The ultrasonic pulse transmission method (100-500 kHz) was adapted to measure compressional (P) and shear (S) wave velocities for synthetic soils fabricated from quartz-clay and quartz-peat mixtures. Velocities were determined as samples were loaded by small (up to 0.1 MPa) uniaxial stress to determine how stress at grain contacts affects ave amplitudes, velocities, and frequency content. Samples were fabricated from quartz sand mixed with either a swelling clay or peat (natural cellulose). P velocities in these dry synthetic soil samples were low, ranging from about 230 to 430 m/s for pure sand, about 91 to 420 m/s for sand-peat mixtures, and about 230 to 470 m/s for dry sand-clay mixtures. S velocities were about half of the P velocity in most cases, about 130 to 250 m/s for pure sand, about 75-220 m/s for sand-peat mixtures, and about 88-220 m/s for dry sand-clay mixtures. These experiments demonstrate that P and S velocities are sensitive to the amount and type of admixed second phase at low concentrations. They found that dramatic increases in all velocities occur with small uniaxial loads, indicating strong nonlinearity of the acoustic properties. Composition and grain packing contribute to the mechanical response at grain contacts and the nonlinear response at low stresses

Aracne-Ruddle, C. M.

Berge, P. A.

Bertete-Auguirre, H.

Bonner, B.

Hardy, E.

Trombino, C. N.

Wildenschild, D.

UNT Digital Library

Approved for public release; further dissemination unlimitedPreprintUCRL-JC-136207Linear and NonlinearUltrasonic Properties ofGranular SoilsB.P. Bonner, P.A. Berge, C.M. Aracne-Ruddle, H. Bertete-Aguirre, D. Wildenschild, C.N. Trombino and E.D. HardyThis article was submitted to theMaterials Research Society MeetingSan Francisco, CAApril 24-28, 2000April 20, 2000LawrenceLivermoreNationalLaboratoryU.S. Department of Energy DISCLAIMER  This document was prepared as an account of work sponsored by an agency of the United StatesGovernment.  Neither the United States Government nor the University of California nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately owned rights. Reference herein to any specificcommercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does notnecessarily constitute or imply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California.  The views and opinions of authors expressed herein do notnecessarily state or reflect those of the United States Government or the University of California, andshall not be used for advertising or product endorsement purposes.  This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may bemade before publication, this preprint is made available with the understanding that it will not be citedor reproduced without the permission of the author.   This report has been reproduced directly from the best available copy.  Available to DOE and DOE contractors from the Office of Scientific and Technical Information P.O. Box 62, Oak Ridge, TN  37831 Prices available from (423) 576-8401 http://apollo.osti.gov/bridge/  Available to the public from the National Technical Information Service U.S. Department of Commerce 5285 Port Royal Rd., Springfield, VA  22161 http://www.ntis.gov/  OR  Lawrence Livermore National Laboratory Technical Information Department’s Digital Library http://www.llnl.gov/tid/Library.html  Linear and Nonlinear Ultrasonic Properties of Granular SoilsBrian P. Bonner, Patricia A. Berge, Chantel M. Aracne-Ruddle, Hugo Bertete-Aguirre,Dorthe Wildenschild, Cosette N. Trombino, and Edgar D. HardyExperimental Geophysics Group, Lawrence Livermore National Laboratory,L-201, PO Box 808,Livermore, CA  94551-9900, U.S.A.ABSTRACTThe ultrasonic pulse transmission method (100-500 kHz) was adapted to measurecompressional (P) and shear (S) wave velocities for synthetic soils fabricated from quartz-clayand quartz-peat mixtures. Velocities were determined as samples were loaded by small (up to 0.1MPa) uniaxial stress to determine how stress at grain contacts affects wave amplitudes,velocities, and frequency content. Samples were fabricated from quartz sand mixed with either aswelling clay  or peat (natural cellulose). P velocities in these dry synthetic soil samples werelow, ranging from about 230 to 430 m/s for pure sand, about 91 to 420 m/s for sand-peatmixtures, and about 230 to 470 m/s for dry sand-clay mixtures. S velocities were about half ofthe P velocity in most cases, about 130 to 250 m/s for pure sand, about 75-220 m/s for sand-peatmixtures, and about 88-220 m/s for dry sand-clay mixtures. These experiments demonstrate thatP and S velocities are sensitive to the amount and type of admixed second phase at lowconcentrations. We found that dramatic increases in all velocities occur with small uniaxialloads, indicating strong nonlinearity of the acoustic properties. Composition and grain packingcontribute to the mechanical response at grain contacts and the nonlinear response at lowstresses.INTRODUCTIONWave propagation in natural granular media applies to a range of problems in near-surfacegeophysics and environmental engineering.   Earthquake strong motion, observations of groundwater movement using seismic methods and slope stability problems in civil engineering aresome examples.  Interest in these problems has generated an extensive literature on elasticproperties of natural granular media [1], but experimental difficulties have preventedmeasurements  at the very low pressures   corresponding to the shallow subsurface.  Newexperimental methods were developed to accomplish the measurements reported here [2].Although these methods can be used for natural materials recovered from in-situ, firstexperiments were conducted on synthetic soils so that critical parameters such as composition,porosity, and packing technique could be controlled and investigated systematically.EXPERIMENTAL DETAILSThe experimental method is ultrasonic pulse transmission, modified for measurements inhighly attenuating materials [3].   The time-of-flight of an ultrasonic pulse launched from atransmitting to a receiving transducer and the sample length (44.9 mm) yields the velocity for Pand S waves as appropriate.  Changes in the path length produce small changes in travel timecompared to concurrent stress-induced changes in the elastic constants and are neglected here invelocity computations.  Transducers with center frequencies ranging from 100 to 500 kHzpolarized for transverse shear were used for S measurements, although sufficient compressionalenergy is available for simultaneous P measurements. Further improvements on previousmethods [3] were made to allow measurements of highly attenuated signals.   The transmittingtransducer was excited with 400 V pulses.  Received signals were as small as 10-5 V, requiringamplification of 60 db for detection and digital averaging of 200-1000 repetitions to pickarrivals.  A sample sleeve was constructed to ensure that the signal travels through the soilmixture, suppressing energy traveling on longer, but faster, paths to the receiving transducer [2].The sample assemblies were closed with latex membranes that transmitted sound from thetransducer and contained the soil mixture.  Accuracy was limited to 20% at the lowest stressesbut improved with signal amplitude to ~3% for compressional waves and 10% for shear waves athigher stresses.  Some of the data scatter is caused by differences in subjective picking ofarrivals, as well as dramatic changes in the character of the waveforms with stress. End-load pressures between 0 and 15 psi (0 to 0.11 MPa), simulating up to several metersof overburden, are applied in increments of approximately 1.5 psi to the sample by air-driven,pneumatic pistons that push on the transducer housing.  Internal stress in the sampleapproximates uniaxial strain, modified by edge effects and minor sleeve deformation.Samples were constructed by mixing pure quartz sand with either clay or peat moss inincreasingly larger percentages by weight.  The sand (Ottawa, IL) and was sieved to a mediangrain diameter of 273 micrometers.   The clay is Na-montmorillonite from Wyoming, a swellingsmectite.  The clay is equilibrated in a 100% humid atmosphere for seven days before samplepreparation to achieve reproducible water content. The peat is commercial Canadian sphagnumpeat moss, composed mainly of natural cellulose fibers.   Typical organic content of peat moss is80 to 95% of the mass fraction.  The peat was equilibrated with air at ambient humidity, typically30%, before sample construction.  The samples were layered to include a central section of puresand to be consistent with earlier experiments that required a high permeability layer to provideaccess for pore fluid.  Measured travel times were corrected to allow for the sand layer, usingdata from a sand sample prepared using the same methods as used for the mixtures.  Velocities,attenuation and load dependence are sensitive to the packing methods (vibration, hand packing)used in sample fabrication for the low stress levels of these experiments.   Packing effectscontribute to the scatter observed in experimental data.RESULTSRepresentative waveforms for sand-clay mixtures are presented in figure 1 to give a generalidea of the quality of the data and to demonstrate the dramatic effects of external loading ontransmitted pulses.  Complete data sets are archived elsewhere [4,5].  Three waveforms plotted atthe same scale (zero gauge units, 7.8 and 15.6 psi) for the 10% clay-sand sample are shown infigure 1.   Both the compressional and shear waves arrive earlier with increasing load. Thecompressional amplitude increases only slightly, if at all. The shear amplitude increases withincreasing load and also becomes sharper, consistent with a decrease in shear attenuation andmore efficient transmission of high frequencies.Velocities for sand-clay and sand-peat mixtures are plotted in figure 2 to demonstrate thatvelocities are low and can increase rapidly with small static loads. Two different pure sand-0.2-0.100.10.20.30 5 10-5 0.0001 0.00015 0.0002 0.00025 0.000310% Na-Montmorillonite Clay with F-50 Ottawa Sand - DryamplitudeamplitudeamplitudeAmplitude (volts)Time (seconds)Zero Gauge7.8 psi15.6 psiFigure 1. Waveforms for 10% clay, 90% sand sample for three different load valuessamples were examined; one after packing by vibration and the other after hand packing.Vibration produces higher velocities and smaller velocity increases with increasing load.  Allmixtures were hand packed.  Adding clay to the sand increases compressional velocity relative tothe hand-packed sand sample and eliminates the velocity increase with pressure at low load.Although trends are less clear for the shear velocity, added clay increases velocity and tends tosuppress the gradient to the highest loads.  Measurements for sand-peat mixtures were generallymore difficult and show more scatter because of sample variability and high ultrasonicattenuation.  Adding peat to sand decreases compressional and shear velocity, except for the 60%sample, which shows the highest P velocity.  Peat mixtures show velocity gradients comparableto hand-packed sand, except for the anomalous 60% peat sample.Figure 2. Compressional and shear velocities for sediments as a function of uniaxial loadDISCUSSION AND CONCLUSIONAlthough the behaviors shown in figure 2 are complicated, several dominant trends areevident.  Packing technique dominates velocity gradient effects.  The stress sensitivity of thesand velocities is altered by the addition of small amounts of clay for loosely packed samples.For these samples, compressional velocity of sand-clay at all concentrations is essentiallyconstant until the stress exceeds 8 psi, at which point P velocity increases at a rate similar to thepure sand.  It appears that the clay binds the sand grains until a critical stress is exceeded, andthen yields and flows away from regions of local high stress at grain contacts.  This behaviorappears to persist to the highest clay fraction tested, 40% clay.  Compressional velocity gradientsof the peat mixtures are variable, and will be investigated further in additional experiments.  Theanomalous behavior of the 60% peat sample coincides with the elimination of air-filled porosityat that concentration.  Discussion of the effects of microstructural parameters and modeling theseresults with effective medium theories is presented elsewhere [6].The velocities observed for the synthetic soils tested in this study are low, with acompressional velocity comparable to the sound speed in air for the slowest sand-peat mixtures.The compressional velocities are lower than typical field values [1] and are slightly higher thanvalues for near-surface sand in situ [7].  The admixed second phase can alter seismic attributeseven for low mass-fractions.  The photomicrograph of 90% sand, 10% clay spread on a glassslide shown in figure 3 suggests that the micromechanics of the small clay particles may explainthis strong influence.   The clay particles adhere electrostaticly to the quartz grains, with theirlong axes perpendicular to the surface, and tend to bridge the gaps between quartz grains.  Thelarge increase in compressional velocity when clay is first added to sand, accompanied by adecrease in shear attenuation, suggests that the clay alters the grain contacts by acting as anadhesive. The packing effects obscure the dependence of velocities on the presence of the secondphase, for our sand-clay mixtures. We do not see monotonic increases in velocities as more clayis added.In contrast, peat does not increase the velocity at low concentrations; instead, even in smallamounts, peat causes both P and S velocities to decrease. Packing is less important for the sand-peat samples than for the sand-clay samples. As peat is added to sand, this soft second phasedisrupts the structure of the sand framework, causing a decrease in velocity.  As the massfraction of the second phase continues to increase, porosity reduction dominates, generallyproducing the highest velocities.  Finally, when the free porosity is eliminated, velocities beginto drop as the slow second phase becomes the framework.  This behavior is similar to thatreported for sand-kaolinite mixtures at high pressures [8].The pressure dependence of the ultrasonic attributes indicates that wave transmission isextremely sensitive to the details of stress transmission through granular media.  The nonlinearbehavior of the elastic properties is an effective probe of conditions at grain contacts, which arefundamental to understanding the granular state.Figure 3. Photomicrograph of 90% Ottawa sand (mean grain diameter 270 microns) and 10%Wyoming bentoniteACKNOWLEDGMENTSD. Hart and C. Rowe made initial measurements and helped with experimental design.  C.Boro designed and constructed the ultrasonic and loading assembly.  This work was performedunder the auspices of the U.S. Department of Energy by the University of California LawrenceLivermore National Laboratory under contract number W-7405-ENG-48 and supportedspecifically by the Environmental Management Science Program of the DOE Office ofEnvironmental Management and the Office of Science.REFERENCES1.   Bourbie, T., O. Coussy, and B. Zinszner, Acoustics of Porous Media (Gulf Publishing, 1987)334 pp.2.   Bonner, B. P., C. Boro, and D. J. Hart, Anti-waveguide for ultrasonic testing of granularmedia under elevated stress, LLNL Patent disclosure IL-10607, and patent application, DOEPatent Docket No. S-94182 (1999).3.   Sears, F. M., and B. P. Bonner, Ultrasonic attenuation measurement by spectral ratiosutilizing signal processing techniques, IEEE Trans. On Geoscience and Remote Sensing,GE-19, 95-99 (1981).4. Aracne-Ruddle, C. M., B. P. Bonner, C. N. Trombino, E. D. Hardy, P. A. Berge, C. O. Boro,D. Wildenschild, C. D. Rowe, and D. J. Hart, Ultrasonic velocities in unconsolidatedsand/clay mixtures at low pressures, LLNL report UCRL-JC-135621, Lawrence LivermoreNational Laboratory, Livermore, CA (1999).5.  Trombino, C. N., Elastic properties of sand-peat moss mixtures from ultrasonicmeasurements, LLNL report UCRL-ID-131770, Lawrence Livermore National Laboratory,Livermore, CA (1998).6.   Berge, P. A., J. G. Berryman, B. P. Bonner, J. J. Roberts, and D. Wildenschild, Comparinggeophysical measurements to theoretical estimates for soil mixtures at low pressures, LLNLreport UCRL-JC-132893, Proceedings of the Symposium on the Application of Geophysicsto Engineering and Environmental Problems, ed. M. H. Powers, L. Cramer, and R. S. Bell,March 14-18, 1999, Oakland, CA, Environmental and Engineering Geophysical Society,Wheat Ridge, CO (1999) pp. 465-472.7.   Bachrach, R., J. Dvorkin,  and A. Nur, High-resolution shallow-seismic experiments in sand,Part II: Velocities in shallow unconsolidated sand, Geophysics, 63, 1233-1240 (1998).8.   Marion, D., A. Nur,  H. Yin,  and D. Han, Compressional velocity and porosity in sand-claymixtures, Geophysics, 57, 554-563 (1992).

Linear and Nonlinear Ultrasonic Properties of Granular Soils

https://digital.library.unt.edu/ark:/67531/metadc741015/m2/1/high_res_d/791377.pdf

Linear and Nonlinear Ultrasonic Properties of Granular Soils

Abstract

Similar works

Full text

Available Versions

UNT Digital Library