唐山地震时不同烈度区液化砂土的特征及其液化判别

本文根据唐山地区的东、中、西三条剖面上的地震喷砂的物质成分和理化性质等分析,探讨了不同烈度区液化砂土的颗粒成分、化学成分、物理性质及砂颗粒形态等变化规律,并根据这些规律,编制出以物质成分和性质为基础的、具有多种因素的砂土液化判别图。该图可供砂土液化预测、砂土液化研究者参考     

砂土地震液化的神经网络预测

BP网络具有很强的非线性映射和自适应学习功能 ,可用于模式识别和预测评估等领域 .在简要分析BP算法的基础上 ,选取砂土的平均粒径 (d5 0 /mm)、相对密度(Dr/% )、标准贯入击数 (N63 .5 /击 )、上覆有效压力 (σv/kPa)、地震烈度 (I0 )作为指标 ,应用BP神经网络的理论与方法 ,预测砂土在地震作用下液化的可能性 ,取得了较好的预测效果 .说明将BP网络用于沙土液化预测是可行的 .     

液化场地区间隧道地震响应分析

为研究液化场地区间隧道的动力响应,利用FLAC3D软件分析一致激励、局部液化和行波激励对隧道结构动力响应的影响.计算结果表明:地基土液化改变了隧道衬砌的受力状态,衬砌的破坏位置由非液化场地的拱肩和拱脚处转成了液化场地的仰拱和拱腰位置;与液化场地相比,隧道穿越局部可液化土层时,破坏位置扩展到衬砌整个断面;与一致激励相比,...     

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。     

场地的地震效应及砂土地基的液化

论述了场地地震效应的表现形式，影响砂土液化的主要因素和砂土液化危害的主要特点。     

黄土场地地震液化研究

通过往返加荷动三轴试验,对兰州某民用机场扩建工程场地的饱和黄土和砂土进行了液化试验。在试验结果的基础上,运用抗液化剪应力判别方法、地震危险性分析计算结果以及根据有限元一维模型计算求得的该场地的地震剪应力,对该场地饱和黄土和砂土的地震液化进行了综合判断。结果表明,在未来遭受到50年10％超越概率的地震作用时,该场地的饱和黄土比饱和砂土更容易发生液化。     

汶上县城区砂土地震液化调查分析

通过钻孔、标准贯入试验等手段,对汶上县城区进行砂土地震液化调查分析。划分砂土液化等级,并给出汶上县城区在Ⅶ度、Ⅷ度时的砂土液化分布图,为城市建设工程抗震设防提供依据。     

砂土地震液化危害及地基处理研究

砂土在地震荷载作用下极易发生液化，液化土地基对建筑物造成的危害不可忽视。分析了液化的机理、影响因素及现行的研究、处理方法。通过实例说明：CFG桩和碎石桩共用，对处理内蒙古地区砂土液化，具有明显的抗液化效果。最后，提出CFG桩和碎石桩抗液化需进一步研究的问题。     

利用qc,D50评价砂土地震液化势研究

本文根据国内外地震液化调查资料，建立了利用双桥静力触探锥头阻力ｑｃ和平均粒径Ｄ５０判别地震液化势的半经验性方法。这种方法对评价地震液化势是颇为有效的。     

基于Monte Carlo模拟技术的液化危险性分析

利用Monte Carlo模拟技术考虑了震源和震级的贡献及抗液化阻力的概率分布,将液化安全系数的倒数作为工程需求参数,对土层的液化危险性作概率估计,可以满足基于性态的抗震设计对于不同设防水平工程场地液化安全判定的要求。在文中应用该方法对北京地区若干场址多种设定土层情况分别进行了液化危险性分析,并在所得的大量模拟样本的基础上,引入液化需求锤击数基准值概念和土层埋深水位影响系数,进行统计分析,提出了本地区简化的液化危险性估计方法。验算表明,该简化方法有一定的合理性,也便于一般设防工程使用。作者认为其它地区也可以依此途径建立适合于该地区的液化危险性的估计方法。     

Probability density evolution method for seismic liquefaction performance analysis of earth dam

To investigate the seismic liquefaction performance of earth dams under earthquake loading, we present a new methodology for evaluating the seismic response of earth dams based on a performance‐based approach and a stochastic vibration method. This study assesses an earthfill dam located in a high‐intensity seismic region of eastern China. The seismic design levels and corresponding performance indexes are selected according to performance‐based criteria and dam seismic codes. Then, nonlinear constitutive models are used to derive an array of deterministic seismic responses of the earth dam by dynamic time series analysis based on a finite element model. Based on these responses, the stochastic seismic responses and dynamic reliability of the earth dam are obtained using the probability density evolution method. Finally, the seismic performance of the earth dam is assessed by the performance‐based and reliability criteria. Our results demonstrate the accuracy of the seismic response analysis of earth dams using the random vibration method. This new method of dynamic performance analysis of earth dams demonstrates that performance‐based criteria and reliability evaluation can provide more objective indices for decision‐making rather than using deterministic seismic acceleration time series as is the current normal practice. Copyright © 2016 John Wiley & Sons, Ltd.     

饱和砂土透镜体液化对建筑物地震反应的影响

采用一种能分析有结构物存在的场地地震液化问题的二维有效应力有限元分析方法,研究饱和砂土透镜体液化对建筑物地震反应的影响。计算中采用了更为合理的迭代方式处理土的非线性,考虑了Ｋｃ对孔压的作用,引入了透射边界。取建筑物为短周期结构。考虑了透镜化宽度、厚度、埋深以及输入地震动类型幅值对结构加速度反应的影响。计算结果表明：（１）所采用的方法与已有的模型实验结果有很好的对应关系,可用于招考莪存在下的场地砂土     

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.     

Aspects of bridge-ground seismic response and liquefaction-induced deformations

Considerable bridge-ground interaction effects are involved in evaluating the consequences of liquefaction-induced deformations. Due to seismic excitation, liquefied soil layers may result in substantial accumulated permanent deformation of sloping ground near the abutments. Ultimately, global response is dictated by the bridge-ground interaction as an integral system. However, a holistic assessment of such response generally requires a highly demanding full three-dimensional (3D) model of the bridge and surrounding ground. As such, in order to capture a number of the salient involved mechanisms, this study focuses on the longitudinal seismic performance of a simpler idealized configuration, motivated by details of an existing bridge-ground configuration. In this model, a realistic multilayer soil profile is considered with interbedded liquefiable/nonliquefiable strata. The effect of the resulting liquefaction-induced ground deformation is explored. Attention is given to overall deformation of the bridge structure due to lateral spreading in the vicinity of the abutments. The derived insights indicate a need for such global analysis techniques, when addressing the potential hazard of liquefaction and its consequences.     

边坡地震稳定性分析探讨

传统的拟静力法和安全系数时程分析法在评价边坡地震稳定性时存在一定的局限性。在提出准确的评价边坡地震稳定性必需因素的基础上,建议对边坡地震稳定性分析方法重新进行分类。根据动力分析得到的边坡在地震作用下的破坏机制和破裂面的性质和位置,提出基于拉-剪破坏的动力时程分析法和强度折减动力分析法。第一种方法将FLAC计算得到破坏时刻的动应力施加到静力情况下边坡上,采用动力分析得到的拉-剪破裂面,结合极限平衡法求解边坡地震安全系数,是一种改进的动力有限元时程分析法;第二种方法考虑了拉-剪破坏的FLAC强度折减动力分析法,是完全动力的方法。最后通过算例分析验证了新方法的可行性,为边坡地震安全系数计算提供了一种新的思路。     

地震边坡稳定性分析的拟静力方法适用性探讨

应用极限平衡拟静力方法进行地震边坡稳定性分析时,对粘聚力较小、内摩擦角较大的浅层滑坡而言,随着地震水平设计加速度值的增大,其临界滑动面基本保持不变;然而,对粘聚力较大而摩擦角较小的深层滑坡而言,随着地震水平设计加速度值的增大,临界滑动面越深、越宽,会呈现出发散现象,选取两个典型的均质土坡,设计了由浅及深的多条滑动面,分别采用极限平衡瑞典方法进行了相应安全系数的计算,同时比较了自重、地震力、粘聚力在抗滑力矩以及滑动力矩中所占比重的变动情况。研究发现:对于浅层滑坡,随着滑动面由浅及深的变化,粘聚力产生的抗滑力矩所占百分比下降幅度较大,自重产生的滑动力矩与总体抗滑力矩的比值下降幅度较大;而对于深层滑坡,随着滑动面由浅及深变化,自重产生的抗滑力矩所占百分比增长的幅度较大,地震力产生的滑动力矩与总体抗滑力矩比值增长幅度也较大。     

Modal analysis of pile‐supported structures during seismic liquefaction

The purpose of this paper is to investigate the effects of liquefaction on modal parameters (frequency and damping) of pile‐supported structures. Four physical models, consisting of two single piles and two 2 × 2 pile groups, were tested in a shaking table where the soil surrounding the pile liquefied because of seismic shaking. The experimental results showed that the natural frequency of pile‐supported structures may decrease considerably owing to the loss of lateral support offered by the soil to the pile. On the other hand, the damping ratio of structure may increase to values in excess of 20%. These findings have important design consequences: (a) for low‐period structures, substantial reduction of spectral acceleration is expected; (b) during and after liquefaction, the response of the system may be dictated by the interactions of multiple loadings, that is, horizontal, axial and overturning moment, which were negligible prior to liquefaction; and (c) with the onset of liquefaction due to increased flexibility of pile‐supported structure, larger spectral displacement may be expected, which in turn may enhance P‐delta effects and consequently amplification of overturning moment. Practical implications for pile design are discussed. Copyright © 2013 John Wiley & Sons, Ltd.     

Wavelet analysis for relating soil amplification and liquefaction effects with seismic performance of precast structures

Precast concrete structures are preferred for facilities with large open areas due to easiness in construction. Such structures are typically composed of individual columns and long‐span beams, and are quite flexible and of limited redundancy. In this paper, nonlinear dynamic analyses of a typical such structure are conducted using as excitation 54 ground motions recorded on top of a variety of soils (hard, soft, and liquefied soil sites). The results show that liquefaction‐affected level‐ground motions systematically impose a greater threat to precast‐concrete structures in terms of seismic demand, even when low values of elastic spectral acceleration prevail, as opposed to soft‐soil records and even more to hard‐soil ones. Thus, elastic spectral acceleration appears to be an insufficient engineering demand parameter for design. Soil effects, the “signature” of which is born on ground motions, are first uncovered using wavelet analysis to detect the evolution of the energy and frequency content of the ground motion in the time domain. From this, the changes in effective (“dominant”) excitation period are noted, persuasively attributed to the nature of the soil, and finally correlated with the observed structural behavior. Copyright © 2016 John Wiley & Sons, Ltd.