Schmalz Larochelle Report 10 07

8/4/2019 Schmalz Larochelle Report 10 07

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Development and Proof-Testing of

a PC-Based Bender Element System for

Shear Wave Measurements in Soft Soil

Research Report

By

Damian Schmalz, Ethan LaRochelle and Thomas C. Sheahan

Department of Civil and Environmental Engineering

Northeastern University

Boston, Massachusetts 02115

October, 2007

Northeastern UniversityDepartment of Civil and Environmental Engineering


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Authors Note

The writing of this report was supported by the National Science Foundation under Grant No.0530151. Any opinions, findings, and conclusions or recommendations expressed in this materialare those of the author(s) and do not necessarily reflect the views of the National Science

Foundation.

This report was prepared under the supervision of Professor Thomas C. Sheahan while the firstauthor was an undergraduate researcher for the NSF project from July through December, 2007,and the second author was working for the project from May through August, 2007.


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Table of Contents

1. INTRODUCTION ......................................................................................................... 41.1 Typical Bender Element System ............................................................................................................ 41.2 System Components and Methods ......................................................................................................... 51.3 Objectives ................................................................................................................................................. 6

2. PREVIOUSLY USED SYSTEM COMPONENTS AND EQUIPMENT ......................... 72.1 Introduction ............................................................................................................................................. 72.2 Function Generators ............................................................................................................................... 72.3 Oscilloscopes ............................................................................................................................................ 72.4 Additional Equipment ............................................................................................................................. 8

3. PC-BASED BENDER ELEMENT SET-UP ................................................................. 83.1 Introduction to the PC-based Set-up ..................................................................................................... 83.2 Bender Element Testing Apparatus ..................................................................................................... 10

3.3 Calibration of Piezoceramic Bender Element Tiles ............................................................................ 113.4 Developing a Software-Based Bender Element System ...................................................................... 123.4.1 Specifying and Sampling a Signal ..................................................................................................... 133.4.2 Analyzing the Signal ........................................................................................................................... 143.4.3 Areas for Future Software Development Work ............................................................................... 15

4. RESULTS AND ANALYSIS ...................................................................................... 154.1 Introduction ........................................................................................................................................... 154.2 Soil Sample Characteristics and Preparation ..................................................................................... 154.3 Testing and Data .................................................................................................................................... 16

5. REFERENCES .......................................................................................................... 205.1 References Cited .................................................................................................................................... 205.2 Website References ................................................................................................................................ 20


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1. Introduction

Piezoceramic bimorphs, also known as bender elements, were first introduced into soilapplications in the late 1970s. Bender element technology was developed to provide a simpleand accurate alternative to other more complicated laboratory small strain shear modulus tests.Bender elements are simply small piezoceramic plates that oscillate when electrically charged by

a variable voltage, either a sine or square wave, or produce a voltage when stimulated by avibration. When placed into a specimen of soil, this oscillation can be utilized to send a shearwave through the soil. As the bender element oscillates, it causes neighboring soil particles toshift past one another resulting in a shear wave transmission. By inserting a bender element atthe top and bottom of the soil sample, a wave can be both sent and received using these elements.When the wave from the sending element reaches the receiving element, the shifting soilparticles surrounding the piezoceramic tile cause the bender element to oscillate in the form of ashear wave. The characteristics of the sent and received waves can then be used to determine thesmall strain shear modulus, Gmax, which is useful when dealing with engineering projects that areadversely affected by vibrations and ground movement (Landon 2004).

The development of this new technology came from the need to develop a simplified,convenient, and cost effective method for determining the small strain shear modulus of soilswithout sacrificing accuracy. Prior to the successful development of bender elements, the mostcommon method for determining Gmax was the resonant column test, performed in the laboratoryusing relatively complex equipment operated by experienced technicians. This is expensive,time consuming, and subject to potential operator error. Thus, bender element testing wasinvestigated to provide an alternative that is simple and manageable.

In the current research, the conventional bender element system has been modified andimproved to allow for much more efficient testing. This enhanced system was achieved byconverting the bender element control and data acquisition system, which typically includes oneor more bulky and expensive external modules (e.g., a function generator and/or oscilloscope)

into one that consists solely of a PC and interface card. This allows the entire testing process tobe controlled from the computer screen, including test activation, data retrieval and dataprocessing. Because of its simplicity and portability, the resulting system is well-suited for bothlaboratory and field testing applications.

1.1 Typical Bender Element System

In a typical bender element set-up, the required equipment consists of a functiongenerator, an oscilloscope, power source and a computer, as shown schematically in Figure 1.Depending on the type of computer used and whether signal filtering or a charge amplifier isrequired, the price of the required set-up may vary. However, a typical cost for an PC,

oscilloscope and function generator1 will generally cost approximately $3,000(www.metrictest.com, www.picotech.com).

1 For example, PicoScope Model 3224, $808, with 500ns/div, 12 bit resolution, +/-1% accuracy,+-10mv to +/-10V range; or PicoScope Model ADC212/100, $1415, 100ns/div, 12 bit resolution,+/-1% accuracy, +/-50mv to +/-20V ranges, 50Ms/s), and a 10-20 MHz function generator (e.g.,Agilent Model 33220A, 20MHz, $1960, or TTi Model TG1010A, 10MHz, $1433).


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Figure 1: Schematic of bender element system (Brocanelli and Rinaldi 1998)

The bender elements must be prepared for use by soldering wires to the bender elementtile itself in order to be able to excite the bender element. This excitation is possible due to thepolarization of the piezoceramic material, which refers to the alignment of positive and negativecharges in the crystal. If the piezoceramic plates are poled in the same direction, meaning bothfaces of the bender element have the same charge, then they are considered to be parallel benderelements. Alternatively, if the two plates are poled in opposite directions, they are referred to asseries bender elements.

Figure 2 illustrates this concept of parallel and series poled bender elements. Typically,series bender elements are used for receiving the transmitted wave since they are able to generatea charge that is double that of parallel bender elements when undergoing the same mechanicaldeflection. Meanwhile, parallel bender elements achieve twice the deflection of a series benderelements deflection under the same applied voltage. Thus, a typical set up will utilize a parallel-poled bender element as the transmitter, and a series-poled bender element as the receiver(Landon 2004). It is important to note that piezoceramic tiles are easily damaged and must notbe exposed to moisture or excessive heat, or else depolarization or other damage to the tile mayoccur.

1.2 System Components and Methods

In a typical set-up, a bender element is excited with a function generator transmitting aninput pulse, either a sine, square, or modified sine wave. Typically, a sine wave is the preferredinput signal due to the fact that sine waves are composed of a single frequency and therefore aremuch easier to interpret (Landon 2004), while square waves are composites of differentfrequency waves. In order to measure the start of the input wave, the function generator mustbe connected to the oscilloscope. As the generated wave moves through the soil specimen it canbe refracted and reflected, resulting in multiple waves being detected by the receiving bender


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element. This receiving bender element must also be connected to the oscilloscope in order torecord the received waves. From these measurements, one is able to find the travel time of theshear wave, i.e., the time for the shear wave to travel from transmitting bender element toreceiving bender element. There are many methods available to determine travel time; however,Lohani et al (1999) reported that for sinusoidal waves, the most consistent and accurate travel

times were found using the peak-to-peak method. The peak-to-peak method refers to the timedifference between the first peak on the transmitted sine wave and the corresponding point on thereceived sine wave.

Figure 2: (a) Bender element poled in series; (b) Bender element poled in parallel(Waanders 1991)

One alternative to the peak-to-peak method is the first zero cross-over method. However,for a given set of conditions, the travel times determined by this method have a tendency to varywhen the frequency is changed. Ideally, travel time should not be dependent on the frequency,and this inherent inconsistency develops from human error and the magnification of near fieldeffects, which can mask the initial arrival of the shear wave. In some cases, the cross-overmethod may be utilized; however, the travel times found using both methods should becompared to make sure there is no significant deviation. By utilizing both the travel time andtravel distance (the measured distance between the tips of the two bender elements when in thetesting position), the shear wave velocity can be calculated. If desired, a frequency filter may beincorporated into the system to clean the signal of disturbances such as ambient noise or static,which can cause the received signal to be unclear and hard to interpret. While frequency filters

do make it easier to interpret the received signals, it is not necessary for transmitting andreceiving a good signal (Landon 2004).

1.3 Objectives

In an attempt to simplify and improve the manner by which signals are transmitted andreceived, the set-up described in this report implemented a system consisting of PC-basedtransmission and receiving of bender signals (i.e., transmission is achieved using a digital-to-


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analog converter, and both transmitted and received waves are measured using an analog-to-digital converter), and software-based data processing. By using this digital system, much of theexpensive and bulky equipment can be eliminated from the typical bender element set-up, thusdiminishing the possible errors that may occur from additional analog circuits and reducing thenumber of systems the signal has to run through before being recorded and displayed. The result

is a system that is able to perform the bender element test more efficiently, and with significantlydecreased processing time for running a test.

2. Previously Used System Components and Equipment

2.1 Introduction

The typical set-up that has been used to run bender element tests usually includes anoscilloscope, function generator, power supply, and computer. In some cases there is a need toinclude a voltage amplifier or filtering devices; however, these are not required to record alegitimate signal and are implemented on as needed basis.

2.2 Function Generators

Transmitted bender element waves are typically actuated using a function generator.Typical systems used have ranged from the Hewlett-Packard Model 3325A (+/- 0.1dB tolerancefor sine wave) used in Dyvik & Madshus (1985) to TTi TG 1010 DDS function generator usedby Pennington et al. (2001). Function generator capabilities typically include the ability togenerate multiple wave forms, which include sine, square, triangle, and ramp, at a frequencyrange of 1 mHz to 21MHz. The TTi function generator used by Pennington et al. had afrequency range of 0.1 mHz to 10 MHz and is able to send 8 different wave forms includingcomplex waves, but this technology is constantly progressing. Typical frequencies transmitted

into the bender element system can range from 500 Hz to 7 kHz; therefore, any functiongenerator able to accommodate this range is acceptable. Outside of this range the signal maybecome difficult to distinguish, and may lead to inconsistent travel times for the same waveform.Also, since the two waves used the most in bender element testing are square and sine waves(more predominately sine waves) the multiple wave capabilities of the function generator aretypically unnecessary unless there are specific needs for the tests being applied. Recently, therehave been advances in function generator technology that have allowed their adaptation to PCcontrol, with specific signals being sent to the function generator via software commands. Forinstance, the Velleman PCG10AU (www.apogeekits.com/function_generator.htm) has thecapability to send a signal to the function generator through a user interface on the computerscreen instead of directly on the panel of the bench-top function generator.

2.3 Oscilloscopes

The task of recording the signal so that it can be displayed and analyzed has, in the past,been handled by an oscilloscope. In a typical set-up, the oscilloscope is connected to both thefunction generator and receiving bender element so that it can record the transmitted andreceived signals. Once the signal is sent, it records the characteristics of the wave at somesampling rate (samples per second). This feature has been continuously improved upon over the


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years as equipment has become more advanced, allowing for a more complete depiction of thewave. For example, the oscilloscope used by Dyvik & Madshus (1985), a Nicolet 4094 digitaloscilloscope, had a sampling rate of 2 million samples per second, but the Pico ADC 212/100, aPC-based oscilloscope utilized by Landon (2004), had a sampling rate of 5 billion samples persecond. Additionally, similar advances in oscilloscope technology have resulted in PC-

controlled devices to replicate the functions of traditional oscilloscopes.

2.4 Additional Equipment

Additional equipment may be implemented into the bender element system; however, thisis specific for each set-up and is not required for producing an analyzable signal. If the benderelement system is housed in a laboratory which includes potential sources of electromagneticinterference, resulting in a significant amount of ambient noise, then filtering devices may berequired. Also, in order to amplify the received signal and reduce the mechanical load (due toexcessive deformation) on the receiving bender element, a charge amplifier may be introducedinto the set-up. Although the implementation of a charge amplifier may not always be effective

enough to make a significant difference, in some cases, it has been successfully implemented(e.g., Pennington et al. 2001 used a Kistler 5011).

3. PC-based Bender Element Set-up

3.1 Introduction to the PC-based Set-up

The set-up used in this research converted the previous standard, consisting of multipleexternal components and manual input adjustments and calculations, to a solely PC-basedarrangement. A data acquisition card installed in the computer is used to transmit and processsignals, which replaces the need for a function generator and oscilloscope. The bender elements

are connected to the PC via this card, which is controlled by a user-friendly software packagedeveloped using MATLAB 7.4. This program, the interface for which is seen in Figure 3, is usedto generate a signal with specified characteristics that can be transmitted to the sending benderelement tile. As seen in Figure 3, these inputs include the frequency, number of pulses,amplitude, periods, wave form, etc. This allows the characteristics of the desired wave to bequickly altered, thus reducing testing time and increasing the systems overall efficiency.Programming methods used to achieve the user interface are discussed in detail in section 3.4.

Once the wave is generated and the received wave collected, plots of voltage versus timedisplay both the transmitted and received waves on the same absolute time axis. Typical plotsfrom a standard test are shown in the viewing window at the bottom of Figure 3, with thetransmitted wave shown in blue and the received wave as red plus signs. Note that in Figure 3,

because the received wave, which is typically on the mV scale, has not been amplified, thewaveform is not readily visible. Figure 4 shows these waves with the received wave amplified by1000. From these plots the user is able to determine the travel time of the shear wave byselecting points on the transmitted and received wave. The program then determines the timedifference between the selected points and displays the travel time. The process by which theprogram generates the graph and calculates the travel time is discussed in section 3.4. Accordingto Landon (2004), the preferred method for determining travel time is a much disputed topic;however, as noted in Section 1.2, one of the more commonly used techniques is the first zero


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Figure 4: Saved graph from completed test with selected times according to the cross-overmethod.

Once the test has been completed and the associated graph generated, the characteristicsof the wave and results of the test, including the graph, are saved. All test specifications andresults are recorded in a log with the date and time of the test noted. This file is appended as eachnew data set is recorded, thus creating a cumulative log of test results. Meanwhile, the graphsare saved to a folder that is available so previously run experiments are easily viewable andaccessible. Section 3.4 provides an in-depth discussion of how the program archives and recordspreviously completed experiments along with files types.

3.2 Bender Element Testing Apparatus

Based on previous work by Landon (2004), an apparatus was fabricated to measure shearwave velocity in soft soil samples using bender elements. The resulting design is shown inFigure 5 and consists essentially of two mounting posts that can hold the existing triaxial top capand base pedestal, each of which has a bender tile installed and wired into it. This allows the


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same cap and base to then be installed in the triaxial cell, where the same set-up can be used tomeasure shear wave velocity during triaxial testing. The transmitting bender element is installedin the top cap, while the receiving bender element is housed in the base pedestal (Figure 5). Thetop cap is supported by a threaded post that moves vertically and rotates in a bearing mounted onthe main support bracket. The base pedestal sits in a cradle that includes a groove and access

port for the bender element wiring. The access port is design so that the bender wiring does notexperience excessive distortion. A detailed schematic, as seen in Figure 6, shows both thetransmitting and receiving bender element specifications and dimensions.

Figure 5: Testing device designed for testing bender elements in correspondence with soilsamples.

3.3 Calibration of Piezoceramic Bender Element Tiles

As noted by Landon (2004), it is important to calibrate the time lag or delay that occurs inthe system due to the circuit and processing speed of the system. This is done by aligning thebender element tiles such that they are in tip-to-tip contact, and transmitting a signal through thesending bender element. The time difference between the initiation of the transmitted wave and

the first arrival of the received wave is determined.The calibration completed on the presented research used an identical signal 25 times in

order to find an average time delay. The standard signal utilized had a frequency of 2000 Hz andconsisted of 20 pulses with a 0.01 second delay between pulses. An average delay time of8.22x10-6 seconds (about 8 sec) was measured with a standard deviation of 9.08x10 -8 (.091sec). Comparing this average to the range of 6 to 9 sec measured by Landon (2004) showsthat the system consistently has a very short delay. Seeing as the current system has fewerexternal components and associated wiring than that used by Landon, it is logical that the delay


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time is very low. Since the average calibrated delay time and its standard deviation isinsignificant, the delay of the system is considered negligible and was not incorporated in thetravel time calculations.

Figure 6: (a) Detailed schematic of top cap with transmitting bender element (b) Detail diagramof bottom mounting with receiving bender element (Sample, 2004).

3.4 Developing a Software-Based Bender Element System

The main goal for developing a software-based system was to eliminate the use of bulkyand expensive hardware external to the PC, while streamlining the data collection and analysisprocess. The current laboratory setup uses a computer with a 3.2 GHz processor, 1Gb of RAM,and a National Instruments 6070E PCI multifunction data acquisition card. This card has 2analog output channels and 16 analog input channels with a maximum sampling rate of 1.25million samples per second. The software developed provides a user-friendly way to interfacewith this card to transmit and receive signals through the bender element setup. Figure 7 providesan overview of the computer-based bender system, where Fig. 7a shows the hardware overviewand Fig. 7b shows the electrical path the signal follows. The signal is first generated based on thespecification given by the user. It then travels from an analog output channel (ao0) to the break-out box (shown in Figure 7a) where the signal is split using a BNC T-connector. At this tee, onebranch is connected to the transmitting bender element, while the other is directly connectedback to an analog input channel (ai0). The receiving bender element is attached to another analog


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input channel (ai3). Connecting the analog output directly to the input and sampling thissimultaneously with the received signal reduces the calibration time to a negligible amountbecause both signals will experience the same delay. The following sections provide additionaldetails about the system.

BNCConnecto r

B lo ck

68-pin

CableComputer w i th DAQ Card

(a)

Figure 7a. Schematic of laboratory setup

Figure 7b. Wiring diagram used in this setup

3.4.1 Specifying and Sampling a Signal

The multi-function card used in the tests is controlled using a program developed usingMATLAB. The program has been customized for the specific needs of this experiment, whichhas provided an efficient and standardized way to record data. The software interface is moreuser-friendly and requires less equipment-specific knowledge than configuring a function


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generator and oscilloscope. This increases the potential for wider use of bender elements fortesting in both the educational and commercial testing lab environment.

To run a test the user must open the program in MATLAB (although it could later becompiled to be a stand-alone program) and specify the desired characteristics of the signal. Theprogram currently is able to generate sine, square, and sawtooth waves. The user can specify the

amplitude, frequency, phase, DC-offset, and number of periods of the desired wave. Additionallythe user can choose to stack the specified wave with a certain delay time between each signal,which helps reduce noise and will be discussed further in section 3.4.2.

When the signal is generated it is sent through the analog output channel (ao0) to thebender element. Simultaneously the analog input channels begin sampling at a specified rate (ai0ai1 ai2 ai3). The multifunction card used has a sampling rate of 1.25 million samples per second.This rate is divided between channels in use, so if all 16 input channels were being used, theactual sample rate per channel would be 78,125 samples per second. In this setup, the minimumrequired number of input channels is two. However, due to cross-talk among the channels, fourchannels were used to isolate the two signals (transmitted and received) being measured. Thisreduces the maximum sampling rate per channel to 312,500 samples per second. Although this isnot as fast as some data acquisition cards or oscilloscopes, the rate is sufficient for our tests.

The software calculates a minimum suggested sampling rate. The sampling rate is that atwhich the computer will convert the incoming analog signal into a digital signal. The programcalculates this rate based on the user-defined output frequency, currently sampling twenty pointsper period per channel. So, for example, if a 5 kHz sinusoidal wave is generated, the samplingrate calculated by the program would be 400,000 samples per second, which is 20 samples perperiod per input channel. This rate is much less than the maximum rate of the acquisition board(1.25MS/s), which leaves room for increasing the sampling rate. Sampling at higher rates willallow for more accurate representations of the analog wave, although this additional informationwill not have a major impact on the final measurements.

3.4.2 Analyzing the Signal

The program gives the user the option of sending consecutive signals with a user-specified time delay between each signal. The program then aligns each of these signals andaverages the result. Stacking these signals helps to reduce the noise. The program then displaysthis result for the user to select the points on the transmitted and received waves used to calculatethe time delay.

There are three methods by which the time delay can be computed. The user can choosebetween simply selecting a point on both the transmitted and received waves, or can choosebetween two methods where the computer interprets the zero cross-over or the positions of thepeaks (see Section 3.1). The interpretation method should improve the precision ofmeasurements taken at different times or by different operators.

With the zero cross-over method, the user selects a region on both the received andtransmitted waves. The program then takes the sampled values in the selected time interval andperforms a linear fit. The zero cross-over for the sent and received signals are calculated byfinding the roots of these lines and the delay is calculated from the time separating these points.


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Similarly, with the peak-to-peak method the user again selects a region around both thetransmitted and received waves. The program performs a second-order polynomial fit on thepoints of each signal in the two selected regions. To find the values at the peaks the roots of thederivative of the curve are calculated. The delay time is then calculated from the time separatingthese points.

Each time a measurement is made, the data concerning the specific test is saved in aMAT file. This file can be opened later in MATLAB for further analysis. In addition to this a logfile is automatically updated with information about the wave. This data is saved in a tabdelimited text format that can be easily imported into a spreadsheet program to analyze trends inthe experiments.

3.4.3 Areas for Future Software Development Work

The current laboratory configuration uses a National Instruments multifunction PCI cardto generate waveforms and receive data. The specific card used in this experiment, the Model6070E, is also manufactured by National Instruments using a Firewire connection, in place of thePCI connection. Using a Firewire connection would allow for this system to be implementedusing a laptop computer, which would have great benefit in field measurements. Otherconnections, such as USB and PCMCIA, can also be used to interface between laptop computersand data acquisition devices.

With some multi-channel data acquisition devices, like the Model 6070E used herein,there can be cross-talk between the channels. Cross-talk can occur in data acquisition cards thatscan through multiple channels. To minimize these effects a grounded channel can be scannedbetween the two input signals, as was done in the current tests. Reducing the sampling rate orincreasing the inter-channel delay will also help minimize cross-talk. To avoid this problemcompletely, a data acquisition card designed for simultaneous sampling, like the National

instruments S Series cards, could be used.

4. Results and Analysis

4.1 Introduction

In order to prove the effectiveness of the digital bender element system, a sample of thesame soil as that used by Landon (2004) was implemented into the digital set-up.

4.2 Soil Sample Characteristics and Preparation

The soil sample used in testing was BBC taken from the Newbury, MA NationalGeotechnical Experimentation Site. The soil was taken from block sample SBS12, which wastaken at a depth of approximately 7.8 m.

The sample was stored in a sealed and controlled environment to prevent moistureexchange and alteration of the soils properties. In order to confirm the samples integrity, thewater content of a sample was taken and compared to that used by Karademir (2006), who usedthe same block sample. Four sections were observed and the average water content was found tobe 60.13%, compared to an average of 59.25% from Karademir (2006). Once the soil was tested


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for water content, a section was cut for use in the testing device. A sample of length 11.84 cm(4.66 in; as obtained using the average of 3 measurements from a depth micrometer) wasprepared and was placed in the set-up. To allow for direct soil-bender contact, it is importantthat the soil sample is placed onto the tile as straight as possible and without excessive smearingor remolding of soil around the bender. The same care must be taken when the top bender

element is lowered down into the soil sample. If any remolding of the soil occurs, the soil-bender element contact is decreased and can lead to lower values of Vs. Once both benderelements are securely embedded in the soil sample, as seen in Figure 8, the set-up is ready totransmit a signal.

Figure 8: Bender element jig with soil specimen installed

4.3 Testing and Data

Since the most difficult parameter to measure in bender element testing is the travel time,the results from the cross-over and peak-to-peak methods are compared. According to Lohani etal (1999), the more precise and efficient method of these two is the peak-to-peak method. This isdue to the fact that, because peaks are not distorted by changing frequencies, there is littlevariation in travel time with various frequencies. Also, there is less human error involved sincethe segment on the received wave where the signal begins to peak is typically more evident thanthe cross-over point on the curves. Referring to Figure 9, many times when a test is run thereceived wave is shifted above the x-axis, and it is not clear whether to choose point A or B as


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the cross-over point. However, when Landon (2004) compared travel times obtained using bothmethods, she found that the average percent difference was only 3%. So, despite the enhancedconsistency offered by the peak-to-peak method, with adequate care, the cross-over method canprovide similar consistency.

Figure 9: Graph from completed test showing the possible human error that can be involvedwhen deciding between point A or B when using the cross-over method

To demonstrate the consistency of the improved set-up, two trials were run at each of the13 frequencies, varying from 1000 Hz. to 7000 Hz. For each trial, the results were analyzedusing both the cross-over and peak-to-peak methods for travel time comparison. The resultingdata, summarized in Table 1, indicate that for a given frequency, comparing the average traveltime found using the peak-to-peak method, 1.042 x 10-3 seconds, with that of the cross overmethod, 1.013 x 10-3 we find an average difference of only 2.9x10-5 seconds.


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Table 1: Bender proof-testing, Newbury test site soil.

Frequency(Hz.) Method

MeasuredTime 1

sec x 10-3

MeasuredTime 2

sec x 10-3

TravelTime

sec x 10-3

ComputedShearWave

Velocity,Vs, m/s

Diff. inAvg.

TravelTimebetw.

Methods,sec x 10

-3

1000

Peak toPeak

1.44 2.73 1.29 77.0

0.092351.43 2.69 1.27 78.7

CrossOver

1.19 2.26 1.08 92.4

1.18 2.47 1.29 77.3

1500

Peak toPeak

0.96 2.09 1.14 87.5

0.002050.96 2.10 1.14 87.1

CrossOver

0.79 1.85 1.06 93.7

0.79 1.85 1.07 93.5

2000Peak toPeak 0.72 1.79 1.08 92.7

0.019550.71 1.77 1.06 94.0

CrossOver

0.58 1.60 1.01 98.3

0.58 1.65 1.07 93.3

2500

Peak toPeak

0.57 1.59 1.01 98.4

0.0285450.57 1.60 1.03 97.2

CrossOver

0.47 1.44 0.97 103.1

0.47 1.51 1.04 96.2

3000

Peak toPeak

0.48 1.49 1.02 98.2

0.01180.48 1.50 1.02 98.1

CrossOver

0.39 1.40 1.01 99.0

0.39 1.38 0.98 101.5

3500

Peak toPeak

0.41 1.42 1.01 98.5

--0.41 1.41 1.01 98.9

CrossOver

-- -- -- --

-- -- -- --

4000

Peak toPeak

0.36 1.36 1.00 99.4

0.0002050.35 1.37 1.02 97.9

CrossOver

0.29 1.24 0.95 104.6

0.29 1.26 0.97 102.9

4500

Peak toPeak

0.32 1.32 1.01 99.0

0.006190.31 1.33 1.01 98.5

CrossOver

0.26 1.19 0.93 106.8

0.26 1.21 0.95 104.8

5000

Peak toPeak

0.27 1.28 1.00 99.2

0.007010.27 1.28 1.01 99.1

CrossOver

0.23 1.20 0.97 102.5

0.23 1.18 0.96 104.1


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Table 1 (contd)

Frequency(Hz.) Method

MeasuredTime 1

sec x 10-3

Measured Time 2

sec x 10-3

TravelTimesec x10

-3

ComputedShearWave

Velocity,Vs, m/s

Diff. inAvg.

TravelTimebetw.

Methods,sec x 10

-

3

5500

PeaktoPeak

0.25 1.24 1.00 99.9

0.020240.24 1.24 1.00 99.9

CrossOver

0.20 1.11 0.91 109.4

0.20 1.15 0.95 104.6

6000

PeaktoPeak

0.22 1.21 0.99 100.5

0.0084750.22 1.22 1.00 100.1

CrossOver 0.18 1.12 0.94 106.00.18 1.10 0.92 108.5

6500

PeaktoPeak

0.20 1.20 1.00 100.0

0.058860.20 1.20 1.00 99.7

CrossOver

0.16 1.14 0.98 101.8

0.17 1.27 1.10 90.6

7000

PeaktoPeak

0.19 1.18 0.99 100.3

0.0024950.19 1.19 1.00 99.8

CrossOver

0.15 1.26 1.11 89.7

0.15 1.25 1.10 90.5

AveragesPeaktoPeak 1.04 96.16.4SD 0.021481

Crossover 1.01 99.07.6SD

Average fromLandon 98.2

Notes: (a) average soil water content, w=60.1%(b) length of benders = 2 cm(c) measured length of soil specimen = 12 cm

Averaging the values obtained using only the cross-over method, an average shear wave velocityof 99.0 m/s is obtained. This compares well with the average shear wave velocity, 98.2 m/s,found from the 6 samples of BBC tested by Landon from the Newbury test site. The shear wavevelocity using the peak-to-peak methods was 95.7 m/s, a difference of only 2.7 m/s from thecross-over method.

By breaking the testing up into intervals of varying frequency, the alteration of traveltime with varying frequencies can be observed. According to Tatsuoka and Shibuya (1992),


20/20

20

since Gmax, and consequently Vs, are elastic properties of the soil, the frequency should beindependent of travel time. Also, Lohani et al (1999) noted that variations in travel time decreasewith increasing transmitted wave frequency, and that this variation increases when calculationsare performed using the cross-over method. Referring to Table 1, the variations in travel time forboth methods can be observed as the frequency is increased and it is evident that data obtained

using the cross-over method is more susceptible to these changes.

5. References

5.1 References Cited

Brocanelli, D. and Rinaldi, V. (1998). Measurement of Low-Strain Material Damping andWave Velocity with Bender Elements in the Frequency Domain, Canadian GeotechnicalJournal, 35(6), pp. 1032-1040.

Dyvik, R and Madshus, C. (1985). Lab Measurements of Gmax Using Bender Elements,Advances in the Art of Testing Soils under Cyclic Conditions, ASCE, pp.186-196

Karademir, T. (2006). Use of a Computer Automated Triaxial Testing System for Assessingand Mitigating Sample Disturbance, M.S. thesis, Department of Civil and EnviromentalEngineering, Northeastern University, Boston, MA.

Lohani, T.N., Imai, G., and Shibuya, S. (1999). Determination of Shear Wave Velocity inBender Element Test, Proceedings of the 2nd International Conference on EarthquakeGeotechnical Engineering, pp. 101-106.

Landon, M.M. (2004). Field Quantification of Sample Disturbance of a Marine Clay usingBender Elements, M.S. Thesis, Department of Civil and Enviromental Engineering,University of Massachusetts, Amherst, MA.

Pennington, D.S., Nash, D.F.T., Lings M.L. (2001). Horizontally Mounted Bender Elementsfor Measuring Anisotropic Shear Moduli in Triaxial Clay Specimens, ASTMGeotechnical Testing Journal, 24(2), pp. 133-144.

Sample, K. (2004). Personal Communication.

Tatsuoka, F. and Shibuya, S. (1992). Deformation Characteristics of Soils and Rocks fromField and Laboratory Tests, Proceedings of the 9th Asian Regional Conference on SoilMechanics and Foundation Engineering, Bangkok, pp. 101-177.

Waanders, J.W. (1991). Piezoelectric Ceramics: Properties and Applications, PhilipsComponents, Eindhoven.

5.2 Website References

MetricTest (2007) www.metrictest.com, accessed on August 29, 2007

Picotech Technology (2007), www.picotech.com, accessed on August 29, 2007

Apogeekits (2007) www.apogeekits.com/function_generator.htm, accessed on August 29, 2007

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