A fundamental procedure and calculation formula for evaluating gravel liquefaction

Field investigations following the 2008 Ms8.0 Wenchuan earthquake identified 118 liquefaction sites, most of which are underlain by gravelly sediment in the Chengdu Plain and adjacent Mianyang area, in the Sichuan Province. Gravel sediment in the Sichuan province is widely distributed; hence it is necessary to develop a method for prediction and evaluation of gravel liquefaction behavior. Based on liquefaction investigation data and in-situ testing, and with reference to existing procedures for sandy soil l...     

碎石桩处理液化地基效果评价

本文在考虑碎石桩排水和应力集中作用的基础上分析了碎石桩处理液化地基后复合地基的抗液化能力,并与自由场液化地基的抗液化能力进行了对比,提出了一种适用于碎石桩复合地基修正“Seed简化法”,可应用标准贯入试验对液化地基的处理效果进行评价。     

砂砾土动力特性的室内试验研究进展

简要回顾了国内外有关砂砾土动剪切模量和阻尼比的试验研究进展,总结了围压、固结比、颗粒形状、含砾量、颗粒破碎等因素对砂砾土动剪切模量、阻尼比及归一化动剪切模量比与阻尼比曲线特征的影响规律,发现砂砾土归一化动剪切模量比和阻尼比的曲线形状主要受围压大小的影响,其他因素的影响不明显;总结了砂砾土抗液化强度、振动孔压发展特征、液化判别方法方面的研究进展,含砾量、试样扰动、膜贯效应是影响砂砾土抗液化强度的主要因素;指出了砂砾土动力特性研究中存在的问题及今后应予以更深入关注的研究方向。     

汶川大地震液化的特点及带来的新问题

总结汶川8.0级大地震液化及其震害科学考察结果,分析此次地震液化带来的新问题.结果表明,汶川大地震中液化及其震害现象显著,是建国以来液化涉及区域最广、液化宏观现象最为丰富的一次;本次地震液化宏观现象特点为喷水高、持时短、喷砂量小但喷砂类型十分丰富;砂砾土液化、Ⅵ度区内场地液化、深层土液化、无液化减震以及液化普遍伴随地裂缝是此次地震液化5个突出特征;砂砾土液化判别技术、液化引起地表裂缝机理和预测方法、Ⅵ度区内场地液化与深层土的液化机理和预测方法、液化加减震发生条件、场地液化现场判定与识别技术等是新的课题.为解决上述问题,应优先发展岩土地震工程观测技术及建设岩土地震工程实验场.     

Predicting seismic liquefaction potential of sands by optimum seeking method

The feasibility of using the optimum seeking method to assess the seismic liquefaction potential of sands has been investigated. Optimization theory is a very important branch of applied mathematics and has a wide application in the practical world. Using the available field sand liquefaction data, the influence of various factors is quantified using the optimum seeking method. The factors considered are: the earthquake magnitude M, the distance of the site from the source of the earthquake L, the depth of the water table D_w, the depth of the sand deposit D_s, and the standard penetration test (SPT) blow count N. The most important factors have been identified as the earthquake magnitude and the SPT blow count. Prediction results show that the proposed method is effective and feasible. Since neither normalization of the SPT blow count nor calculation of the seismic shear-stress ratio are required, the proposed method is simpler and more direct than the conventional methods of evaluating liquefaction potential.     

A semi-empirical formula for evaluating residual strain of soils under earthquake loading

A new semi-empirical formula for evaluating the residual strain of soils under earthquake loading is presented in this paper based on the incremental method and the increment model proposed by the authors. When the incident loading is uniform, the results calculated by the new formula are nearly the same as those by the existing formula. For excitation of the random earthquake loading, the results calculated by the new formula are compared to the results obtained by dynamic triaxial tests. The dynamic triaxial tests had been performed considering different seismic waves, confining stresses,consolidation ratios, and types of cohesive soils. The comparison between the calculated and tested results indicate that the presented formula can efficiently and practically describe the time-dependent process of the soil residual strains under actual seismic loads.     

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。     

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.     

徐州市棠张镇饱和粉土液化性能试验研究

徐州市棠张镇高速铁路路基的粉土粘粒含量少于1．5％、粉粒含量约为80％,在强烈地震作用下存在着液化可能性,为了分析该路基粉土的液化特性,进行了多组振动三轴液化试验。对试验结果进行分析发现,该地区的粉土振动孔压发展模式与Seed提出的用反正弦三角函数拟合的砂十振动孔压发展模式不同,可以用双曲线进行拟合。采用最大往返剪切作用面上的应力条件确定了该地区的孔压发展模式的拟合参数,为该地区粉土地震液化动力计算分析提供了所需的地震孔隙水压力增长模型。同时,本文指出粉土的密度和结构是振动孔压发展的重要影响因素。     

Behavior of stratified sand–silt–gravel composites under seismic liquefaction conditions

Laboratory seismic liquefaction studies have generally dealt with homogeneous soil conditions only, although stratified soils exist for various soil deposits. The main objective of this research project was to compare the behavior of stratified and homogeneous sand–silt–gravel composites during seismic liquefaction conditions for various silt and gravel contents. An experimental program was undertaken in which a total of eighty stress-controlled undrained cyclic triaxial tests were performed. Two methods of sample preparation were used for each soil type. These methods included moist tamping (representing homogeneous soil conditions) and sedimentation (representing layered soil conditions). The silt contents ranged from 0 to 50%, and soils with 10 and 30% gravel contents were tested. The confining pressure in all test series was 100 kPa. The results indicate that the liquefaction resistances of layered and uniform soils are not significantly different, despite the fact that the soil fabric produced by the two methods of sample preparation is totally different. This finding justifies applying the laboratory tests results to the field conditions for the range of variable studied.     

动载作用下饱和土壤液化的研究述评

从土壤液化的机理、影响因素、液化的判别、液化的分析方法以及液化后土壤的性质等方面详述了动载(地震荷载和爆炸荷载)作用下的饱和土壤液化的国内外研究成果(主要是近十年的研究),并对其作出了述评。最后,对今后土壤液化的研究工作作出了展望,认为以下问题需要开展深入研究:⑴液化分析中的土骨架的动力本构模型;⑵Rayle igh波对地震液化的影响;⑶建构物的存在对液化的影响;⑷液化后土的性质和液化引起的建构物破坏;⑸尤其爆炸液化问题。     

An improved method of evaluating liquefaction potential with the velocity of shear-waves

According to the results of cyclic triaxial tests, a linear correlation is presented between liquefaction resistance and elastic shear modulus, which shows the relation of G _max (kPa) with (σ_d/2)^1/2(kPa)^1/2. When applied to soils from different sites, the correlation can be normalized in reference to its minimum void ratio (e _min). Accordingly, an improved method is established to evaluate the liquefaction potential with shear-wave velocity. The critical shear-wave velocity of liquefaction is in linear relation with 1/4 power of depth and the maximum acceleration during earthquakes, which can be used to explain the phenomenon that the possibility of liquefaction decreases with the increment of the depth. Compared with previous methods this method turns out simple and effective, which is also verified by the results of cyclic triaxial tests. Foundation item: State Natural Science Foundation (59678020) and Natural Science Foundation of Zhejiang Province (RC9609).     

关于砂土液化判别的若干意见

简要回顾了我国抗震规范中液化判别方法的发展历史,并对我国若干规范中的液化初步判别的条文进行了比较,提出了修改意见;对我国有代表性的几种液化判别方法及基于Seed简化方法修正的美国NCEER法进行了评述和综合对比,指出了我国现行若干规范中有关液化判别规定方面的错误;此外,还将GB50011－2001《建筑抗震设计规范》液化判别公式转变为类似于NCEER法的表达式,拓宽了该规范方法的应用范围。最后,结合某长江大桥工程的地基液化判别工作,探讨了深层土液化判别的可能性,结果表明,本文建议的方法是可行的。     

Probabilistic evaluation of liquefaction potential under earthquake loading

This paper presents a procedure to perform the risk analysis for ground failure by liquefaction. The first part of this study describes the differential equation of a smooth hysteretic model to characterize the behavior of the soil under random loading. The parameters of the proposed model to represent the experimental relationship are discussed. The second part of this study is to develop a method to calculate the probability that a specified volume of soil will liquefy at a given depth in the deposit. The liquefaction is defined as the result of cumulative damage caused by seismic loading. The fatigue life of soil can be determined on the basis of the N---S relationship and Miner's cumulative damage law. The rain-flow method is used to count the number of cycles of stress response of the soil deposit. Finally, the probability of liquefaction is obtained by integration over all the possible ground motion and the fragility curves of liquefaction potential. The sensitivity of the reliability against liquefaction to soil system parameters is also examined.     

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。     

Seismic liquefaction potential assessment by using Relevance Vector Machine

Determining the liquefaction potential of soil is important in earthquake engineering. This study proposes the use of the Relevance Vector Machine (RVM) to determine the liquefaction potential of soil by using actual cone penetration test (CPT) data. RVM is based on a Bayesian formulation of a linear model with an appropriate prior that results in a sparse representation. The results are compared with a widely used artifi cial neural network (ANN) model. Overall, the RVM shows good performance and is proven to be more accurate than the ANN model. It also provides probabilistic output. The model provides a viable tool for earthquake engineers to assess seismic conditions for sites that are susceptible to liquefaction.     

饱和粉土液化特性的大型振动台模型试验研究

京沪高速铁路徐沪段路基的粉土粘粒含量少于1.5%、粉粒含量约为80%,在强烈地震作用下存在着液化可能性.为充分研究这一饱和粉土地层的液化特性,本文作者利用大型地震模拟振动台,进行了模拟自由场地饱和粉土的地震液化模型试验,试验结果再现了自然地震触发的粉土液化的各种宏观震害现象,揭示了饱和粉土的地震液化规律和特征。试验结果为京沪高速铁路徐沪段路基的抗震设计提供了参考依据。     

Neural network model for liquefaction potential in soil deposits using Turkey and Taiwan earthquake data

Simplified methods have been practiced by researchers to assess nonlinear liquefaction potential of soil. Derived from several field and laboratory tests, various simplified procedures such as stress-based, strain-based, Chinese criteria, etc. have been developed by utilizing case studies and undisturbed soil specimens. In order to address the collective knowledge built up in conventional liquefaction engineering, an alternative general regression neural network model is proposed in this paper.To meet this objective, a total of 620 sets of data including 12 soil and seismic parameters are introduced into the model. The data includes the results of field tests from the two major earthquakes that took place in Turkey and Taiwan in 1999 and some of the desired input parameters are obtained from correlations existing in the literature.The proposed GRNN model was developed in four phases, mainly: identification phase, collection phase, implementation phase, and verification phase. An iterative procedure was followed to maximize the accuracy of the proposed model. The case records were divided randomly into testing, training, and validation datasets.Generating a model that takes into account of 12 soil and seismic parameters is not feasible by using simplified techniques; however, the proposed GRNN model effectively explored the complex relationship between the introduced soil and seismic input parameters and validated the liquefaction decision obtained by simplified methods. The proposed GRNN model predicted well the occurrence/nonoccurrence of soil liquefaction in these sites. The model provides a viable tool to geotechnical engineers in assessing seismic condition in sites susceptible to liquefaction.     

液化地基自由场振动台模型试验研究

进行了液化场地自由场振动台试验,试验采用柔性容器以减小边界影响,采用上覆黏土层的饱和砂土作为模型土。试验中再现了液化场地土的震害现象。得出的主要规律有:随着振次的增加,地基的频率迅速降低,阻尼比迅速增大;砂土对地震动起滤波作用;土体的加速度峰值反应在高度上呈"K"形分布;当最初加速度峰值到达前,砂土层中的孔压比存在负值;震后土中振动孔隙水压力不一定随振动的停止而立即开始消散,在短期内可能继续增长。