Seismic evaluation of concrete dams via continuum damage models

In this paper a general methodology for the analysis of large concrete dams subjected to seismic excitation is outlined. It is valid both for gravity dams (2D representation) and arch dams (3D representation). The method allows for non-linear material behaviour of the dam, ‘transparent fictitious boundaries’ for dealing properly with in-coming and out-going seismic waves, and an efficient procedure to deal with dam-soil-fluid interaction. The mechanical behaviour of concrete is modelled using an isotropic damage model which allows for tension and compression damage, and exhibits stiffness recovery upon load reversals. Emphasis is placed in the treatment of fluid-structure interaction, regarding both formulation and efficiency aspects. A gravity dam and an arch dam are analysed subjected to artificially generated earthquakes of different intensities, and the results are used to study the degree of (un)safety of the dams.     

The use of 3D modeling for the prediction of the seismic demands on the gravity dams

Seismic behavior of gravity dams has long been evaluated using a representative two‐dimensional (2D) system. Formulated for the gravity dams built in wide canyons, the assumption is nevertheless utilized extensively for almost all concrete dams due to the established procedures as well as the expected computational costs of a three‐dimensional model. However, a significant number of roller‐compacted concrete dams, characterized as such systems, do not conform to the basic assumptions of these methods by violating the conditions on canyon dimensions and joint‐spacing/details. Based on the premise that the 2D modeling assumption is overstretched for practical purposes in a variety of settings, the purpose of this study is to critically evaluate the use of 2D modeling for the prediction of the seismic demands on these systems. Using a rigorous soil–structure interaction approach, the difference between the two and three‐dimensional response for gravity dams was investigated first in the frequency domain for a range of canyon widths and foundation to dam moduli ratios. Then, the time domain differences between the crest displacements and the maximum principal stress were obtained using 70 different ground motions in order to show the possible bias introduced into the analysis results due to the modeling approach. The results of the study show that even for relatively wide canyons, the 2D analysis can lead to misleading predictions. Copyright © 2017 John Wiley & Sons, Ltd.     

A continuum damage concrete model for earthquake analysis of concrete gravity dam–reservoir systems

In this study, the earthquake damage response of the concrete gravity dams is investigated with considering the effects of dam–reservoir interaction. A continuum damage model which is a second-order tensor and includes the strain softening behavior is selected for the concrete material. The mesh-dependent hardening technique is adopted such that the fracture energy dissipated is not affected by the finite element mesh size. The dynamic equilibrium equations of motion are solved by using the improved form of the HHT-α time integration algorithm. Two dimensional seismic analysis of Koyna gravity dam is performed by using the 1967 Koyna earthquake records. The effects of damage on the earthquake response of concrete gravity dams are discussed. Comparison of the Westergaard and Lagrangian dam–reservoir interaction solutions is made. The effects of viscous damping ratio on the damage response of the dam are also studied.     

Earthquake safety assessment of concrete arch and gravity dams

Based on research studies currently being carried out at Dalian University of Technology, some important aspects for the earthquake safety assessment of concrete dams are reviewed and discussed. First, the rate-dependent behavior of concrete subjected to earthquake loading is examined, emphasizing the properties of concrete under cyclic and biaxial loading conditions. Second, a modified four-parameter Hsieh-Ting-Chen viscoplastic consistency model is developed to simulate the rate-dependent behavior of concrete. The earthquake response of a 278m high arch dam is analyzed, and the results show that the strain-rate effects become noticeable in the inelastic range. Third, a more accurate non-smooth Newton algorithm for the solution of three-dimensional frictional contact problems is developed to study the joint opening effects of arch dams during strong earthquakes. Such effects on two nearly 300m high arch dams have been studied. It was found that the canyon shape has great influence on the magnitude and distribution of the joint opening along the dam axis. Fourth, the scaled boundary finite element method presented by Song and Wolf is employed to study the dam-reservoir-foundation interaction effects of concrete dams. Particular emphases were placed on the variation of foundation stiffness and the anisotropic behavior of the foundation material on the dynamic response of concrete dams. Finally, nonlinear modeling of concrete to study the damage evolution of concrete dams during strong earthquakes is discussed. An elastic-damage mechanics approach for damage prediction of concrete gravity dams is described as an example. These findings are helpful in understanding the dynamic behavior of concrete dams and promoting the improvement of seismic safety assessment methods.     

Damage evaluation of concrete gravity dams under mainshock–aftershock seismic sequences

A large mainshock may trigger numerous aftershocks within a short period, and large aftershocks have the potential to cause additional cumulative damage to structures. This paper investigates the effects and potential of aftershocks on the accumulated damage of concrete gravity dams. For that purpose, 30 as-recorded mainshock–aftershock seismic sequences are considered in this study, and a typical two-dimensional gravity dam model subjected to the selected as-recorded seismic sequences is modeled. A Concrete Damaged Plasticity (CDP) model including the strain hardening or softening behavior is selected for the concrete material. This model is used to evaluate the nonlinear dynamic response and the seismic damage process of Koyna dam under mainshock–aftershock seismic sequences. According to the characteristics of the cracking damage development, the local and global damage indices are both established to study the influence of strong aftershocks on the cumulative damage of concrete gravity dams. From the results of this investigation, it is found that the as-recorded sequences of ground motions have a significant effect on the accumulated damage and on the design of concrete gravity dams.     

The effects of reservoir geometry on the seismic response of gravity dams

Conventional seismic analysis of gravity dams assumes that the behaviour of the dam–water–soil system can be represented using a 2‐D model since dam vertical contraction joints between blocks allow them to vibrate independently from each other. The 2‐D model assumes the reservoir to be infinite and of constant width, which is not true for certain types of reservoirs. In this paper, a boundary element method (BEM) model in the frequency domain is used to investigate the influence of the reservoir geometry on the hydrodynamic dam response. Important conceptual conclusions about the dam–reservoir system behaviour are obtained using this model. The results show that the reservoir shape influences the seismic response of the dam, making it necessary to account for 3‐D effects in order to obtain accurate results. In particular, the 3‐D pressure and displacement responses can be substantially larger than those computed with the 2‐D model. Copyright © 2007 John Wiley & Sons, Ltd.     

Numerical simulation of reinforcement strengthening for high‐arch dams to resist strong earthquakes

This paper focuses on analyzing the nonlinear seismic response of high‐arch dams with cantilever reinforcement strengthening. A modified embedded‐steel model is presented to evaluate the effects of the strengthening measure on alleviating the extension and opening of cracks under strong earthquakes. By stiffening reinforced steel, this model can easily consider the steel–concrete interaction for lightly reinforced concrete (RC) members without the need of dividing them into RC and plain concrete zones. The new tensile constitutive relations of reinforced steel are derived from the load–deformation relationship of RC members in direct tension. This model has been implemented in the finite element code and its applicability is verified by two numerical simulations for RC tests. Subsequently, numerical analyses for a 210‐m high‐arch dam (Dagangshan arch dam) are conducted with and without the presence of cantilever reinforcement. Numerical results show that reinforcement strengthening can reduce the nonlinear response of the arch dam, e.g. joint opening and crest displacement, and limit the extension and opening width of concrete cracks. Copyright © 2008 John Wiley & Sons, Ltd.     

混凝土重力坝FRP片材表面加固抗震性能研究

如何提高混凝土重力坝薄弱位置的抗震性能是国内外大坝抗震研究的热点问题。本文基于等价静力非线性方法,采用考虑混凝土细观非均匀特性的混凝土损伤模型,研究FRP片材表面加固大坝薄弱位置的抗震有效性。以2座不同形态的混凝土重力坝A、B为例,分别进行坝踵FRP片材表面加固研究和折坡处FRP片材表面加固研究,分析加固前后坝体的应力状态、裂缝扩展情况和破坏形态。数值模拟结果表明：坝踵处采用FRP片材加固可以很好地增强坝体的抗震性能,有效地抑制裂缝的产生和发展;折坡处采用FRP片材加固在一定程度上可以提高坝体的抗震性能,下游坝身加固与否对提高大坝的抗震性能影响不大。     

Seismic failure modeling of concrete dams considering heterogeneity of concrete

Study on the failure process of high concrete dams subjected to strong earthquakes is crucial to reasonable evaluation of their seismic safety. Numerical simulation in this aspect involves dynamic failure analysis of big bulk concrete dam subjected to cyclic loading. The Rock Failure Process Analysis (RFPA) proposed by C.A. Tang, with successful applications to failure modeling of rock and concrete specimens mainly subjected to static loading, is extended for this purpose. For using the proposed model, no knowledge on the cracking route needs to be known beforehand, and no remeshing is required. Simulation of the whole process of elastic deformation, initiation and propagation of microcracks, severe damage and ultimate failure of concrete dams in earthquakes with a unified model is enabled. The model is verified through a shaking table test of an arch dam. Finally a practical gravity dam is employed as a numerical example. Considering the uncertainty in ground motion input and concrete material, typical failure process and failure modes of gravity dam are presented. Several small cracks may occur due to tension particularly at dam neck, dam faces and dam heel, and a few of them evolve into dominant ones. Relatively smaller earthquake may cause damage to the dam neck while a bigger one may bring on cracks at lower parts of the dams. Cracking at the dam bottom may incline to a direction almost perpendicular to the downstream face after propagating horizontally for a certain distance when the shaking is strong enough.     

Seismic failure modeling of concrete dams considering heterogeneity of concrete

Study on the failure process of high concrete dams subjected to strong earthquakes is crucial to reasonable evaluation of their seismic safety. Numerical simulation in this aspect involves dynamic failure analysis of big bulk concrete dam subjected to cyclic loading. The Rock Failure Process Analysis (RFPA) proposed by C.A. Tang, with successful applications to failure modeling of rock and concrete specimens mainly subjected to static loading, is extended for this purpose. For using the proposed model, no knowledge on the cracking route needs to be known beforehand, and no remeshing is required. Simulation of the whole process of elastic deformation, initiation and propagation of microcracks, severe damage and ultimate failure of concrete dams in earthquakes with a unified model is enabled. The model is verified through a shaking table test of an arch dam. Finally a practical gravity dam is employed as a numerical example. Considering the uncertainty in ground motion input and concrete material, typical failure process and failure modes of gravity dam are presented. Several small cracks may occur due to tension particularly at dam neck, dam faces and dam heel, and a few of them evolve into dominant ones. Relatively smaller earthquake may cause damage to the dam neck while a bigger one may bring on cracks at lower parts of the dams. Cracking at the dam bottom may incline to a direction almost perpendicular to the downstream face after propagating horizontally for a certain distance when the shaking is strong enough.     

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.     

拱坝非线性地震反应分析

以乌东德拱坝为对象,进行了拱坝非线性地震反应分析。结果表明:坝体损伤区域明显大于坝体混凝土按线弹性材料计算时最大主应力超过抗拉强度的区域,说明传统方法低估了拱坝的开裂范围,偏于危险;考虑基岩的非线性后,坝体损伤总体上要有所减小;坝基岩体剪胀角对坝基岩体的屈服和坝体混凝土的损伤都有影响,但是规律不明确,因此应该进行基岩剪胀角的敏感性分析。     

Nonlinear earthquake analysis of high arch dam–water–foundation rock systems

A nonlinear finite element model for earthquake response analysis of arch dam–water–foundation rock systems is proposed in this paper. The model includes dynamic dam–water and dam–foundation rock interactions, the opening of contraction joints, the radiation damping of semi‐unbounded foundation rock, the compressibility of impounded water, and the upstream energy propagating along the semi‐unbounded reservoir. Meanwhile, a new equivalent force scheme is suggested to achieve free‐field input in the model. The effects of the earthquake input mechanism, joint opening, water compressibility, and radiation damping on the earthquake response of the Ertan arch dam (240 m high) in China are investigated using the proposed model. The results show that these factors significantly affect the earthquake response of the Ertan arch dam. Such factors should therefore be considered in the earthquake response analysis and earthquake safety evaluation of high arch dams. Copyright © 2011 John Wiley & Sons, Ltd.     

Seismic assessment of concrete gravity dams using capacity estimation and damage indexes

A new concept to determine state of the damage in concrete gravity dams is introduced. The Pine Flat concrete gravity dam has been selected for the purpose of the analysis and its structural capacity, assuming no sliding plane and rigid foundation, has been estimated using the two well‐known methods: nonlinear static pushover (SPO) and incremental dynamic analysis (IDA). With the use of these two methods, performance and various limit states of the dam have been determined, and three damage indexes have been proposed on the basis of the comparison of seismic demands and the dam's capacity. It is concluded that the SPO and IDA can be effectively used to develop indexes for seismic performance evaluation and damage assessment of concrete gravity dams. Copyright © 2012 John Wiley & Sons, Ltd.     

A unified approach for long-term behavior and seismic response of AAR-affected concrete dams

Concrete dams suffering from alkali-aggregate reaction (AAR) exhibit swelling and deterioration of concrete or even cracking over a long period. The deterioration of concrete may significantly affect the dynamic behavior of the structures, and it is necessary to estimate seismic safety of the deteriorated dams subjected to strong earthquakes. A unified approach is presented in this paper for long-term behavior and seismic response analysis of AAR-affected concrete dams by combining AAR kinetics, effects of creep and plastic-damage model in the finite element method. The proposed method is applied to a gravity dam and an arch dam. The long-term behavior of the AAR-affected dams is first predicted in terms of anisotropic swelling, spatially non-uniform deterioration of concrete, and cracking initiation and propagation with the development of AAR. The seismic response of the deteriorated dams is subsequently analyzed based on the state of the structures at the end of the long-term analysis. The AAR-induced expansion displacements obtained from the proposed method are in good agreement with the measured ones in the long-term operation. The simulated cracking patterns in the dams caused by the continuing AAR are also similar to the field observation. The results from the seismic analysis show that AAR-induced deterioration of concrete and cracking may lead to more severe damage cracking in the dams during earthquake. The dynamic displacements are also increased compared with the dams that are not suffering from AAR. The seismic safety of the AAR-affected concrete dams is significantly reduced because of the AAR-induced deterioration of concrete and cracking.     

Seismic performance sensitivity and uncertainty analysis of gravity dams

Uncertainties in structural engineering are often arising from the modeling assumptions and errors, or from variability in input loadings. A practical approach for dealing with them is to perform sensitivity and uncertainty analysis in the framework of stochastic and probabilistic methods. These analyses can be statically and dynamically performed through nonlinear static pushover and IDA techniques, respectively. Of the existing structures, concrete gravity dams are infrastructures which may encounter many uncertainties. In this research, probabilistic analysis of the seismic performance of gravity dams is presented. The main characteristics of the nonlinear tensile behavior of mass concrete, along with the intensity of earthquake excitations are considered as random variables in the probabilistic analysis. Using the tallest non‐overflow monolith of the Pine Flat gravity dam as a case study, its response under static and dynamic situations is reliably examined utilizing different combinations of parameters in the material and the seismic loading. The sensitivity analysis reveals the relative importance of each parameter independently. It will be shown that the undamaged modulus of elasticity and tensile strength of mass concrete have more significant roles on the seismic resistance of the dam than the ultimate inelastic tensile strain. In order to propagate the parametric uncertainty to the actual seismic performance of the dam, probabilistic simulation methods such as Monte Carlo simulation with Latin hypercube sampling, and approximate moment estimation techniques will be used. The final results illustrate the possibility of using a mean‐parameter dam model to estimate the mean seismic performance of the dam. Copyright © 2014 John Wiley & Sons, Ltd.     

SV波斜入射下混凝土重力坝易损性分析

易损性分析是评估不同强度地震作用下混凝土重力坝各级破坏概率的有效方法。目前重力坝易损性分析通常假定地震波为垂直入射,然而在近断层区域,地震波往往是倾斜入射的,地震波斜入射对重力坝地震响应有显著影响。从太平洋地震工程研究中心数据库选取16条地震动记录,采用黏弹性人工边界结合等效节点荷载实现SV波斜入射波动输入。采用增量动力分析方法对地震动峰值加速度进行调幅,以印度Koyna混凝土重力坝为研究对象,以坝顶相对位移为抗震性能指标,建立SV波斜入射下重力坝不同震损等级的易损性曲线。结果表明,与垂直入射相比,相同震损等级和相同地震动强度下,斜入射时重力坝破坏概率减小;当PGA接近重力坝实际遭受的地震动强度时,入射角为15°和30°时破坏概率与垂直入射相比最大减小率分别为27.3%和68.2%;各地震强度下,15°和30°斜入射相对于垂直入射的破坏概率差异值最大分别达36.6%、83.9%。因此,混凝土重力坝抗震性能分析应考虑地震波斜入射的影响。研究结果也可为近断层区域混凝土重力坝安全风险评估提供参考。     

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.     

Integrative seismic safety evaluation of a high concrete arch dam

An integrative seismic safety evaluation of an arch dam should include all sources of nonlinearities, dynamic interactions between different components and the external loads. The present paper investigates the calibration procedure and nonlinear seismic response of an existing high arch dam. The first part explains the conducted analyses for the static and thermal calibrations of the dam based on site measurements. The second part investigates the nonlinear seismic analysis of the calibrated model considering the effect of joints, cracking of mass concrete, reservoir–dam–rock interaction, hydrodynamic pressure inside the opened joints and the geometric nonlinearity. Penetration of the water inside the opened joints accelerates the damage process. The integrative seismic assessment of a case study shows that the dam will fail under the maximum credible earthquake scenario. The dam is judged to be severely damaged with extensive cracking and the joints undergo opening/sliding. A systematic procedure is proposed for seismic and post-seismic safety of dams.     

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.