Soil Mechanics Report
November 2013
Preston Merrell
Abstract
Soil Mechanics is a very important
part of the engineering world. How soil
experiments mentioned in this report help
accomplish this goal.
Objective
reacts under loads, with various water
contents, and movement will be critical in
how a project should be planned. Students
are to use a sieve analysis to help classify
two types of soil: a sample from the Teton
Dam, and a road base sample. They also
measure the ultra-fine particles that cannot
be measure in the sieve analysis through a
hydrometer test. These values are
tabulated and graphed together to show
the grain diameter vs. the percent passing.
The Plastic Limit, Liquid Limit, and Plastic
Index are also found through two types of
The objective of these experiments is to
acquire data that will allow us to classify the
two types of soil being used, as well as the
moisture content which will allow us to
note the best moisture content to allow the
best compaction. Actual compaction tests
will not be included in this lab. However,
the students will calculate the water
content of the soil in its natural state which
would be used to know how much water is
allowable for safety. A sieve analysis will
also be used in this lab to help classify the
soil. From the analysis we will tabulate the
experiments.
percent passing of soil in each sieve. These
Introduction
The study of soil behavior is a critical
values will be used to classify the soil and
give it a “name”. The Unified Soil
part of construction for any type of project.
Classification System is used most widely in
Geotechnical engineers are needed in
the geotechnical world for classification of
almost every project to ensure the safety
soils, which is what will be used in this
and integrity of a structure by sampling the
experiment. Any soil passing the #200 sieve,
soil it will rest on to be able to make correct
or the .075mm sieve, is then analyzed
engineering decisions. Attention to the
further by a hydrometer test. The grain
minutest details of how the soil being dealt
diameter can be calculated from the
with reacts to moisture, weight, and
distance and time of fall of the particles.
movement, will save thousands of dollars
and even lives. Various methods used in the
The last experiments that will be
basis for this test is Stokes’ law for falling
included in this report will be the Liquid
spheres in a viscous fluid. This law relates
Limit and the Plastic Limit tests, which then
the terminal velocity of the grains in
allows us to calculate the Plastic Index.
suspension, their density, and the density of
These values, part of the family Atterberg
the fluid. The diameter can be found with
limits, will be very important to know from
this equation:
an engineering standpoint because they
𝐷(𝑚𝑚) = 𝐾√
correlate with the engineering properties
𝐿
𝑇
and engineering behavior of fine-grained
D = Diameter of soil particles in suspension
soils.
K = Constant which is a function of specific
gravity and temperature
Theory
The range of particle size
distributions in soils is enormous. Soils can
L = Effective Depth (found in attached table)
T= Time of reading (minutes)
You can now compare these grain
range from boulders or cobbles several
sizes of the particles with the percent of the
centimeters in diameter down to ultrafine-
soil still remaining in suspension for each
grained colloidal materials. Grain sizes
reading. The percent finer in suspension is
between 5 mm and 0.074 mm are classified
calculated by the equation
according to U.S. Standard sieve number,
which can be directly related to a specific
𝑃=
𝑅ℎ 𝛼
𝑊
× 100
diameter of a soil grain. Since soil particles
P = Percent of soil in suspension for a given
hydrometer reading
are rarely perfect spheres, when we speak
Rh = Hydrometer reading + Cm
of particle diameters, we really mean an
α = Correction factor
equivalent particle diameter as determined
by the sieve analysis.
The sieve analysis is impractical for
sieve openings less than about 0.05 to
0.075 mm. Thus for the fine-grained soils
the hydrometer analysis can be used. The
These found particle sizes can now
be treated as if we used a tiny sieve to
analyze them. They can now be imposed on
a grain-size distribution graph along with
the larger sieved particles.
The presence of water in the voids
increases, the state of the soil changes from
of a fine-grained soil, as shown in figure 1,
a brittle solid to a plastic solid and then to a
can remarkably affect its engineering
viscous liquid. Thus we can see how critical
behavior. It is not only important to know
it is to know these parameters when
the water content of the soil, but we need
construction projects are underway.
to be able to compare or scale this water
Methods
content against some standard of
engineering behavior.
In these three experiments there
will be two types of soil being tested; one
fine-grained and the other a coarse-grained.
The fine-grained soil is a sample from the
actual Teton Dam site. The course-grained
material is a type of road base given to the
students to test.
Fig. 1-Water Content representation
Plasticity is one the most obvious
The sieve analysis is performed by
obtaining 9 different sieves, cleaning them
characteristics of clayey soils. This is what
to make sure no extraneous particles are
distinguishes between clays and silts, and
accounted for, and weighing and recording
plasticity is directly related to the water
each one. The sieve classification and
content.
diameters are ¾”, ¼”, #4 (4.75 mm), #10 (2
Albert Atterberg developed the
mm), #20 (850 µm), #40 (425 µm), #60 (250
Atterberg limits, 7 limits that describe a
µm), #120 (125 µm), #200 (75 µm), and Pan.
fine-grained soil, and included in this family
Then 1280.9g of the fine-grained material is
of soil characteristics are the Plastic Limit,
obtained and slowly poured into the
Liquid Limit, and the Plasticity Index. These
stacked sieves. A cap is placed on the top
parameters are required to define the
and the entire collection of sieves is set into
plasticity of clays. The Plasticity Index
a sieve shaker which then mechanically
represents the range of water content
distributes the soil particles according to
where the soil is plastic. As water content
their grain diameter. Each sieve is then
weighed and the mass of soil retained on
off the hydrometer, you can then calculate
each sieve is then calculated. These values
the percent finer is suspension.
are important because the Percent Passing
can now be calculated, which we need in
order to classify the soil. The same process
is then completed with the coarse-grained
material, but with 1961.5g of this type.
The soil passing the #200 sieve is the
The liquid and plastic limits are
calculated in a more eccentric way that will
require a little more detail. A device
developed by Casagrande called the LiquidLimit device as shown in Figure 2, and it
allows one to calculate the LL.
ultra-fine soil that will be used for the
hydrometer to test particle size distribution.
Two groups performed these experiments,
however, one group failed to complete the
experiment of the course-grained material
for the hydrometer test. The 152H
hydrometer was first calibrated by filling it
with a dispersing agent and distilled water
Fig. 2 – Casagrande’s LL device
up to the 1000mL mark and recording the
hydrometer’s value at equilibrium which
became the correction factor. 50g of soil
passing the #200 sieve was added to the
glass cylinder with the hydrometer and
distilled water, filling it up to the 1000 mL
mark. Readings from the hydrometer were
immediately taken starting at 4 seconds. 11
other readings ranging between this and
1450 min were taken. It is important to do
this over a relatively large period of time to
get a good idea of what is happening to the
particles as time goes on. From the readings
He defined the LL as that water
content at which a standard groove cut in
the remolded soil sample by a grooving tool
will close over a distance of 13 mm (1/2 in.)
at 25 blows of the LL cup falling 10 mm on a
hard rubber plastic base. In practice, it is
difficult to mix the soil so that the groove
closure occurs at exactly 25 blows, so we
generally mix and test the soil at 5 to 6
different water contents, each resulting in
the ½-in. groove closing at blow counts
higher and lower than 25. If you plot the
water content versus the logarithm of the
weighing them the next day to take the
number of blows, you get a slightly curved
difference.
relationship called the flow curve. Where
the flow curve crosses 25 blows, that water
content is defined as the liquid limit.
The Plastic Limit is a bit more
arbitrary, and it requires more practice to
get consistent and reproducible results. The
PL is defined as the water content at which
a thread of soil just crumbles when it is
carefully rolled out to a diameter of 3 mm.
It should break up into segments about 3 to
10 mm long. If the thread can be rolled to a
smaller diameter, then the thread is too
wet. If it crumbles before you reach 3 mm
diameter, then the soil is too dry and you
are below the PL. Properly rolled out PL
thread should look like Figure 3.
Fig. 3 Plastic Limit test
After correctly performing these two
experiments, the water contents are found
by weighing the wet samples and putting
them in an oven overnight to dry and then
Results
Using the Unified Soil Classification
Standard chart in the textbook, a flow type
method is used that leads to a specific
conclusion on the type of soil. The flow on
the chart for the Teton fine-grained soil is as
follows: Course Grained->Sands->Clayey
Sand. 𝐶𝑢 was calculated to be 4.38. For the
coarse-grained soil, the flow is Coarse
Grained->Gravel->Clean Gravel->Poorly
Graded Gravel with Sand. 𝐶𝑢 for this soil
was calculated to be 105.56.
Table 1 represents the tabulated
values of grain size and percent passing.
Grain Size (mm) % Passing
19.050
100.000
6.350
98.800
4.750
98.100
2.000
93.700
0.850
80.800
0.425
64.600
0.250
53.800
0.125
30.900
0.075
8.300
0.075
3.104
0.072
1.103
0.052
0.379
0.039
0.118
0.033
0.033
0.029
0.009
0.020
0.002
0.012
2.036E-04
0.009
2.371E-05
0.006
2.617E-06
0.003
2.406E-07
0.001
1.972E-08
Table 1 – Tabulated Grain Size & % passing
Figure 4 represents the fine-grained soil
used in the sieve analysis. The specific
results for the sieve analysis and the
gravity of this soil is 2.698. When touching
hydrometer test for percent passing vs.
the soil, it feels very powdery and fine.
grain diameter.
An attempt was made to perform
the PL and LL tests on the road base, but it
is too course to perform these experiments
with it. The instructor gave the students a
different type of finer soil for experimental
purposes.
The Liquid and Plastic Limit tests
were performed and then the water
Figure 4 - Percent Passing vs. Grain
Diameter for fine grained soil
content was found. For the LL of the Teton
Dam soil the graph is shown in figure 6.
Figure 5 represents the course-grained
(road base) soil data for percent passing vs.
grain diameter.
Figure 6 – Graph of LL test for Teton Soil
Drawing a line up from 25 blows, then
horizontally to the water content, we find
the LL to be about 24.75%. For the finer soil
Figure 5 – Percent Passing vs. Grain
Diameter for course grained soil
For the hydrometer analysis, the
largest grain size used is just smaller than
75 µm since this was the smallest diameter
given to the students, the graph is shown in
Figure 7. Drawing the two lines, the LL
comes out to be about 83%.
types were used; one fine grained soil from
the Teton Dam, and another road base
material. The sieve analysis helped classify
orur Teton Dam and road base soils which
were Clayey Sand and Poorly Graded Gravel
Figure 7 – Graph of LL test for fine clay
There was an error in calculating the
PL for the finer soil given to us by the
instructor because of the initial weight of
the sample, or it was recorded wrong, so
the Teton Dam soil was the only one with a
PL value calculated. After rolling out this soil
with the correct amount of water, the
water content came out to be 24.2%. The
with Sand.
The Hydrometer analysis takes what
is not measurable through the sieve analysis
and measures the ultra-fine particles in
suspension, from which we can find the
diameters. The values were tabulated and
graphed with the values from the sieve
analysis.
The PL and LL tests allow us to know
Plastic Index can now be calculated using
the water content range in which the soil
the equation PI=LL-PL. This value came out
remains plastic. This range is called the
to 0.5 for the fine, Teton Dam soil. The PI
Plastic Index. It is important to know these
for the other soil could not be calculated
values because soil becomes useless when
because PL was not available. The natural
it is out of the plastic range. Studying Soil
water content was calculated in the very
Mechanics is extremely useful in
beginning and these values are 1.61% for
engineering because the integrity of any
the fine, Teton Dam soil, and 2.45% for the
structure depends on it.
road base soil. The percent of the sample
retained on the #40 sieve was 35.4%.
Conclusion
The purpose of these experiments
are to help one understand the physical
properties and mechanics of a soil. Two soil