Analysis of seismic disaster failure mechanism and dam-break simulation of high arch dam

Based on a Chinese national high arch dam located in a meizoseismal region, a nonlinear numerical analysis model of the damage and failure process of a dam-foundation system is established by employing a 3-D deformable distinct element code(3DEC) and its re-development functions. The proposed analysis model considers the dam-foundation-reservoir coupling effect, infl uence of nonlinear contact in the opening and closing of the dam seam surface and abutment rock joints during strong earthquakes, and radiation damping of far fi eld energy dissipation according to the actual workability state of an arch dam. A safety assessment method and safety evaluation criteria is developed to better understand the arch dam system disaster process from local damage to ultimate failure. The dynamic characteristics, disaster mechanism, limit bearing capacity and the entire failure process of a high arch dam under a strong earthquake are then analyzed. Further, the seismic safety of the arch dam is evaluated according to the proposed evaluation criteria and safety assessment method. As a result, some useful conclusions are obtained for some aspects of the disaster mechanism and failure process of an arch dam. The analysis method and conclusions may be useful in engineering practice.     

Improvement of overtopping risk evaluations using probabilistic concepts for existing dams

Hydrologic risk analysis for dam safety relies on a series of probabilistic analyses of rainfall-runoff and flow routing models, and their associated inputs. This is a complex problem in that the probability distributions of multiple independent and derived random variables need to be estimated in order to evaluate the probability of dam overtopping. Typically, parametric density estimation methods have been applied in this setting, and the exhaustive Monte Carlo simulation (MCS) of models is used to derive some of the distributions. Often, the distributions used to model some of the random variables are inappropriate relative to the expected behaviour of these variables, and as a result, simulations of the system can lead to unrealistic values of extreme rainfall or water surface levels and hence of the probability of dam overtopping. In this paper, three major innovations are introduced to address this situation. The first is the use of nonparametric probability density estimation methods for selected variables, the second is the use of Latin Hypercube sampling to improve the efficiency of MCS driven by the multiple random variables, and the third is the use of Bootstrap resampling to determine initial water surface level. An application to the Soyang Dam in South Korea illustrates how the traditional parametric approach can lead to potentially unrealistic estimates of dam safety, while the proposed approach provides rather reasonable estimates and an assessment of their sensitivity to key parameters.     

Seismic hazard analysis for engineering sites based on the stochastic finite-fault method

Seismic hazard analyses are mainly performed using either deterministic or probabilistic methods. However, there are still some defects in these statistical model-based approaches for regional seismic risk assessment affected by the near-field of large earthquakes. Therefore, we established a deterministic seismic hazard analysis method that can characterize the entire process of ground motion propagation based on stochastic finite-fault simulation, and we chose the site of the Xiluodu dam to demonstrate the method. This method can characterize earthquake source properties more realistically than other methods and consider factors such as the path and site attenuation of seismic waves. It also has high computational efficiency and is convenient for engineering applications. We first analyzed the complexity of seismogenic structures in the Xiluodu dam site area, and then an evaluation system for ground motion parameters that considers various uncertainties is constructed based on a stochastic finite-fault simulation. Finally, we assessed the seismic hazard of the dam site area comprehensively. The proposed method was able to take into account the complexity of the seismogenic structures affecting the dam site and provide multi-level parameter evaluation results corresponding to different risk levels. These results can be used to construct a dam safety assessment system of an earthquake in advance that provides technical support for rapidly and accurately assessing the post-earthquake damage state of a dam, thus determining the influence of an earthquake on dam safety and mitigating the risk of potential secondary disasters.     

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.     

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.     

Quantitative failure metric for gravity dams

Quantitative failure monitoring is a critical tool for safety assessment of concrete dams. This includes damage occurrence, intensity, location, number, size, and propagation pattern. Such an assessment is essential for a quantifiable prioritization of repair and will thus reduce overall cost and improve safety. This paper will address this timely topic through the nonlinear transient analysis of a dam and failure will be ascertained through a multi‐scale damage index. A damage‐plastic model for mass concrete is used, Drucker‐Prager elasto‐plastic one for the foundation, and infinite elements are used for far‐field boundaries. Water‐dam interaction is accounted for through fluid finite elements. It is determined that the proposed damage indices can indeed provide a quantitative metric for the degree of failure in gravity dams in terms of the input dynamic motion. Copyright © 2014 John Wiley & Sons, Ltd.     

Study of the flood control scheduling scheme for the Three Gorges Reservoir in a catastrophic flood

The Three Gorges Project is the world's largest water conservancy project. According to the design standards for the 1,000‐year flood, flood diversion areas in the Jingjiang reach of the Yangtze River must be utilized to ensure the safety of the Jingjiang area and the city of Wuhan. However, once these areas are used, the economic and life loss in these areas may be very great. Therefore, it is vital to reduce this loss by developing a scheme that reduces the use of the flood diversion areas through flood regulation by the Three Gorges Reservoir (TGR), under the premise of ensuring the safety of the Three Gorges Dam. For a 1,000‐year flood on the basis of a highly destructive flood in 1954, this paper evaluates scheduling schemes in which flood diversion areas are or are not used. The schemes are simulated based on 2.5‐m resolution reservoir topography and an optimized model of dynamic capacity flood regulation. The simulation results show the following. (a) In accord with the normal flood‐control regulation discharge, the maximum water level above the dam should be not more than 175 m, which ensures the safety of the dam and reservoir area. However, it is necessary to utilize the flood diversion areas within the Jingjiang area, and flood discharge can reach 2.81 billion m³. (b) In the case of relying on the TGR to impound floodwaters independently rather than using the flood diversion areas, the maximum water level above the dam reaches 177.35 m, which is less than the flood check level of 180.4 m to ensure the safety of the Three Gorges Dam. The average increase of the TGR water level in the Chongqing area is not more than 0.11 m, which indicates no significant effect on the upstream reservoir area. Comparing the various scheduling schemes, when the flood diversion areas are not used, it is believed that the TGR can execute safe flood control for a 1,000‐year flood, thereby greatly reducing flood damage.     

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.     

Reliability and sensitivity analysis of wedge stability in the abutments of an arch dam using artificial neural network

In this study, the seismic stability of arch dam abutments is investigated within the framework of the probabilistic method. A large concrete arch dam is considered with six wedges for each abutment. The seismic safety of the dam abutments is studied with quasi-static analysis for different hazard levels. The Londe limit equilibrium method is utilized to calculate the stability of the wedges in the abutments. Since the finite element method is time-consuming, the neural network is used as an alternative for calculating the wedge safety factor. For training the neural network, 1000 random samples are generated and the dam response is calculated. The direction of applied acceleration is changed within 5-degree intervals to reveal the critical direction corresponding to the minimum safety factor. The Latin hypercube sampling (LHS) is employed for sample generation, and the safety level is determined with reliability analysis. Three sample numbers of 1000, 2000 and 4000 are used to examine the average and standard deviation of the results. The global sensitivity analysis is used to identify the effects of random variables on the abutment stability. It is shown that friction, cohesion and uplift pressure have the most significant effects on the wedge stability variance.

     

Dynamic centrifuge tests of concrete dam

Dynamic tests of a concrete gravity dam are, for the first time, performed inside a centrifuge. Details of the experimental procedure, data interpretation, and results are presented. It is shown (in conjunction with a parallel paper) that these tests cannot only provide a direct assessment of certain aspects of dam safety, but more importantly provide a data base for possible non‐linear finite element code validation. Copyright © 2005 John Wiley & Sons, Ltd.     

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.     

Seismic upgrade of hydraulic fill dam by buttressing

The vulnerability of hydraulic fill dams under strong earthquake shaking has long been recognized. When located in areas of high seismic hazard, seismic upgrading of these types of dams is often required to meet current dam safety standards. Selection of an appropriate design concept for seismic upgrading of such dams requires careful consideration of seismically induced deformations when the hydraulic fill is to remain as part of the dam. This paper presents a case history of the seismic upgrade of Butt Valley Dam, a hydraulic fill dam located in Northern California. The dam was strengthened to withstand the Maximum Credible Earthquake (MCE) by buttressing of its upstream and downstream slopes. The paper discusses the evaluation of alternatives to upgrade the dam, the design criteria, and the design and analysis of the seismic upgrade. It is shown that a conservative and robust design was developed based on well-established engineering principles and multiple lines of defense, and sound use of analysis procedures including finite-difference non-linear dynamic deformation analyses.     

Seismic safety analysis of tall concrete dams,investigation and insights on critical challenges

This paper discusses critical and potentially controversial issues related to the seismic safety of tall concrete dams. These include the seismic input at a dam site, the effective treatment of the damage-rupture process, and the consideration of compressibility of reservoir water for hydrodynamic pressure. Major challenges to currently popular but questionable treatments of these critical problems are presented. Insights and additional research on these critical challenges are emphasized and explained based on prior published works of the author. More reasonable alternatives to dealing with these potentially controversial problems are provided in light of engineering practice in China. First, the design seismic input at depth as deconvoluted from an arbitrarily selected recorded accelerogram at a control point of an artificially developed free-field surface with the elevation of the dam crest is difficult for engineering projects to accept as appropriate. It may be more reasonable to use the design seismic incident motions as half of the ground surface motions from seismic safety analyses obtained from deterministic or probability approaches conducted by seismologists according to approved standards or guidelines. Second, since seismic damage to the dam must be estimated separately following uniaxial tensile and compressive experimental damage evolution rules, a simplified and realistic nonlinear elastic model is proposed as an alternative to the plastic-damage coupling model, which is very complex and includes assumptions based on a number of uncertainties. Finally, the effect of the reflection coefficient for compressibility of reservoir water on hydrodynamic pressures is very sensitive. The notion that the applied unified reflection coefficient at the reservoir bottom could be frequency-dependent and exhibit a significant variability in space as confirmed by field tests is questionable. To neglect the compressibility of reservoir water it may be closer to engineering practice at present.     

A hybrid information fusion approach to safety risk perception using sensor data under uncertainty

This paper presents a hybrid information fusion approach that integrates the cloud model and the D–S evidence theory to perceiving safety risks using sensor data under uncertainty. The cloud model provides an uncertain transforming tool between qualitative concepts and their quantitative expressions and uses the measurement of correlation to construct Basic Probability Assignments. An improved evidence aggregation strategy that combines the Dempster’ rule and the weighted mean rule is developed to get rid of counter-intuitive dilemma existing in a combination of high-conflict evidence. A three-layer information fusion framework consisting of sensor fusion, factor fusion, and area fusion is proposed to synthesize multi-source information to get the final fusion results. The developed cloud D–S approach is applied to the assessment of the safety of a real tailings dam in operation in China as a case study. Data information acquired from 28 monitoring sensors is fused in a continuous manner in order to obtain the overall safety level of the tailings dam. Results indicate that the developed approach is capable of achieving multi-layer information fusion and identifying global sensitivities of input factors under uncertainty. The developed approach proves to perform a strong robustness and fault-tolerant capacity, and can be used by practitioners in the industry as a decision tool to perceive and anticipate the safety risks in tailings dams.     

基于分布式光纤传感的堤坝安全监测技术及展望

及时、准确地掌握堤坝的性态，是保证堤坝安全运行的基础.因此，需要一种实时监测系统对堤坝进行监测.分析了堤坝健康监测的主要内容，介绍了分布式光纤传感技术的工作原理及使用现状；探讨了分布式光纤传感技术在我国堤坝渗流和形变中的应用；指出了分布式光纤传感技术在土堤坝使用中存在的问题，并给出了新的堤坝长期安全监测系统设计建议，结论有益于指导构建新的基于分布式光纤技术的堤坝安全监测系统.     

水库地震预测与风险评估系统研制

水库地震预测与风险评估系统(RISaRiskAS 2007 for Window),是专门用于水库地震数据管理和信息查询、最大震级预测和潜在地震危险性与风险分析的系统,该系统具有很强的分析处理功能,可用于水库地震研究、大坝安全运行决策辅助支持和水库地震应急信息查询等。文中介绍了该系统的设计思路和主要功能     

Seismic stability assessment of an arch dam-foundation system

A seismic stability assessment of arch dam-foundation systems is presented using a comprehensive approach,in which the main factors that significantly influence the seismic response of an arch dam-foundation system are considered.A large scale finite element model with over 1 million degrees of freedom is constructed for the Baihetan arch dam(289 m high),which is under construction in the Southwest of China.In particular,the complicated geological conditions with faults intersecting interlayer shear weakness zones at the dam base and the dam abutment resisting force body is modeled in the analysis.Three performance indices are adopted to assess the seismic stability of the arch dam.The results demonstrate that the opening of the joints of the Baihetan arch dam is small and the water stop installed between the joints would not be torn during a design earthquake.The yielding formed in the interface between the dam and foundation does not reach the grouting curtain that would remain in an elastic state after an earthquake.The yielding zones occurring on the upper portion of the dam faces extend 1/8 thickness of block section into the dam body and thus cantilever blocks need not be concerned with sliding stability.The faults and interlayer shear weakness zones in the near field foundation exhibit severe yielding,and a potential sliding surface is penetrated.Although the factor of safety against sliding of the surface fluctuates with a decreased trend during an earthquake,the minimum instantaneous value reaches 1.02 and is still larger than 1.0.Therefore,a conclusion is drawn that the Baihetan arch dam-foundation system will remain stable under the design earthquake.     

基于PSO-AHP的大坝致灾因子权重计算

提出了一种基于改进粒子群优化-层次分析法（SELPSO-AHP）的致灾因子权重计算模型,该模型依据大坝现场检测、原型监测和安全定期检查等成果,由层次分析法构建大坝风险分析的层次结构体系。为保证判断矩阵的一致性,引入改进粒子群算法和罚函数法用于层次分析法的权值计算,取得了比遗传算法更精确、更稳定的结果。将所建立的模型用于某混凝土重力坝的运行风险分析,得到了影响该坝运行风险的主要因素,为保障大坝安全提供了依据。     

Risk analysis for clustered check dams due to heavy rainfall

Check dams are commonly constructed around the world for alleviating soil erosion and preventing sedimentation of downstream rivers and reservoirs.Check dams are more vulnerable to failure due to their less stringent flood control standards compared to other dams.Determining the critical precipitation that will result in overtopping of a dam is a useful approach to assessing the risk of failure on a probabilistic basis and for providing early warning in case of an emergency.However,many check dams are built in groups,spreading in several tributaries in cascade forms,comprising a complex network.Determining the critical precipitation for dam overtopping requires a knowledge of its upstream dams on whether they survived or were overtopped during the same storm,while these upstream dams in turn need the information for their upstream dams.The current paper presents an approach of decomposing the dam cluster into(1)the heading dam,(2)border dams,and(3)intermediate dams.The algorithm begins with the border dams that have no upstream dams and proceeds with upgraded maps without the previous border dams until all the dams have been checked.It is believed that this approach is applicable for small-scale check dam systems where the time lag of flood routing can be neglected.As a pilot study,the current paper presents the analytical results for the Wangmaogou Check Dam System that has 22 dams connected in series and parallel.The algorithm clearly identified 7 surviving dams,with the remaining ones being overtopped for a storm of 179.6 mm in 12 h,which is associated with a return period of one in 200 years.     

Sliding response of gravity dams including vertical seismic accelerations

Seismic safety assessment of gravity dams has become a major concern in many regions of the world while the effects of vertical seismic accelerations on the response of structures remain poorly understood. This paper first investigates the effect of including vertical accelerations in the sliding response analysis of gravity dams subjected to a range of historical ground motion records separated in two groups according to their source-to-site distance. Analyses showed that the incidence of vertical accelerations on the sliding response of gravity dams is significantly higher for near-source records than for farsource records. The pseudo-static 30% load combination rule, commonly used in practice to account for the non-simultaneous occurrence of the peak horizontal and vertical accelerations, yielded good approximations of the minimum safety factors against sliding computed from time-history analyses. A method for empirically estimating the vertical response spectra based on horizontal spectra, accounting for the difference in frequency content and amplitudes between the two components is investigated. Results from analyses using spectrum compatible horizontal and vertical synthetic records also approximated well the sliding response of a gravity dam subjected to series of simultaneous horizontal and vertical historical earthquake records.