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。     

研究地基液化的新方法

介绍一种研究地基液化的新方法，利用地震时可能液化的土层的地面加速度，地下钻孔加速度及孔隙水压力方面的连续资料，通过计算处理，先后获得土层的水平位移反应，平均剪应变和有效应力，在此基础上，对平均剪应力－应变和有效应力路径历史作出评价，由此获得地震振动时有关地基液化过程的直接和有价值的资料。     

碳酸盐岩振动液化地震序列

新建立的振动液化地震序列系碳酸盐软沉积物中的强地震事件记录，包括地震液化阶段产生的泄水脉、液化变形；液化停止后的地面下沉构造；地震引起的津浪丘状层与浊积岩等序列单元。这个序列来自大区域的野外实践总结，反映了一次强地震事件的全过程，并在理论上得到统一的解释。序列将为野外地质学家在碳酸盐层中识别事件提供对比的标准。根据振动液化地震记录，本文首次提出古郯庐带概念，并对其地质意义进行了初步讨论。     

地震砂土液化预测模型研究

本文通过对砂土液化机制的研究,建立了临界液化状态下的超静孔隙水压力随地震地面加速度及地震持续时间的关系式,利用该模型可以确定砂土层达到临界液化状态所需地震持续时间及液化可能发生的深度,并根据唐山地区的砂土液化资料进行了实例分析。     

基于地面运动强度及标准贯入试验的上海地区砂土地震液化评价

上海市地处长江三角洲前缘,黄浦江和苏州河交汇区域,特殊的地理环境与沉积环境导致浅部砂层广泛发育。随着城市建设的不断推进,上海城市区域范围的砂土地震液化风险评价成为亟待研究的课题。文章基于上海市工程钻孔数据,结合地震地面运动加速度分布与标准贯入试验,建立区域性地震液化危险性评价模型,对上海市进行了地震液化危险性评价。研究认为当发生50年超越概率10%的地震条件下,上海市陆域面积的66.0%将不会产生地震砂土液化灾害,21.8%的陆域面积仅发生轻微液化,只有崇明、横沙、长兴三岛,黄浦江及苏州河两岸地震液化等级达到中等甚至严重,占全市陆域面积12.3%;50年超越概率2%的地震条件下,随着峰值地面运动加速度整体升高,全市范围内轻微—严重液化区域明显增多,可能发生地震液化的总面积达到全市陆域面积46.25%。上海市存在砂土地震液化的危险性,但是发生概率较低。研究认为,目前的抗震设计规范中上海市的设防烈度偏高,可能导致不必要的建设成本。同时研究中的不同超越概率下的地震液化危险性评价结果为上海市工程建设相关标准的合理化改进的提供了建议和参考。      

基于应变空间多机构CG模型的地震液化大变形数值分析

砂土液化导致的地基侧向大变形是地震中许多重要的工程设施和建筑物破坏的主要原因之一。简要介绍了可进行液化大变形分析的散粒体材料本构模型--应变空间多机构CG模型,基于FLIP ROSE程序平台,建立了预测和研究倾斜地基砂土液化导致侧向大变形的二维有限元数值分析方法。采用该模型对相同工况的土工动态离心模型试验进行了模拟,通过对比超孔隙水压力、剪切波水平加速度以及地基侧向位移发现,数值预测与试验结果吻合良好,从而验证了该有限元数值分析模型的可靠性。最后利用该数值分析模型预测了倾斜率不同的地基受到相同剪切波作用时,倾斜地基不同深度产生的侧向位移。预测结果显示,随着地基深度的减小,倾斜率对于地震液化导致倾斜地基侧向大变形的影响越来越显著。     

乌鲁木齐二电厂断裂地震工程地质效应研究

强烈构造活动区重大工程场址的评价是目前国内外研究的难题。本文以新疆乌鲁木齐二电厂为例,应用风险度和专家系统对该厂区及附近断裂的活动性及其地震工程地质交效应进行了研究。采用Ｗｅｉｂｕｌｌ函数,综合考虑了裂活动可能产生的振动、错动,蠕滑,地面裂缝,液化和崩塌等６种因素,建立了该区断裂地震工程地质模型;运用ＣＲＵＳＴＡＢ专家系统,综合了地质条件,地震条件和地震球理对厂区断断裂可能产生的地震地面运动,断及     

液化场地桩-土-桥梁结构地震相互作用简化分析方法

液化场地桩-土-桥梁结构地震相互作用分析属于桩基桥梁抗震设计中的一个关键科学问题,而目前尚缺乏合理的简化分析方法。鉴于此,直接针对振动台试验,基于Penzien模型,建立了液化场地桩-土-桥梁结构地震相互作用的数值分析模型与相应的简化分析方法。通过振动台试验验证了数值建模途径与简化计算分析方法的正确性,可用于液化场地桩基桥梁结构地震反应的分析,并且特别考虑砂层中孔压升高引起的砂土承载力衰减效应,推荐了计算参数的合理选取方法;据此进行了桩径、桩土初始模量比、砂土内摩擦角、上部桥梁结构质量等重要参数对液化场地桩-土地震相互作用影响的敏感性分析。研究表明：在液化场地条件下,随桩径和桩土初始模量比的增大,桩的峰值加速度、峰值位移减小,而桩的峰值弯矩则增大;随砂土内摩擦角增大,桩的峰值加速度、峰值弯矩、峰值应力均增大,而桩的峰值位移则减小;随上部结构配重增大,桩的峰值位移、峰值弯矩均增大。     

基于CFD的地震液化研究新进展

综述了近年来关于液化土体流体动力学特征的试验发展状况,以及基于计算流体动力学（简称CFD）的地震液化数值模拟现状,重点介绍了目前比较活跃的可以较高精度模拟液化土体流动状态的三次伪质点数值方法（简称CIP法）。通过对CFD和传统固体力学在地震液化研究中的应用比较,指出了应用CFD的三大优势,即土体大变形问题、液化土体参数分析以及液化土体中结构物的变形应用CFD分析,均可获得较好的结果。进一步提出,在地震液化应用中,未来CFD的发展应该考虑整合液化前的土体性状研究和地震液化中桩-土-结构物的综合分析。     

最近20年地震中场地液化现象的回顾与土体液化可能性的评价准则

回顾了1994年美国Northridge地震、1995年日本阪神地震、1999年土耳其Kocaeli地震、1999年台湾集集地震、2008年中国汶川地震、2010年智利Maule地震、2010～2011新西兰Darfield地震及余震、2011年东日本地震中大量的、不同类型的液化实例调查与研究,发现这些地震的液化具有以下特点：（1）罕见的特大地震（Mw9.0）使远离震中300～400 km的新近人工填土发生严重的大规模液化;（2）特大地震（Ms8.0、Mw8.8）使远离震中的低烈度Ⅴ～Ⅵ度地区发生严重液化;（3）海岸、河岸附近地区的新近沉积冲积、湖积土,填筑时间不到50年的含细粒、砂砾人工填土,容易发生严重液化;（4）天然的砂砾土层液化发生严重液化;（5）发生了深达20 m的土层液化现象;（6）松散土层液化后可以恢复到震前状态并再次发生液化;（7）高细粒（粒径≤75 ?m）含量≥50%或高黏粒（粒径≤5 ?m）含量≥25%的低-中塑性土严重液化,对介于类砂土与类黏土之间的过渡性态土,有时地表未见液化现象;（8）液化土层的深度较深或厚度较小时,容易出现地面裂缝而无喷砂现象;有较厚的上覆非液化土层时,场地液化不一定伴随地表破坏。液化实例证明,第四系晚更新世Q3地层可以发生严重液化;黏粒含量不是评价细粒土液化可能性的一个可靠指标;低液限、高含水率的细粒土易发生液化,采用塑性指数PI、含水率wc与液限LL之比作为细粒土液化可能性评价的指标是适宜的。综合Boulanger和Idriss、Bray和Sincio、Seed和Cetin等的液化实例调查与室内试验研究成果,建议细粒土液化可能性的评价准则如下：PI ＜12且wc/LL＞0.85的土为易液化土,12＜PI≤20和/wc/LL≥0.80的土为可液化土;PI ＞20或wc/LL＜0.80的土为不液化土。     

Evaluation of seismic soil-liquefaction at Guwahati city

Great earthquakes in the past (e.g. 1869 Cachar earthquake, 1897 great Assam earthquake) have caused large scale damage and ground liquefaction in the Guwahati city. Moreover, seismologists are of opinion that a great earthquake might occur in the unruptured segment of the North-East Himalaya that is near to Guwahati city. In this paper, the liquefaction hazard due to these events have been simulated. The obtained results are in general agreement with the reported damages due to the past earthquakes. The central part of the city (i.e. Dispur, GS road), that has large thickness of soft soil deposit and shallow ground water table, is highly vulnerable to liquefaction.     

An energy-based interpretation of sand liquefaction due to vertical ground motion

In several recent earthquakes, high vertical ground accelerations accompanied by liquefaction were observed. Downhole records have also shown that large vertical accelerations do not necessarily originate from the source, but rather get amplified towards the ground surface. Given the advantages of energy-based interpretation of liquefaction triggering due to S-waves, this approach is used together with finite element analyses to investigate vertical motion amplification and ensuing liquefaction. The results show the importance of the post-resonance response cycles, while hysteretic damping based on total stresses, accounting for the water in the pores, is shown to be very low, explaining the observed amplification.     

Geotechnical aspects and ground response studies in Bhuj earthquake, India

The extent of damage and affected areas in Bhuj earthquake (26th January 2001) has provided a unique opportunity to evaluate a wide range of geotechnical issues. A large area in the Rann of Kutch experienced massive liquefaction resulting in ground subsidence and lateral flow. A large number of dams in the Kutch district suffered moderate to severe damages. Many buildings were damaged and collapsed in the city of Ahmedabad situated on the bank of the Sabarmati River. In this paper, the ground response studies at a site in Ahmedabad City along with observations of geotechnical aspects such as ground cracking, sand volcanoes and liquefaction of soils associated with the Bhuj earthquake are discussed. The ground response studies indicate that the varying degree of damage to multistorey buildings in Ahmedabad in the close proximity of Sabarmati river area was essentially due to the collapse and undesirable settlement of partly saturated silty sand deposits. Large settlements are attributed to amplification of the ground and the near resonance condition. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Explanation of liquefaction in after shock of the 2011 great east Japan earthquake using numerical analysis

During the 2011 Great East Japan Earthquake, severe liquefaction occurred in reclaimed ground in Urayasu city, Chiba prefecture. This liquefaction provided important lessons for us to re-recognize the liquefaction mechanism. A distinct feature of the liquefaction in this earthquake is that severe liquefaction happened not only in the main shock but also in an aftershock with a maximum acceleration of 25 gal. In some areas, liquefaction happened in the aftershock is even more serious than that happened in the main shock. In this paper, focus is placed on the characteristic features in the occurrence of liquefaction and consequent ground settlement. Based on the observed data, a series of dynamic–static analyses, considering not only the earthquake loading but also static loading during the consolidation after the earthquake shocks, are conducted in a sequential way just the same as the scenario in the earthquake. The calculation is conducted with 3D soil–water coupling finite element–finite difference analyses based on a cyclic elasto-plastic constitutive model. From the results of analyses, it is recognized that small sequential earthquakes, which cannot cause liquefaction of a ground in an independent earthquake vibration, cannot be neglected when the ground has already experienced liquefaction after a major vibration. In addition, the aftershock has great influence on the long-term settlement of low permeability soil layer. The observed and predicted liquefaction and settlements are compared and discussed carefully. It is confirmed that the numerical method used in this study can describe the ground behavior correctly under repeated earthquake shocks.     

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.     

高原季节性冻土区砂土液化探讨——以阿克陶MS6.7地震为例

2016年11月25日新疆阿克陶县木吉乡发生MS6.7地震,发震构造为公格尔山拉张系北端的木吉断裂,断裂总长度超过100 km,以右旋走滑为主兼有一定的拉张分量。文章在对震区进行了初步的地震地质灾害调查,总结砂土液化和地裂缝在高原季节性冻土地区的分布及发育特点的基础上,发现:1)在研究区Ⅰ维日麻村的砂土液化主要沿原有泉眼或沿地裂缝发育,沿泉眼形成的砂土液化其喷砂锥的覆盖面积达36.1 m2,占总液化面积的60%,研究区Ⅱ布拉克村的砂土液化则主要是沿草甸的根系喷出,在地表形成大面积的最新涌水结冻特征;2)对研究区Ⅱ布拉克村地裂缝的深度进行统计,反演出区域冻土层厚度,结合探槽揭露的地层剖面,推断冻土层发生大面积地裂缝是因为地震引起冻土层下部融土层发生砂土液化导致土层变形失稳,从而使冻土层发生形变产生一系列规律性的地裂缝。     

Assessment of liquefaction and lateral spreading on the shore of Lake Sapanca during the Kocaeli (Turkey) earthquake

The 1999 Kocaeli earthquake of Turkey (M_w = 7.4) caused great destruction to buildings, bridges and other facilities, and a death tall of about 20,000. During this earthquake, severe damages due to soil liquefaction and associated ground deformations also occurred widespread in the eastern Marmara Region of Turkey. Soil liquefaction was commonly observed along the shorelines. One of these typical sites is Sapanca town founded on the shore of Lake Sapanca. This study was undertaken as quantitative measurement of ground deformations induced by liquefaction along the southern shore of Lake Sapanca. The permanent lateral ground deformation was measured through the aerial photogrammetry technique at several locations both along the shoreline and in the town. In situ soil profiles and material properties at Sapanca area were obtained based on the data from 55 borings and standard penetration tests (SPT), and laboratory tests, respectively. The data and the empirical methods recommended by an NCEER workshop were employed to evaluate the liquefaction resistance of the soils. In addition, simple shaking tests on a limited number of samples were also performed. The permanent ground displacements were estimated from the existing empirical models, sliding block method and residual visco-elastic finite element methods. Then these estimations were compared with the observed ground displacements. The assessments suggested that liquefaction at Sapanca have occurred within Quaternary alluvial fan deposits at depths 1 and 14 m, and the major regions of liquefaction and associated ground deformations were located along the shore and creeks. The evaluations also indicated that for sites with no sand boils but with ground displacement greater than 1 m, thickness of the non-liquefiable layer was large. It is also noted that no liquefaction-induced ground surface disruption is expected at the site when the thickness of the liquefiable and non-liquefiable layers vary between 0.5 and 1.5 m, and 3.5 and 5.5 m, respectively. Except one model, all the empirical models employed in the study over-predicted the observed lateral ground displacements, while sliding block method and residual visco-elastic finite element methods yielded reasonably good results if the known properties of liquefied soils are used.     

A performance-based framework for assessing liquefaction potential based on CPT data

This article describes a new performance-based approach for evaluating the return period of seismic soil liquefaction based on standard penetration test (SPT) and cone penetration test (CPT) data. The conventional liquefaction evaluation methods consider a single acceleration level and magnitude and these approaches fail to take into account the uncertainty in earthquake loading. The seismic hazard analysis based on the probabilistic method clearly shows that a particular acceleration value is being contributed by different magnitudes with varying probability. In the new method presented in this article, the entire range of ground shaking and the entire range of earthquake magnitude are considered and the liquefaction return period is evaluated based on the SPT and CPT data. This article explains the performance-based methodology for the liquefaction analysis – starting from probabilistic seismic hazard analysis (PSHA) for the evaluation of seismic hazard and the performance-based method to evaluate the liquefaction return period. A case study has been done for Bangalore, India, based on SPT data and converted CPT values. The comparison of results obtained from both the methods have been presented. In an area of 220 km² in Bangalore city, the site class was assessed based on large number of borehole data and 58 Multi-channel analysis of surface wave survey. Using the site class and peak acceleration at rock depth from PSHA, the peak ground acceleration at the ground surface was estimated using probabilistic approach. The liquefaction analysis was done based on 450 borehole data obtained in the study area. The results of CPT match well with the results obtained from similar analysis with SPT data.