Chuhan Zhang

Non‐linear seismic response of arch dams with contraction joint opening and joint reinforcements

A comprehensive study of non-linear seismic response of arch dams with contraction joint opening and joint reinforcements has been conducted. A numerical model of contraction joint reinforcements is presented for optimization control of the joint opening. The objective of this control is to reduce the joint opening and expectantly to balance the sustained loads between the horizontal and the vertical components of the dam, thus avoiding an overstress in the cantilever while retaining the release of arch tensile stresses to some extent. Several parameter studies such as critical element size and required number of joints to be simulated for convergence are also performed. As an engineering application, a 292-m high arch dam (the Xiaowan arch dam) and the Big Tujunga dam are analysed in detail. The results demonstrate that the joint opening and the corresponding load transfer from the arch to cantilever components of the dam during strong earthquakes are substantial. It is also evident that by providing sufficient strength and reinforcement flexibility, the joint opening can be controlled to some extent. However, the stress redistribution due to reinforcement control is not sufficient to avoid the overstress in the cantilever for the Xiaowan arch dam. Thus, alternative measures are discussed.

Study on the Heterogeneity of Concrete and Its Failure Behavior Using the Equivalent Probabilistic Model

Taking the heterogeneity character of concrete into account, this paper presents an equivalent probabilistic model for failure study of concrete in which the heterogeneity of concrete is considered by assuming that the material properties conform to the Weibull distribution law and by using mesoscale mesh of finite elements. The study is divided into three parts. In the first part, a spatial correlation length factor is developed into the Weibull distribution formula so that the spatial correlation of local continuity of material properties can be considered. The second part presents a series of numerical analyses for investigating the size effect of self-compacting concrete (SCC) based on the equivalent probabilistic model, and a comparison is made between the simulation results and compression test measurements for illustrating the size effect on uniaxial compression strength and failure pattern of the SCC concerned. It is shown that the numerical model can provide reasonable predictions in the analysis of the size effect of SCC. In the final part, as an engineering application of the presented model, the damage and fracture behavior of the Koyna gravity dam during the 1967 earthquake are analyzed. Influences of concrete parameters on the crack pattern and failure modes of the dam prototype during the event are discussed.

Reflection and Transmission of Plane Waves at a Water–Porous Sediment Interface with a Double-Porosity Substrate

This paper investigates the wave propagation at the interface between the ocean and the ocean floor. The ocean floor is assumed to be composed of covered porous sediment with an underlying double-porosity substrate. For this purpose, plane wave reflection and transmission in the coupled water–porous sediment–double-porosity substrate system are analytically solved in terms of displacement potentials. Using numerical examples, the effects of the material properties of the underlying double-porosity substrate on the reflection coefficients are discussed in detail. Variations in pore and fracture fluid, fracture volume fraction, and permeability coefficients are considered. In addition, two cases of boundary conditions at the porous sediment–double-porosity substrate interface, i.e., sealed-pore boundary and open-pore boundary, are compared in the numerical calculations. Results show that material property variations in the double-porosity substrate may significantly affect the reflected wave in the overlying water if the sandwiched sediment depth is less than the critical value.

Numerical prediction of swelling in concrete arch dams affected by alkali-aggregate reaction

A chemo-damage model is presented for anisotropic swelling analysis of concrete arch dams affected by alkali-aggregate reaction (AAR). The model combines the AAR kinetics and the plastic-damage model, and the chemical and mechanical phases are coupled. A redistributing weight function, determined by the applied stresses in the concrete, is introduced to control the AAR-induced anisotropic expansion of the concrete. Creep strain is also included in the approach using the Kelvin–Voigt model. Accelerated tests, in which the specimens are confined with steel rings and subjected to axial loads, are first analyzed using the proposed model. The computed strains of the specimens are in good agreement with the experimental measured strains. The application to the AAR-affected Kariba dam is then carried out. The radial and vertical displacements of the dam due to AAR are reproduced with sufficient accuracy. The stresses within the dam are significantly redistributed during the AAR process. Severe cracking and damage appear to occur in the dam heel and the downstream face on both sides of the dam-foundation interface. It demonstrates Kariba dam is facing an increased risk of collapse associated with the increasing compressive stresses within the dam and developing cracks on both sides of the downstream face during the development of AAR.

Accuracy of the half-power bandwidth method with a third-order correction for estimating damping in multi-DOF systems

A third-order correction was recently suggested to improve the accuracy of the half-power bandwidth method in estimating the damping of single DOF systems. This paper analyzes the accuracy of the half-power bandwidth method with the third-order correction in damping estimation for multi-DOF linear systems. Damping ratios in a two-DOF linear system are estimated using its displacement and acceleration frequency response curves, respectively. A wide range of important parameters that characterize the shape of these response curves are taken into account. Results show that the third-order correction may greatly improve the accuracy of the half-power bandwidth method in estimating damping in a two-DOF system. In spite of this, the half-power bandwidth method may significantly overestimate the damping ratios of two-DOF systems in some cases.

Comparison of different fracture modelling approaches to gravity dam failure

Purpose – There is not a unified modelling approach to finite element failure analysis of concrete dams. Different behaviours of a dam predicted by different fracture methods with various material constitutive models may significantly influence on the dam safety evaluation. The purpose of this paper is to present a general comparative investigation to examine whether the nonlinear responses of concrete dams obtained from different fracture modelling approaches are comparable in terms of crack propagation and failure modes. Design/methodology/approach – Three fracture modelling approaches, including the extended finite element method with a cohesive law (XFEM-COH), the crack band finite element method with a plastic-damage relation (FEPD), and the Drucker-Prager (DP) elasto-plastic model, are chosen to analyse damage and cracking behaviour of concrete gravity dams under overloading conditions. The failure process and loading capacity of a dam are compared. Findings – The numerical results indicate that the...

Nonlinear Spectral‐Element Method for 3D Seismic‐Wave Propagation

Abstract This study extends the linear spectral‐element method (SEM) program SPECFEM3D to include nonlinear constitutive relationships. The iterations for solving the nonlinear equations are bypassed considering that the explicit integration used in the code requires a very small time increment; as a result, the high efficiency of the original linear procedure is preserved. This nonlinear code allows us to perform a deterministic seismic‐hazard analysis that considers the nonlinear behaviors of the propagation path and local site for an engineering site. After checking its accuracy against the commercial finite‐element method program ABAQUS, the extended nonlinear SEM is used to investigate the effect of noncausative faults on the seismic ground motion at the Dagangshan dam site in Southwest China. The analysis of the study case shows that the isolation effect of noncausative faults is dependent on the fault‐source relative position, and the nonlinear responses of the faults are sensitive to earthquake magnitude.

Novel method for groyne erosion stability evaluation

ABSTRACT This paper concentrates on the simulating methodology for the reliability analyses of the groyne erosion stability with the randomness, including the probabilistic turbulences of the mean value, the variation, and the correlation of the pertinent parameters. On the basis of the interpretation for the mechanism of the groyne erosion, the backslope gradient of the local eroded pit is adopted as the major factor for the establishment of the reliability function. Created for the reliability analyses in this paper is the systematic algorithm that can solve the complicated reliability function of the groyne erosion stability and compute the corresponding failure probability. The reliability function here incorporates the random parameters that are non-Gaussian and statistically correlated. These complicated parameters exist in most coastal engineering cases. The comprehensive sensitivity of the groyne erosion stability is investigated based on the measured data of the random parameters and their designed randomness that includes the designed variation and the designed correlative coefficients. Results show that the probabilistic turbulences of the velocity of the natural main stream flow and the mouth radius of the crateriform shape of the local eroded pit affect significantly the groyne erosion stability; the optimal conformation interval of the assembly average diameter of the particles in the local river bed sediments is 0.62, 0.68 mm, where the value of the failure probability is the minimal one; the statistic correlation between the behaviors of the natural main stream and other random parameters deteriorates certainly the performance of the coastal engineering structures. According to the sensitivity analyses, the suggested values of the random parameters are offered in this paper. The methodology developed here is a generalized approach to the reliability simulation for the pertinent structures of coastal engineering and can be applied directly in the n-dimensional independently standardized Gaussian space. On the basis of the comparison with the Monte Carlo simulation, the methodology developed here shows the super efficiency.

Analysis of parameters in granular solid hydrodynamics for triaxial compression tests

Granular materials are omnipresent in industries and in nature. For small strains, elastic-plastic and hypoplastic constitutive relations are widely used in engineering practice, but they are not a significant reflection of the underlying physics. Under a unified thermodynamics framework explaining the physics of materials, granular solid hydrodynamics (GSH) was an extension towards describing granular materials, not only solid-like, but also fluid-like behaviors. In this paper, the fundamentals of GSH are briefly treated and then simplified to analyze quasi-static deformations in triaxial compressions. The calculated stress-strain relations and volumetric strain are compared with experimental results. The influences of the major parameters in GSH, especially their cross coupling influences, are analyzed and their physical meanings are further clarified. After parameters were calibrated, the calculated stress values in the characteristic stress state are found to be within 22% of tested values. Meanwhile, the energy dissipation during triaxial compression is analyzed. The above results support and partially quantify GSH.

Chapter 11 – Influence of Seismic Input Mechanisms and Radiation Damping on Arch Dam Response1

Two earthquake input models are introduced: the massless foundation model and the viscous-spring boundary model considering radiation damping. Linear elastic and nonlinear contraction joint opening analyses of the 210 m high Dagangshan arch dam are performed using the two models. The responses of the three-dimensional canyon without the dam are analyzed with massless-truncated foundation and viscous-spring boundary; then, linear and nonlinear analyses of the dam–foundation system are performed and compared using the two models. Hydrodynamic effects are considered using finite element discretization for incompressible reservoir fluid. Stresses, displacements and contraction joint opening in the dam are significantly reduced in linear and nonlinear analyses using the viscous-spring boundary model. In linear analysis, the massless foundation model with a high damping ratio of 10% leads to a similar response to that using the viscous-spring boundary model. Maximum tensile stresses from nonlinear analysis are 10–25% larger than those from linear analysis owing to a partial release of the arch action.