Energy dissipation and seismic liquefaction in sands

A statistical representation of seismic liquefaction is advanced based on the postulate that pore water pressure increases are proportional to the dissipated seismic energy density. The representation, based on approximately fifty case histories, relates the pore pressure increase to earthquake magnitude, distance to centre of energy release, initial effective overburden stressand standard penetration value. The model may be used for analysis ofseismic liquefaction risk. An example analysis for the ‘South of Market Zone’ in San Francisco is carried out in relation to earthquakes on the San Andreas fault.     

Requirements for soil-specific correlation between shear wave velocity and liquefaction resistance of sands

The application of the simplified method for evaluating the liquefaction potential based on shear wave velocity measurements has increased substantially due to its advantages, especially for microzonation of liquefaction potential. In the simplified method, a curve is proposed to correlate the cyclic resistance ratio (CRR) with overburden stress-corrected shear wave velocity (V_s1). However, the uniqueness of this curve for all types of soils is questionable. The objective of this research is to study whether the correlation between CRR and V_s1 is unique or not. Besides, the necessity of developing the soil-specific correlations is also investigated. Based on laboratory test data, a new semi-empirical method is proposed to establish the soil-specific CRR–V_s1 correlation. To validate the proposed method, a number of undrained cyclic triaxial tests along with bender element tests were performed on two types of sands. Similar experimental data for six other types of sands reported in the literature was also compiled. Applying the proposed method, soil-specific CRR–V_s1 correlation curves were developed for these eight types of sands. It is shown that the correlation is not unique for different types of sands and the boundary curve proposed in the available simplified method can only be used as an initial estimation of liquefaction resistance. Finally, using the results of this study as well as previous ones, a chart is suggested to be used in engineering practice showing the conditions for which a detailed soil-specific CRR–V_s1 correlation study needs to be performed.     

Threshold seismic energy and liquefaction distance limit during the 2008 Wenchuan earthquake

Estimating the possible region of liquefaction occurrence during a strong earthquake is highly valuable for economy loss estimation, reconnaissance efforts and site investigations after the event. This study identified and compiled a large amount of liquefaction case histories from the 2008 Wenchuan earthquake, China, to investigate the relationship between the attenuation of seismic wave energy and liquefaction distance limit during this earthquake. Firstly, we introduced the concept of energy absorption ratio, which is defined as the absorbed energy of soil divided by the imparted energy of seismic waves at a given site, and the relationship between the energy absorption ratio and the material damping ratio was established based on shear stress–strain loop of soil element and the seismic wave propagation process from the source to the site. Secondly, the threshold imparted seismic energy of liquefaction was obtained based on existing researches of absorbed energy required to trigger liquefaction of sandy soils and the ground motion attenuation characteristics of the 2008 Wenchuan earthquake, and the liquefaction distance limit of this earthquake was estimated according to the proposed magnitude–energy–distance relationship. Finally, the field liquefaction database of 209 sites of the 2008 Wenchuan earthquake was used to validate such an estimation, and the field observed threshold imparted seismic energy to cause liquefaction in recent major earthquakes worldwide was back-analyzed to check the predictability of the present method, and several possible mechanisms were discussed to explain the discrepancy between the field observations and the theoretical predictions. This study indicates that seismic energy attenuation and liquefaction distance limit are regional specific and earthquake dependent, and 382 J/m³ is the average level of threshold imparted seismic energy to cause liquefaction for loose saturated sandy soils, and the corresponding liquefaction distance limit is approximately 87.4 km in fault distance for a M_w?=?7.9 event in the Chengdu Plain. The proposed regional energy attenuation model and threshold imparted seismic energy may be considered as an approximate tool in evaluating the liquefaction hazard during potential earthquakes in this area.     

Local magnitude,surface wave magnitude and seismic energy

Summary The local magnitude M_L at the seismological station Pruhonice (PRU) was converted into surface wave magnitude M_S using the formula M_S=–3.2+1.45 M_L and the seismic wave energy was estimated using the relation log E (Joule)=1.2+2.0 M_L. It was proposed to apply the same conversion formulae at seismological stations Kaperské Hory (KHC) and Berggiesshübel (BRG) where the calibrating functions for local magnitudes were determined for the same set of earthquakes with common reference magnitudes as in the case of the PRU station.     

Strain energy based evaluation of liquefaction and residual pore water pressure in sands using cyclic torsional shear experiments

In this study, cyclic hollow cylinder torsional tests were conducted on the reconstituted specimens of Toyoura sand in a practical range of initial density and stress states. The results were employed to evaluate the liquefaction resistance and residual pore water pressure of sand using the strain energy concept. A simple pore water pressure (PWP) model with two calibration parameters was developed for the prediction of residual pore pressure as a function of cumulative strain energy density and the capacity energy of sand. Capacity energy is defined as the cumulative strain energy that is required for liquefaction onset. Based on the results of the tests, an equation is then presented for the estimation of capacity energy in terms of relative density and initial effective confining pressure of sand. This equation is shown to work well as a state boundary curve, which can discriminate between the liquefied and non-liquefied field case histories. Several extra tests were also performed to investigate the effect of initial static shear stress on the proposed PWP model and capacity energy. The results show that initial shear stress has a minor effect on the trend of the proposed PWP model; however, it definitely affects the capacity energy. The final part of the paper aims to confirm reasonable performance of the proposed PWP model by the available observations of seismically induced pore water pressure in shaking table, centrifuge, and real site conditions.     

Characterization of liquefaction resistance in gravelly soil: large hammer penetration test and shear wave velocity approach

Gravelly soil is generally recognized to have no liquefaction potential. However, liquefaction cases were reported in central Taiwan in the 1999 Chi-Chi Taiwan earthquake and in the 1988 Armenia earthquake. Thus, further studies on the liquefaction potential of gravelly soil are warranted. Because large particles can impede the penetration of both standard penetration test and cone penetration test, shear wave velocity-based correlations and large hammer penetration tests (LPT) are employed to evaluate the liquefaction resistance of gravelly soils. A liquefied gravelly deposit site during the Chi-Chi earthquake was selected for this research. In situ physical properties of soil deposits were collected from exploratory trenches. Instrumented LPT and shear wave velocity (V_s) measurements were performed to evaluate the liquefaction resistance. In addition, large-scale cyclic triaxial tests on remolded gravelly soil samples (15 cm in diameter, 30 cm in height) were conducted to verify and improve LPT-based and V_s-based correlations. The results show that the LPT and shear wave velocity methods are reasonably suitable for liquefaction assessment of gravelly soils.     

Investigation of liquefaction and pore water pressure development in layered sands

This paper presents the results of shaking table model tests which were carried out to investigate the pore water pressure generation and related liquefaction mechanism in layered sand deposits. The experiments were performed on uniform sand columns, silt interlayered sand columns and two layered sand columns deposited at various relative densities and subjected to different input excitations. During the experiments excess pore water pressures were measured by pore pressure transducers installed at three different depths and, surface settlements and thickness of water film developed under less permeable inclusions were measured by a digital camera. The experimental results are discussed and compared to demonstrate the effects of relative density, input acceleration and presence of a silt seam on the generation of excess pore water pressure in sand deposits subjected to dynamic loading. The results showed that the presence of a less permeable silt interlayer within the sand deposit and existence of a loose sand layer underlying dense sand deposits can have significant effect on the pore water pressure generation mechanism.     

用能量累积法检测地震波雷达信号

将具有高度重复性的地震波雷达长时间向地壳内发射线性调频信号,经过地下介质的传播后到达地面,用地震仪器在监测点和检测点记录下来,通过分析数据来了解地壳速度结构及波速变化.线性调频信号是一种非平稳信号,它的频率随时间线性变化,有很好的能量聚集性,非常适合做时间-频率分析.本文用短时傅里叶变换对监测点的信号进行时间-频率分析,以检验地震波雷达发射信号的时间和频率是否和控制系统一致.通过Wigner-Ville分布将地震波雷达发射的信号能量聚集在线性调频直线上,再用Hough变换累积聚集的能量形成波峰,按照线性调频直线的倾角提取波峰所在行,计算到时后构成地震波走时曲线图.用靠近本次实验地点的H-21剖面得到的地壳速度结构正演该测线的Pg、Sg、PmP和SmS的折合走时曲线,并与用能量累积法提取出的地震波走时曲线进行对比,分析结果表明:地震波雷达发射线性调频信号的时间和频率都符合控制要求,重复性高达99.9%以上,可以清晰地分辨出Pg、Sg震相,并且PmP和SmS震相可辨.     

Effects of seismic input,fine crust and existing structure on liquefaction from centrifuge model tests

The results of dynamic centrifuge tests carried out to study the effects of seismic input, fine crust and existing structure on liquefaction triggering and manifestations are presented. The basic concept of the experimentation was to analyse the seismic behaviour of level ground, saturated, 14 m deep sandy deposits, homogeneous or stratified, subjected to increasing seismic excitations up to liquefaction, with or without a one degree of freedom structure on shallow foundations. The study was performed in the framework of the European project Horizon2020 “LIQUEFACT”.

     

Simultaneous determination of the source mechanism and the seismic wave energy

Summary It is necessary to know the source mechanism for the determination of the seismic energy of seismic waves from the seismogram recorded at one station. For sparse data (selected events of the 1985/86 Western-Bohemia earthquake swarm) the source mechanisms cannot be determined fromP-wave first motions. Therefore, a new method of determining the source mechanism simultaneously with the energy calculated from observations at two stations has been developed. The method is described and tested. Advantages, disadvantages and the stability of the method are discussed.     

Application of Hilbert-Huang transform to characterize soil liquefaction and quay wall seismic responses modeled in centrifuge shaking-table tests

In this paper, a Hilbert-Huang Transform data-processing technique is successfully used to characterize the seismic responses of soil–quay wall systems using measured data in a series of geotechnical centrifuge shaking-table tests. The predominant frequency of a liquefied deposit shifts down to a low frequency level; however, “de-liquefaction” leads to frequent, local higher-frequency spikes in the time histories of predominant instantaneous frequency (PIF). A lower amount of seaward displacement was found if the combined translation and rotation modes resulted in lower excess-pore-water pressure when the wall accelerated seaward. Cyclic changes in the PIF of the wall during shaking are directly related to the stiffness of the soil in which the wall is embedded. Thus, PIF at any given instant provides a superior indicator for characterizing the occurrence of liquefaction and the time-varying soil dynamic property. This advantage assists in evaluating the degradation of soil dynamic properties at any given instant based on the acceleration time histories measured in model tests or even in the field.     

Applicability of existing empirical shear wave velocity correlations to seismic cone penetration test data in Christchurch New Zealand

Seismic piezocone (SCPTu) data compiled from 86 sites in the greater Christchurch, New Zealand area are used to evaluate several existing empirical correlations for predicting shear wave velocity from cone penetration test (CPT) data. It is shown that all the considered prediction models are biased towards overestimation of the shear wave velocity of the Christchurch soil deposits, demonstrating the need for a Christchurch-specific shear wave velocity prediction model (McGann et al., 2014) [1]. It is hypothesized that the unique depositional environment of the considered soils and the potential loss of soil ageing effects brought about by the 2010–2011 Canterbury earthquake sequence are the primary source of the observed prediction bias.     

Inverse determination of soil density and stress state using dispersion wave measurements and cone penetration tests in a non-layered soil

The determination of the void ratio and stress state distribution in undisturbed soil is still an ambitious aim which cannot be reached by sampling from bore holes. Therefore, an alternative method is proposed to determine soil density and stress state using dispersion wave measurements and cone penetration tests. Dispersion wave measurements and resonant-column tests are carried out to measure the shear wave velocity and shear modulus distribution with depth. Using finite element calculations a relationship between cone penetration resistance, stress state and void ratio is derived. From these results the void ratio distribution and the stress state can be calculated inversely. The inverse method is applied to real test data. The results of the inverse parameter determination are shown and assessed regarding the possibilities and the limitations of the presented method.     

我国标贯液化判别方法对黄土适用性研究

鉴于我国20世纪50年代以来,地震液化现场很少有黄土液化的实例,而饱和黄土甚至高含水率的黄土也具有很高的液化势和流态破坏势.为工程设计准确预测饱和黄土在设计地震动的作用下是否会液化,本文用Seed-Idriss简化判别法对兰州某民用机场的饱和黄土和砂土进行液化判别,并以此结果为依据检验规范判别式对黄土液化判别的适用性.结论证实用Seed-Idriss简化判别法结果检验规范方法对黄土的适用性是可行的,规范方法对于黄土液化判别过于保守.     

基于BP神经网络的近震中地震波和干扰波的识别

在ＢＰ神经网络的基础上,利用传统的ＳＴＡ／ＬＴＡ阈值分类法建立了一个用于区分地震波和干扰波的神经网络分类器,并做了检验。结果表明,具有良好的分类效果。     

地震波的场方程矩阵和能量的正定二次型及其意义

场方程是能够表述半空间地震波场的整体特征.波动方程、速度方程和能量方程.通过分析可知每个场方程都具有各自的“场方程矩阵”.能量方程能够对所有场方程矩阵进行综合和贯通,给出了能量方程以“弹性矩阵”为核心的普适性表达形式.最后,运用矩阵的正定二次型理论阐述了“能量矩阵与弹性矩阵”之间一致的对称性和正定性.能量矩阵蕴含的动态力的平衡关系、速度的时间_空间分布和能量的传播及变化的物理意义,能够从能量矩阵的正定二次型特性表述出来.本文研究分析问题的方法完全适用于复杂介质模型,相关的认识和结论可以拓展到均匀黏弹性各向同性介质、均匀弹性各向异性介质、均匀黏弹性各向异性介质以及比奥饱和流体介质.     

山东临沂地区砂土剪切波速与标准贯入击数关系统计分析

剪切波速(V_S)与标贯击数(N)之间存在相关关系,受地区土壤条件影响很大。对临沂地区场地实测得到的砂土的剪切波速和标贯击数之间的关系进行了统计分析,得到了砾砂、粗砂、中砂和细砂相应的关系曲线。结果发现受沉积环境的影响,砂土层粒径与埋深呈正相关,砂土粒径越大其密实段样本点数量越多。为消除不同密实程度段之间的相互影响,以密实度为划分标准进一步进行分区段统计分析,得到了不同密实程度的四类砂土的相关关系方程,通过实际钻孔数据对比了分段与不分段的统计分析结果,分段模拟能更好地反映两者之间的相关关系。将砂土根据密实度进行划分再给出剪切波速和标贯击数的回归关系可以提高分析结果的准确性,同时可以考虑土体特性的影响,更为科学。此研究为临沂地区提供了一种简便预估剪切波速的方法,对相关地区的工程建设和科学研究也具有参考价值。     

Wavelet analysis for relating soil amplification and liquefaction effects with seismic performance of precast structures

Precast concrete structures are preferred for facilities with large open areas due to easiness in construction. Such structures are typically composed of individual columns and long‐span beams, and are quite flexible and of limited redundancy. In this paper, nonlinear dynamic analyses of a typical such structure are conducted using as excitation 54 ground motions recorded on top of a variety of soils (hard, soft, and liquefied soil sites). The results show that liquefaction‐affected level‐ground motions systematically impose a greater threat to precast‐concrete structures in terms of seismic demand, even when low values of elastic spectral acceleration prevail, as opposed to soft‐soil records and even more to hard‐soil ones. Thus, elastic spectral acceleration appears to be an insufficient engineering demand parameter for design. Soil effects, the “signature” of which is born on ground motions, are first uncovered using wavelet analysis to detect the evolution of the energy and frequency content of the ground motion in the time domain. From this, the changes in effective (“dominant”) excitation period are noted, persuasively attributed to the nature of the soil, and finally correlated with the observed structural behavior. Copyright © 2016 John Wiley & Sons, Ltd.     

Shear wave velocity-based evaluation and design of stone column improved ground for liquefaction mitigation

The evaluation and design of stone column improvement ground for liquefaction mitigation is a challenging issue for the state of practice. In this paper, a shear wave velocity-based approach is proposed based on the well-defined correlations of liquefaction resistance (CRR)-shear wave velocity (V _s)-void ratio (e) of sandy soils, and the values of parameters in this approach are recommended for preliminary design purpose when site specific values are not available. The detailed procedures of pre- and post-improvement liquefaction evaluations and stone column design are given. According to this approach, the required level of ground improvement will be met once the target V _s of soil is raised high enough (i.e., no less than the critical velocity) to resist the given earthquake loading according to the CRR-V _s relationship, and then this requirement is transferred to the control of target void ratio (i.e., the critical e) according to the V _s-e relationship. As this approach relies on the densification of the surrounding soil instead of the whole improved ground and is conservative by nature, specific considerations of the densification mechanism and effect are given, and the effects of drainage and reinforcement of stone columns are also discussed. A case study of a thermal power plant in Indonesia is introduced, where the effectiveness of stone column improved ground was evaluated by the proposed V _s-based method and compared with the SPT-based evaluation. This improved ground performed well and experienced no liquefaction during subsequent strong earthquakes.