岩质高陡边坡地震动响应规律的振动台试验研究

设计并完成了1∶10大比例尺的边坡大型振动台模型试验,试验模型尺寸为4.4m×4.4m×1.8m(长×宽×高),斜坡模型表面包含30°、45°、50°、60°四个不同坡度的坡面,模拟岩体材料采用重晶石粉、河砂、石膏、黏土和水按比例配制而成。通过输入不同类型、幅值、频率的地震波来研究模型边坡的动力响应规律,在试验数据分析中采用三维局部坐标系。试验结果表明:边坡临空面方向和竖直方向的加速度高程放大效应随坡面角度的增大而增强,在坡面角度由45°→50°变化时增长趋势呈明显"台阶状"形式,而边坡走向方向的峰值加速度高程放大效应基本不随坡面角度变化;边坡各向的峰值加速度的高程放大效应随着输入地震波幅值的增大而减小,表现出"量级饱和"特性;加速度傅里叶谱的频谱成分随着高程的增大,边坡岩体对于试验模型自振频率f周围的频率成分具有显著的放大作用,而对于其他频率成分则具有滤波作用;加速度反应谱沿高程的形状基本一致,并且卓越周期对应的反应谱幅值沿高程具有一定的放大作用,而在其他周期T处,尤其是长周期部分(低频部分)则存在一定的减小作用,对于临空面方向来讲,具有明显的波峰现象。试验结果有助于揭示边坡在地震作用下的失稳机制,为边坡工程的抗震设计提供有益的参考。     

坡面角度对岩质边坡加速度高程放大效应的影响

基于现场调查、振动台试验及理论计算等结果,由宏观现象定性的分析到理论求解的定量计算,全面、系统地研究坡面角度对岩质高陡边坡的加速度高程放大效应的影响。现场调查结果表明,边坡的失稳破坏主要发生在45°及以上的斜坡,从宏观现象上间接说明了随着坡面角度的增大,边坡的加速度放大效应会逐渐增强;振动台试验和理论计算结果从半定量和定量的角度揭示随着坡面角度的增大,临空面方向、竖直方向峰值加速度的高程放大效应逐渐增强,而在增强的过程中同时存在突然增大的拐点45°和渐趋平缓的拐点50°,即在45°以下加速度放大效应的增长较为缓慢,在45°~50°之间突然增大,在50°以上增长又逐渐变缓,这充分解释了“滑坡灾害主要发生在45°以上的斜坡”这一汶川地震震害现象。边坡走向方向的峰值加速度高程放大效应基本上不随坡面角度的增大而变化,其台阶也相对较为平缓。     

土质边坡地震动力响应规律研究

地震往往会触发大量的山体滑坡,给人类和社会带来巨大灾难。为研究探索土质边坡在地震作用下的动力响应规律,本文建立了二维均质边坡有限元模型,模拟计算了其在地震作用下的动力响应,对地震响应中学术界和工程界最为关心的加速度、位移和频谱特性响应进行了研究分析,并得到以下几点结论:①在坡顶和坡面处,与输入地震动加速度时程比较,输出加速度峰值出现的时刻有滞后现象;②坡体内部对输入地震动PGA存在垂直和临空面放大作用,同时垂直放大作用呈现出节律性变化的特点;③地震作用下坡体的最大变形在水平方向上出现在坡脚处,在竖直方向上出现在坡肩处;④边坡体内部对输入地震动的频谱成份产生滤波作用,滤掉了输入地震动中的高频成份,且随着高度的增加,这种滤波作用呈现增强的趋势。     

组合支护结构作用下反倾层状岩质边坡加速度响应振动台试验研究

设计并完成一个1∶30的大比例尺高陡反倾层状岩质边坡的振动台模型试验,坡体内部有6个软弱泥化夹层,研究在组合支护体系作用下EL Centro地震波和汶川-清屏地震波激振下泥化夹层含水量发生变化时边坡的加速度动力响应规律。试验结果表明:(1)坡面X、Z向加速度放大系数均具有非线性高程放大效应,但前者大于后者;(2)泥化夹层含水量的变化对坡面加速度放大效应影响显著,注水后X向减小而Z向增大;(3)支护体系作用下边坡临空面放大效应的现象受限制,预应力锚索抗滑桩以下边坡基本不存在加速度放大效应;边坡分级支护可有效降低X向加速度放大系数的高程增大效应,但对Z向会产生不利作用;(4)边坡的破坏模式为上部受软弱夹层滑动牵引而发生倾倒-拉裂变形,导致顶部框架梁有可能最先发生破坏,且破坏类型可能以绕坡顶为支点向坡体内侧转动,引起上部的锚索产生拔出破坏。     

岩层倾角对顺倾向边坡地震效应的影响

利用FLAC3D软件模拟地震作用下不同岩层倾角的顺倾向边坡,对比坡面峰值加速度放大系数、峰值位移、地震作用结束后坡体剪应变增量的变化规律,探讨岩层倾角对顺倾边坡地震效应的影响。研究表明:(1)在水平地震波作用下,坡面水平峰值加速度放大作用随岩层倾角增大而线性减小;(2)当岩层倾角小于软弱岩层内摩擦角时,坡面峰值位移较小且变化规律受岩层倾角影响不明显,当岩层倾角大于软弱岩层内摩擦角且小于30°时坡面峰值位移增大,大于60°时减小;(3)岩层倾角小于坡角时,残余剪应变增量最大值集中在坡面中下部软弱岩层处,反之,剪应变增量最大值出现在整个坡面并呈弧形区。     

单体边坡地形的地震动力响应及其放大效应的数值分析

本文采用隐式动力有限单元法研究了不同的边坡角度和边坡高度对地形放大效应的影响,并以位移峰值放大系数为衡量地震动放大效应的标准,计算了不同边坡角度和边坡高度条件下的地震响应,在此基础上对模型关键监测点的输出波形以及位移峰值放大系数的变化趋势进行了分析,获得了不同监测点处的地震动时程曲线,揭示了坡角和坡高对单体边坡地震动放大效应的定量作用规律。数值结果表明,相同高度处坡面监测点的水平向位移峰值放大系数大于坡内监测点的,地形放大效应在水平方向具有趋表效应。由于坡面存在入射波和反射波的叠加,因此竖直向位移峰值放大系数的最大值出现在坡体内部。      

地震作用下均质土坡动力特性的振动台试验研究

设计并完成比例尺1∶100的边坡振动台模型试验,讨论模型的相似关系、传感器的布置及模型的建造,并编制相应的动荷载加载方案。通过输入不同类型、不同幅值、频率的动荷载,分析模型边坡在地震作用下的动力响应规律以及地震动参数对边坡动力特性和动力响应的影响。试验结果表明,在坡体的表面和坡内的竖直方向上,加速度放大系数均随着高程增加而明显增大。当输入不同压缩比的地震波时,压缩比越大坡体的动力响应越明显,即随着输入动荷载的频率增加,越接近土体的自振频率加速度放大效应越明显;在坡体的同一高程处,坡面的加速度放大系数略大于坡体内的加速度放大系数,表现出一定的趋表效应,同时随着输入地震波振幅的增加,加速度放大系数整体出现递减的趋势。实验结果分析有助于揭示土质边坡在地震作用下的失稳破坏机制,为今后边坡工程的抗震设计提供积极的参考。     

黄土隧道边仰坡动力响应的大型振动台模型试验研究

设计并完成了1∶80比例尺的高陡黄土边坡大型振动台模型试验。通过输入不同类型、幅值、频率的地震波,探讨地震作用下模型洞口仰坡动力响应规律,以及地震动参数对动力响应的影响。试验结果表明,不同地震波加载的过程中,坡面的PGA随着测点高程的增大而增大,在隧道仰拱和拱顶所处的位置,PGA有突然减小的过程。其中仰拱周围边坡坡面的PGA减小更大,说明隧道的存在对坡面加速度的放大作用具有一定的抑制作用,其中仰拱位置比拱顶位置抑制作用更强。台面峰值加速度越大,仰拱对加速度的抑制作用更加明显,坡顶面的PGA最大,因此坡面越高,加速度的放大效应越明显,并没有出现加速度放大系数饱和的这种状态。试验结果有助于揭示黄土隧道仰坡在地震作用下的失稳机制,为工程的抗震设计提供有益的参考。     

斜入射地震作用下坡面形态对碎石土边坡稳定性影响研究

本文研究了在震源近场时,地震波斜入射条件下,不同坡面形态对碎石土边坡稳定性的影响。利用FLAC3D有限差分软件,建立了多种坡面形态的边坡模型。采用对比方法,分析了边坡在地震波作用下的稳定性。结果表明:临空坡面对输入地震波有放大效应;坡面的水平位移与加速度峰值随高度增加而增大,随坡度增加而增大;随着坡面凹度的增大,剪应力增量向坡肩处集中,坡肩愈加不稳定;随着坡面凸度的增大,剪应力增量向坡腰处集中,坡腰愈加不稳定;台阶对坡面的加速度放大效应有削弱作用,台阶数越多越稳定。同时给出了边坡防治的几点建议:减小坡面的坡度;对凹型坡加强坡肩位置处的防治,对凸型坡加强坡腰位置处的防治;对高边坡可设置多级台阶增加稳定性。     

仰坡坡度对隧道洞口段动力响应的影响分析

采用动力有限元数值模拟的方法,设置四种不同仰坡坡度的模型,沿隧道轴向方向输入地震波,探究隧道洞口段及仰坡在不同仰坡坡度影响下的动力响应,并通过对无隧道通过的纯边坡模型与相同条件下有隧道通过的边坡模型的动力响应进行对比分析,研究隧道的存在对坡面动力响应的影响。研究结果表明:(1)由于临空面的存在,隧道洞口段的位移与加速度具有明显的放大效应。不同坡度模型的位移峰值皆位于y=0m断面的拱顶处,在距洞口y=40m后的各控制点位移差值迅速减小。(2)随着仰坡坡度的增加,同一断面处隧道的位移值随之增加,洞口段的截面变形也随之增大。(3)当坡度α≥60°时,坡面位移随着坡面高程的增加而增大;当坡度α60°时,坡面位移随着坡面高程的增加先增大后减小,在0.4~0.6倍坡高处达到最大,即随着坡度的增加,坡面的位移峰值也就越靠近坡顶。(4)隧道的存在对于坡面的稳定性有重要影响,这种影响在洞口附近尤为明显。     

SCIENCE CHINA Earth Sciences SUBJECT INDEX TO VOLUME 57(2014)

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A Monte Carlo study of rainfall forecasting with a stochastic model

A procedure for short-term rainfall forecasting in real-time is developed and a study of the role of sampling on forecast ability is conducted. Ground level rainfall fields are forecasted using a stochastic space-time rainfall model in state-space form. Updating of the rainfall field in real-time is accomplished using a distributed parameter Kalman filter to optimally combine measurement information and forecast model estimates. The influence of sampling density on forecast accuracy is evaluated using a series of a simulated rainfall events generated with the same stochastic rainfall model. Sampling was conducted at five different network spatial densities. The results quantify the influence of sampling network density on real-time rainfall field forecasting. Statistical analyses of the rainfall field residuals illustrate improvement in one hour lead time forecasts at higher measurement densities.     

Distribution of throughfall and stemflow in multi‐strata agroforestry,perennial monoculture,fallow and primary forest in central Amazonia,Brazil

The partitioning of rain water into throughfall, stemflow and interception loss when passing through plant canopies depends on properties of the respective plant species, such as leaf area and branch angles. In heterogeneous vegetation, such as tropical forest or polycultural systems, the presence of different plant species may consequently result in a mosaic of situations with respect to quantity and quality of water inputs into the soil. As these processes influence not only the water availability for the plants, but also water infiltration and nutrient leaching, the understanding of plant effects on the repartitioning of rain water may help in the optimization of land use systems and management practices. We measured throughfall and stemflow in a perennial polyculture (multi‐strata agroforestry), monocultures of peach palm (Bactris gasipaes) for fruit and for palmito, a monoculture of cupuaçu (Theobroma grandiflorum), spontaneous fallow and primary forest during one year in central Amazonia, Brazil. The effect on rain water partitioning was measured separately for four useful tree species in the polyculture and for two tree species in the primary forest. Throughfall at two stem distances, and stemflow, differed significantly between tree species, resulting in pronounced spatial patterns of water input into the soil in the polyculture system. For two tree species, peach palm for fruit (Bactris gasipaes) and Brazil nut trees (Bertholletia excelsa), the water input into the soil near the stem was significantly higher than the open‐area rainfall. This could lead to increased nutrient leaching when fertilizer is applied close to the stem of these trees. In the primary forest, such spatial patterns could also be detected, with significantly higher water input near a palm (Oenocarpus bacaba) than near a dicotyledonous tree species (Eschweilera sp.). Interception losses were 6·4% in the polyculture, 13·9 and 12·3% in the peach palm monocultures for fruit and for palmito, respectively, 0·5% in the cupuaçu monoculture and 3·1% in the fallow. With more than 20% of the open‐area rainfall, the highest stemflow contributions to the water input into the soil were measured in the palm monocultures and in the fallow. Copyright © 1999 John Wiley & Sons, Ltd.     

Anthropogenic Eutrophication and Red Tide Outbreak in Lacustrine Systems of the Kashmir Himalaya

Red tide, a recurrent phenomenon has become conspicuous in several Kashmir lake ecosystems since 1991. The responsible organism (Euglena pedunculata), a rare flagellate rediscovered in the Kashmir Himalaya (Khan 1993) caused first and unprecedented red tide outbreak, constituting a maximum of 96% of resident numerical phytoplankton density in Dal Lake. At present, conflicting hypotheses exist on the generation of causal assemblage(s) imparting redness to waters: Jeeji Bai (1991) linked its origin to acid precipitation – a fallout of burning oil‐fields during the Gulf War – whilst Khan (1993) holds local factor(s) responsible. Field/experimental studies support the latter contention that the influx of untreated sewage, in unison with warm temperatures, high levels of PhAR, iron and interruption to hydrological flow‐pattern together with absence/or reduction in grazing activity created conducive environmental milieu for red tide outbreak. Dal Lake “red tide” drifted the bloom‐inoculum to other waters, including Lake Wular, where additional ecological niches were carved out, threatening the aesthetic value and biological diversity of Kashmir lakes. Ecological monitoring indicates frequent seasonal red tide occurrence in Dal Lake (including summer‐autumn event of 1998) which testifies its unabated eutrophication status. Further studies are needed on ecological adaptability and biogeographic distribution of this rare and unique red tide‐causing flagellate.