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311.
Initial fabric of a soil induced by its cyclic strain history is an important parameter among others like void ratio, state of stress and amplitude concerning the further accumulation of deformations under cyclic loading. It is of high importance to determine methods in order to estimate the initial fabric of the grain skeleton or the cyclic loading history of the soil. An attempt is made in this paper to correlate small strain stiffness of non-cohesive soil with its cyclic loading history. For this purpose several cyclic triaxial tests with specimens subjected to cyclic axial loading were performed and changes of small strain soil properties due to this cyclic loading were studied. All these tests showed only moderate changes of small strain stiffnesses independently of the different boundary conditions. Thus, a correlation between fabric or strain history and small strain stiffnesses seems not to be possible. 相似文献
312.
双峰具下寒武统灰绿色板岩、硅质板岩中产大量小壳动物化石,为研究湘中南地区震旦一寒武系界面划分提供了古生物资料。 相似文献
313.
A 235.5 km2 headwater catchment of the Krishna River in the Deccan plateau lavas is dry for eight months of the year but receives intense monsoonal rains during four months. High initial suspended sediment concentrations fall as rapid vegetation growth provides increasing protection. During a six-year period annual suspended sediment yields from the deeply weathered kaolinitic soils ranged between 36.9 and 275.3 t km?2 in dry and wet years respectively. 相似文献
314.
We use beaver-pond deposits and geomorphic characteristics of small streams to assess long-term effects of beavers and climate change on Holocene fluvial activity in northern Yellowstone National Park. Although beaver damming has been considered a viable mechanism for major aggradation of mountain stream valleys, this has not been previously tested with stratigraphic and geochronologic data. Thirty-nine radiocarbon ages on beaver-pond deposits fall primarily within the last 4000 yr, but gaps in dated beaver occupation from ~ 2200–1800 and 950–750 cal yr BP correspond with severe droughts that likely caused low to ephemeral discharges in smaller streams, as in modern severe drought. Maximum channel gradient for reaches with Holocene beaver-pond deposits decreases with increasing basin area, implying that stream power limits beaver damming and pond sediment preservation. In northern Yellowstone, the patchy distribution and cumulative thickness of mostly < 2 m of beaver-pond deposits indicate that net aggradation forced by beaver damming is small, but beaver-enhanced aggradation in some glacial scour depressions is greater. Although 20th-century beaver loss and dam abandonment caused significant local channel incision, most downcutting along alluvial reaches of the study streams is unrelated to beaver dam abandonment or predates historic beaver extirpation. 相似文献
315.
Semi-arid areas are often considered to be environmentally and economically marginal, a situation which has been exacerbated by economic change, shifts in agricultural production and land use, and changing state policy. These themes are explored with reference to a semi-arid landscape, namely the Karoo, which covers some 40% of the geographic space of South Africa and is used primarily for extensive livestock farming. Despite long-term decline in agricultural output, the traditional mainstay of the region, and weakening small town economies, the Karoo's population and the economies of its largest service centres are growing. There are, real socio-economic needs and development backlogs, and the situation has been exacerbated by recent political marginalisation. In this study, the small towns of the region are focal points of investigation and provide a lens to investigate the changing demographic and economic dynamics of the Karoo. Most of the region's population lives in these centres which are service, collection, and distribution points for what traditionally has been an agriculture-based regional economy. This paper explores the concept of marginalisation with specific reference to historical, economic, and demographic change. 相似文献
316.
形变观测中小数据集PSInSAR的误差分析与初步应用 总被引:1,自引:0,他引:1
PS InSAR技术在微小形变的监测中有着独特的优势, 但同时受到数据量的严重制约。 本文以形变速率稳定的怀来县城为实验区, 利用15景ASAR数据, 依据主影像选择和高程改正选取了三种略有差异的模型分别进行PS处理, 并对形变提取中的相位解缠、 基线改正和高程改正进行误差分析。 结果表明, 小数据集的相位解缠和高程改正在部分PS点上容易发生错误, 但形变估计整体上能够正确反映实际情况。 三个模型中MS(Multi-Reference Single Regression) 模型得出结果最为可靠。 小数据集解算的整体PS点质量不高, 可以在满足PS点密度要求的前提下, 降低相位标准偏差阈值以提高解算的可靠性。 相似文献
317.
318.
This paper is the first part of the general paper dealing with effects of constitutive modeling of cyclic stress–strain behavior of soils on site amplification. The paper concentrates on modeling of pseudo-static cyclic soil behavior in small to medium strain range. In order to fit the small strain data accurately, the chosen analytical stress–strain relationship should satisfy the specific small strain condition formulated for soils using the small strain data from the pseudo-static cyclic tests. Analysis of conventional relationships, in particular the Ramberg–Osgood (R–O) relationship, indicated that a failure to satisfy this condition lead to low accuracy of prediction of both tangent stiffness and damping ratio at small and medium strains. The logarithmic function originally proposed to describe static monotonic stress–strain behavior is applied to fit experimental cyclic backbone curves. Constructed to satisfy the formulated small strain condition for soils, this function has proven to be free from the limitations of the R–O and other relationships. When applied in combination with the Masing rules to predict damping ratios, it gives a good prediction in the small to medium strain range, where the Masing hypothesis is supported by experimental evidence. 相似文献
319.
目的:探讨螺旋CT在腰大肌病变的应用价值。方法:回顾性分析手术病理或临床随访证实的110例腰大肌病变的螺旋CT资料。结果:腰大肌血肿37例(其中急性期血肿30例,慢性期血肿7例),急性期血肿表现为腰大肌筋膜增厚、体积增大、伴有不均匀高密度和横突等骨折,随访中体积逐渐变小、密度渐减低,慢性期血肿液化呈水样密度;累及腰大肌的感染28例(其中结核21例,其他感染7例),CT表现为腰大肌膨隆、轮廓模糊,周围脂肪间隙可见索条、积液,CT随访可因病变的转归而发生相应变化,结核可伴相应椎体及附件骨质破坏;腰大肌肿瘤45例,其中腹腔恶性肿瘤直接侵犯15例,肿瘤转移30例(单纯性腰大肌转移3例,同时伴脊椎转移27例),CT表现为腰大肌肿大,明显强化的肿物。结论:认识腰大肌病变的CT特征对诊断具有重要意义。 相似文献
320.