Kenneth H. Stokoe

Measurement of Shear Waves in Laboratory Specimens by Means of Piezoelectric Transducers

The use of piezoelectric ceramics to measure shear wave velocity in laboratory soil specimens is discussed. The experimental technique and interpretative methodology are described. Both bender and flat-plate ceramic elements were employed, and each performed well. Interpretation of the shear waveforms generated with each transducer improved as the number of wavelengths between the source and receiver increased, with four or five wavelengths being optimum. Shear wave velocities measured using both types of transducers compared closely with the results of a parallel program performed with the torsional resonant column. Based on this work, flat-plate shear transducers show significant potential for future laboratory use because of their robustness and noninvasive nature.

Comparison of Shear-Wave Slowness Profiles at 10 Strong-Motion Sites from Noninvasive SASW Measurements and Measurements Made in Boreholes

The spectral-analysis-of-surface-waves (SASW) method is a relatively new in situ method for determining shear-wave slownesses. All measurements are made on the ground surface, making it much less costly than methods that require boreholes. The SASW method uses a number of active sources (ranging from a commercial Vibroseis truck to a small handheld hammer for the study conducted here) and different receiver spacings to map a curve of apparent phase velocity versus frequency. With the simplifying assumption that the phase velocities correspond to fundamental mode surface waves, forward modeling yields an estimate of the subsurface shear-wave slownesses. To establish the reliability of this indirect technique, we conducted a blind evaluation of the SASW method. SASW testing was performed at 10 strong-motion stations at which borehole seismic measurements were previously or subsequently made; if previously made, the borehole results were not used for the interpretation of the SASW data, and vice-versa. Comparisons of the shear-wave slownesses from the SASW and borehole measurements are generally very good. The differences in predicted ground-motion amplifications are less than about 15% for most frequencies. In addition, both methods gave the same NEHRP site classification for seven of the sites. For the other three sites the average velocities from the downhole measurements were only 5-13 m/sec larger than the velocity defining the class C/D boundary. This study demonstrates that in many situations the SASW method can provide subsurface information suitable for site response predictions. Manuscript received 18 January 2002.

Guide for Shear-Wave-Based Liquefaction Potential Evaluation

Small-strain shear-wave velocity measurements provide a promising approach to liquefaction potential evaluation. In some cases, where only seismic measurements are possible, it may be the only alternative to the penetration-based approach. Various investigators have developed relationships between shear wave velocity and liquefaction resistance. Successful application of any liquefaction evaluation method requires that procedures used in their development also be used in their application. This paper presents detailed guidelines for applying the procedure described in Andrus and Stokoe that was developed using suggestions from two workshops and following the general format of the Seed-Idriss simplified procedure. Correction factors to velocity and liquefaction resistance for soil aging are suggested. Based on the work by Juang et. al., factors of safety of 1.0, 1.2, and 1.5 correspond to probabilities of liquefaction of about 0.26, 0.16, and 0.08, respectively. Additional field performance data are needed from all soil types, particularly denser and older soil deposits shaken by stronger ground motions, to further validate the recommended procedure.

Generation and Measurement of Shear Waves In Situ

Field procedures used to measure in situ shear wave velocity by crosshole and downhole seismic methods are presented along with typical travel time records. Identification of the initial shear wave arrival is enhanced by use of a reversible torsional source in the crosshole method and a reversible embedded source in the downhole method. Correct triggering of recording equipment is critical in these measurements. Characteristics of three triggering systems-a velocity transducer, a resistance-capacitance (RC) circuit, and an electrical step trigger-are presented. Incorrect triggering can cause errors greater than 50 percent in field measurement of shear wave velocity. Other variables such as borehole casing, borehole disturbance, and source and sensor configuration also affect velocity measurement. The effects of many of the variables can be minimized by basing wave velocity computations on interval travel times of the initial arrival. It is recommended that any ASTM standards for crosshole and downhole seismic methods include field check procedures for correct timing and triggering of recording equipment and field measurement of borehole verticality.

Strong Motion Station Characterization and Site Effects During the 1999 Earthquakes in Turkey

The 1999 Kocaeli and Duzce earthquakes in Turkey generated a moderate amount of strong ground motion data. This paper describes the shear-wave velocity profiles measured at a number of strong motion stations in Turkey using the spectral-analysis-of-surface-waves (SASW) method. The shear-wave velocity profiles from SASW testing compare well with deeper profiles developed by microtremor surface wave inversion, but SASW provides more shear-wave velocity resolution near the ground surface. The developed shear-wave velocity profiles are used to define site classifications for each station. For the Kocaeli earthquake, event-specific attenuation relationships are developed. These relationships show considerable amplification of peak ground acceleration and spectral acceleration (at a period of 0.3 s) at deep soil sites in the far field, but no amplification in the near-fault region. For spectral accelerations at longer spectral periods (1.0 and 2.0 s), amplification is indicated in both the near field and far field. Amplification factors derived from the Kocaeli earthquake strong motion data are generally larger than those used in current attenuation relationships and building codes. The short-period amplification factors derived from the regression decrease with increasing rock motion intensity (PGArock), and the derived long-period amplification factors increase with increasing PGArock. These trends are most likely due to soil nonlinearity. The increase in long-period amplification factors with PGArock is not taken into account in current building codes.

Torsional Motion Monitoring System for Small-Strain (10 −5 to 10 −3 %) Soil Testing

For accurate measurements of stress-strain hysteresis loops at strains below 10−3%, a torsional motion monitoring system was modified to incorporate micro-proximitors. A micro-proximitor system with enlarged target arms resulted in about 50 times higher resolution in motion monitoring than previously possible. The four proximitor signals (instead of two) were measured, compared, and averaged to insure that pure torsion of the system was generated and that any bending did not enter the measurement. Ambient noise was controlled by using a low-pass filter and/or a vibration isolation table. With this new system, shear modulus was measured at strains as low as 2 · 10−5%, and hysteretic damping was measured at strains as low as 6 · 10−5%. Below the elastic threshold strain, shear modulus of dry sand measured in the cyclic torsional shear test is independent of strain amplitude. Hysteretic damping exists below the elastic threshold and is independent of strain amplitude, even though the value is quite small.

Development of an in situ dynamic liquefaction test

A new field-testing technique has been developed in which liquefaction and pore pressure generation characteristics of soil are measured in situ. The in situ dynamic liquefaction test utilizes a large, hydraulic shaker to load dynamically a soil deposit. The soil response is measured with embedded instrumentation. The embedded instrumentation includes newly developed liquefaction test sensors that incorporate velocity transducers (geophones) and pore pressure transducers in a single case. The recorded data are used to describe pore pressure generation and liquefaction characteristics in terms of the relationship between shear strain and induced pore pressure ratio. The analytical techniques used to compute shear strain from particle velocity measurements are discussed and compared. The results from testing a 1.2-m by 1.2-m by 1.2-m reconstituted field test specimen are presented. The shear strain—pore pressure relationships at selected numbers of loading cycles that were determined from the in situ dynamic liquefaction test are compared with those measured by other investigators in the laboratory. The field-measured relationships show the same shape as the lab-measured relationships, but the field data indicate a smaller threshold strain for pore pressure generation (∼0.005 %). This difference is attributed to the low effective stresses in the reconstituted field specimen.

Nondestructive evaluation of pavements by surface wave method

The spectra-analysis of surface waves (SASW) method is a seismic method that can be used to determine elastic modulus profiles of pavement sections nondestructively. This method is based upon the theory of stress wave propagation in layered media. The dispersive characteristics of measured surface waves are used for determining the elastic modulus and thickness of each layer in the profile. This paper contains a description of the in situ testing technique and the in-house data reduction procedure used in conducting SASW tests. Emphasis is placed on demonstrating situations where SASW tests can be utilized more effectively than other nondestructive tests. For example, when bedrock is present at shallow depths or when the pavement layers are thin, use of the SASW method may be preferred. Two case studies illustrating the accuracy, sensitivity, and limitations of the method are also included.

Deep Shear Wave Velocity Profiles from Surface Wave Measurements in the Mississippi Embayment

Shear wave velocity ( VS ) profiles to depths of approximately 200 m were developed from active-source surface wave velocity measurements in the Mississippi Embayment region of the Central United States. Soil deposits in this region are hundreds of meters thick, but are poorly characterized at depths below 60 m. Measurements were performed at five locations in Arkansas and Tennessee with a maximum distance between sites of approximately 130 km. The median VS profile calculated from the five profiles is in good agreement with a generic reference VS profile for the Mississippi Embayment that has been used in recent site response studies. The near-surface VS profiles at the five sites were remarkably consistent with average shear wave velocities in the top 30 m (VS30), varying by less than 10%. Increasing variability between the VS profiles was observed at greater depths. The variability between VS profiles was shown to be correlated with changes in lithology at two of the sites where nearby lithologic information was available.

Development of Model for Shear-Wave Velocity of Municipal Solid Waste

AbstractThe shear-wave velocity and associated small-strain shear modulus of municipal solid waste (MSW) are important engineering parameters in evaluating the seismic response of MSW landfills as well as in characterizing the waste material and its response to static loads. Semiempirical and empirical models for the shear-wave velocity are presented. The semiempirical model is a more comprehensive model that aims to separately capture the effect of waste density and confining stress on the shear-wave velocity of MSW. It is based on similar models for soils, and its mathematical expression is formulated using data generated from large-scale laboratory studies on reconstituted MSW. The empirical model has a simpler mathematical expression that is a function of depth only. The parameters of both models are derived by calibrating them against a total of 49 in situ shear-wave velocity profiles at 19 MSW landfills, i.e., 13 profiles from four landfills in Michigan generated as part of this study and 36 additio...