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.