A review of considerations on seismic safety of embankments and earth and rock-fill dams

An attempt has been made to summarise the methods of approach used in assessing the dynamic behaviour and safety of earth and rock-fill dams under seismic shaking until about the present time, from the soil engineer's point of view. Shortcomings of the pseudo-static method, procedures to estimate the permanent deformations, liquefaction effects, and experience gained from the previous events have been reviewed. Observing that the most important cause of instability is the occurence liquefaction during ground motions, cyclic approach and steady-state approach in assessing the liquefaction potential have been addressed and recent practical approaches of analysis and design have been referred. It has been found noteworthy to recall that incidence of failure or serious damage to well-engineered dams has not been experienced, even under strong ground shaking.     

Seismic shear vibration of embankment dams in semi-cylindrical valleys

A rigorous analytical solution is developed for the lateral linear shear response of embankment dams in semi-cylindrical valleys. Closed-form algebraic expressions are presented pertaining to both free and base-induced oscillations, and extensive parametric and comparative studies elucidate the prominent effects of canyon geometry (shape and aspect ratio) on dynamic response. Harmonic steady-state as well as earthquake-induced accelerations, displacements and shear strains in the dam are studied and compared with those obtained from 3-Dimensional analyses for other canyon geometries, as well as from 2-Dimensional (2-D) analyses of the dam mid-section. It is shown that such 2-D analyses may provide significantly lower values of near-crest accelerations, but slightly higher values of shear strains and stresses than the 3-D analyses. The proposed method of analysis is at least three orders of magnitude less expensive than other presently available numerical procedures.     

Methodology for estimating seismic coefficients for performance-based design of earthdams and tall embankments

Following an overview of pertinent literature, this paper presents a new methodology for estimating seismic coefficients for the performance-based design of earth dams and tall embankments. The methodology is based on statistical regression of (decoupled) numerical data for 1084 potential sliding masses, originating from 110 non-linear seismic response analyses of 2D cross sections with height ranging from 20 to 120 m. At first, the methodology estimates the peak value of the seismic coefficient k_hmax as a function of: the peak ground acceleration at the free field, the predominant period of the seismic excitation, the non-linear fundamental period of dam vibration, the stiffness of the firm foundation soil or rock layer, as well as the geometrical characteristics and the location (upstream or downstream) of the potentially sliding mass. Then, it proceeds to the estimation of an effective value of the seismic coefficient k_hE, as a percentile of k_hmax, to be used with a requirement for pseudo-static factor of safety greater or equal to 1.0. The estimation of k_hE is based on allowable permanent down-slope deviatoric displacement and a conservative consideration of sliding block analysis.     

洱源地震台数字地震记录S波分裂研究

利用2008年1月—2013年5月洱源地震台数字地震资料进行S波分裂研究,得到洱源地区应力优势方向和S波延迟时间,结果表明,洱源地区快剪切波偏振优势方向为NE160°,与活动断裂走向一致,与GPS主压应力方向一致,与该区主压应力场方向基本一致。在洱源地震台300 km范围内发生M≥4.5地震前,快剪切波偏振方向发生明显偏转。部分地震前,延迟时间增加;部分地震发生前短时间内,延迟时间有减小现象,符合震前应力长时间积累和短时间应力释放的特征。可见,S波分裂参数可以反映区域应力场的动态变化信息,为应力场研究与地震预测提供有用信息。     

土石坝地震响应的非线性剪切条模型与对比分析

对一维剪切条计算模型进行改进,提出了土石坝非线性地震反应的简化计算方法。首先将坝体沿坝高离散为一系列的具有不同剪切刚度与阻尼比等参数特性的层状体系,建立了各层的振动控制方程及其边值条件,进而采用数学物理方程方法进行了求解,确定了体系的振动特性,并根据振型叠加原理和Duhamel积分确定了坝体地震反应的线弹性解。采用等价线性化方法考虑坝料的动力非线性性质,通过对线弹性地震响应的反复迭代计算,使得各层土的模量和阻尼比与其相应的剪应变水平相协调,确定出与非线性坝体系统相等效的线性解答,并将所得到的地震响应作为非线性地震响应的近似解。最后,以均质坝和心墙坝作为算例进行了具体的数值计算,将所得结果与有限元数值解进行对比分析,论证了所提方法的适用性和合理性。     

Analytical seismic fragility functions for highway and railway embankments and cuts

A fundamental tool in seismic risk assessment of transportation systems is the fragility curve, which describes the probability that a structure will reach or exceed a certain damage state for a given ground motion intensity. Fragility curves are usually represented by two‐parameter (median and log‐standard deviation) cumulative lognormal distributions. In this paper, a numerical approach, in the spirit of the IDA, is applied for the development of fragility curves for highways and railways on embankments and in cuts due to seismic shaking. The response of the geo‐construction to increasing levels of seismic intensity is evaluated using a 2D nonlinear finite element model, with an elasto‐plastic criterion to simulate the soil behavior. A calibration procedure is followed in order to account for the dependency of both the stiffness and the damping to the soil strain level. The effect of soil conditions and ground motion characteristics on the response of the embankment and cut is taken into account considering different typical soil profiles and seismic input motions. This study will provide input for the assessment of the vulnerability of the road/railway network regarding the performance of the embankments and cuts; therefore, the level of damage is described in terms of the permanent ground displacement in these structures. The fragility curves are estimated based on the evolution of damage with increasing earthquake intensity, which is described by PGA. The proposed approach allows the evaluation of new fragility curves considering the distinctive features of the element's geometry, the input motion, and the soil properties as well as the associated uncertainties. A relationship between the computed permanent ground displacement on the surface of the embankment and the PGA in the free field is also suggested based on the results of the numerical analyses. Finally, the proposed fragility curves are compared with existing empirical data and the limitations of their applicability are outlined. Copyright © 2015 John Wiley & Sons, Ltd.     

Seismic safety of high concrete dams

Peak ground acceleration(PGA) estimation is an important task in earthquake engineering practice.One of the most well-known models is the Boore-Joyner-Fumal formula,which estimates the PGA using the moment magnitude,the site-to-fault distance and the site foundation properties.In the present study,the complexity for this formula and the homogeneity assumption for the prediction-error variance are investigated and an effi ciency-robustness balanced formula is proposed.For this purpose,a reduced-order Monte Carlo simulation algorithm for Bayesian model class selection is presented to obtain the most suitable predictive formula and prediction-error model for the seismic attenuation relationship.In this approach,each model class(a predictive formula with a prediction-error model) is evaluated according to its plausibility given the data.The one with the highest plausibility is robust since it possesses the optimal balance between the data fi tting capability and the sensitivity to noise.A database of strong ground motion records in the Tangshan region of China is obtained from the China Earthquake Data Center for the analysis.The optimal predictive formula is proposed based on this database.It is shown that the proposed formula with heterogeneous prediction-error variance is much simpler than the attenuation model suggested by Boore,Joyner and Fumal(1993).     

Non-linear seismic response of arch dams

An experimental study of non-linear mechanisms that may occur during intense seismic response of arch dams is described in this paper. The presentation deals with three types of non-linearity that were observed during shaking table model studies: monolith joint opening, cantilever cracking, and reservoir cavitation at the dam face. The monolith joint opening phenomenon was represented by a segmental arch ring model that simulated a horizontal slice of a prototype dam. The cantilever cracking and reservoir cavitation mechanisms were studied using a model gravity dam section. The principal conclusion of the investigation was that shaking table experiments provide a practical means of studying the non-linear earthquake response of concrete arch dams, including their actual failure mechanisms.     

Seismic cracking and energy dissipation in concrete gravity dams

A finite element method for seismic fracture analysis of concrete gravity dams is presented. The proposed smeared crack analysis model is based on the non-linear fracture behaviour of concrete. The following features have been considered in the development of the model: (i) the strain softening of concrete due to microcracking; (ii) the rotation of the fracture band with the progressive evolution of microcrack damage in finite elements; (iii) the conservation of fracture energy; (iv) the strain-rate sensitivity of concrete fracture parameters; (v) the softening initiation criterion under biaxial loading conditions; (vi) the closing-reopening of cracks under cyclic loading conditions. The seismic fracture and energy response of dams and the significance of viscous damping models to take account of non-cracking structural energy dissipation mechanisms are discussed. The influences of global or local degradation of the material fracture resistance on the seismic cracking response of concrete dams were also studied. Two-dimensional seismic response analyses of Koyna Dam were performed to demonstrate the application of the proposed non-linear fracture mechanics model.     

Seismic stability of concrete gravity dams

Linear finite element analyses are commonly used to simulate the behaviour of gravity dam—foundation systems. However, the foundation is generally unable to develop any significant tensile stresses. Therefore any tension occurring in the vicinity of the dam—foundation interface is largely fictitious. Moreover, the traditional overturning and sliding stability criteria have little meaning in the context of the oscillatory response of dams during earthquakes. In this study, time domain analyses using non-linear contact elements located at the dam—foundation interface have been used to determine the dynamic sliding and uplifting response of gravity dam monoliths considering various elastic foundation properties. The magnitudes of the relative interface displacements, of the percentage of base not in contact (PBNC) and of the compressive stresses at the heel or toe of the dam have been used to monitor the seismic stability. The numerical results have shown that the non-linear behaviour of the dam—foundation interface reduces the seismic response of the system, indicating the possibility of more rational and economical designs. The PBNC was identified as the critical seismic stability response parameter for all analyses except for very flexible foundation conditions where the maximum values of relative interface displacements need to be considered.     

Seismic safety of high concrete dams

China is a country of high seismicity with many hydropower resources. Recently,a series of high arch dams have either been completed or are being constructed in seismic regions,of which most are concrete dams. The evaluation of seismic safety often becomes a critical problem in dam design. In this paper,a brief introduction to major progress in the research on seismic aspects of large concrete dams,conducted mainly at the Institute of Water Resources and Hydropower Research(IWHR) during the past 60 years,is presented. The dam site-specific ground motion input,improved response analysis,dynamic model test verification,field experiment investigations,dynamic behavior of dam concrete,and seismic monitoring and observation are described. Methods to prevent collapse of high concrete dams under maximum credible earthquakes are discussed.     

1-G model tests and hollow cylindrical torsional shear experiments on seismic residual displacements of fill dams from the viewpoint of seismic performance-based design

This paper concerns technological efforts for the general acceptance of performance-based seismic design principle of geotechnical structures. Among many problems to be solved, a particular emphasis was placed on the prediction of residual displacement that remains after a strong earthquake. Because of the complicated behavior of soils undergoing cyclic loading, the prediction is often either complicated/costly or not very accurate. The aim of this study is to examine the capability of existing prediction measures and propose some future scopes. To achieve these goals, shaking table model tests and laboratory shear tests were conducted by taking fill dams as an example target structure. It is concluded that performance-based design is possible if the necessary time and cost are spent and if the required accuracy of prediction is reasonable.     

Efficient non-linear seismic analysis of arch dams

An approximate procedure for seismic analysis of concrete arch dams which is based on the smeared crack method is described. Features include construction sequence modelling, water and foundation interaction, crack formation, opening and closing of joints and cracks, frictional sliding, presence of shear keys, action of internal water pressure, and a reliable solution algorithm. Complete solutions can be obtained in an hour on a fast workstation computer, allowing parameter studies to be run. Results suggest that an arch dam can suffer significant cracking during strong ground shaking and still remain stable. © 1998 John Wiley & Sons, Ltd.     

Higher order model for the seismic response of bridge embankments

The paper presents a new higher order model for the dynamic analysis of embankments. By considering a Legendre polynomial expansion to describe the motion at a generic point of the embankment, the application of the Lagrange-D’Alembert principle in conjunction with a through-the-width closed-form integration allows reducing the 3D physical domain into a 2D analytical domain. 4-node isoparametric elements with linear interpolating functions are used to numerically solve the problem. The model is suitable for bridge embankments by introducing a kinematic rigid constraint to account for the presence of the abutment. The embankment frequency dependent impedances and the displacements to be imposed to the abutment in bridge seismic analyses are obtained by condensation. The model has been validated comparing results with those furnished by high-fidelity 3D finite element models. The application to the approach embankment of an instrumented bridge subjected to a severe earthquake has demonstrated the model capability to capture both occurrence and intensity of main response peaks, as well as the frequency content of the response.     

Nonlinear seismic response analysis of earth dams

The objective of this paper is to propose a general and efficient numerical procedure for analysing the dynamic response of geotechnical structures, which are considered as both nonlinear and two phase systems. In Section 2, the appropriate coupled dynamic field equations for the response of a two-phase soil system are briefly reviewed. The finite element spatial discretization of the field equations is described and time integration for the resulting nonlinear semi-discrete finite element equations is discussed. In Section 3, iterative techniques are examined for the solution of the global nonlinear system of finite element equations. A large amount of computational effort is expended in the iterative phase of the solution and so the iterative procedure used must be both reliable and efficient. The performance of three iterative procedure is examined: Newton Raphson, Modified Newton Raphson and Quasi-Newton methods, including BGFS and Broyden updates. Finally, in Section 4, the elasto-plastic earthquake response analysis of a two phase nonhomogeneous earth dam is presented. Extensive documentation exists¹ for the particular problem selected including recorded earthquake motions at the base and crest of the dam. The results of the numerical calculations are compared to the recorded response of the dam.     

Seismic fracture analysis of concrete gravity dams

A numerical procedure for evaluation of the fracture process of gravity dams during strong earthquakes is presented. The BEM is used to discretize the dam reservoir system including the crack surfaces, and stress intensity factors at the crack tip are employed in a stage by stage procedure which simulates the crack extension. For each stage of constant crack length the mode superposition technique is applied; this is made possible by simulating the impact process of crack closing by a load pulse applied at the contact points which permits the structural stiffness to be assumed unchanged. To verify the proposed procedure, a cantilever beam model structure made of gypsum was tested on a shaking table. Good correlation with the numerical results was obtained, from which it is concluded that the procedure can be employed for evaluation of the crack propagation process in concrete structures subjected to dynamic loadings.     

Seismic fragility assessment of concrete gravity dams

Many concrete gravity dams have been in service for over 50 years, and over this period important advances in the methodologies for evaluation of natural phenomena hazards have caused the design‐basis events for these dams to be revised upwards. Older existing dams may fail to meet revised safety criteria and structural rehabilitation to meet such criteria may be costly and difficult. Fragility assessment provides a tool for rational safety evaluation of existing facilities and decision‐making by using a probabilistic framework to model sources of uncertainty that may impact dam performance. This paper presents a methodology for developing fragilities of concrete gravity dams to assess their performance against seismic hazards. The methodology is illustrated using the Bluestone Dam on the New River in West Virginia, which was designed in the late 1930s. The seismic fragility assessment indicated that sliding along the dam–foundation interface is likely if the dam were to be subjected to an earthquake with a magnitude of the maximum credible earthquake (MCE) specified by the U.S. Army Corps of Engineers. Moreover, there will likely be tensile cracking at the neck of the dam at this level of seismic excitation. However, loss of control of the reservoir is unlikely. Copyright © 2003 John Wiley & Sons, Ltd.     

Seismic behaviour of cracked concrete gravity dams

A finite element model of incremental displacement constraint equations (IDCE), based on an existing node‐to‐surface concept, is implemented to deal with dynamic contact surfaces in the seismic behaviour analysis of cracked concrete gravity dams. After verification for sliding, rocking and impact, the IDCE model is applied to study the seismic responses of concrete gravity dams with different profiles and crack locations for a variety of parameters, such as coefficient of friction, water level and type of earthquake, as well as impact damping based on the concept of coefficient of restitution. It is revealed that cracked concrete gravity dams can experience not only sliding and rocking modes, but also the drifting mode in some cases of crack either at the base or at a height. Downstream sliding is normally accompanied by rocking, especially for the cases of crack at a height. Due to rocking and drifting, a cracked dam may still acquire a certain amount of residual sliding even if the effective coefficient of friction is relatively high. Copyright © 2005 John Wiley & Sons, Ltd.     

Seismic risk ranking for large dams in South Africa

Earthquake motion is one of the extreme loads acting on large dams. Dam owners and regulators must therefore ensure that dams are safely operated and present minimal risk to the public in case of extreme loads such as floods and earthquakes. Owners of many dams or officials in charge of dam safety programs may consider comparative assessment of the seismic risk associated with their dams and establish priorities for detailed evaluation. South Africa has in excess of 100 large state-owned dams and the characteristics of these dams have been used to perform a basic seismic hazard assessment and rank the vulnerability of these dams from the lowest to highest. One of the most decisive factors that contributes to the risk of a dam is the wall type; with gravity and earthfill dams being the most vulnerable to earthquake motion. Another aspect that needs further investigation is the downstream hazard potential which, if known to a better degree of accuracy, can provide more reasonable estimates of the risk factors.     

Quantification of seismic potential failure modes in concrete dams

A comprehensive framework for potential failure modes (PFM) identification and quantification of concrete dams subjected to seismic excitation is presented. A quantifiable indicator of PFM is presented in the context of both linear and nonlinear analyses. As an illustrative example, a thin arch dam subjected to a set of ground motions at different seismic intensity levels is investigated and corresponding PFM quantified. An outcome of this analysis is the probabilistic‐based correlation between linear and nonlinear analyses and identification of the optimal intensity measure parameter. This study, is an adaptation and extension of well‐accepted procedures defined by the performance‐based earthquake engineering paradigm in buildings. Copyright © 2015 John Wiley & Sons, Ltd.