NEESR Payload:
Characterization of Dynamic Soil-Pile Interaction by Random Vibration Methods
EXPERIMENTAL SETUP REPORT
Jeramy C. Ashlock and Mohammad K. Fotouhi
Iowa State University
This Experimental Setup Report contains descriptions of the experimental plan, site conditions,
test specimen geometry and material properties, sensor properties and locations, and NEEShub
data directory structure.
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BRIEF PROJECT DESCRIPTION
Full-scale elastodynamic vibration tests were performed on two steel HP 10x42 piles installed to
an approximate depth of 20 ft in a soil profile featuring soft clay in Miami, Oklahoma. One pile
was tested in the natural soil profile while the other featured a cement deep soil mixed (CDSM)
improved zone near the surface. A newly developed servo-hydraulic inertial shaker system and
dynamic signal analyzer were used to deliver various forms of broadband excitation to a pile cap
mounted on the test piles. Separate tests of the vertical and coupled horizontal-rocking modes
were performed (referred to as VC and HC tests, respectively), as well as multi-modal tests of
general planar motion via vertical eccentric excitation (VE tests). To determine the optimal
testing configuration, three different excitation waveforms were examined for a range of forcing
intensities as explained below. One of the goals of the multi-mode tests was to validate their use
as a replacement for separate vertical and horizontal-rocking tests, which will greatly improve
efficiency while simultaneously reducing experimental uncertainties compared to the use of the
two traditionally separate modal tests. Similar to common nondestructive surface and borehole
geophysical tests, the experimental approach developed in this study uses small-strain wave
propagation to infer information about the soil properties, but also inherently includes the effects
of pile installation and soil-foundation interaction with actual pile-soil contact conditions.
2
TEST SETUP AND INSTRUMENTATION
2.1 Sensors
The measurement approach was designed with consideration of the analytical and computational
phases of the project. In the latter, the experimental broadband dynamic response will be used to
validate and calibrate existing analytical approaches used in engineering practice, as well as 3D
computational continuum models describing the pile-soil interaction and wave propagation. The
required measurements are the time-histories of dynamic force applied to the pile cap and its
resulting vertical, horizontal and rotational accelerations, from which spectral quantities of
interest are calculated. To obtain the dynamic force, the acceleration of the moving mass of the
shaker was measured. The primary spectral measurements of interest1 are a collection of
1
Further defined and discussed in J.C. Ashlock and M.K. Fotouhi, “Characterization of Dynamic Soil-Pile
Interaction by Random Vibration Methods: Experimental Design and Preliminary Results”, Proc. 2011 NSF
Engineering Research and Innovation Conference, Atlanta, Georgia 11 pp., January 2011. This paper is available
for download from the project‟s NEEShub page.
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complex-valued frequency dependent transfer functions, defined as the ratios of directional
acceleration at selected points of the pile cap to the applied force. For each transfer function, a
coherence function is calculated which provides an indication of nonlinearity and measurement
quality in each test. All frequency domain measurements were calculated during testing by the
signal analyzer using ensemble averages from 15 separate time windows of auto-spectral and
cross-spectral densities. The spectral averaging statistically minimizes the effects of random
errors. To allow further calibration of the computational models towards a more complete
description of the dynamic soil response, accelerations near the soil surface were also measured
in two directions at seven locations around the test piles. These measurements will be useful to
NEEShub users for evaluation of different approaches to the problem (e.g. time domain versus
frequency domain approaches, numerical versus analytical methods). The locations, orientation,
and notation for the sensors on the pile cap and soil are shown in Figures 1 and 2, respectively.
Figure 2 shows the right-handed coordinate system used, with the origin at the center of the pile
in the unimproved soil, the positive x-axis pointing to the West and positive z-axis pointing up.
Detailed sensor specifications and coordinates for the 6 experiments are given in Tables 1
through 6 of the Appendix.
2.2 Inertial Shaker
A servo-hydraulic inertial shaker and controller were used to deliver broadband excitation of up
to 2,000 lbf over a nominal bandwidth of 1-200 Hz. The shaker is capable of producing
excitation outside this frequency range, although at reduced levels of peak force. The shaker
features a feedback control system which accepts a user-generated input voltage signal. As
indicated in Table 1, the gain was fixed for stability of the feedback control loop, while the span
and input voltage were varied to obtain the desired level of forcing excitation in the various tests.
The shaker consists of a moving mass weighing 499 lbf, and a fixed frame which together with
the attached hose couplings and pressure accumulators weighs 225 lbf. Details on the inertial
properties of the composite pile-cap and shaker system (mass, rotational moment of inertia and
centroid location) can be found in the documentation at the Experiment level.
2.3 Data Acquisition Systems and Data File Formats
2.3.1 National Instruments Dynamic Signal Analyzer
For data acquisition and control of the shaker, a 20 channel dynamic signal analyzer was built
using National Instruments (NI) hardware and programmed using LabVIEW. The signal analyzer
enabled real-time display of spectral data as well as feedback on the quality of the tests, so that
parameters could be adjusted during testing for optimum data quality. Data from the NI analyzer
are saved in engineering units of m/s2 in binary Matlab (.mat) files, which have the advantages of
reduced file sizes and pre-defined variable names for use in plotting and analysis. For each trial,
a single .mat file containing all 20 channels of acceleration data was saved with the test names
shown in the test log in Table 7. For each test, the Matlab data has also been converted to CSV
text format. To maintain throughput of the dynamic signal analyzer and avoid unnecessarily
large file sizes, only the final (15th) window of time-domain data was saved along with the
averaged spectral measurements by the NI analyzer. If needed, the complete time-histories can
be recovered from the Kinemetrics Granite System data files described in the following section.
As shown in the test log, measurement bandwidths of 250 Hz and 1 kHz were used. A
combination of analog and digital anti-aliasing filters were employed to ensure that the
measurement bandwidths were alias-free. For both bandwidths, the sampling rate was fixed at
2.56 times the measurement bandwidth.
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Figure 1: Elevation view showing orientation and notation for accelerometers on shaker and pile
cap (shaker shown in VE test position).
Figure 2: Plan view showing layout of the two test piles and soil accelerometers.
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2.3.2 Kinemetrics Granite Seismic Monitoring Systems
In addition to the data recorded by the NI analyzer, complete time-histories of the test data were
simultaneously recorded by the nees@UCLA Kinemetrics Granite seismic monitoring systems
using the maximum possible sampling rate of 1 kHz. Anti-aliasing filters were used, limiting the
maximum usable frequency to 400 Hz. The complete time histories were recorded for three
purposes: (1) to enable determination of transfer functions between any desired pair of sensors
from their time histories via post-processing, (2) to provide a backup in the event of loss of data
from the NI analyzer, and (3) so users of NEEShub could analyze the problem data using their
own chosen techniques and re-sampling parameters. The Granite systems continuously logged
data throughout the day, with separate data files written at approximately one hour intervals. To
recover the time histories from a given test, users must load the appropriate hour-long data file
and extract the portion of the time-histories corresponding to the start of the desired test.
Each file from the Granite systems contains 12 channels of data in tab-delimited ASCII text
format. The first column contains the sample times in „Unix time‟ format, which is the number of
seconds elapsed since January 1 st, 1970. The sample times can be converted to Central Daylight
Time (CDT) or Greenwich Mean Time (GMT) for comparison with the times in the test log of
Table 7, or zeroed to obtain elapsed seconds from the start of the data file. Columns 2-13 in the
files contain the measured data in units of bits (or „counts‟), which can be converted to Volts
using the conversion factor 224 bits=40 Volts, then converted to acceleration using the calibration
constants in the sensor tables (Tables 1-6), located in the Experiment level documentation
folders. The raw Granite data will also require corrections for zero offset, and a few channels
have significant 60 Hz noise which will require filtering.
The names for the Granite system data files correspond to the time at which they were first
created (typically on the hour), with the naming conventions YYYYMMDDHHMMSS.OK01.txt
and YYYYMMDDHHMMSS.OK02.txt in Greenwich Mean Time. Because individual trials
lasted only a few minutes while the raw Granite data files span approximately one hour, the data
from a single trial must be extracted from the appropriate pair of Granite data files. For this
purpose, the approximate start and stop times for each trial are listed on the test log in GMT and
local Central Daylight Time (CDT=GMT-5).
For ease of analysis and smaller file sizes, the Granite ASCII text data files were also converted
to Matlab binary format. The ASCII and Matlab data files are stored in Experiment-7/Trial1/Rep-1/Unprocessed_Data.
2.4 Material Properties and Test Specimen Geometry
This Payload project uses the soil site and reaction piles from the NEESR-SG project
“Understanding and Improving the Seismic Behavior of Pile Foundations in Soft Clays” (Grant
#0830328), which involves dynamic and cyclic lateral tests of open-ended pipe piles in soft
clays. The test specimens used in this Payload project are two of the H-piles from the reaction
frame of the NEESR-SG project. Design and installation of the H-piles were therefore performed
by the NEESR-SG project team.
2.4.1 Site Conditions
Testing was performed next to the Neosho River in Miami, Oklahoma on September 27th through
29th, 2010. The soil profile consists of a 3.6 ft thick layer of lean clay with gravel and occasional
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W
E
Figure 3: Soil profile and improved soil zone dimensions.
construction debris overlying 11 ft of soft, silty clay to clayey silt and 6.4 ft of sandy gravel, with
limestone bedrock at a depth of 21 ft. The cylindrical CDSM improved zone around Pile I has a
diameter of 48 inches and a depth of 13 ft. The soil profile and dimensions of the cylindrical
CDSM improved soil zone are shown in Figure 3. Site investigation and in-situ testing
performed for the NEESR-SG project included SPT tests, CPT and DMT soundings and piston
samples. Laboratory tests were performed for grain-size analysis, Atterberg limits and
unconfined compressive strength. It is anticipated that results and interpretation of in-situ and
laboratory testing will be made available on the NEEShub page for the NEESR-SG project.
2.4.2 Pile Cap
A two-piece modular pile cap was designed with epoxied threaded steel rods embedded at
multiple locations for mounting the exciter in the VC, VE and HC orientations. The two halves
of the pile cap are bolted together with six ¾-inch threaded rods. The assembled pile cap is 3 by
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Figure 4: Rebar cages for the modular pile cap.
3 by 3 feet in size, with a 28 inch deep recess that forms a compression fit around the H-pile. To
ensure rigid body motion of the pile cap, a dense framework of reinforcing steel was designed
using #5 bars with #3 J-hooks, as shown in Figure 4. The design drawings for the pile cap are
shown in Figure 5, and the assembled pile cap is shown in Figure 6 with the shaker in the VC,
HC and VE orientations.
2.4.3 H-Piles
The HP 10x42 piles are made of grade 50 steel and have the following section properties:
As=12.4 in2 , d=9.7 in, tw=0.415 in, w=10.075 in, t f=0.420 in. Both have a total length of
approximately 25 feet and were installed by a vibratory hammer to embedment depths of 245
inches for pile I and 242 inches for pile U. The above-ground unembedded lengths are 55.75
inches for pile I and 57.75 inches for pile U. The estimated capacity of the pile in unimproved
soil is 168 kips in compression and 35 kips in tension, which increases to 374 kips in tension and
240 kips in compression for the pile in improved soil.
3
TEST PLAN AND NEES PROJECT DIRECTORY STRUCTURE
Experimental data including the test log, sensor measurements, specimen properties, sensor
tables, and documentary images and videos are available on the NEEShub. Results of analyses
and numerical simulations will also be uploaded periodically. This section describes the
organization of the experimental data within the framework of the NEES User Data Model.
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Figure 5: Design drawings for reinforced concrete pile cap (dimensions in inches).
(a) VC test
(b) HC test
(c) VE test
Figure 6: Pile stem, cap and shaker illustrating shaker locations for the three test types.
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3.1 Project
The project includes 6 experiments and 109 trials as detailed in the test log (Table 7) and
explained below.
3.2 Documentation
Project level documentation includes the test log, experimental setup report (this document),
photos and video. Experiment level documentation includes sensor tables, specimen properties,
setup drawings, sensor layouts and relevant photos/video. Sensor tables in the Documentation
directory of each Experiment contain complete sensor descriptions including channel numbers
for both the NI analyzer and the Granite DAQ, measurement type, manufacturer, model number,
serial number, orientation, location, calibration constants, and data units (see Tables 1-6).
3.3 Experiments
Each of the three shaker excitation locations (VC, VE and HC) on the pile-cap in Figure 6
constitutes a separate experiment. As detailed in Table 1, the three excitation types were used on
Pile I in improved soil followed by Pile U in unimproved soil, giving a total of six separate
experiments. The setup drawings for Experiments 1 through 6 are shown in Figures 7 through
12. The test names shown in Table 7 indicate the test pile and the excitation configuration and
type. For example, I-VC-S1 is a test of the pile in improved soil (I) with VC excitation at the first
level of sinusoidal excitation (S1). Experiments were performed in the following sequence: (1) IVC, (2) I-VE, (3) I-HC, (4) U-VC, (5) U-VE, and (6) U-HC.
3.4 Trials
For each experiment, three types of excitation were applied using the inertial shaker; random (R),
chaotic impulse (C), and swept-sine (S). Excitation was first applied at low intensity, for which
the test name suffixes R1, C1 and S1 are used. The three excitation types were then repeated at
medium intensity (tests R2, C2, S2) and high intensity (tests R3, C3, S3). The nine excitation
combinations were applied for each of the six experiments, resulting in 54 separate trials.
Beyond these tests, 55 additional trials were carried out in which experimental parameters were
varied, giving a total of 109 unique trials. For example, the excitation/measurement bandwidth
was decreased from 1,000 to 250 Hz, the sinusoidal sweep period and excitation level were
varied parametrically, and ambient vibration tests (with suffix Amb#) were performed to
measure the level of background ambient vibration noise. Due to time constraints as well as the
small-strain nature of the tests, multiple repetitions were not saved for each trial. However, a few
repetitions performed throughout testing were seen to give unchanged results. A graphical
illustration of the data hierarchy is shown in Figure 13 of the Appendix.
Relative to the raw data continuously recorded throughout the day by the Granite DAQ, the NI
analyzer files do not contain long periods of data between tests. Data files from the NI analyzer
in Matlab binary and CSV text format are therefore located in the Corrected_Data folders at the
Trial level. Due to the different lengths of the time and frequency domain data vectors contained
in the Matlab files, separate CSV files were created with _time.csv and _freq.csv suffixes.
3.5 Repetitions
As noted above, many unique trials were performed but multiple repetitions were not carried out.
Each trial therefore has only one repetition.
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3.6 Analysis
Numerical simulations by finite element, boundary element and analytical techniques are
currently being performed and results will be uploaded to the Analysis folders at the project,
experiment or trial level as appropriate.
3.7 Metadata
Metadata including setup drawings, specimen details, sensor tables, photos and video are
uploaded in the Experiment level Documentation folder. The test log and experimental setup
report are located in the Project level Documentation folder.
3.8 File Formats
Documentation files are in PDF, Microsoft Excel, JPG, and PNG formats. Configuration files for
the NI analyzer are provided in Labview .vi format. Photos and videos are provided in JPG and
WMV or AVI formats. As described above, data files are provided in tab-delimited and CSV text
in addition to Matlab binary formats.
4
ACKNOWLEDGMENTS
This material is based upon work supported by the National Science Foundation under Grant No.
0936627. Any opinions, findings, and conclusions or recommendations expressed in this material
are those of the author(s) and do not necessarily reflect the views of the National Science
Foundation.
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Figure 7: Experimental setup drawing for Experiment 1: I-VC.
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Figure 8: Experimental setup drawing for Experiment 2: I-VE.
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Figure 9: Experimental setup drawing for Experiment 3: I-HC.
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Figure 10: Experimental setup drawing for Experiment 4: U-VC.
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Figure 11: Experimental setup drawing for Experiment 5: U-VE.
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Figure 12: Experimental setup drawing for Experiment 6: U-HC.
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5
APPENDIX
Full-Scale Vibration
Test of Piles
U-VC
Project
U-VE
U-HC
I-VC
I-VE
I-HC
I-VC-R1
I-VE-R1
I-HC-R1
U-VC-R1
U-VE-R1
U-HC-R1
I-VC-R2
I-VE-R2
I-HC-R2
U-VC-R2
U-VE-R2
U-HC-R2
I-VC-R3
I-VE-R3
I-HC-R3
U-VC-R3
U-VE-R3
U-HC-R3
I-VC-R4
I-VE-R4
I-HC-R4
U-VC-R4
U-VE-R4
U-HC-R4
I-VC-C1
I-VE-C1
I-HC-C1
U-VC-R5
U-VE-C1
U-HC-C1
I-VC-C2
I-VE-C2
I-HC-C2
U-VC-R6
U-VE-C2
U-HC-C2
I-VC-C3
I-VE-C3
I-HC-C3
U-VC-R7
U-VE-C3
U-HC-C3
I-VC-C4
I-VE-C4
I-HC-C4
U-VC-C1
U-VE-C4
U-HC-C4
I-VC-S1
I-VE-S1
I-HC-S1
U-VC-C2
U-VE-S1
U-HC-S1
I-VC-S2
I-VE-S2
I-HC-S2
U-VC-C3
U-VE-S2
U-HC-S2
I-VC-S3
I-VE-S3
I-HC-S3
U-VC-C4
U-VE-S3
U-HC-S3
I-VC-S4
I-VE-S4
I-HC-S4
U-VC-C5
U-VE-S4
U-HC-S4
I-VC-S5
I-VE-S5
I-HC-S5
U-VC-C6
U-VE-S5
U-HC-S5
I-VC-S6
I-VE-S6
I-HC-S6
U-VC-C7
U-VE-S6
U-HC-S6
I-VC-S7
I-VE-S7
I-HC-S7
U-VC-S1
U-VE-S7
U-HC-S7
I-VC-Amb1
I-HC-S8
U-VC-S2
U-VE-S8
U-HC-S8
I-VC-Amb2
I-HC-S9
U-VC-S3
Experiments
Trials
U-HC-S9
U-VC-S4
U-VC-S5
U-VC-S6
U-VC-S7
U-VC-S8
U-VC-S9
U-VC-S10
U-VC-S11
I-VC-Amb1
I-VC-Amb2
Figure 13: Organization of the test data within the framework of the NEES User Data Model.
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Table 1: Sensor table for Experiment 1.
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Table 2: Sensor table for Experiment 2.
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Table 3: Sensor table for Experiment 3.
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Table 4: Sensor table for Experiment 4.
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Table 5: Sensor table for Experiment 5.
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Table 6: Sensor table for Experiment 6.
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Table 7: Test log
Figure 7: Organization of the test data within the framework of the NEES User Data Model.
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Table 7 (continued): Test log.
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Table 7 (continued): Test log.
Page Model.
25 of 25
Figure 7: Organization of the test data within the framework of the NEES User Data