in
there the very last test we're gonna do is with a normal force of 1,500 grams
1.5 kilograms I'm gonna put that in there and watch our scale now let's see
what we get to like this static one and it looks like that was right about
between 700 and 800 it was kind of hard to see cuz it was bouncing around let's
try it again get a second leading on that one because
that one was kind of difficult to see
look for me it was like between 700 and 800 some sand in there my piece of paper
on there
I've got 1,500 grams here yeah yeah it looks to me like it was bouncing here
between 600 and 800 but was closer to 700 so when we had zero grams on top we
were reading zero when we had 500 grams on top of rereading 200 when we had a
thousand we were reading about 500 and we had 1500 we were reading about 700 so
those are the numbers we're going to plot okay that tabletop test was really
exciting let's plot the data and see what we get now remember we tested with
normal forces of this is in grams zero 500 1000 and 1500 and we measured zero
shear force at zero we measured 200 at 500
we measured 500 at a thousand and we measured 700 at 1500 normal force that's
as good as we could do with our little tabletop test but let's plot those data
and our plot so here I have a plot of the shear force required to feel the
soil as a function of normal force and the first point was at zero zero so
we're going to plot a point right there and then we had 500 200 as 255 so that's
about right there and we had 1500 that's an easy one to
plot and then we had 1500 700 so here's 750 down a little bit from that about
right there and now we can fit a best fit line through those and just going to
look right up here and it's gonna look just like this that looks pretty much
like my best fit line sir there and then the slope of that line is going to be
the friction angle and here the cohesion is going to be 0
there was zero force there and I've got a really good eyes here and I can tell
that that friction angle not about equal to 25 degrees and you can actually put
those points in your calculator and calculate a best fit line and you'll
find out that that friction angle is right at 25 degrees so this was for a
dry sand okay and we found out that it was cohesionless and had a friction
angle of about 25 degrees let's look at some cohesive soils and see how they
behave hey watch this oh my god what just happened I just threw two
different kinds of soil onto the whiteboard the first was this silty soil
that comes down from by Big Bend and the second was this sand this clean dry sand
now notice when I threw the silty soil on the wall it's stuck
and when I threw the loose sand nothing happened well how come this is sticking
what forces are there that is allowing this to stay there let's draw a Freebody
diagram all right so here's the wall and here's my blob of silty st. silty soil
this is silt okay so let's draw up a free body
diagram of just the silt so I'm going to take that here it is let's draw our free
body diagram but what forces do I have acting here well I have the weight here
and that's it that's the only thing that's acting on this side one so what's
happening on the wall here there's no force there's no horizontal force being
applied here I just have to wait and so that's you know the mass times gravity
well I must be having a reaction here in this direction
I'm gonna call that s I must have a force s here and notice that there is no
force in this direction this is equal to zero so how can i generate as this is a
shear force right this this wants to shear along there how can i generate a
shear force when there's no normal force well this is one of the characteristics
of cohesive soils cohesive soils when the normal force is zero have some shear
strength in this case it's s we call that the cohesion so cohesive soils at
zero normal stress have a cohesion see now I threw my sand on there right
through my sand on here I threw a big blob of sand and the red just fell down
so why did it fall down well cuz the sand was sitting there like this
it had mg here but there was no force here at all this s was equal to zero and
so it fell down and that's exactly what we found out when we tested our sand on
the tabletop that the cohesion was actually zero when there was zero normal
force there was zero shear strength cool let's go test our silt on our little
shear table okay here I am back at my little simple direct shear table I've
made up here I'm going to test our silty soil here on our direct shear device
here first we're gonna test it with no normal force on it I'm gonna see what
the shear force is required to fail it and that one do that again a couple
times that looks pretty close to 300 grams to me it's really fast you're
gonna have to watch carefully it looks like it's failing at about 300
grams so we're gonna write that down I have
all right so now we're gonna test it with 500 gram normal force and I want to
get that sliding friction on here so we want to see so it looks to me at 500
grams of normal force that it's failing at right about 500 grams that that
static is about it's between 400 and 500 and if you it one more time
see if their 600 goes down to 400 so let's call that 500 so at 500 grams of
normal force and hit 500 grams of shear force
so now let's let's go up to a thousand
so I'm put a thousand grams of normal force on here see how much shear force
we get oh that's right at a thousand look at that okay so at 1000 grams of
normal force so let's check that one out at 1000 grams of normal force we'll see
what we get
so at 1,000 grams of normal force well it looks like it's between one kilogram
and 800 oh that looks right at 900 so let's call that at a thousand grams of
normal force we're getting 900 grams of shear force and then finally you want to
do a test at 1,500 grams of normal force so now I got 1500 grams of normal force
on there you'll see what we get here so let me give it a little go here so it
looks like it's between I'm gonna it looks like that's right between 1500 and
1300 so I'm going to call it 1400 grams and that was at 1500 grams of normal
force had 1400 grams of shear force so that's what I get with his with his
silty soil so let's go back over to the graph and plot that up okay
let's plot our data from our silty soil the one that's stuck to the wall so for
that soil a pot I'm in red at the same normal forces FS was 300 500 900 1400 so
we'll plot those points here so at zero it says 250 so there's 300 about right
there and 500 500 that one's easy 1900 there's 1900 is probably about right
there and at 1,500 1,400 1,500 right there are 14 hundreds about right there
all right and then I can draw a best-fit line
through those it looks like it's going to be
something like do a very good job there's something like this but clearly
an intercept here well that intercept is gonna be my C
Prime it looks like it's about we measured 400 here a 300 here but it
looks like it's lower than that looks look like it's closer to 200 if I do the
intercept and then this is my friction angle it looks a little steeper than my
other one that looks like it's in 2025 maybe that's about 30 degrees I don't
know you have to measure that one so what have we learn about that where we
have a cohesive soil this is an ml it was a low plasticity silt and we had a
finite intercept here we had a C prime is equal to that in this case it was
we're gonna say 200 and then we had a fee prime was equal to 30
now I'm fibbing to you a little about this he's a really really simple test
and the truth is these really aren't effective stress parameters because I
did that test really really fast and later we'll talk about how that's gonna
affect the test but the basic principle is correct here so when I have
cohesionless soils so I have a sand or a gravel that has no cohesive material in
it I'm gonna have a material that has no cohesion and it has a positive friction
angle here in the case of cohesive soils I'm gonna get some kind of intercept
here they're gonna have some strength at zero normal force that'll be the
cohesion and then they too may increase with strength now these are really rough
measurements that was a really simple test and they're not particularly
accurate but the basic principles are showing there so that's cohesive soil
and the sand isn't