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1.
Earthquake-induced liquefaction is a major concern for embankment dam safety. Many liquefaction-induced earth embankment failures or near failures have been reported around the world during various earthquakes. Such embankment damages were particularly destructive when the underlying saturated granular soils liquefied, resulting in cracking, settlement, lateral spreading, and slumping of the embankment. Through a series of four highly instrumented geotechnical centrifuge model tests, seismic behavior of a zoned embankment dam with saturated sandy soil foundation was studied under moderate earthquake conditions. The beneficial effects of foundation densification were investigated. Valuable insights into the dynamic behavior of the employed embankment–foundation systems are provided. Test results suggest that there may be an optimum depth of densification treatment beneath an earth dam beyond which the reduction of the earthquake-induced deformations are relatively minor and that relatively small and isolated zones (e.g., at depth) of loose material within a densified volume of soil may not impair the overall effectiveness of treatment and do not necessarily result in damaging displacements.  相似文献   

2.
Surface rolling was employed to fabricate a densified layer on a powder metallurgy (PM) Fe–2Cu–0.6C piece. A densified surface layer with a depth of 335?μm and a surface hardness of 330?HV0.1 was obtained, in which the lamellar spacing of pearlite and grain size of ferrite were refined. Friction and wear behaviours of the surface densified material were studied. Results indicated that friction coefficient of the rolled material decreased as the load increased, which was lower than that of the un-rolled material. Wear volumes were lower than that of the un-rolled material, which increased as the load increased. Wear loss was caused by flake spalling and grooves, and the wear mechanism mainly was abrasive wear. The surface densified layer with higher hardness and lower porosity can hinder the cracks initiation and propagation on the surface and under the surface, which enhance the wear resistance of the PM material.  相似文献   

3.
This work presents numerical computations for the analysis of Dam-Break Flow using two-dimensional flow equations in a vertical plane. The numerical model uses the general approach of the simplified marker and cell method combined with the volume of fluid approach for the surface tracking. The time evolution of flow depth at the dam site and the evolution of the pressure distribution are investigated for both wet and dry bed conditions. The effect of the initially nonhydrostatic state on the long term surface profile and wave velocity are studied. These long term effects are found to be marginal in the case of wet-bed conditions, but are significant in dry-bed conditions. The dry-bed tip velocity immediately after the dam break, computed numerically, compares well with analytical results published previously. The time taken to obtain a constant flow depth at the dam site increases with decreasing initial depth ratio. The numerical result for this time elapse for dry-bed conditions is close to the experimentally obtained value.  相似文献   

4.
In the present paper a numerical model is developed to predict the exact electric current paths in the slag region of an Electro‐Slag‐Remelting (ESR) process. The model solves the momentum and energy equations. The solidification of the slag at the mould is modelled with an enthalpy‐porosity approach. The magnitude of the Joule heating and the Lorentz force are derived from the computed electric current lines. The localization where the Joule heating occurs controls the temperature distribution. The electric current distribution is in turn influenced by the temperature field through the temperature dependant electric conductivity. With this numerical tool the electric current paths are exactly computed by choosing the less resistive way to the liquid pool, or to the mould. For a given electric intensity the model predicts the power generated by the system and the solidified slag thickness at the mould. The model is validated by comparing its results with experiments on a small scale ESR process with high current density.  相似文献   

5.
Using the Vlasov foundation model, a modified approach of the continuous beam on elastic supports, leading to both a mechanical model and the proper foundation parameters of the generalized foundation is shown. Two formulations of the beam finite-element with shear deformation effect, resting on a two-parameter elastic foundation, characterized by distinct contributions of normal and rotary reactions are presented. The behavior of the second foundation parameter in the two formulations is governed by the bending cross section rotation of a beam. The first formulation, yielding a free-of-meshing stiffness matrix and equivalent nodal load vector, is based on the transcendental or “exact” solution of the governing differential equation of the beam resting on the elastic layer of constant thickness. Considering a linear variation of the layer thickness along the beam, the second formulation is based on the assumed polynomial displacement field. Numerical comparisons with the exact approach show that the cubic formulation leads to better results when the foundation parameters are variables. The practical utility of the analogy between a tensile axial force and the second foundation parameter is exemplified, too.  相似文献   

6.
A finite element method (FEM) and an artificial neural network (ANN) model were developed to simulate flow through Jeziorsko earthfill dam in Poland. The developed FEM is capable of simulating two-dimensional unsteady and nonuniform flow through a nonhomogenous and anisotropic saturated and unsaturated porous body of an earthfill dam. For Jeziorsko dam, the FEM model had 5,497 triangular elements and 3,010 nodes, with the FEM network being made denser in the dam body and in the neighborhood of the drainage ditches. The ANN model developed for Jeziorsko dam was a feedforward three layer network employing the sigmoid function as an activator and the back-propagation algorithm for the network learning. The water levels on the upstream and downstream sides of the dam were input variables and the water levels in the piezometers were the target outputs in the ANN model. The two models were calibrated and verified using the piezometer data collected on a section of the Jeziorsko dam. The water levels computed by the models satisfactorily compared with those measured by the piezometers. The model results also revealed that the ANN model performed as good as and in some cases better than the FEM model. This case study offers insight into the adequacy of ANN as well as its competitiveness against FEM for predicting seepage through an earthfill dam body.  相似文献   

7.
A series of highly instrumented, large-scale centrifuge models have been tested to investigate the extent of remediation required to control settlement and lateral sliding of soil deposits at a hypothetical bridge site. The baseline model represents a prototype with a 9-m-thick layer of fine sand having a relative density (Dr) of 50%. The sand layer is overlain by clay floodplains with a free face at a river channel. One nearly level floodplain surface supports a bridge abutment. The other floodplain has a 9% slope toward the river. In different models, different amounts of the 50% relative density sand was densified to Dr = 80%. Full depth improvement reduced settlements and lateral sliding of the sand by about a factor of 3. Due to the effects at the clay-sand interface, lateral sliding of the surficial clay deposit was not controlled by densification of the sand. Tests in which the width of the densified zone was only about 75% of the thickness of the loose sand indicated that relatively narrow zones of improvement can control settlement and sliding of the sand. Differences in shear resistance, pore pressures, dilatancy, and energy dissipation in loose and dense sands are presented.  相似文献   

8.
The flow and scalar field in the cross section of a bent-over momentum jet in cross-flow has been investigated via a 2D numerical solution of the governing equations with similarity transformation. The semianalytical model employs a free shear layer model for turbulent closure and the assumption of a constant velocity in the direction of the cross-flow; the entire solution depends solely on a dimensionless turbulent mixing parameter λ that measures the relative importance of advection and diffusion. The computed streamlines and vorticity field clearly indicate a vortex-pair flow for all λ. The shape of the scalar distribution, however, grows from a circular cell to a kidney-shaped double peak structure. Using a value of λ = 55, the computed jet trajectory and spreading and the cross-sectional shape are in good agreement with the experiments, although the dilution is somewhat overpredicted. The results suggest a free shear layer model of ε = 0.0272Wml*, where ε, Wm, and l* are the eddy viscosity, maximum velocity, and half-width in the bent-over jet cross section, respectively.  相似文献   

9.
We investigated the behavior of microgrooved implants in soft tissue using polystyrene implantable disks, either smooth or microgrooved (1-10 microm) on both sides. The implants were placed subcutaneously in a goat for 1, 4, or 12 weeks. Light and transmission electron microscopy showed that fibrous capsule formation around the implants was fairly uniform. After 1 week the implants were covered with a fibrous capsule about 80 microm thick. The collagen matrix was loose, and many inflammatory cells were present. After 4 weeks the matrix was more dense and contained many newly formed blood vessels. At the implant surface a layer of inflammatory cells about 10 microm thick had accumulated. Finally, after 12 weeks the matrix had densified. One cellular layer of inflammatory cells was present at the implant surface. We carried out histomorphometric measurements of capsule thickness, inflammatory layer thickness, and the number of blood vessels. Capsule thickness appeared not to decrease with time. Further, these measurements showed that there were no differences in tissue reaction between smooth and microgrooved implants. On the basis of our observations, we suggest that 1 microm deep and 1-10 microm wide microgrooves do not influence tissue response around polystyrene implants in soft tissue.  相似文献   

10.
INTRODUCTION: For many decades, the interpretation of unipolar electrograms (EGs) and ECGs was based on simple models of the heart as a current generator, e.g., the uniform dipole layer, and, more recently, the "oblique dipole layer." However, a number of recent and old experimental data are inconsistent with the predictions of these models. To address this problem, we implemented a numerical model simulating the spread of excitation through a parallelepipedal myocardial slab, with a view to identifying the factors that affect the shape, amplitude, and polarity of unipolar EGs generated by the spreading wavefront. METHODS AND RESULTS: The numerical model represents a portion of the left ventricular wall as a parallelepipedal slab (6.5 x 6.5 x 1 cm); the myocardial tissue is represented as an anisotropic bidomain with epi-endocardial rotation of fiber direction and unequal anisotropy ratio. Following point stimulation, excitation times in the entire volume are computed by using an eikonal formulation. Potential distributions are computed by assigning a fixed shape to the action potential profile. EGs at multiple sites in the volume are computed from the time varying potential distributions. The simulations show that the unipolar QRS waveforms are the sum of a "field" component, representing the effect of an approaching or receding wavefront on the potential recorded by a unipolar electrode, and a previously unrecognized "reference" component, which reflects the drift, during the spread of excitation, of the reference potential, which moves from near the positive to near the negative extreme of the potential distribution during the spread of excitation. CONCLUSION: The drift of the reference potential explains the inconsistencies between the predictions of the models and the actual shapes of the EGs. The drift modifies the slopes of EG waveforms during excitation and recovery and can be expected to affect the assessment of excitation and recovery times and QRS and ST-T areas. Removing the drift reestablishes consistency between potential distributions and electrographic waveforms.  相似文献   

11.
Staged Construction and Settlement of a Dam Founded on Soft Clay   总被引:1,自引:0,他引:1  
Alibey Dam is located near Istanbul in Turkey on the Alibey Stream, 4.5?km north of its point of confluence with Golden Horn, an ancient submerged river mouth. It was constructed as an earthfill dam over 30-m-thick soft valley sediments. Before the construction of the dam, field and laboratory tests were performed to determine the geotechnical characteristics of the foundation soils. During the construction and many years after the construction of the earthfill embankments, including the cofferdams and the intermediate fills, the response of the foundation soils was monitored by extensive field instrumentation generating a unique long-term (over 25 years) database. With proper instrumentation and careful monitoring of the collected data, field construction rates could be adjusted and the earth dam was safely constructed on the thick soft deposits. Approaches to settlement prediction were evaluated in a historical context, starting with the simplified one-dimensional approach available at the time of construction to more sophisticated analyses including the employment of modern numerical methods, in terms of the recorded data. Standard subsurface exploration and field testing supplemented with conventional laboratory testing provided the relevant material parameters that were used in the finite element method. The only exception to this was the overall hydraulic conductivity of the deposit, which controlled the rate of consolidation. Early field observations were used to assign the appropriate hydraulic conductivity. An elastoplastic soil model in a coupled analysis of consolidation was employed in the analysis that yielded realistic predictions of field behavior in response to the complex construction history. The accurate prediction and monitoring of the behavior of soft and thick soil layers subjected to staged construction, as in the case of Alibey Dam, is very important for planning of the construction as well as the expected behavior after construction.  相似文献   

12.
Seasonal temperature displacements are an important component of the total displacements recorded by pendulum measurements at gravity dams located in northern regions. A hybrid dam displacement model is presented in this paper to interpret these displacements and extrapolate the response for an extreme thermal event not yet experienced by the dam. The hybrid model uses a simplified deterministic structural dam representation with beam elements in complement to a hydrostatic seasonal time (HST) statistical displacement model. Comparisons are first established between 1D heat transfer analyses of typical gravity dam sections, and 2D finite-element (FE) analyses. Thermomechanical displacements are compared to show the validity of the proposed simplified deterministic beam model for typical dams. A case study of an actual 40?m gravity dam located in Quebec, Canada is then presented. It is shown that the deterministic model can be calibrated using the pendulum displacements and the HST model. The calibrated deterministic model is then used to extrapolate the displacement response for extreme thermal events not yet experienced by the dam. The proposed methodology represents a simple extension of the gravity method, widely used to verify gravity dam stability, as a first step to interpret recorded pendulum displacements and set appropriate warning and alarm levels on a rational basis before developing 2D and 3D thermomechanical FE deterministic dam models that require a lot of resources and expertise to be used effectively.  相似文献   

13.
地下开采条件下干堆尾矿库稳定性分析   总被引:1,自引:0,他引:1  
湖北三鑫金铜股份有限公司(简称"三鑫公司")拟建干堆尾矿库位于生产采场之上,影响其稳定性的主要因素有不良工程地质条件、早期采矿遗留下的采空区、地下开采引起的岩层移动。在工程地质分析的基础上,提出了地基改良和工程优化方案。通过建立的三维有限元数值分析模型,对干堆尾矿库设计方案进行评价,提出适合三鑫公司干堆尾矿库的第三种工况,并建议尾矿库堆积高度为20 m。  相似文献   

14.
This paper investigates the reduction of vibration due to the foundation slab both in horizontal and vertical directions. Similar reduction efficiency of the foundation slab was obtained among three-dimensional finite element analyses, field experiments, and the averaging scheme of the rigid slab theory. Then, two formulas fitted from numerical simulations were generated. Using those two simple formulas, one can estimate the reduction efficiency of horizontal and vertical vibrations due to foundation slabs. In summary, a suitable mat foundation can significantly reduce the horizontal vibration transformed from soil. The reduction efficiency depends on the size of the mat foundation over the soil wavelength. Enlarging the thickness of the foundation slab can decrease the vertical vibration transferred from soils. However, it is difficult to reduce vertical low-frequency vibrations for a reasonable slab thickness.  相似文献   

15.
Approximate Displacement Influence Factors for Elastic Shallow Foundations   总被引:2,自引:0,他引:2  
Displacement influence factors for calculating the magnitudes of drained and undrained settlements of shallow foundations are approximated by simple numerical integration of elastic stress distributions within a spreadsheet. Influence factors for circular foundations resting on soils having homogeneous (constant modulus with depth) to Gibson-type (linearly increasing modulus) profiles with finite layer thicknesses are obtained by summing the unit strains from incremental vertical and radial stress changes. The effects of foundation rigidity and embedment are addressed by approximate modifier terms obtained from prior finite-element studies. Results are compared with closed-form analytical and rigorous numerical solutions, where available. A new solution for Gibson soil of finite thickness is presented.  相似文献   

16.
Presented herein is a procedure for arriving at an optimal design of a homogeneous earth dam laid on an impervious foundation and provided with a drain. The procedure, heavily dependent on variably saturated flow modeling, involves optimizing a multiobjective function comprising a weighted summation of four objective functions, viz., the dam section area, seepage discharge, wetted area of the dam section and the drain area. The design variables considered in the optimization are the upstream and downstream slopes and the drain dimensions. The optimization is carried out subject to the constraints ensuring safe upstream and downstream slopes and sufficient distance between the free surface and the downstream face. Two of the objective functions (viz., the seepage discharge and the wetted area) and the constraints are implicit functions of the design variables. Their values are obtained by employing a numerical model of two-dimensional (vertical plane) variably saturated flow in a homogeneous earth dam. Optimization, conducted by the sequential unconstrained minimization technique procedure is preceded by several runs of the model for various combinations of the design variables. The discrete values of the implicit functions so generated are invoked during optimization to compute the implicit objective functions and constraints. The results are presented in the form of nondimensional design tables/curves. The design procedure is illustrated with the help of few examples.  相似文献   

17.
Numerical Study of Compressive Behavior of Concrete at High Strain Rates   总被引:1,自引:0,他引:1  
High strain rate unconfined compressional tests on concrete are simulated by a 3D discrete-element method. The laboratory data set was provided by three unconfined experiments on a split Hopkinson pressure bar apparatus at very high strain rates (350–700 s?1). This numerical method was chosen because it is well adapted to problems involving the characterization of fracturing and fragmentation in geomaterials. The simulations input data are the recorded experimental velocities, whereas the simulations output data are the computed forces that are compared with the experimental ones. The fit between the experimental and the numerical data is quite good. Based on this fit, it is shown that the strain rate dependency of the material strength can be explained by inertial effects.  相似文献   

18.
The stochastic response of the Santa Felicia earth dam, in southern California, to spatially varying earthquake ground motion (SVEGM) is analyzed. An SVEGM model that accounts for both incoherence and propagation of seismic waves is used, the results are compared with those for various simplified excitations, and the sensitivity of the responses to coherency models proposed by different researchers is investigated. A 3D inhomogeneous finite-element model is used to represent the dam. Finite element-based random vibration analysis is performed and the statistical moments of Cartesian displacement, stress, and strain responses are computed. Statistical moments of the maximum shear stress are computed using Monte Carlo simulation that utilizes results from the random vibration analysis. The results indicate that the stress response of stiff material near the base of the dam can be significantly increased due to SVEGM, and that the increase is sensitive to the low-frequency variation of ground motion coherency.  相似文献   

19.
A three-dimensional numerical model was applied to compute uniform and nonuniform sediment transport and bed deformation in an S-shaped laboratory channel located at the University of Innsbruck, where detailed measurements of the velocity field and bed elevation changes were made. The channel had two bends, a trapezoidal cross section, and a slope of S = 0.005. Gravel with a mean diameter of 4.2?mm was used as movable bed material and for sediment feeding. Wu’s formula for multiple grain sizes was compared with van Rijn’s formula using one grain size. Fairly good agreement was found between the computed and measured bed elevations for both approaches, whereas Wu’s formula could further improve the numerical results. Looking at the physics of the erosion pattern, the computed scour areas were located slightly more downstream than what was observed in the physical model. The current study also includes several parameter tests: grid distribution in vertical, lateral, and longitudinal direction; time step; number of inner iterations/time step; active sediment layer thickness; and the Shields coefficient. The variation of those parameters gave some differences in the results, but the overall pattern of bed elevation changes remained the same.  相似文献   

20.
Based on the experimental results, a mathematical kinetic model of the reaction process to synthesize TiC particles has been built, and a reaction rate expression is developed in this article based on the following mechanism: a Ti-rich Al-Ti complex layer is formed around carbon particles and titanium atoms diffuse from melt, across the layer, to react with carbon to form TiC; the TiC particles precipitate out of melt and diffuse all over the melt. The calculated results have shown that there are four major effect elements—the temperature of the system, the aluminum content of the preform, the thickness of the titanium-rich layer, and the size of carbon particle—to decrease the aluminum content, the thickness of titanium-rich layer, and the size of carbon particle and to increase the temperature, accelerate the reaction rate, and reduce the complete reaction time. In the end, some experiments have been done to investigate the effect of temperature, size of C powder, and Al content on the reaction. The experimental results were in good agreement with the numerical calculated result.  相似文献   

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