Liquefaction hazard mapping at the town of Edessa,Northern Greece

The town of Edessa is located on Northern Greece at a region that is characterized as low seismicity zone due to the fact that few moderate events of M < 6 occurred during the last century. According to the Greek Seismic Code, the expected acceleration having a 10% probability of being exceeded in 50 years is equal to 0.16g. However, an amplification of ground motion is likely to occur due the local geology that is consisted of Holocene fluvio-torrential deposits. The basic aim of this paper is to evaluate the site amplification due to geological conditions and to assess the liquefaction hazard. In order to achieve this, 1-D site response analyses were performed. The data that were employed for the construction of the numerical models have been collected from borings with standard penetrations tests (SPT) that were drilled for construction purposes. Afterward, the liquefaction potential of the subsoil layers was evaluated taking into consideration two seismic scenarios. The first scenario was based on the seismic parameters, earthquake magnitude and PGA, assigned by the Greek Seismic Code. On the second seismic model, we employed the values of acceleration, resulted from the 1-D analyses and the earthquake magnitude as it was defined by the Greek Seismic Code. In order to compile the liquefaction hazard maps, we initially estimated the liquefaction potential index (LPI) of the soil columns using the parameters provided by SPT, for both seismic loadings, and afterward we correlated these values with the proposed classification of the severity of liquefaction-induced deformations. In addition, having computed the value of probability based on the LPI, liquefaction manifestations probability maps were compiled for both scenarios. The result of this study was that liquefaction-induced ground disruptions are likely to occur at the center of the city, among the branches of Voda River, only when the amplified values of acceleration are taken into account to the computation of liquefaction potential.     

脉冲地震动中竖向加速度对场地液化的影响

为研究近断层脉冲地震动中竖向加速度对砂土场地液化的影响,基于有限元平台OpenSees开发的边界面塑性本构模型,建立了动单剪单元试验模型和饱和砂土三维有限元模型。选取台湾Chi-Chi地震中10条具有速度脉冲特性的地震波,对比分析了水平双向脉冲波与三向脉冲波作用下土柱竖向位移、循环应力比、孔压比及等效循环周数的差异性,继而明确了脉冲地震动中竖向加速度对砂土液化的影响规律。研究表明,三向脉冲地震波中竖向加速度分量对场地永久位移值影响较小,但使永久位移的发展持时明显增大;土柱循环应力比受竖向地震动影响较小,因此分析脉冲地震动对场地剪切特性的影响时,可将三向脉冲地震动简化为水平双向地震动;考虑竖向地震动的三向脉冲地震波引起的孔压比变化幅度较大,孔压消散时间较长;三向脉冲地震波对应的等效循环周数较大,地震动发展持时长,可认为竖向加速度对场地液化有促进作用。     

Liquefaction-induced settlement,site effects and damage in the vicinity of Yalova City during the 1999 Izmit earthquake,Turkey

Yalova City (Turkey) is in a tectonically active location that is particularly affected by the northern branch of the North Anatolian Fault Zone. Magnitudes 7.4 and 7.2 earthquakes in 1999 caused great destruction in Yalova. The heavy damage to buildings and other civil engineering structures was mainly due to liquefaction-induced settlement and site effects such as resonance and amplification. In the first phase of this study, the soil liquefaction potential index (PL) and the induced soil settlement were estimated. In the second phase, the effects on sites in Yalova soil were investigated using microtremor and earthquake data. The fundamental periods and amplification in soft soil were compared with microtremor data and strong ground motion records obtained by a local array of eight accelerograph stations deployed in Yalova. Thirty-seven ‘single site’ ambient noise measurements were taken in a dense grid of points covering the centre of the city. A comparison between fundamental periods obtained from strong ground motion records and from microtremor measurements showed similarities, in the 0.1–5 Hz range. Finally, soil liquefaction and amplification (or resonance) were divided into regions according to the extent of damage and the geotechnical/geophysical results.     

Ground motion estimation at Guwahati city for an Mw 8.1 earthquake in the Shillong plateau

In this paper, the ground motion at Guwahati city for an 8.1 magnitude earthquake on Oldham fault in the Shillong plateau has been estimated by stochastic finite-fault simulation method. The corresponding acceleration time histories on rock level at several sites in the epicentral region have been computed. These results are validated by comparing them with the estimates obtained from Medvedev–Sponheuer–Karnik (MSK) intensity observations of 1897 Shillong earthquake. Using the local soil parameters, the simulated rock level acceleration time history at Guwahati city is further amplified up to the ground surface by nonlinear site response analysis. The results obtained are presented in the form of peak ground acceleration (PGA) contour map. The maximum amplification for PGA over Guwahati city is as high as 2.5. Based on the simulated PGA, the liquefaction susceptibility at several locations in the city has been estimated. The results are presented in the form of contours of factor of safety against liquefaction at different depths below the ground surface. It is observed that over a large part of the Guwahati city, the factor of safety against liquefaction is less than one, indicating that the city is highly vulnerable to liquefaction in the event of this earthquake. The contour maps obtained can be used in identifying vulnerable areas and disaster mitigation.     

Nonlinear soil response to ground motions during different earthquakes in Nepal,to arrive at surface response spectra

Catastrophic damages reported during an earthquake include building damages, excessive ground shaking, uneven settlements and liquefaction. While most of the seismic hazard studies map the probable level of ground shaking at the bedrock level, their use in assessing the above damages is very limited until the response of the local soil is also taken into account. Determination of the local soil response needs regionally recorded ground motions, dynamic soil properties, in situ geotechnical details, etc., which most of the time are not readily available for the region under study. In the present work, the response of local soil for Nepal has been studied indirectly taking into account the surface level of ground shaking during various past as well as recent EQs observed at various locations. Based on the present analysis, a low value of amplification factor for high peak horizontal acceleration and vice versa is observed in central, western as well as southern parts of Nepal. These observations suggest nonlinear soil behavior and are in accordance with the available literature. Further, the ground motion records during 2015 Nepal EQ show maximum soil response at 0.3 s which is exactly matching with the site class C obtained from in situ data for the above locations. Based on the above observations, various correlations between the high peak horizontal acceleration and the surface spectral acceleration are proposed to obtained site specific surface response spectrum for Nepal.     

Equivalent-Linear Seismic Ground Response Analysis of Some Typical Sites in Mumbai

Any earthquake event is associated with a rupture mechanism at the source, propagation of seismic waves through underlying rock and finally these waves travel through the soil layers to the particular site of interest. The bedrock motion is significantly modified at the ground surface due to the presence of local soil layers above the bedrock beneath the site of interest. The estimation of the amplifications in ground response due to the local soil sites is a complex problem to the designers and the problem is more important for mega cities like Mumbai in India, where huge population may get affected due to devastations of earthquake. In the present study, the effect of local soil sites in modifying ground response is studied by performing one dimensional equivalent-linear ground response analysis for some of the typical Mumbai soil sites. Field borelog data of some typical sites in Mumbai city viz. Mangalwadi site, Walkeswar site, BJ Marg near Pandhari Chawl site are considered in this study. The ground responses are observed for range of input motions and the results are presented in terms of surface acceleration time history, ratio of shear stress to vertical effective stress versus time, acceleration response spectrum, Fourier amplitude ratio versus frequency etc. The typical amplifications of ground accelerations considering four strong ground motions with wide variation of low to high MHA, frequency contents and durations are obtained. Results show that MHA, bracketed duration, frequency content have significant effects on the amplification of seismic accelerations for typical 2001 Bhuj motion. The peak ground acceleration amplification factors are found to be about 2.50 for Mangalwadi site, 2.60 for Walkeswar site and 3.45 for BJ Marg site using 2001 Bhuj input motion. The response spectrum along various soil layers are obtained which will be useful for designers for earthquake resistant design of geotechnical structures in Mumbai for similar sites in the absence of site specific data.     

远场大地震作用下大尺度深软场地的非线性地震效应分析

基于ABAQUS软件自行研制的并行计算显式算法集群平台,针对苏州城区典型地层剖面,建立了大尺度深软场地的二维精细化非线性有限元分析模型,对人工地震波和大地震远场地震动作用下深软场地的非线性地震效应进行了比较研究。研究结果表明：（1）与人工地震波作用时深软场地的地表峰值加速度放大效应相比,大地震远场地震波作用时的放大效应尤为显著,由于土介质的横向不均匀性及其非线性,使不同地表的峰值加速度放大效应存在明显的变异性。（2）深软场地对周期小于0.3 s的高频地震波均具有显著的滤波效应;大地震远场地震波作用时,深软场地对周期0.85～1.65 s的长周期地震波的放大效应非常显著,但对2.5～7.0 s的长周期地震波呈现出明显的滤波效应。（3）地震动峰值加速度PGA值沿土层深度和横向的分布形态呈现出明显的高低起伏现象,在不同成因的土层更迭面附近及土介质横向不均匀性显著的区域,地震波的局部聚焦放大和过滤减小现象尤为明显,且大地震远场地震动作用时,20 m以浅土层的PGA值呈现出非常显著的放大效应。（4）地震波的频谱特性、土层的横向不均匀性对深软场地地表加速度反应谱? 谱的谱形有显著影响;给出了描述加速度反应谱沿土层深度变化特征的三维谱形曲线,可以直观地展示出深软场地中细长地下结构地震反应可能存在类共振现象的土层深度。     

Soft sediments and damage pattern: a few case studies from large Indian earthquakes vis-a-vis seismic risk evaluation

India is prone to earthquake hazard; almost 65 % area falls in high to very high seismic zones, as per the seismic zoning map of the country. The Himalaya and the Indo-Gangetic plains are particularly vulnerable to high seismic hazard. Any major earthquake in Himalaya can cause severe destruction and multiple fatalities in urban centers located in the vicinity. Seismically induced ground motion amplification and soil liquefaction are the two main factors responsible for severe damage to the structures, especially, built on soft sedimentary environment. These are essentially governed by the size of earthquake, epicentral distance and geology of the area. Besides, lithology of the strata, i.e., sediment type, grain size and their distribution, thickness, lateral discontinuity and ground water depth, play an important role in determining the nature and degree of destruction. There has been significant advancement in our understanding and assessment of these two phenomena. However, data from past earthquakes provide valuable information which help in better estimation of ground motion amplification and soil liquefaction for evaluation of seismic risk in future and planning the mitigation strategies. In this paper, we present the case studies of past three large Indian earthquakes, i.e., 1803 Uttaranchal earthquake (Mw 7.5); 1934 Bihar–Nepal earthquake (Mw 8.1) and 2001 Bhuj earthquake (Mw 7.7) and discuss the role of soft sediments particularly, alluvial deposits in relation to the damage pattern due to amplified ground motions and soil liquefaction induced by the events. The results presented in the paper are mainly focused around the sites located on the river banks and experienced major destruction during these events. It is observed that the soft sedimentary sites located even far from earthquake epicenter, with low water saturation, experienced high ground motion amplification; while the sites with high saturation level have undergone soil liquefaction. We also discuss the need of intensifying studies related to ground motion amplification and soil liquefaction in India as these are the important inputs for detailed seismic hazard estimation.     

液化自由场地震响应大型振动台模型试验分析

开展了含上部黏土层、饱和砂土层、密实砂土层的可液化自由场地在水平地震动激励下的大型振动台模型试验研究,分析了地震动激励时饱和砂土液化后场地加速度、位移、孔压比时空响应等动力响应。试验结果表明：在小震激励时,场地动力反应较小,加速度反应自下而上不断放大,各深度处孔压比均较小,模型地基整体处于弹性反应阶段;0.3g汶川地震卧龙台地震记录输入时,孔压积累迅速,可液化土体最上部土层孔压比达到1,饱和土体液化,模型地基表现出明显的非线性反应特征,加速度反应在饱和砂土层中未有明显放大,土体卓越周期对应的反应加速度自下而上有不断增大趋势。该研究是土-群桩-上部结构体系大型振动台系列试验中可液化自由场动力反应部分,可供今后做对比分析和验证数值模拟参考。     

地震液化条件下地面的大变形三维数值分析

地基液化条件下地面大变形是造成工程结构破坏的主要原因之一。考虑地形、地震、土层、地下水等影响因素,针对典型的岸坡场地3层土地基模型,利用有限差分法FLAC3D,对可液化场地在地震作用下发生地面大变形的过程进行了数值模拟。结果表明,临空面坡比愈大、地表坡度越陡,地基液化地表侧向位移值愈大;变坡度的场地在地震作用下发生的侧移要比单一倾斜率的场地大;地震最大加速度越大、地震持续时间越长,地基液化侧向位移、地表沉陷和隆起现象越严重;液化层的埋深、厚度以及地下水位都对地面大变形的产生有着不同程度的影响,应选择合理的地基处理方案进行处理。     

基于动力离心试验的软基尾矿库地震响应研究

基于离心机振动台,分别针对典型软基尾黏土尾矿库及加高扩容后的尾矿库开展了动力离心模型试验,重点探究了加速度分布规律、软土及尾矿内部孔压变化规律以及“软基?库内尾矿?尾矿坝”系统的变形规律等内容。试验结果表明：软基对地震动的放大效应较为微弱,而坝体加速度沿高程逐渐放大,高层子坝加速度响应最为强烈,加高扩容后的现坝顶加速度响应可达原坝顶的2.2倍;地震作用下软土与库内尾矿内部均会产生一定的孔压增量,但未达到液化状态。地震动强度、软基及库内尾矿的固结状态对尾矿库的变形模式影响较大。当固结不充分时,在强震作用下易发生尾矿的水平滑移,进而造成坝顶及下游软土隆起。在固结较为充分且地震动强度较弱的情况下,变形模式以震陷为主。该试验成果将为此类尾矿库的动力稳定分析及抗震加固设计提供一定参考。     

液化土层地震动模拟计算方法及验证

针对目前土层反应分析方法难以模拟液化土层地震动时程的缺欠,在工程力学研究所原有有效应力分析程序基础上提出了一个改进的计算方法并进行了验证。改进方法中,通过每个应力循环模拟土的非线性进程以及液化导致的土刚度衰减过程,并引入了新建立的适于非均等固结随机地震荷载作用下的孔压增长模型。将计算方法与实际地震记录及大型振动台实验结果进行了对比,结果表明：在峰值加速度、液化后波形变化、时频曲线及加速度反应谱等主要特征上,计算结果均与现场实际记录及振动台实验基本一致;在液化引起地震动特征变化上,计算出的液化较非液化土层加速度反应谱的增量与实验结果一致。说明所提出的改进方法可以用于液化土层地震动的模拟。     

CFG桩加固饱和粉土地基的动力特性试验研究

以京沪高速铁路液化土地基加固为背景,利用大型堆叠式剪切变形模型箱,进行了模型比例1:10的CFG桩桩网结构地基加固饱和粉土地基的振动台模型试验,得出各级加载情况下地基路基响应加速度幅值放大系数的分布以及响应加速度幅值放大系数与各级加载加速度幅值的关系,分析了CFG桩桩网结构地基加固饱和粉土地基在地震作用下的加速度传递规律,得到了以下结论：（1）在加载加速度小于0.101g时,地基路基响应加速度幅值基本不放大,且加速度放大系数云图呈现水平状分布;（2）当加载加速度为0.161g时,桩间土的液化引起地基路基的响应加速度幅值放大系数的较大增加;（3）当加载加速度为0.210g时,桩间土液化的同时部分CFG桩发生剪断破坏,从而引起了地基路基的各响应加速度幅值放大系数的增大;（4）当加载加速度为0.324g和0.363g时,地基整体刚度的大幅度减小,在一定程度上降低了桩网结构地基传递振动的能力,从而导致加速度幅值放大系数相对前一级加载时减小;（5）整个加载过程中,粉土层的放大效应显著,而黏土层对加载加速度几乎没有放大作用。     

河口盆地非线性地震效应及设计地震动参数

针对基岩明显起伏、土层非均匀分布的典型河口盆地场地,考虑土体非线性特征,采用黏弹性人工边界模拟无限域对地震波动的影响,建立大尺度精细化二维有限元模型,分析了盆地地表地震动幅值、频谱、持时、传递函数特征,探讨了基岩起伏土层的地震动聚集效应及盆地边缘效应。结果表明：（1）盆地近地表土层表现出不同程度的地震动放大效应,且随土层深度增加呈非单调递减特征,基岩突变处地震动聚集效应明显,盆地两侧产生较为显著的边缘效应;场地中、长周期地震动的放大作用显著;（2）多遇地震、偶遇地震和罕遇地震水平时,场地卓越周期依次介于0.35～0.65 s、0.40～0.75 s和0.50～1.05 s之间;给出了盆地地表PGA（地表峰值加速度）、卓越周期均值等值线图及地表加速度反应谱放大因子建议值,地表设计地震动参数amax（地震影响系数）与Tg（特征周期）明显大于现行《建筑抗震设计规范》取值;（3）盆地特殊位置地表地震动持时得到不同幅度增长,且与输入地震动特性相关;（4）该盆地对0.5～2.0 Hz频段基岩地震动的放大效应比较显著,对小于0.2 Hz或大于2.5 Hz的基岩地震动,该盆地地震动放大效应不明显;（5）福州城区及其邻近区域地震动放大效应普遍较大。大尺度二维非线性分析一定程度上能合理反映微地形起伏、土层分布及土体非线性对地震波传播过程的影响。     

Nonlinear Analysis of Local Site Effects on Seismic Ground Response in the Bam Earthquake

The influence of local geologic and soil conditions on the intensity of ground shaking is addressed in this study. The amplification of the ground motion due to local site effects resulted in severe damage to dwellings in the Bam area during the 2003 Bam Earthquake. A unique set of strong motion acceleration recordings was obtained at the Bam accelerograph station. Although the highest peak ground acceleration recorded was the vertical component (nearly 1 g), the longitudinal component (fault-parallel motion) clearly had the largest maximum velocity as well as maximum ground displacement. Subsurface geotechnical and geophysical (down-hole) data in two different sites have been obtained and used to estimate the local site condition on earthquake ground motion in the area. The ground response analyses have been conducted considering the nonlinear behavior of the soil deposits using both equivalent linear and nonlinear approaches. The fully nonlinear method embodied in FLAC was used to evaluate the nonlinear soil properties on earthquake wave propagation through the soil layer, and compare with the response from the equivalent linear approach. It is shown that thick alluvium deposits amplified the ground motion and resulted in significant damage in residential buildings in the earthquake stricken region. The comparison of results indicated similar response spectra of the motions for both equivalent and nonlinear analyses, showing peaks in the period range of 0.3–1.5 s. However, the amplification levels of nonlinear analysis were less than the equivalent linear method especially in long periods. The observed response spectra are shown to be above the NEHRP building code design requirements, especially at high frequencies.     

场地土对基岩峰值加速度放大效应分析

通过实际土层地震反应结果的统计分析和强震加速度观测结果的对比, 讨论了不同场地条件对基岩峰值加速度的放大效应及其特点。该分析可为地震动参数区划图编制和地震安全性评价中场地效应的估计、由基岩地震动估算场地地面地震动提供参考。     

核岛结构设计地基场地及计算基底效应研究

针对新型核电工程结构AP1000核岛结构设计地基中的5类非坚硬岩场地,即硬岩场地、软岩场地、上限软-中等土场地、软-中等土场地和软土场地,采用一维土层场地模型开展场地土和计算基底条件对设计地震动影响计算分析。分析中,场地模型的计算基底剪切波速分别取为700、1 100、2 438 m/s,计算基底输入地震动分别选择基于核电建设相关技术文件和规范规定的反应谱RG1.60谱、AP1000谱和HAD101/01谱（5个阻尼比）合成的人工地震动时程。计算分析表明：非坚硬岩场地会导致场地地震动峰值加速度及频谱特性显著变化,场地越软影响程度越显著;除软土场地外,场地对地震动峰值加速度和反应谱的影响均为放大作用,软土场地对地震动较低频段反应谱有放大作用,但对峰值加速度和较高频段反应谱具有强烈的减小作用;对于各类场地,计算基底及其剪切波速的变化均会导致地表地震动峰值及频谱特性明显甚至显著变化,其影响程度与计算基底剪切波速成正比;随着场地由硬变软,计算基底剪切波速的变化对场地地震动的影响程度大为减小,至软土场地几乎不产生影响。考虑到场地类型及计算基底选取对场地地震动的显著影响,我国核电厂建设引用AP1000标准设计时应合理分析场地的适宜性。     

地下商场结构对地面运动影响的振动台试验研究

为了研究地下空间结构对场地地震反应的影响,以一个三层地下商场结构为原型,设计并开展了土-地下空间结构动力相互作用振动台模型试验,研究了在6条不同卓越频率输入地震波在不同峰值加速度下场地地震反应的变化规律。试验研究结果表明:(1)地表水平加速度放大系数以结构中轴线为对称轴呈对称分布,地下结构显著影响上方及邻近场地的地表水平加速度。同时,由于地下空间结构的局部场地效应影响,即使在水平地震输入下也将导致场地产生竖向振动,地表竖直加速度放大系数呈M形分布。地下空间结构的影响范围可达到其两侧各一倍宽度距离。(2)地表加速度放大系数受输入地震波卓越频率的影响明显,且随着输入地震波的峰值加速度(PGA)的增大而降低,但空间放大系数随着PGA的增大而增大。(3)地表加速度Fourier谱的峰值频率受输入地震波的卓越频率、场地水平方向的固有频率和竖向固有频率的综合影响。     

可液化场地微型桩地震响应特性研究

可液化场地微型桩的地震响应分析是确保工程安全和优化抗震设计的前提。应用动态离心机试验和三维有效应力数值分析方法,研究了微型单桩桩台的侧向变形和加速度、不同埋深桩身弯矩、可液化场地的加速度及超孔隙水压力等响应特征。首先开展了相对密实度为57%饱和土层、输入波是频率为1 Hz和峰值加速度为1.516 m/s2正弦波的微型桩40 g动态地震响应离心机试验,进而应用基于多重剪切机构塑性模型和液化前缘状态面概念的三维有效应力分析方法,反演了试验结果,并进行了对比分析,结果表明,数值模拟与离心机试验结果吻合,液化场地特性控制着建于其中微型桩的地震响应特征,微型桩桩台的水平变形和残余变形可达78、30 mm,桩身最大弯矩和最大残余弯矩呈现向桩身底部迁移特点,同时表明,基于动态土工离心机试验和数值分析相结合的研究方法,分析可液化场地微型桩地震响应特性是有效可行的,研究结论为可液化场地微型桩的抗震设计提供了可靠的依据和参考。     

Evaluating the effects of ground motion parameters on response spectra in Uttarakhand Himalayas,India

Uttarakhand, a state of India, is located in seismically active Himalayan region and in the proximity of plate boundaries. The effects of important ground motion parameters like magnitude, distance, and local geology and site conditions on acceleration response spectra are examined in Uttarakhand Himalayas in this work. A total of 447 strong ground motion histories (horizontal and vertical) from 42 earthquakes were selected. The results show that the shape of the acceleration response spectra is influenced by the local site conditions and regional geology. The studies are carried out for two categories of sites, i.e., rock sites and soft soil sites. The maximum average horizontal spectral amplification for rock sites is 2.7 at 0.1 s, while for soft soil sites, it is found to be 3.2 at 0.2 s. In the same way, the maximum average vertical spectral amplification for rock is found to be 2.7 at 0.1 s, while for soft soil, it is found to be 2.95 at 0.1 s. The average spectral amplification in vertical component also shifts from low period (rock) to high period (soft soil). The level of spectra increases with decrease in distance for rock sites as well as soft soil sites. When comparing different magnitude earthquakes in different geological conditions, the response spectra are found to follow each other up to 0.04 s, while for period greater than 0.04 s, the spectra of higher magnitude earthquake is observed on the higher side. For soft soil sites, spectra from different magnitude earthquakes are observed to follow each other up to 0.1 s, beyond which they get separated.