Brent L. Rosenblad

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.

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.

Performance of Active and Passive Methods for Measuring Low-Frequency Surface Wave Dispersion Curves

This paper examines the consistency between surface wave dispersion curves measured at wavelengths of up to 600 m using active and passive methods at sites in the Mississippi Embayment. Large-diameter (200 m) circular receiver arrays were deployed at five deep soil sites located in Tennessee, Arkansas, and Missouri to record ambient ground vibrations at low frequencies. Measurements were performed at the same locations using linear receiver arrays and actively generated low-frequency energy using the recently developed Network for Earthquake Engineering Simulation (NEES) field vibrator (termed Liquidator). Characteristics of the ambient wavefield measured at the five sites in the Mississippi Embayment are presented along with comparisons between the surface wave dispersion curves obtained from the active and passive measurements at each site. The ambient wavefield measurements exhibited peak levels in the frequency range of 1–5 Hz. Surface wave dispersion curves developed from frequency-wavenumber (f-k) processing of the active and passive methods were in good agreement at four of the sites, with phase velocities from the passive measurements within 5–10 % of the active-source measurements out to wavelengths of about 550 m. Improved comparisons were obtained at the fifth site by applying high-resolution f-k processing.

Potential Phase Unwrapping Errors Associated with SASW Measurements at Soft-Over-Stiff Sites

Results from simulations of spectral-analysis-of-surface-waves (SASW) measurements are presented for the common geotechnical condition of soft surficial soils overlying a stiffer half-space. Several recent experimental and numerical studies have demonstrated the dominance of higher-mode surface wave propagation at low frequencies for profile conditions where strong, shallow contrasts in elastic properties exist. This transition to higher-mode propagation has proven problematic for methods utilizing a fundamental-mode forward model in the inversion procedure, resulting in overprediction of the shear wave velocity (VS) profile. The objective of this study is to investigate the effectiveness of the SASW methodology, which uses an effective-velocity forward model, in dealing with the soft-over-stiff profile condition. Specifically, this study focuses on the phase unwrapping procedure used in the SASW method. Nine VS profiles were simulated in this study, representing different conditions of VS contrast and depth to the stiffer layer. The results show that in some cases the phase unwrapping procedure produces an experimental dispersion curve that is inconsistent with both the fundamental mode and the true effective-velocity dispersion curve for the profile. The resulting VS profile is substantially under-predicted. Experimental results from measurements at a soft-over-stiff site are presented that support the findings from the simulated measurements.

Comparative Study of Refraction Microtremor (ReMi) and Active Source Methods for Developing Low-Frequency Surface Wave Dispersion Curves

Obtaining high-quality dispersion curves is a critical step in the development of reliable shear wave velocity profiles from surface wave measurements. Because of its limited equipment and space requirements, the refraction microtremor (ReMi) method has become a popular approach for determining surface wave dispersion curves, and is increasingly being used for estimating low-frequency (long wavelength) surface wave velocities that are beyond the range of most active sources. The recent development of a low-frequency field vibrator as part of the Network for Earthquake Engineering Simulation (NEES) program has made it possible to actively generate surface wave energy down to frequencies of less than 1 Hz. This paper presents a comparative study of the ReMi method and the active-source frequency-wavenumber (f‐k) method (using the NEES vibrator) for developing low-frequency dispersion curves. Linear arrays of 1-Hz seismometers were deployed at eight deep soil sites in the Mississippi Embayment. Using both am...

Super-Accelerated Testing of Flexible Pavement with Stationary Dynamic Deflectometer

The Texas Department of Transportation (TxDOT), in partnership with the Center for Transportation Research, has implemented the Texas mobile load simulator (TxMLS) as a tool for accelerated testing of in-service pavements. Although the TxMLS has been used successfully to test in-service pavements in the Yoakum and Fort Worth districts, the fact that only one TxMLS machine exists limits the number of accelerated pavement tests (APT) that can be performed. Therefore, TxDOT is evaluating a modification of the rolling dynamic deflectometer (RDD) for use as a super-accelerated pavement tester. In this application, the truck-mounted dynamic loading system is operated in a stationary mode, with the loading rollers and rolling sensors of the RDD removed from operation. The servohydraulic actuator is used for application of harmonic loading to a wheel footprint on the pavement surface. Hundreds of thousands of load repetitions are applied in a matter of hours; hence the designation as super-accelerated testing. This stationary dynamic deflectometer (SDD) is being studied as a possible tool for use in expanding TxDOT’s APT program. The SDD may allow TxDOT to increase, in a cost-effective manner, the number of accelerated tests that can be performed. Preliminary tests have been performed with the TxMLS and SDD on two different pavement recycling strategies constructed on the northbound and southbound lanes of US-281 in the Fort Worth District. That the same conclusion was reached about the relative performance of the test sections with both machines indicates the potential usefulness of the SDD.

Influence of Local Site Conditions on the Reliability of Fundamental-Mode Surface Wave Inversion Methods

The development of reliable shear wave velocity (VS) profiles from surface wave velocity measurements requires compatibility between the measured and theoretical dispersion curves used in the inversion procedure. The most common approach in geotechnical applications is to use a fundamental-mode theoretical dispersion curve based on the assumption that the measured dispersion curve is representative of the fundamental mode of surface wave propagation. Analyses of low-frequency surface wave measurements presented in this paper show that this widespread assumption is invalid at low frequencies (long wavelengths) for some profile conditions. Results are presented from measurements performed at two deep soil sites in the central United States where surface wave analyses were performed using both the common fundamental-mode approach as well as an “effective-velocity” approach. At one site these two methods produced essentially the same estimate of the VS profile, while at the second site the estimated VS values at depth differed by more than 40 %. Using soil profile information for these sites and simulations of surface wave propagation, it is shown that the shallower depth to a higher-velocity formation at the second site created dominant higher-mode energy at wavelengths of 300–600 m, resulting in an overestimation of deep VS values.