Estimation of Local Site Conditions in Kushiro City Based on Array Observation of Microtremors

Possible use of short-period microtremors is explored for estimating the effects of subsurface soil conditions on the ground motion characteristics. For this purpose, microtremor measurements are conducted using arrays of sensors at two strong motion stations (Kushiro Japan Meteorological Agency and Kushiro Harbor, Hokkaido, Japan), which are located nearby but on different soils. Based on the F-k spectrum analysis of microtremors, dispersion curves of Rayleigh waves for the sites are determined. The inverse analysis of these dispersion curves results in shear wave velocity profiles down to a depth of 300 m. With these profiles, the spectrum ratio of ground surface motions between the two sites is computed, and compared with that of the observed records. The computed and observed spectrum ratios show a fairly good agreement, indicating that the array observation of microtremors is an economical and yet reliable means of estimating local site conditions

Effects of Soil-Structure Interaction on Stress Distribution within a Pile Group under Multi-Dimensional Loading

To investigate inertial and kinematic effects on stress distributions within a pile group, physical model tests were conducted at EDefense, one of the largest shaking table facilities in the world. A 3x3 steel pile group supporting a foundation with a superstructure was set in a dry sand deposit prepared in a cylindrical laminar box with a height of 6.5 m and a diameter of 8.0 m. Natural periods of superstructures were variable in the tests. The tests were conducted under one-, two- or three-dimensional shaking. The test results have shown that pile stresses were mainly controlled by the inertial force when the natural period of superstructure was shorter than or close to that of the ground. In this case, the pile group effects were remarkable, in which pile stresses were the largest in the leading pile and the smallest in the following pile. In contrast, the pile stresses were mainly controlled by the ground displacement when the natural period of superstructure was longer than that of the ground. In this case, the pile group effects were insignificant in such a way that the pile stresses were almost the same within the pile group

Site Effects Estimated From Microtremor Measurements at Selected Strong Motion Stations in Taiwan

Microtremor measurements are conducted using arrays of sensors at six strong motion stations in Taiwan where the peak ground accelerations over 400 cm/s2 were recorded during the 1999 Chi-Chi earthquake. Shallow shear wave velocity profiles of these stations are estimated based on an inverse analysis of microtremor dispersion characteristics and H/V spectra. At Wufeng (TCU065) where building damage was the most extensive among others, a thick soft surface layer with a shear wave velocity less than or equal to about 200 m/s overlies a stiff layer with Vs greater than 400 m/s at a depth of about 30 m. At other stations, stiff layers with Vs greater than 300-400 m/s occur from the ground surface or at a depth less than 20 m. Linear and equivalent linear analyses are conducted using the estimated Vs profiles. The natural site periods computed from an equivalent linear analysis are generally consistent with the peak periods of the response acceleration spectra of the recorded strong motions, but they are considerably longer than those estimated from a linear analysis, regardless of the value of the shallow shear wave velocity. This suggests that local soil conditions including nonlinear soil behavior might have had significant effects on the ground surface motion and performance of buildings during the earthquake

Liquefaction Potential Evaluation Based on Rayleigh Wave Investigation and Its Comparison with Field Behavior

A simplified method is presented for evaluating liquefaction potential of sand deposits using shear wave velocity. Effectiveness of the proposed method is evaluated through field tests at 17 sites in Niigata city where field performance during the 1964 Niigata earthquake is known. A modified version of steady state Rayleigh wave method is used in which the amplitude ratio between vertical and horizontal ground surface motions can be measured in addition to the phase velocity. Based on the measured phase velocity vs. wavelength relationship, shear wave velocity profile is determined using an inverse analysis. The liquefaction potential of each site is then evaluated using the shear wave velocity. The estimated results are reasonably consistent with the actual field behavior during the earthquake, indicating that the proposed method is effective

Back-Calculated p-y Relation of Liquefied Soils from Large Shaking Table Tests

Time histories of the p-y behavior during soil liquefaction, defined as the relation between subgrade reaction and relative displacement between pile and soil, are back-calculated based on shaking table tests using a large-scale laminar box. The results show that, if the pile pushes the soil, the subgrade reaction is correlated with the relative displacement between pile and soil. In contrast, if the soil liquefies and pushes the pile, the subgrade reaction becomes correlated with the relative velocity between pile and soil. The p-y curve of loose sand shows stress-softening behavior after liquefaction, while the p-y curve of medium dense to dense sand shows stress hardening behavior. The stress-hardening behavior tends to diminish with cyclic loading after liquefaction if the sand is not sufficiently dense or the input acceleration is high. The coefficient of subgrade reaction is affected by such factors as the pore pressure ratio, relative displacement, and soil density

Centrifuge Tests and Simple Analyses for Seismic Soil-Structure Interaction

Dynamic centrifuge tests were performed on a superstructure-footing model that was placed on a dry sand surface and subjected to two different input motions having peak accelerations of 60 cm/s2 and 249 cm/s2. Two simple analyses, equivalent linear analysis (SHAKE) and dynamic response of a structure using a sway-rocking model (SR-model) were performed. The following conclusions were drawn: (1) SHAKE and SR-model analyses can simulate the recorded response of the soil and superstructure. However, the shear wave velocity of the ground that can simulate the superstructure response by an SR-model for amax=249 cm/s2 is much smaller than that of the free field estimated using SHAKE. (2) The observed relation of the base friction force with relative displacement between the footing base and the ground surface shows strong nonlinearity when amax=249 cm/s2, which probably results from the large shear deformation of the thin layer beneath the footing

Effective Stress Analysis of Pile Foundations Showing Various Damage Patterns in Liquefied Deposits During 1995 Hyogoken-Nambu Earthquake

Effective stress analysis is conducted for six buildings that suffered various patterns of damage during soil liquefaction in the 1995 Hyogoken-Nambu earthquake, in order to examine major causes of the damage as well as the effectiveness of the analytical procedure. A comparison of the computed result with the filed observation indicates that the effective stress analysis is capable of discriminating damaged from undamaged foundations as well as of estimating the damage portion and severity with a reasonable degree of reliability. The analytical result also shows that: (1) the damage to pile heads is mainly due to the inertia force from the superstructure and the damage at depths below the pile head is mainly due to the kinematic force resulting from ground displacements; (2) because of their ductile behavior, steel reinforced concrete piles are immune from extensive damage; and (3) to enclose a pile foundation with diaphragm walls can reduce pile damage but can increase the response of the superstructure as well as the shear force and moment particularly in the lower levels of buildings

Soil-Pile-Structure Interaction During Liquefaction

An analytical method is presented for evaluating dynamic response of soil-pile-structure system during soil liquefaction. The method consists of a modified Penzien\u27s model combined with an effective stress analysis for free-field soil response and a horizontal subgrade reaction model which connects free field response with pile-structure response. Shaking table tests are conducted to study the effectiveness of the proposed procedure. A model sand deposit-pile-structure system is constructed in a large container which can permit shear deformation of the soil. Soil density, pile diameter, pile rigidity, and input motion are controlling variables in the tests. Analysis is made for both liquefied and non-liquefied cases, and the results are compared with the measured values. The analytical results including the time histories of excess pore water pressures, accelerations, and displacements, and the Fourier spectra of the ground surface and the pile head, are all in good accord with the observed values, showing that the proposed analysis is effective

トカマク型核融合動力炉の経済性及び環境適合性に関する定量的評価研究）

University of Tokyo (東京大学

Preliminary Report on the Geotechnical Aspects of the Philippine Earthquake of July 16, 1990

The Philippine earthquake of July 16, 1990 (MS = 7.8), of which epicenter is about 100 km north of Manila city, was one of the most costly single natural disasters in Philippine history. The loss of life of over 1,600 persons resulted. Extensive damage to buildings, roads, embankments, natural slopes, and bridges was observed in a widespread area of approximately 20,000 square kilometers. One of the major causes of the damage was liquefaction of various sandy soils including artificially fills, alluvial deposits of river delta, and sandbars. This paper presents a preliminary overview of damage aspects of the earthquake, with emphasis on liquefaction-induced damage of various structures