汶川地震六度区内场地液化问题初步研究

对汶川地震中VI度区内出现液化的场地进行实地调查和勘察。结果表明,以周围非液化场地房屋地震中的表现衡量,这些液化场地的确处于Ⅵ度区,有些比Ⅵ度区还要低。这些VI度区内液化场地,地下水位较高,剪切波速值低,具备液化基本条件,但并没有超出常规范围;其地层中砾石层含量占优,部分夹杂有砂土透镜体,液化土既有细粒土(砂土)液化可能也有砂砾土液化可能。此次地震Ⅵ度区内出现显著液化及液化破坏的现象,与我国现有抗震规范有关条文规定相悖,其机理需要深入研究。     

场地液化震害的影响因素探讨

<正>研究表明,只有当某个地区发生震级足够大的地震,这一地区才有可能产生场地液化现象。震级小于或者等于5.5级的地震,即使极震区烈度为Ⅶ度,其场地液化则难以发生或者说场地液化震害的影响有限。但是,对于7.5级左右的(特)大地震,由于地震动持续时间长,震动次数多,需要考虑远场甚至是Ⅵ~Ⅴ度区内液化的发生及其引起的破坏。因此,要反映地震强度和地震持续时间对液化的确切影响,仅以烈度概括是不够的,应以震级定量化,这样可以为地震现场工作人员评估场地液化震害奠定基础。结合地震震     

汶川地震砂砾土液化现象及现场确认实验

通过汶川8.0级地震中不同地区、不同烈度的4个典型液化场地宏观现象分析以及现场测试,确认了汶川地震天然砂砾土液化的真实性并初步研究了其特征。结果表明汶川地震中砂砾土液化是真实存在的,4个场地均为砂砾土液化。地表喷出物与实际液化土类差异显著,而上覆非液化层厚度是影响地表是否可能喷出砾石的主要因素。重型动力触探击数是值得推荐的砂砾土液化判别的指标,若采用剪切波速指标则需要将砂砾土进行分类处理,需考虑含砾量、平均粒径等指标的影响。     

汶川Ms8.0级大地震中成都地区液化特征研究

汶川大地震中成都地区液化及其震害现象较为显著.通过现场调查和工程地质资料分析,成都地区的液化特点为:液化带主要集中都江堰市液化在烈度Ⅵ、Ⅶ、Ⅷ、X和X度区均有出现,但Ⅶ度区最为集中液化喷水高度多在1-3m之间,最高一处超过10m液化场地喷出物涵盖了多种土类,约67％为粉细砂,且有卵砾石,约占11％液化带普遍伴随地裂缝,...     

新疆巴楚地震液化宏观现象的对比分析

2003年2月24日新疆巴楚-伽师地区发生6.8级地震,出现了唐山、海城地震后近30年我国最具规模的砂土液化现象。本文通过现场调查和资料分析,并与以往大地震液化比较,提出了此次地震液化宏观现象的认识,包括液化分布特征、喷水冒砂形态、液化震害和工程地质条件等几个方面。结果表明,此次地震液化宏观现象与我国以往几次大地震有很多不同,具有很大的研究价值。     

汶川地震液化特征及砂砾土液化预测方法研究

地震液化震害调查是获取液化震害经验最直接的手段,是抗震理论和分析方法发展的重要基础,而地震液化场地的现场测试与分析,是建立液化预测方法最可靠的途径。以往国内几次大地震液化现场深入的考察和分析研究,对我国乃至世界工程抗震技术的发展都起到了很大的推动作用。     

基于巴楚地震调查的液化判别方法研究

震害预防是目前减轻地震灾害最直接和最有效的手段之一,液化震害预防的第一步就是对工程场地进行液化预测和判别。液化判别方法的合理性和可靠性对工程震害防御和工程造价影响很大,是土动力学和地震工程学研究的重要课题之一,历来受到国内外学者和工程界的高度重视。2003年2月24日我国新疆巴楚-伽师地区发生了MS6.8地震,是新疆自建国以来人员和财产损失最严重的一次地震。灾区属新疆喀什地区,位于叶尔羌河流域,场地土主要为全新世-晚更新世晚期厚度较大的粉砂、细砂、粉土和粉质黏土,地下水埋深浅,在烈度为Ⅶ、Ⅷ、Ⅸ度区产生了大面积的砂土液化、地裂缝、河流陡岸滑塌等地震地质灾害,液化规模与1975年海城地震相当,是继1976年唐山地震后近30多年来中国大陆出现砂土液化现象最显著的     

自由场地基液化大型振动台模型试验研究

目前，大型振动台模型试验是研究液化场地的地基动力特性及土-结动力相互作用的一种有效手段。以1976年唐山地震中倒塌的胜利桥的地基为原型，开展1：lO模型的自由场地基液化的大型振动台模型试验研究，再现了自然地震触发地基砂土液化的各种主要宏观震害现象，尤其是模型地基的试验破坏与其原型的实际震害情况比较吻合。主要介绍有关此次大型振动台模型试验的相似设计、操作技术及试验结果等方面的若干关键科学问题。     

汶川大地震中德阳地区液化特点分析

汶川大地震中八个主要地区均有液化现象出现,其中德阳地区液化现象及其震害最为显著.通过现场调查和工程地质资料分析,德阳地区的液化特点为:液化带主要集中在绵竹市、什邡市和德阳市,绵竹市最为严重;液化在烈度Ⅶ、Ⅷ、Ⅸ度区均有出现,但Ⅷ度区最为集中;液化喷水高度多在几公分到2m之间,最高一处超过10 m;液化场地喷出物基本涵盖...     

汶川特大地震汉源震害异常研究

正2008年5月12日在汶川发生的8.0级特大地震,是新中国成立以来破坏最强、波及范围最广、救灾难度最大的一次地震。这次历史罕见的地震灾害所造成的巨大破坏,举国震惊,举世关注。距震中近250km的汉源县城为这一特大地震Ⅵ度区内唯一的Ⅷ度异常区,是十分典型而又罕见的远震高烈度异常区。汶川地震发生后,这一现象引起了国内外学者的广泛关注。探索汉源县城震害异常成因,总结其形成条件和地震动特征,对促进场地条件工程抗     

Liquefaction macrophenomena in the great Wenchuan earthquake

On May 12, 2008 at 14:28, a catastrophic magnitude M 8.0 earthquake struck the Sichuan Province of China.The epicenter was located at Wenchuan (31.00°N, 103.40°E). Liquefaction macrophenomena and corresponding destruction was observed throughout a vast area of 500 km long and 200 km wide following the earthquake. This paper illustrates the geographic distribution of the liquefaction and the relationship between liquefaction behavior and seismic intensity, and summarizes the liquefaction macrophenomena, including sandboils and waterspouts, ground subsidence, ground fissures etc., and relevant liquefaction features. A brief summary of the structural damage caused by liquefaction is presented and discussed. Based on comparisons with liquefaction phenomena observed in the 1976 Tangshan and 1975 Haicheng earthquakes, preliminary analyses were performed, which revealed some new features of liquefaction behavior and associated issues arising from this event. The site investigation indicated that the spatial non-uniformity of liquefaction distribution was obvious and most of the liquefied sites were located in regions of seismic intensity Ⅷ. However, liquefaction phenomena at ten different sites in regions of seismic intensity Ⅵ were also observed for the first time in China mainland. Sandboils and waterspouts ranged from centimeters to tens of meters, with most between 1 m to 3 m. Dramatically high water/sand ejections,e.g., more than 10 m, were observed at four different sites. The sand ejections included silty sand, fine sand, medium sand,course sand and gravel, but the ejected sand amount was less than that in the 1976 Tangshan earthquake. Possible liquefaction of natural gravel soils was observed for the first time in China mainland.     

Distribution and characteristics of gravelly soil liquefaction in the Wenchuan M_s 8.0 earthquake

In this paper,a distribution map of gravelly soil liquefaction that was caused by the Wenchuan M_s 8.0 earthquake in China is proposed based on a detailed field investigation and an analysis of geological soil profiles. The geological background of the earthquake disaster region is summarized by compiling geological cross sections and borehole logs. Meanwhile,four typical liquefied sites were selected to conduct sample drillings,dynamic penetration tests (DPT),and shear wave velocity tests,to understand the features of liquefied gravelly soil. One hundred and eighteen (118) liquefied sites were investigated shortly after the earthquake. The field investigation showed:(1) sandboils and waterspouts occurred extensively,involving thousands of miles of farmland,120 villages,eight schools and five factories,which caused damage to some rural houses,schools,manufacturing facilities and wells,etc.; (2) the Chengdu plain is covered by a gravelly soil layer with a thickness of 0 m to 541 m according to the geological cross sections; (3) there were 80 gravelly soil liquefied sites in the Chengdu plain,shaped as five belt areas that varied from 20 km to 40 km in length,and about ten gravelly soil liquefied sites distributed within Mianyang area; and (4) the grain sizes of the sampled soil were relative larger than the ejected soil on the ground,thus the type of liquefied soil cannot be determined by the ejected soil. The gravelly soil liquefied sites are helpful in enriching the global database of gravelly soil liquefaction and developing a corresponding evaluation method in further research efforts.     

Distribution and characteristics of gravelly soil liquefaction in the Wenchuan <Emphasis Type="Italic">M</Emphasis><Subscript>s</Subscript> 8.0 earthquake

In this paper,a distribution map of gravelly soil liquefaction that was caused by the Wenchuan M_s 8.0 earthquake in China is proposed based on a detailed field investigation and an analysis of geological soil profiles. The geological background of the earthquake disaster region is summarized by compiling geological cross sections and borehole logs. Meanwhile,four typical liquefied sites were selected to conduct sample drillings,dynamic penetration tests (DPT),and shear wave velocity tests,to understand the features of liquefied gravelly soil. One hundred and eighteen (118) liquefied sites were investigated shortly after the earthquake. The field investigation showed:(1) sandboils and waterspouts occurred extensively,involving thousands of miles of farmland,120 villages,eight schools and five factories,which caused damage to some rural houses,schools,manufacturing facilities and wells,etc.; (2) the Chengdu plain is covered by a gravelly soil layer with a thickness of 0 m to 541 m according to the geological cross sections; (3) there were 80 gravelly soil liquefied sites in the Chengdu plain,shaped as five belt areas that varied from 20 km to 40 km in length,and about ten gravelly soil liquefied sites distributed within Mianyang area; and (4) the grain sizes of the sampled soil were relative larger than the ejected soil on the ground,thus the type of liquefied soil cannot be determined by the ejected soil. The gravelly soil liquefied sites are helpful in enriching the global database of gravelly soil liquefaction and developing a corresponding evaluation method in further research efforts.     

The use of liquefaction features in paleoseismology: Lessons learned in the New Madrid seismic zone, central United States

A recent study in the New Madrid seismic zone demonstrates that large uncertainties, often involved but rarely expressed, in paleoliquefaction studies can be reduced by conducting detailed investigations at the most promising sites for dating liquefaction features. During the site investigations, care must be taken to collect samples that will provide close maximum and minimum dates for liquefaction features. It is advisable to use two-sigma calibrated dates, rather than one-sigma calibrated dates or radiocarbon ages, when estimating ages of liquefaction features. Well-constrained ages of individual liquefaction features should provide the basis for estimating the timing of paleoearthquakes and correlating features across a region. As uncertainty in ages of liquefaction features decreases, confidence in estimates of timing, source areas and magnitudes of paleoearthquakes increases. The New Madrid study also shows that modern or historic earthquakes that induced liquefaction in the same region and whose locations and magnitudes are fairly well know can serve as calibration events for paleoearthquakes. Future efforts that could further improve the usefulness of liquefaction feature in paleoseismology include (1) the development of new techniques for dating liquefaction features directly, (2) case studies of modern earthquakes that focus on the site and spatial distributions of liquefaction feature as well as geotechnical properties of liquefaction sites and (3) more rigorous quantification of uncertainties associated with estimates of timing, source areas and magnitudes of paleoearthquakes.     

下辽河地区砂土液化形成的震害地质问题

海城地震引起下辽河平原大面积砂土液化,液化震害有喷水冒砂、陷落砂坑、地滑移等等。液化区是全新世海退之地,震害严重区地表粘性土层薄,承压水头高;震害轻微区地表粘性土层厚,承压水头低。喷砂颗粒的单粒结构表明液化砂层具有松散的特点,砂颗粒的滚圆度在液化难易上不起主要作用     

Recording-based identification of site liquefaction

Reconnaissance reports and pertinent research on seismic hazards show that liquefaction is one of the key sources of damage to geotechnical and structural engineering systems. Therefore, identifying site liquefaction conditions plays an important role in seismic hazard mitigation. One of the widely used approaches for detecting liquefaction is based on the time-frequency analysis of ground motion recordings, in which short-time Fourier transform is typically used. It is known that recordings at a site with liquefaction are the result of nonlinear responses of seismic waves propagating in the liquefied layers underneath the site. Moreover, Fourier transform is not effective in characterizing such dynamic features as time-dependent frequency of the recordings rooted in nonlinear responses. Therefore, the aforementioned approach may not be intrinsically effective in detecting liquefaction. An alternative to the Fourier-based approach is presented in this study, which proposes time-frequency analysis of earthquake ground motion recordings with the aid of the Hilbert-Huang transform （HHT）, and offers justification for the HHT in addressing the liquefaction features shown in the recordings. The paper then defines the predominant instantaneous frequency （PIF） and introduces the PiF-related motion features to identify liquefaction conditions at a given site. Analysis of 29 recorded data sets at different site conditions shows that the proposed approach is effective in detecting site liquefaction in comparison with other methods.     

唐山地震时北京及邻区砂土液化深度的探讨

砂土液化深度历来是工程界颇为关心的问题.以往研究砂土液化深度多靠间接或宏观的方法, 因而取得的结果彼此间差别很大, 本文提出一种建立在喷砂同地下砂层直接对比基础上的矿物学方法.唐山地震之后, 作者等收集北京通县的西集、王庄(以上为 Ⅷ 度烈度区), 河北香河县的骡子王、渠口、县城大气所(以上为 Ⅶ 度区)等五个钻孔剖面上的砂样及地表喷砂、并进行矿物与颗粒成分的研究.结果表明, 西集喷砂同地下12.26米砂层类似;王庄喷砂同地下11.02米砂层类似;骡子王喷砂同地下12.3米砂层类似;大气所喷砂同地下9.2米砂层类似;渠口喷砂同地下11.28米砂层类似.据此可以认为, 唐山地震时北京通县与河北香河等地的 Ⅶ 和 Ⅷ 度烈度区的最大液化深度不超过12.30米, 变化在9.2——12.30米之间, Ⅶ 度区有可能更浅些.      

Earthquake induced liquefaction features in the Karewas of Kashmir Valley North-West Himalayas,India: Implication to paleoseismicity

Earthquake induced liquefaction features found in the Karewas of Kashmir Valley are potential tools for estimating energy center, magnitude and peak ground acceleration of the paleoearthquakes. Size, pattern and spatial distribution of liquefaction features and in-situ geotechnical data were collected at the paleoliquefaction sites to assess the magnitude and peak ground acceleration of the paleoearthquake. The magnitude of paleoearthquakes was estimated to be of the order of (Mw = 6.2) using “Magnitude bound method” and “In-situ testing of geotechnical properties and assessment of liquefaction potential of liquefied beds (source stratum) using cyclic stress method”. The peak ground acceleration computed were in the range of 0.27 g to 0.83 g using cyclic stress method and 0.30 g using attenuation equation NDMA [21].