Kohji Tokimatsu

S-Wave Velocity Profiling by Inversion of Microtremor H/V Spectrum

A method for estimating the S -wave velocity ( V S ) profile of subsurface soils is proposed, based on inversion of the horizontal-to-vertical (H/V) spectrum of microtremors observed with a three-component sensor. For this purpose, theoretical formulas are derived for computing the H/V spectrum of surface waves propagating on a layered half-space in which the effects of the fundamental and higher modes are taken into account. An inverse analysis using microtremor H/V spectrum is then presented for estimating S -wave velocity profiles of subsurface soils. Assuming that either the V S values or the thicknesses of the shallow soil layers are known, the remaining unknowns are sought. The inverse analyses are performed using the H/V spectra observed at six sites, and their shallow V S profiles are estimated. The inverted S -wave velocity profiles are consistent with available downhole velocity logs at the sites. The standard error ratios of the inverted values are less than about 0.1, with a maximum of 0.2.

S-Wave Velocity Profiling by Joint Inversion of Microtremor Dispersion Curve and Horizontal-to-Vertical (H/V) Spectrum

A joint inverse analysis using both microtremor dispersion curve and horizontal-to-vertical (H/V) spectrum is proposed for estimating the S-wave velocity (VS) profiles of subsurface soils. In the inversion, both microtremor dispersion and H/V data are assumed to be the Rayleigh-wave dispersion curve and the surface (both Rayleigh and Love) wave H/V spectrum that have been theoretically derived by taking into account the effects of their fundamental and higher modes. The proposed joint inversion as well as the conventional one using dispersion data alone is per- formed at four sites where shallow VS profiles down to engineering bedrock are available. The VS profiles estimated by the proposed joint inversion are more con- sistent with available down-hole velocity logs than those by the conventional method. In particular, the proposed inversion shows significant improvement in estimating bedrock VS structures compared to the conventional inversion. Sensitivity analyses indicate that the surface-wave H/V ratio is sensitive to the bedrock VS structure more than the Rayleigh-wave phase velocity, confirming that the proposed joint inversion including H/V spectrum is promising.

Shear-Wave Velocity-Based Probabilistic and Deterministic Assessment of Seismic Soil Liquefaction Potential

AbstractShear-wave velocity (Vs) offers a means to determine the seismic resistance of soil to liquefaction by a fundamental soil property. This paper presents the results of an 11-year international project to gather new Vs site data and develop probabilistic correlations for seismic soil liquefaction occurrence. Toward that objective, shear-wave velocity test sites were identified, and measurements made for 301 new liquefaction field case histories in China, Japan, Taiwan, Greece, and the United States over a decade. The majority of these new case histories reoccupy those previously investigated by penetration testing. These new data are combined with previously published case histories to build a global catalog of 422 case histories of Vs liquefaction performance. Bayesian regression and structural reliability methods facilitate a probabilistic treatment of the Vs catalog for performance-based engineering applications. Where possible, uncertainties of the variables comprising both the seismic demand an...

New charts for predicting large residual post-liquefaction ground deformation

Abstract Significant ground settlements and lateral spreading occurred extensively during past strong earthquakes, not only in liquefied soils where static driving shear stresses were present, but also in liquefied level sandy ground with a sufficiently large lateral extent. Their generic causes were revealed based on a new post-liquefaction stress–strain constitutive analysis. It was found that the residual post-liquefaction ground settlement and lateral spreading are not independent of each other, so, in principle, both cannot separately be evaluated. A new, unified methodology and related pragmatic charts were developed to evaluate simultaneously the two types of ground deformation. The effectiveness of the proposed method was confirmed favorably with actual observations during the 1995 Hyogoken-Nambu earthquake.

Elastic and Large-Strain Nonlinear Seismic Site Response from Analysis of Vertical Array Recordings

AbstractStrong ground motions from the Mw=6.6 2007 Niigata-ken Chuetsu-oki earthquake were recorded by a free-field downhole array at a nuclear power plant. Site conditions consist of about 70 m of medium-dense sands overlying clayey bedrock, with groundwater located at 45 m. Ground shaking at the bedrock level had a geometric mean peak acceleration of 0.55g, which reduced to 0.4g at the ground surface, indicating nonlinear site response. One-dimensional ground response analysis of relatively weak motion aftershock data provides good matches of the observed resonant site frequencies and amplification levels, provided small-strain damping levels somewhat larger than those from laboratory tests are applied. Nonlinear ground response analyses of strong-motion data using laboratory-based modulus reduction and damping relations valid up to moderate strain levels (<∼0.5%) produce unrealistic strain localization at a velocity contrast. A procedure is presented to more realistically represent the large-strain por...

Liquefaction-Induced Lateral Load on Pile in a Medium Dr Sand Layer

One-g shake-table experiments are conducted to explore the response of single piles due to liquefaction-induced lateral soil flow. The piles are embedded in saturated Medium Relative Density (Dr) sand strata 1.7–5.0 m in thickness. Peak lateral pile displacements and bending moments are recorded and analyzed by uniform and triangular pressure distributions. On this basis, the observed levels of pile bending moment upon liquefaction suggest a hydrostatic lateral pressure approximately equal to that due to the total overburden stress. Using the experimental data, comparisons with current recommendations are made, and the Showa Bridge case history is briefly assessed.

Liquefaction at Strong Motion Stations and in Urayasu City during the 2011 Tohoku-Oki Earthquake

The 2011 MW = 9.0 Tohoku-oki earthquake generated a large number of unique soil liquefaction case histories, including cases with strong ground motion recordings on liquefiable or potentially liquefiable soils. We have compiled a list of 22 strong motion stations (SMS) where surface evidence of liquefaction was observed and 16 SMS underlain by geologically recent sediments or fills where surface evidence of liquefaction was not observed. Pre-earthquake standard penetration test data and borehole shear wave velocity (Vs) profiles are available for some stations, but critical information, such as grain size distribution and fines plasticity, are often lacking. In the heavily damaged city of Urayasu, we performed post-earthquake cone penetration testing at seven SMS and Vs profiles, using surface wave methods at 28 additional locations to supplement existing geotechnical data. We describe the liquefaction effects in Urayasu, the available site characterization data, and our initial data interpretations.

Probabilistic Liquefaction Triggering Based on the Cone Penetration Test

Performance-based earthquake engineering requires a probabilistic treatment of potential failure modes in order to accurately quantify the overall stability of the system. This paper is a summary of the application portions of the probabilistic liquefaction tri ggering correlations proposed recently proposed by Moss and co -workers. To enable probabilistic treatment of liquefaction triggering, the variables comprising the seismic load and the liquefaction resistance were treated as inherently uncertain. Supportin g data from an extensive Cone Penetration Test (CPT)-based liquefaction case history database were used to develop a probabilistic correlation. The methods used to measure the uncertainty of the load and resistance variables, how the interactions of these variables were treated using Bayesian updating, and how reliability analysis was applied to produce curves of equal probability of liquefaction are presented. The normalization for effective overburden stress, the magnitude correlated duration weighting factor, and the non-linear shear mass participation factor used are also discussed.

Soil Liquefaction–Induced Uplift of Underground Structures: Physical and Numerical Modeling

AbstractUnderground structures located in liquefiable soil deposits are susceptible to floatation following a major earthquake event. Such failure phenomenon generally occurs when the soil liquefies and loses its shear resistance against the uplift force from the buoyancy of the underground structure. Numerical modeling accompanied with centrifuge experiments with shallow circular structures has been carried out to investigate the floatation failure at different buried depths of the structure. The influence of the magnitude of input sinusoidal earthquake shaking was also studied. Both numerical and experimental results showed matching uplift response of the structures and acceleration and pore-pressure measurements in the liquefied soil deposit. A higher uplift displacement of the structure was observed for shallower buried depth, thereby indicating the influence of overlying soil weight against floatation. Results also showed that the structures commenced floatation in the presence of high excess pore pr...

Seismic Earth Pressure Acting on Embedded Footing Based on Large-Scale Shaking Table Tests

This paper examines earth pressure acting on an embedded footing and its effects on pile forces, based on both liquefaction and non-liquefaction tests using a large-scale laminar shear box. The following conclusions are drawn: (1) The total earth pressure defined by the difference in earth pressure between passive and active sides in the non-liquefaction tests varies significantly depending on its phase relative to the soil inertia around the embedded footing as well as on the relative displacement between soil and footing; (2) The total earth pressure in the liquefaction test, by contrast, depends mainly on the relative displacement because the soil inertia gets small in liquefied soil; (3) The total earth pressure in the non-liquefaction tests tends to be out of phase by 180 degrees with the superstructure inertia, reducing the shear force and bending moment at the pile head; and (4) The total earth pressure in the liquefaction tests tends to be in phase with the superstructure inertia, making the bending moment at the pile head large. A method for estimating the total earth pressure considering its phase relative to the superstructure inertia as well as the effects of soil inertia has been proposed. The proposed method gives a reasonable explanation of the difference in earth pressure between different tests.