INTERMEDIATE SOILS: SPECIMEN PREPARATION AND
CHARACTERIZATION FOR GEOTECHNICAL CENTRIFUGE TESTING
Mark Jones* (Ohio University)
Alan Abad* (Florida International University)
University of California, Davis
Advisor: Jason DeJong*, PhD
Abstract
The liquefaction of intermediate soils, namely silty soils, is of significant concern
and importance to the geotechnical community. This project’s focus was twofold. The
first goal of the project was the development of reliable method to prepare uniform silt
specimens in containers for centrifuge testing. The study focused on pluviation as a
method of sample preparation using SIL-CO-SIL #106 silt. Dry and wet pluviation were
also compared. The second goal was the development of in-situ tools, namely cone
penetrometers, T-Bar, and Ball penetrometers to characterize such soil properties as
uniformity, undrained and remolded shear strength, and liquefaction resistance. Silt was
found to contain properties of both sand and clay. Hopefully the research provided in this
report will assist future researchers in developing better safety practices when
constructing on silty soils in earthquake-vulnerable areas.
Problem Studied
Silt Background
Soils are often categorized by grain size. Soils with large grain sizes ( > 0.075
mm) are called Granular soil, and consist of gravels and sands. Fine grained soils (
<0.075 mm) are classified in two groups: Silts and Clays. (Das 2005)
Silts are microscopic soil fractions that consist of very small quartz crystals and
some flake-shaped particles that are fragments of micaceous minerals (Das 2005). For
this reason, silts have properties resembling both clays and sands. Silts can be created
through mechanical weathering of rock. This weathering could be caused by winds,
glaciers or water on the bottom of rivers and streams. Silt is especially important in the
study of earthquake engineering due to its potential for liquefaction. One popular method
of data retrieval to determine the properties of silty soil is using a device known as a
Penetrometer.
Background on In-situ Testing
There is evidence that shows that since the beginning of the 1900s, devices known
as penetrometers have been used to determine certain characteristics of sands and clays.
Penetrometers can be used to obtain a wealth of geotechnical information about soil,
including the following: shear strength, density, elastic modulus, rates of consolidation,
undrained and remolded strength, and uniformity. Another advantage of penetrometers is
that they can collect all this information without disturbing the ground. Since the first
penetrometers were used centuries ago, penetrometers technology has improved
drastically. For example, certain penetrometers can now be equipped with tiny cameras
to allow testers to see the soil deep under the ground.
Despite advances in penetrometers technology, the simplest type of
penetrometers, the Cone Penetrometer, is most frequently used in field testing today.
Cone penetrometers consist of a long cylindrical rod ending in a small cone. The cone
penetrometer is very versatile and inexpensive when compared to more advanced
penetrometers. When tested using a constant rate, the Cone Penetrometer can measure
the forces applied to the tip of the cone equivalently through the full stroke of
penetration. Besides the cone penetrometer, the T-bar and Ball Penetrometers are also
very useful in determining properties of soils.
Testing with penetrometers is very common and well documented for sands and
clays. However, they have not yet been reliably tested in intermediate soils, silt in
particular. Learning to use penetrometers in silty material will be an important step in
geotechnical engineering, as most soils are a mix of fine and coarse grained materials.
Objectives
The main objectives of this research project can be broken down into two parts:

Developing a reliable method to prepare uniform silt specimens in
containers for centrifuge testing (focusing on pluviation)

Develop in-situ testing tools, namely the cone penetrometers, T-bar and
Ball Penetrometers to characterize uniformity, undrained and remolded
strength, and liquefaction potential of silty material
Research Approach
Silt Used
The silt used in this experiment was SIL-CO-SIL #106, manufactured by U.S.
Silica. Figure 1 shows an average grain-size distribution for SIL-CO-SIL #106.
Figure 1: SIL-CO-SIL #106 Grain Size Analysis
SIL-CO-SIL #106 was chosen primarily for three reasons. SIL-CO-SIL #106 is primarily
silt, with only small amounts of clay and sand. Also, it was important to choose a soil
that would have, on repeated trials, very consistent qualities. Finally, SIL-CO-SIL #106
was readily available at the UC Davis Centrifuge Facility, making it a very convenient
choice.
Mold Design
The first part of sample preparation is choosing a mold in which the soil will be
tested. The two most important factors considered in choosing a mold were size and
depth. The mold had to be deep enough to accommodate the penetrometers. It also must
have a large enough surface area for multiple tests to be run on one sample. The mold
used was a cylinder 16” high and had a diameter of 24”.
A few modifications were made to the mold for the purpose of this experiment.
The first modification was the addition of a Piezometer on the side of the tank to monitor
water level in the silt. Next, a layer of gravel was added to the bottom, with a layer of
geosynthetic. The gravel was added so when water entered the bottom of the tank, it
would spread evenly before rising through the silt. The geosynthetic was added to
prevent the silt from mixing with the gravel. Finally, a hose attachment was added to the
valve at the bottom of the mold to fit the hose used for saturation. Figure 2(a) and (b)
show pictures of the mold after all modifications.
Preparation Method and Sample Saturation
The method of sample preparation chosen for this experiment was pluviation.
Pluviation, or raining, is a method commonly used to prepare sand samples. Soil is
rained down from a designated height, through a mesh, and into the sample. Mesh size
and raining height depend on the size and type of soil particles. As sand fills up in the
sample, the pluviator is also raised to maintain consistent impact energy. Figure 3 shows
a picture and a cross section of the pluviator used.
Pluviation can be done in two ways (i) dry pluviation and (ii) wet pluviation. In
dry pluviation, soil is simply rained into the soil sample as described above. In wet
pluviation, the soil is first filled with water, and the silt is rained through the water. Wet
pluviation tends to give a lower relative density because the soil particles have a slower
falling velocity, and therefore lower impact energy.
After dry pluviation, the sample must be saturated. Saturation was accomplished
by bringing the sample under vacuum. Water was then very slowly sucked into the
attachment at the bottom of the mold. The vacuum caused the water to slowly seep
through the sample, filling all the voids.
(a)
(b)
Figure 2: Modifications to the mold including (a) the geosynthetic and
(b) the Piezometer and hose attachment.
Figure 3: A picture of the large pluviator at UC Davis
Testing
There were many factors that played an important role in our decision making
process throughout the development of the whole project. Knowing the goal of our
research, we had to take into consideration the tools and resources we had available to us
in order to be able to map out a successful way to achieve this goal. The tools, with
which we were going to do all our penetration testing to the almost untested silt, had to fit
the hydraulic actuator that we had to our disposition. This was done not only for our own
sake, but also to allow the continuity of our research to the rest of the scientific
community interested in geotechnical aspects.
We had to start out by doing a series of measurements and calculations trying to
establish the appropriate ratio of the rod that will accurately fit into the actuator. Once
we had this established we went ahead and worked out other calculations that would
allow us to make all of the different types of penetrometers we initially had in mind to
test, only this time using different ratios of each to gain an understanding of how each
behaved during penetration. We then designed a cone penetrometer with a standard 60
degree tip, and T-bars and Ball penetrometers with ratios of 2/1, 5/1, 10/1 and 15/1 with
relation of the projected area of the rod to the projected area of the T-bar or Ball. After
all the math had been successfully worked out, the next step in the design process was to
make computer models of these penetrometers with the aid of Solidworks. Engineering
drawings, and sketches of the tools were finished, and they now had to be made.
Figure 4: T-bar and Ball Penetrometers
After an extensive training, safety lessons and rigorous practice on the use and
implementation of the tools in the machine shop, our penetrometers were now ready to be
made. One of the tools that played a very important role in the creation of our tools was
the lathe which requires a lot of practice and precision in order to sculpt detailed patterns
into the metal being worked with. Not all attempts were successful, but we had enough
material to work around a highly large margin of error and finally all the probes were
made to a very high level of resemblance to those originally sketched out.
When first faced to this challenge, we knew very little about hydraulic actuators,
load cells, penetrometers. To gain the background knowledge necessary, we had to go
and do some further independent research into each of these devices. We had to learn
what they were used for, how they were used and how to properly calibrate and set up for
testing these devices. All the different calibration processes required extensive repetitive
measurements and analysis of data the collected, and in a few occasions even further
research or a loud cry-out for help in that particular subject area. A deep understanding
of all the tools and software such as Labview, and the units it worked with, used to
analyze the findings were required in order to properly manipulate and effectively use
and command these tools.
Finally after all the research had been done and all the required obstacles over
came, the next step in the itinerary was to proceed with the actual testing. This was the
step that was going to put everything we had done so far to the test, and we hoped
everything worked out as our scientific predictions had dictated.
Every test that we conducted had to have a different set up to make the offsets
created by the different probes in relation to the silt the same. Another thing that we
learned to pay attention to was the anchorage placement and amounts that were required
for the rack mounting the hydraulic actuator in each of the varying rates of penetration,
because to higher penetration rates the anchorage had to be strong enough to maintain the
rack and actuator in place instead of lifting the whole thing up in really dense silt.
With every test there were a series of things that had to be checked off a checklist
prior to starting to ensure proper data gathering. Hydraulic pumps had to be on, actuators
had to be in home position and ready to start, load cells had to be zeroed out to prevent
the interference of electrical noise in our readings, the rack mounting the actuator had to
be properly secured to outcome the force generated by the silt during penetration, the
excel template created to acquire the right commands to the actuator updated with the
necessary information for that given test to be evaluated and many other little things that
if not added into the equation the results would not add up as necessary to come up with
satisfactory results.
The testing was conducted at different rates determined by dividing the velocity
over the diameter of the probe. The rates used were 0.005, 0.05, 0.5 and 5.0. This was
made in an effort to determine which of all the testing rates will provide the best and
most useful data for the purposes of our research.
Apart from testing the silt with the three different probes and with different ratios
of each, and each at different penetration rates, we also designed a testing profile that was
uniform in every test. The profile implemented consisted of an initial penetration to a
depth of half of the full penetration, followed by a cyclic motion of 10 up and down
strokes within a specified height, then by a second penetration further down to the
maximum allowed stroke by the actuator and finalized with an extraction of the probe
from the silt. With the cyclic motion in the middle of our testing profile we were trying
to disturb the ground to characterize its remolded strength.
This final step, the actual testing, indicated an integration of both parts of the
project, the geotechnical and mechanical aspects of it. Compatibility factors were
carefully studied to ensure that all the components, the testing tools and the sample being
tested, would properly align.
During testing many problems arose, each requiring a solution before research
could continue. Some of these problems included network failures due to the extreme
temperatures that all the equipment was exposed to in the poorly ventilated area of the
centrifuge during record breaking summer heat and limited availability to some of the
equipment. Undesirable test results due to an excess or lack of data points also caused a
problem. Depending on the rate of penetration, tests could take no more than a few
minutes, or could last over the length of an entire day.
Outcomes
Pluviation
Pluviation did not initially work as planned. The methods used for pluviating
sand do not work with silt. A few simple modifications to the pluviator, however,
allowed the process to continue successfully. The first problem was friction with the
sides of the pluviator. Silt would catch on the sides and build up, eventually stopping the
flow completely. To fix this, two aluminum sheets were added to the slanted sides of the
pluviator at the same steepness as the wooden walls. Adding the aluminum sheets
reduced friction and allowed the silt to flow normally.
The second problem was the buildup of silt as it flowed through the meshes. The
best method found to solve this problem was to attach a vibrator onto the side of the
pluviator. The vibrator kept particles from sticking to the mesh and allowed for them to
fall freely into the sample.
Soil After Saturation
There were several noticeable differences between the silt sample before and after
saturation. The dry sample had a very powdery feel, much like baking soda. After
saturation, however, the sample consolidated, which reduced the void space. The
reduction in voids and the bonding with water caused the silt to lose its powdery feel for
a much harder, stronger texture. The total volume of the soil in the sample changed as
well. The calculation of volumetric strain is shown below:
Volumetric Strain, εv = Δh/H = 2.3in/10.5in
Eq. 1
εv = 0.238 in/in
Testing Outcomes
Once we had concluded with all the strenuous testing we were able to analyze
significantly large amounts of data summarized in graphs and charts. According to our
observations we came up to several conclusions in which we tried to analytically
compare our results to previous studies devoted to sands and clays specifically.
Common traits that we observed throughout all of our graphs and charts in every test
included increased penetration resistance with increased penetration rates, which when
compared to the characteristics of sands and clays it was the behavior that we expected.
The results of the cyclic penetrations performed indicated a rapid degradation in strength
of the silt most likely due to the quick increase in pore water pressure resulting from
these cyclic motions. Such strength degradation is very similar to that of very sensitive
clays. The rapid loss of strength resulting from cyclic loading is of particular importance
when evaluating the potential for ground movement resulting from an earthquake. One
other thing that was prevalent in our results was the fact that the silt posed almost no
resistance to the probe during the final stage of the profile, the extraction. This is in fact
very different from clays which tend to do completely the opposite.
Tbar 3
Tbar 1
Penetration Resistance (kPa)
Penetration Resistance (kPa)
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Depth (mm)
Depth (mm)
-5000
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(a)
(b)
Figure 5: Graph of test conducted with (a) 10/1 ratio T-bar at a rate of 5
and (b) 10/1 ratio T-bar at a rate of 0.005
Possible Future Work
The greatest limiting factor of this experiment was time. In the time available,
only one method of sample preparation, pluviation was tested. In the future, testing of
other methods would be invaluable in truly finding the best method of silt preparation.
One method discussed was moist tamping. Moist tamping is a technique commonly used
in soil compaction. Moist Tamping involves multiple layers of silt of a known moisture
content being pressed to a desired relative density. Another possible preparation method
is vibration; however, this method is generally better for granular soils.
Another goal is to eventually use the methods described in this paper for
geotechnical centrifuge testing. However, due to the immense cost of running centrifuge
tests, much more time must be put into developing a definitive and precise saturation
method before centrifuge testing can begin. Centrifuge testing will be able to aide in
answering questions regarding the properties of silt during earthquakes. With time,
centrifuge testing could lead to an increase in safety for buildings, highways, and other
structures built on silty soil.
References:
Das, B. Fundamentals of Geotechnical Engineering Second Edition. Toronto, Ontario,
Canada. 2005.
DeGregorio, V. B. “Loading Systems, Sample Preparation, and Liquefaction” Journal of
Geotechnical Engineering Vol. 116, No. 5, (May 1990), 805-821
U.S Silica Home Page 2005. www.u-s-silica.com. U.S. Silica Company, 9/14/06
Vaid, Y. P. and Negussy, D. “Relative Density of Pluviated Sand Samples”. Soils and
Foundations Vol. 24, No. 2, (June 1984), 101-105.