Breaching in fine sands and the generation of sustained turbidity currents in submarine canyons

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.     

Estimation of potential failure risks in a mine slope using indicator kriging

Indicator kriging has been applied to the study of failure mechanisms in a mine slope in Minas Gerais, Brazil, to estimate potential failure risks in limited areas along this slope. Timbopeba Mine, Vale Company, is an open pit iron mine situated in the Quadrilátero Ferrífero, a very important mining district in Minas Gerais. A slope excavated in quartzite with a maximum height of 200 m at the time of this study, has presented many failure problems involving the sliding of blocks formed by discontinuities. These blocks are of limited size in comparison to the dimensions of the overall slope. They appear along the entire slope, wherever discontinuity orientations have led to the kinematic feasibility of these blocks. Geostatistics permits the estimation of local failure probability distributions associated to these local failures, which would not be possible with traditional statistical models. The geostatistical method employed in this study, indicator kriging, is quite suitable because it is unnecessary to assume a particular global distribution of the phenomena being modeled. The model was used for locating areas with a great tendency for sliding failure, as it considers the local spatial variability of discontinuity orientations. Copyright © 2010 John Wiley & Sons, Ltd.     

深圳某填土滑坡破坏机理研究

2002年9月18日13:40,在连降暴雨的影响下,深圳一填土边坡发生滑坡,约2.5×104m3的松散填土体在水平地面高速滑动了140m,造成5人死亡3、1人受伤。文章首先介绍了滑坡区的地质环境特点及滑坡特征;为了揭示该滑坡的发生机理,开展了松散击实填土的等压固结不排水剪(ICU)及常剪应力排水剪(CS)试验。ICU试验结果表明,土体具有明显的应变软化特性并伴随孔隙水压力的上升;CS试验结果表明,土体在低围压条件下破坏具有突然性,破坏过程中孔隙水压力急剧上升,表明土体出现突然性结构丧失。ICU及CS试验均表明松散填土具有静态液化特性。从现场孔隙比与稳态线(SSL)的关系来看,原位填土的状态参数位于稳态线上方,因此该填土边坡是由于地下水位上升造成土体出现静态液化、液化土体形成流滑所致。无限斜坡稳定分析表明造成该填土边坡破坏需要有较高的地下水位。     

A critical review of rock slope failure mechanisms: The importance of structural geology

Geological structures such as folds, faults, and discontinuities play a critical role in the stability and behaviour of both natural and engineered rock slopes. Although engineering geologists have long recognised the importance of structural geology in slopes, it remains a significant challenge to integrate structural geological mapping and theory into all stages of engineering projects. We emphasise the importance of structural geology to slope stability assessments, reviewing how structures control slope failure mechanisms, how engineering geologists measure structures and include them in slope stability analyses, and how numerical simulations of slopes incorporate geological structures and processes.     

某码头软黏土岸坡破坏机制分析及重建

在某吹填软黏土岸坡上修建码头时发生了较大规模的滑坡,待滑体稳定后采取了一系列的工程措施,实现了码头的重建。详细阐述了岸坡发生滑动的全过程,通过现场勘察和有限元模拟分析了岸坡发生滑动破坏的原因。分别考虑了打桩与交通荷载作用,导致地基土中产生超静孔压,从而对岸坡的稳定性造成影响。分析结果表明,岸坡发生失稳破坏是多种不利工况叠加的结果,其主要原因是在低潮位时的超挖引起的,打桩及交通荷载作用也是造成滑坡的不利因素。在破坏的岸坡上重建码头,采用了振动砂桩加固地基,同时在地表铺碎石垫层作为预压荷载,加速地基土的固结。有限元分析和码头的成功重建表明,使用砂桩加固地基可以同时达到提高地基承载力和加速土体排水固结的目的,对于码头的重建是有效的技术措施。     

Progressive failure mechanism in one-dimensional stability analysis of shallow slope failures

In many slopes, overstressed zones can develop where the shear stress is larger than the available shear strength. Along a shear surface within a soil exhibiting a strain-softening behavior, the shear displacement increases while the available shear strength decreases. The excess shear stress is transferred from the overstressed zone to the adjacent zones, providing more shear strength. This stress-transferring mechanism induces stress redistribution within the slope and could enlarge the overstress zone. A one-dimensional model that satisfies the strain compatibility and force equilibrium is proposed for the stability analysis of a slope of strain-softening behavior. This paper’s objective is to facilitate the application of this model to estimate stress distribution along the failure surface of a strain-softening slope and thereafter the stability status. The study presents a set of specific solutions to this model by describing and demonstrating procedures to identify the pattern of a stress state and to calculate stress distribution within a one-dimensional, strain-softening slope. The progressive failure mechanism is also investigated by using the proposed approach. As the magnitude of released stress gets large enough, it induces an overstressed zone adjacent to the initial unstable zone and progressive failure develops. The proposed approach is also applied to study the pattern of stress redistribution. It is found that the pattern of stress redistribution is affected by the magnitude of released stress. It is too complex to be reasonably expressed by simple models. Though some limitations exist, the proposed approach serves as a simple tool for a better understanding of the progressive failure mechanism.     

The response of subaqueous dunes to floods in sand and gravel bed reaches of the Dutch Rhine

Abstract The branches of the River Rhine in the Netherlands, characterized by a sand–gravel bed in the upstream part and a sand bed in the downstream part of the river system, show migrating dunes, especially during floods. In the last 20 years, these dunes have been studied extensively. High-resolution echo-sounding measurements of these dunes, made with single and multibeam equipment, were analysed for three different sections of the Rhine river system during several floods. This analysis was done to quantify the growth, decay and migration rates of the dunes during floods. In addition, the migrating dunes were used to calculate bedload transport rates with dune tracking. The results of dune growth and decay and migration rate are shown to be very different for the various sections during the various floods, and these differences are related to differences in grain size of the bed and to differences in the distribution of discharge over the main channel and the floodplain. The relations are used to show that the growth and migration rate of dunes, and the calculated bedload transport rates during the rising stage of a flood wave can be predicted from the mobility of the bed material with simple power relations.     

正断层诱发砂土中群桩基础破坏及避让距离研究

地震中发震断层诱发桩基失效,导致上部结构破坏甚至坍塌,相关破坏机制和避让距离缺乏系统研究。通过离心机试验和数值模拟,针对基岩正断层活动诱发上覆砂土中群桩基础的静力破坏展开研究,考察不同群桩断层相对位置下群桩的破坏特征。试验与计算结果均表明,当群桩跨越断层时,正断层活动使群桩向上盘一侧倾斜,并使基桩弯向上盘一侧。基桩桩顶荷载的重分布进一步使基桩形成受拉和受压两种破坏模式。数值参数分析表明,在不同桩位上,群桩的变形响应可划分为5个特征区域。对于埋深为20.0 m的基岩正断层,群桩在上盘和下盘一侧的安全避让距离分别为23.5 m和15.9 m,其中下盘一侧离开断层7.9 m至上盘一侧离开断层4.1 m的区域需要进行重点避让。     

Assessing the impacts of a large slope failure using 3DEC: The Chiu-fen-erh-shan residual slope

This paper assesses the impact of Chiu-fen-erh-shan residual slope failure using a three-dimensional distinct element program. The simulation results indicate that rocks will severely damage the Lung-Nan path. The Taanshan syncline generates a depression zone adjoining to the slope toe and acts as a buffer zone to trap failing rocks. Some rocks will travel through the syncline, which poses a risk to visitors at the monument and the Shihmen observation deck. Few rocks will pass though the memorial park near the northern boundary of the slope. Visitors should be evacuated out of the impact area during a heavy rainfall event.     

三峡库区顺层灰岩岸坡劣化−溃屈灾变机制研究

三峡库区巫峡段发现多处顺层岸坡滑移−弯曲变形迹象,库水循环涨落加剧了岸坡前缘劣化损伤与失稳破坏。以巫峡段青石 6号坡为例构建室内概化模型,开展顺层灰岩岸坡在消落带岩体劣化下的灾变机制研究。研究结果表明：蓄水前岸坡整体长期处于稳定状态。随着劣化进行,蓄水后岸坡变形加剧直至溃屈破坏,岩体劣化缩短了劣化−溃屈失稳进程。运动学分析显示,溃屈破坏时同一岩层达到速度峰值近似。岩层“弯折点”后部运动特征较为一致,前部较为离散。溃屈破坏点是岸坡能量释放的转折点和顶点;随劣化演变位移、应力逐渐递增,呈现提前破坏征兆,溃屈破坏前后应力产生“集中−释放”。整体来看,应力变化提前于位移,表明应力监测更有效。应力监测的核心在于关键区段的确定,对于劣化−溃屈型岸坡来讲,前缘“挠曲段”处应力陡增可作为岸坡临界失稳的重要表征;“劣化−溃屈”演化进程中后缘推挤始终存在,它是岸坡灾变的前提。但岸坡失稳的主导因素却是消落带岩体持续不断的劣化。青石 6 号坡当前处于向强烈弯曲隆起演化进程中,由于消落带岩体持续劣化,可能由稳定/基本稳定逐渐演变为欠稳定状态。     

汉源大渡河大桥巨厚砂层大直径灌注桩施工技术

汉源大树大渡河大桥2号桥墩12根钻孔灌注桩,钻孔直径2500 mm,钻孔深度为63~75 m不等,桩径大,桩孔深,属于河水位以下的巨厚砂层桩基,具有一定的施工难度。2号墩基岩上覆覆盖层存在多层砂土及卵石夹土（质土）,且砂层巨厚,在11.5~49.4 m之间除厚5.5 m的卵石夹土外均为砂土,对桩基成孔施工极为不利。采用冲击钻进成孔,泥浆净化除渣系统,通过对钻头直径的控制、内护筒的使用、灌注时导管埋深的控制等措施,确保了工程的顺利完工。相关技术措施可为其他类似工程提供借鉴。     

Wavy lamination in a mixed sand and gravel foreshore facies of the Pleistocene Hosoya Sandstone, Aichi, central Japan

Although sandy foreshore facies are generally characterized by parallel lamination, wavy lamination is predominant in the mixed sand and gravel foreshore facies of the Pleistocene Hosoya Sandstone, which crops out along the Pacific coast of the Atsumi Peninsula, Aichi, central Japan. The foreshore facies consists of three sedimentary subfacies; interbeds of gravel and parallel laminated sand of the lower foreshore facies, parallel laminated fine to medium sand beds containing scattered pebbles and cobbles of the middle foreshore facies, and wavy laminated fine to medium sand beds containing scattered pebbles and cobbles of the upper foreshore facies. A lack of erosional surfaces in the middle foreshore facies indicates the continuous accumulation of sand in flat beds under upper plane bed flow. The wavy laminated sands of the upper foreshore facies exhibit erosional surfaces indicative of repeated deposition and erosion. The erosional surfaces are undulatory, with depressions (10 cm wide and 3 cm deep) that contain scattered pebbles and cobbles. These depressions reflect backwash erosion of sand around and below the pebbles and cobbles. Sand draping over the undulating erosional surfaces forms the wavy lamination. The wavy laminated sand with scattered pebbles and cobbles is a key facies of an upper foreshore or swash zone, and is a good sea-level marker.     

Depth–transport functions and erosion–deposition diagrams as indicators of slope inclination and time-averaged traction forces: applications in tropical reef environments

The trigonometric relationship between slope inclination, the horizontally acting time‐averaged traction force and the vertical depth of transport allows the estimation of one factor, when both others are known. Depth–transport functions can be deduced by comparing the depth distributions of living organisms and their skeletal remains, and this paper simplifies this comparison using foraminifera in which a single test represents an individual. Differences in distribution parameters between living individuals and empty tests allow depth–transport functions to be determined; these functions differ between species at a single transect according to the varying buoyancies of the tests. Within a single species, differences in depth–transport functions between locations are based on either slope inclination or traction intensities. After establishing a mean depth–transport function by averaging species‐characteristic functions, the time‐averaged traction force acting on the studied transect can be calculated. Transport intensities are also estimated using an erosion–deposition diagram that combines the relative frequency distributions of living individuals and empty tests. The proportion of ‘eroded’, ‘parautochthonous’ and ‘allochthonous’ tests mirrors the influence of both slope inclination and traction force for the deposition of empty tests. To test the model, six species of symbiont‐bearing benthic foraminifers were investigated at two transects in front of a NW Pacific coral reef. One transect is distinguished by a strong slope flattening below the steep reef slope (30 m), whereas further steepening characterizes the equivalent part in the other transect. These differences are mirrored in the depth–transport functions as well as in the erosion–deposition diagrams of all species. The time‐averaged traction forces differ in intensities between transects, because of the position of the reef front with respect to the predominant wind direction. However, the form of the functions is identical and distinguished by an increase from the surface to 35 m depth, followed by a decrease down to 105 m. This can be explained by successive onshore and offshore forces acting on the shallow slope, such as the tropical cyclones that cross the region every summer.     

节理型黄土开挖边坡塌滑破坏机理

黄土节理控制着黄土区地下水或者地表水的运输通道,黄土节理的发育致使黄土节理扩张为局部大的拉张裂隙,在拉张力作用下使得黄土高原地区塌滑灾害频繁发生。通过野外调查和分析,提出了黄土开挖边坡塌滑破坏可分为4个阶段:坡脚侵蚀剥落阶段、坡顶拉裂阶段、垂直裂隙扩展阶段、边坡整体塌滑破坏阶段。结合工程实例,通过概化的三维数值分析模型,使用岩土工程软件FLAC3D模拟分析了在3种降雨量条件(40 mm/d、80 mm/d、100 mm/d)下,降雨2 d后概化模型的塌滑破坏情况,这可为工程地质条件相同地区的黄土开挖边坡塌滑变形破坏的分析以及防治提供一种有效分析范例。     

东营凹陷北部陡坡带西段水下扇体储集层特征

东营凹陷北部陡坡带发育多种类型的砂砾岩扇体，近年来一直是寻找隐蔽性油气藏的有利地区。其西段古近纪沙河街期四段时期形成以砂岩、砾岩为主的近岸水下扇沉积体。储集层岩石结构成熟度和成分成熟度均较低，多为双模态和复模态结构。本文首先从宏观上对储集层的岩石类型、沉积特征和平面分布进行描述，再从微观上对影响储集层物性的主要成岩作用进行描述，分析该砂砾岩体储集层的孔隙类型和孔隙结构特征。最后，对储层进行综合评价，认为该类储集层孔隙类型多样，孔隙结构复杂，为中低孔隙度、低渗透率储层类型。     

陆相盆地水下扇扇中亚相的微相细分--以东营凹陷史南地区沙二9梁家楼水下扇为例

通过对东营凹陷史南地区沙二9砂层组的小层精细对比和微相细分,可以将梁家楼水下扇扇中亚相进一步划分出辫状水道、辫状砂坝、“边滩”、侧缘砂坝、侧缘分支沟道、末端砂坝、冲溢扇、低阶地、高阶地等9个微相。不同微相之间的砂体粒度变化、平面发育位置都有明显的不同,尤其是其中的辫状砂坝、“边滩”、侧缘砂坝是在水下地形较平坦,坡度较小的情况下,由辫状沟道侧向迁移形成的。对水下扇扇中亚相的微相细分和重新厘定,不仅有助于对水下扇沉积过程的进一步了解,而且也将有助于油田对水下扇砂体剩余油的分布研究和开发方案的调整。      

Geotechnical reconnaissance and liquefaction analyses of a liquefaction site with silty fine sand in Southern Taiwan

Geotechnical reconnaissance of a recurrent liquefaction site at a Quaternary alluvial deposit in southern Taiwan was conducted to establish a comprehensive case history for liquefaction on silty fine sand with high fines content. The liquefaction occurred at a silty fine sand layer with D₅₀ = 0.09 mm and fines content greater than 35% and was triggered by a M_w = 6.4 earthquake on March 4, 2010, which induced maximum horizontal acceleration up to 0.189 g at the site. In situ subsurface characterizations, including standard penetration test, cone penetration test, and shear wave velocity measurement, were performed as well as cyclic simple shear tests on undisturbed specimens retrieved by a modified hydraulic piston sampler. Comparisons of cyclic resistance ratios (CRRs) indicate that CPT sounding with standard penetration rate could overestimate the resistance ratio and drainage conditions during penetration should be considered for high fines content soil in the liquefaction analysis. Additionally, variations of CRRs from different in situ tests indicate that correlations among in situ tests and CRR could be soil specific and precautions should be taken when using these curves on silty fine sands.     

Depositional setting of the 2·1 Ga Francevillian macrobiota (Gabon): Rapid mud settling in a shallow basin swept by high‐density sand flows

The depositional setting of the 2·1 Ga fill of the Franceville Basin of Gabon is important for understanding the habitat (energy and availability of light and oxygen) and taphonomy of recently discovered early macro‐organisms buried in black shales in Unit FB . The available data bearing on the stratigraphy and sedimentology of Unit FB provide new insight into processes acting on the palaeo‐sea floor. The shales are interpreted to have formed as fluid mud deposits interstratified with structureless sands. The latter (Poubara sandstones) were emplaced during a forced regression during the terminal infill of fault‐bounded sub‐basins following a stage characterized by a ferruginous to anoxic water column. The structureless sandstones were deposited from high‐density gravity currents along with a locally strong bottom oscillation of the water column. Tuft structures preserved in cyanobacterial mats, together with the position of the macro‐organisms at the top of the sandstone beds within associated black shales, point to a water depth of less than 80 m. The relative sea‐level fall that drove deposition of the Poubara sandstones controlled the rise of a phototrophic ecosystem and also possibly favoured the supply of oxygen and nutrients via density flows.