饱和土沉积谷场地对平面Rayleigh波的散射

把沉积谷软土场地用饱和多孔介质的Biot动力学理论模拟,周围的场地用单相介质弹性动力学理论模拟,利用波函数展开法,给出具有饱和土沉积层的沉积谷对Rayleigh波散射问题的解析解。通过数值计算,分析了入射波波长、沉积谷的高宽比、沉积层的软硬(利用波速描述其软硬)对地面运动的影响。     

横观各向同性饱和层状土中垂直受荷群桩的动力阻抗

给出了稳态振动时,垂直受荷群桩在 横观各向同性 饱和层状土中动力阻抗计算的一般方法。首先,基于 Biot 饱和介质的三维波动理论,借助 Hankel 变换,得到横观各向同性饱和土轴对称动力问题的通解,利用通解,给出饱和土层在 Hankel 变换域内的精确动力刚度矩阵,进而构建出饱和土层在竖向柱面荷载作用下的动力 Green 函数;其次,根据桩–土界面的位移协调及平衡条件,利用 Kynia 的方法,建立了饱和地基–群桩纵向耦合振动的边界元–有限元方程,给出了垂直受荷群桩动力阻抗的计算公式。算例表明：场地土的各向异性及土层刚度的变化,对摩擦型群桩动力刚度的影响要远大于对端承型群桩的影响。     

基于黏性流体力学的液化土中桩基桩顶阻抗研究

将液化后的土体视为流体，考虑液化土的黏性，研究了黏性液化土体中端承桩的水平振动问题。用黏性流体运动方程模拟液化后土层的运动，用饱和多孔介质模型模拟饱和未液化土层，联系桩-流体耦合条件运用势函数法、分离变量法得到黏性流体水平振动解析解。利用上层液化土和下层饱和非液化土分界面的位移、转角和内力连续条件得到黏性液化土-饱和土分层条件下的桩顶阻抗解析表达式，将理论解与有限元解进行对比验证了解的正确性，对端承桩桩顶阻抗进行参数分析，表明在分析液化土中桩基桩顶阻抗时，需要考虑液化土的黏滞特性，以免造成刚度阻抗的高估和阻尼阻抗的低估。     

层状饱和地基上刚性条形基础的动力刚度

利用饱和土层的精确动力刚度矩阵求解了动力Green函数,将基础和场地交界面离散,根据交界面上的位移边界条件得到层状饱和场地上刚性条形基础的动力刚度。对工程中常用的四种场地(饱和均匀半空间、基岩上单一饱和土层、饱和半空间上单一饱和土层、层状饱和场地)上刚性条形基础的动力刚度进行了对比和分析。研究表明:层状场地结果与其他场地结果有着明显的不同;当上覆土层较厚时,其他三种模型结果基本一致。本文方法可进一步推广到层状饱和场地中任意埋置基础动力刚度的计算中。     

层状横观各向同性饱和地基中桩基的纵向耦合振动

基于层状横观各向同性饱和土地基模式,研究了桩基在饱和土体中的纵向耦合振动问题。首先,根据饱和介质三维波动理论的Biot模型,利用Hankel变换,给出横观各向同性饱和土波动方程的轴对称通解;利用通解,建立了饱和土层的精确动力刚度矩阵,进而构建出盘面和竖向柱面荷载作用下,土体与结构动力相互作用的Green函数;其次,根据桩-土界面的位移协调及平衡条件,用边界单元法构建了层状饱和地基与桩相互作用的动力方程和计算公式,给出求解层状横观各向同性饱和地基与桩基纵向耦合振动的一般解法。算例表明:土介质的各向异性参数、饱和土孔隙率和桩-土模量比,对低频振动桩的动力刚度影响不大,但对高频振动影响显著。     

流体饱和层状半空间中沉积谷地对地震波的散射— IBIEM求解

针对流体饱和层状半空间中任意形状沉积谷地对地震波的散射问题,基于单层位势理论,建立一种新的间接边界积分方程法（IBIEM）：首先推导饱和层状半空间中两类膨胀波源和剪切波源动力格林函数,进而由分布在沉积谷地和层状半空间交界面附近两虚拟波源面上的P_I、P_II和SV波源分别构造沉积内外的散射波场。由交界面连续性条件建立方程并求解确定虚拟波源密度,总波场即可由自由波场和散射波场叠加而得。然后通过边界条件验算、退化解答与现有结果的比较,验证了方法的计算精度。通过一组典型算例,初步探讨了平面SV波在饱和层状半空间中沉积谷地周围散射的基本规律。研究表明：饱和层状沉积对地震波的散射和相应干土情况存在显著差异,边界渗透条件对沉积内部地震响应具有重要影响;饱和层状场地中波的散射和均质半空间情况有着本质不同,总体反应体现了饱和土层的共振放大效应和沉积谷地对波的散射效应。实际场地中地震波传播的精确模拟需考虑岩土介质的多相性和层理特征。     

复杂介质对地震波传播的影响

本文基于两相介质动力学方程组,利用显式集中质量有限元结合透射人工边界,研究了水平自由场地单相土、欠饱和土和饱和土及不同模量的土层排列结构对地震波传播的影响。结果表明介质的性质对P 波传播有明显的影响;而土层结构对SV 波传播有明显影响。同时,本文还研究了横向不均匀场地地震反应,结果进一步表明,仅用一维模型研究横向不均匀场地会引起较大误差。     

半球形凹陷谷场地对Rayleigh波的三维散射

利用半空间场地用单相介质动力学理论模拟，并略去阻尼项的影响，首次推导出了三维半球形凹陷谷场地在平面Rayleigh波入射下散射问题的解析解。确定局部场地的三维波场分布，给出半球形沉积谷场地地表位移分量，分析了入射波频率对场地震动的影响，并进一步与单相弹性介质情况下的地表运动进行了对比。     

基于多孔介质理论的饱和土中桩基水平振动研究

将土体视为液固两相多孔介质,饱和土的力学行为运用饱和多孔介质理论来描述,将土层视为带有一圆孔的无穷多的薄土层组成,在Novak平面应变模型的基础上,借助势函数求得了饱和土层的水平动力阻抗,并利用初参数法求解了饱和土中单桩的水平振动问题,通过与三维模型结果的对比验证了本文方法的正确性与可行性,分析了主要桩土力学参数对水平动力阻抗、水平-摇摆动力阻抗和摇摆动力阻抗的影响,为桩基动力检测和抗震设计提供了理论依据。     

SH波作用下粘弹性半空间场地土地震反应分析

本文考虑稳态简谐SH波激励下的谐和振动和场地土的材料阻尼,建立弹性动力反平面应变模型,采用一维粘弹性波动理论进行土层地震反应分析,得出基岩运动输入下半空间上覆粘弹性场地土体系的解析解.利用此解来分析SH波激励下粘弹性场地土动力效应,讨论不同场地土的动力特性参数对粘弹性场地土动力效应的影响.土-结构物动力相互作用和材料阻尼对场地土地震反应有很明显的影响,这为工程中的地震场地效应评价、震害分析和结构抗震设计都提供一定的参考依据.     

Assessment of dynamic site response of the peat deposits at an industrial site (Turkey) and comparison with some seismic design codes

Peat is an organic soil type with properties of high compressibility and high water content. Peat deposits are often unsuitable for supporting structures of any kind due to their susceptibility to cause a number of geo-engineering problems, such as settlement, slope instability and soil amplification. There are peatlands in Turkey, although these are not so extensive as in other countries, such as Canada, where peatlands are very common. One of the peat deposits in Turkey is located close to the city of Kayseri (Central Anatolia), and industrial structures have been built on these deposits. This paper focuses on one part of a comprehensive research project conducted by the authors on various geo-engineering aspects of the peat deposits in this industrial site, with the main aims to investigate and assess some of the dynamic properties of the peat deposits in this area, to determine and assess their seismic site response and to compare them with the design spectra recommended in the current seismic codes of Turkey and Europe. In the study shear wave velocities of the peat deposits were determined with seismic measurements, and some dynamic properties of the peat were obtained from the resonant column and torsional shear tests. Then site response analyses were carried out and the results were compared with the design spectra recommended for loose-soft soils by the Turkish Seismic Design Codes and European Construction Design Codes. The comparisons suggest that the response spectra of the investigated peat deposits are greater than those recommended in the Turkish Seismic Codes and that at periods of > 0.3–0.5 s the peat will cause an increase in amplification. Although the response spectra of the peat tested show a better agreement with those recommended in the European codes, they show differences at periods of > 1 s.

     

Statistical analyses of inherent variability of soil strength and effects on engineering geology design

Clayey soil strata, as all natural deposits, generally show variability in the values of their geotechnical properties. This is due mainly to geological and environmental processes such as deposition and diagenesis, which introduce heterogeneity, anisotropy and variability to soil properties. Other causes of variability, and thus uncertainty, are the representativeness of samples and errors related to testing procedure, measurement and data processing procedures. To improve our knowledge about the inherent variability in the geomechanical properties of clays, this work presents a case study related to the analysis of the strength variability along a log of marine stiff clay deposits, which are apparently quite homogeneous. The analysis was based on pocket penetrometer strength measurement, performed both punctually and across the whole deposit. The adopted testing procedure, which is fast and reliable, provides a really wide dataset of the investigated soil property, with more than 800 data points. These allow for detailed variability analysis, and a reliable estimation of the coefficient of variation as well as research into the best fitting probability density functions, which are key factors for robust design. The presented case study allows discussion of the inherent variability of soil properties, and its influence on the characteristic values of soil strength in geotechnical design.     

上海软土场地的地震反应特征分析

近年来的宏观震害资料和强震观测记录已经证明：场地条件不仅对地震动的峰值加速度和频谱形状具有重要的影响,而且与不同类型工程结构的地震反应和震害机理也具有相当密切的关系.上海地区的第四纪沉积土层呈水平层状分布,基岩埋深可达300m左右,且浅部普遍发育有滨海沼泽相软土.因此,上海地区上覆深厚、软弱的沉积土层,对场地的地震反应特性无疑具有重要的影响.文章基于一维场地地震反应的等效线性化频域分析方法,建立了上海软土场地的动力分析模型,以El Centro地震波为例,重点分析了上海地区场地土的地震反应加速度反应和频谱特征.研究表明,在7度抗震设防即场地地面峰值加速度（PGA）等于0.1 g的情况下,上海软土场地的地表峰值加速度具有放大特性,地表加速度的频谱组成具有低频放大,高频过滤的特征,地面运动加速度反应谱的卓越周期也具有向长周期方向移动的趋势.     

地基自由场离心机振动台模型试验研究

在清华大学土工离心机实验室50g 离心场环境中进行了一系列地基自由场离心机振动台试验。试验包括1个砂土地基、1个黏土地基和2个成层土地基振动台试验。试验中模型箱采用叠环式模型箱,其平面尺寸为0.5 m和0.2 m,试验中输入地震波采用Kobe波和Parkfield波。给出了部分试验结果分析,包括加速度时程及频谱反应、加速度峰值放大系数和土层中剪应力-剪应变变化曲线。试验结果发现,不同类型地基中加速度反应存在差异,在低频部分加速度峰值放大系数从底部到顶部逐渐增大。土体中剪应力-剪应变变化规律在砂土地基和黏土地基中存在差异。     

Long-term impact of acid resin waste deposits on soil quality of forest areas II. Biological indicators

In this study, we evaluated the effects of two acid resin deposits on the soil microbiota of forest areas by means of biomass, microbial activity-related estimations and simple biological ratios. The determinations carried out included: total DNA yield, basal respiration, intracellular enzyme activities (dehydrogenase and catalase) and extracellular enzyme activities involved in the cycles of C (beta-glucosidase and chitinase), N (protease) and P (acid-phosphatase). The calculated ratios were: total DNA/total N; basal respiration/total DNA; dehydrogenase/total DNA and catalase/total DNA. Total DNA yield was used to estimate soil microbial biomass. Results showed that microbial biomass and activity were severely inhibited in the deposits, whilst resin effects on contaminated zones were variable and site-dependant. Correlation analysis showed no clear effect of contaminants on biomass and activities outside the deposits, but a strong interdependence with natural organic matter related parameters such as total N. In contrast, by using simple ratios we could detect more stressful conditions in terms of organic matter turnover and basal metabolism in contaminated areas compared to their uncontaminated counterparts. These results stress that developed ecosystems such as forests can buffer the effects of pollutants and preserve high functionality via natural attenuation mechanisms, but also that acid resins can be toxic to biological targets negatively affecting soil dynamics. Acid resin deposits can therefore act as contaminant sources adversely altering soil processes and reducing the environmental quality of affected areas despite the solid nature of these wastes.     

Geo-environmental issues induced by the 2011 off the Pacific Coast of Tohoku Earthquake and tsunami

The 2011 off the Pacific Coast of Tohoku Earthquake of March 11, 2011, caused devastating geotechnical and geo-environmental issues mainly in the coastal area of the Tohoku and North-Kanto Regions, Japan. As a result of the earthquake and subsequent tsunami, approximately 23,000 Gg (23,000,000 t) of disaster debris was generated, with more than 12 million m³ of tsunami deposits left in the flooded area. The geotechnical utilization of the soil fraction in the disaster debris and tsunami deposits has presented a huge challenge to geotechnical engineers since (1) the clearance of debris and tsunami deposits is an urgent task which must be completed within a few years and (2) although a large amount of waste-mixed soil can be used in the construction of new embankments and levees to protect the coast from future tsunamis, their geotechnical properties have temporal and spatial variations. This paper summarizes the current status on the generation, clearance, and treatment and utilization of disaster debris and tsunami deposits from geotechnical and geo-environmental viewpoints. In addition, the environmental monitoring data on soil and groundwater quality conducted over the affected area is briefly reviewed. The results of several wide-area monitoring efforts conducted by different organizations indicate that no significant soil or groundwater contamination has occurred. However, the localized contamination near industrial plants where toxic chemicals leaked due to the earthquake and tsunami needs to be carefully taken into consideration during future revival and redevelopment works. Another serious geo-environmental issue is the management of the radioactive contamination of surface soils caused by the accident at the Fukushima First Nuclear Power Plant. This paper summarizes the distributions and expected behaviours of radioactive Caesium in soils and groundwater, and outlines the possible remediation options for dealing with this contamination.     

Review of collapse triggering mechanism of collapsible soils due to wetting

Loess soil deposits are widely distributed in arid and semi-arid regions and constitute about 10% of land area of the world.These soils typically have a loose honeycomb-type meta-stable structure that is susceptible to a large reduction in total volume or collapse upon wetting.Collapse characteristics contribute to various problems to infrastructures that are constructed on loess soils.For this reason,collapse triggering mechanism for loess soils has been of significant interest for researchers and practitioners all over the world.This paper aims at providing a state-of-the-art review on collapse mechanism with special reference to loess soil deposits.The collapse mechanism studies are summarized under three different categories,i.e.traditional approaches,microstructure approach,and soil mechanics-based approaches.The traditional and microstructure approaches for interpreting the collapse behavior are comprehensively summarized and critically reviewed based on the experimental results from the literature.The soil mechanics-based approaches proposed based on the experimental results of both compacted soils and natural loess soils are reviewed highlighting their strengths and limitations for estimating the collapse behavior.Simpler soil mechanics-based approaches with less parameters or parameters that are easy-to-determine from conventional tests are suggested for future research to better understand the collapse behavior of natural loess soils.Such studies would be more valuable for use in conventional geotechnical engineering practice applications.     

Cracking mechanism of secondary lining for a shallow and asymmetrically-loaded tunnel in loose deposits

Tunnels constructed in loose deposits with low strength and complex composition are usually subjected to asymmetrical stresses at the entrance and exit. The secondary tunnel lining is prone to excessive deformation, cracking, or even collapse, seriously affecting the safety of tunnel construction and operation. In this paper, a large shallow highway tunnel in loose deposits is used as an example to study the cracking mechanism of secondary lining. Triaxial consolidated-drained shear tests are carried out on large remolded specimens to obtain the mechanical parameters of the surrounding soil. Three-dimensional numerical modeling is conducted based on the field monitoring data to simulate the process of tunnel construction and to analyze the mechanical mechanism of cracking in the secondary lining. It is shown that even with the 30 m advance pipe roof at the tunnel entrance, the apparent difference in stiffness between the retaining wall and the surrounding soil results in an obvious stress concentration at the spring of the secondary lining near the end of the retaining wall, due to the effect of highly asymmetrical stresses. In addition, loose deposits are very sensitive to construction disturbances. Large horizontal deformation towards the lower topography occurs during tunnel construction. With increasing overburden depth, the stress concentration at the spring level and the horizontal deformation in the secondary lining increases, which are the main reasons for cracking in the secondary lining. These findings can be useful for tunnel design and construction in the similar type of loose deposits.     

易液化深厚覆盖层地震动放大效应台阵观测与分析

基于美国加州Caltrans/CDMG的两个井下台阵加速度记录,从时域和频域分析了百米级易液化深厚覆盖层在不同地震下的地震动放大效应,揭示了地震动从基岩向覆盖层传播的4个重要特征:(1)加速度放大效应受土层深度影响,近地表20~30 m以内放大效应突出;(2)地震动三分量的放大效应具有方向性,水平向与竖向地震动放大效应差异显著;(3)基岩加速度呈现"小震放大、大震衰减"的规律;(4)加速度放大规律与频率相关,深厚覆盖层放大频带较宽。初步分析了造成上述放大效应的可能原因。在此基础上,基于平面波动假设提出了考虑层间波阻抗比放大和传播路径衰减的深厚覆盖层加速度放大效应简化函数,改进了1/4波长法的参数取值,并结合自由表面效应,对前述台阵记录的放大效应进行了估算,发现无论时域还是频域,预测结果与实际观测都较接近。本文研究成果可为深厚覆盖层液化判别和抗震设计的加速度选取提供理论依据和简化分析方法。