Advanced seismic slope stability analysis

The objective of this study was to present an advanced methodology for assessing seismic slope stability by taking into account the uncertainties related to the main input parameters. The methodology was applied on a real landslide in order to show the advantages of using the proposed procedure and establish the baseline trends of dynamic response and calculated permanent seismic displacements. It involves the following steps: preliminary analysis, probabilistic static and seismic factor of safety analysis, and permanent seismic displacement analysis. Estimating post-failure maximum seismic deformation of landslide mass and sounding properties is the most important part of this study. It involves both Newmark sliding block method and continuum mechanics approach, applied for characteristic set of input values in order to have more accurate assessment of slope performance and determine the relative importance of input parameters. The results of the analysis showed the benefits of using the proposed step-by-step methodology. The obtained difference in the results between the two methods depends strongly on the set input data for a particular analysis.     

Stochastic seismic slope stability assessment using polynomial chaos expansions combined with relevance vector machine

This paper presents probabilistic assessment of seismically-induced slope displacements considering uncertainties of seismic ground motions and soil properties.A stochastic ground motion model representing both the temporal and spectral non-stationarity of earthquake shakings and a three-dimensional rotational failure mechanism are integrated to assess Newmark-type slope displacements.A new probabilistic approach that incorporates machine learning in metamodeling technique is proposed,by combining relevance vector machine with polynomial chaos expansions(RVM-PCE).Compared with other PCE methods,the proposed RVM-PCE is shown to be more effective in estimating failure probabilities.The sensitivity and relative influence of each random input parameter to the slope displacements are discussed.Finally,the fragility curves for slope displacements are established for sitespecific soil conditions and earthquake hazard levels.The results indicate that the slope displacement is more sensitive to the intensities and strong shaking durations of seismic ground motions than the frequency contents,and a critical Arias intensity that leads to the maximum annual failure probabilities can be identified by the proposed approach.     

Performance-based assessment of earthquake-induced catastrophic landslide hazard in liquefiable soils

This paper outlines a methodology for evaluating the likelihood of catastrophic landslide occurrence on gentle slopes in liquefiable soils during earthquake. The approach is based on a modified Newmark sliding block model of assessing the earthquake-induced undrained landslide displacements for conditions of no shear stress reversals on the sliding surface. By employing the shear resistance-displacement relationship from undrained monotonic ring shear tests, the simulation model incorporates the sensitivity of computed displacements to variations in yield acceleration. The proposed approach involves an examination of undrained seismic slope performance under various horizontal seismic waveforms scaled to different specific values of the peak earthquake acceleration. An example problem illustrates how the proposed methodology may be used to demarcate, based on the magnitude of permanent seismic displacement, the levels of low, moderate and high risk of catastrophic landslide on a gentle slope in a saturated cohesionless soil susceptible to liquefaction during earthquake.     

地震作用下堆积体边坡的坡面变形与失稳机制

地震作用下堆积体边坡的动力响应特性十分复杂,单一抗震安全系数不足以评价其动力稳定性。通过大型振动台试验,研究了连续多级地震荷载作用下,地震波的类型、卓越频率及峰值加速度对堆积体边坡坡面永久位移的影响,并初步分析其失稳机制。试验结果表明,相同峰值加速度下振动型地震波比冲击型地震波更容易产生坡面永久位移,地震波卓越频率对坡面永久位移也有重要影响;堆积体边坡在峰值加速度apeak=0.2g时开始有大颗粒石砾滚落,对应的坡面永久位移在apeak=0.2g～0.3g之间开始产生并显著增大,另外利用考虑坡面几何形态变化的改进Newmark法对坡顶的永久位移进行了估算。通过坡面永久位移评价堆积体边坡的动力稳定性有一定合理性。     

A simplified approach for studying the influence of vertical accelerations on seismic response of slopes

ABSTRACT

A simplified approach is presented for estimating permanent displacements in slopes as a result of both vertical and horizontal seismic accelerations. A study of 52 earthquake records showed that the time difference between maximum horizontal and vertical accelerations varied between 0 and 10.3 s. The approach is illustrated for an earth dam embankment by analysing the effects of five of the above earthquake records. The approach combines a pseudo-static slope stability analysis for estimation of the critical (or yield) horizontal-vertical acceleration combinations, and a Newmark type displacement analysis. Guidelines are presented for conservative choice of soil strength parameters of saturated clays for use in the stability analysis. While permanent displacements of up to 40 cm were predicted without considering the vertical acceleration component, no additional displacement above 3.5 cm resulted when this component was included. The predicted additional displacement was consistently less than 10%, and in 50% of the analyses, vertical acceleration led to smaller predicted displacements. The simple approach may be applied in analysis for any slope using real earthquake records. Using existing, empirical expressions for permanent displacement, based only on horizontal accelerations, the effect of the vertical accelerations may be conservatively estimated by increasing the displacement by 10%.     

On the stability of fissured slopes subject to seismic action

A set of analytical solutions achieved by the upper bound theorem of limit analysis and the pseudo‐static approach is presented for the assessment of the stability of homogeneous c, ϕ slopes manifesting vertical cracks and subject to seismic action. Rotational failure mechanisms are considered for slopes with cracks of either known or unknown depth and location. A validation exercise was carried out based on numerical limit analyses and displacement‐based finite‐element analyses with strength reduction technique. Charts providing the stability factor for fissured slopes subject to both horizontal and vertical accelerations for any combination of c, ϕ and slope inclination are provided. The effect of the direction of the vertical acceleration on slope stability is specifically analysed. Yield seismic coefficients are also provided. When the presence of cracks within the slope can be ascertained with reasonable confidence, maps showing the zones within the slope where they have no destabilising effect are provided. Finally, Newmark's method was employed to assess the effect of cracks on earthquake induced displacements. To this end, displacement coefficients are provided in chart form as a function of the slope characteristics. Two examples of slopes subjected to known earthquakes are illustrated. © 2016 The Authors. International Journal for Numerical and Analytical Methods in Geomechanics published by John Wiley & Sons Ltd.     

基于静动力有限元的边坡抗震稳定分析方法

由人工截断边界输入地震波,通过时程积分法确定地震荷载作用下边坡的应力和变形,在人工截断边界上采用黏-弹性人工边界条件模拟地基辐射阻尼的影响。采用有限元强度折减法求解边坡稳定安全系数,以塑性区贯通时刻特征点的位移突变作为边坡失稳的评判标准。非线性有限元和黏-弹性人工边界条件结合运用,建立了基于静动力有限元的边坡抗震稳定分析方法。该方法在稳定安全系数的定义上和传统的刚体极限平衡法是一致的。以十里铺水电站为工程实例,分析了库区边坡的抗震稳定性,得出了边坡动力位移时程和动力稳定安全系数,计算结果合理评价了实际工程在地震荷载下的稳定性。     

Empirical-based seismically induced slope displacements in a geographic information system environment: a case study

Landslides are one of the most damaging and threatening hazards associated with seismically induced slope movements. Estimations of support conditions for slope displacements are important for taking preventive measures to avoid landslide events in future. California's Division of Mines and Geology (DMG) procedure is utilised in the present paper for estimating the slope failure mechanism under seismic conditions. In this study, the DMG procedure has been explained and has also been incorporated in a Geographic Information System (GIS) using Arc-GIS software from Environmental Systems Research Institute. Further, it is utilised for establishing a seismically induced slope displacement map for the Skien municipality area of Telemark County in Norway. The motivation for selecting this site was the availability of geotechnical parameters for the site. Three different displacement maps have been produced for earthquake scenarios of magnitude 5, 6 and 7, respectively. The maximum displacement of 133 cm is estimated for earthquake scenario of magnitude 7. It is noticed that the sensitive areas for slope failure remain the same under different earthquake scenarios. A displacement tool based on the DMG procedure has been created in the Arc-tool box in Arc-GIS software. This tool minimises the efforts for inserting formulas for making raster displacement maps. By using the displacement tool one can generate final products like displacement maps automatically at high accuracy and in quick time. The prepared slope displacement maps of study area are used for landslide susceptibility zonation (LSZ) map preparation. The LSZ maps are useful for landslide hazard assessment and further can be utilised by planners, civil engineers and local administrators for town planning and policy-making.     

地震载荷下斜坡稳定性数值模拟分析

地震载荷是影响复杂结构斜坡塌滑的一个重要因素。本文以汶川震区一边坡为实例,利用有限差分法FLAC-3D对地震动力下斜坡进行了数值模拟。并且在用FLAC-3D进行斜坡动力分析时探讨了边界条件设置、动力时程的合成以及输入和转化。从位移场与速度场进行了详细的动力分析,获得该斜坡地震载荷下位移场、速度场的变化规律,指出山体斜坡在非连续介质状态下软弱面区域位移和速度最大,为后续进行复杂的山体破坏力学研究提供理论依据。     

Seismic landslide analysis: Gurpinar (Istanbul) as a case history

Slope failures triggered by earthquakes are among the most important soil mechanics problems. In this study, static and pseudostatic analyses of slope stability for earthquake conditions were carried out in the Gurpinar area. In situ testing (SPT) was carried out and laboratory samples were obtained from six boreholes (maximum depth 50.0 m) to determine soil classification and strength characteristics. Geophysical studies (seismic refraction and MASW) were also carried out in the area to estimate the structure and stiffness strength characteristics of the slope to 50.0 m depth. All field and laboratory data were used to determine the mechanical and structural (geometrical) behavior of the slope. In order to solve the slope stability problem, three soil slope models were considered. Pseudostatic analysis was carried out to estimate the earthquake acceleration seismic hazard in the region. These analyses showed that, while there was no potential slope instability under static load conditions, some problems would appear with increasing earthquake acceleration. A geotechnical slope improvement project is proposed for the study area.     

滑坡位移的多模态支持向量机模型预测

将支持向量机（support vector machine,SVM）方法与信号分析中的经验模态分解（empirical mode decomposition, EMD）方法相匹配,提出了一种通过多模态支持向量机函数回归分析建模预测滑坡位移的理论方法。以边坡位移历史观测数据为基础,应用EMD方法获得滑坡形成过程中位移演化的几个特征时间模态,构成了多模态信息统计学习样本,确定了边坡位移演化的自适应多尺度变化信息。对应于每个经验模态的位移变化信息,引入了多模态SVM建模方法,然后合成不同经验模态下边坡位移的计算结果,得到滑坡位移的预测值。以卧龙寺新滑坡和新滩滑坡的监测数据为基础的理论预测结果表明,与采用遗传算法的神经网络方法的预测结果相比,支持向量机经验模态方法具有更强的预测能力,理论预测结果与实际监测值具有很好的一致性     

Regression analysis for seismic slope instability based on a double phase viscoplastic sliding model of the rigid block

Semi-empirical models based on Newmark’s sliding block permit the estimation of expected co-seismic displacements in relation to one or more parameters which characterize the ground motion that theoretically caused them. Taking this into consideration, a regression analysis, based on a double-phase viscoplastic (DPV) model, was developed using 96 Italian ground motion accelerograms for a total of 1,448 combinations obtained for different parametric conditions of the indefinite slope model. Repeated stability analysis, performed by means of the DPV model, allows for the assessment of the seismic instability of a slope in relation to different reached behaviour levels, as well as seismically induced permanent displacements. At these behaviour levels, co-seismic increases and possible subsequent decreases of viscoplastic shear strengths are associated. This implies that the post-seismic persistent mobility (collapse) of the slope can be obtained from the computation. On the other hand, coherently with the increasing of shear resistances during fast sliding displacements in clay soils, the seismic-forced displacements result substantially lower than corresponding values obtained by means of the rigorous Newmark’s sliding block. In addition, in relation to some seismic ground motion parameters, regression and functional border and separation curves were obtained with the aim of providing an expeditious seismic slope stability evaluation in reference to the co-seismic and post-seismic behaviour of clayey slopes. Regarding this, the real behaviour of two historical landslide events is discussed in the light of the results of the regression analysis outlined in this work.     

Seismic landslide simulations in discontinuous deformation analysis

Discontinuous deformation analysis (DDA) is a numerical approach used to simulate the post-failure behavior of a blocky assembly. Three available algorithms incorporate seismic impacts into DDA simulations for earthquake-induced slope failure. The following methods are used: directly applying time-dependent accelerations to falling/sliding blocks (Method 1); adding time-dependent accelerations to base block (Method 2); and time-dependently constraining seismic displacements of the base block (Method 3). However, incorrect absolute movements of falling/sliding and base blocks were obtained using Method 1. Additionally, relative movements between falling/sliding blocks and the base block are opposite to those simulated by the other two algorithms—Methods 2 and 3. Since locating an earthquake-induced landslide before an earthquake is extremely difficult, the seismic movements of base rock are recorded. Method 1 applies recorded seismic data to sliding blocks in conflict with d’Alembert’s principle of mechanics. Additionally, in Method 2, when the computation time step must be longer than the time in seismic data, computational results reveal abnormal base block displacements due to the non-zero velocity recorded at the end time of seismic data in seismic DDA. In this study, a novel algorithm to diminish the velocity of the base rock in the seismic analysis is utilized to modify Method 2. Furthermore, this work confirms that DDA with the modified Methods 2 and 3 is a practical approach for earthquake-induced landslide simulations.     

基于振动台试验的加速度积分位移方法可靠性研究

土体应变变形是土工测试技术的重要难题之一,加速度积分位移是目前动荷载下原位监测和模型试验中认识土体剪应变响应的一种普遍方法,但缺乏验证与可靠性研究。设计砂质边坡?摇臂?质量块振动台模型试验,实测两地震荷载下多组工况的质量块加速度与位移响应时程,探讨了4种代表性方法求解动态、永久位移的可靠性及偏转影响。结果表明,动态位移求解采用高通滤波是一种可靠途径且受积分方式影响较小,各方法在动态位移工况中积分位移与实测相比,峰值平均误差小于等于25%;双拟法积分位移发生严重偏离,与函数形式和处理过程相关。永久位移求解采用高通滤波导致加速度低频成分缺失而无法得到永久位移;双拟法积分位移与实测相比吻合较好,两地震荷载中误差分别为7%和5%;偏转对动态位移求解可靠性影响较小,但对永久位移影响严重,双拟法呈现发散而失真。试验方法与结论,对认识加速度积分位移求取土体剪应变的可靠性和合理选取数据分析方法具有重要科学价值和指导意义。     

Stability assessment of a circular earth dam

Stability of a circular earth dam is assessed for radial cracking potential and static slope stability using continuum mechanics‐based three‐dimensional numerical models. Comparisons of numerical model results for a circular water tank with vertical walls and different radii with their analytical counterparts are included to support the validity of the ideas and their implementation in the continuum mechanics‐based computer program used. Effects of sloping wall faces and Poisson's ratio on computed deformations and stresses are also included. The same numerical models are used to assess stability of a circular dam in terms of factor‐of‐safety and associated failure surface. Three‐dimensional slope stability analysis results are compared with continuum based two‐dimensional slope stability analysis results to assess the magnitude of 3D effects. Example problems are included to illustrate the use of ideas presented. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

A NEW MODEL FOR THE ANALYSIS OF SETTLEMENT OF DRILLED PIERS

A variational model for the analysis of axially loaded piers is presented. A closed-form solution technique employing an iterative procedure, is developed to obtain the displacement and forces in the pier along its axial direction. The method is suitable for similar analyses of pile foundations. It is shown that displacements and the load distribution along the axis of the pier compare well with a more sophisticated finite element solution. Furthermore, the new model complements the well-known Reese model employing t–z curves for the analysis of settlement of axially loaded piers. This new formulation using continuum mechanics principles, distributes the work done by the applied load as compressive strain energy in the pier, and as shear strain energy in the soil, as well as, the compressive strain energy in the soil surrounding the pier and at the bottom of the pier.     

Vulnerability assessment of reinforced concrete buildings subjected to seismically triggered slow-moving earth slides

The objective of this paper is to develop an efficient analytical method for assessing the vulnerability of low-rise reinforced concrete buildings subjected to seismically induced slow-moving earth slides. Vulnerability is defined in terms of probabilistic fragility curves, which describe the probability of exceeding a certain limit state of the building, on a given slope, versus the Peak Horizontal Ground Acceleration (PHGA) at the assumed “seismic bedrock”, allowing for the quantification of various sources of uncertainty. The proposed method is based on a two-step, uncoupled approach. In the first step, the differential permanent landslide displacements at the building’s foundation level are estimated using a dynamic non-linear finite difference slope model. In the second step, the calculated differential permanent displacements are statically imposed at the foundation level to assess the building’s response to differing permanent seismic ground displacements using a finite element code. Structural limit states are defined in terms of threshold values of strains for the reinforced concrete structural components. The method is applied to typical low-rise reinforced concrete frame buildings on shallow foundations with varying strength and stiffness characteristics (isolated footings and continuous slab foundation), standing near the crest of a relatively slow-moving earth slide. Two different slope models are selected representing a cohesive and a purely frictional soil material. The paper describes the method and the derived fragility curves for the selected building and slope typologies that could be used in quantitative risk assessment studies at site-specific and local scales.     

Seismically induced landslide displacements: a predictive model

Newmark's model for predicting earthquake-induced landslide displacements provides a simple way to predict the coseismic displacements affecting a sliding mass subject to earthquake loading. In this model, seismic slope stability is measured in terms of critical acceleration, which depends on the mechanical soil properties, pore-pressure distribution, and slope geometry. The triggering seismic forces are investigated in terms of energy radiation from the source, propagation, and site effects, based on 190 accelerometric recordings from 17 Italian earthquakes with magnitudes between 4.5 and 6.8. The method is based on the calibration of relations having the general form of an attenuation law that relates the energy of the seismic forces to the dynamic shear resistances of the sliding mass to propagate the expected landslide displacements as an inverse function of the distance from the fault rupture; the amount of displacement computed through these relations provides a criterion to predict the occurrence of slope failures. Finally, maps showing, in a deterministic and a probabilistic way, the potential of seismically induced landslide displacements are displayed as a tool to provide seismic landslide scenarios and earthquake-induced landslide hazard maps, respectively.     

边坡位移演化的多尺度分析

通过相邻两个时间区间位移观测结果均值差分的积分描述和Harr小波基函数的小波变换方程的对比分析,得到了一个将不同时间尺度下边坡位移均值变化表示为相应尺度下小波变换系数的基本关系。根据这一关系,提出了可以采用小波变换方法确定两个相邻时间区间位移均值在不同时间尺度下变化规律的边坡位移演化的多尺度分析方法。针对卧龙寺新滑坡、三峡永久船闸边坡开挖和隔河岩水电站进水口边坡变形的观测结果,讨论了它们的位移演化多尺度特征。当边坡位移呈现较规则的变化趋势时,在一个尺度上就可以提取它们的时间演化特征。开挖剧烈扰动的影响可以通过给定时间尺度支撑区端点与位移突变点之间递增连的线进行近似。     

边坡土体侧移模式对抗滑桩影响分析

在开挖、降雨或地震等外部因素作用下,边坡土体很容易进入局部或瞬态大变形乃至失稳滑动,使抗滑桩产生附加位移及弯矩。基于两阶段分析方法,采用Winkler模型模拟抗滑单桩与土之间的相互作用,建立单桩水平位移控制方程组,根据内力与位移的连续条件得到考虑不同土体侧移模式下求解桩身响应的矩阵解析表达式,并采用现场监测数据及Poulos弹性理论进行验证,证明该方法是合理可行的,并通过参数分析土体侧移对抗滑桩水平承载性状的影响程度。分析结果表明,土体侧移模式包括最大侧移值、分布形状及重心、侧移势等,对抗滑桩的挠度和弯矩均有显著影响,在工程设计中应予以充分重视。