Browsing by Subject "SASW"

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Experimental investigation of near-field effects on the SASW dispersion curve
(2014-08) Hwang, Sungmoon; Stokoe, Kenneth H.
When any method of surface wave testing that involves Rayleigh waves is performed, one important assumption is that plane Rayleigh waves are being measured. In the forward modeling or inversion procedure that is used to analyze the field dispersion curve to determine the field V[subscript s] profile, the analysis is based on the wave field consisting of plane Rayleigh waves. Therefore, field dispersion curves that contain near-field data could adversely distort the field V[subscript s] profile. To minimize the influence of near-field effects, several criteria have been recommended in the past. However, most of the criteria were based on empirical equations that implicitly assumed zones of influence, or numerical simulations. There is a lack of experimental investigation, particularly full-scale field investigations. Even, the numerical solutions have been based on simple soil profiles without significant velocity contrasts between soil layers and/or varying thicknesses of soil layers which can significantly influence near-field effects. Data from full-scale field test using the Spectral-Analysis-of-Surface-Waves (SASW) method was used in this thesis research. SASW tests performed at two stages in the construction of a deep, 90-ft thick backfill were studied. The V[subscript s] profiles were normally dispersive, with a substantial increase in the velocity of the layer beneath the backfill. The study shows the adverse distortions that can occur in the field dispersion curve from near-field effects when the spacing of the receiver pair is: (1) above the zone of rapidly increasing V[subscript s] near the surface and (2) less than the depth to the stiffer layer in deeper measurements. Other factors that affect the results are discussed and recommendations are presented to minimize the introduction of near-field effects, at least in these relatively simple V[subscript s] profiles.
Field studies comparing SASW, beamforming and MASW test methods and characterization of geotechnical materials based on Vs
(2011-08) Yuan, Jiabei; Stokoe, Kenneth H.; Gilbert, Robert B.; Kallivokas, Loukas F.; Wilson, Clark R.; Cox, Brady R.; Joh, Sungho
Estimating S-wave velocities (Vs) from Rayleigh-wave velocities (VR) is widely used in field seismic testing for geotechnical engineering purposes. In this research, two widely used surface-wave methods, the Spectral-Analysis-of-Surface-Waves (SASW) and Multichannel-Analysis-of-Surface-Waves (MASW) methods, are evaluated and compared in field experiments. An experimental parametric study was undertaken of the SASW and MASW methods. Conventional seismic sources in the SASW method are sledge hammers, bulldozers and vibroseises. For MASW testing, sledge hammers and small shakers are usually used as the seismic sources. In this research, MASW testing was performed with traditional and non-traditional sources at a site owned by the City of Austin, Texas. Experimental dispersion curves and Vs profiles from SASW tests are used as references for the field parametric study with the MASW method. The source type, source offset, receiver spacing and number of receivers were varied to evaluate the impact of each variable on the field experimental dispersion curve. Two type of receivers, 1-Hz and 4.5-Hz natural-frequency geophones, were also compared in these tests. A second part of this research involved studying the use of characterizing geotechnical materials based on Vs. This work included two projects. The first project involved basalt on the Big Island of Hawaii. To develop empirical ground motion prediction models for the purpose of earthquake hazard mitigation and seismic design on the Big Island, the subsurface site conditions beneath 22 strong motion stations were investigated by SASW tests. Vs profiling was performed to depths of more than 100 ft. Vs30, the average Vs in the top 30 m, was also calculated to assign NEHRP site classes to different testing locations. Different materials, mainly thought to be stiff basalt, were characterized and grouped based on the Vs values. These groups were then compared with reference curves for sand and gravel (Menq, 2003) to differentiate the groups. The second project dealing with charactering geotechnical materials based on Vs involved of soil/rock profiles at a project site in British Columbia, Canada. The goals in terms of this research were to: (1) compare the Vs profiles from the different test locations to investigate the stiffnesses of different geologic materials, the variability in the material stiffnesses, and the estimated depth to bedrock, and (2) to compare the Vs profiles to existing geological and geotechnical information such as nearby boreholes, cone penetration tests and seismic cone penetration tests. Good agreement between SASW Vs profiles and boring records is expected when lateral variability at the site is low. However, when lateral variability is significant, then the difference between localized measurements, like borings and CPT results, and global measurements, like SASW Vs results, can further contribute to understanding the site conditions as shown at the site in British Columbia, Canada.
Measurements of intact and cracked concrete structural elements by the SASW method
(1994) Kalinski, Michael E. (Michael Edward), 1963-; Stokoe, Kenneth H.
Research was conducted to apply the Spectral-Analysis-of-Surface-Waves (SASW) method to the identification of internal cracking in concrete structural elements such as beams and columns. The SASW method is a nondestructive technique which involves the propagation of surface stress waves along the exposed face of a structural element. By yielding the relationship between wave velocity and wavelength of the surface wave energy, the SASW method provides information about how the stiffness of a material varies with depth. SASW measurements were made on intact and cracked concrete structural elements. Surface waves propagated through cracked concrete at consistently lower velocities. These velocity reductions corresponded to the presence of visibly detectable cracking and, in some cases, occurred in the absence of visible cracking. This result demonstrated the ability of the SASW method to detect otherwise hidden damage. When the cracks were reclosed by the application of a compressive load, the surface waves propagated at velocities comparable to those of uncracked concrete. In addition to performing velocity measurements, material damping measurements were made on intact and cracked concrete structural elements. These measurements revealed that the presence of cracking can be qualitatively assessed through an increase in observed surface wave material damping, with material damping ratios ranging from less than 1% in undamaged concrete to around 3% in damaged concrete