Simplified multi-block constitutive model predicting earthquake-induced landslide triggering and displacement along slip surfaces of saturated sand

Slopes consisting of saturated sand have recently moved down-slope tens or hundreds of meters under the action of earthquakes. This paper presents a simplified but accurate method predicting the triggering and displacement of such landslides. For this purpose, a simplified constitutive model simulating soil response of saturated sands along slip surfaces is proposed and validated. Then, this constitutive model is coupled with the multi-block sliding system model to predict the triggering and displacement of such slides. The multi-block model considers a general mass sliding on a trajectory which consists of n linear segments. The steps needed to apply this method are described in detail. The method was applied successfully to predict the triggering, the motion and the final configuration of the well-documented (a) Higashi Takezawa, (b) Donghekou and (c) Nikawa earthquake-induced slides.     

Numerical evaluation of slope topography effects on seismic ground motion

This paper presents results of numerical analyses for the seismic response of step-like ground slopes in uniform visco-elastic soil, under vertically propagating SV seismic waves. The aim of the analyses is to explore the effects of slope geometry, predominant excitation frequency and duration, as well as of the dynamic soil properties on seismic ground motion in a parametric manner, and provide qualitative as well as quantitative insight to the phenomenon. Among the main conclusions of this study is that this kind of topography may lead to intense amplification or de-amplification variability at neighboring (within a few tens of meters) points behind the crest of the slope, especially for high frequency excitations. Nevertheless, a general trend of amplification near the crest and de-amplification near the toe of the slope seems to hold for the horizontal motion. As a result of these two findings, it becomes evident that reliable field evidence of slope topography aggravation is extremely difficult to establish. Furthermore, this study highlights the generation of a parasitic vertical component of motion in the vicinity of the slope, due to wave reflections at the slope surface, that under certain preconditions may become as large as the horizontal. Criteria are established for deciding on the importance of topography effects, while approximate relations are provided for the preliminary evaluation of the topographic aggravation of seismic ground motion and the width of the affected zone behind the crest.     

Seismic stability of earth-rock dams using finite element limit analysis

In this study, a finite element limit analysis method is developed to assess the seismic stability of earth-rock dams. A pseudo-static approach is employed within the limit analysis framework to determine the lower and upper bounds on the critical seismic coefficients of dams. The interlocking force in the soil is considered, and the rockfill material is assumed to follow the Mohr–Coulomb failure criterion and an associated flow rule. Based on the native form of the failure criterion, the lower and upper bound theorems are formulated as second-order cone programming problems. The nonlinear shear strength properties of rockfill materials are also considered. The developed finite element limit analysis is applied to two different types of earth-rock dams. The results indicate that the rigorous lower and upper bounds are very close even for rockfill materials with large internal friction angles. The failure surfaces are easily predicted using the contour of the yield function and the displacement field obtained by the limit analysis method. In addition, the pore water pressures are modelled as external forces in the limit analysis to assess the seismic stability of earth-rock dams in the reservoir filling stage.     

Earthquake-induced landslide displacement attenuation models and application in probabilistic seismic landslide displacement analysis

A landslide displacement (DLL) attenuation model has been developed using spectral intensity and a ratio of critical acceleration coefficient to ground acceleration coefficient. In the development of the model,a New Zealand earthquake record data set with magnitudes ranging from 5.0 to 7.2 within a source distance of 175 km is used. The model can be used to carry out deterministic landslide displacement analysis,and readily extended to carry out probabilistic seismic landslide displacement analysis. DLL attenuation models have also been developed by using earthquake source terms,such as magnitude and source distance,that account for the effects of earthquake faulttype,source type,and site conditions. Sensitivity analyses show that the predicted DLL values from the new models are close to those from the Romeo model that was developed from an Italian earthquake record data set. The proposed models are also applied to an analysis of landslide displacements in the Wenchuan earthquake,and a comparison between the predicted and the observed results shows that the proposed models are reliable,and can be confidently used in mapping landslide potential.     

Geotechnical design with apparent seismic safety factors well-bellow 1

The paper demonstrates that whereas often in seismic geotechnical design it is not realistically feasible to design with ample factor of safety against failure as is done in static design, an “engineering” apparent seismic factor of safety less than 1 does not imply failure. Examples from slope stability and foundation rocking illustrate the concept. It is also shown that in many cases it may be beneficial to under-design the foundation by accepting substantial uplifting and/or full mobilization of bearing capacity failure mechanisms.     

Seismic analysis of sliding wedge: extended Francais–Culmann's analysis

Pseudo-static analysis is commonly used to design earth structures. Most pseudo-static methods of analysis require a computer program. This paper presents a simple closed form solution of seismic stability analysis by extending Francais–Culmann's analysis. The analysis is valid for a slope with the most critical planar mechanism and under the influence of horizontal and vertical earthquake accelerations. The resulting permanent displacement is discussed and demonstrated using several real earthquake records. The simplicity of extended Francais–Culmann's solution allows it to be used for preliminary design and classroom instruction.     

盾构隧道抗震分析的整体强制反应位移法适用性研究

整体强制反应位移法采用土-结构相互作用模型,将地震作用下的土层位移以强制位移的方式施加到整个土层上,从而计算结构在地震作用下的响应。本文以天津某地铁盾构隧道为例,采用整体强制反应位移法进行地铁盾构隧道结构的抗震分析计算。为了验证该方法在复杂软土场地下盾构隧道结构抗震分析中的适用性,以时程分析法为基准,对比分析整体强制反应位移法、反应位移法和时程分析法3种方法计算出的盾构隧道结构内力图和内力峰值。研究表明,整体强制反应位移法具有较好的适用性及易操作性,具有较高的计算精度,是一种方便有效的抗震设计拟静力方法。     

Ground motion selection for seismic slope displacement analysis using a generalized intensity measure distribution method

Selecting ground motions based on the generalized intensity measure distribution (GIMD) approach has many appealing features, but it has not been fully verified in engineering practice. In this paper, several suites of ground motions, which have almost identical distributions of spectral acceleration (SA) ordinates but different distributions of non‐SA intensity measures, are selected using the GIMD‐based approach for a given earthquake scenario. The selected ground motion suites are used to compute the sliding displacements of various slopes. Comparisons of the resulting displacements demonstrate that selecting ground motions with biased distribution of some intensity measures (ie, Arias intensity) may yield systematic biases (up to 60% for some slope types). Therefore, compared to the ground motions selected based only on the distribution of SA ordinates, the ground motion suite selected by the GIMD‐based approach can better represent the various characteristics of earthquake loadings, resulting in generally unbiased estimation in specific engineering applications.     

基于拟动力法的抗滑桩加固边坡地震稳定性分析

地震力作用下土质边坡动态稳定性研究对实际边坡工程有着重要的意义。采用拟动力法结合简化毕肖普法研究坡顶抗滑桩加固土质边坡在地震力作用下的动态稳定性。尽管拟静力法是目前处理地震力最为广泛的方法之一,但其局限性在于无法考虑地震力随时间变化且忽略了地震波在土体中的传播。而拟动力法采用正弦波模拟地震波在土中传播,并考虑地震波从坡脚传递到坡顶的相位差以及阻尼力对边坡稳定性的影响,通过边坡安全系数的变化揭示土质边坡在地震力作用下的稳定性变化规律。将得到的结果与拟静力法进行对比,突出了拟动力法的优势。最后,考虑水平地震加速度系数、加速度幅值放大系数以及土体内摩擦角对边坡稳定性的影响,以期对实际工程提供理论借鉴。     

Three-dimensional curvature analysis of seismic waveforms and its interpretational implications

The idea of curvature analysis has been widely used in subsurface structure interpretation from three-dimensional seismic data (e.g., fault/fracture detection and geomorphology delineation) by measuring the lateral changes in the geometry of seismic events. However, such geometric curvature utilizes only the kinematic information (two-way traveltime) of the available seismic signals. While analysing the dynamic information (waveform), the traditional approaches (e.g., complex trace analysis) are often trace-wise and thereby fail to take into account the seismic reflector continuity and deviate from the true direction of geologic deposition, especially for steeply dipping formations. This study proposes extending the three-dimensional curvature analysis to the waveforms in a seismic profile, here denoted as the waveform curvature, and investigates the associated implications for assisting seismic interpretation. Applications to the F3 seismic dataset over the Netherlands North Sea demonstrate the added values of the proposed waveform curvature analysis in four aspects. First, the capability of the curvature operator in differentiating convex and concave bending allows automatic decomposition of a seismic image by the reflector types (peaks, troughs and zero crossings), which can greatly facilitate computer-aided horizon interpretation and modelling from three-dimensional seismic data. Second, the signed minimum curvature offers a new analytical approach for estimating the fundamental and important reflector dip attribute by searching the orientation associated with least waveform variation. Third, the signed maximum curvature makes it possible to analyse the seismic signals along the normal direction of the reflection events. Finally, the curvature analysis promotes the frequency bands of the seismic signals and thereby enhances the apparent resolution on identifying and interpreting subtle seismic features.     

Seismic stability analysis of reinforced slopes

In this paper, the seismic stability of slopes reinforced with geosynthetics is analysed within the framework of the pseudo-static approach. Calculations are conducted by applying the kinematic theorem of limit analysis. Different failure modes are considered, and for each analytical expressions are derived that enable one to readily calculate the reinforcement force required to prevent failure and the yield acceleration of slopes subjected to earthquake loading. Several results are presented in order to illustrate the influence of seismic forces on slope stability. Moreover, a suitable procedure based on the assessment of earthquake-induced permanent displacement is proposed for the design of reinforced slopes in seismically active areas.     

Three-dimensional analysis of rows of piles as passive barriers for ground vibration isolation

This paper presents a theoretical study on the ground vibration isolation efficiency by multi-rows of piles as passive barrier in a three-dimensional context. Integral equations governing Rayleigh wave scattering are derived according to the Green's solution of Lamb problem. The integral equations are solved accurately and efficiently with an iteration technique. They are used to predict the complicated Rayleigh wave field that is generated by a number of irregular scatterers embedded in an elastic half-space solid. The method is verified with a numerical solution available in the literature for a simplified Rayleigh wave scattering problem. Passive isolation effectiveness of ground vibration with two or three rows of small piles is further studied in detail. Effects of relevant parameters on the effectiveness of vibration isolation are analyzed and presented.     

Three-dimensional finite element analysis of the seismic behavior of inclined micropiles

This paper presents a thorough study of the behavior of inclined micropiles under seismic loading. Analysis is carried out using a full three-dimensional finite element modeling. The soil media is assumed to be elastic with Rayleigh damping, while micropiles are modeled as 3D elastic beam elements. The structure is described by a single degree of freedom system composed of a concentrated mass and a column. The paper is composed of four parts. The first part includes a literature survey on the behavior of inclined micropiles. The second part presents the numerical model used in this study. The third part concerns analysis related to the influence of micropiles inclination on the seismic behavior of a group of micropiles embedded in a homogeneous soil with a uniform stiffness. The last part deals with the seismic behavior of inclined micropiles embedded in a soil layer with a depth-based increasing stiffness. The results of this study provide valuable information about the influence of micropiles inclination on dynamic amplification and on the seismic-induced internal forces in micropiles.     

地震危险性分析中潜在震源区范围与震级上限的不确定性分析

目前进行的地震危险性分析计算中, 潜在震源区范围和震级上限的确定过程中存在很大的不确定性。 采用二级划分和三级划分潜在震源区的方法, 对潜在震源区范围不确定性进行了研究; 用震级上限的单边正态分布模型代替固定的震级上限模型, 分析了这种改进对地震危险性分析结果的影响及其工程意义。 结果表明, 用三级划分潜源的方法代替二级划分潜在震源方法可降低局部地区的地震危险性, 对于在潜在震源区附近选址和降低工程成本有重要意义。 震级上限的改进, 使得地震危险性降低, 这对百年使用年限的工程更具有实用性。     

Seismic displacement analysis of embankment dams with reinforced cohesive shell

Suitable materials for use as shell of embankment dams are clean coarse-grained soils or natural rockfill. In some sites these materials may not be available at an economic distance from the dam axis. The use of in-situ cohesive soils reinforced with geotextiles as the shell is suggested in this study for such cases. Dynamic behavior of reinforced embankment dam is evaluated through fully coupled nonlinear effective stress dynamic analysis. A practical pore generation model has been employed to incorporate pore pressure build up during cyclic loading. Parametric analyses have been performed to study the effect of reinforcements on the seismic behavior of the reinforced dam. Results showed that reinforcements placed within the embankment reduce horizontal and vertical displacements of the dam as well as crest settlements. Maximum shear strains within the embankment also decreased as a result of reinforcing. Furthermore, it was observed that reinforcements cause amplification in maximum horizontal crest acceleration.     

A GLE-based model for seismic displacement analysis of slopes including strength degradation and geometry rearrangement

Seismic performance of natural slopes, earth structures and solid-waste landfills can be evaluated through displacement-based methods in which permanent displacements induced by earthquake loading are assumed to progressively develop along the critical sliding surface as a result of transient activation of plastic mechanisms within the soil mass. For sliding mechanisms of general shape the earthquake-induced displacements should be computed using a model that provides a closer approximation of sliding surface. When large permanent displacement are induced by seismic actions, due to substantial shear strength reduction, and significant changes in ground surface occur, an improved estimate of permanent displacement can be obtained using a model which accounts for shear strength reduction and mass transfer between adjacent portions of the slope resulting from geometry changes of ground surface during the seismic event.In this paper, a GLE-based model is proposed for seismic displacement analysis of slopes that accounts for shear strength degradation and for geometry rearrangement. Model accuracy is validated against experimental results obtained from shaking table tests carried out on small scale model slopes. Comparison of computed and experimental results demonstrates the capability of the proposed approach in capturing the main features of the observed seismic response of the model slopes.     

Reliability analysis of three-dimensional dynamic slope stability and earthquake-induced permanent displacement

Different models were developed for evaluating the probabilistic three-dimensional (3D) stability analysis of earth slopes and embankments under earthquake loading using both the safety factor and the displacement criteria of slope failure. In the 3D analysis, the critical and total slope widths become two new and important parameters.The probabilistic models evaluate the probability of failure under seismic loading considering the different sources of uncertainties involved in the problem, i.e. uncertainties stemming from the discrepancies between laboratory-measured and in-situ values of shear strength parameters, randomness of earthquake occurrence, and earthquake-induced acceleration. The models also takes into consideration the spatial variabilities and correlations of soil properties.Five probabilistic models of earthquake-induced displacement were developed based on the non-exceedance of a limited value criterion. Moreover, a probabilistic model for dynamic slope stability analysis was developed based on 3D dynamic safety factor.These models are formulated and incorporated within a computer program (PTDDSSA).A sensitivity analysis was conducted on the different parameters involved in the developed models by applying those models to a well-known landslides (Selset landslide) under different levels of seismic hazard.The parametric study was conducted to evaluate the effect of different input parameters on the resulting critical failure width, 3D dynamic safety factor, earthquake-induced displacement and the probability of failure. Input parameters include: average values and coefficients of variations of water table, cohesion and angle of friction for effective stress analysis, scales of fluctuations in both distance and time, hypocentral distance, earthquake magnitude, earthquake strong shaking period, etc.The hypocentral distance and earthquake magnitude were found to have major influence on the earthquake-induced displacement, probability of failure (i.e. probability of allowable displacement exceedance), and dynamic 2D and 3D safety factors.     

Fragility estimation and sensitivity analysis of an idealized pile-supported wharf with batter piles

The main objective of the present study is to develop seismic fragility curves of an idealized pile-supported wharf with batter piles through a practical framework. Proposing quantitative limit states, analytical fragility curves are developed considering three engineering demand parameters (EDPs), including displacement ductility factor (µ_d), differential settlement between deck and behind land (DS) and normalized residual horizontal displacement (NRHD). Analytical fragility curves are generated using the results of a numerical model. So, the accuracy and reliability of resulted fragility curves directly depend on how accurate the seismic demand quantities are estimated. In addition, the seismic performance of pile-supported wharves is highly influenced by geotechnical properties of the soil structure system. Hence, a sensitivity analysis using the first-order second-moment (FOSM) method is performed to evaluate the effects of geotechnical parameters uncertainties in the seismic performance of the wharf.Herein, the seismic performance of the wharf structure is simulated using the representative FLAC2D model and performing nonlinear time history analyses under a suit of eight ground motion records. Incremental dynamic analysis (IDA) is used to estimate the seismic demand quantities. As a prevailing tool, adopted fragility curves are useful to seismic risk assessment. They can also be used to optimize wharf-retrofit methods. The results of sensitivity analysis demonstrate that uncertainties associated with the porosity of loose sand contribute most to the variance of both NRHD and µ_d. While in the case of differential settlement, the friction angle of loose sand contributes most to the variance.     

钢筋混凝土圆截面桥墩抗震加固方式对比分析

为研究不同加固方式对钢筋混凝土(RC)圆截面桥墩抗震性能的影响,利用OpenSees有限元软件建立了普通RC桥墩以及分别采用钢套管、碳纤维增强聚合物(CFRP)、体外预应力筋进行加固的桥墩数值分析模型,对模型输入远断层地震动,进行增量动力分析。以墩顶峰值位移角与震后残余位移角为指标,对比分析了桥墩加固前后的地震响应。结果表明:采用钢套管、体外预应力筋和CFRP加固后,RC桥墩的峰值位移与震后残余位移均减小,钢套管加固方式对桥墩峰值位移的降低幅度最大,体外预应力筋加固方式对抑制桥墩震后残余位移的效果最好;随着剪跨比的增大,3种加固方式对桥墩在地震动作用下位移响应的抑制作用均逐步减小;随着轴压比的增大,3种加固方式对RC桥墩峰值位移的抑制作用逐步降低。     

基于GIS的地下管网抗震分析系统

本文提供了一个地下管道震害快速预测方法,给出了供水管网和供气管网地震影响范围判定分析方法以及关键环节判定方法,完成了一个基于Arc View GIS的多功能的地下管网抗震分析系统的研制。