基于ANN和GIS耦合模型的场地地震液化势评价系统研究

采用组件式GIS (COMGIS)技术开发了结合BP神经网络分析模型的场地地震液化势评价系统,调用水平成层土地震反应分析程序SHAKE91实现设定地震下地震动影响场的模拟。在VB下调用Matlab神经网络工具箱来完成场地地震液化势评价模型在COMGIS系统中的模块化;利用GIS技术对评价结果,即液化势等级进行空间复合,给出场地潜在的地层液化势空间分布图。研究表明,SHAKE91应用程序在系统菜单下可直接调用,实现地震动影响场计算的模块化;BP神经网络技术应用于场地地震液化势评价中能达到较为理想的效果;系统的GIS空间分析功能可使评价结果与场地信息进行空间匹配,实现目标场地潜在地震液化势的快速评估。     

基于土层常规参数的剪切波速液化概率计算公式

分析国际上现有液化场地剪切波速473组数据进行参数特征后,以此为基础,采用分区方法,利用成熟的Logistic模型,提出了以地表峰值加速度、剪切波速、地下水位、可液化层埋深等4个参数表达的土体液化概率计算公式和不同概率下液化临界值计算公式,研究了不同概率水平下公式的适用性,并与现有主要方法进行比较。研究表明:地震动强度为液化判别首要影响参数,液化层与非液化层剪切波速区分度不显著,采用以往一个公式构造液化判别公式的方式难以要达到基本要求;现有Andrus公式和石兆吉公式会将很多明显为非液化的场地误判为液化场地,违背了现有认识,达到了不可接受的程度,需要改进。文中公式取50%概率时液化点和非液化点回判成功率符合对等原则,不同烈度下成功率均在70%左右;总体看,公式表现良好,可为中国工程建设提供一个合理、可操作性强的剪切波速液化概率判别方法。     

基于Logistic回归模型的砂土液化概率评价

以国内外23次地震中200组场地液化实测数据为基础,通过Logistic回归分析,建立关联修正标准贯入击数N160cs与循环应力比CSR的液化概率模型。以50 %液化概率水平为液化与非液化的临界点,建立了指数形式的抗液化应力比CRR计算式,新建概率模型预测饱和砂土液化与非液化的成功率分别为85.71 %和76.14 %,具有较高的可靠性。与已有模型比较,使用了新的数据和修正系数,消除了一些不合理的偏差,总体判别结果偏于安全。为了将确定性分析方法与概率分析方法联系起来,建立了抗液化安全系数FS与液化概率PL的关系式。算例结果表明,新建概率模型简单、实用、可靠。     

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。      

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。     

我国规范液化分析方法的发展设想

通过分析我国地震形势、液化震害潜在威胁以及震害防御工作现状,阐明了我国发展液化分析方法的客观需求,简要评述了我国规范中现有液化判别方法,总结了我国近来两次大地震液化考察经验,提出了与规范修订相关的研究设想。通过我国地震安全性评价工作的发展现状以及我国大陆地震重点监视区与第四纪沉积分布性态分析,说明了我国发展液化分析方法的必要性和紧迫性。根据2003年新疆巴楚地震和2008年汶川大地震中液化震害考察结果,比较我国规范现状,提出了与未来规范的完善和发展相关的10个研究课题,包括：液化对设计谱的影响、 特殊土类液化判定标准、区域性的液化判别标准、场地液化概率评价、基于液化土层PGD的结构物损害估计、液化引起地裂缝的生成条件、基于剪切波速的液化判别方法、VI度区内场地液化及危害性判定、深层土液化判定以及场地液化的现场判定和识别技术     

利用地震动强度指标评价场地液化的离心模型试验研究

场地地震液化灾变(液化触发与震后变形)评价对基础设施抗震设计和安全服役有重要意义。为研究目前常用地震动强度指标IM的液化灾变评价能力,开展水平饱和砂土场地离心机振动台模型试验,对模型在50 g离心加速度下进行了20次不同幅值的振动,得到了液化与非液化响应数据。基于振动台台面输入建立峰值加速度a_(max)、地震剪应力比CSR、阿里亚斯强度I_a和累积绝对速度CAV_5的算法,并利用模型试验数据检验了这几种IM与液化触发和震后变形的相关性。分析表明,几类IM对液化触发的评价能力接近,且从超静孔压产生到初始液化触发都存在明显的IM阈值;几类IM对震后沉降评价能力有一定差异,其中I_a和CAV_5优于a_(max)和CSR,并对造成上述差异的原因做了初步分析。文中研究为选择合理的地震动强度指标评价场地液化灾变提供了科学依据。     

考虑地震随机特征的液化侧向变形超越概率

基于液化侧向变形实用统计模型和地震概率模型,建立了可以考虑地震随机特征和土体性质不确定性的液化侧向变形超越概率模型框架,通过实际案例初步探讨了模型的有效性,并将超越概率模型与现有统计模型的预测结果进行了对比。分析结果表明,若液化侧向变形的条件概率满足正态分布,标准差在5%到20%期望值范围内变化时,对位移超越概率影响不大;若满足对数正态分布,标准差对超越概率有一定影响。实用统计模型只能预测指定地震水平下的液化侧向变形值,而超越概率模型考虑了指定时间内所有可能地震的发生概率,可以同时预测变形值及发生概率,更加适合用于区域性的地震液化灾害评估。     

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。     

不同地震动强度下珊瑚礁砂地基中桩-土-结构地震响应试验研究

珊瑚礁砂在地震作用下的场地响应受其特殊的工程性质影响。为揭示珊瑚砂场地桩基-地基-上部结构在地震作用下的动力响应特性,开展了不同地震动强度下珊瑚砂地基上三层框架结构群桩基础的振动台模型试验,对地基土和结构物的动力响应进行测试与分析,同时与可液化福建砂场地进行对比研究。结果表明,0.1 g正弦波激励下,两种砂模型地基内各处超孔压比均远小于1,模型地基未发生液化。0.2 g正弦波激励下,两种砂模型地基发生液化,珊瑚砂场地液化程度小于福建砂场地,液化后的珊瑚砂场地模型地基相比福建砂场地仍具有一定的剪切传递能力和刚度。0.1 g和0.2 g振动强度下珊瑚砂场地建筑物沉降、水平位移和立柱动弯矩相比福建砂场地较小。不同振动强度下桩基础出现动弯矩峰值的位置不同。     

Probabilistic evaluation of seismic soil liquefaction potential based on SPT data

The performance-based liquefaction potential analysis was carried out in the present study to estimate the liquefaction return period for Bangalore, India, through a probabilistic approach. In this approach, the entire range of peak ground acceleration (PGA) and earthquake magnitudes was used in the evaluation of liquefaction return period. The seismic hazard analysis for the study area was done using probabilistic approach to evaluate the peak horizontal acceleration at bed rock level. Based on the results of the multichannel analysis of surface wave, it was found that the study area belonged to site class D. The PGA values for the study area were evaluated for site class D by considering the local site effects. The soil resistance for the study area was characterized using the standard penetration test (SPT) values obtained from 450 boreholes. These SPT data along with the PGA values obtained from the probabilistic seismic hazard analysis were used to evaluate the liquefaction return period for the study area. The contour plot showing the spatial variation of factor of safety against liquefaction and the corrected SPT values required for preventing liquefaction for a return period of 475 years at depths of 3 and 6 m are presented in this paper. The entire process of liquefaction potential evaluation, starting from collection of earthquake data, identifying the seismic sources, evaluation of seismic hazard and the assessment of liquefaction return period were carried out, and the entire analysis was done based on the probabilistic approach.     

以标贯试验为依据的砂土液化确定性及概率判别法

核电厂址非基岩场地的地基液化问题是核电厂选址的关键问题,亟需建立核电厂址地基液化判别方法。回顾了以标贯试验和地表峰值加速度为依据的砂土液化判别方法的演化历史,依据Idriss-Boulanger确定液化临界曲线的基本方法,提出了确定液化临界曲线的基本原则,分别依据美国液化数据库、中国抗震规范液化判别式所用的液化数据及综合两者的液化数据资料,给出了相应的液化临界曲线,验证了液化临界曲线的位置对不同的细粒含量、有效上覆压力、现场试验方法的液化数据的合理性,分析了测量或估计土层循环应力比和修正标贯击数各种因素的不确定性对液化临界曲线的敏感性,结果表明：所提的液化临界曲线不易受各种因素的影响。利用Monte Carlo模拟、加权最大似然法和加权经验概率法,给出了液化临界曲线的名义抗液化安全系数与液化概率的经验关系式及概率等值线,并对核电厂Ⅰ类、Ⅱ类和Ⅲ类抗震物项地基,给出了相应的液化临界曲线。     

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.     

Geotechnical reconnaissance and liquefaction analyses of a liquefaction site with silty fine sand in Southern Taiwan

Geotechnical reconnaissance of a recurrent liquefaction site at a Quaternary alluvial deposit in southern Taiwan was conducted to establish a comprehensive case history for liquefaction on silty fine sand with high fines content. The liquefaction occurred at a silty fine sand layer with D₅₀ = 0.09 mm and fines content greater than 35% and was triggered by a M_w = 6.4 earthquake on March 4, 2010, which induced maximum horizontal acceleration up to 0.189 g at the site. In situ subsurface characterizations, including standard penetration test, cone penetration test, and shear wave velocity measurement, were performed as well as cyclic simple shear tests on undisturbed specimens retrieved by a modified hydraulic piston sampler. Comparisons of cyclic resistance ratios (CRRs) indicate that CPT sounding with standard penetration rate could overestimate the resistance ratio and drainage conditions during penetration should be considered for high fines content soil in the liquefaction analysis. Additionally, variations of CRRs from different in situ tests indicate that correlations among in situ tests and CRR could be soil specific and precautions should be taken when using these curves on silty fine sands.     

Seismic microzonation of a nuclear power plant site with detailed geotechnical,geophysical and site effect studies

This paper highlights the seismic microzonation carried out for a nuclear power plant site. Nuclear power plants are considered to be one of the most important and critical structures designed to withstand all natural disasters. Seismic microzonation is a process of demarcating a region into individual areas having different levels of various seismic hazards. This will help in identifying regions having high seismic hazard which is vital for engineering design and land-use planning. The main objective of this paper is to carry out the seismic microzonation of a nuclear power plant site situated in the east coast of South India, based on the spatial distribution of the hazard index value. The hazard index represents the consolidated effect of all major earthquake hazards and hazard influencing parameters. The present work will provide new directions for assessing the seismic hazards of new power plant sites in the country. Major seismic hazards considered for the evaluation of the hazard index are (1) intensity of ground shaking at bedrock, (2) site amplification, (3) liquefaction potential and (4) the predominant frequency of the earthquake motion at the surface. The intensity of ground shaking in terms of peak horizontal acceleration (PHA) was estimated for the study area using both deterministic and probabilistic approaches with logic tree methodology. The site characterization of the study area has been carried out using the multichannel analysis of surface waves test and available borehole data. One-dimensional ground response analysis was carried out at major locations within the study area for evaluating PHA and spectral accelerations at the ground surface. Based on the standard penetration test data, deterministic as well as probabilistic liquefaction hazard analysis has been carried out for the entire study area. Finally, all the major earthquake hazards estimated above, and other significant parameters representing local geology were integrated using the analytic hierarchy process and hazard index map for the study area was prepared. Maps showing the spatial variation of seismic hazards (intensity of ground shaking, liquefaction potential and predominant frequency) and hazard index are presented in this work.     

A genetic algorithm approach for assessing soil liquefaction potential based on reliability method

Deterministic approaches are unable to account for the variations in soil’s strength properties, earthquake loads, as well as source of errors in evaluations of liquefaction potential in sandy soils which make them questionable against other reliability concepts. Furthermore, deterministic approaches are incapable of precisely relating the probability of liquefaction and the factor of safety (FS). Therefore, the use of probabilistic approaches and especially, reliability analysis is considered since a complementary solution is needed to reach better engineering decisions. In this study, Advanced First-Order Second-Moment (AFOSM) technique associated with genetic algorithm (GA) and its corresponding sophisticated optimization techniques have been used to calculate the reliability index and the probability of liquefaction. The use of GA provides a reliable mechanism suitable for computer programming and fast convergence. A new relation is developed here, by which the liquefaction potential can be directly calculated based on the estimated probability of liquefaction (P _L ), cyclic stress ratio (CSR) and normalized standard penetration test (SPT) blow counts while containing a mean error of less than 10% from the observational data. The validity of the proposed concept is examined through comparison of the results obtained by the new relation and those predicted by other investigators. A further advantage of the proposed relation is that it relates P _L and FS and hence it provides possibility of decision making based on the liquefaction risk and the use of deterministic approaches. This could be beneficial to geotechnical engineers who use the common methods of FS for evaluation of liquefaction. As an application, the city of Babolsar which is located on the southern coasts of Caspian Sea is investigated for liquefaction potential. The investigation is based primarily on in situ tests in which the results of SPT are analysed.     

砂土地震液化的模糊概率评判方法

利用模糊数学中的模糊概率理论,建立了砂土地震液化的模糊概率综合评判模型。在此模型中,提出了模糊权重的概念,可充分考虑权重的模糊性,从而避免权重取值带来的不确定性。结合砂土地震液化特点,选取地震烈度、标准贯入击数、平均粒径和上覆有效压力作为主要评价影响因子,同时将液化程度划分为不液化、轻微液化、中等液化和严重液化4个等级,进而使其评判结果更为精细化。通过算例分析,表明文中方法对砂土液化评判的合理性与有效性。     

Assessing probability of surface manifestation of liquefaction at a given site in a given exposure time using CPTU

This paper is a follow-up to a previous paper on the subject of liquefaction potential index (LPI), a parameter that is often used to characterize the potential for surface manifestation of liquefaction at a given site subjected to a given shaking level (represented by a pair of peak ground surface acceleration a_max and moment magnitude M_w). In the previous paper by Juang and his coworkers, the LPI was re-calibrated for a piezocone penetration test (CPTU) model, and a simplified model based on LPI was created for computing the conditional probability of surface manifestation of liquefaction (P_G). In this paper, the model for this conditional probability P_G is extended into a complete framework for assessing the probability of surface manifestation of liquefaction in a given exposure time at a given site subjected to all possible ground motions at all seismic hazard levels. This new framework is formulated and demonstrated with an example site in 10 different seismic regions in the United States.