development and design of centrifuge model container · mechanisms of liquefaction-induced...
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DEVELOPMENT AND DESIGN OF
CENTRIFUGE MODEL
CONTAINER
Nahom Micael
San Jose State University (SJSU)
U.C. Davis Centrifuge Research Facility
Site Coordinators: Dr. Dan Wilson, Dr. Bruce Kutter
PhD Mentor: Shideh Dashti
Abstract
Centrifuge modeling has become a powerful and important experimental tool in
geotechnical engineering. The appropriate development and design of the centrifuge
model container is absolutely crucial in centrifuge model testing. With regards to the
invention and the purpose of the device, the device is very important in the study of soil
mechanics in attempt to appropriately model soil behavior. By achieving uniformly
distributed soil particles, we can reach consistent soil density properties in our container.
The pluviation device is a breakthrough in how to uniformly distribute fine sand into a
receptor container. After weeks of pluviation, the large receptor container is spun in a
centrifuge with a maximum possible speed of 75g in order to achieve the max
compressive stress of overlying soil or fluids. There are many key engineering
instruments attached in the model container to measure physical variables. Designing
and developing the model container is a time-consuming process that takes several weeks
in preparation of the centrifuge spin.
Introduction/Literature Review
The literature review below focuses on the damaging effects of liquefaction on the built
environment. My readings have included the design of the centrifuge model container by
pluviation techniques and liquefaction. Pluviation consists of the process of dry sand
raining down from a pluviator device into a receptor container; by this process, uniformly
distributed soil particles can be established throughout the container. Several practice
pluviation tests were formed with a cylindrical container in order to become familiar with
the process before beginning the actual test in the much larger rectangular container
approximately 1600 mm x 700 mm x 700 mm. Above the container, there is a supply
vessel with four vertical slide walls where dense Nevada sand flows by gravity through
perforation. The machine has a simple on-and-off switch to control the flow of sand
pluviation into the receptor container.
A geotechnical centrifuge is used to accurately conduct model tests in studying
geotechnical problems such as strength, stiffness and capacity of foundations for bridges
and buildings. It makes use of centrifugal acceleration to match soil stresses in a 1/50
scale model. So, for a model container 1 m deep filled with soil, subjected to a
centrifugal acceleration of 50 g, the pressures and stresses will be increased by that factor
of 50. The purpose of the centrifuge machine is to shake the receptor in a controlled
manner to simulate a dynamic event similar to an earthquake. But, most importantly, it is
useful to study ground-shaking effects without risking public safety.
The design of the centrifuge machine consists of the drive system, a swinging bucket, the
arm (lifting the receptor container), the hydraulic rotary joint, the electronic slip ring
assembly and all the hardware and software necessary for control electronics and data
acquisition. From completing the necessary centrifuge preparation steps and collecting
the data of centrifuge spin, we can develop several important conclusions about
liquefaction effects during seismic activity.
Mechanisms of Liquefaction-Induced Settlement of Buildings on Shallow
Foundation
The purpose of this centrifuge modeling and testing is to discuss the problem and results
of liquefaction on building soil. Recent earthquakes have accurately provided indication
on the damaging effects of liquefaction on the built environment. The series of
centrifuge tests involving buildings placed on top of a uniformly layered soil deposit have
been performed to determine the dominant mechanisms involved with liquefaction-
induced buildings. One dominant mechanism includes building-induced shear
deformations combined with localized volumetric strains during partially drained cyclic
loading. The centrifuge results described the likely effects of the major parameters; for
example, building settlements are not proportional to the thickness of the liquefiable layer
concluding that building settlement occurs during earthquake strong shaking.
In order to properly determine liquefaction-induced building settlement, there are several
important parameters that must be developed. These parameters include the development
of high excess pore pressures, localized drainage in response to the high transient
gradients and earthquake-induced ratcheting of the building into the softened soil. The
journal by Shideh Dashti, Jonathan D. Bray, Juan M. Pestana, Michael Reimer and Dan
Wilson provided a wealth of information about important effects of liquefaction-induced
settlement of building with shallow foundations.
A total of three centrifuge tests were performed to generate accurate historical cases of
building responses on liquefied ground. Examples of historical cases are the 1964
Niigata Earthquake and the 1990 Luzon Earthquake where the majority of the buildings
were multiple stories high and founded on shallow foundations. These buildings are
being supported on thick uniform sand deposits and the footing dimensions were found to
affect the structural displacements. The article explained that many of the damaged
structures were indeed affected by liquefaction of shallow and thin deposits of loose silty
and saturated sand. Within the description of the centrifuge testing program, it provided
detailed information about the geotechnical facility, instrumentation and applications. It
provided and explained all the errors and results for each experiment; for example, it
discussed information regarding the effect of the liquefiable layer thickness in the first
experiment.
Additional important journal areas included an in-depth analysis of the response under
and around structures, the importance of liquefiable layer thickness, a stronger emphasis
on the primary liquefaction-induced building settlement mechanisms, the volumetric
deformation modes, deviatoric deformations, and lastly a well-developed emphasis on all
the effects of the key parameters on the mechanisms of liquefaction-induced settlement.
For volumetric deformation modes, it explained the importance of localized volumetric
strains during partially drained cyclic loading, the displacements of settlements due to
sedimentation after liquefaction, and the consolidation-induced volumetric strains as a
result of pore pressure dissipation. With regards to deviatoric deformations, the journal
discusses how strength and stiffness loss in the building foundation material can lead to
punching settlement of the structures or better known as building tilt.
The journal study on all the effects of key parameters on the mechanisms of liquefaction
settlement is crucial for future experiments. Many of these key parameters include
seismic demand, sand initial relative density (from sand pluviation), liquefiable layer
thickness, foundation width (to preventing relatively substantial structural
displacements), static shear stress ratio, the building height/width ratio, building pressure
and 3-D drainage. These topics are very important in developing an interpretation for
liquefaction-induced building settlements. The centrifuge experiments helped determine
the dominant settlement mechanisms involved in liquefaction-induced settlement
specifically of buildings with shallow foundations.
Methods and Materials
Method and materials were a critical area in the design and development of the centrifuge
model container. Before explaining the various model instruments, parts and software
involved with the model, the equipment for the centrifuge machine must first be clearly
explained. The materials and methods are described below for the centrifuge machine
and the centrifuge model.
Centrifuge Machine
The centrifuge machine is driven by a motor and rotates the model container around the
fixed, applying a force perpendicular to the axis. The machine is comprised of an arm,
the drive system, the adjustable counterweight assembly, the swinging bucket, the
hydraulic rotary joint, the electronic slip ring assembly and the data acquisition and
control electronics mounted on the arm.
Figure 1: A 3-D model of the UC Davis centrifuge machine is shown with many
electronic types of equipment detailed.
Within the centrifuge machine, there is a servo-hydraulic shaking table upon which the
model container is mounted on. Figure 1 shows a view of the flexible-shear-beam model
container mounted above the shaker and the shaker actuators are controlled by
conventional closed-loop feedback control system. Many of the model instruments
include accelerometers, sensors, displacement transducers and many others, explained
below. In figure 2, MEMS accelerometer and WIDAW module are ready for use. The
WIDAQ module is configured with four Silicon 1221L-100 accelerometers and a cost-
efficient MEMS accelerometer are plotted into a model pile tip. The last important
electronic equipment includes the custom-printed circuit board and cable strain relief
shown with the analog devices.
Figure 2: The electronic components of the centrifuge model container are clearly
illustrated in the figure above.
Model Instruments
The model instruments are very important because we have many accelerometers,
sensors, displacement transducers and other instruments all working collectively in the
experiment. Thus, we cannot have a successful project with the failure or improper
calibration of one instrument. An accelerometer is a device used for measuring change in
acceleration and gravity-induced forces. It is a very thin cable with two ends, a
monoaxial connector and more importantly a sensor on the other end. There are several
accelerometer devices used during in the model container and all devices must be
calibrated before being installed.
Figure 3: The accelerometer, shown above, is an integral device in the design of the
centrifuge model container.
The pressure sensor is a device used for monitoring and controlling our viscous fluid, the
methylcellulose that slowly enters the model container shortly before the shaking
experiment. Pressure sensors can help measure variables such as fluid/gas flow, speed
and water level.
The next three are all relatively similar: DTs, LPs and LVDTs. Displacement
Transducers (DTs) are position sensors that measure various types of displacement.
Linear Potentiometers (LPs) are devices used to measure linear position and velocity
using a flexible cable, more appropriately for a moving object. Linear Variable
Displacement Transformer (LVDTs) are well known sensors that have numerous
applications, including use in position-sensing probes to measure displacement and
velocity on co-ordinate measuring machines and machine tools. In the experiment, it was
used as a linear variable differential transformer to measure the shaft’s displacement.
Figure 4: The displacement transducer, similar to the other position sensors, measures
important physical variables such as displacement and velocity of the soil in the model
container.
Each of the three products has advantages and disadvantages among each other. For
example, one reason to use displacement position transducer products is for the small size
and light weight and size and weight are well-considered in engineering designs. In the
sensor, test and instrumentation world, this is very evident. Because of the important
aspect of cable-actuated position transducer being stainless steel cable, they immediately
have size and weight advantages over the other choices. Other considerable aspects
include cost, resistive technology, material properties, rotational life, AC circuit
applications (reliability), mechanical travel and other engineering elements (linear and
conformity).
Figure 5: The above is a linear potentiometer (LPs), made by etisystems, which shows
the various physical and chemical elements involved to create this effective system
within the model container.
The next instrument, pore pressure transducers (PPTs) are used as instruments to measure
distributions of pore water pressure in the model ground and to observe the consolidation
process. Pore water pressure refers to the pressure of groundwater held within a soil or
rock. Following the PPT is an amplifier, which is an electronic device that increases or
amplifies the size of a voltage or current signal without altering the signal’s basic
characteristics. Because they play an important role in earthquake simulation, it is
important to know how they work before calibration. Basically, amplifiers will allow
researchers to analyze sensor signals.
Results/Methods
The successful testing of the centrifuge machine relies heavily on a successful design of
the centrifuge model container, which depends on accurate pluviation. Pluviation, as
explained above, is the slow raining of the fine Nevada sand into our model container
while maintaining a balanced level of sand throughout the container. We can reach
consistent soil density properties by achieving uniformly distributed soil particles which
results from accurately pluviating the container.
Pluviation
The early stages of the pluviation process before pouring sand involves inserting long
poles as shown in Figure 6 and carefully cleaning the container. The long poles represent
where colored sand will be placed into, in order to maintain the desired testing
coordinates in the container. The blue clay is used to hold the poles up, preventing them
from being placed in this precise experiment. The locations in the model container are
read by three coordinates, in the x-axis, y-axis and z-axis. There are many position-
sensors and accelerometers taped to the bottom of the container and subsequently taped at
each particular level (100 mm up, 200 mm up, etc.) There are many silver poles in the
end used for vacuuming sand from the container to balance the levels after pluviation; the
vacuum hose is placed over the silver poles.
Figure 6: The figure above illustrate the beginning stage in the design of the model
container, shortly before the pluviation process.
Pluviation took several weeks with many stops to level the sand and insert instruments at
the balanced levels. Figure 7 displays the closing stages of the pluviation process after
much repetitive pluviation work. The fine Nevada sand has been carefully balanced at a
level of over 600 mm. The small checkered-board objects are the bridge-structures that
we’re testing on uniformly distributed sand. The pluviating project is just about complete
and ready to place into the centrifuge machine with minor checks of the electronic
equipment in the container.
Figure 7: The figure above illustrate the beginning stage in the design of the model
container, shortly before the pluviation process.
Methylcellulose
In seismic centrifuge modeling, there is a time-scaling conflict occurring between
dynamic and dissipative phenomena. In order to resolve this issue, a substitute pore fluid
must be slowly poured into the model container. Metolose, consisting of powder, has
been a great solution when mixed with water to yield methylcellulose. An experimental
program to test the suitability of methylcellulose and other possible fluid substitutes were
examined; these tests included compression tests, permeability tests, and a seismic
centrifuge experiment on level ground models. Results from these tests showed that the
scaling requirements were satisfied with the methylcellulose mixture.
Methylcellulose is used in geotechnical modeling as a viscous fluid and is much better
than water because its viscosity is far greater. Methylcellulose is used to satisfy the
scaling laws relating to movement of pore fluid through the soil during the movement of
pore fluid during dynamic loading events or liquefaction. Older alternatives to
methylcellulose were silicone oil or mixtures of water and glycerol. Methylcellulose
remains as the most effective solution because of its viscosity. The relative performance
of the fluid was illustrated with data from two centrifuge model tests, one with pure water
as the pore fluid and one with an HPMC solution having viscosity ten times that of water.
Discussions/Conclusion
The most powerful earthquakes are uncommon and unrepeatable but are very destructive.
These associated factors have made it very difficult to study the ground-shaking effects
by earthquake field experts. The UC Davis centrifuge modeling provides the capability
of studying the results of full scale structures, specifically bridge structures without
risking the safety of public. Centrifuge modeling is a time-consuming process, but yet a
valuable tool in measure accurate data for dynamic events such as an earthquake while
considering financial implications. Engineers can study the failure modes of structures
placed on uniformly distributed soil particles using a scale model container.
The appropriate design and development of the centrifuge container is significant in
completing a successful centrifuge testing project. The objective through the design of
the model container is to obtain consistent soil density properties and insert key
engineering instruments at the same time. These instruments will provide valuable
information for physical variables such as position and displacement for the soil particles.
After weeks of pluviation and pouring the sand into the model container, the receptor
container is spin at a maximum speed of 75g in order to achieve the maximum
compressive stresses and pressures of overlying fluids and soil. The engineers can then
gather the data and results and verify their assumptions in learning more the powerful
effects of earthquakes.
Acknowledgments
Without the many beneficial parties involved, this research opportunity would not have
been possible. I would first like to thank the National Science Foundation (NSF) and the
Network for Earthquake Engineering Simulation (NEESinc) for providing the NEESreu
program. I would like to thank all the staff at the UC Davis centrifuge facility most
importantly the site coordinators, Dr. Bruce Kutter and Dr. Dan Wilson and my PhD
mentor, Shideh Dashti. Also, I would like to acknowledge Alicia Lyman-Holt for all her
hard work in preparing site activities and meeting. It was a great experience getting
familiar with graduate school research and it is certainly under consideration for the near
future.
References
Adaachi T., Iwai, S., Yasui, M. and Sato Y. (1992) Settlement and Inclination of
Reinforced Concrete Buildings in Dagupan City Due to Liquefaction During the
1990 Philippine Earthquake, Earthquake Engineering, Tenth World Conference,
147-152.
Dashti, Shideh, Bray, Jonathan D., Pestna, Juan M., Reimer, Michael and Wilson, Dan
Bolt, Bruce A. (2007) Mechanisms of Liquefaction-Induced Settlement of
Buildings on Shallow Foundation, Neesinc, Davis, CA
Dashti, S., Bray, J.D., Riemer, M.R. and Wilson, D. (2007) Centrifuge Experimentation
of Building Performance on Liquefied Ground, Proc., 5th NEES Annual Mtg.,
June 19-21, 16 pp.
Fiegel, G.L., and Kutter B.L. (1994). Liquefaction-induced Lateral Spreading of Mildly
Sloping Ground,J. Geotech. Eng., 120(12), 2236-2243
Ishihara, K., and Yoshimine, M., (1992) Evaluation of Settlements in Sand Deposits
Following Liquefaction during Earthquakes, Soils and Foundations, 32(1), 173-
188.