This is professor Kitch. Welcome to this webcast on part 2 of soil classification
using the Unified Soil classification system. There’s only one learning outcome for this lesson,
which is to properly classify coarse grained soils using the USCS.
We will be following ASTM standard D2487 which codifies the unified soil classification system.
Before we start the classification system, let’s review the important characteristics
of coarse-grained soils. First is particle size,
that is the average particle size. This will tell us if the soils sands or gravels.
Next is the distribution of soil particles. This will tell us if the soil is very uniform in size
or has a large distribution of different sized particles. We’ll use this to determine
if the soil is well or poorly graded. The percent of fines, that is percent of
clays and silts in the otherwise coarse-grained soil. If there are a lot of fines, we’ll need to
include that information in the classification of the soils. If there are very little fines,
then they won’t be in the classification. And if there are a modest amount of fines,
we’ll have to describe both the coarse and fine grained parts of the soil in the classification.
The type of fines will be important, that is whether they are clays of silts.
Finally, there are some important characteristics which are not part
of the unified soil classification system. One is particle shape. While it’s not technically
included in the USCS, we can add a particle shape description to our USCS classification.
Finally, the relative density of a soil is important to its behavior.
However, this is an in situ property and USCS classification is based on measurements of
disturbed soils, so we can’t say anything about relative density in this system.
However, if we have information about density from a field test
such as the standard penetration test, we can add that to the official USCS classification.
Let’s start by looking at the grain size distribution of the three different soils
show in this figure. In addition to the grain
size curves for the soils, this figure show size of the number 200 sieve,
which is the dividing line between fine grained soils and coarse grained soils
and the number four sieve, which is the dividing line between sands and gravels.
A quick look at this figure shows that all three soils are coarse grained.
That is, well more than 50% of each soil is larger than the openings in the number 200 sieve.
Looking at soil A, we see that this is a soil with a very limited range of particle sizes. We can see
that over 70 percent of particles are between 0.2 and 0.4 millimeters in diameter. So the particles
of this soil are a very uniform size. We call a soil like this poorly graded,
because the gradation of particles covers only a small range of sizes.
In contrast, the size of particles in soil B covers a much wider range. We can see that over
80 percent of the soil is spread across the entire rage of sand particles form less than one tenth of
a millimeter to five millimeters in diameter. We call a soil like this, well graded,
because its soil particles are spread over a large range of sizes in a relatively uniform fashion.
Soil C has an interesting grain size distribution. It covers a wide range of particle sizes, from
less than a tenth of millimeter in diameter to over 30 millimeters in diameter.
That is from fine sand to coarse gravel. However, the distribution isn’t uniform across this range.
Close observation shows that over 40 percent of this sample is in a range
from 20 to 40 millimeters in diameter. Another 40 percent is in the range from
fine to medium sand, one tenth to one and a half millimeters in diameter.
Only a small portion of the particles in this soil are in the coarse sand to fine gravel range.
We call a soil like this, gap graded, because it consists largely of two different size ranges
with a gap between them. This is a special case of poorly graded soil. In this case
the particle size covers a large range, but isn’t well distributed across that range.
To be able to quantify the gradation of a soil,
ASTM Standard D2487 uses two measures of gradation.
The first is the coefficient of uniformity c-sub-u, which is defined as D-60 over D-10,
where D-60 is the diameter for which 60 percent of the soil particles are finer,
and D-10 is the diameter for which 10 percent of the particles are finer.
For example, looking at soil B, D-60 is one millimeter and D-10 is 0.12 millimeters.
The coefficient of uniformity is a measure of the slope of the grain size distribution curve
between 10% passing and 60% passing. Soil A has a very steep slope in this region.
Soil B has a shallower slope. and soil C has a yet a shallower slope.
C-sub-u is actually proportional to the inverse of the slope of the curve, so soil C
has the largest coefficient of uniformity and soil A the smallest. For a soil to be well graded, we
want it to have a relatively shallow slope, like soil B rather than the steeper slope of soil A.
ASTM Standard D2487 specifies minimum values required for a soil be classified as well graded.
For sands C-sub-u must be greater than 6. For gravels, it must be greater than
four. If we only used the coefficient of uniformity to define being well graded,
soil C would a well graded soil. However, it’s gap graded rather than well graded.
To identify gap graded soils we use a second measure call the coefficient of concavity,
c-sub-c. Which is defined as D-30 squared divided by the quantity D-10 times D-60.
This measures whether the grain size curve is concave up between D-10 and D-60, as seen
in soil B, or concave down as seen in soil C. ASTM Standard D2487 requires well graded soils
to have coefficient of concavity between one and three. Gap graded soils like soil C
will have a C-sub-c value outside this range. So for a soil to be well graded,
it must have coefficient of uniformity greater the 6 for a sand or greater than 4 for a gravel,
and its coefficient of concavity must be between one and three. Before moving on,
I’d like to contrast geotechnical engineering terms with geology terms as they relate to gradation.
Geologist use the terms well sorted and poorly sorted when discussing the grain size
distribution of soils. This language is just the opposite of that used by geotechnical engineers.
To a geotechnical engineer, a well sorted soil will be poorly graded.
and a poorly sorted soil will be well graded. So be carefully when you’re talking to a geologist
about soils—it’s easy to be misunderstood. It’s not that one of these descriptions is right and
the other is wrong. It’s just that geologists are talking about the process that formed the soils,
while engineers are talking about the quality of the soil itself.
To understand how coarse soils are classified in the USCS system,
it’s helpful to think of these soils as being grouped into three categories.
The first category is what we can call clean sand and gravels.
These are soils with very little fine material. Essentially, they’re all sand and/or gravel.
We classify the soils based solely on their particle size and gradation.
These soils will be classified as either SW, well graded sand; SP, poorly graded sand;
GW, well graded gravel; or GP poorly graded gravel.
The second category are those sands and gravels with a lot of fine material.
In this case the behavior of the soils will be largely governed by the size of the coarse
material and the type of fine material. These soils will be classified as
SM, silty sand; SC clayey sand, GM silty gravel, or GC clayey gravel.
The third category are soils with a moderate amount of fine material.
These are in between soils and will be classified both on their gradation
and on the type of fine material they contain. These soils will get a dual classification such as
SW-SC, well graded sand with clay, or GP-GM, poorly graded gravel with silt.
These three categories aren’t an official part of the ASTM D2487 standard,
but they help us to understand the classification process. With these three categories in mind,
let’s take a detailed look at the coarse-grained soil classification process
We will use this flow chart to classify coarse grained soils. This is Figure 5.7 in your text
or Figure 3 from ASTM standard D2487. Remember to start using this flow chart,
we must have already determined that the soil is either a sand or gravel.
That is, more that 50 percent of the soil must be retained on the #200 or larger sieve.
Once that’s determined the first question we need to ask is, “is there more gravel or more sand.” .
If there’s gravel than sand, we’ll be using the top half of the chart.
If there’s sand than gravel, we’ll be using the bottom half of the chart.
The second question we’ll ask is how much fine material is there in the soil?
If less than 5% passes the number 200 sieve,
then then it’s a category 1 soil with very little fines (a clean sand or gravel).
If more than 12% passes the number 200 sieve, then it’s a category 2 soil with a lot of fines.
Otherwise it’s a category 3 soil with a moderate amount of fines.
The third step is to determine if the soil is poorly or well graded.
This applies only to soils with less than 12 percent fines.
The fourth step is to determine the classification of the fine grained portion of the soil.
This applies only to soils with more than 5 percent fines.
Once we’ve completed this step, we can determine the soil’s group symbol.
The final step is to look at the percent sand, if the soil is a gravel, or the percent gravel, if
the soil is a sand. This will allow us to give the soil its complete and official group name. If you
think about breaking this chart down into these five steps, it’s really not that complicated.
So lets summarize the steps in classifying a coarse grained soil.
First we ask, is the soil mostly sand or mostly gravel?
Next we look about the amount of fine material and determine is there just a little,
less than 5 percent? Is there a lot,
more than 12 percent? Or is it in between?
The third step is just for soils with less than 12 percent fines. Here, we’re
asking if the soil is well graded. For sands the coefficient of uniformity must be greater than 6,
for gravels it must be greater than 4. And for both, the coefficient of
concavity must be between one and three. In the fourth step, we look at the fine
material for soils with more than 5 percent fines to determine if it is clay or silt.
And in the final step we ask if there’s more than 15 percent sand in a gravel, or more than
15 percent gravel in a sand. Once we’ve completed all five steps, we can complete the classification
and give the soil both its two-letter group symbol and its full group name. Well this gives you an
overview of the process of classifying coarse grained soils in the USCS system. However,
it’s not really possible to understand the process without actually classifying some soils. In the
next webcast, I’ll go through a number of example classifications of coarse grained soils.