Earthquake imprints on a lacustrine deltaic system: The Kürk Delta along the East Anatolian Fault (Turkey)

Deltas contain sedimentary records that are not only indicative of water‐level changes, but also particularly sensitive to earthquake shaking typically resulting in soft‐sediment‐deformation structures. The Kürk lacustrine delta lies at the south‐western extremity of Lake Hazar in eastern Turkey and is adjacent to the seismogenic East Anatolian Fault, which has generated earthquakes of magnitude 7. This study re‐evaluates water‐level changes and earthquake shaking that have affected the Kürk Delta, combining geophysical data (seismic‐reflection profiles and side‐scan sonar), remote sensing images, historical data, onland outcrops and offshore coring. The history of water‐level changes provides a temporal framework for the depositional record. In addition to the common soft‐sediment deformation documented previously, onland outcrops reveal a record of deformation (fracturing, tilt and clastic dykes) linked to large earthquake‐induced liquefactions and lateral spreading. The recurrent liquefaction structures can be used to obtain a palaeoseismological record. Five event horizons were identified that could be linked to historical earthquakes occurring in the last 1000 years along the East Anatolian Fault. Sedimentary cores sampling the most recent subaqueous sedimentation revealed the occurrence of another type of earthquake indicator. Based on radionuclide dating (¹³⁷Cs and ²¹⁰Pb), two major sedimentary events were attributed to the ad 1874 to 1875 East Anatolian Fault earthquake sequence. Their sedimentological characteristics were determined by X‐ray imagery, X‐ray diffraction, loss‐on‐ignition, grain‐size distribution and geophysical measurements. The events are interpreted to be hyperpycnal deposits linked to post‐seismic sediment reworking of earthquake‐triggered landslides.     

厦门砂土的动力特性研究

为了探讨厦门砂土的动力特性,对厦门金秋豪园场地的中粗砂和细粉砂做了一系列振动三轴试验,并对同一场地的含泥中砂做了4个共振柱试验。根据试验结果,确定了中粗砂与细粉砂的液化曲线和动本构关系,提出了相应的动孔压模型,得到了含泥中砂在小应变条件下动剪切模量和阻尼比的变化规律。     

饱和粉土振动液化分析

液化是造成场地地震破坏的首要原因之一。自Casagrande的经典工作以来,对地震液化的研究已经取得了很大的进展。然而这些研究大多是针对于砂土而进行的,对于粉土液化研究的相对较少,且粉土的液化特性也有别于砂土。因此,在已有研究的基础之上利用粉土液化试验得出的结果,分析了粉土液化的机理、影响因素以及在振动过程中粉土中孔隙水压力的增长规律,认为粉土中的粘粒含量、密实度以及土的结构性对其抗液化能力有较大的影响。考虑到试验中振动次数的离散性,引入了时间参数的概念,根据动三轴试验结果提出了孔隙水压力增长的经验公式,可以比较方便地应用于计算液化的有限元程序中去。     

杭州钱塘江防洪堤抗震分析

通过动力试验研究了杭州钱塘江防洪堤基的动力特性,并用原位和室内分析法判别了堤基液化的可能性,在有限元分析时,改进了工程上常用的排水有效应力动反应分析方法,在地震期间采用Biot动力固结方程进行动力分析,可以同时考虑地震期间孔压的上升及固结消散对防洪堤的抗震液化和稳定性进行分析。采用非线性成层地基反演技术,反演分析基岩地震加速度曲线,对堤坝地震反应进行分析。地震响应的各种结果对于了解今后防洪堤在地震期间的运行状态具有一定指导意义。     

饱和砂砾料振动孔压与轴向应变发展模式研究

利用自行研制、开发的高精度中型动三轴仪,对文选定的砂砾料液化过程中的孔隙水压力增长和轴向应变发展模式进行了研究。同时,对剔除粒径大于5 mm后的模拟料,采用相似级配法,运用DS-2T型动三轴仪进行了一组平行试验。着重比较了砂砾料和模拟料在振动荷载作用下孔压、应变发展模式的区别。结果表明：在均压固结条件下,砂砾料在循环应力作用下,其孔压的发展模式与模拟料不同,可以用修正的反正弦进行拟合,而模拟料振动孔压的发展模式可用反正弦拟合;二者振动孔压均达不到初始有效固结围压,约为初始围压的90 %左右。模拟料与砂砾料在振动荷载作用下,应变发展模式不同。前者在初始液化前应变幅值较小且无明显增长,临近初始液化时应变大幅度增加导致试样突然破坏;后者在循环荷载作用过程中应变幅值一直稳定增加。     

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。     

栗西沟尾矿坝地震响应及液化分析

采用不排水有效应力法,利用Geo-slope软件,对金堆城栗西沟尾矿坝进行了地震响应的综合分析与液化计算。结果显示: a .栗西沟尾矿坝在静态条件下是稳定的; b .在7度地震条件下,尾矿坝的加速度反应较小,其放大倍数为2.204; c .栗西沟尾矿坝内动剪应力和动孔压绝大部分是随着地震历时的增加而逐渐增大; d .坝内孔压比都较小,抗液化安全系数较大,但局部液化区的存在,仍可能影响到整个坝体的安全性,应在液化区采取加固措施。      

Steady state and liquefaction characteristics of gravely sands

Liquefaction of loose and saturated soils during earthquakes and strong ground motions has been a major cause of damage to buildings and earth embankments as well as other civil engineering structures. In order to evaluate the liquefaction potential and steady state characteristics of gravely sand of south west Tehran,a subsoil exploration program conducted dividing the region into 10 zones. In each zone of 500 m × 500 m a borehole of 20 m deep was drilled. SPT was performed at one meter intervals in each borehole and a total of 200 samples were recovered. Soils of similar grain size distribution have been considered to have similar steady state characteristics,therefore consolidated undrained triaxial tests were performed on these soils of similar grain size distribution to evaluate the steady state strength. The steady state line for each soil type was derived. Comparing the steady state strengths with the shear stress due to an earthquake with a PGA of 0.35 g,the potential of sand liquefaction and .ow failure in soil layers has been evaluated and the settlement of soil due to the liquefaction phenomena is calculated. Finally some recommendations for estimating the steady state strength from simple SPT test in gravely sands are presented.     

近岸水平场地液化侧向大变形机理及软化模量分析方法

本文依据震害现象和实验探讨近岸水平场地地面液化侧向大变形机理，改进现有软化模量分析技术，给出一套地面液化侧向大变形的分析方法。近岸水平场地侧向大变形机理因地基中孔隙水压力升高、土体模量衰减、土骨架变软使偏应变得到充分发展所致，其水平永久侧移可用从底部到顶部呈增加形式的整体变形描述。利用本文方法，对1995年阪神地震中近岸沉箱岸壁和土体液化侧向大变形进行了数值模拟，结果与震后实测结果和试验结果在主要特征上一致，说明改进的软化模量法可以用于地面液化侧向大变形的分析。     

Liquefaction evaluation discrepancies in tropical lagoonal soils

Field penetration tests and shear wave velocity measurements are both established and accepted methods for evaluating liquefaction potential in soils. The results produced by the two methods are generally well correlated. However, recent studies have shown that when investigating tropical lagoonal deposits, the same accepted methods for evaluating liquefaction potential often produce significant discrepancies in results. This discrepancy is most apparent in saturated lagoonal deposits of calcareous gravelly sand (or sandy gravel), which tend to exhibit low penetration resistance values but relatively high shear wave velocities. These disparate test results can suggest different soil classifications under current building codes. Ambiguity in the code may allow for a potentially unconservative classification, which may in turn allow for the use and construction of less costly, lighter weight foundation systems than warranted. Equally as important, the potential for unconservative design as related to liquefaction appears to be high when shear wave velocity measurements are used as a basis for evaluation in these types of lagoonal deposits. Because of this, it is strongly recommended that caution should be excercised when determining seismic design parameters in these types of geologic environments. A hypothesis to explain the discrepancies in the results of evaluation methods and a suggested design protocol is proposed.