Centrifuge DAQ System Reinforces New Orleans Levees
by Peter Blume and Greg Burroughs, Bloomy Controls and
Inthuorn Sasanakul, Ph.D., Rensselaer Polytechnic Institute
Figure 1. London Canal Levee Break
Courtesy of http://911review.org/Hurricane_Katrina
Hurricane Katrina produced a massive surge of
water on the U.S. Gulf Coast that overtopped and eroded away
more than 50 levees and floodwalls that compose the New Orleans
and Southeast Louisiana Hurricane Protection System (Figure 1).
In the aftermath of this catastrophic disaster, several
investigations were conducted into the performance of the levees
and floodwalls and the causes of the damage and failures.
For one study, the U.S. Army Corps of
Engineers assembled an Interagency Performance Evaluation Task
Force (IPET) that included Rensselaer Polytechnic Institute.1
The task force used centrifuge models of the levees on 17th
Street, London Avenue, and the Orleans canals to provide
detailed insights into the mechanisms that led to the breaches.
A geotechnical centrifuge consists of an arm
that rotates a basket containing a payload around a central
vertical axis. The payload is a geotechnical specimen or model
constructed within a strong box that is subject to high
acceleration forces during rotation.
The behavior, including the failure modes, of
any structure or material dependent on its self-weight can be
simulated on a reduced size and time scale using centrifuge
testing.2 Geotechnical centrifuges are commonly used
for modeling the response of geotechnical material or structures
such as soil, dams, and foundations to natural and man-made
hazards like earthquakes, floods, and explosions.
As the behavior of soil is stress dependant,
it is crucial in studying the performance of a geotechnical
system to ensure that the correct stresses are applied to each
element in the levee and foundation. This is difficult to
achieve in a scale model under earth’s gravity alone because the
weight of the model is not sufficient to bring the soil to the
correct state.
In a centrifuge, however, a model at a scale
of 1/N can be subjected to a steady acceleration field
equivalent to N times earth’s gravity. In this state, the same
stress conditions that exist in the field can be effectively
reproduced at all points in the model.
By applying appropriate data acquisition
techniques, these stresses can be verified throughout the model
during centrifuge testing. Moreover, detailed information can be
collected on the response of a geotechnical system including
measurements of water pressure in the ground at different
locations, the movement of a structure such as the flood wall
and ground surface, and video imagery of a sequence of events
leading to a breach.
Data Acquisition Hardware
Rensselaer Polytechnic Institute’s Center for
Earthquake Engineering Simulation maintains a 150g-ton
centrifuge with a 3.0 meter arm radius and maximum payload of
1.5 ton spinning at 100g and maximum acceleration 150g.
To capture the necessary data, Rensselaer
uses a highly configurable, high-performance DAQ system
developed by Bloomy Controls. (www.bloomy.com/centrifuge) As shown in Figure 2, the
system includes National Instruments’ PXI and SCXI hardware to
condition signals from various sensor types. The controller runs
Bloomy centrifuge DAQ application software and communicates to
the control room PC via Ethernet.
Figure 2. Block Diagram of the Centrifuge DAQ System Hardware
Each DAQ module is cabled to a SCXI
signal-conditioning chassis. The DAQ modules perform high-speed
analog-to-digital conversion of the conditioned analog signals
and analog output control of an external shaker table and
camera. The SCXI chassis contain modules for conditioning strain
gages, pore pressure transducers, accelerometers, LVDTs,
thermocouples, and analog voltages. A single model can be
instrumented with more than 128 sensors.
The SCXI chassis are mounted in the
centrifuge basket adjacent to the steel box containing the
model. This helps to minimize wire length and signal attenuation
between sensors inside the model and the SCXI chassis.
Consequently, the SCXI chassis are subject to similar
accelerative forces as the model.
The PXI chassis mounts in a cabinet at the
center of the centrifuge and connects to the control-room
computer and the LAN via an optical fiber running through a
rotary optical coupling. The centrifuge DAQ software is operated
from the control-room computer via a Windows remote desktop.
Signals from multiple SCXI chassis are
synchronized using a common sample clock shared by all DAQ
modules via the PXI timing and synchronization bus.
Synchronization is very important in centrifuge testing because
acceleration compresses the time scale of geological responses
and events.
Data Acquisition Software
The centrifuge DAQ GUI allows you to select
the type of sensors conditioned by the SCXI module using the
sensor selection control. Supported sensors include
accelerometers; LVDTs, quarter-, half-, and full bridge strain
gages; pore pressure sensors; thermocouples; and analog
voltages. Each sensor selection has a corresponding form that
allows you to configure the channels of a module based on the
applicable parameters.
For strain gages, the bridge completion,
excitation voltage, and filter setting may be individually set
for each channel, and the lead resistance, nominal resistance,
and gage factor may be selected for each module. Additionally,
the channel name, location within the model, sensor serial
number, scaling factor, and engineering units are common
settings applicable to all sensor types.
A waveform chart can be displayed that
represents live data from the active sensors connected to any
SCXI module. It assists with preparation of the model and
identification of any problems with the sensors during
centrifuge spin-up.
Data acquisition is initiated via the start
button and ended either manually via the stop button or
automatically based on completion of the acquisition time
interval. The sample rate is adjustable on the fly using the
sample rate control. Also, you can control pumps and valves
simulating geological events such as flooding.
After testing, you can open any data file,
view selected channels on one graph or on separate graphs by
sensor type, and convert the data from binary to text files or
XML. Figure 3 shows the response of two pore pressures to
increasing water levels at the levee.
Figure 3. Graph of Two Pore Pressures Responding to Increasing
Water Level at the Levee
Levee Tests
Rensselaer researchers built small-scale
models of typical levee sections from several locations in New
Orleans including the 17th Street Canal and the
London Avenue Canal.3,4 The models were 50 times
smaller than the actual levees and tested in the centrifuge
spinning at 50g acceleration, accurately simulating the field
conditions during Katrina. The models of the levees and soil
profiles were constructed of clay, metal, and peat as shown in
Figure 4.
Figure 4. Centrifuge Model Container and Setup
During the test, water was pumped from the
reservoir through a pipe into the model at the canal side of the
levee to simulate flood conditions. Pore pressure sensors were
installed in each soil layer to measure soil pressure, and laser
displacement sensors were installed behind the sheetpile to
measure the rotation of the sheetpile.
The centrifuge DAQ software was configured to
acquire approximately 25 pore pressure sensors, three laser
displacement gages, and two water-level sensors, and a relay was
configured to control the water pump. Data was acquired at 100
samples/s while the water level in the canal side of the levee
was raised at 100-s intervals for 30 minutes.
The IPET study shows that, in the 17th
Street model, the wall in the middle of the earthen structure
started to move before the water reached the top. The weak clay
directly underneath the peat layer sheared first, causing the
whole levee to slide.
Summary
According to officials with the IPET
Geotechnical Structure Performance Analysis Team, the
independent centrifuge modeling experiments conducted at
Rensselaer using Bloomy Controls’ centrifuge DAQ system greatly
assisted with the repairs and improvements of the New Orleans
hurricane protection system following Katrina. The Rensselaer
centrifuge experiments, coupled with those conducted by the U.S.
Army Corps of Engineers, discovered and validated floodwall
failure mechanisms. These lessons learned were factored
into the system improvements to provide much better protection
for the citizens of New Orleans.
References
- Performance Evaluation of the New
Orleans and Southeast Louisiana Hurricane Protection System,
U.S. Army Corps of Engineers, June 1, 2006.
- Turner, P., Geotechnical Centrifuges,
University of Cambridge Department of Engineering.
- Sasanakul, I., Sharp, M., Abdoun, T.,
Ubilla, J., Steedman, S., and Stone, K., "New Orleans Levee
System Performance During Hurricane Katrina: 17th
Street Canal and Orleans Canal North," Journal of
Geotechnical and Geoenvironmental Engineering ASCE, May
2008.
- Sasanakul, I., Sharp, M., Abdoun, T.,
Ubilla, J., Steedman, S., and Stone, K., "New Orleans Levee
System Performance During Hurricane Katrina: London Avenue and
Orleans Canal South," Journal of Geotechnical and
Geoenvironmental Engineering ASCE, May 2008.
About the Authors
Peter Blume is founder and president of
Bloomy Controls. He is the author of The LabVIEW Style Book and
has published articles for various trade magazines. e-mail:
peter.blume@bloomy.com
Greg Burroughs is senior project engineer
with Bloomy Controls. He has 16 years experience developing
systems for automated test, data acquisition, and control and is
a National Instruments certified LabVIEW architect and
professional instructor. Mr. Burroughs graduated from Rochester
Institute of Technology with a B.S. in electrical engineering.
e-mail: greg.burroughs@bloomy.com
Bloomy Controls, 839 Marshall Phelps Rd.,
Windsor, CT 06095,
860-298-9925.
Inthuorn Sasanakul, Ph.D., is a research
assistant professor and technical manager for the Center for
Earthquake Engineering Simulation in the Civil and Environmental
Engineering Department at Rensselaer Polytechnic Institute.
Previously, she was a graduate research assistant at Utah State
University and the University of Texas and a project assistant
in the Asian Center of Soil Improvement and Geosythetics at the
Asian Institute of Technology in Bangkok. Dr. Sasanakul is the
author of many publications and a member of several professional
societies. Rensselaer Polytechnic Institute, Center for
Earthquake Engineering Simulation, Troy, NY 12180, 518-276-6944,
e-mail: sasani@rpi.edu