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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
(e.g., Dyvik and Olsen (1991), Zeng and Ni (1998), Lee and
Santamarina (2005), Lee et al. (2007)). The bender element tip-
to-tip separation is 6.2 cm, yielding a ratio of test specimen
diameter to sensor separation of approximately 2.45. The
sample height is 15.8 cm. Figure 2 contains a view of the test
system including key ancillary equipment.
Glass beads used for the granular testing matrix are 0.5 mm
in diameter. Consideration is given to optimize uniformity and
repeatability of the placement of the glass beads, in order to
minimize unwanted acoustic impedance contrasts in the
medium, to discourage preferential fluid flow pathways, and to
facilitate comparisons among tests. The oven-dried glass beads
are vibrated into place using a vibratory table operating at 60 Hz
for 20 minutes with a surcharge load of 28 kg. The surcharge
mass was selected based on the methodology laid out by ASTM
D4253 - 00(2006) Standard Test Methods for Maximum Index
Density and Unit Weight of Soils Using a Vibratory Table. The
surcharge mass is kept in place throughout testing. It
approximates a vertical overburden stress of 15 kPa.
The glass beads are flooded with de-ionized (DI) water
which is introduced through upward flow distributed across the
cross-section of the experimental column. The water is plumed
into the bottom of the test chamber to reduce the amount of
entrapped air and is plumed under gravity flow with total head
not exceeding 0.3 meters. The fluid-flow gradient is kept low to
discourage entrainment of air bubbles and approximate laminar
flow. The fluid passes through a set of baffles and a perforated
disc before entering the sample (Figure 1). This process is
intended to encourage a uniform wetting front and discourage
fingering and the formation of preferential flow pathways. A
somewhat loose-fitting top cap allows the cell to be completely
flooded
and allows the overburden stress to act directly on the
specimen
. Excess water exits the test apparatus through a port
above the cap. Satisfactory dispersion of liquid through the
glass-bead-filled test chamber was demonstrated using colored
dye (Figure 3). The capacity of the fluid-delivery system
including test cell is approximately 1.7 liters.
Data are collected with a Data Physics (Data Physics, Inc.)
dynamic signal analyzer. Single sine pulses are created using a
function generator. The sampling interval is 9.301
microseconds. The signal is not filtered during data capture.
The reported result is an average from 350 pulses, which are
repeated at 0.7-second intervals.
3 BASELINE TESTING
Through resonance testing and experimental trials, optimum
frequencies to test for shear and compression were determined
to be 1 kHz and 8 kHz, respectively. The compression
measurements have not yet been resolved satisfactorily because
of complications with electrical crosstalk between source and
receiver and are not presented here. Results of baseline testing
for shear (using DI water, with no experimental treatment) are
shown in Figure 4. Four datasets are collected. The first dataset
is collected after initial inundation of the sample. Repeat
collections occur after 4, 8 and 12 additional liters of DI water
are flushed through the sample. The time history from the initial
measurement is dramatically different from those collected
later. Results demonstrate that up to four liters of fluid need to
be flushed through the system before the response stabilizes.
The flushing process is likely to improve the signal by expelling
entrained air.
The zero time in Figure 4 corresponds to source initiation.
Based on first arrival by visual interpretation, the shear wave
velocity of the specimen is approximately 170 m/s.
Considering the timing for the first troughs in the signals,
velocity estimates among the three measurements (after flushing
4, 8 and 12 liters of DI water) would vary by approximately +/-
3 %. Amplitudes at the first trough vary by +/- 17%.
4 NANOPARTICLE EFFECTS
The test sequence was repeated on a new sample, in which a
solution of 0.05% (by weight) nZnO solution was introduced.
The nanoparticle treatment solution is made through sonication
to uniformly disperse the nanoparticles in DI water. Eight liters
of clean DI water were flushed through the test chamber first,
followed by 4 liters of the nanoparticle solution.
Results are shown in Figure 5. Again, the dataset conducted
before flushing was not representative of results after flushing.
The time histories collected after flushing but before
introducing the nZnO treatment would ideally be the same as
those collected in the baseline test. However, comparison with
Fig. 4 demonstrates that the tests are not closely repeatable
between samples, despite careful efforts to replicate test
conditions. For example, consider the difference in signal
amplitude between the two figures. This is an unfortunate
discrepancy that requires further investigation. From Figure 5,
considering the time histories gathered after flushing 4 and 8
liters of DI water, the shear wave velocity is approximately 150
m/s, which represents a decrease of 12 % with respect to
baseline (Figure 4). Also, for the later test (Figure 5), results do
not stabilize as fluid flushing occurs to the extent seen in
baseline testing. Repeating analyses of the timing and amplitude
of the first trough, velocity estimates among the measurements
after flushing 4 and 8 liters would vary by approximately +/- 9
% and amplitudes by +/- 3 %. With respect to the baseline test,
the percentage variabilities are higher for velocity and lower for
amplitude.
Despite the variability observed with DI water, the trial
conducted after introducing the nZnO experimental treatment
(labeled in Figure 5 as “12”) shows distinctly different results
from those collected earlier on the same test specimen. Signal
amplitude is dramatically increased, and timing is delayed. To
effectively quantify differences, cross-correlation of traces,
signal matching or coda wave analysis might be applied (Lee
and Santamarina 2005, Dai et al. 2012).
The significant increase in signal amplitude might be due to
agglomeration of nZnO onto the glass bead matrix (Klaine et.
al., 2008) which would enhance contacts in the skeletal
structure of the glass bead matrix, thereby reducing signal
attenuation. The smaller decrease in velocity is not as readily
explained. One hypothesis (which is yet untested) is that the
flushing process allows sufficient displacement of the glass
beads to permit introduction of nanoparticles between contacts.
This possibility seems unlikely given the slow influent flow rate
and the amplitude of the surcharge pressure.
5 CONCLUSIONS
A laboratory apparatus has been developed to study seismic
response to the presence of nanoparticles dispersed in the pore
fluid of saturated granular media. Uniform placement of the
solid sample matrix is encouraged through the use of a vibratory
table and a surcharge load. A perforated plate and baffle system
for introduction of fluid at the bottom of the test cell encourage
laminar flow and uniform fluid distribution which is
demonstrated using dye. Baseline testing in clean water using
sine pulses at 1 kHz, chosen to optimize transmission of shear
energy, demonstrated that by first flushing several liters of fluid
through the sample, repeated measurements would yield similar
results. Testing a new sample demonstrated that close
repeatability of results is not assured between samples.
The measurements revealed sensitivity of the nanoparticle
treatment to shear wave energy propagation. After flushing a
sample with a treatment containing 0.05% nZnO, measurements
revealed a dramatic increase in signal amplitude and a slight
increase in travel time. Changes might be due to agglomeration
of nanoparticles on the granular matrix which enhances grain-
to-grain contacts, possibly coupled with insertion of
nanoparticles between grains which reduce system stiffness.
Further experimentation is required to assess if the observed
responses are indeed trends. Once accomplished, the research
will expand to increase experimental sensitivity and broaden
scope, and to more closely approximate conditions in nature.
