干湿循环路径下弱膨胀土峰值及残余强度演化研究

膨胀土具有胀缩性、多裂隙性和超固结性,在自然条件下极易受到降雨和蒸发的干湿循环效应,土的抗剪强度会随着时间的延续而衰减,造成边坡失稳。以荆门弱膨胀土为研究对象,对经历不同干湿循环次数的荆门弱膨胀土开展环剪试验,并探讨分析其峰值强度和残余强度的变化规律。试验结果表明,试样的抗剪强度与法向压力的大小有关,无论是峰值强度还是残余强度均随着法向压力的增大而增大,同时法向压力越大,试样达到残余强度时所需要的剪切位移也会越小;随着干湿循环次数的增加,膨胀土的峰值强度明显衰减,残余强度虽略有变化但并不明显,可近似认为稳定;经历3次干湿循环后膨胀土的峰值黏聚力和残余黏聚力指标已经近乎一致,峰值内摩擦角和残余内摩擦角之间始终保持在2o左右的差异,基本不受循环次数的影响。     

黄土边坡可靠度的随机有限元分析

为了研究黄土边坡的可靠度,在对黄土强度参数进行变异性统计、概率分布检验及垂直向自相关距离计算的基础上,将黄土强度参数黏聚力和内摩擦角作为随机变量,将其在二维边坡剖面的空间变化作为随机场,用局部平均细分法将符合一定均值和标准差的强度参数按随机场分布在边坡剖面的有限元网格上,再采用弹-理想塑性有限元计算边坡稳定系数,用MonteCarlo法计算其失效概率。结果表明:黄土黏聚力的变异系数多在0.30以上,内摩擦角的变异系数多在0.20以下,且黄土高原从西向东两个参数的变异性都增大;黏聚力和内摩擦角的概率分布类型对边坡失效概率计算结果影响显著,统计检验发现黏聚力采用对数正态分布、内摩擦角采用正态分布为宜;黄土地层的垂直向自相关距离多在0.7m以内,与计算厚度的比值多小于0.2;在大部分黄土的变异系数与自相关距离水平下,采用确定场进行可靠度分析,多数情况将极大地高估了黄土边坡的失效概率,而在极端高变异系数水平下,采用确定场进行可靠度分析,又将极大地低估其失效概率。     

Vulnerability and resilience of remote rural communities to shocks and global changes: Empirical analysis from Solomon Islands

Successful management of socio-ecological systems not only requires the development and field-testing of robust and measurable indices of vulnerability and resilience but also improved understanding of the contextual factors that influence societal capacity to adapt to change. We present the results of an analysis conducted in three coastal communities in Solomon Islands. An integrated assessment map was used to systematically scan the communities’ multiple dimensions of vulnerability and to identify factors affecting households’ perception about their capacity to cope with shocks (resilience). A multivariate probit approach was used to explore relationships amongst factors. Social processes such as community cohesion, good leadership, and individual support to collective action were critical factors influencing the perception that people had about their community's ability to build resilience and cope with change. The analysis also suggests a growing concern for a combination of local (internal) and more global (external) contingencies and shocks, such as the erosion of social values and fear of climate change.     

水平地震力作用下浅埋偏压隧道围岩压力的简化理论分析

考虑到传统浅埋偏压隧道围岩压力的分析仅以计算摩擦角体现围岩材料特性,没有将内摩擦角和黏聚力作为独立参数分开研究,基于规范法,提出水平地震作用下独立考虑黏聚力的浅埋偏压隧道围岩压力的简化解析分析方法,获得隧道顶部竖直围岩压力、隧道两侧水平侧压力以及滑动面破裂角的理论表达式,并对影响顶部竖直围岩压力、水平侧压力和破裂角的因素进行了研究。结果表明,竖直围岩压力与滑裂面摩擦角、地面倾角呈正相关,与水平地震效应系数、滑裂面黏聚力呈负相关;滑裂面内摩擦角、黏聚力、地面倾角越大,破裂角越大,水平地震加速度系数越大,破裂角越小;水平侧压力随滑裂面内摩擦角和黏聚力的增大而减小,随水平地震效应系数和地面倾角的增大而增大。研究成果可为浅埋偏压隧道的围岩应力计算提供一定的理论依据。     

边坡稳定性温度效应数值模拟研究

气候变化引起环境温度改变,岩土体的性质随之产生相应变化。文章分析了温度对边坡稳定性产生的影响,于不同 温度状况下开展粘性土直剪试验,探究粘聚力及内摩擦角随温度变化的特征。进一步利用有限元软件ANSYS 结合实测地温 数据模拟边坡温度场演变过程,基于试验结果将温度场推导出粘聚力场,利用瑞典条分法结合粘聚力分布特征求得边坡安 全系数。通过一系列室内试验发现,下蜀土粘聚力受温度影响变化规律较为明显;土体含水量和密度不同使土体随温度升 高相应的产生热软化或热固结现象。通过ANSYS 模拟南京城郊边坡夏季高温期间温度场的演变,推算出了边坡安全系数的 变化规律。结果表明,对于高含水量的土体,在夏季高温环境下边坡稳定性降低,而城区温度更高相应边坡的安全系数通 常更低,高温对边坡稳定性具有负面作用。     

饱和土体原位大型剪切试验方法与实践

由于室内剪切试验结果存在试样结构性扰动和尺寸效应等带来的不确定性问题,且以往的常规原位大型剪切试验是针对天然状态土体,不能获得饱和土体的强度参数,导致岩土结构稳定性分析和工程设计中大多采用反演方法求取强度参数,而很少直接运用这些试验参数.在本次试验方法研究中,试验仪器仍然采用常规的天然土体原位大型剪切试验仪器,针对土体饱和问题,设计了“制样后饱和法”和“饱和后制样法”2种浸水饱和装置,并采用这2套装置在甘肃省黑方台地区进行了7个饱和黄土原位大型剪切试验.结果表明,2种饱和方法都能够使土样饱和,“饱和后制样法”容易达到饱和,试验历时短,但对试样扰动大,甚至导致试样在制样时破坏,试验成功率低;“制样后饱和法”达到饱和历时稍长,对试样扰动小,试验成功率高.与原位天然大剪试验相比,粘聚力c值由44.65kPa下降至17.35kPa,φ值由14.18°降至11.95°;与室内饱和固结快剪相比,饱和黄土原位大剪的c值增加约4kPa,φ值增加3°左右.     

Effect of Moisture on Attraction Force in Beach Sand

Beach sands located above the sea water level exist in an unsaturated, rather than a fully saturated or dry state. Within the unsaturated zone, a steep excavated surface can be sustained for some unknown but finite time, and some slopes may remain stable for extended time periods due to capillary forces. These observations clearly indicate small but nonzero values for attraction strength (tensile strength and cohesion) in unsaturated beach sands, especially apparent but not confined to settings where there are low stress levels. Thus, experiments were carried out to quantify the magnitude of attraction strength in moist sands (D _r ?=?30%) and to examine the variation of these values as a function of moisture content and presence of a small amount of fines. Tensile strength, which is significantly different from zero, increases with increasing moisture content and fines. However, the influences of fines on the tensile strength are substantially dependent on the water content. Apparent cohesion strength is also identified in moist sands. A simple relationship between tensile strength and apparent cohesion is proposed using the obtained data. This study would help to further understand the phenomenon of stability of beach sands.     

Depositional processes,bedform development and hybrid bed formation in rapidly decelerated cohesive (mud–sand) sediment flows

Flows with high suspended sediment concentrations are common in many sedimentary environments, and their flow properties may show a transitional behaviour between fully turbulent and quasi‐laminar plug flows. The characteristics of these transitional flows are known to be a function of both clay concentration and type, as well as the applied fluid stress, but so far the interaction of these transitional flows with a loose sediment bed has received little attention. Information on this type of interaction is essential for the recognition and prediction of sedimentary structures formed by cohesive transitional flows in, for example, fluvial, estuarine and deep‐marine deposits. This paper investigates the behaviour of rapidly decelerated to steady flows that contain a mixture of sand, silt and clay, and explores the effect of different clay (kaolin) concentrations on the dynamics of flow over a mobile bed, and the bedforms and stratification produced. Experiments were conducted in a recirculating slurry flume capable of transporting high clay concentrations. Ultrasonic Doppler velocity profiling was used to measure the flow velocity within these concentrated suspension flows. The development of current ripples under decelerated flows of differing kaolin concentration was documented and evolution of their height, wavelength and migration rate quantified. This work confirms past work over smooth, fixed beds which showed that, as clay concentration rises, a distinct sequence of flow types is generated: turbulent flow, turbulence‐enhanced transitional flow, lower transitional plug flow, upper transitional plug flow and a quasi‐laminar plug flow. Each of these flow types produces an initial flat bed upon rapid flow deceleration, followed by reworking of these deposits through the development of current ripples during the subsequent steady flow in turbulent flow, turbulence‐enhanced transitional flow and lower transitional plug flow. The initial flat beds are structureless, but have diagnostic textural properties, caused by differential settling of sand, silt and cohesive mud, which forms characteristic bipartite beds that initially consist of sand overlain by silt or clay. As clay concentration in the formative flow increases, ripples first increase in mean height and wavelength under turbulence‐enhanced transitional flow and lower transitional plug‐flow regimes, which is attributed to the additional turbulence generated under these flows that subsequently causes greater lee side erosion. As clay concentration increases further from a lower transitional plug flow, ripples cease to exist under the upper transitional plug flow and quasi‐laminar plug flow conditions investigated herein. This disappearance of ripples appears due to both turbulence suppression at higher clay concentrations, as well as the increasing shear strength of the bed sediment that becomes more difficult to erode as clay concentration increases. The stratification within the ripples formed after rapid deceleration of the transitional flows reflects the availability of sediment from the bipartite bed. The exact nature of the ripple cross‐stratification in these flows is a direct function of the duration of the formative flow and the texture of the initial flat bed, and ripples do not form in cohesive flows with a Reynolds number smaller than ca 12 000. Examples are given of how the unique properties of the current ripples and plane beds, developing below decelerated transitional flows, could aid in the interpretation of depositional processes in modern and ancient sediments. This interpretation includes a new model for hybrid beds that explains their formation in terms of a combination of vertical grain‐size segregation and longitudinal flow transformation.