塑性混凝土立方体和圆柱体试件的力学特性数值模拟研究

为研究塑性混凝土立方体和圆柱体试件力学特性的差异,采用 FLAC3D 有限差分软件模拟了塑性混凝土单轴压缩试验,数值模拟得到的塑性混凝土单轴应力 - 应变关系与试验结果基本一致,说明采用 FLAC3D 开展塑性混凝土力学特性数值分析是可行的。在此基础上,分别开展了立方体和圆柱体塑性混凝土试件的三轴试验数值模拟,获得了塑性混凝土试件应力 - 应变曲线、峰值强度和峰值应变,并分析了两种试件位移云图。研究表明在相同参数下立方体试件峰值强度普遍比圆柱体试件偏大,峰值强度随着围压近似线性增长。     

塑性混凝土三轴受压本构关系影响因素分析

基于15种配合比制作21组塑性混凝土立方体试件,进行定侧压三轴试验,实测三轴受压荷载作用下试件的应力应变曲线,分析不同因素对塑性混凝土三轴受压应力应变关系的影响。结果表明:当应力水平较低时,水胶比、砂率、黏土用量及膨润土用量对塑性混凝土的应力应变关系影响不大,塑性混凝土的应变随着围压的增大而增大;当应力水平较高时,在应力相同的情况下,水胶比、黏土用量、膨润土用量的增大导致塑性混凝土应变逐渐增大,砂率的增大导致塑性混凝土应变逐渐减小;当应变相同时,塑性混凝土应力水平随着砂率、水泥用量、围压的增大而升高,随着水胶比、黏土用量及膨润土用量的增大而降低。     

真三轴试验下真实应变对塑性混凝土应力 －应变特性的影响

目前国内外学者对于塑性混凝土的研究工作大部分都是基于其小应变假定的基础上进行的, 为了分析塑性混凝土大极限应变对其应力－应变特性的影响,设计了4组塑性混凝土配合比及12个围 压环境进行真三轴试验。研究了塑性混凝土的峰值强度及峰值后曲线特征。结果表明:小应变假定和 真实应变下峰值强度与围压环境的相对变化关系一致,当第三主应力相同时,塑性混凝土的峰值强度随 第二主应力的增大而增大,但增幅越来越小;当膨润土掺量为零时,小应变假定和真实应变下应力－应 变峰值后曲线特征一致,呈现近似塑化或理想塑化特征;当膨润土掺量不为零时,真实应变下塑性混凝 土应力－应变曲线峰值后出面明显下降段,出现明显峰值点。     

混凝土试件单轴及常规三轴压缩试验研究

利用SAW-2000型电液伺服微机控制岩石力学试验系统,进行了混凝土试件单轴压缩试验、常规三轴压缩试验,研究了围压在0 MPa和1 MPa时的应力应变曲线,得到了该混凝土试件的峰值强度、弹性模量和泊松比等参数,随围压的增加,混凝土试件的抗压强度将提高21.6%,验证了低围压下混凝土应力应变曲线的五阶段特征.     

静水压力下塑性混凝土力学性能研究

塑性混凝土力学性能具有明显的静水压效应,对复杂应力状态下力学性能的研究具有重要的工程应用价值和学术研究价值。文章利用三轴试验,对塑性混凝土试件施加不同的围压,然后轴向冲击压缩加载,系统进行不同围压条件下塑性混凝土轴向压缩试验,得到了相应的应力-应变曲线。研究结果表明,塑性混凝土的静水压力效应比较明显,塑性混凝土具有明显的应变硬化现象,轴向抗压强度增长较为明显。     

循环荷载后围压水对混凝土力学特性影响

对不同围压(0, 2, 5, 10 MPa)下的水饱和混凝土试件,历经25次循环荷载作用后(荷载下限140 kN,上限260 kN,频率0.1 Hz),进行了不同加载速率(地震荷载作用下的混凝土应变速率响应范围10-5/s~10-2/s)下的静动态常三轴抗压性能试验。分析了混凝土峰值应力、峰值应变以及损伤特性。结果表明:历经循环荷载后,随着加载速率的增加,混凝土峰值应力增大,峰值应变整体上逐渐增大,损伤变量D增长速度减缓;随着围压增加,混凝土峰值应力和峰值应变逐渐增大,损伤变量D增长速度降低,但损伤极限差值越来越大。基于上述试验结果,得到了历经荷载循环后混凝土动态峰值应力与围压、加载速率有关的经验计算式及历经荷载循环后混凝土动态峰值应变与围压、加载速率有关的经验式。     

循环加载下塑性混凝土的应力-应变曲线分析

通过设置4组配合比浇筑圆柱体和棱柱体试件,进行了抗压强度和弹性模量试验。试验分析了应力-应变曲线、预压段和未预压段以及循环段的应力-应变曲线的特点,比较了预压段与未预压段的应力-应变曲线。通过对曲线斜率的分析,了解试件孔隙存在的微观性,明确预压时循环次数对应力-应变曲线的影响。试验结果表明:塑性混凝土应力-应变曲线的上升段较普通混凝土有明显的不同,预压段的应力-应变曲线的斜率先增加后减小,而未预压段的曲线斜率一直增加。同时,可以宏观地明白试件内部孔隙存在的微观性,得出预压次数对应力-应变曲线的影响并不大。所得出的结论对塑性混凝土今后的科学研究与工程应用具有一定的借鉴。     

不同期龄塑性混凝土力学性能试验研究

对养护期龄为7,14,28,90 d的塑性混凝土进行三轴压缩试验,实测不同围压状态下试件的应力-应变曲线及强度参数,分析不同养护期龄及试验围压对强度、应力-应变关系的影响。试验结果表明:塑性混凝土在低围压时表现出软化性,破坏偏应力与破坏应变较小,随着围压提高塑性混凝土应力应变曲线软化趋势减弱,并且随着围压的提高其破坏偏应力显著提高。塑性混凝土在不同围压下均表现出先剪缩后剪涨的特点,且随着围压升高其剪缩的趋势增大剪涨的趋势减小。双曲线模型参数K值随养护期龄的发展而增大,双曲线模型参数n、F、破坏比Rf值随养护期龄的发展而减小。相同养护期龄内塑性混凝土破坏偏应力随围压的增大而增大,相同围压下其破坏偏应力随养护期龄的发展而增大。凝聚力随期龄发展而增大,内摩擦角受养护期龄变化影响较小。     

塑性混凝土单向受压应力-应变关系的试验研究

通过棱柱体试件的单向受压试验,研究了水胶比和龄期对塑性混凝土单向受压的应力-应变曲线的影响。研究结果表明,单向受压下塑性混凝土的峰值应力随水胶比增加而减小,随龄期的增加而增大,其峰值应变基本在3.000με以上,平均值为3.528με;塑性混凝土应力应变曲线上升段斜率、下降段曲率及陡峭程度均随水胶比的减小或龄期的增加而增大;塑性混凝土单向受压中吸收的能量随峰值应力的增加而线性增加。通过对试验结果的分析,建立了塑性混凝土弹性模量、吸收能量、应力-应变曲线参数a1和a2与峰值应力间的关系式及峰值应力、a2与水胶比和龄期间的关系式。在此基础上,提出了考虑水胶比和龄期影响的单向受压下塑性混凝土应力-应变关系数学表达式。     

渠道水工混凝土防渗材料破坏性能试验

以新疆某灌区输水渠道防渗为背景，通过水工混凝土材料三轴试验系统，展开混凝土力学破坏试验分析，主要包括加载速率、围压、水灰比等参数对混凝土试件三轴应力应变特征的影响；根据分析结果，随着加载变形速率的提高，水工混凝土试件三轴强度随之提升，但在屈服变形阶段力学特征差异才明显显现；三轴强度、峰值应力等与围压呈正相关关系；在加载速率、围压、水灰比等因素的耦合作用下，水灰比的增大对围压正向促进作用有抑制和减缓作用，但试件变形速率间力学特征愈加明显。相关分析成果和研究结论可作为混凝土防渗材料在水利工程中应用的借鉴参考。     

Call for Papers

正As the eldest and most distributed professional journal of water industry,Water Wastewater Engineering(W WE)gained very wide acceptance from the colleagues working in this field in this country since start publication in 1964.Now it is administrated by the Ministry of Construction of the People’s Republic of China and sponsored by the China Civil Engineering Society etc.     

地下厂房施工过程的三维非线性数值模拟分析

根据冗各电站地下厂房开挖支护的施工顺序,以隐式杆单元模拟锚杆,以壳单元模拟混凝土喷层,采用三维非线性有限元技术,对洞室群的开挖支护动态过程进行了模拟计算。通过三维非线性有限元计算分析得到:洞室群动态开挖过程中围岩应力变形的分布规律,围岩塑性区的发展规律,锚杆受力变化情况。计算结果表明:整个开挖过程中,洞周围岩位移变化规律正常,量值较小,塑性区主要分布在洞室周边,围岩稳定性较好;但在洞室交叉口处围岩的变形较大,出现较大范围的塑性区,在此基础上提出了支护措施优化方案,并对优化方案的效果进行了评价。     

Evaluation of freshwater mussel relocation as a conservation and management strategy

The relocation of unionacean mussels is commonly used as a conservation and management tool in large rivers and streams. Relocation has been used to recolonize areas where mussel populations have been eliminated by prior pollution events, to remove mussels from construction zones and to re-establish populations of endangered species. More recently, relocation has been used to protect native freshwater mussels from colonization by the exotic zebra mussel Dreissena polymorpha. We conducted a literature review of mussel relocations and evaluated their relative success as a conservation and management strategy. We found that 43% of all relocations were conducted because of construction projects that were forced to comply with the Endangered Species Act 1973 and that only 16% were monitored for five or more consecutive years. Most (43%) relocation projects were conducted from July to September, presumably a period when reproductive stress is relatively low for most species and the metabolic rate is sufficient for reburrowing in the substrate. The mortality of relocated mussels was unreported in 27% of projects; reported mortality varied widely among projects and species and was difficult to assess. The mean mortality of relocated mussels was 49% based on an average recovery rate of 43%. There is little guidance on the methods for relocation or for monitoring the subsequent long-term status of relocated mussels. Based on this evaluation, research is needed to develop criteria for selecting a suitable relocation site and to establish appropriate methods and guidelines for conducting relocation projects.     

基于模型试验的河床砂砾石层基本特性研究

为了经济可靠地确定深厚覆盖层的基本特性参数,以大渡河双江口水电站坝基河床砂砾石层基本特性研究为工程背景,进行大尺寸模型试验,研究河床砂砾石层的旁压模量和动探击数随密度、级配以及上覆压力的变化规律。根据室内模型试验结果,结合现场旁压试验及钻孔试验成果,推断出坝址区河床砂砾石相关层位的密度,并依此进行砂砾石室内力学试验,获得设计所需力学参数。研究成果可为深厚覆盖层基本特性测试提供新的方法。     

SPATIAL AND TEMPORAL WATER QUALITY VARIABILITY IN AQUATIC HABITATS OF A CULTIVATED FLOODPLAIN

The floodplains of lowland rivers contain diverse aquatic habitats that provide valuable ecosystem services but are perturbed when intensively cultivated. Hydrologic, water chemistry and biological (fish) conditions in five aquatic habitats along the Coldwater River, Mississippi, were measured for more than 4 years: the river, two severed meanders that functioned as backwaters, a managed wetland and an ephemeral channel draining cultivated fields. Off‐channel habitats were connected to downstream regions 0.10% to 32% of the dry season and 24% to 67% of the wet season. The median temperatures for the five monitored sites ranged from 18°C to 23°C, the median total solids concentration for all sites was 135 mg L^?1, the median total phosphorus was 0.29 mg L^?1 and the median total nitrogen was 1.56 mg L^?1. Chemical and physical water quality displayed strong seasonal differences between the wet winter/spring and the dry summer/fall periods so that temporal variation consisted of gradual seasonal trends superimposed on strong diurnal variations. All off‐channel habitats exhibited periods of hypoxia and temperatures >30°C during the dry season. Between‐site gradients of water and habitat quality were strongly coupled to water depth and runoff loading. The rehabilitation of one backwater by increasing water depth and diverting agricultural runoff was associated with improved water quality and fish species richness relative to an adjacent untreated backwater. The diversion of polluted runoff and the use of water control structures to maintain greater water depth were observed to be effective management tools, but the former reduces the water supply to habitats that tend to dry up and the latter reduces connectivity. Published in 2011 by John Wiley & Sons, Ltd.     

Spatio‐temporal variation of gross CO2 and CH4 diffusive emissions from Australian reservoirs and natural aquatic ecosystems,and estimation of net reservoir emissions

Carbon dioxide (CO₂) and methane (CH₄) diffusive emissions were measured during two field surveys in Queensland and Tasmania, Australia, using the floating chamber method. Bubbling and degassing emissions in 2010 were estimated in Koombooloomba Dam reservoir using only inverted funnels and gas concentrations, respectively. A total of 14 reservoirs and 16 rivers and lakes were sampled from 2006 to 2010. Spatial variation was substantial within each water body, as well as between them. The main drivers of diffusive emission variation were physiographic region and climate, with a clear demarcation being observed between diffusive emissions from tropical Queensland and temperate Tasmania, and between the humid West Coast Range (Tasmania) and dry Central Plateau (Tasmania). Higher CO₂ and CH₄ diffusive emissions were observed during the dry season, when long water residence times would promote organic matter degradation. Estimated total gross emissions, including diffusive, bubbling and degassing emissions, for Koombooloomba Dam reservoir were about 1.5 × 10⁶ t CO₂eq km² per year, or 24 × 10⁶ t CO₂eq per year. This corresponds to a plant emission factor of 3.18 kg CO₂eq MWh^?1. Using an estimate of terrestrial emissions derived from literature data for the Tully River catchment area, rough estimated net emissions from the catchment area are about 44 kt CO₂eq per year, or 5.83 kg CO₂eq MWh^?1, which is in the lower range of the studied reservoirs.     

Automated system for removal and pneumatic transport of fly ash from electric precipitator hoppers

A system for removal and pneumatic transport of fly ash is examined, in which air pulses act on batches (pistons) of ash formed in a duct. Studies are made of the effect of several physical parameters on the force required to displace a piston of ash and these serve as a basis for choosing a system for removal and pneumatic transport of ash simultaneously from several hoppers of an electric precipitator. This makes it possible to separate the ash particles according to size without introducing additional components. Formulas are given for calculating the structural and dynamic parameters of this system and measurements of indirect dynamic parameters are used to calculate the input-output characteristics of the system. In order to optimize the system, configurations for summing several ducts into a single transport duct for pneumatic ash transport are proposed. Some variants of dry ash utilization and the advantages of producing of size-separated particles are considered. __________ Translated from élektricheskie Stantsii, No. 2, pp. 26–30 (2008).     

The flood pulse advantage and the restoration of river-floodplain systems

The ‘flood pulse advantage’ is the amount by which fish yield per unit mean water area is increased by a natural, predictable flood pulse. Evidence for this increase is presented from tropical and temperate fisheries. It is argued that increasing multispecies fish yield by restoring the natural hydrological regime is consistent with increasing production of other trophic levels and with restoration from ecological and aesthetic viewpoints. When applied to a river-floodplain system, this restoration would provide a large, self-sustaining potential for recreation, commercial exploitation, and flood control. An interim ‘natural flood pulse’ restoration approach is proposed for systems modified for navigation. This approach approximates the natural hydrological regime in a river reach and is intended as a first step in the long process of restoring the watershed.     

基于BP神经网络-马尔科夫链模型的隧道围岩位移预测

在隧道工程施工中,围岩位移预测起着很重要的作用。将BP神经网络-马尔科夫链模型引入到隧道围岩位移预测中来,通过对训练样本的学习,利用BP神经网络实现了对位移时间序列的滚动预测,同时得到了实测值与预测值的相对误差;在此基础上利用马尔科夫链对相对误差进行修正,有效地提高了预测结果的精度。并将该模型应用于某公路隧道拱顶下沉位移时序预测中,结果表明该模型具有精度高、科学可靠的特点,为隧道围岩变形的预测提供了新的途径。     

Short communication. Pipelines as a refuge for freshwater snails

In the lower River Murray, Australia, populations of gastropods have declined markedly over the past 40 years. Of the native fauna of about 18 species, only the ancylid Ferrissia petterdi remains common (but in patches), and another five species feature in sporadic local resurgences. Until recently the viviparid Notopala was thought to be extinct. The introduced physid Physa acuta is widespread, but in low numbers. The decline probably is associated with flow regulation, high turbidity and the introduction of common carp. Snail populations have persisted, however, in irrigation pipelines fed from the river, where they periodically cause blockages. This paper reports the discovery of the only known populaion of Notopala hanleyi in a pipeline where the inner wall accumulates microbial and other organic matter, providing high quality food for snails. In the Murray before regulation most of the periphytic biomass was probably microbial, as fluctuating water levels would have maintained littoral communities in a highly productive, early state of succession his postulated that, by stabilizing seasonal water levels, regulation has promoted the production of filamentous green algae, an inferior food source, and so displaced snail populations from the open aquatic environment.