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941.
942.
对挡土墙背离填土绕墙脚转动时墙后滑裂土体的应力状态进行了详细分析,建立了墙后滑裂体水平土层墙面反力、滑裂面反力、土层间剪力和土层竖向土压力强度之间的关系式。为了考虑挡土墙绕墙脚转动时墙脚局部土体并未达到极限状态,对墙面摩擦角、滑裂面土体的内摩擦角予以折减。在水平土层单元法的基础上,考虑水平土层间剪力作用、每一土层的墙面摩擦角和滑裂面水平倾角等的变化,建立了土层竖向土压力强度的逐层渐近的计算方法,并给出了挡土墙主动土压力强度、土压力合力及其作用位置的计算公式。经比较表明:挡土墙主动土压力分布曲线与试验结果基本一致,计算得的主动土压力系数与试验结果很接近,比库仑解大;计算得出的滑裂面为一曲面,其顶部开裂宽度比库仑滑裂面小,与工程实际相符。 相似文献
943.
本文指出了我国水资源存在的严重问题,提出把搞好水利建设作为我国的基本国策,并为解决我国的水利问题提出解决办法:要运用科学的系统工程作好总体规划,逐步在我国建成水利网络,做到合理利用并防止水源污染,使每一寸土地都能得到水的滋润。 相似文献
944.
干旱区绿洲土地利用与覆被变化过程 总被引:88,自引:6,他引:82
应用遥感和GIS技术及数理统计学方法,以新疆三工河流域冲洪积扇-冲积平原型绿洲为研究区,利用1978年、1987年和1998年的遥感数据,深入研究了70年代中后期以来干旱区绿洲LUCC的过程和趋势。结果表明:(1) 遥感和GIS在绿洲土地利用与覆被变化数据获取和空间分析方面是一种非常强大而有效的技术手段;(2) 通过建立绿洲LUCC的幅度、速度、区域差异和变化趋势数学模型,可以较好的研究绿洲土地LUCC的过程和趋势,如变化的双向性、单向性、正向性和反向性,变化的涨势或落势等,在三工河流域绿洲应用取得良好效果;(3) 自70年代中后期以来三工河流域绿洲LUCC趋缓,并存在显著的时空差异:前期绿洲LUCC处于单向转换过程的非平衡状态,并呈现一定的地段分异,其中绿洲上部处于准平衡状态,而绿洲中部和下部处于非平衡状态。后期绿洲LUCC呈现双向转换为主的发展态势,使绿洲LUCC处于准平衡状态;绿洲上部和中部处于非平衡状态,绿洲下部处于平衡状态,均表现出不同的变化过程和趋势。 相似文献
945.
Lukáš Gábor Vojtěch Barták Vincent Lecours 《International journal of geographical information science》2020,34(8):1567-1584
ABSTRACT Species distribution models (SDMs) are widely used in ecology and conservation. However, their performance is known to be affected by a variety of factors related to species occurrence characteristics. In this study, we used a virtual species approach to overcome the difficulties associated with testing of combined effects of those factors on performance of presence-only SDMs when using real data. We focused on the individual and combined roles of factors related to response variable (i.e. sample size, sampling bias, environmental filtering, species prevalence, and species response to environmental gradients). Results suggest that environmental filtering is not necessarily helpful and should not be performed blindly, without evidence of bias in species occurrences. The more gradual the species response to environmental gradients is, the greater is the model sensitivity to an inappropriate use of environmental filtering, although this sensitivity decreases with higher species prevalence. Results show that SDMs are affected to the greatest degree by the species response to environmental gradients, species prevalence, and sample size. Models’ accuracy decreased with sample size below 300 presences. Furthermore, a high level of interactions among individual factors was observed. Ignoring the combined effects of factors may lead to misleading outcomes and conclusions. 相似文献
946.
Predicting the initiation of static liquefaction of cross‐anisotropic sands under multiaxial stress conditions
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Experimental evidence has shown that the liquefaction instability of sands can be affected by its material density, stress state, and inherent anisotropy. In order to predict the initiation of the static liquefaction of inherent cross‐anisotropic sands under multidimensional stress conditions, a rational constitutive model is needed. An elastoplasticity model able to capture the influences of intermediate principal stress ratio (b = (σ 2 ? σ 3)/(σ 1 ? σ 3)) and loading direction on stress–strain relationships and volumetric properties was proposed. The yield function was formulated to be controlled by Lode angle, loading direction, and material state; the stress–dilatancy was a material state‐dependent function. After using the existing drained hollow cylinder tests to validate the proposed model, this model was used to simulate the existing undrained hollow cylinder tests. During this simulation, the second‐order work criterion was used to determine the initiation of static liquefaction. The results showed that an increase in both the intermediate principal stress ratio and the loading angle induces a decrease in the second‐order work. Static liquefaction is initiated more easily at a stress state with a large intermediate principal stress ratio and a large loading angle, and the mobilized friction angle at the instability points decreases with the intermediate principal stress ratio and the loading angle. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
947.
Alexandros L. Petalas Yannis F. Dafalias Achilleas G. Papadimitriou 《国际地质力学数值与分析法杂志》2019,43(1):97-123
SANISAND is the name of a family of bounding surface plasticity constitutive models for sand within the framework of critical state theory, which have been able to realistically simulate the sand behavior under conventional monotonic and cyclic loading paths. In order to incorporate the important role of evolving fabric anisotropy, one such model was modified within the framework of the new anisotropic critical state theory and named SANISAND-F model. Yet the response under continuous stress principal axes rotation requires further modification to account for the effect of ensuing noncoaxiality on the dilatancy and plastic modulus. This modification is simpler than what is often proposed in the literature, since it does not incorporate an additional plastic loading mechanism and/or multiple dilatancy and plastic modulus expressions. The new model named SANISAND-FN is presented herein and is validated against published data for loading that includes drained stress principal axes rotation on Toyoura sand. 相似文献
948.
Jin-Tae Hwang 《The Professional geographer》2016,68(4):554-560
Using a multiscalar approach, there has been a recent increase in geographic critiques of the East Asian developmental state thesis (DST). These critiques argue that the existing DST literature is epistemologically caught in a territorial trap. There has been less concern, however, about the methodology related to this research. In this article, I suggest that scholars consider not only the epistemological perspective but also the methodological dimension to effectively exploit the less territorially trapped DST research. Based on these analyses, this article emphasizes that interdisciplinary research using a multiscalar methodology and epistemology is required to better explain economic development in East Asia. 相似文献
949.
950.