2846
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
4 DISCUSSION
Table 2 lists the scatter in travel time estimates for source
frequencies from 14.3 kHz to 3.3 kHz, as this provides an
indication of the robustness of each analysis method. Note the
lower limit of 3.3 kHz was chosen as this value corresponds to
an approximate propagation distance-to-wavelength ratio equal
to two, avoiding data which may include near field effects
(Sánchez-Salinero et al. 1986). The values in Table 2 indicate
significant variation in estimates obtained using the cross-
spectrum calculation and first deflection observational
algorithm (both ± 138 µs), suggesting estimates taken from
either method may be unreliable if not validated by other data.
In particular the scatter observed from cross-spectrum analyses
has been previously been reported following other studies
(Yamashita et al. 2009), with the sensitivity of estimates to the
frequency window used within the analysis method also being
highlighted (Viana da Fonseca et al. 2009). As such it is
recommended the cross-spectrum method be used with caution,
and it is recognized that a more advanced technique for
determining the frequency window may be required for
implementation within the Batch Analysis tool.
Conversely the scatter in estimates obtained from the cross-
correlation function (± 7 µs) and first bump maximum
observation (± 9 µs) were relatively minimal. These values
suggest each method is relatively robust, an observation also
made for the cross-correlation function after reviewing recent
studies comparing analysis methods (Styler and Howie 2012).
Whilst the
V
S
and
G
0
values presented in Figure 4
demonstrate the carry-on effect of scattered travel time
estimates produced by the cross-spectrum calculation, they also
display good comparison between values obtained via BEAT
and the subjective WU observational analyses. It can
specifically be seen that travel times based on a determination
of point C (i.e. the start-to-start method) lead to
V
S
values
generally within 2 m/s of each other, whilst the peak-to-peak
estimates follow the same trend with variation in source
frequency (e.g. increased
V
S
and
G
0
values when
f
< 3.3 kHz).
Such preliminary results suggest the use of BEAT may decrease
subjectivity when interpreting travel times using standard
observational techniques, whilst still allowing accurate
estimates of the shear wave velocity and small-strain shear
modulus to be calculated.
5 CONCLUSIONS
The subjectivity and lack of a satisfactory model for interpreting
shear wave travel times from bender element test data has led
GDS Instruments to develop BEAT, a tool designed to automate
the interpretation process using a number of recommended
analysis methods in both the time and frequency domains. The
tool is accessed via two easy-to-use Microsoft Excel Add-Ins,
allowing data derived from almost any bender element system
to be analysed, either one test at a time, or in batches when
organised using the GDS .bes file format. Outputs from the tool
include numerical values of the travel time estimates and
analysis metrics, as well as visual representations of the source
and received bender element signals to assist with rapid data
validation by the user.
An initial assessment of BEAT was made by conducting
bender element tests on a saturated, isotropically consolidated
triaxial specimen of Leighton Buzzard sand. During these tests
the single sine-wave source frequency was systematically varied
from 14.3 kHz to 1.0 kHz, allowing the robustness of each
analysis method to be investigated. Results from BEAT when
f
≥
3.3 kHz showed significant scatter in travel time estimates
obtained from a cross-spectrum calculation (± 138 µs), whilst
the cross-correlation function produced relatively consistent
estimates (± 7 µs) with variation in the source frequency.
Observational analyses of the received bender element signals
conducted by BEAT were also compared with subjective
estimates provided by a geotechnical academic, demonstrating
good agreement between calculated
V
S
and
G
0
values. This has
led to the preliminary conclusion that BEAT can provide
accurate, objective interpretation of bender element test data via
a simple user interface, however caution and engineering
judgment are still recommended when making final decisions
regarding the most suitable shear wave travel time estimate for
further geotechnical calculations.
GDS BEAT is available for free download from
,
which also includes further technical information and video
demonstration of the software tool.
6 ACKNOWLEDGEMENTS
The authors wish to express their sincere thanks to Tomasz
Szczepański of Warsaw University for providing observational
analyses of the bender element test data presented in this paper,
and Sophie Laliat for providing a French translation of the paper
abstract.
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