基于中国砂土液化数据库的标准贯入试验液化判别方法研究

历史地震液化调查数据是液化判别方法构建、改进以及验证的重要基础,也是检验现有液化认识和理论的主要标准。通过吸收我国集集、巴楚和松原等地震液化数据,将基于标准贯入试验（standard penetration test,简称SPT）锤击数的液化数据量库从121个增至465个。采用该液化数据库,对比研究了我国建筑抗震设计规范液化判别方法（简称建规法）、2个双曲线模型以及循环剪应力比CSR简化方法等4种基于SPT液化判别方法的可靠性。结果显示：2个双曲线模型能够很好地判别液化和非液化数据,成功率超过85%,对液化数据和非液化数据的判别成功率保持均衡;建规法以及CSR简化方法判别结果存在不足;地震烈度为7度区液化数据和非液化数据混合情况较多,不同液化判别公式判别效果均不够理想,而对8度区和9度区液化数据,4种方法判别成功率较高;与以往液化概率判别公式对比,基于CSR计算方法,采用新液化数据给出的液化风险评估概率判别公式虽然采用数据量不同,但公式预测临界线匹配良好;建规法存在明显不足,对饱和砂层埋深大于10 m的深层土液化判别结果过于保守。研究成果可为改进我国规范中液化判别方法提供参考。     

液化判别临界曲线的变化模式与一般规律

液化临界值与砂层深度的关系是液化判别方法的基本表征,但对比分析表明,现有液化判别方法的临界曲线有不同的表现模式,甚至定性相反。以Seed-Idriss模型为基础,推导出了砂层埋深对液化势影响的理论解答,提出了砂层埋深与液化临界值的普遍关系,得到了液化判别临界曲线的变化模式和一般规律。结果表明：一般而言,随砂层深度增加,水平地震剪应力和土体抗液化强度同时增大,但前者增大速率大于后者,液化势及液化临界值与砂层深度呈正相关关系;液化临界值与砂层深度呈非线性递增关系,浅埋处饱和砂层液化势递增较为剧烈,深埋处趋于平缓;我国规范CPT液化判别公式的液化临界值与砂层深度呈递减关系,存在定性错误,需要纠正;现有一些液化判别公式中,液化临界值与砂层深度呈线性递增关系,定性正确但模型需要改进。所得结果可为液化判别方法正确发展提供理论基础和借鉴。     

基于台湾集集地震数据的CPT与SPT液化判别方法比较

利用集集地震静力触探试验(CPT)数据,对基于CPT测试的Robertson液化判别方法和Olsen方法进行了检验,两个方法对液化点判别成功率分别为82.61%和80.43%,对非液化点判别成功率分别为31.82%和44.32%。CPT液化判别方法对液化点判别基本可靠,但对非液化点判别准确性较差。对集集地震标准贯入试验(SPT)数据,美国地震工程研究中心(NCEER)推荐的SPT液化判别方法对液化点和非液化点判别成功率分别达到92.41%和94.35%。SPT方法判别成功率非常高,整体准确性远高于CPT方法。另一方面,CPT的土分类图可以同时反映土的种类与强度,甚至可以对集集地震液化土与非液化土进行区分。对于细粒土的液化初判,CPT土分类图也优于SPT方法中的黏粒含量指标。因此,土分类图是CPT的优势所在。     

水电站坝的砂层地基地震液化可靠度研究

对四川地区江河上数座水电站坝基砂层的26组动力三轴试验资料进行了统计分析,基于动剪应力比法的液化判别方法推导了的地震液化的极限状态方程,使用蒙特卡洛随机抽样的方法计算了砂层液化的失效概率,并对某水电站的厂房地基砂层的液化可靠度进行了计算分析。研究表明,统计按粉砂样总体和中细砂样总体划分较为合理;砂层的动剪应力比可采用正态分布;电站砂层地基地震液化的最危险工况为,闸坝盖重加稳定的向上渗流及遭遇Ⅶ度地震荷载,为高液化风险,其液化概率随埋深加大而增大,最危险部位为砂层底板,对坝基砂层应进行抗液化处理。     

砂土液化的地质综合判别法

本文根据地震烈度七、八、九度区的实际资料,分别建立了包括砂土厚度、砂土密实度、砂土埋深及地下水位埋深等四项因素的藏化判别式和严重液化与轻微液化的判别式。根据对唐山、海城地震区131个钻孔162组资料的统计结果,本判别法的有效率为:七度区91％,八度区88％,九度区95％。     

砂土液化预测的Fisher判别分析模型及应用

针对砂土地震液化预测问题,基于Fisher判别分析原理(FDA),选用平均粒径、不均匀系数、标贯击数、地下水水位、砂层埋深、剪应力与有效上覆应力比、地震烈度.震中距等8个实测指标为判别因子,建立Fisher线性判别函数模型,对砂土液化进行预测.研究结果表明,Fisher判别分析结果与神经网络输出结果一致,优于规范法和Seed法判别结果,验证了该模型的合理性和可靠性.运用该模型进行判别分析,简易方便,分类效率高,对砂土液化判别快速、有效,模型适用性强,具有一定的工程应用前景.     

砂土液化预测的Fisher判别模型及应用

基于Fisher判别理论建立了砂土液化可能性的Fisher判别分析（FDA）模型。在分析砂土液化影响因素的基础上,选取烈度、震中距、地下水位、砂层埋深、标贯击数、平均粒径、不均匀系数、剪应力比等8个实测特征指标作为FDA模型的预测指标。利用砂土液化的实测数据作为训练样本进行训练,建立FDA模型对砂土液化进行预测,并用其他未参加训练的实测数据进行了验证。研究结果表明,FDA模型简便可行、预测精度高,是解决砂土液化预测问题的有效方法之一。     

砂土液化判别方法可靠性评价

在进行液化评价简化法与规范液化判别方法对比基础上，利用液化概率的对数回归方程，讨论了规范液化判别方法的可靠性。研究结果表明，我国规范液化判别方法其液化概率变动较大，对于烈度 Ⅶ 度，规范法的液化概率在0.17～0.42间，小于简化法概率0.36～0.43，偏保守。对于烈度 Ⅷ 度和 Ⅸ 度近地表场地，规范法的液化概率高达0.65～0.70，远远高于简化法的液化概率，安全裕度不够。对于含粘粒土质液化评价，规范方法的液化概率总体上都较简化法高，特别是烈度Ⅷ度高粘粒含量其液化概率为0.9。     

静力触探法对巴楚地震液化判别的适用性

2003年2月24日新疆巴楚-伽师地区发生的6.8级地震中出现了自1996年唐山大地震以来我国大陆境内最为严重的砂土液化现象。以此地震液化调查为基础,检验包括我国规范液化判别方法、Robertson方法和Olsen方法的国内外现有静力触探试验为指标的液化判别方法的适用性。结果表明,我国规范、Robertson方法、Olsen方法在对巴楚地震液化场地判别中,总体上非液化场地判别成功率高于液化场地判别成功率,非液化场地的判别成功率分别为88%、71%和88%,但对液化场地判别成功率分别为55%、73%和45%,明显偏于危险,原因需要进一步查明,建立适合局部地区的液化判别方法应是未来必须进行的工作。     

汶川地震中甘肃清水田川黄土液化的试验研究

2008年5·12特大地震中,位于甘肃省清水县郭川乡的田川村发生了饱和黄土的液化滑移灾害。本文首先在对田川场地进行考察的基础上,综合田川在汶川地震中的震害情况以及滑移区的地形条件,将该地区在汶川地震中的烈度进行了修正。其次对田川黄土进行了物性指标测试以及室内动三轴液化试验,根据试验结果,综合考虑产生液化所需的场地及土性条件、黄土的动强度和液化特性,对田川黄土液化灾害进行了分析,并采用反应分析的方法对其进行了液化判定。研究结果证明了田川黄土液化的事实存在性,为低烈度区黄土液化提供了新的震害依据。

     

基于双桥静力触探的饱和砂土液化判别方法刍议

饱和砂土液化是地震引发地基变形失稳、建筑受损破坏的主因之一,合理判定液化可能性是勘察实践和理论研究的重大课题。国内规范要求砂土液化判别仍以标准贯入试验为主,静力触探等原位测试方法为辅,静力触探较其他原位测试手段,在操作便利性、数据采集连续性、测试成本和抗干扰能力等方面更具优势。针对国内规范常用静力触探砂土液化判别方法进行了分析研究,并结合勘察实例进行了计算和对比,探讨了静力触探判别液化方法的土层适用性,并提出有关改进建议。     

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.     

常德张家界高速公路某大桥桥基砂土液化评价

结合常德张家界高速公路某大桥桥基工程,在DSD160型电磁式振动三轴试验仪上,通过往返加荷三轴试验,对饱和砂土进行了液化试验研究,探讨了基于动三轴液化试验结果判断饱和砂土液化的方法。并尝试了这种室内研究反应分析的液化可能性估计方法与地震剪应力时程相结合的综合判断方法。在该高速公路大桥桥基的饱和砂土液化评价中,采用这种综合判断方法,对大桥桥基砂土液化进行了判断。在判断场地是否液化后,对其液化危害程度进行了等级划分,给出了该工程场地在未来遭受到不同超越概率下的地震作用时发生液化的危害程度,得到了一些有工程实用意义的结果。     

基于MPL神经网络的地震作用下砂土液化评估及预测

砂土在地震的作用下会产生剧烈的液化现象,液化引发的地基失稳会对道路、建筑物、堤坝等各类设施造成严重危害。因此,地震作用下的砂土液化判别预测一直是地质灾害领域研究的热点问题。本文使用过去几十年发生在世界各地的166组地震作用下砂土液化实例数据,通过大量数据训练和参数分析建立了基于机器学习的地震作用下砂土液化判别模型。结果表明,当网络结构为6(输入层)-15(隐藏层)-1(输出层)、训练函数为Levenberg-Marquardt时,对地震液化预测效果较好,最大准确率可达96%。参数分析结果表明不同参数对网络预测准确率影响程度不一:锥端阻力、地表归一化峰值水平加速度影响相对较大;地震震级、总垂向应力、有效垂向应力影响中等;贯入深度对其影响较小。因此在不同网络预测精度要求的条件下,可考虑适当简化输入参数。     

Review of soil liquefaction characteristics during major earthquakes of the twenty-first century

Liquefaction, which can be defined as a loss of strength and stiffness in soils, is one of the major causes of damage to buildings and infrastructure during an earthquake. To overcome a lack of comprehensive analyses of seismically induced liquefaction, this study reviews the characteristics of liquefaction and its related damage to soils and foundations during earthquakes in the first part of the twenty-first century. Based on seismic data analysis, macroscopic phenomena of liquefaction (e.g., sand boiling, ground cracking, and lateral spread) are summarized, and several new phenomena related to earthquakes from the twenty-first century are highlighted, including liquefaction in areas with moderate seismic intensity, liquefaction of gravelly soils, liquefaction of deep-level sandy soils, re-liquefaction in aftershocks, liquid-like behavior of unsaturated sandy soils. Additionally, phenomena related to damage in soils and foundations induced by liquefaction are investigated and discussed.     

Probabilistic and spatial liquefaction analysis using CPT data: a case study for Alameda County site

Soil liquefaction is one of the major concerns causing damage to the structures in saturated sand deposits during earthquakes. Simplified methods for the assessment of liquefaction potential rely on the limit states that are generally established with built-in conservatism and a great deal of subjectivity. Well-known SPT- and CPT-based methods are widely used in the design practice for this purpose due to their simplicity and reasonable predictive capability. However, these methods do not account for various sources of uncertainties explicitly. Moreover, evaluations are made only at the locations of test results and are generalized for the whole region, which may not give accurate results where spatial variation of soil properties is significant. The present study focuses on the probabilistic evaluation of liquefaction potential of Alameda County site, California, considering spatial variation of soil indices related to CPT soundings. A stochastic soil model is adopted for this purpose using random field theory and principles of geostatistics by developing 2D exponential correlation functions. It has been observed that the probability of liquefaction is significantly underestimated as much as 34 %, if the spatial dependence of soil indices is not considered. Further, the effect of spatial variation is more prominent in low-level earthquakes compared to the high-level earthquakes, showing a 41.5 % deviation for magnitude 8.1 and a 60.5 % deviation for magnitude 5.0 earthquake at a depth of 10 m.     

Triaxial tests on the fluidic behavior of post-liquefaction sand

Liquefaction-induced ground deformation is a major cause of structural damage during earthquakes. However, a better understanding of seismic liquefaction is needed to improve earthquake hazard analyses and mitigate structural damage. In this paper, a dynamic triaxial test apparatus was employed to investigate the fluidic characteristics of post-liquefaction sand. The specimens were vibrated to the point of liquefaction by dynamic loading, and then the liquefied sand was further sheared by triaxial compression in an undrained manner. It was found that a non-Newtonian fluid model can accurately describe the shear stress and the shear strain rate of post-liquefaction sand during undrained triaxial compression. The apparent viscosity, a major parameter in a constitutive model of a non-Newtonian fluid, decreases with an increase in the shear strain rate.     

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.     

Three-Dimensional Interpolation and Lithofacies Analysis of Granular Composition Data for Earthquake-Engineering Characterization of Shallow Soil

In Japan, many major cities are located on tectonic basins which are surrounded by faults and underlain by soft alluvial materials. Because these areas are subject to earthquake damages, it is important to determine their seismic engineering characteristics. Geotechnical databases which contain many borehole logs are useful information sources for this type of analysis. Each datum stored in the database has a value or an attribute, and its location is irregular in both horizontal and vertical directions. A new interpolation method based on the optimization principle is proposed here to deal with such three-dimensionally distributed data. Susceptibility of unconsolidated ground to liquefaction is known to be related to the content of loose and saturated sand. The mixture ratio of several soil types in a deposit, i.e., granular composition, is strongly influenced by the sedimentary environment. There are two numerical methods: the optimization principle method (OPM) used to determine three-dimensional distribution of granular composition and the model used to evaluate liquefaction. The application of the proposed methods to two locations in Japan indicated that the zones with high susceptibility to liquefaction were indeed those that had suffered from liquefaction during past earthquakes.     

地震成因软沉积物变形记录的地震强度研究进展

〗确定地震震级对中长期地震预报、震后应急救援和地震危险性评价具有重要意义。古地震学是研究地质记录中的地震事件,特别是它们的位置、时间和震级大小。然而,传统由地表破裂参数确定的古地震震级仍存在不确定性（大多数地震事件不会导致地表破裂,或位移小于0.3m）,尤其是由湖泊沉积记录的古地震事件。为了解决未发现明显位错地震震级问题,本文依据软沉积物变形构造的类型和形式,对确定地震震级/强度的方法（经验估算、最大液化距离、扰动层厚度、软沉积物变形类型,经验公式,快速沉积砂层厚度）进行总结和讨论,并分析其理论基础、优缺点、误差大小、适用性、存在问题等。并以中东死海盆地利桑组晚更新世湖相沉积中的震积岩(混杂层)和岷江上游萝卜寨晚第四纪湖相沉积中地震成因的液化底劈为例,利用上述6种方法推断,其代表的震级分别为M5.5~6.5和M6.0~7.0,进一步证实了前人的研究结果。这6种方法的结合,为确定地震震级/强度,特别是湖泊沉积中的地震事件提供了一种新的、相对便捷的方法。该研究可为基于地表破裂参数确定的古地震震级提供可靠的参考,为更好地认识构造活跃的地震活动性和危险性提供数据支持。