Lecture 39 – Index testing of soil & rocks


Here, we are going to give the demonstrations
on different simple laboratory testing for classification and identification of key soil
parameters and rock parameters. So, with that
said, we are going to move on to the first laboratory experiment involving sieve analysis
which is carried out to find out what is the distribution of grain size or representation
of different grain sizes within the soil matrix
for coarse grain soils. . These are the higher standard sieves used
for finding out the grain size distribution, as I
was saying before, for coarse grained soils. What we have got here is a stack of different
standard sieves which has got different opening sizes mentioned on the labels here. And
you can see the opening of individual sieve from this pore set sieve that is being shown
now and we can show now what is the appearance of the finest sieve used for finding out
the grain sized using sieve analysis. .. This is the 75 micron opening size sieve,
whereas the first one that you saw was for it
had an opening size of 4.75 millimeter as mentioned on the label. And now, you can get
the idea and you look at the labels of different standard sieves and you can see that it
actually goes down in size from the top; starting from the top are the coarsest sieves at
the top and the finest sieve size is at the bottom. And at the bottom of the 75 micron
sieve is a pan which will collect all those soils, all those particles which are finer
than 75 microns. So, what we are going to do? . .Here we are going to take about 500 grams
of coarse grained soils composed mainly of sand here. As it is shown here, this is basically quartz
and feldspar sand. We are going to
dump it on the coarsest sieve like this and then we are going to subject this set of sieves,
we are going to cover it up because we do not want to lose any finer soils into the
atmosphere being blown away by wind. We are going to take this sieve set, place
it on a mechanical shaker and shake it for ten minutes
and find out what is the weight of soil retained on each individual sieves. And from that we will get a grain sized distribution
of this particular soil sample. . Now, you see that we have put the sieve set
that you saw earlier, on which we took 500 grams of soil sample, on the mechanical shaker
and we are going to subject it to mechanical shaking for about 10 minutes and
after that we are going to find out what is the weight of soil retained on each individual
sieve. And from that we are going to
construct the grain sized distribution. So, just we will start the shaking now. You just
watch it and then we are going to come back . .. So, you see now after shaking of 10 minutes,
we got the sieves back and as you recall that this was the coarsest sieve and in this
one we do not have any coarse soil retained. . Then we move on to the next one which is a
2 millimeter opening size IS standard sieve and here you can see that we have got a little
bit of soil on retained here. So, what we are
going to do? We are going to take the weight of this much
of soil and whatever is the weight we are going to divide it by the initial
total weight of the soil taken, and from that we can find out what is the gravimetric percent
of soil retained on sieve size 2 millimeter. So, then we are going to proceed to other
sieve sizes similarly. .. You can see here that there is a little bit
of soil retained passing past actually 75 micron
size sieve and the grain size of this particular fraction is going to be determined not from
sieve analysis typically. We are going to need to go on for another
laboratory experiment which is called involving hydrometer for finding
out the grain size distribution of the fraction of soil which is finer than 75 micron. So, then as I mentioned, we are going to
proceed to weighing of the percent of soil retained on each one of the individual sieve
size. . .Finally, using the data, we are going to
be able to generate a distribution, grain size
distribution curve like this which plots the percent finer by weight on the vertical scale
and the opening size of the sieve on the horizontal scale, as you see on this particular
plot. So, using that we get a, we construct a grain
size distribution and this allows us to classify coarse grain soils as I indicated
earlier in one of the theoretical sessions in the
course. . So, here is the setup for hydrometer analysis
for finding out the grain size distribution of
soil samples, which has substantial amount of particles finer than 75 micron and this
one actually is one such sample of silt and clay. .. And what we are going to do with this particular
sample is we are going to prepare a slurry of this particular soil sample, which
has got, for which we want to find out the grain size distribution by taking 50 grams
of the soil and making it mixing it up with water to make slurry of approximately one,
exactly one liter. And we are going to place
it, place the slurry making sure that soils are not readily settling in a sedimentation
jar. We also actually mix it up with about 20 ml
of sodium Hexametaphosphate solution, which acts as a dispersing agent, so that
individual soil particles did not cling together forming clots, so that the settlement rate
is wrongly measured, because the clots are going to settle at a faster rate than the
individual fine grained soil particles. So, we mix
up a slurry involving 50 grams of approximately 50 grams of soil and substantial amount
of water to make a slurry of one liter and that includes about 20 ml of dispersing agent
as I just mentioned. .. Then what we are going to do? . We are going to, before soil starts settling,
we are going to place a hydrometer like this into the slurry and find out what is the reading
that we observe at a particular time on the stem of this hydrometer. And from that reading, we can get, we can
calculate the reading to the specific gravity of the slurry at the
center of the hydrometer bulb and from that we
can find out what is the representation of a certain grain size within the soil sample
using Stokes law of settling spheres. Approximating the soil particles as settling
sphere, we .have got theoretical solution available for
that type of problem and from that one using that solution, Stroke solution, we find out
what is the grain size distribution of the slurry. . Now, you see that the hydrometer is being
inserted very gently into the soil and what we
are going to do? We want to make sure that the hydrometer does
not bob up and down when the when it reached its equilibrium position. So, we have to be very careful with
this thing and you can see the reading at the meniscus for this hydrometer position
is about 1009 for the level for the water, for
the level of the slurry that intersects the hydrometer stem. And this reading is, you should note that
this reading is going to change with time because the density of the slurry at this
center, at the elevation of the center of the
hydrometer bulb is continuously changing with more and more soil settling near the
bottom of the sedimentation jar and the reading at a given time actually correlates to the
percentage of a given soil sample, of a given size of soil that was there in that soil sample
taken originally. You should also note that the reading has
to be corrected for the temperature at the date
and time of testing because that is going to affect the specific gravity of the or the
viscosity of the slurry which is required in application of Stoke solution to this particular
problem. .. So, here we have got the demonstration for
the experiment which we use for finding out the liquid limit of fine grain soils. This test is applicable for soils which has
got grain sizes finer than 425 micron typically. So, if you have got in situ soil which has
got some representation of coarser particles, the soil
is washed to remove particles which are coarser than 425 micron and what we have here
is a sample which is already screened through 425 micron and this is the sample. . And what we are going to do? We are going to make a pat of this particular
soil sample in this pan here. .. 4
This is the casagrande apparatus which includes a hard rubber base, black hard rubber
base and it has got a brass pan. This pan is of standard dimension and it has
got a certain standard fall. So, it goes up; when it is cranked up using
this particular handle, it goes up a standard amount of height and that height
is given by this dimension here. This is the
standard tool used for grooving. As you are going to see in the next little
bit, it is called the grooving tool and this thickness at the
end of the handle of the grooving tool is used
for adjusting the height of fall of this particular pan. And what we are going to do? We are going to prepare a pat and we are going
to count the number of blows. We are going to, actually we are going to,
open up a groove near the center of the pat as you are going to
see in the next little bit and we are going to find
out the number of blows required for the groove to close over a distance of about half an
inch which is about 12.5 millimeter as you are going to see in the next little bit. And we
are going to count, we are going to prepare a plot against the number of blows required
for a given amount of moisture content within the sample versus the number of blows
required for the closure of 12.5 millimeter. You can imagine that as the soil becomes drier,
you are going to require more and more blows for closing the groove and we are going
to plot it up in a semi logarithmic scale. And we are going to find out what is the number
of blows required for what is the moisture content required for 25 number of
blows closing the groove by 12.5 millimeter. You are going to see that in the next little
bit. .[FL]. . So, now, you see now we are preparing the
pat in the pan making sure that there is no air
entrapment within the volume of soil and the surface of this particular pat should be
approximately horizontal. And then we use the grooving tool, as you
see here, to cut a groove at the center of the pat, of the clay
pat or pat of fine grained soil, and this is the
groove that we construct, that we have constructed here using the standard grooving tool. . .And then we are going to bounce it, bounce
the pan up and down, bounce the pan up and down and count the number of blows that is
required for the closure of the pat by an
amount of 12.5 millimeter. You can see that the pat is gradually closing. . And you see here the pat is closed by an amount
of approximately 12.5 millimeter and closure is approximately counted from there
to about there and the number of blows in this case is much larger than 25. And what we are going to do? We have to keep on
adding a little bit of water to the sample until we get the groove to close by 12.5
millimeter exactly on the 25th blow. And for doing that, what do we do? . .Actually, we are going to go on adding. We are going to add water and we are going
to get. We are going to plot the number of blows required
for the closure of 12.5 millimeter in this type of plot. This is called the flow curve where the number
of blows are plotted on a logarithmic scale in the horizontal axis
and the moisture content of the soil sample for that particular number of blows is plotted
on the vertical axis like this one. So, from
this pack, in fact, we are going to save some sample for finding out the moisture content
that we are going to plot in the vertical axis, as I was mentioning. . And then, we are going to prepare a plot like
this and this is called a flow curve and you see that on the third, these are the data
points; the data points are shown with the solid
diamonds here. And for this particular sample you see that
25 number of blows was obtained at a moisture content of between
of got 47.8 percent which is from this particular chart flow curve, and so the liquid
limit of this particular sample becomes equal to 47.8 . So, that is how we get the
liquid limit from Casagrande apparatus. .. So, here we have got the setup of determination
of plastic limit. So, what we do here is
more proved actually in comparison with the setup that we used, I should say. For liquid
limit, it is probably a little bit more difficult because of the subjectivity involved in this
one. So, here what we do? We take a sample of soil and we try to roll
a thread which has got a diameter of 3 millimeter exactly; means
if we can roll a thread with the sample of soil which has got 3 millimeter diameter,
then we say that the soil is at plastic limit. And
in fact, if the soil is at plastic limit, then it needs to just start cracking as soon
as we reach 3 millimeter diameter, in fact, we will not
be able to roll a thread which is finer than 3
millimeter size. So, how do you roll a thread? That is being demonstrated by my
colleague here. .. And this is the pat of the soil that we that
we have here and we just try to roll a thread by
the procedure like that, and you can see that as the thread becomes finer and finer, it
will have a greater likelihood of cracking. So, here we have got a thread which is probably
about twice as thick as the standard 3 millimeter diameter piece of wire here. .. And let us see a little bit more of that rolling
action here. What we want to see is when at
what diameter it starts to crack. So, probably it can go a little bit finer
still because it is yet to crack, and you can see that this could
be actually rolled to 3 millimeter size and the
sample did not crack. So, we need to have a little bit of drier
sample and if it is a little bit slightly drier than that one, then we will
not be able to roll a thread of 3 millimeter size. You will see that in the next little bit. . And you see here that the sample has been
dried up and it started to crack. The thread has
started to crack as soon as it has reached about 3 millimeter diameter and the cracks
are .starting to appear on the thread, as you
can see from the from the thread that my colleague was rolling that I am holding now
in my hand. So, this moisture content is barely allowing
us to, the moisture content of this specimen is barely allowing us to a roll a thread of
3 millimeter diameter as soon as we try to go
finer starting to crumble. So, the moisture, this moisture content, moisture
content of this piece of thread is going to be the plastic
limit. . We are going to save enough of such sample
in moisture content pans like this one and we are going to find out what is the representative
value of moisture content from a large amount of soil by rolling several different
threads because that is going to allow us to
minimize the amount of error in the experiment because if we just take this much of
sample, then the amount of soil is going to be very much less. So, the amount of error is
going to be large. So, we take, we roll several different threads
and we try to determine the moisture content based on the several
different threads like this one. So, that is it. .. So, here we have got the setup for finding
out the specific gravity of soil solids. So, this
is the sample that we take in this particular experiment. So, what we do with this one is
the procedure involves taking about, actually comparing the weight of some known
volume of distilled water. . 4
In fact, this particular flask is calibrated to be of 250 milliliter of volume at 27 degree
Celsius and you can see that there is a mark on the stem of this particular flask and we
are going to take distilled water exactly up to that mark. We are going to find out what is
the weight of the flask plus distilled water and then we are going to take the distilled .water out of there. We are going to take a representative amount
of soil in the flask and then we are going to top it off with distilled
water upto the mark that we saw there on the stem of this flask. We are going to keep it under suction to deair
the water so that there is no bubble of air included in between individual
grains of the soil because that is going to introduce some error in the reading. By comparing the weight of the distilled water
and the weight of distilled water plus soil, we
can calculate what is the specific gravity of the
soil. So, that is how we obtain this specific gravity
of soil solids. And as you have seen that this particular
property is required in finding out several different index properties such as the total
unit weight of soil and void ratio of the soil,
and in fact for saturated soil, that is also required for finding out the, for relating
the moisture content and the void ratio. So, that is about the specific gravity. . So, here you see that the soil is kept under
a vacuum pump kept under vacuum here and that is being shaken by my colleague here,
so that the water, so that the mixture does not
have any intrusion of air because that might actually, as I was mentioning that might,
introduce some error in the estimated specific gravity of solids.. .(Refer Slide Time: 26:03min) This equation, this is the equation that we
use for finding out the specific gravity of soil
solids and which is given by w subscript s divided by w subscript s plus w subscript
bw minus w subscript bws multiplied a quantity
alpha. So, the explanation of these different
symbols are also there at the bottom. w subscript s means the weight of soil solid
taken in the flask. w subscript bw means weight of bottle or the
flask that we had there plus water and w subscript bws is weight of bottle
plus water plus soil solid and alpha as I was saying is temperature correction. There are standard charts available to standard
charts available for alpha because if you recall from your under graduate, other under
graduate courses, the volume of water expands with temperature increase, change
in temperature, and alpha is a factor that accounts for that volume expansion. Thank you. .. So, this is the demonstration of actually
undrained unconsolidated triaxial test. So, what
we have here is a loading frame. . And we have already mounted the triaxial cell
on the loading frame. What we do here is
we apply a displacement in the axial direction and you can see that inside actually. So,
this is the demonstration of un consolidated undrained triaxial test. What you see here is
a triaxial cell and the triaxial cell is essentially a flexi glass cylinder which is in between,
sandwiched in between the top cap and the bottom cap and it is filled with water as
you can see here. And inside the triaxial cell also is a cylindrical
soil sample which is .wrapped up in the white membrane. In order to seal it off from the water which
is there in the triaxial cell is a couple of orings
and you can see one oring, dark oring at the at the
bottom and the top oring is actually covered by the membrane here in this particular
case. And you have there is a brass cap through
which the load or displacement is applied axially to the sample and there is
a bottom plating also which is covered in this
particular case by the membrane. . Now, we need to measure the axial stress on
the triaxial sample for which we make use of a proving ring here and that is calibrated
in such a manner that from the reading on this particular dial gauge, we can find out
what is the stress, actual stress on the triaxial sample. Then we have got the displacement. We make use of this particular dial gauge
for finding out what is the axial displacement of this particular triaxial set up. We also have got another connection from the
bottom of the cell and that connects to a pressure gauge. As you can see here, this allows us to keep
track of the pressure that is there in the water inside the triaxial cell
and we can pressurize the water to our required level using a compressor, which is further
to the left of this particular set up. So, you see now that we are going to connect
the pressure, connect the compressor, compressed air, connect compressed air to
the triaxial cell and then we have applied here
a load of about 10 pound per square inch which you can see from the pressure gauge. We
are going to see that the axial displacement is going to start climbing and that is being
measured on this particular dial gauge and the load also is increasing, as you can see .from the movement of the proving ring dial
gauge there. But the cell pressure in this
particular case is kept constant as you can see here. There is no movement on the
pressure gauge and we are going to continue this test until we see no further increase
in the proving ring dial gauge. . . You can see that the loading range here is
gradually decreasing. In other words, it is
taking more and more movement to mobilize a certain amount of soil reaction. It is
virtually stand still; it is not increasing any more. So, we are going to terminate the test .here and you can see the deformed pattern
from the bulging out as I was mentioning before of the sample. . The way the data are analyzed is by calculating
what is the axial stress the sample is being subjected to and plotting the axial
stress versus the axial strength which is the
changing length of the sample in the axial direction divided by the original length of
the sample. . .We prepare a plot of this type which is a
plot of the axial stress minus the cell pressure that gives us the deviated stress, in this
particular case, versus the axial strain. And as I
was mentioning before we read the streak of this particular plot and that is, that gives
us an estimate of that. From that, we can get an estimate of the undrained
shear strength of the sample. . So, here you have got the demonstration on
standard penetration testing. If you recall, I
told you in the class that we drive a split spoon sampler into the ground using a standard
spt hammer. So, this is the split spoon sampler which
is of dimension which I have already mentioned and we are going to disassemble
the sampler in order to illustrate what are the internal parts. .. So, this is the cutting shoe
which comes of like that and then the socket which connects
the sampler to the spt rod. That also comes from the other side and then
the portion in between splits into two different parts as
you are going to see in the next little bit. You
need a rod to do that and this is the two portions which split up like this one. . And then we assemble the stuff to and then
then drive it into the ground to retrieve the
soil sample. Now we can reassemble the stuff. So, this is how the sampler is assembled. .And what we are going to do? We are going to drill a hole in the ground
and we just lower the sampler to the depth like this and
drive it, connect it to the hammer, spt hammer with the standard rods and drive the
setter, drive the assembly with standard spt hammer as you are going to see in the next
little bit. . So, now, we have got the setup into the ground
and what we have got here is a standard spt hammer mounted on the spt rod. . .This is a hammer called donut hammer. This hammer has got a weight of 65kg. It is
going to fall through 750 millimeters onto the hand wheel here which is underneath the
hammer, which may not be visible from where the camera is. . And what you have, what you also see is that
we have got three markings at 150 millimeter spacing. One at the top of the hole, then 150 millimeter
above, next one 300 millimeter above, and the third one at 450
millimeter above the top of the hole. So, what
we are going to measure? We are going to measure the number of blows
required for the penetration of this length from here to here;
then the number of blows required for penetration from here to here, and finally,
the number of blows required for penetration from here to here. So, now you just be with us and we are going
to take help from two f my colleagues who are going to lift the hammer up to 750 millimeter
markings which is also marked on the rod and they are going to drop it on the hand
wheel and that is going to drive the sampler into the ground and we are going to count
the blows as you are going to see in the next little bit. .. So, you see that the first 150 millimeter
penetration is completed with only one blow; the
soil is quit soft here. Then we are going to go with the next penetration
of 150 millimeter. So, that one also basically is penetrated
for with one blow only. Yes
So, this one did not get penetrated in one blow. So, we are going to perhaps need a few
more blows. Let us see how many. Yes. .. So, that is it. So, the number of blows if you recall, we
required only one blow for the penetration of first 150 millimeter. Then we required one blow for the penetration
of next 150 millimeter, and finally, we required two
blow two blows for penetration of the final 150 millimeter. So, in this particular case, the spt, raw
spt blow count is going to be recorded as the total number of blows required
for the penetration of the second and the third 150 millimeter blow count, 150 millimeter
penetration, which is 3. So, in this
particular case the uncorrected spt blow count is 3. So, that is the demonstration of the
standard penetration testing and we are going to move on to the next demonstration now. Now, we have got the soil sample from out
from the ground and we are going to disassemble the sampler, and we are going
to see how the sampler looks, how the soil sample looks. So, this is the soil sample that we got from
down there. .. This particular sample is visually identified
in the field and it is also saved. Some of the
samples is also going to be saved for moisture content determination. So, if you recall
this is the bottom end of the sample and that is the top end of the sample. . And you can see that the sample was moist
near the top and it was becoming dry as we went downward. So, this has got a lot of sand and silt and
other fine grain soils. It has got
some cohesion as well and that is how we identify the sample and this is the sample
which is near the bottom end of the sample. .. So, this sample is saved for grain size distribution
and finding out the natural moisture content of the specimen. That is it. . Then, we are now into the demonstration of
point load index testing. In this particular
test, if you recall from our earlier laboratory, earlier classroom lesson we have we
basically do we try to break a core sample in between two points by applying a
compressive load through the points. We are going to see that in the next little
bit how and from the load at which the sample fails
we have got a correlation between that load .and the unconfined compressive strength. Now, basically the calculation, if you recall,
that is involved in this particular case. . The calculation to jog your memory involved
in this particular case first requires calculation of the point load index i subscript
s given simply by p over d square, where p is the point load at which the sample fails. That is going be recorded in force units
although in the next little bit we will see that we are going to read that thing of a
pressure gauge. So, we have to multiply the pressure recorded
by the diameter of the ram of the hydraulic cylinder to get the value of p and
then d is the diameter of the sample and this one here is in length unit. Finally, from the point load index, we calculate
the uniaxial compressive stress sigma subscript c using the formula at the bottom
near the bottom of this particular handwritten note which is 14 plus 0.175 times the diameter
of the sample multiplied by the point load index, where d is taken in millimeter. We are going to get into the testing now. These are the core samples that we are going
to test today in the point load index testing
machine. Now, both of them are basically
sandstone samples. .. If you look at this particular sample carefully
from the knowledge that you have got on rock samples, you will see that this is a
fairly coarse grained, grey colored sandstone and
this one here on my in my right hand is a relatively finer grained sample which has
got a few quartz grains. What is also to be noted here is that the
samples that we have chosen has got a length of at least 1.5 times the
diameter of the specimen; that is one of the requirements of point load testing. So, if you recall, if you recall, the point
load index is basically point load divided by
square of the diameter of the samples. So, we need to find out what is the diameter. Although both of these samples were obtained
from a drilling investigation, using (( )) bit. So, we know roughly the diameter of the sample
will be about, will be a 55 millimeter. But we need to we need to take a careful measurement
of the diameter. So,
for that what we are going to do? .. We are going to use this pair of calipers
here and try to determine the diameter. So, here,
what we got? The main scale reading of the calipers is
53 millimeter for this sample and then the fractional reading I am getting is
0.36. . So, the sample diameter in this particular
case is 53.36. I want to take a few more
readings. So, here I am getting a value in this direction. In other orthogonal direction, I
am getting a value, different value. Here it is 53.26. So, by taking such measurement in
at least four orthogonal directions, whatever average diameter you get, you need to use
that average diameter in your point load index calculation. .. So, here again we are getting a readings of
roughly 53.26 millimeter. all right. So, let us
say, let us say the diameter of this sample is about 53.3 millimeter and we are going
to use that 53.3 millimeter in our point load
index calculation. So, this one, this sample here
is 53.3. Let us measure the diameter of the other one. Notice that this sample was also obtained
using the same, the same, actually same bit. Here, what we are getting? The diameter is
likely larger, in fact, than 55 millimeter, getting 55.08 and other direction again I
am getting 55.1. .. So, this sample, although it was obtained
using the same reading, similar very similar reading, that similar reading equipment, the
diameter is slightly larger than the one on my left hand and that is because of the durability
of this sample. This, the sample that I
am holding in my left hand is slightly more friable and slightly less durable than the
one that I am holding on my right hand here. all right. Now, let us do the testing. . The sample is in between two points. You just observe the sample. It is going to crack
once the point load reaches the failure value; the sample failed and the reading was
roughly about 14; 14 kg per square kg per square centimeter at which the sample failed. .. So, this is the face where the sample broke
and we are going to record the point load, actually the pressure which is 14.7 kg per
square centimeter at which the sample failed. So, I am recording it here. . So, this is 14 kg per square centimeter. I am going to actually use it in the calculator. I
am going to show the calculation as well later on. Now, what we are going to do? We are
going to test the second sample. .. We have got the second sample and now we are
trying to note the point load. You just
look at the dial gauge reading. It is going to start mounting and when the
sample fails, it is going to suddenly drop. . It actually did not even go upto the extent
of the previous sample. So, here, we got a
reading of approximately is 8 kpa; 8 kg per square centimeter. .. . This is the instrument that we are going to
use for nondestructive testing of a rock mass. It is very simple index really. So, what happens here is that
we are going to push the instrument in and we have to make sure that
this particular knob is stressed when the rebound occurs in order to measure the rebound
value and the reading is going to appear on this particular scale here. .. And we did the testing on the floor. . So, the strength, the strength here index
is 52. The value of the value is 52. .. And then this value gets converted from a
standard chart which is also given by the manufacturer. You have to read off the value of
the ucs from looking at the appropriate curve. For example, this curve the one at the top
this curve is appropriate for the configuration of the testing that we did in this particular
case. And here we had a demand value of 52
and 52 comes roughly about here and that translates to a uniaxial compressive stress of
about 56 mva for this instrument. So, that is a nondestructive testing in which
we do not have to bring in a sample and subject it to
a set of deformation process which destroys the sample. So, this is, this type of testing is called
nondestructive testing. .

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