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1.
通过黄河上游沙漠宽谷河段现场考察及实测资料分析,依据河岸物质来源及组成将黄河上游崩塌河岸划分为粘性河岸及非粘性风沙堆积河岸两大类,前者可分为平面崩塌、弧形滑动崩塌和复合式崩塌3种类型,后者表现出非粘性河岸的表层滑移及平面崩塌两种形式。进一步以磴口、乌海河段为例,分析粘性河岸和风沙堆积河岸的塌岸特征,并结合近10年的遥感影像解译分析河岸线崩退变化规律,揭示塌岸引起河道横向变化特征。结果表明粘性河段的塌岸后退距离大于风沙堆积河段,局部河段短期出现凹退凸淤的动态岸线变化特点,但全河段长期仍然处于总体淤积的态势。 相似文献
2.
三峡水库蓄水后下荆江河段河床冲刷下切, 局部河段崩岸险情频繁发生。为研究下荆江二元结构河岸的土体特性及崩岸机理,结合近期该河段崩岸情况,现场查勘了6个崩岸点,并对河岸土体进行了室内土工试验。试验结果表明下荆江河岸土体的垂向组成具有典型的二元结构特征:下部非粘性土（沙土）层较厚,上部粘性土（低液限粘土）层较薄且松散。以河岸崩塌过程分析为基础,提出了二元结构河岸发生绕轴崩塌时上部土层稳定性的计算方法。结合近岸水动力条件计算及土工试验结果,定量分析了二元结构河岸的崩塌机理及其影响因素:① 下部沙土层的起动流速比近岸流速小得多,故该土层容易受水流冲刷;②上部粘性土层崩塌前抗冲强度很大,但多为低液限粘土且相对松散,崩塌后堆积在岸边容易分解并被水流带走;③ 河岸稳定安全系数在一个水文年内呈周期性变化,落水期安全系数最小,故容易引发崩岸,该计算结果与近期崩岸实际统计结果一致。 相似文献
3.
Many lowland stream channels have dramatically widened over the last two centuries. There has been considerable debate about whether this widening was caused by an unusually large flood, by a series of large floods, or by decreased bank stability caused by clearing of riparian vegetation. The relative effects of floods and vegetation can be disentangled in southeastern Australia where streams have undergone both clearing of bank vegetation, and decadal sequences of relatively higher and lower flood magnitude and frequency. Archival aerial photographs of the Nepean River, in southeastern Australia, suggest that banks did not erode during periods of low flood magnitude (drought-dominated regime: from 1901–1949) whether they were cleared or not. However, during periods of flood-dominated regime (1950 to 1970s) only cleared stream banks eroded. Thus, on the upper Nepean River, clearing alone was insufficient to trigger erosion by small floods, and even large floods were unable to erode vegetated banks. The conclusion is that substantial channel widening in this river required both clearing of bank vegetation, and periods of unusually large and frequent floods. This conclusion is supported by geomechanical modelling that examine the reduction in bank shear strength arising from the loss of tree-root reinforcement. The modelling also suggests that bank instability arising from devegetation amplifies the potential for bank failure during the drawdown phase of a flood, leading to channel widening. 相似文献
4.
二元结构河岸下部非黏性土层持续冲刷侵蚀,上部黏性土体崩塌并堆积于凹岸坡脚,改变弯道水流结构并影响河岸二次侵蚀过程。为探究崩塌体对急弯河道水力特性的影响,以荆江石首河段为背景,建立弯道三维水流数学模型,模拟不同崩塌体尺寸下的弯道水流结构,并对比分析壁面剪切力的变化规律。结果表明：(1)崩塌体堆积于凹岸坡脚驱离水流动力轴线,减小凹岸次环流强度并改变环流方向。(2)崩塌体对弯道不同区域壁面剪切力的影响不同,其中,床面及凸岸坡面的平均壁面剪切力增大;凹岸坡面平均壁面剪切力在崩塌体头部及上游区域减小,在崩塌体尾部区域增大。(3)崩塌体越大,剪切力变化幅度越大,剪切力相对变化幅度为崩塌体截面积变化幅度的2%～10%。研究成果可为河道整治及岸线规划利用提供技术支撑。 相似文献
5.
Failure mechanisms and pore water pressure conditions: analysis of a riverbank along the Arno River (Central Italy) 总被引：1，自引：0，他引：1
Mechanisms of failure occurring in two portions of a riverbank along the Arno River (Central Italy), are investigated in detail starting by a series of periodic field observations and bank profile measurements. Two dominant mechanisms involving the silty sand portion of the bank have been observed: (a) alcove-shaped failure in the middle portion of the bank; (b) slab failure involving the middle–upper bank.
A portion of the riverbank was subject to laboratory (grain size analysis; phase relationship analysis; triaxial tests) and in situ tests (borehole shear tests (BSTs)) to characterise the geotechnical properties of the overbank deposits. Two different procedures of bank stability analysis have been performed: (1) a complete analysis, coupling seepage analysis with the limit equilibrium method; (2) two simplified analyses, through the limit equilibrium method with simple assumptions on pore water pressures distribution.
For the complete analysis, saturated/unsaturated flow within the riverbank was modelled by finite element seepage analysis in transient conditions, using as boundary conditions eight hydrographs with increasing water stage. Riverbank stability analyses have been conducted by the Morgenstern–Price rigorous method, dividing each of the eight hydrographs in 21 time steps and calculating the safety factor for each step. The analysis revealed the occurrence of two possible mechanisms of failure (slab-type and alcove-shaped sliding failures), according to the field observations, related to different river stages and pore water pressures within the riverbank: alcove failures are likely to occur with moderate flow events, while slab failures are favoured by flow events with higher peak river stage.
A first type of simplified analysis, representing critical conditions reached during a rapid flow event, was based on the main hypothesis of the occurrence of a zero pore water pressures zone within the portion of the bank between the low-water stage and the peak stage reached. A second type of simplified analysis was applied in order to represent rapid drawdown conditions following a prolonged flow event (worst case), with the main assumption of total saturation of the material up to the same elevation of the peak river stage. The first simplified analysis has given similar results to the complete seepage/stability analysis, confirming slab-type and alcove-shaped failure as the two main mechanisms of instability, while the second type of simplified analysis has conducted to too conservative results compared to the other previous analyses.
Field observations regarding different characteristic bank geometries in adjacent sub-reaches have been summarised in a conceptual cyclic sketch, that include all the possible paths of bank evolution depending on the succession of river stages reached during flow events and related pore pressure conditions. 相似文献
6.
Upper bound analysis of tunnel face stability in layered soils 总被引：3，自引：3，他引：0
The working face of tunnel constructions has to be kept stable during tunneling to prevent large soil deformations or fatal failure. In layered soils with lower cohesion, failures happen more often and more abrupt than in cohesive soils. Therefore, the maintenance of a proper support pressure at the tunnel working face is of high importance. In this paper, an upper bound analysis is introduced to investigate the minimum support pressure for the face stability in layered soils. A three-dimensional kinematically admissible mechanism for the upper bound analysis is improved to model potential failure within different soil layers. An analytical solution for the support pressure assessment is achieved. The influence of the crossing and cover soil on the face stability is analyzed, respectively. This solution provides an analytical estimation of the minimum support pressure for the face stability. It may be used as a reference for projects under similar conditions. 相似文献
7.
针对河岸崩塌问题分析和研究,在考虑江河水位升降引起坡外水压力变化及坡内非稳定渗流基础上,同时考虑水流冲刷引起的河床冲深及河岸后退,提出了水流冲刷过程中的边坡临界滑动场和适用于天然江河崩岸的数值模拟,并对水流冲刷过程中的崩岸问题进行了分析。通过对两类不同土质岸坡的崩岸数值模拟,分析了水流冲刷引起的河床冲深及河岸后退过程中坡体的稳定性变化,探讨了不同土质岸坡的崩岸类型及崩塌模式。结果表明,坡度较陡的黏性岸坡崩塌时趋近于平面破坏且通过坡脚;坡度较缓的粉土岸坡崩塌时沿曲面破坏,且在水位骤降过程中易发生局部崩塌。 相似文献
8.
JianWenxing YinKunlong XuYixian 《中国地质大学学报(英文版)》2005,16(1):45-50
INTRODUCTIONGeological investigations have illustrated that in stability problems exist in the bank slopes of nearly10 % of the total length of the 5 000 km YangtzeRiver and its tributaries when water level rises to 175m in the Three Gorges … 相似文献
9.
The design against failure of an embankment resting upon a soft soil improved by a group of columns is investigated with the help of the yield design homogenization approach. Assuming that both constituents of the reinforced ground are purely cohesive materials (‘lime column’ technique), an upper bound estimate for the macroscopic strength condition of the reinforced soil as a homogenized medium is first obtained, providing definite evidence of a shear strength anisotropy associated with the reinforcement preferential orientation. The kinematic method of yield design is then performed on the basis of such a criterion, making use of rotational failure mechanisms involving slip circles in the reinforced ground. Upper bound estimates are finally obtained for the embankment stability factor, as functions of the degree of reinforcement and relative thickness of the soil layer. These results are compared with those derived from a simplified analysis, where the reinforced soil is assumed to exhibit an averaged isotropic cohesion. This comparison clearly indicates that the latter simplified analysis may produce quite unsafe estimates for the embankment stability, which can be attributed to the fact that it fails to capture the inherent strength anisotropy of the reinforced soil. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
10.
Soil exploration, collection of hydrologic and meteorologic data, and sequential comparative photography were used to investigate erosion/deposition on alluvial banks of an island in the Ohio River from April 1980 through April 1982. Major erosion episodes coincided with flood events and were caused primarily by exit of bank recharge waters from pervious bank soil layers after flood recession. Waves created by wind and/or vessel passage were of little significance in causing bank failure. Floodwater tractive forces, runout of precipitation-derived waters, and wave action were effective only in removing soft sediments or loose debris from upper bank failures. Sediments deposited by spring floods, if allowed to dry for two to three months, strongly resisted subsequent wave attack. 相似文献
11.
Breaching in fine sands and the generation of sustained turbidity currents in submarine canyons 总被引：6，自引：0，他引：6
Abstract Natural, moderately loosely packed sands can only erode from the surface of the bed after an increase in pore volume. Because of this shear dilatancy, negative pore pressures are generated in the bed. In cases of low permeability, these negative pressures are released relatively slowly, which retards the maximum rate of erosion. This effect is incorporated in a new, analytically derived, pick‐up function that can explain the observation of gradual retrogressive failure of very steep subaqueous slopes, sometimes more than 5 m high, in fine non‐cohesive sands. This process, termed ‘breaching’ in the field of sediment dredging, may produce large failures in sand bars or river banks. The analytical function that describes the breaching process in fine sand is incorporated in a one‐dimensional, steady‐state numerical model of turbidity currents describing the spatial development of flow. This model is applied to simulate a large ‘flushing’ event in Scripps Submarine Canyon, Pacific coast of California. Breach retrogradation and the successive evolution in time of the resulting turbidity current in the canyon are predicted in a sequence of discrete steps. Predicted velocities are compared with values measured during a flushing event. Implications for the interpretation of deep‐water massive sands are discussed. 相似文献
12.
为研究河型转化过程机理,建立了考虑弯道二次流影响与边岸崩塌过程的平面二维河流数学模型,包括水流模型、泥沙模型和边岸崩塌模型。通过在水流动量守恒方程中增加弥散应力项以考虑弯道二次流的影响,并采用室内水槽实验结果对水流模型进行了验证;利用上荆江沙市至石首天然长河段的水沙过程和河道演变资料,对泥沙模型进行了验证;结果表明本模型数值计算量合适,有较好的适应范围。模型中提出了边岸崩塌过程的模拟技术,相对于传统平面二维水沙模型而言,可以更好地模拟天然河道的横向摆动以及洲滩消长过程。 相似文献
13.
以二元结构河岸崩塌模式为研究背景,在弯道水槽中展开系列试验,研究凹岸坡脚处成型崩塌体在水力作用下输移过程及其对岸坡稳定性与河床冲淤的交互影响。试验成果表明,崩塌体经水力分解破碎呈块状、片状或颗粒状起动,部分随水流带至凸岸或下游,堆积在坡脚附近的崩塌体残留量随水力作用大小及土体特性不同而变化。崩塌体临水面周围尤其上下游端水流紊动强烈易形成较大剪切力区,临坡面上下游端附近剪切力较无崩塌体时减小;崩塌体体积越大,对剪切力区特征的改变也越明显。崩塌体的存在虽不能制止附近岸坡的再次崩塌,但可能抑制崩岸发展及附近河床淤积的程度,崩塌体的粘性或体积越大,这种抑制作用越显著;相同崩塌体抑制附近河床淤积的程度较抑制岸坡崩塌的程度大。 相似文献
14.
为明确盾构施工掌子面滑移破坏机制并确定掌子面支护力的合理范围,基于滑移线理论和极限分析上限定理,利用空间离散技术提出了一种盾构施工掌子面三维滑移破裂模型。依据大主应力拱理论计算滑移区顶部竖向土压力值,并以此作为滑移破坏区上部的竖向荷载计算掌子面极限支护力。研究表明,土拱效应显著影响掌子面前方土体竖向应力的大小及分布规律;将本模型与已有研究方法进行比较,验证了本模型获取的掌子面极限支护力极限分析上限解在黏性土地层以及摩擦土地层中的适用性。同时本模型构建的掌子面破坏区域形态更加贴近离心试验结果与数值计算结果。 相似文献
15.
The effects of failure mode transition from tensile to shear on structural style and fault zone architecture have long been recognized but are not well studied in 3D, although the two modes are both common in the upper crust of Earth and terrestrial planets, and are associated with large differences in transport properties. We present a simple method to study this in physical scale models of normal faults, using a cohesive powder embedded in cohesionless sand. By varying the overburden thickness, the failure mode changes from tensile to hybrid and finally to shear. Hardening and excavating the cohesive layer allows post mortem investigation of 3D structures at high resolution. We recognize two end member structural domains that differ strongly in their attributes. In the tensile domain faults are strongly dilatant with steep open fissures and sharp changes in strike at segment boundaries and branch points. In the shear domain fault dips are shallower and fault planes develop striations; map-view fault traces undulate with smaller changes in strike at branches. These attributes may be recognized in subsurface fault maps and could provide a way to better predict fault zone structure in the subsurface. 相似文献
16.
在弯道水槽中开展6组试验,分别用非黏性土及黏性土填筑河床,研究相同水力作用下近岸河床组成对黏性岸坡崩塌的影响规律。研究发现,在试验给定的岸坡及河床组成情况下,非黏性河床凹冲凸淤且总体表现为淤积,近岸河床及凹岸岸坡冲刷强度大,凸岸附近床面上泥沙掺混较明显;黏性河床及凹岸岸坡均被冲刷,河床主流区冲刷强度比近岸河床及凹岸岸坡大。相较于黏性河床,非黏性河床近凹岸处较易冲刷,水流结构重新调整,凹岸坡脚处水流流速及紊动能可增至2倍左右,环流强度可增至11倍,加速了岸坡崩塌及崩塌体的分解输移;非黏性河床近凹岸坡脚处变形以及凹岸岸坡崩塌量均相对较大,岸坡崩塌强度为河床淤积强度的2~4倍,崩塌物质可充分补给河床的泥沙来源;经水力冲刷后非黏性河床组成情况下形成的河道滩槽高差相对较小,河道横断面相对宽浅。 相似文献
17.
倾斜基岩上的边坡破坏模式和稳定性分析 总被引：6，自引：0，他引：6
应用离心模型试验对工程中被颇为关注的带有与边坡走向一致的倾斜基岩面,且在该基岩面存在软弱夹层的边坡的稳定性和破坏模式进行了比较详细地研究,并用极限平衡分析方法对试验结果进行了计算分析。模型试验结果表明,该类边坡失稳时,紧贴岩面的软弱夹层就成为滑动破坏面,因而边坡整体沿基岩面向下滑动,且侧向水平位移各处基本一致,表现出典型的平移滑动破坏模式。将稳定安全系数的实测结果与按平移滑动破坏模式的极限平衡分析方法的计算结果相比较后发现,两者相当地吻合,证实了按平移滑动破坏模式所作的极限平衡分析,能良好地预测边坡平移滑动破坏情形下的稳定安全系数。 相似文献
18.
M. Al Mamunul Haque C. S. Jahan Q. H. Mazumder S. M. S. Nawaz G. C. Mirdha P. Mamud M. I. Adham 《Journal of the Geological Society of India》2012,79(1):77-84
The Rajshahi city is the fourth largest metropolitan city in Bangladesh on the bank of the River Padma (Ganges). Here an upper
semi-impervious layer overlies aquifer — the source for large-scale groundwater development. The groundwater resource study
using Visual MODFLOW modeling shows that recharge occurs mainly due to infiltration of rainfall and urban return flow at low
rate, and water level fluctuates seasonally in response to recharge and discharge. Hydraulic connection between river and
aquifer which indicates inflow from high river water levels beyond its boundaries. The total groundwater abstraction in 2004
(15000 million liters) is lower than total input to aquifer reveals an ample potentiality for groundwater development with
increasing demand. But groundwater shortage (1000 million liter/year) especially in the vicinity of the River Padma in dry
season happens due to its increasing use and fall of river water level resulting in reduced inflows and hence decline in groundwater
level. The conjunctive use of surface water-groundwater and its economic use will help for sustainable groundwater supply
to avoid adverse impact. 相似文献
19.
Large bank failures, comprising up to 106 m3 of sediment, are common features along steep channel banks in estuaries and large rivers that consist of clean, fine sands, and are mostly assumed to be generated by sudden liquefaction of large masses of very loosely packed sand. Another less commonly recognized type of failure is manifested by the gradual retrogression of a very steep wall, steeper than the angle-of-repose. Instead of the voluminous surging plastic sediment-water flow, or hyperconcentrated density flow (sensu Mulder & Alexander, 2001 ) generated by liquefaction, this type of failure, known as breaching by dredging companies and hydraulic engineers, produces a sustained quasi-steady, turbidity current. To date, sedimentologists have not recognized the process of breaching as such. In this paper, it is suggested that breaching may be the origin of many thick, massive sand layers known from ancient deposits from various environments, notably in some turbidite successions. Possible differences in the sedimentary structure of the deposits produced by breach failures vs. liquefaction slope failures (=liquefaction flow slides) can be deduced from a knowledge of the sediment transport processes initiated by the failure. A field study is presented on some poorly structured beds in the Eocene Vlierzele Sands in Belgium, which are supposed to have originated from liquefaction failures, but are reinterpreted to be the products of breaching. It is postulated that the local steep slope disturbance required to initiate an active breach can be produced by a small liquefaction slope failure (=liquefaction flow slide failure) or local erosion by river or tidal channel flow at the initial stage of the failure event. 相似文献
20.
Continuous velocity fields for collapse and blowout of a pressurized tunnel face in purely cohesive soil 总被引：1，自引：0，他引：1
Face stability analysis of tunnels excavated under pressurized shields is a major issue in real tunnelling projects. Most of the failure mechanisms used for the stability analysis of tunnels in purely cohesive soils were derived from rigid block failure mechanisms that were developed for frictional soils, by imposing a null friction angle. For a purely cohesive soil, this kind of mechanism is quite far from the actual velocity field. This paper aims at proposing two new continuous velocity fields for both collapse and blowout of an air‐pressurized tunnel face. These velocity fields are much more consistent with the actual failures observed in undrained clays. They are based on the normality condition, which states that any plastic deformation in a purely cohesive soil develops without any volume change. The numerical results have shown that the proposed velocity fields significantly improve the best existing bounds for collapse pressures and that their results compare reasonably well with the collapse and blowout pressures provided by a commercial finite difference software, for a much smaller computational cost. A design chart is provided for practical use in geotechnical engineering. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献