Failure analysis method of concrete arch dam based on elastic strain energy criterion

The arch dam is a type of massive water-retaining structure made of concrete. The overall failure mechanism and corresponding analysis criterion are key issues of concern in the dam engineering field. In this paper, the energy evolution of arch dams in the failure process is studied first, which can be decomposed as energy dissipation accompanied by concrete damage and elastic strain energy absorption and release during elastic deformation. An evaluation criterion for failure analysis of concrete arch dams is then established based on elastic strain energy. An orthotropic damage constitutive model for dam concrete is then proposed along with its numerical simulation method, which is established for structural failure analysis. Numerical simulations show that the elastic strain energy in elements increases with increasing overload safety coefficient and finally converges to the concrete material surface energy, at which time the locally plastic damage area develops rapidly and finally leads to cracking failure of structures. The proposed failure analysis criterion for concrete dams under integrated loads is suitable for analyzing dam instability failure, which has great operability and value in engineering applications in the future.     

Interval risk analysis for gravity dam instability

Fuzziness and randomness are two inseparable uncertainty attributes of most factors influencing stability of gravity dam. Moreover, there is a fuzzy transition from stability status into failure status. Risk analysis and fuzzy mathematics are conducted to evaluate the stability problems of gravity dams, where the process of stability failure is studied as a fuzzy event. Membership functions are used to describe the extent of stability failure risk for gravity dams. The fuzziness of both the design parameters and failure criterion are accordingly eliminated through a transformation by use of the concept of the Level Set. Corresponding analysis procedures are then provided to calculate the fuzzy risk and its probability of the stability failure for gravity dams. Based on a real dam section, a detailed example is further provided to illustrate the proposed risk analysis approach. The results show that it is feasible to apply the present method to analyze the fuzzy risk of stability failure for gravity dams. The modeling approach is sound and the findings do improve the current understanding of this important problem. The conventional risk is a determinate value but the proposed fuzzy risk is an interval value. The obtained conclusions can reflect more reasonably the actual dam engineering.     

基于改进云图法的高土石坝抗震可靠度分析

高土石坝抗震可靠度研究对大坝抗震防灾和震害风险研究具有重要意义。通过考虑地震动和筑坝料参数双重随机性,建立基于地震易损性和地震峰值加速度概率密度函数的高土石坝抗震可靠度模型,为研究不同设计使用年限的高土石坝抗震可靠度提供依据。通过拉丁抽样方法选取筑坝料参数样本并与选择地震动组合成样本对,选取坝顶相对震陷率作为性能参数,提出考虑抗震设防标准的高土石坝性能水平了;采用SWANDYNE Ⅱ程序进行动力计算,并根据改进云图法得到不同地震峰值加速度下坝顶相对震陷率的地震易损性三维曲面;结合糯扎渡高土石坝不同设计年限的概率分布函数与地震易损性曲面,确定不同设计年限失效概率和抗震可靠度。分析结果表明:随着设计使用年限增加,大坝各个性能水平可靠度不断减小,对于严重破坏状态下不同设计年限可靠度均能满足《水利水电工程结构可靠性设计统一标准》规范要求,说明糯扎渡高土石坝在变形方面抗震设计是合理的。     

Meshless study of dynamic failure in shells

The dynamic fracture of a thin-walled structure that is mainly due to impact and explosive loading is studied. Use is made of a meshless SPH shell formulation based on Mindlin–Reissner’s theory. The formulation is an extension of the continuum-corrected and stabilized SPH method allowing a thin structure to be modelled using only one particle characterizing the mean position of the shell surface. Fracture is based on an effective criterion similar to that of the visibility method. Four numerical examples are studied among which tearing of pre-notched plates, fracture due to impact loading and dynamic fracture of cylindrical shells.     

我国高坝建设和科技攻关

介绍了我国大坝建设的发展和特点,概括了高坝建设中的地质勘探、坝工技术、泄洪消能、坝基处理和高边坡处理等方面的科技成果和应用水平,展望了我国高坝发展前景并指出今后高坝科技攻关重点。     

Dynamic analysis of an inflatable dam subjected to a flood

A dynamic simulation of the response of an inflatable dam subjected to a flood is carried out to determine the survivability envelope of the dam where it can operate without rupture, or overflow. The free-surface flow problem is solved in two dimensions using a fully nonlinear mixed Eulerian-Lagrangian formulation. The dam is modeled as an elastic shell inflated with air and simply supported from two points. The finite element method is employed to determine the dynamic response of the structure using ABAQUS with a shell element. The problem is solved in the time domain which allows the prediction of a number of transient phenomena such as the generation of upstream advancing waves, the dynamic structural response and structural failure. Failure takes place when the dam either ruptures or overflows. Stresses in the dam material were monitored to determine when rupture occurs. An iterative study was performed to find the serviceability envelope of the dam in terms of the internal pressure and the flood Froude number for two flood depths. It was found that existing inflatable dams are quite effective in suppressing floods for a relatively wide range of flood velocities.     

基于断裂力学的混凝土坝裂缝灌浆压力计算模型

根据大坝工程上对裂缝种类的分类以及经常采用的化学灌浆方法,以某在建高拱坝中出现的几种裂缝为实例,考虑了浆液流动时的沿程阻损及重力影响,建立了基于断裂力学的化学灌浆计算模型。运用权重法,通过积分计算了灌浆时3种模型下的裂缝尖端应力强度因子。在不同的大体积混凝土断裂韧度条件下,对不同缝长及3种裂缝类型进行化学灌浆的容许灌浆...     

基于XFEM的强震区砼重力坝开裂与配筋抗震措施研究

已有震害表明,混凝土坝遭遇强烈地震将不可避免地产生开裂。扩展有限元法（XFEM）通过在相关节点的影响域上富集非连续位移模式,使得对非连续位移场的表征独立于单元边界,可以有效描述混凝土中的裂纹扩展。基于扩展有限元模型,采用合理的地震波动模型对国内某混凝土重力坝强震下的动力破坏过程进行了分析;针对大坝破坏情况,应用嵌入式滑移模型模拟了混凝土重力坝配筋前后的地震响应和破坏状况,据此评价局部配筋的抗震效果。研究表明,局部配置抗震钢筋虽无法防止裂缝的发生,但可有效限制坝体裂缝的开裂扩展范围及深度,减少裂缝的开度,有效改善坝体的抗震性能。     

重力坝水下接触爆炸的数值分析

采用显式程序AUTODYN中的Euler-Lagrange耦合算法模拟混凝土重力坝水下接触爆炸。研究在不同炸点的水下接触爆炸荷载作用下大坝的动力响应以及损伤裂纹分布特点。针对上述特点采用泡沫铝作为抗爆材料,研究了泡沫铝的抗爆效果。研究结果表明:采用泡沫铝作为大坝水下爆炸抗爆材料能大幅降低爆炸冲击波幅值,减小大坝的动力响应,同时能减小爆炸对坝体产生损伤及裂纹分布范围,具有良好的抗爆性能。     

Non‐linear seismic behaviour of concrete gravity dams using coupled finite element–boundary element technique

In this paper, the seismic response of concrete gravity dams is presented using the concept of Continuum Damage Mechanics (CDM) and adopting the hybrid Finite Element–Boundary Element technique (FE–BE). The finite element method is used for discretization of the near field and the boundary element method is employed to model the semi‐infinite far field. Because of the non‐linear nature of the discretizied equations of motion modified Newton–Raphson approach has been used at each time step to linearize them. Damage evolution based on tensile principal strain using mesh‐dependent hardening modulus technique is adopted to ensure the mesh objectivity and to calculate the accumulated damage. The methodology employed is shown to be computationally efficient and consistent in its treatment of both damage growth and damage propagation in gravity dams. Other important features considered in the analysis are: (1) realistic damage modelling for concrete that allows isotropic as well as anisotropic damage state and exhibits stiffness recovery upon load reversals. (2) softening initiation and strain softening criteria for concrete, and (3) proper modelling of semi‐infinite foundation using FE–BE method that allows to consider dam–foundation interaction analysis. As an application of the proposed formulation a gravity dam has been analysed and the results are compared with different foundation stiffnesses. The results of the analysis indicate the importance of including rock foundation in the seismic analysis of dams. Copyright © 1999 John Wiley & Sons, Ltd.     

Analysis of hydraulic fracturing in concrete dam considering fluid–structure interaction using XFEM-FVM model

High-pressure hydraulic fracturing (HF) of cracks is the crucial factor in the safety-assessment of high concrete dams, and fluid–structure interaction in HF is the key to accurately calculate the fluid pressure within cracks. In this study, a hybrid approach combining the extended finite element method (XFEM) and the finite volume method (FVM) is introduced to simulate HF in concrete dams. Through the hybrid approach XFEM-FVM, we can obtain water pressure distribution within cracks, which reflects the coupling relationship between water pressure and fracture width. This approach is verified by an example, and the numerical results agree well with the corresponding analytic solutions. Then, we use it to simulate the HF in a concrete gravity dam. Results show that, in contrast to the constant water pressure distribution within cracks, the distribution calculated by XFEM-FVM varies with time and leads to the bigger crack extension angle, contrary to growth length. It also illustrates the significance of considering fluid–structure interaction in HF progress. This study reveals that the approach is a very efficient tool for HF simulation in concrete dam.     

Numerical modelling of fracture of particulate composites using SPH method

Simplicity of mesh generation and robustness against mesh entanglement during large deformations are key attractive features of particle based methods. These features can be exploited in number of engineering problems where traditional techniques suffer due to aforementioned limitations. Numerical modelling of particulate composites is one of such ideal engineering applications where particle based methods can be effectively used due to their simplicity and robustness. Complicated geometrical configurations of particulate composites obtained from techniques such as scanning electron microscopy (SEM) can be easily converted to particle based mesh without loosing much information. This enables more accurate analysis of the chosen composite materials. Therefore, a smooth particle hydrodynamics (SPH) based numerical technique is developed here to investigate the mechanical properties and evolution of debonding process in particulate composites. To perform the numerical study, a Lagrangian corrected SPH (CSPH) method is presented together with an appropriate numerical model for treating material interface discontinuity within the particulate composites. The material interface discontinuity is enforced using an innovative method which combines penalty formulation with a bilinear interface cohesive model for SPH method. The proposed SPH methodology is used in a number of numerical examples involving composite materials and related interface problems. The effect of penalty value on the interface model and of the smoothing length of the SPH method are also analysed during these simulations. The results illustrate the effectiveness, robustness and potential of the developed methodology. It is concluded that the proposed numerical techniques can be easily and effectively applied to simulate multi-phase composites with various interface conditions and, can provide useful information regarding the inherent mechanism of damage evolution and fracture of particulate or fibre reinforced composites.     

不同高度重力坝动水压力分析及Westergaard修正公式研究

对5种不同高度的重力坝进行了地震作用下坝体-库水系统耦合计算,将所得结果与Westergaard公式解析解相比较发现,两种方法得到的动水压力结果沿坝高度上的分布有很大不同,流固耦合结果在坝面上有上部偏大,下部偏小的趋势;随着坝体高度的增加流固耦合模型的这种趋势越为明显,坝面上出现动水压力最大值的位置也逐渐升高。据此,对Westergaard公式做出修正,考虑到多方面因素对动水压力的影响,对原公式引入了坝体高度修正项、坝体弹性修正项、库底吸收修正项。修正的Westergaard公式解与流固耦合结果及以往文献的试验和计算结果吻合良好。     

Large-scale testing on specific fracture energy determination of dam concrete

The specific fracture energy of dam concrete is a basic material characteristic needed for the prediction of concrete dam behavior. Data on fracture properties of dam concrete are quite limited to date. A series of tests was carried out based on the size effect due to a number of geometrically similar notched specimens of various sizes. Experimental tests include three-point bending tests. The specimens were of square cross section with a span to depth ratio of 2/5. Three different specimens with depth of 200, 400 and 800 mm were considered for the purpose of testing. Concrete mixtures are provided from the Caroon 3 dam project site using river gravel or commonly crushed stones from quarries. To compare the fracture properties of dam concrete with normal concrete five types of concrete mixes of 65, 50, 40, 30 and 20 mm maximum aggregate size were considered. For all mixes, value of relative notch depth of 0.2 is used. Experimental results show that aggregate size has an important effect on specific fracture energy values of dam concrete. According to specific fracture energy definition based on size effect, the specific fracture energy of concrete mixes of 65, 50, 40, 30 and 20 mm M.S.A. are 178, 129, 88, 79 and 72 N/m, respectively. This indicates that the high specific fracture energy of dam concrete is the result of the nature, size and properties of its aggregate. The difference between specific fracture energy of normal and dam concrete should be noticed in nonlinear analysis of concrete dams.     

Discussion on construction and type selection of China high dams

At the beginning of 21st century, with the rapid and steady development of China economy, a lot of large scale hydropower projects with large dam from 200 m to 300 m high are being or to be built. China dam constructions are reaching the level of 300 m high arch dam, 250 high CFRD and 200 m high RCC gravity dam. Due to the safety and the economy, the type selection for high dams have becomes the key issue during the argumentation for the hydropower projects, and further efforts are still needed in this aspect for high dams. After reviewing the high dam constructions in China and abroad, authors proposed some advices for the selection of dam types, and hope that it can provide some helpful information for the researches and the design of high dams.     

基于能量耗散碾压混凝土重力坝地震损伤分析

摘　要：采用塑性损伤力学对混凝土重力坝进行非线性动力分析,通过研究塑性损伤本构中滞回曲线的特点以及地震中重力坝裂缝发展特征和结构能量耗散机理,建立了包含能量特性的大坝整体损伤评价指标。通过分析发现强震作用下坝体上部的损伤是结构的主要损伤,地震中的能量以结构阻尼耗散能量为主,混凝土损伤和塑性耗散的能量所占比例不大,但与裂缝的发展有直接关系。提出的大坝整体损伤指标可以综合的反应结构的整体损伤程度,以此对结构进行抗震设计,可以提高结构的抗震性能。     

A new boundary condition for energy radiation in covered reservoirs using BEM

In this paper, a new boundary condition accounting for energy radiation at the far end of covered reservoirs is proposed. Using boundary element modelling (BEM), the boundary condition is investigated through analysis of the hydrodynamic pressure within a two-dimensional ice-covered reservoir impounded by a gravity dam. The proposed boundary condition accounts for reservoir bottom absorption effects and the presence of an ice cover at infinity. Seismic excitation is introduced by subjecting the dam and the reservoir to a horizontal harmonic ground motion. The effectiveness of the proposed method and the accuracy of the boundary condition are examined through a parametric study. The boundary condition is shown to be accurate even when placed near the dam upstream face, and the results obtained are in excellent agreement when compared to those from a mathematical model developed by the authors in a previous work. Some fundamental aspects of hydrodynamic pressure within ice-covered reservoirs are also discussed.     

Modelling of equal channel angular pressing using a mesh-free method

Severe plastic deformation (SPD) processes are widely recognised as efficient techniques to produce bulk ultrafine-grained materials. As a complement to experiments, computational modelling is extensively used to understand the deformation mechanisms of grain refinement induced by large strain loading conditions. Although considerable research has been undertaken in the modelling of SPD processes, most of the studies have been accomplished using mesh-based methods, such as the finite element method (FEM). Mesh-based methods have inherent difficulties in modelling high-deformation processes because of the distortions in the mesh and the resultant inaccuracies and instabilities. As an alternative, a mesh-free method called smoothed particle hydrodynamics (SPH) is used. The effectiveness of this technique is highlighted for modelling of one of the most popular SPD techniques, equal channel angular pressing. A benchmark between SPH and FE calculation is performed. Furthermore, a number of simulations under different processing conditions are compared to existing literature data. A satisfactory agreement is found, which indicates that SPD processes can be approached by mesh-free methods, such as SPH.     

Segmented impact fracture of toppling truncated cones and tall trees

Critical dynamic stresses and segmented fracture lengths are predicted for a tall, initially upright, truncated right circular cone as it rotates about its base point and as the cone tip impacts the ground plane. This cone can represent a tall masonry structure such as a tapered stone tower or a brick chimney which has a foundation failure; or the trunk of a tree which topples in a windstorm. In any case, ground impacts can induce sequential fractures along the cone's length, resulting in multiple in-line cone segments that come to rest on the ground plane. A dynamic analysis for the cone's bending moment and stress distributions during impact is used to predict the primary fracture points for the broken segment lengths. The dynamic bending stresses controlled the failures since they were found to be about two orders of magnitude higher than the longitudinal stresses caused by the self-weight and the centrifugal acceleration of a rotating cone. In a case study, this dynamic analysis is shown to correlate well with observed fracture patterns of fallen segments for a dead tree toppled by wind. An unexpected secondary fracture point observed in this case study is also explained using the dynamic theory.     

Dynamic particle refinement in SPH: application to free surface flow and non-cohesive soil simulations

In this paper, we present a dynamic refinement algorithm for the smoothed particle Hydrodynamics (SPH) method. An SPH particle is refined by replacing it with smaller daughter particles, which positions are calculated by using a square pattern centered at the position of the refined particle. We determine both the optimal separation and the smoothing distance of the new particles such that the error produced by the refinement in the gradient of the kernel is small and possible numerical instabilities are reduced. We implemented the dynamic refinement procedure into two different models: one for free surface flows, and one for post-failure flow of non-cohesive soil. The results obtained for the test problems indicate that using the dynamic refinement procedure provides a good trade-off between the accuracy and the cost of the simulations.