This is professor Kitch.
Welcome to this webcast where I’ll be going through examples of using the USCS to classify
coarse grained soils I’ll be classifying six different soils
in the webcast.
The grain size distribution for the first three soils, A through C, are shown here.
Let’s start with Soil A. Recall, our first question is whether this
soil is a sand or a gravel.
In this case, we can see that the entire grain size curve lies within the range of sand sized
particles, so this is all sand.
Next, we need to look at the percent fines.
In this case there are well less than 5 percent passing the number 200 sieve.
So our next question is whether or not the soil is well graded.
First, we’ll check the coefficient of uniformity, C-u, which is D-60 divided by D-10.
For this soil, D-60 is 0.35 millimeters and D-10 is 0.25 millimeters.
This gives us a C-u of 0.35 divided by 0.25 which equals 1.4.
This is less that the value of 6 required for sands to be well graded, so this is a
poorly graded sand and there’s no need to check the coefficient of concavity, C-sub-c.
The symbol for this soil is SP.
We can skip step four since this is a clean sand with very few fines.
Our last step is to check the percent gravel, which is zero and therefore less that 15 percent.
So our final classification of this soil is SP, poorly graded sand.
Notice that we were able to skip a number of the steps in the classification process.
Next, well look at soil B. Our first step is to determine if there’s
more gravel or more sand.
We see that 92 percent of the soil passes the number 4 sieve.
So, the percent gravel is 100 minus 92 or 8 percent.
The percent finer than then number 200 sieve is 4 percent.
So, the percent sand is 92 minus 4 or 88 percent.
So this soil is a sand.
(Which was already obvious from the grain size distribution curve.)
Question 2 is about the percent fines.
In this case there are 4 percent fines, which is less than 5 percent, so this is a clean
sand.
Now we need to determine if it’s well graded.
D-60 is one millimeter, and D-10 is 0.16 millimeters.
So the coefficient of uniformity is one divided by 0.16 which equals 6.25.
So this soil passes the first check since C-sub-u is greater than six.
Now we need to check the coefficient of concavity.
C-sub-C is D-30 squared divided by D-10 times D-60.
D-30 for this soil is 0.5 millimeters.
And C-sub-c is 0.5 squared, divided by the quantity 0.16 times 1, which equals 1.56.
This is in the range of one to three, so this soil is well graded and it’s two letter
designation is SW.
We can skip step four because the fines are less than 5 percent.
Our final step is to check the percent gravel, which is less than 15%.
So our final classification of this soil is SW, well graded sand.
Our last soil in this group is Soil C.
We first need to compare the amount of sand with the amount of gravel.
48 percent of this soil is finer than the number four sieve.
So the percent gravel is 100 minus 48 or 52 percent.
Two percent of the soil passes the number 200 sieve.
So, the percent sand is 46.
There’s more gravel than sand, so this soil is a gravel.
There are two percent fines, so this is a clean gravel.
Now we need to check to see if it’s well graded.
D-60 is 20.5 millimeters.
D-1- is 0.2 millimeters.
So the coefficient of uniformity is 20.5 divided by 0.2 which equals 102.
102 is greater than the 4, so now we need to check the coefficient of concavity D-30
is 0.61.
So, C-sub-C is 0.61 squared divided by the quantity 20.5 time 0.2 which is equal to 0.09.
0.09 is well less than 1.
Therefore, this soil is poorly graded, and it’s symbol is GP.
We don’t need to check the fines, since they’re less than 5 percent of the soil.
Our final check is the percent sand, which is well over 15 percent.
Therefore, the final classification of this soil is GP, poorly graded gravel with sand.
OK, so much for the easy soils.
Let’s classify some more difficult soils.
Next we’ll classify these three soils, which are coarse grained soils, but with each with
a significant amount of fines.
Let’s start with soil D. First we’ll determine if it’s a sand or
gravel We look at the top of the graph and see there’s
zero percent gravel The percent passing the number 200 sieve is
28, So, the percent sand is 100 minus 28 which
is 72 percent.
So this is a sand.
Checking the percent fine, we know they are 28 percent.
So this sand has well more the 12 percent fines.
And it must classify as either SM or SC We don’t need to check the gradation since
the fines content is over 12 percent.
So we can move on to checking the type of fines.
To do that we’ll need to know the liquid limit and plasticity index of the fine part
of the soil.
From our Atterberg limit test we find that the liquid limit is 45 and plastic limit is
32 This gives us a plasticity index of 45 minus
32 which is 13.
We’ll need the Casagrande chart.
And plot a point with liquid limit equal to 45 and PI equal to 13.
This point plots below the A-line and the fine material classifies as ML, silt, so the
group symbol for this soil is SM The last step is to check the percent gravel,
which is less than 15.
And this soil classifies as SM, Silty sand.
Notice that we were able to skip the gradation check because of the large amount of fine
material in this soil.
Next let’s classify soil E First checking the percent gravel and percent
sand.
88 percent passes the number 4 sieve.
So, the percent gravel is 100 minus 88 or 12 percent.
10 percent passes the number 200 sieve.
So, the percent sand is 88 minus 10 or 78 percent.
And this soil is sand.
Checking the percent fines, we find it’s between 5 and 12 percent.
Therefore this soil will have a dual classification.
We’ll have to check the gradation, starting with the coefficient of uniformity.
D-60 is 1.5.
D-10 is 0.075.
C-sub-u is 1.5 divided by 0.075.
Which equals 20.
This is greater than 6 so we passed the first check for well graded and need to check the
coefficient of concavity.
D-30 is 0.3.
And C-sub-c is 0.3 squared divide by 1.5 times 0.075.
Which is 0.8 0.8 is less than one, so this soil is poorly
graded.
And the first symbol in the dual classification is SP.
We now need to check the fine material.
So we do our Atterberg limit test and find that the liquid limit is 63 and the plastic
limit is 22.
Which gives us a plasticity index of 63 -22 or 41.
Using the Casagrande chart we plot the point and find that it falls above the A-line, and
the fines classify as CH.
So the second symbol for this soil is SC.
Our final check is the percent gravel, which is less than 15.
So, the final classification of this soil s SP-SC, Poorly graded sand with clay.
Notice that we had to check both the gradation and type of fines for this soil, because the
percent fines was between 5 and 12 and the soil has a dual classification.
Our final soil is Soil F. Again, we start with the determination of
the amount of sand and gravel.
We find the percent passing the number four sieve is 57 percent.
So, the percent gravel is 100 minus 57 or 43 percent.
The percent passing the number 200 sieve is 18 percent
The percent sand is 57 minus 18 or 39 percent.
So, this soil is a gravel.
Notice that less than half of the soil is gravel, but it still classifies as a gravel,
because the percent gravel exceeds the percent sand.
Next we check the percent fines and find that they’re greater than 12 percent.
Therefore this soil will be either SM or SC.
And we won’t need to check the gradation of the soil.
We do need to check the fines.
So we perform Atterberg limits on the fines and find they have a liquid limit of 41 and
a plastic limit of 37.
This gives us a PI of 41 minus 37 or 4.
Using our Casagrande chart, we plot the point, and find that the fines classify as ML.
Soil the group symbol for the soil is GM.
We do our final check on the percent sand and find that it’s much greater than 15
percent.
So, the final classification of this soil is GM, silty gravel with sand.
The tricky part of classifying this soil is to realize that it’s a gravel even though
less than 50 percent of the soil is gravel.
Remember when determining whether a coarse grained soil is a sand or a gravel, the question
is whether there’s more gravel or sand in the coarse grained fraction of this soil.
Not whether more than half of the total amount of soil is sand or gravel.
We’ll that’s the end of this set of example problems for classifying coarse grained soils.
I think I’ve taken you through almost all the nuances of classifying coarse grained
soils using ASTM D2487 and the unified soil classification system.
Here are some keys to remember.
First, follow the flow chart.
Second, check the percent fines first.
This will tell you if you can skip part of the flow chart.
If the percent fines is less than 5 percent, you only need to do the grain size analysis,
you can skip checking the Atterberg limits.
If the percent fines is greater than 12 percent, you only need to check the Atterberg limits,
you can skip checking to see if the soil is well graded or poorly graded.
If the percent fines in between, you’re out of luck and will have to check everything.
When determining if the soil is gravel or sand,
remember that it’s based on which is the greater percent of the coarse grained portion
of the soil.
It’s not based on the percent of the total soil sample.
We’ll that pretty much wraps it up.
I hope you found these example problems helpful.