Recent Earthquakes in Japan

The paper presents highlights of case histories during earthquakes in Japan in 2003. One is a river embankment of the Naruse river in Northern Miyagiken, in which the earthquake with Richter magnitude 6.2 caused failure. A particular interest in this case history is the timing of the earthquake and failure; the earthquake was coincided with the oncoming risk of flooding, with the river suffering a high water level due to continuous raining for three days before the earthquake. This warns us not to disregard the low probability event of combined risks that pose high consequence. The other case history is a gravity quay wall in Kushiro port, Hokkaido, in which the earthquake with Richter magnitude 8.0 caused minor damage. Of a particular interest in this case history is the performance of a quay wall with backfill treated with cement for solidification, which suffered settlements in the order of 0.5m. Other quay walls in the vicinity treated with densification and gravel drains suffered no damage. The investigation is under way with respect to the difference in the performance of these quay walls

Numerical Analysis of Trampoline Effect in Extreme Ground Motion

Very large vertical surface acceleration of nearly four times gravity was measured at a strong motion observation station in Iwate Prefecture during the 2008 Iwate-Miyagi Inland, Japan, earthquake (Mw 6.9). The station is located about 3 km southwest of the epicenter and equipped with three-component accelerometers, installed at both the free surface and the bottom of a 260-m borehole. The wave form of the vertical acceleration shows a clearly asymmetric form with large amplitude in the upward direction. Aoi et al. (2008) reported and qualitatively explained the mechanism of this phenomenon by the analogy of bouncing a piece of matter on a trampoline, and thus they called it the “trampoline effect.” To simulate this recently discovered nonlinear behavior of the surface ground motion, numerical analysis with a finite-element method has been employed with parameters derived from the borehole data at the station. The analysis successfully simulates the asymmetric vertical motion. Results indicate that the asymmetric motion may be characterized by the existence of a lower bound of negative acceleration, which in most cases corresponds to the acceleration of gravity, and high positive pulses caused by the compression stress of the disturbed surface ground material

Scaling law and centrifuge modelling in geotechnical engineering

科学研究費助成事業基盤研究(B)(26282103,2014-2016)基盤研究(C)(25420502,2013-2015)遠心力載荷模型実験と数値解析に関する国際プロジェクト(LEAP)解

Non-linear Site Response Analysis of Soil Sites in Northern Thailand during the Mw 6.8 Tarlay Earthquake

On 24 March 2011, the 6.8 Mw Tarlay Earthquake occurred at the border of Thailand and Myanmar. The earthquake not only resulted in structural building damage but also triggered liquefactions on sandy soils in Northern Thailand. Several site investigations including SPT and shear wave velocity measurements are conducted to study subsoils condition in this area. Ground motions at several seismic stations were recorded during the earthquake. In this study, four soil sites in Chiang Rai and Chiang Mai are selected, including a site at the border of Thailand and Myanmar. Next generation attenuation models are employed to generate the input ground motions for each site. Non-linear finite element analysis is employed to observe soil behaviour under the earthquake. The results showed that liquefaction could happen in the investigated area during an earthquake. The result is confirmed by the liquefaction evidence found in Chiang Rai during the 6.8 Mw Tarlay Earthquake. This research can help raise awareness of the impacts of earthquakes to this region

LEAP-2017: Comparison of the Type-B Numerical Simulations with Centrifuge Test Results

This paper presents comparisons of 11 sets of Type-B numerical simulations with the results of a selected set of centrifuge tests conducted in the LEAP-2017 project. Time histories of accelerations, excess pore water pressures, and lateral displacement of the ground surface are compared to the results of nine centrifuge tests. A number of numerical simulations showed trends similar to those observed in the experiments. While achieving a close match to all measured responses (accelerations, pore pressures, and displacements) is quite challenging, the numerical simulations show promising capabilities that can be further improved with the availability of additional high-quality experimental results

Performance-Based Design of Geotechnical Structures: Recent Advances

The paper presents an overview of recent advances in earthquake geotechnical engineering with respect to the seismic design of geotechnical structures. The modern principles in seismic design are described along the framework of performance-based design as adopted in the International Standard (ISO23469). With the growing awareness of the need to understand the effect of non-linearity in soils and soil-structure interaction, the paper discusses the highly non-linear response of ground during strong earthquake motions with a peak ground acceleration exceeding 1g, and the highly non-linear behavior of soil-pile interaction, including soil-pile separation. The modern principles in seismic design described in this paper allow a sophisticated approach to deal with the uncertainty. Discussion on this issue is provided through the life-cycle cost approach. The paper also discusses the combined hazards, such as those during the Sumatra, Indonesia, earthquake of 2004, posing a new challenge to seismic design of geotechnical structures

Dilatancy of granular materials in a strain space multiple mechanism model

A granular material consists of an assemblage of particles with contacts newly formed or disappeared, changing the micromechanical structures during macroscopic deformation. These structures are idealized through a strain space multiple mechanism model as a twofold structure consisting of a multitude of virtual two-dimensional mechanisms, each of which consists of a multitude of virtual simple shear mechanisms of one-dimensional nature. In particular, a second-order fabric tensor describes direct macroscopic stress–strain relationship, and a fourth-order fabric tensor describes incremental relationship. In this framework of modeling, the mechanism of interlocking defined as the energy less component of macroscopic strain provides an appropriate bridge between micromechanical and macroscopic dilative component of dilatancy. Another bridge for contractive component of dilatancy is provided through an obvious hypothesis on micromechanical counterparts being associated with virtual simple shear strain. It is also postulated that the dilatancy along the stress path beyond a line slightly above the phase transformation line is only due to the mechanism of interlocking and increment in dilatancy due to this interlocking eventually vanishing for a large shear strain. These classic postulates form the basis for formulating the dilatancy in the strain space multiple mechanism model. The performance of the proposed model is demonstrated through simulation of undrained behavior of sand under monotonic and cyclic loading

飽和地盤に対する拡張型相似則の適用性

"Modeling of models"の手法を用い, 動的遠心模型実験に用いられる拡張型相似則(Iai, et al. 2005)の適用性を検討する。実物の100分の1の飽和砂質水平成層地盤を対象に, 遠心加速度を5gから70gに変化させ実験を行う。模型地盤に対し, プロトタイプスケールで最大入力加速度振幅2.5m/s2と3.1m/s2, 振動数0.65Hzの正弦波を与えた。いずれの実験ケースにおいても, 振動中の応答加速度, 水圧の挙動については, プロトタイプ換算値でほぼ一致した。しかし, 振動後の過剰間隙水圧の消散過程において, 10g以下の低い遠心加速度を用いた場合に, 50g以上の高い遠心場の場合と比較して, 消散時間が4倍以上長くなる現象が見られた。低い遠心場では, 有効拘束圧が小さいため地盤の弾性係数が小さく, 圧密理論から予測されるように, 圧密時間(=水圧消散時間)が長くなるものと推察される。A series of centrifuge model tests are conducted under the scheme of the modeling of models technique to find issues on the generalized scaling law for dynamic centrifuge tests (Iai et al., 2005). In a series of dynamic tests on a flat, saturated sand layer of 1/100 scale, four different centrifugal accelerations from 5 g to 70 g are employed on the scaled models for which the prototype is uniquely given. The models are exposed to sinusoidal input accelerations with 0.65 Hz and amplitudes of 2.5 m/s2 and 3.1 m/s2 in prototype scale. For response during shaking, nearly identical accelerations and excess pore-water pressure buildups are recorded for all the cases in the prototype scale. Discrepancies are found on surface settlements and duration time for dissipation of excess pore-water pressure. The major causes of the discrepancy may be (1) the duration time for the initial consolidation, (2) small value of the shear modulus due to low confining stress in model ground under low centrifugal acceleration, and (3) reduced permeability due to adsorption of viscous fluids on sand particles."Modeling of models"の手法を用い, 動的遠心模型実験に用いられる拡張型相似則(Iai, et al. 2005)の適用性を検討する。実物の100分の1の飽和砂質水平成層地盤を対象に, 遠心加速度を5gから70gに変化させ実験を行う。模型地盤に対し, プロトタイプスケールで最大入力加速度振幅2.5m/s2と3.1m/s2, 振動数0.65Hzの正弦波を与えた。いずれの実験ケースにおいても, 振動中の応答加速度, 水圧の挙動については, プロトタイプ換算値でほぼ一致した。しかし, 振動後の過剰間隙水圧の消散過程において, 10g以下の低い遠心加速度を用いた場合に, 50g以上の高い遠心場の場合と比較して, 消散時間が4倍以上長くなる現象が見られた。低い遠心場では, 有効拘束圧が小さいため地盤の弾性係数が小さく, 圧密理論から予測されるように, 圧密時間(=水圧消散時間)が長くなるものと推察される。A series of centrifuge model tests are conducted under the scheme of the modeling of models technique to find issues on the generalized scaling law for dynamic centrifuge tests (Iai et al., 2005). In a series of dynamic tests on a flat, saturated sand layer of 1/100 scale, four different centrifugal accelerations from 5 g to 70 g are employed on the scaled models for which the prototype is uniquely given. The models are exposed to sinusoidal input accelerations with 0.65 Hz and amplitudes of 2.5 m/s2 and 3.1 m/s2 in prototype scale. For response during shaking, nearly identical accelerations and excess pore-water pressure buildups are recorded for all the cases in the prototype scale. Discrepancies are found on surface settlements and duration time for dissipation of excess pore-water pressure. The major causes of the discrepancy may be (1) the duration time for the initial consolidation, (2) small value of the shear modulus due to low confining stress in model ground under low centrifugal acceleration, and (3) reduced permeability due to adsorption of viscous fluids on sand particles