Influence of earthquakes on the stability of slopes

Earthquakes are a major trigger for instability of natural and man-made slopes. Often the instability of slopes due to an earthquake causes more destruction and kills more people than the actual earthquake itself. A comparison is made between different methodologies to analyze the potential stability of slopes during earthquakes. Theoretically, it seems simple to calculate the stability of a slope during an earthquake. In reality, however, the stability is influenced by so many parameters that are either not known or which influence is so poorly known that a decent estimation of stability cannot be made. Offshore the situation is worse because proper data required for stability calculations are even less available than onshore. On- and offshore, erosion and weathering create continuously slopes that may become unstable during a future earthquake, offshore also sedimentation creates continuously new slopes. Another fundamental problem in stability analysis is the complicated and largely unknown behavior of seismic waves in three-dimensions in natural materials. The lack of accurate data and the unknown behavior of seismic waves in three-dimensions make estimations of slope stability during an earthquake unreliable.     

Progress in stability analysis of submarine slopes considering dissociation of gas hydrates

Gas hydrates have the potential to be a new energy source and a submarine geohazard. Though researchers generally agree about the association between gas hydrate dissociation and submarine slope failures, the processes and mechanism of submarine slope failure caused by gas hydrate dissociation are not clearly understood. In the last few years, some authors have tried to analyse submarine slope stability by considering the existence and dissociation of gas hydrate, and a few researchers have presented quantitative models. This paper presents a review of the various causes of submarine slope failures associated with gas hydrate dissociation. Also, analysis models of submarine slope stability associated with gas hydrate dissociation that are documented from the literatures including the infinite slope model, wedge model, slump and retrogressive failure model are interpreted and illustrated, respectively.     

Influence of rain infiltration on the stability of compacted soil slopes

Stability analyses for a homogeneous compacted embankment were undertaken considering infiltration of water into the embankment. The analyses include several different practical scenarios: (i) saturated condition, (ii) ponding (or runoff) along with saturated condition, (iii) short term analysis for unsaturated conditions, and (iv) long term analysis for unsaturated conditions. The appropriate shear strength parameters of the compacted soil required for analyzing different practical scenarios were determined using conventional and modified triaxial shear apparatus. The results of the study show that typically shallow circular failures above water front occur due to infiltration rather than the conventional infinite slope type failures.     

Stability analysis of submarine slopes in the area of the test production of gas hydrate in the South China Sea

In this paper, the mechanical properties of gas hydrate-bearing sediments (GHBS) were summarized and the instability mechanism of submarine hydrate-bearing slope (SHBS) was analyzed under the background of the test production of gas hydrate in the northern part of the South China Sea. The strength reduction finite element method (SRFEM) was introduced to the stability analysis of submarine slopes for the safety of the test production. Two schemes were designed to determine the physical and mechanical parameters of four target wells. Through the division of the hydrate dissociation region and the design of four working conditions, the range and degree of hydrate dissociation at different stages during the test production were simulated. Based on the software ABAQUS, 37 FEM models of SHBS were set up to analyze and assess the stability of the submarine slopes in the area of the test production. Necessary information such as safety factors, deformation, and displacement were obtained at different stages and under different working conditions. According to the calculation results, the submarine slope area is stable before the test production, and the safety factors almost remains the same during and after the test production. All these indicate that the test production has no obvious influence on the area of the test production and the submarine slopes in the area are stable during and after the test production.     

Influence of rainfall-induced wetting on the stability of slopes in weathered soils

Shallow failures of slopes in weathered soil are caused by infiltration due to prolonged rainfall. These failures are mainly triggered by the deepening of the wetting band accompanied by a decrease in matric suction induced by the water infiltration. This paper reports trends of rainfall-induced wetting band depth in two types of weathered soils that are commonly found in Korea. Both theoretical and numerical analyses for wetting band depth are presented based on the soil–water characteristic curve obtained using filter paper as well as tensiometer tests. It is found that the magnitude of wetting front suction plays a key role in the stability of slopes in weathered soils. Theoretical analysis based on modified Green and Ampt model tends to underestimate the wetting band depth for typical Korean weathered soils. It was also deduced that for Korean weathered soils, the factor of safety drops rapidly once the wetting band depth of 1.2 m reached.     

Influence of discontinuities on the stability of cut slopes in weathered meta-sedimentary rocks

The meta-sedimentary rocks along Pos Selim Highway in Perak State, Malaysia showed a gradational weathering profile based on differences particularly in textures, hardness, lateral changes in colour and consistency of material extension. Both large and small scale discontinuities observed in the investigated rocks reduce the physical and mechanical properties of the rocks, and provide slip surfaces for failures. Rock and soil samples were tested using established standards to determine their characteristics and responses under a wide variety of disturbances. Kinematic analyses were also carried out to determine the modes and likely modes of failures.

Petrographic analyses revealed associated micro-structures, and the implications of these micro-structures showed shearing components along planes of weakness. From the determined index properties of the tested soil samples, the weathered quartz mica schist is not suitable for structural support. Further study involving unconsolidated undrained direct shear box tests carried out under total stress, revealed a non-uniform response of the rocks to shearing disturbance along discontinuity planes, and the rate and depth of deformation. The shear strength components of the investigated rocks were thereby prescribed in terms of cohesion and friction angle. From the kinematic analyses across this extended cut slope, there are possibilities of wedge and planar failures.     

Failure mechanism of submarine slopes based on the wave flume test

Natural Hazards - Although submarine landslides rarely occur in shallow or coastal areas, they are intensely violent and dangerous, causing serious damage to submarine structures and cables and...     

岩土体斜坡稳定性分析

本文据醪糟坪斜坡变形破坏特点对岩土体的岩性、结构、地质构造、地表水及测试结果等进行综合分析,认为该斜坡是潜伏不稳定斜坡,通过整治治理工程,效果良好.     

岩质边坡稳定性分析的探讨

通过最大剪应力等值线来确定边坡的滑动面,并与其结构面抗剪强度相比较,给出了确定边坡的稳定系数的方法。     

沉桩超孔隙水压力对码头边坡稳定的影响

沉桩会对码头边坡稳定产生不利影响,一是引起桩周土体超孔隙水压力的急剧上升,导致土体有效应力降低;二是沉桩的振动加速度会产生对边坡稳定不利的瞬时惯性力。对于灵敏度低的土质岸坡来说,前者是影响其稳定性的主要因素。考虑沉桩时初始超孔隙水压力的分布,根据Biot固结方程超孔隙水压力消散解的一般表达式,建立了沉桩引起的超孔隙水压力随时间消散的解析式,在条分法的基础上考虑沉桩产生的超孔隙水压力的不利影响,建立了沉桩时边坡稳定安全系数的计算公式。根据沉桩顺序对某码头进行边坡稳定分析,结果表明：考虑打桩作用的岸坡稳定安全系数明显降低,沉桩产生的超孔隙水压力逐渐消散,边坡稳定安全系数随沉桩工序历时变化,施工中期由于超孔隙水压力叠加,岸坡最危险,沉桩结束3个月以后,超孔隙水压力基本消散,边坡稳定安全系数接近不考虑沉桩时的值。工程中要根据打桩计划进行边坡稳定计算。     

Geomechanical issues in the exploitation of natural gas hydrate

As a clean fossil fuel with great reserves, natural gas hydrate (NGH) is widely regarded as an important future alternative energy source. NGH is widely distributed in onshore tundra and shallow sedimentary layers in the deep sea. These sedimentary layers typically exit shallow burial depth, poor diagenesis and low strength characteristics; moreover, the decomposition of NGH can also greatly reduce reservoir strength. Therefore, NGH development can easily causes many geomechanical problems, including reservoir instability, sand production and seabed landslides, etc., which may further trigger a series of environmental disasters such as tsunamis, natural gas leakage and the acceleration of global warming. This study mainly reviews the research progress regarding geomechanical issues in NGH development, including mechanical properties of NGH-bearing sediments, borehole stability, hydraulic fracturing, sand production, reservoir settlement and seabed landslides. In addition to previous research achievements regarding geomechanical problems in NGH exploitation, the limitations and challenges are also discussed, and several questions and insightful suggestions are put forward for future research from our point of view.     

水下岩质边坡稳定性的模型试验研究

随着三峡水利建设工程的进行,许多原本位于水面以上且已趋于稳定的人工或自然岩质边坡将被淹没,导致边坡稳定性降低,而水的压力、冲击、渗流和侵蚀等综合作用,给下岩质边坡的稳定性带来大量的不确定性。基于已有的水下土质边坡稳定性试验和水下岩质边坡有限元分析,进行水下岩质边坡模型试验研究,探讨在水位升降水过程中和波浪作用下水下岩质边坡的稳定性和破坏机制。将结构面为30°和50°的两种岩质边坡模型布置在人工水槽中,采用波流系统进行水位升降水波浪冲击试验,量测岩质边坡的应力变化。通过分析边坡各测点应力变化,得出了一些有意义的结论：（1）把边坡前部岩体划分成3个区域,每个区域内应力变化规律相同;（2）水下岩质边坡坡脚处应力集中最大,破坏始于坡脚,并由此产生的塑性区沿结构面逐渐同坡顶发展,这与一般边坡从坡顶逐渐向下发展的开裂破坏形式完全不同;（3）波浪对水下岩质边坡结构面的影响,将随其倾角的变小而减弱。     

非饱和土边坡稳定分析方法探讨

在对非饱和土边坡进行稳定分析时,应该全面地考虑基质吸力对边坡稳定的贡献。首先,分别探讨了基于Fredlund非饱和土强度表达式和Bishop非饱和土有效应力强度公式将强度折减有限元法推广到非饱和土边坡稳定分析中的具体方法;然后,开发了可以考虑基质吸力两种处理方法的强度折减有限元计算程序;最后,给出了一个非饱和土边坡稳定分析的对比算例,说明了二者的不同特点。     

水-力耦合及干湿循环效应对浅层残积土斜坡稳定性的影响

降雨入渗过程中,土体吸力降低,体积明显改变。天然浅层土体长期受到季节性气候变化的影响,因此,开展水-力耦合及干湿交替对浅层残积土坡稳定性影响的数值分析,分析浅层土坡孔隙水压力、湿润锋及安全系数的时空演变规律,并对水-力耦合及干湿交替条件下的浅层土坡失稳破坏机制进行探讨显得尤为必要。研究结果表明：随着干湿循环次数的增加,水-力耦合分析下孔隙水压力以及湿润锋的迁移速度增加更快,边坡也更易失稳破坏;干湿交替初期,雨水入渗易引起地下水位上升,边坡可因正孔隙水压力的增加而失稳;干湿交替后期,湿润锋的快速推进加剧基质吸力迅速丧失及土体强度下降,边坡安全系数显著降低,发生失稳破坏的时间缩短。因此,可将湿润锋处的安全系数(局部最小值)作为控制边坡长期稳定性的临界值。     

坡积松散体稳定性模型试验研究

云南西北部三江源高海拔、高深切地区,坡积松散体稳定性是公路边坡面临的主要问题之一,模型试验是研究松散体稳定性各影响因素最有效的手段。根据相似原理,设计模型试验的主要几何相似常数,采用一系列手段改造模型试验材料,使其达到材料强度、重度相似。一系列模型破坏条件真实再现现场坡积松散体的滑坡条件,试验结果分析对相关设计、施工和科研人员有较好的借鉴作用。模型试验材料从松散状态到较密实状态,开始破坏的自然坡角提高42%～100%,c值提高约50%～120%, 值提高约6%～8%,密实状态提高,坡体的强度提高,相应坡体的稳定性提高,起始破坏的自然坡角也显著提高;降雨使坡体含水率提高近饱和时,充分软化的c值下降约70%, 值下降约7%,表明雨水下渗软化是导致坡积松散体产生滑动的最根本、最直接的因素。模拟降雨的土体强度变化过程对松散体边坡稳定性研究有较好的参考价值。     

海域天然气水合物开发的地球物理监测

作为极具潜力的未来清洁能源,海域天然气水合物开发难度大、环境保护要求高,需要大范围地对水合物储层进行动态监控。然而,虽然在国外的天然气水合物试开发时,曾尝试对储层进行监测,但主要是局限于开发井周边小范围监测。对天然气水合物藏的大规模监测,目前国内外都还没有建立起相关的技术系统,即使有关的研究都还很少。在未来的几年内,我国以及日本、韩国、印度等将在海域开展天然气水合物的长期试开发,在开发过程中对储层的大规模监测必不可少。本文从水合物藏的岩石物理特征出发,讨论了沉积物中水合物的饱和度和孔隙赋存状态对其声学和电学性质的影响规律,比较了基于水合物的地震和电磁性质的多种地球物理勘探手段应用于海域水合物藏大规模监测的可行性和利弊。针对我国南海天然气水合物生产试验和未来商业开发的储层监测需求,为实现对大区域水合物储层和海底变形的定量监测,我们设计的水合物试采过程监测系统由井中地球物理参量监控、三维全光纤4分量海底时移地震和海底多功能监测站三部分构成。该方案大量利用油气行业现有成熟技术,并对其进行优化设计,可实施性强,能够较全面地收集海域水合物开采过程中储层相关的基础数据,从而可为后续技术优化、水合物资源评价、海底环境保护、工程灾害预防等方面提供科学依据。     

General stability of two-dimensional slopes based on Sarma's method

Sarma's method of taking a fictitious accerelation as a measure of safety is used to study a two-dimensional slope. The slope is divided into arbitrary slices. The relations among forces acting on the slices and Sarma's acceleration are assumed to be linear. Consequently, a general analytical expression of Sarma's acceleration is derived by means of Cramer's rule. Furthermore, for four commonly used slice methods with simple relations among the forces on each slice and Sarma's acceleration, a general closed-form solution of Sarma's acceleration is given. An example is calculated and the results agree well with those of Hoek. Copyright © 1999 John Wiley & Sons, Ltd.     

A review on simulation models for exploration and exploitation of natural gas hydrate

Simulation models have been used widely to help design, operate, control and optimize the processes of exploration and exploitation of natural gas hydrates and been responsible for many of the most important technology breakthroughs. Currently, a rich body of literature exists and is still evolving. This paper presents a critical review of the most influential works that are recognised as representative and important simulation models and links to the techniques commonly used in natural gas hydrate exploration and exploitation. Model background, ideal assumptions and main results are presented. Models are broadly classified into two categories: physically and empirically based models. Models are reviewed with comprehensive, although not exhaustive, publications. The strengths and limitations of the models are discussed. The paper is concluded by outlining open questions and new directions for future work. The review is useful for understanding the innovation process and the current and future status of simulation models on exploration and exploitation of natural gas hydrate and highlights the key aspects of model improvement.     

地下水对库岸边坡稳定性的影响

针对库岸边坡在库水位陡降时易发生失稳破坏这一特点,分析了地下水引发库岸边坡失稳的机理,指出在地下水作用下,边坡岩土物理力学性质恶化、库水浮托力以及坡体内渗透力是影响库岸边坡稳定性的重要因素,给出了考虑地下水影响的库岸边坡稳定性计算公式。对三峡库区的某边坡工程进行了分析,结果表明：边坡岩土体介质因饱水软化作用,其滑动面的力学参数值降低;库水浮托力在某种程度上有利于边坡的稳定。当库水位陡降时,坡体内产生较大的渗透力是导致边坡失稳的一个很关键的原因。同时指出,在进行库岸边坡治理设计时,采取合理的防冲刷、反滤及疏导措施,能有效地降低地下水对边坡稳定性带来的消极影响。