Seismic performance of earth-core and concrete-faced rock-fill dams by large-scale shaking table tests

Two of China's highest earth-core rock-fill dams (ECRDs) and concrete-faced rock-fill dams (CFRDs) were simulated by large-scale earthquake simulation shaking table tests in this work. A series of staged tests were performed, including white noise, different types of earthquake excitations with different magnitudes etc. The seismic performance of the ECRD and CFRD models were analyzed and investigated. The test results indicated that reservoir impoundment influenced the structure and seismic characteristics of the ECRD model much more than the CFRD model. The average fundamental frequency of the CFRD decreased less than the ECRD model when subjected to strong excitation. The acceleration amplification factors decreased as the input peak acceleration increased. The maximum acceleration occurred at the top of the ECRD model, while it occurred at 0.6–0.9 dam height of the CFRD model. Seismic residual deformations of the two models were very small. When subjected to strong earthquake excitation, the residual deformation of the CFRD model was smaller than that of the ECRD model. The dominant failure pattern of the two models was shallow sliding at the height of 3/4 on the downstream slope. The above analysis indicated that seismic performance of CFRD was superior to ECRD.     

Seismic behavior of an inverted T-shape flexible retaining wall via dynamic centrifuge tests

In the design procedure for a retaining wall, the pseudo-static method has been widely used and dynamic earth pressure is calculated by the Mononobe–Okabe method, which is an extension of Coulomb’s earth pressure theory computed by force equilibrium. However, there is no clear empirical basis for treating the seismic force as a static force, and recent experimental research has shown that the Mononobe–Okabe method is quite conservative, and there exists a discrepancy between the assumed conditions and real seismic behavior during an earthquake. Two dynamic centrifuge tests were designed and conducted to reexamine the Mononobe–Okabe method and to evaluate the seismic lateral earth pressure on an inverted T-shape flexible retaining wall with a dry medium sand backfill. Results from two sets of dynamic centrifuge experiments show that inertial force has a significant impact on the seismic behavior on the flexible retaining wall. The dynamic earth pressure at the time of maximum moment during the earthquake was not synchronized and almost zero. The relationship between the back-calculated dynamic earth pressure coefficient at the time of maximum dynamic wall moment and the peak ground acceleration obtained from the wall base peak ground acceleration indicates that the seismic earth pressure on flexible cantilever retaining walls can be neglected at accelerations below 0.4 g. These results suggest that a wall designed with a static factor of safety should be able to resist seismic loads up to 0.3–0.4 g.     

Dynamic failure numeric simulations of model concrete-faced rock-fill dam

Conventional numeric simulations of rock-fill dams are generally performed finite element method (FEM), in which the rock-fill body is treated as continuum material. But the rock-fill body possesses strong discontinuity and FEM based on continuum idealization cannot simulate its failure process. The discontinuous deformation analysis (DDA) method is just the right tool of solving this problem satisfactorily. In this paper, two kinds of model dams, i.e. homogeneous rock-fill dam and concrete-faced rock-fill dam, are simulated using DDA method, their characteristics of response and failure process are presented. The results from numerical simulations are consistent with those from the author's previous dynamic experiments.     

Large-scale shaking table model tests of aseismic measures for concrete faced rock-fill dams

The aseismic design of concrete faced rock-fill dams (CFRDs) has become vitally important in recent years. A series of 1-g large-scale shaking table model tests were conducted, applying similitude laws to evaluate the effectiveness of various aseismic measures. The following four reinforcing measures are discussed in this study: reinforcing the top zone with geogrid, protecting the downstream slope surface using stone block, replacing the top zone of the dam with cemented rock-fill material or concrete layers. The failure developments were visualised using the particle image velocimetry (PIV) technique. The mechanisms of these reinforcement measures were analysed in detail. The effectiveness of these measures was evaluated by comparing the crest settlement rates and key accelerations corresponding to surface grain yield, slab fracture and slab dislocation. Based on the model test results, a combination of reinforcement measures was proposed for the prototype CFRDs.     

An investigation into the seismic behaviour of dams using dynamic centrifuge modelling

Dynamic response of dams is significantly influenced by foundation stiffness and dam-foundation interaction. This in turn, significantly effects the generation of hydrodynamic pressures on upstream face of a concrete dam due to inertia of reservoir water. This paper aims at investigating the dynamic response of dams on soil foundation using dynamic centrifuge modelling technique. From a series of centrifuge tests performed on model dams with varying stiffness and foundation conditions, significant co-relation was observed between the dynamic response of dams and the hydrodynamic pressures developed on their upstream faces. The vertical bearing pressures exerted by the concrete dam during shaking were measured using miniature earth pressure cells. These reveal the dynamic changes of earth pressures and changes in rocking behaviour of the concrete dam as the earthquake loading progresses. Pore water pressures were measured below the dam and in the free-field below the reservoir. Analysis of this data provides insights into the cyclic shear stresses and strains generated below concrete dams during earthquakes. In addition, the sliding and rocking movement of the dam and its settlement into the soil below are discussed.     

Influence of seismic wave type and incident direction on the dynamic response of tall concrete-faced rockfill dams

Owing to the stochastic behavior of earthquakes and complex crustal structure, wave type and incident direction are uncertain when seismic waves arrive at a structure. In addition, because of the different types of the structures and terrains, the traveling wave effects have different influences on the dynamic response of the structures. For the tall concrete-faced rockfill dam (CFRD), it is not only built in the complex terrain such as river valley, but also its height has reached 300 m level, which puts forward higher requirements for the seismic safety of the anti-seepage system mainly comprising concrete face slabs, especially the accurate location of the weak area in seism. Considering the limitations of the traditional uniform vibration analysis method, we implemented an efficient dynamic interaction analysis between a tall CFRD and its foundation using a non-uniform wave input method with a viscous-spring artificial boundary and equivalent nodal loads. This method was then applied to investigate the dynamic stress distribution on the concrete face slabs for different seismic wave types and incident directions. The results indicate that dam-foundation interactions behave differently at different wave incident angles, and that the traveling wave effect becomes more evident in valley topography. Seismic wave type and incident direction dramatically influenced stress in the face slab, and the extreme stress values and distribution law will vary under oblique wave incidence. The influence of the incident direction on slab stress was particularly apparent when SH-waves arrived from the left bank. Specifically, the extreme stress values in the face slab increased with an increasing incident angle. Interestingly, the locations of the extreme stress values changed mainly along the axis of the dam, and did not exhibit large changes in height. The seismic safety of CFRDs is therefore lower at higher incident angles from an anti-seepage perspective. Therefore, it is necessary to consider both the seismic wave type and incident direction during seismic capacity evaluations of tall CFRDs.     

Nonlinear seismic response of tall concrete-faced rockfill dams in narrow canyons

The seismic behavior of tall concrete face rockfill dams in narrow canyons is investigated, based on numerical simulation of the staged construction, creep settlements, reservoir impoundment and seismic shaking of the dam. The study takes into account the flexibility of the canyon rock, the hydrodynamic effects and potential dynamic rockfill settlements. The static analysis uses a hyperbolic model for the rockfill, whereas the dynamic analysis uses a nonlinear hysteretic model, which accounts for the initial dynamic stiffness and produces hysteresis loops in agreement with the experimental data regarding the shear modulus and damping ratio. A damage plasticity model is used for the reinforced concrete, whereas frictional contact behavior is considered at the base and vertical walls of the concrete slab panels. An existing 150-m-high dam is used to investigate some key issues on the seismic behavior of such dams subjected to upstream−downstream and vertical excitation. Emphasis is placed on the evaluation of the tensile stresses within the slab panels, the compressive stresses at the slab-to-slab vertical interfaces and the opening of the joints. Moreover, the effect of potential dynamic settlements on both the slab stresses and joint openings is investigated. Recommendations for increasing the dam safety and reducing the water leakage through the dam body are given.     

Comparison between fe prediction and results from dynamic centrifuge tests on tilting gravity walls

An analytical model is developed to analyze the seismic response of gravity walls retaining and founded on dry sand, with special emphasis on tilting behaviour. A well verified two-dimensional finite element code is used for this purpose. The analytical model is verified by comparing predictions to results from three dynamic centrifuge tests, with satisfactory agreement. Moreover, sensitivity analyses are carried out for one of the centrifuge test conditions to understand how the results would change if the boundary conditions and rotational stiffness of the wall were changed.     

猴子岩高面板坝振动台模型试验——大坝结构动力特性研究

利用大型振动台模型试验,测得了猴子岩高面板堆石坝缩尺模型坝的动力特性参数,包括大坝结构的自振频率、阻尼比和振型系数等;分析了多种因素对坝体动力特性的影响规律;根据模型试验相似率推算得到原型坝的相应动力特性参数。研究表明:大坝结构有相对稳定的振型;坝体的动力特性参数值受激振白噪声强度和振动历史等因素影响;水库蓄水使得模型坝结构自振频率小幅提升。这些试验结果和研究结论,可以为该坝的动力分析提供基本资料和定性参考。     

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.     

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

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).     

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.     

Seismic stability of concrete gravity dams strengthened by rockfill buttressing

Rockfill buttressing resting on the downstream face of masonry or concrete gravity dam is often considered as a strengthening method to improve the stability of existing dam for hydrostatic and seismic loads. Simplified methods for seismic stability analysis of composite concrete-rockfill dams are discussed. Numerical analyses are performed using a nonlinear rockfill model and nonlinear dam-rockfill interface behavior to investigate the effects of backfill on dynamic response of composite dams. A typical 35 m concrete gravity dam, strengthened by rockfill buttressing is considered. The results of analyses confirm that backfill can improve the seismic stability of gravity dams by exerting pressure on the dam in opposition to hydrostatic loads. According to numerical analyses results, the backfill pressures vary during earthquake base excitations and the inertia forces of the backfill are the main source for those variations. It is also shown that significant passive (or active) pressure cannot develop in composite dams with a finite backfill width. A simplified model is also proposed for dynamic analysis of composite dam by replacing the backfill with by a series of vertical cantilever shear beams connected to each other and to the dam by flexible links.     

Using centrifuge tests data to identify the dynamic soil properties: Application to Fontainebleau sand

Knowledge of the dynamic properties of the soil is of great importance as the dynamic shear modulus and damping ratio are necessary input data in finite element modeling programs. This paper presents a post-processing strategy to identify the shear modulus and damping ratio vs. shear strain curves using the experimental results of a dynamic centrifuge program. Application is presented for the Fontainebleau sand. The proposed methodology is fast, robust and able to capture the nonlinear hysteretic behavior of the material. Based on the results, specific parameters for the Fontainebleau sand are identified for the empirical equation of shear modulus and damping ratio proposed by Ishibashi and Zhang [1]. It is found that confining pressure has an important influence on both shear modulus evolution and damping ratio.     

Performance of an equivalent shear beam (ESB) model container for dynamic geotechnical centrifuge tests

In dynamic centrifuge tests, appropriate boundary conditions are required in order to simulate the seismic semi-infinite soil layer responses within the confines of a finite size model container. An ESB (equivalent shear beam) model container first designed at the University of Cambridge was built with a stack of light-weight aluminum frames separated by rubber to experimentally achieve this goal. In this paper, a significant number of dynamic centrifuge tests and the corresponding seismic response analyses were performed to evaluate the dynamic performance of a newly constructed ESB model container and to shed light on the range of testable soil conditions. In the set of conducted tests, it appears that the end walls of the ESB model container behave in accordance with the dynamic response of the soil deposit, despite a difference in the natural period depending on the relative density of the sand deposit. This is attributed to the differences in mass and stiffness of the end walls compared to those of the contained soil model. For partially filled model container, significant differences in seismic responses are observed in the end walls and in the soil deposit due to seismic interaction caused by the upper unfilled frames of the container. These findings suggest that dynamic model tests using this ESB model container should be conducted with the container completely filled. In addition, on the basis of a comparison with the seismic soil behavior inside a rigid-walled model container, it is clear that the ESB model container can provide a more representative lateral boundary configuration for dynamic site response studies.     

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.     

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。