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University of Colorado-Boulder
University of Wisconsin-Madison
Centrifuge Testing to Evaluate Seismic Soil-Structure-Interaction and Lateral Earth Pressures near Buried Water Reservoir Structures in Southern California
Miguel Frias
Dr. Shideh Dashti
Geotechnical Laboratory
SMART PROGRAM 2013
Aug. 6, 2013
Contact Information
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Summer Internship Program: SMART Program 2013
Student InformationStudent’s name: Miguel Frias
Phone No.: (414) 719-1905
Fax No.: none
E-mail: [email protected]
Employer Information
Laboratory name: Geotechnical Engineering Laboratory
Supervisor’s name: Shideh Dashti
Phone No.: 310.500.9721
Fax No.: 303.492.7317
E-mail: [email protected]
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Table of Contents
I. Abstract_____________________________________________________________5
II. Introduction_______________________________________________________6Summer Research Goals______________________________________________________________6
Research Description________________________________________________________________6
Research Objective__________________________________________________________________8
III. Hypothesis_________________________________________________________9
IV. Soil Properties Tests/Results__________________________________________9Soil Backfill________________________________________________________________________9
Sieve Analysis______________________________________________________________________10
Specific Gravity (Gs)________________________________________________________________12
Pluviation Test_____________________________________________________________________12
V. Prototype: Model Design____________________________________________14Scaling Laws______________________________________________________________________14
Centrifuge________________________________________________________________________15
VI. Set up : Sensor Installation__________________________________________16Strain Gauges______________________________________________________________________16
Pressure Sensors___________________________________________________________________19
Box Modifications__________________________________________________________________22
VII. Calibration_______________________________________________________25Introduction to calibration___________________________________________________________25
Determination of Calibration Process__________________________________________________26
Calibrating sensors/ setup___________________________________________________________26
Centrifuge Results__________________________________________________________________29
VIII. Executive Summary______________________________________________34
References_____________________________________________________________35
List of TablesTable 1 U.S. Sieve Sizes..............................................................................................................................10Table 2 Data obtained from specific gravity test to carry out equation 1 and 2..........................................12Table 3 Strain gauge configurations............................................................................................................17Table 4 Resistance of Strain Gauges 1-15 Note: Strain gauge 5, 12 and 16 not included-broken/damage 18Table 5 Data Table for Pressure Sensor Resistance Check........................................................................21Table 6 Soldering Process............................................................................................................................21
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List of FiguresFigure 1 Headworks Construction Site..........................................................................................................7Figure 2 Sample of Nevada Sand...................................................................................................................9Figure 3 Set of sieve while in shaking table................................................................................................11Figure 4 Graph plotting the percent passing vs. grain size..........................................................................11Figure 5 Graph plotting drop height vs. density..........................................................................................13Figure 6 Remote used to raise and lower drop bucket for test.....................................................................14Figure 7 Conducting a pluviation test for 60% relative density..................................................................14Figure 8 Scale Factor for Centrifuge Model Test (Kutter 1992).................................................................15Figure 9 Wheatstone Bridge Circuit Diagram.............................................................................................17Figure 10 Placing the Ethernet cap on data cable........................................................................................18Figure 11 Wires inside ethernet cap in their respected color order.............................................................19Figure 12 Soldering process by color oder..................................................................................................19Figure 13 Finished project wall...................................................................................................................20Figure 14 Insulator tubs on wires.................................................................................................................20Figure 15 Retaining wall showing pressure sensor and strain gauge locations...........................................22Figure 16 Section view of container with cut-off wall and reinforcement..................................................23Figure 17 Plan View- Container box with cut-off wall and reinforment.....................................................24Figure 18 Base plate plan view of hole size and locations..........................................................................24Figure 19 Retaining wall to sand side view Figure 20 Retaining wall top view...............................25Figure 21 Retaining wall rotated at a 90 degree angle.................................................................................27Figure 22 Leveling off sand to insure equal pressure..................................................................................27Figure 23 Process of getting our model ready for spinning.........................................................................28Figure 24 The final setup inside centrifuge. Ready for spin.......................................................................28Figure 25 Total of 7 pressure sensors used from portal 0-6, 1 strain gauge used in portal 7......................29Figure 26 Voltage/ Strain vs. Time Test 1...................................................................................................29Figure 27 Voltage/ Strain vs. Time Test 2...................................................................................................30Figure 28 Voltage/ Strain vs. Time Test 3...................................................................................................31Figure 29 Voltage/ Strain vs. Time Test 4...................................................................................................31Figure 30 Theoretical Pressure vs. Voltage Test 1......................................................................................32Figure 31 Theoretical Pressure vs. Voltage Test 2......................................................................................32Figure 32 Theoretical Pressure vs. Voltage Test 3......................................................................................33Figure 33 Theoretical Pressure vs. Voltage Test 4......................................................................................33
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I. AbstractThe Los Angeles Department of Water and Power has decided to make immediate changes to its
open reservoir system in Southern California, due to emerging State and Federal water quality
regulations. Regulations have been implemented to replace the existing open reservoirs with
buried, reinforced-concrete, water reservoir structures. These structures will be surrounded by a
number of active faults. The performances of these types of underground structures restrained at
the base and roof during earthquake loading is currently not well understood. There are no
reliable analytical tools for evaluating seismic lateral earth pressures acting on these structures,
which is an important design parameter. To address this gap, centrifuge experiments were
performed to study soil-structure-interaction effects and seismic earth pressures near model
reservoir structures under earthquake loading. The primary objective of the research focuses on
evaluating the reliability of different types of pressure sensing technologies in capturing static
lateral earth pressures imposed by the backfill of Nevada Sand. These tests were conducted using
a simple, cantilever retaining structure with a fixed base under an increased gravity load induced
by a centrifuge. Previous research has shown that obtaining reliable pressure measurements are
difficult, especially in the high-frequency environment of the centrifuge due to scaling laws. The
data obtained will compare the pressure sensors and their reliability in capturing static earth
pressures. This will test the hypothesis that tactile pressure sensors with minimum stiffness and
high sampling rate are the only sensors at this time that are capable of providing reliable pressure
measurements in the geotechnical centrifuge.
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II. Introduction
Summer Research Goals Become familiar with design of retaining walls under static conditions Understand how centrifuge testing and pressure sensors work Calibrate and install instruments within soil and on structure Pluviate sand using dry pluviation method Record and analyze measurement of lateral earth pressures in centrifuge at the
sand-metal interface Complete a series of simplified static tests in centrifuge to compare different
types of sensors and their reliability in capturing the static earth pressures
Research DescriptionThe Los Angeles Department of Water and Power (LAWDP) considered the need to make
immediate changes to its open reservoir system due to emerging State and Federal water quality
regulations (Hushmand et al. 2011). In order to comply with these new water quality regulations,
the LADWP is planning on bypassing each of its current open reservoirs and replacing them with
buried reinforced concrete reservoirs. The area’s drinking water is mainly stored in the Los
Angeles Aqueduct, the Metropolitan Water District, and Silver Lake. The treated water that
enters these open reservoirs is exposed to environmental pollutants that can be dated back to the
early 1990s. These contaminants result from surface runoff, animals and even human resources.
During the hot summer months, high elevated temperatures and extreme sunlight can also play a
major role in decreasing the quality of the water by promoting the growth of algae around the
area.
These problems can be eliminated by bypassing these open reservoirs and constructing a new
reservoir called Headwork’s Reservoir. The project is titled to cover a 43-acre site located next to
the Los Angeles River. The proposed project has been divided into four separate construction
phases (LADWP News Room, 2012). The biggest two phases of the project include the
construction of a 56-million gallon East Reservoir with a follow up construction of a 54-million
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gallon West Reservoir. Refer to Figure 2.1 for an overview of the construction site. Estimating
an average total cost of around $25 million, the East Reservoir is expected to begin operation in
late 2014 whereas the West Reservoir is to be completed by 2017.
Figure 1 Headworks Construction Site
In order to determine the seismic resistance of these structures, tests that study soil-structure-
interaction near these buried reservoirs under static conditions and earthquake loading were
conducted. The geotechnical facility used for these tests includes state of the art centrifuges at
the University of Colorado. Instruments available in the geotechnical facility are able to
determine the static and seismic performance of these retaining structures: accelerometers, strain
gauges, and pressure transducers. A series of eight large centrifuge experiments have been
performed last year on different types of tunnel structures, soil profiles, and ground motion
characteristics to evaluate these effects on the performance of the tunnel structure.
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The National Earthquake Information Center (NEIC) records approximately 50 different
earthquakes per day (Earthquake Hazards Program. N.p., n.d. Web. 14 June 2013.). That alone
shows that earthquakes can vary substantially and arrive without warning. Their intensity
depends on the size of the fault and the amount of slip on the fault. Their sudden burst of high
energy is what causes them to be catastrophic. The energy released by an earthquake originates
from the accumulation of elastic strain energy in the crust around a fault due to deformation. As
the stress and resulting strain begin to accumulate, it will ultimately exceed the shear strength of
the rock mass in the fault zone and cause a rupture. The soil acceleration can then be measured
by placing a device known as an accelerometer in specific areas of the soil and the retaining
structure. As the wall of the structure will begin to experience static earth pressures, it becomes
quite difficult to measure this seismic shockwaves as they begin to increase in pressure. Strain
gauges will help determine bending moment distributions acting on the cantilever retaining wall.
Research ObjectiveThe primary testing objective this summer was to evaluate the reliability of different types of
pressure sensing technologies in capturing static lateral earth pressures imposed by the backfill
soil on a simple, cantilever retaining structure fixed at the base. Previous research at the
University of Colorado has shown that obtaining reliable pressure measurements are difficult,
especially in the high-frequency environment of the centrifuge due to scaling laws. For this
investigation, the 15 g-ton size centrifuge was used. By using a model that is reduced to the Nth
factor, we can study the behavior of this structure only after it has been under increased gravity.
Increased gravity produces an identical self-weight stress in the model and prototype (Kutter,
1992).
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III. HypothesisThe data obtained from the measurement of lateral earth pressures in centrifuge at the sand-
metal interface will allow us to compare different types of pressure sensors and their reliability in
capturing static and later dynamic earth pressures. This will eventually test the hypothesis that
tactile pressure sensors with minimum stiffness and high sampling rate are the only sensors at
this time that are capable of providing reliable pressure measurements in the centrifuge.
Additionally, further data analysis of these tests will give us an insight into the fundamentals of
soil mechanics and soil-structure-interactions.
IV. Soil Properties Tests/Results
Soil BackfillThe soil backfill used to carry out this project was Nevada Sand. Nevada Sand with a density of
60% was selected for this testing due to its uniformed, well characterized, and fine angular
material. Various tests were done to determine its properties throughout the summer. These tests
included a specific gravity test, sieve analysis test and a pluviation test. Further information on
these different tests will be discussed in the upcoming sections. A sample of Nevada Sand is
shown in Figure 4.1.
Figure 2 Sample of Nevada Sand
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Sieve AnalysisOne needs to know the distribution of size particles of a given soil mass in order to classify a soil
for engineering purposes. A sieve analysis test was used to determine the grain size distributions
of the Nevada Sand. A sieve analysis test consists of shaking the soil sample through a set of
sieves that have progressively smaller openings (Das, 2006). It’s important to note that the sieves
are made of woven wires with square openings that decrease in size as the seize number
increases. Table 4.1 contains a table with U.S. standard sieve numbers with their respected size
openings.
Table 1 U.S. Sieve Sizes
Conducting a sieve analysis required shaking the Nevada Sand through a set of sieves with
openings of decreasing size from top to bottom. Figure 4.2 shows the set of sieves used in
conducting this test while in shaking motion. The smallest sieves used in the test were U.S No.
40 sieve where the largest was U.S No. 200 sieve. Once all the soil was shaken for
approximately 15 minutes, the mass of soil retained on each sieve was measured and analyzed.
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Figure 3 Set of sieve while in shaking table
Upon further analysis of the data obtained, a plot illustrating a particle size distribution curve
was created as seen in Figure 4.3. This plot resulted from the calculations from the percent finer
of each sieve. While a particle size distribution curve can be used to determine various
parameters for the Nevada Sand, it helped in finding the range of particle sizes as well as any
distribution of various size particles in the soil. The curve represents a poorly graded soil
meaning that most of the grains within the soil were all of similar size. Our tests as shown as
Test 1 and Test 2 compare pretty similar to previous tests.
Figure 4 Graph plotting the percent passing vs. grain size
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Specific Gravity (Gs)Specific gravity is defined as the ratio of the unit weight of a given material to the unit weight of
water (Bas, 2006). In dealing with soil testing, the value of specific gravity is necessary to
compute the soil’s void ratio and for determining the grain-size distribution in other tests such as
a hydrometer analysis. The specific gravity of Nevada Sand was determined accurately in the
laboratory. The specific gravity can be calculated using the following equation
Gs= WsWs+Wfw−Wfws (1)
Gs= 60 g60 g+677.9 g−715.3 g
≡ 2.65 %
(2)
Where Weight of the soil (Ws), Weight of the flask + water (Wfw), Weight of flask + water+ soil (Wfws)
Equation 1 is used and results are shown from data obtained from Table 4.2
Table 2 Data obtained from specific gravity test to carry out equation 1 and 2
Object Mass (g)Filter .5Filter + Sand 60.5Sand 60Empty dry 500 mL volumetric flask 180.2Flask + Sand from filter 240.3Flask with water + sand after de-airing process
715.3
Flask with water up calibration mark (water only)
677.9
Pluviation TestVarious pluviation tests were done to determine at what heights will be needed to calibrate the
sensors. The height is needed to obtain certain relative densities for calibration. Once
determined, Nevada Sand was dropped at this certain height onto the retaining wall to calibrate
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the sensors. The targeted relative densities for this test are the following: 40%, 60% and 80%
densities. Relative density is commonly used to indicate the in situ denseness or looseness of
granular soil (Das, 2006). Relative density is defined as
Dr=emax−e
emax−¿ emin¿ (3)
Where Dr = relative density (%), e= in situ void ratio of soil, emax= void ration of coarse grained soil (cohesionless) in its loosest state, emin= void ration of coarse grained soil (cohesionless) in its densest state
The targeted relative densities for this test were the following: 40%, 60% and 80%. Obtaining
these densities was a major challenge. After numerous attempts, it was determined that a height
of .5 meters would give a 40% density, a height of .99 meters would give the 60% density while
the 1.45 meter height would give the 80% density.
Figure 5 Graph plotting drop height vs. density
Conducting a pluviation test required time and patience. With the ability to control the crane
machine as seen in Figure 4.5, we were able to carry out successful tests with consistent results.
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In the pluviation test, the ideal goal was to make to sand as evenly distributed and compacted as
possible. Raising the drop bucket to various consistent height locations from the top of sand was
one of the most important factors in carry out these tests.
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Figure 6 Remote used to raise and lower drop bucket for test Figure 7 Conducting a pluviation test for 60% relative density
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V. Prototype: Model Design
Scaling LawsCantilever retaining walls are routinely used to support moderate heights of earth (Springman,
Sarah, and Jan Laue, 2010). Retaining walls should be designed to withstand lateral earth and
water pressures. To determine what forces were acting on this wall, a centrifuge becomes useful
for scale modeling of any large nonlinear problem for which gravity is a primary driving force
(Kutter, 1992). Thus, by using a much smaller scale model, we can obtain the results one would
expect to obtain in the normal prototype under two conditions: (1) the same soil with same mass
density is used and (2) reduce length by Nth factor while increasing gravity by the same Nth
factor. Soil with the same density is used to obtain similar behavior while increasing
“gravitational” acceleration (g force) to produce an identical self-weight stress in the model and
prototype (Kutter, 1992). Scaling relations are used to extrapolate measurements made on
models to the appropriate prototype magnitudes (Stadler, 1996). The benefits of using a much
smaller scale model are that it is more economical, and it is more accurate to obtain the data.
Quantity Symbol Units Scale FactorLength L
Volume v
Mass m
Acceleration, Gravity a, g
Force FStress Moduli EStrength sTime (dynamic) tdyn
Frequency F
Time (diffusion)a tdif
Figure 8 Scale Factor for Centrifuge Model Test (Kutter 1992)
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Centrifuge The Civil, Environmental, and Architectural Engineering Department at the University of
Colorado-Boulder is home to state of the art centrifuges used for various kinds of research,
industry design, and instructional purposes. For this specific project, a 15 g-ton centrifuge
equipped with a symmetrical payload swing basket was used to carry out the tests. This
centrifuge has a 25 horse power electric motor and is capable of accelerating a 300 lb. payload to
100g. Similar to the 400 g-ton centrifuge also located within the University, this centrifuge is
equipped with a high performance modular data acquisition system that is mounted at the center
of the centrifuge arm (CEAE, 2013). Having the ability to interface with various types of analog
transducers, this will allow data to be obtained from the strain gauges and pressure sensors which
will be discussed in the following section. Figure 5.2 shows the 15 g-ton centrifuge inside the
geotechnical lab.
Figure 5.2 CU Boulder 15 g-ton centrifuge
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VI. Set up : Sensor Installation
Strain GaugesIn order to determine the bending distributions acting on the retaining wall induced by an
increase in gravity, strain gauges were installed on both sides of our retaining wall. All strain
gauges were tested and checked for accuracy which were configured in a half-bridge
configuration. Strain gauges configurations are based off a network of four resistive legs known
as a Wheatstone bridge. Among the four active legs, any of them can be considered to be an
active sensing element. Figure 6.1 shows a Wheatstone bridge circuit diagram.
Figure 9 Wheatstone Bridge Circuit Diagram
While there are three types of strain gauges, the ones used in this test were the half-bridge
configurations. The other two are a quarter bridge and a full bridge. Depending on the number of
active element legs will determine the kind of bridge configuration. Table 6.1 shows the different
types of strain gauge configurations with their respected number of active elements.
Table 3 Strain gauge configurations
All strain gauges were checked for their resistance. First, the strain gauges were labeled from 1
to 16, starting at the bottom of the retaining wall to the top of the wall. Next, the resistance of
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each strain gauge was tested using a voltmeter. The resistance of each strain gauge on both the
back-fill side and excavated side were found to be anywhere between 118-123 ohms. Table 6.2
shows the resistance for each strain gauge. Each strain gauge was then connected to the
complaining strain gauge on the other side of the retaining wall. This was done by connecting
one wire for each side together.
Table 4 Resistance of Strain Gauges 1-15 Note: Strain gauge 5, 12 and 16 not included-broken/damageStrain Gauge # Sand Side (orange) No Sand
(orange/white)Both (blue)
1 122 Ω 120 Ω 241 Ω2 120 Ω 120 Ω 240 Ω3 120 Ω 120 Ω 240 Ω4 120 Ω 120 Ω 240 Ω6 119 Ω 121 Ω 238 Ω7 120 Ω 120 Ω 240 Ω8 121 Ω 121 Ω 240 Ω9 120 Ω 120 Ω 240 Ω10 120 Ω 120 Ω 240 Ω11 120 Ω 120 Ω 240 Ω13 120 Ω 120 Ω 240 Ω14 120 Ω 120 Ω 240 Ω15 120 Ω 120 Ω 241 Ω
Next, the data cords were prepared by placing data point Ethernet caps on one end. The wires
were placed in the Ethernet cap in the following order: white/orange, orange, white/green, blue,
white/blue, green, white/brown, brown as seen in Figure 6.2 and 6.3. The other end of cord was
prepared by removing all but the white/orange, orange and blue wires which will be used to
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solder the data cord to the strain gauges. And insulator tub was placed around each core and each
of the white/orange, orange and blue wires.
Figure 10 Placing the Ethernet cap on data cable
Figure 11 Wires inside ethernet cap in their respected color order
The gauges were connected to the data cord by soldering the wire from the back fill side to the
orange wire, the wire from the excavated to the orange/white wire and the connected wire (wire
from back fill and excavated side) to the blue wire. This can be seen in figure 6.4. To check if the
wires are connected properly, they were checked once again using the volt meter. However,
strain gauges 5, 12, and 16 were no longer used due to bad connections or possible wire damage.
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Figure 12 Soldering process by color oder
Pressure SensorsAnother type of sensors used were pressure sensors. These determined the total amount of
pressure that was distributed along the sand face of the wall. Before installation, the retaining
wall was cleaned using rubbing alcohol. Double-sided sticky tape was used to hold the pressure
sensors in place. The stiffness and thickness of the tape was weak to the extent that it would have
no effect on the pressure sensor whatsoever. A total of 13 strain gages were installed on the sand
side as seen in figure 6.5. Unlike the strain gauges, these pressure sensors were configured in a
full bridge configuration. As seen from Table 6.1, full bridge wires have a total of 4 active
elements.
Figure 13 Finished project wall
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Figure 14 Insulator tubs on wires
Just like the strain gauges, we checked the pressure sensors for their resistance. All pressure
sensors were labeled from 1–16 starting from the bottom of the wall and moving up towards the
top of the retaining wall. Pressure sensors 12, 14, and 16 were not included as there is was no
room to install them. The resistance for each pressure sensor was determined in the following
order: red and black measured input resistance, while white and green measured the output
resistance. All pressure sensors measured a resistance higher for both input and output than the
expected resistance. This may be due to the older wires having rust within the whole wire.
Nevertheless, the tested measurements were still fairly close to the original expected values.
Figure 6.6 shows the soldering process before insulator was placed on each wire. Below is a
table with the resistance of each pressure sensor.
Table 5 Data Table for Pressure Sensor Resistance CheckPressure Sensor
Pressure Sensor Serial #
Input TestΩ
Expected Input Test Ω
Output TestΩ
Expected Output Test Ω
1 27F9F-D1-1 230 223 2611 26092 27F9F-D2-2 231 219 2921 29113 27F9F-D3-3 21 219 2622 26054 27F9F-D4-4 230 225 2925 29245 27F9F-D5-5 228 217 2664 26546 27F9F-D6-6 234 220 3126 30277 27F9F-D7-7 230 217 3025 30048 27F9F-D8-8 228 216 2980 2972
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9 27F9F-D9-9 237 225 3080 307410 27F9F-D10-10 231 201 3060 2988
11 5F9F-D2-2 303 287 3570 3548- - - - - -
13 5F9F-D3-3 305 289 3910 3853- - - - - -
15 5F9F-D4-4 304 284 4230 4212- - - - - -
Table 6 Soldering ProcessPressure Sensor Wire Goes to... Data cable
Red orangeBlack white/orangeWhite white/blueGreen blue
Box Modifications One of the biggest challenges was modifying the retaining wall box. The aluminum retaining
wall measured 12 inches whereas the centrifuge measured a total of 16 inches. The goal was to
design an extension wall/ supportive wall to make up for those 4 inches so no valuable data
would be lost once spinning inside the centrifuge. Using a computer design software called
AutoCAD; numerous detailed drawings of the retaining wall were designed with exact
dimensions up to a tenth of an mm. The exact location of each pressure sensor and strain gauge
was also determined as well as the dimension of the container, both outside and the inside. These
drawings can be as seen in figure 6.8-6.11. Thickness of the base plate was also taken into deep
consideration. The lengths of the bolts and how thick the base plate should be were further
discussed with the experts at the machine shop.
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Figure 15 Retaining wall showing pressure sensor and strain gauge locations
Figure 16 Section view of container with cut-off wall and reinforcement
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Figure 17 Plan View- Container box with cut-off wall and reinforment
Figure 18 Base plate plan view of holes sizes and locations
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Once all drawings were competed, the next goal before sending it off to the machine shop was to
determine if there were already pieces of aluminum that could use for the design. Luckily, all
pieces were found. One of the next challenges was determining whether the pieces would be
bolted down or welded. At first, it was believed that welding was the best choice however,
aluminum was vulnerable to melting/ shrinking, especially under the detailed specifications of
the drawings made. Bolting was determined to be the better fit. Figures 6.12 and 6.13 show the
final project.
Figure 19 Retaining wall to sand side view Figure 20 Retaining wall top view
VII. Calibration
Introduction to calibrationEvery single project involving centrifuge testing requires some sort of calibration for any given
sensors used in the test. For this project, the main objective was to determine the reliability of
different type of pressure sensing technology in capturing static lateral earth pressures imposed
by the backfill of Nevada Sand. This is required to calibrate each type of sensor to convert the
voltage output. The pressure sensors worked with gave out a given voltage. The goal was to
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relate that given voltage to an actual pressure which was known already. Once the known
pressure was applied to the sensors, the voltage was than multiplied by a calibration factor to get
that know pressure or in other terms, a theoretical pressure.
Determination of Calibration ProcessThere are numerous ways of calibrating pressure sensors. Determining which way would be best
was a task given the fact that many offer their advantages. At the same time, they also offer some
disadvantages.
One form of calibrating was the usage of water. The advantage of using water on sensors is that
there is an even distribution acting on the sensors throughout the whole retaining wall. The
second option was using soil. While soil may be more challenging due to its lack of uneven
distribution, the biggest advantage soil offers over water is that it ultimately will measure
pressure that soil exerts on the sensors. Therefore, using soil to calibrate instead of water will be
consistent. So while soil particles may not be of the same size, it will be a bigger benefit for a
better consistency in the final results.
Calibrating sensors/ setupThe main objective was to determine pressures in a vertical position. Before that was done,
calibrating the sensors at a 90 degree angle was done first. In other words, the wall was rotated
90 degrees so that the pressure sensors and strain gauges faced upwards. This will allow all
sensors to experience an equal amount of pressure from the Nevada Sand above. Figure 7.1
illustrates the retaining wall inside the centrifuge box.
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Figure 21 Retaining wall rotated at a 90 degree angle
By placing the wall at a horizontal position with all sensors facing up, it allows for every sensor
to eventually experience similar forces acting upon them from the soil. Once this process was
completed, the process of pluviating sand at a known relative density and height was completed.
The density was at 60% which we had obtained a height of approximately 1 meter high from the
top of the soil. Next and final step was to set up the box for spinning at certain g levels induced
from the centrifuge.
Figure 22 Leveling off sand to insure equal pressuredistribution on sensors
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Figure 23 Process of getting our model ready for spinning
Figure 24 The final setup inside centrifuge. Ready for spin
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Figure 25 Total of 7 pressure sensors used from portal 0-6, 1 strain gauge used in portal 7
Centrifuge ResultsA total of 4 centrifuges tests for a 60% relative density sand were conducted. While some test
proved to be worthless, they did offer potential valuable data that could address a current
issue with the model or the pressure sensors. Below are the 4 graphs, ordered from test 1 –
test 4. These graphs provide data from voltage/strain vs. time in seconds.
0 500 1000 1500 2000 25000
0.005
0.01
0.015
0.02
0.025
Voltage/Strain vs Time for Test 1
Pressure Sensor 2Pressure Sensor 4Pressure Sensor 6Pressure Sensor 8Pressure Sensor 10Pressure Sensor 13Pressure Sensor 15Strain Gage 7
Time (sec)
Vol
tage
/ Str
ain
of
pres
sure
sens
or a
nd
stra
in g
auge
s
Figure 26 Voltage/ Strain vs. Time Test 1
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0 500 1000 1500 2000 2500
-0.004-0.002
00.0020.0040.0060.008
0.010.0120.0140.016
Voltage/ Strain vs Time for Test 2
PS 1
PS 3
PS 5
PS 7
PS 9
PS 11
SG 7
SG 11
Time (sec)
Vol
tage
/ Str
ain
of p
ress
ure
sens
ors a
nd st
rain
gau
ges
Figure 27 Voltage/ Strain vs. Time Test 2
0 500 1000 1500 2000 2500 30000
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
Voltage/ Strain vs Time Test 3
PS 1PS 2PS 4PS 6PS 8PS 10PS 15SG 1
Time (sec)
Vol
tage
/ Str
ain
of p
ress
ure
sens
ors a
nd st
ragn
gag
e
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Figure 28 Voltage/ Strain vs. Time Test 3
0 200 400 600 800 1000 1200 1400 16000
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
Voltage/ Strain vs Time for Test 4
PS 1PS 2PS 4PS 6PS 8PS 10PS 15SG 1
Time (sec)
Vol
tage
/ Str
ain
of p
ress
ure
sens
ors a
nd st
rain
gau
ges
Figure 29 Voltage/ Strain vs. Time Test 4
For the most part, all pressure sensors seemed pretty reliable. Test 1 and Test 2 were by far the
best two tests. The plateau region seen in every graph indicates that the centrifuge was kept
stable at a given g level. However, not every pressure sensor seemed to work at certain tests.
Taking Test 3 for example, pressure sensor 15 seemed to be worthless data until it reached g
level 40. It than followed the same trend as the rest of the cables until it reached g level 20 at
spin down. The reason for this is still yet unknown. The very same trend was seen for the same
exact sensor in Test 4. As a result, the average voltage at each g level for each sensor used in the
test was taken. This average voltage was used to plot it against theoretical pressure. The goal was
to obtain a linear trend.
By taking the average voltage of each sensor, t following equation for theoretical pressure could
be solved for.
Therotical Pressure=psoil∗hsoil∗g level at center of gravity of model∗9.81 (3)
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Where psoil=density of soil, hsoil= height of soil above sensors
Upon solving the theoretical pressure of each sensor at the sand g level, the following tables
were created.
Figure 30 Theoretical Pressure vs. Voltage Test 1
Figure 31 Theoretical Pressure vs. Voltage Test 2
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Figure 32 Theoretical Pressure vs. Voltage Test 3
Figure 33 Theoretical Pressure vs. Voltage Test 4
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VIII. Executive SummaryThe main objective of this research project was to determine the reliability of different types of
pressure sensing technology in capturing static lateral earth pressures imposed by the backfill of
Nevada Sand on a simple cantilever retaining wall. The first types of pressure sensors to be
tested were a type of cell pressure sensors. These sensors and their locations are depicted in
Figure 15. Each sensor responds independently given their location.
Modifications to a pre-existing centrifuge container were made to fit the retaining wall without
affecting the natural displacement from lateral earth pressures. All tests on retaining wall were
done with a specific soil called Nevada Sand. Numerous tests were conducted to determine their
properties. The results of these tests were compared to literature and previous conducted tests.
The results from these test fell within the acceptable range of the pre-determine Nevada Sand
properties confirming that the sand used for testing is clean Nevada Sand.
With the know properties of the Nevada Sand, the cell pressure sensors were calibrated using
only vertical stresses to relate the voltage output of each sensor to its experienced pressure. After
conducting four centrifuge tests, it was determined that these cell pressure sensors responded
well. Their voltage outputs remained consistent when the centrifuge stabilized at specific g-
levels. The calculated calibration factor for each pressure sensor was found to be reliable because
these calculated factors were similar to the manufactures calibration factors.
Given the accuracy of the cell pressure sensors under vertical loading, the calculated calibration
factors will also be used to confidently measure pressures under lateral loading when the
retaining wall is placed vertically. These cell pressure sensors can then be used as a comparison
for the accuracy of other types of sensors such as the Tekscan sensor in the future.
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References
Ben Hushmand, Ali Bastani,Naz Mokarram. (2011). Phase I Technical Review and Support for Geotechnical Centrifuge Modeling of Lateral Seismic Earth Pressure Development on Restrained Walls of Underground. 11 (001).
Bridge, using two strain gages in the, you can further minimize the effect of temperature. For example, and Figure 5 illustrates a strain gage configuration where one gage is active (R. "Measuring Strain with Strain Gages - National Instruments." National Instruments: Test, Measurement, and Embedded Systems. N.p., n.d. Web. 12 June 2013. <http://www.ni.com/white-paper/3642/en/>.
B.L. Kutter, "Dynamic Centrifuge Modeling of Geotechnical Structures", Transportation Research Record 1336, TRB, National Research Council, pp. 24-30, Washington, D.C., 1992.
Das, Braja M.. "Origin of Soil and Grain Size." Principles of geotechnical engineering. 6th ed. Southbank, Vic., Australia: Thomson, 2006. 30. Print.
"Are Earthquakes Really on the Increase?." Earthquake Hazards Program. N.p., n.d. Web. 12 June 2013. <http://earthquake.usgs.gov/learn/topics/increase_in_earthquakes.php>.
"Civil, Environmental, and Architectural Engineering » Geotechnical Centrifuge Laboratory." Civil, Environmental, and Architectural Engineering . N.p., n.d. Web. 12 June 2013. <http://ceae.colorado.edu/facilities-centers/facilities/geotechnical-centrifuge-laboratory/>.
Springman, Sarah, and Jan Laue. "Dynamic earth pressures and earth pressure cell measurements." Physical modelling in geotechnics. S.l.: CRC Press, 2010. 493. Print.
Stadler, Alan Thomas. Static and dynamic behavior of cantilever retaining walls. University of Colorado: Department of Civil, Environmental and Architectural Engineering, 1996. Print.
"Strain Gauge Configuration Types - National Instruments." National Instruments: Test, Measurement, and Embedded Systems. N.p., 6 Oct. 2006. Web. 6 Aug. 2013. <http://www.ni.com/white-paper/4172/en/>.
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