جستجوی مقالات مرتبط با کلیدواژه "finite element" در نشریات گروه "آب و خاک"

The stress created in the soil significantly affects its engineering behavior. Changing its value during the construction of earthen dams causes volume change and shear strength, causing rupture, soil compaction and settlement in earthen dams. So measuring soil stresses of dams is essential that it is done by instrumentation installed. Artificial intelligence models such as artificial neural networks for modeling many engineering applications. Also by extending the meta-heuristic algorithms, combined with neural networks have become very popular due to more accurate results.

In this study, the cross-section 19 (cross-section of the middle part of the body and dam foundation) for the modeling of soil stress were used during the construction of the dam Kaboud-val. Also Kaboud-val dam instrumentation data (derived from Golestan Regional Water Co.) was used at the time of construction during the period of 4 years. Type and number of input data is the most important thing in modeling artificial intelligence. By examining data TPC19.1 cells in section 19 in Kaboud-val dam, embankment alignment (F), the water level of the reservoir (R), the construction of the dam (T), speed filling and dewatering speed was selected for input. The soil stress (P) in the body of the dam during construction, intelligent model was selected for output. This process is most effective in a subset of features from the set of input features according to the least error, selected and additional features will be removed. In this research, a meta-algorithm (artificial neural network (NNA) algorithm) is combined with an artificial neural network (ANN) that has the ability to predict complex and nonlinear relationships and extracts effective features for modeling soil stress with appropriate accuracy. In this study, the most effective features in soil stress modeling were determined in a case study (Kaboud-val Dam) using the NNA-ANN hybrid algorithm and a comparison was made between the results of the hybrid model and the numerical model. Five features include reservoir level, fill level, dam construction time, impounding velocity and fill velocity was selected for the input.

Using hybrid algorithm and feature selection method, a combination of three features, including reservoir level, fill level, dam construction time (with RMSE equal to 0.5024) were the most effective features in modeling soil stress in the selected cell. The results showed that the hybrid model in Kaboud-val Dam (with values of R^2, RMSE, MAE and NS equal to 0.9943, 2.5653, 1.9973 and 0.9999, respectively) has better performance in modeling soil stress than the numerical model. (With values of R^2, RMSE, MAE and NS are equal to 0.9625, 26.2567, 16.6725 and 0.9772, respectively). The results showed that the reduction in the input features to reduce the time and cost reduction is more economical and more effective. Because with the increase in the number of features in the hybrid model, the increase in modeling accuracy did not occur. Sensitivity analysis showed that the dam construction time and fill level, of the highest sensitivity factor, the most important feature of the model is the total stress in cells. Modeling with the mentioned features, in another dam with a new construction site and new geotechnical specifications (Masjed Soleiman Dam) showed that the use of artificial intelligence model according to statistical indicators has more accurate answers than the numerical model.

The results showed that the use of artificial intelligence methods in the design and initial estimates of soil stress parameters in earthen dams instead of using numerical methods has high reliability and accuracy. The combination of input data in the hybrid model under study is suitable for Kaboud-val dam and Masjed Soleiman dam and the appropriate combination should be used for each construction site. By completing the number of data in different sections of the dam and the number of construction sites in areas with similar climate and geotechnical conditions, a design software can be obtained to predict the amount of soil stress during construction in the body and foundations of earth dams.

In this paper, an optimization simulation hybrid model is used to evaluate the optimum pumping policy in an artificial aquifer. In this study, the groundwater level drop was investigated in a hypothetical free aquifer with a surface area of 1.5 square kilometers and three different hydraulic conductivity with a thickness of 100 meters, as well as ten pumping wells. In order to estimate the groundwater level of the aquifer, the finite element method was used and to optimize the position of the wells, used the algorithm of the ant colony and finally the FEM-ACO model was presented. The position of wells with a specific discharge is optimized to minimize groundwater level losses in the aquifer. In this regard, the number of different number of ants, the effect of Foramen evaporation and the effect of the elite percentage of an ant collection on the value of the objective function were investigated. The results showed that sequencer antler algorithms and elite ant anchor algorithms with almost identical performance have the best performance among the antler algorithms, and after that, the antler community algorithm and the max-minimal ant antler algorithm are in the next ranks. All of the ant colony algorithms arrived too quickly into convergence, which makes this early convergence a suitable global optimum due to the use of chain constraints. Finally, after considering the proposed model, the proper position of the wells was determined. The results also showed that the maximum water loss in the aquifer is about 2.5 m.

Seismic study of canyon sites has always been one of the important fields of seismic studies because of massive structures such as dams that are built in such sites. Jointed rock mass in rock canyon sites is one of the main site effects that can change the seismic waves. In this research, we studied the influence of this factor on the scattering of seismic waves. To fulfil this goal, we employed a coupled method combining the finite element method in the near field with boundary elements in the far field. To simulate the behaviour of jointed rock mass, we used the linear elastic model. Based on the results of the numerical analyses, jointed rock mass could have significant effects on the seismic waves in some special conditions. These conditions are the angle of incident wave, the thickness of layers and the material properties of the jointed rock mass. So it is necessary to consider this factor in the seismic structure design in the canyon sites.

The spillway is one of the most important elements of dam building. So studying and designing of the spillway hydraulic should be considered in dams and other similar projects. The ogee spillway, because of its superb hydraulic characteristics, has been widely studied. Its ability to pass flow efficiently and safely, when has been properly designed, with relatively good flow measuring capabilities, has enabled engineers to use it in a wide variety of situations. In the last two decades, there has been an increasing interest in the stepped spillways in various laboratory experiments around the world. This is partly because of technical advances in the construction of Roller Compacted Concrete (RCC) dams and considerable amount of energy dissipation along the chute, leading to reduction in the size of the stilling basin. Flow analysis using laboratory experiments usually involves considerable time and cost requirements .Nowadays, availability of computational fluid dynamic (CFD) programs and powerful computers has resulted in increasing usage of numerical methods of flow analysis. Free surface flows are encountered in hydraulic engineering problems including water jets, weirs and around gates. For the numerical modelling of a steady flow, the area of calculation and therefore the position of the free surface have to be known. The main object of this study is numerical analysis of free surface profiles on ogee and stepped spillways by finite volume and finite element methods and comparison of the two methods. The physical laws governing a fluid flow problem are represented by a system of partial differential equations regrouping the continuity equation, the Navier-Stokes equations and any additional conservation equations. The numerical analysis resolves these equations by accurate and complex numerical schemes. A program or code, where the numerical algorithm is implemented, is then solved on a computer. In recent study experimental results offered by Tabara and CHatila (2005) have been used to investigate stepped spillway physics models of Tabara and Chatara (2004) for studying ogee spillway. FLUENT and ADINA software have been used to simulate flow field of types of stepped and ogee spillways. It should be noted that k-£ model has been applied in order to modeling turbulence. Gambit preprocessor software is a tool for networking flow field. In recent study we have used PRESTO scheme for discretization of pressure, quick plan for discretization of terms of displacement of momentum equations, turbulence formulas and PISO algorithm for coupling velocity and pressure. In finite element method, Galerkin relation has the second order accuracy. SUPG method is used in momentum equations and turbulence for discretization of term advection which is effective in problem convergence. Meshing type has been selected as triangle form in ADINA code, but quadrilateral in FLUENT code regarding the spillway geometry. In some area structured mesh and in some other ones non-structured mesh have been chosen. Due to existence of rotatory flows over the steps, more fine meshes have been used in both codes near to body of stepped spillway as well as ogee spillway, because of the velocity gradient nearby wall, where flow enters spillway channel from reservoir. In recent study, water surface profile has been simulated in four types of stepped spillways and 3 types of ogee spillways on the basis of finite element method using FLUENT code and finite volume method using ADINA code. Relatively close consistency has been observed between water surface profiles in both codes by comparing experimental results. Note that free surface profile simulated over the ogee spillway was closer to water surface profile evaluated in laboratory than the profile by finite element method. There were observed better consistency in results from free surface profile over stepped spillway with finite volume method. Maximum differences between results of water surface profile in numerical model of FLUENT and ADINA were nearly 1.2 cm and 1.6 cm, comparing with laboratory results respectively. This value in numerical simulation is acceptable regarding to different meshing in model networking. On this basis, numerical methods of FLUENT and ADINA are proper in order to simulate similar structures for saving costs due to construction of physical model. Flow surface can be determined considering the lower error of FLUENT model in measuring free surface flow; thereby, designing lateral walls of spillway is realized with more precise.

Due to accuray of numerical methods and constitutive equations, a comparison of these methods with the classical methods of dynamic analysis of earthfill dams seems necessary. The results of Seed-Makdisi, Sarma and Ambrasys-Sarma analysis of a homogenous dam are compared with those of Quake/W and Slope/W softwares. A code is developed for calculating permanent displacements based on Newmark double integration method. The results of linear and equivalent linear models show that, due to unrealistic linear assumption of the behaviour of the material, the calculated accelerations are high and the analytical methods are overdesign. This is due to realistic constitutive models. Also, in homogenous dam, as the assumed critical wedge in analytical methods is different from that calculated in numerical analyses, the critical acceleration estimated is different too (0.45g in Ambrasys-Sarma method compared to 0.26g in equivalent linear model for a wedge of 0.6h depth). This shows that realistic models must be utilized. In the developed code first the average aceleration of assumed wedge is calculated using acceleration calculated with finite element codes utilizing modern constitutive models and consequently the permanent displacements are calculated. The results show that the current methods are overestimating. Also, the assumed critical wedges are unrealestic and pass through the dam core. While the critical wedges pass through downstream shell. The permanent displacement of a wedge with 0.6h depth is estimated to be 3 cm compared to zero in Sarma method. The developed code was used for dynamic analysis of Balarud earthfill dams.

Variation of temperature in concrete dams has a direct effect on the thermo-elasticity properties and creep phenomenon of concrete. These variations also, the stress and strains from them should be evaluated as initial loading used in the stability analysis of the dam. In this study, ABAQUS three-dimensional finite element model used to determine the thermal response of a dam under the effect of air temperature variations, solar radiation and water behind of the dam. So, a structural meshing with tetrahedral element block was used that each element had 20 nodes with 6 degrees of freedom in which there was the ability to analyze the stress and displacement.
In this research, the air temperature in the format of average daily air temperature was introduced closely as a boundary condition in ABAQUS model. The water temperature is other boundary conditions that were estimated based on the method proposed by Bofang to predict the water temperature at different depths. Also, in this model, the water level as the average daily water level was applied. The amount of displacement and moment of rotation at the level of dam foundation, both essential as boundary conditions, were used as fixed support and equal to zero. The attenuation model of the concrete was considered using the concrete damage plasticity (CDP) criteria with the coefficient of 3%. The modeling of dam foundation was performed according to a ratio of bedrock modulus of elasticity to the modulus of elasticity of concrete dam and using cubic model. The acoustic element was used to reservoir modeling. This element is the best choice in the modeling of water behind of dam. In this study, the considered hypotheses in the analyzing the dam models were as follow: A) Analysis carried out in a time domain, in which the time range of solving problem would be considered from zero to the alternation time. In dynamic analysis (earthquake), the real-time would be entered into the equation's solution. But in the statical analysis, the alternation time would be considered virtual time, and depending on the amount, the time of performing the program could be lower or higher. The software needs this virtual time to solve the equations step by step in different time periods. Generally, to solve a non-linear statically complex problem such as the provided model, it is recommended to analyze these problems by explicit analyzer and it is suggested to consider the entire time of the analysis longer than the alternation time or natural frequency corresponds to the first figure of the vibration mode of problem. Since time or the necessary frequency period for analyzing arc dam commonly is between to 10 to 30 seconds in dynamical analysis; therefore, this time was considered from zero to 20 seconds in the present study. B) Constituent material of the dam was linear, homogenous and viscoelastic.
The results showed that by thermal modeling, the final displacement direction was toward the reservoir while without thermal analysis and only by hydrostatic force modeling, the hydrostatic pressure tended to displace the dam in the opposite direction. This indicated the effect of thermal changes on the dam body that not only removed the effect of hydrostatic force but also it displaced the body toward the upstream reservoir. Also, the maximum displacement occurred during the warmest month of July and toward the reservoir. Therefore, due to high solar radiation, the slope of the heat changes in areas near the dam’s crest and the downstream areas exposed to sunlight were at high level. The dam body displacements toward the downstream in hot months were more than cold months. This was due to the simultaneous displacements caused by the thermal and hydrostatic loads in cold months. The results of analyzing finite element in different 12 months showed that upstream tensile stress was at low level and as a result, the possibility of cracking due to the static loads is excluded in this area. Based on the average level of the reservoir in 2009, it could be expected that as the water level decrease under the minimum level of the safety operation, the stress level would move from pressure to tensile state and increase the level of the main stresses in the dam body. So the effect of temperature on the creating critical states was assessed more than the effect of the water pressure.
The results obtained from model parameters such as displacement of dam crest, depth of cracked areas and analyzing temperature variation between different points indicated the high ability of this method for evaluation of the stability of the dam. The analysis showed that the maximum temperature and temperature gradient happen in the shallow area of the downstream level. Also, the temperature difference between different points of the downstream level and the same level point was considerable because of the effects of solar radiation. This could reveal that a lack of attention to operation of dam would lead to the occurrence of instability and cracks in whole arc dam body. The result of the proposed model showed that developed methodology had suitable accuracy in prediction of dam thermal behavior under the environmental actions.

General assumption in designing cut off walls is that the seepage be held in a limited amount. In order to control the seepage, the foundation drains are located in parallel to cut off wall at downstream. The main aim of foundation drains construction is to collect seepage water and decrease uplift force. Numbers, distance, and diameters of these drains need technical judgment about characteristics of foundation stones. The depth of drains depends on cut off wall characteristics. In this study, the performance of drains in foundation of concrete gravity dam was investigated using finite element method (FEM) via Seep/w software. Among influential parameters in studying the drain effects, one might consider diameters, center to center distance, and distance of the drains from upstream of dam. In this paper effective parameters on uplift force were analyzed and it was found that increasing the drain diameter had lesser effect on decrease of the uplift force slightly compared to the other parameters. In the other words, selecting the best diameters depends on executive consideration. But decreasing the distance of drains and also their distance from upstream of dam would play an important role in decreasing the uplift force. Focusing to the simulated range of parameters, the drains with 15 centimeter diameter and distance of 3 meters had the optimum performance and the best operation in decreasing uplift force. For validation, the numerical method used in this research was compared with the other analytical methods and the proper agreement with them was observed.

Seepage in earth fill dams is one of the important factors that may cause the erosion and slope instability if it is ignored. Therefore, it is crucial to calculate the leakage discharge through the body of dam and foundation, to prevent economic and life hazards by considering technical and economical issues. In this research, the seepage analysis of the body and the stability of the upstream and downstream slopes of Golfaraj dam after construction, during steady state seepage, rapid drawdown of reservoir and OBE, MCE earthquakes have been investigated and compared by applying different limit state equilibrium methods including: Spencer, Bishop, Janbu, and Mohergenstren-Price, using built-in Geo studio software. The results showed that seepage discharge in a deep section without dike was 18.2 m3day-1 per unit width which was reduced in the case of applying a cutoff or a concrete blanket by 95 and 10 percent, respectively. Data obtained from the slope analysis indicated that the slope was stable under any circumstances and the limit equilibrium methods were convergent.

Cracking of earth dams is a one of the main threat causes of stability of embankment dams. In this research by modeling of the behavior of an embankment dam and employing conditions of the earthquake, the reasons of cracking were inspected using by modeling of earth dam behavior. Based on the literature, one of the main causes of dam failures is sliding and cracking of the dam structure during earthquake. Localized liquefaction of foundation soils was one of the causes of the observed post-earthquake distress within these dams.
Material and In order to study the causes and the results of crack on earth dams, Mahmoodabad earthen dam with a height of 19 m, is located in Zanjan province, northwest of Iran, which suffered a longitudinal crack on the crest and slight sliding of the upstream slope due to 2001 Avaj earthquake was studied. This dam has faced earthquake two times with an interval of two years. During the first earthquake with the magnitude about 6.6 in Richter scale small longitudinal cracks had created on the crest. The developed cracks had been repaired by injecting the cement and then has been hidden by passing the time. After the second earthquake with the magnitude about 6.5 in Richter scale the hidden cracks had been appeared again and the slight movement of the upper slopes of dam reported. Based on the site investigation and documented information about dam, including maps and parameter data, the behavior of the dam has modeled by using Plaxis as a finite element model. In order to check the accuracy of the design of dam, the stability analysis has been conducted using by Xslope as a limit equilibrium model. The foundation conditions and the Geotechnical properties of the layer beneath the dam has been inspected by open excavation.
Underground investigation about Geotechnical properties of dam foundation has showed that there is a thin sandy layer confined in alluvium material of the river base beneath the dam structure. In fact , this layer has not been considered in the analysis as well in design. Because of fully saturated condition of this layer in an operation period of dam it might subjected to liquefaction during the happening of the earthquake. Evaluation of liquefaction potential of this layer based on Seed and Idriss (1971) diagram showed probability of this phenomenon. To prove this hypothesis, the stability analysis had been conducted in two different conditions by including the thin sandy layer and without considering the mentioned layer. The analysis showed in the case of absence of sandy layer the required safety factor was satisfied, but including the sandy layer leads cause the safety factor dropped to 0.84 that means accruing of liquefaction in the thin layer would lead to structural instability of the studied dam. The simulation of the behavior of dam by employing the accrued earthquake acceleration confirmed the liquefaction has been accrued in the thin sandy layer. The results of finite element simulation showed the depth of the cracks on the crest is about 2 meters and also the upstream slope has slipped about 81 mm to the reservoir of the dam. These results was consistent with the observed values. To overcome the next risks, also to repair the damaged parts of the dam, 3 different methods had been proposed. The curing technics was deploying of the reservoir and removing of the damage part of the dam and as well the thin sandy layer and reconstructed that part of dam, Deploying of reservoir of the dam and adjusting the slope of the upper shoulder to stable condition and at least repairing the developed cracks by injecting cement slurry and tolerate the current condition without imposition any additional costs to the project. The third method has been selected, but for any probable risky condition monitoring of the dam has been advised.
Based on the overall results of the investigations, it was concluded that cracking and sliding of upstream shell is mainly due to the presence of a loose, fine sandy layer under the base of the dam which was liquefied during the earthquakes. In order to overcome the problem, one of the three offered solutions, including modification of cracks by injecting cement slurry, was applied and after roughly 9 years, the provided reports confirmed the adequacy of the applied solution.

In this research, the shallow-water equations are described and the governing equations are discretized using Rayleigh–Ritz method in order to assemble the required local and global stiffness matrices. The depths and the velocities of fluid flow throughout the channel and at different times are acquired by solving the governing equations using non-explicit methods. The obtained results from the numerical model with three, five and seven node elements were compared to the pre-existing results of other researchers, and it was observed that they are in a reasonably good agreement. It was concluded that employing the non-linear elements, oscillations induced by using linear models can be efficiently reduced. Subsequently, the model was utilized to investigate Ardak dam-break as a case study. In this study, three equivalent-roughnesses (Horton, Lotter and Pawłowski) were used for the numerical simulation of the flow induced by dam-break and to calculate water depths and velocities at different times and places through the river. The maximum average-depth throughout the river was 24.78m with using Pawłowski roughness, and the maximum average-velocity was calculated 15.05m/s by using Lotter roughness at t=100s.In this research, the shallow-water equations are described and the governing equations are discretized using Rayleigh–Ritz method in order to assemble the required local and global stiffness matrices. The depths and the velocities of fluid flow throughout the channel and at different times are acquired by solving the governing equations using non-explicit methods. The obtained results from the numerical model with three, five and seven node elements were compared to the pre-existing results of other researchers, and it was observed that they are in a reasonably good agreement. It was concluded that employing the non-linear elements, oscillations induced by using linear models can be efficiently reduced. Subsequently, the model was utilized to investigate Ardak dam-break as a case study. In this study, three equivalent-roughnesses (Horton, Lotter and Pawłowski) were used for the numerical simulation of the flow induced by dam-break and to calculate water depths and velocities at different times and places through the river. The maximum average-depth throughout the river was 24.78m with using Pawłowski roughness, and the maximum average-velocity was calculated 15.05m/s by using Lotter roughness at t=100s.In this research, the shallow-water equations are described and the governing equations are discretized using Rayleigh–Ritz method in order to assemble the required local and global stiffness matrices. The depths and the velocities of fluid flow throughout the channel and at different times are acquired by solving the governing equations using non-explicit methods. The obtained results from the numerical model with three, five and seven node elements were compared to the pre-existing results of other researchers, and it was observed that they are in a reasonably good agreement. It was concluded that employing the non-linear elements, oscillations induced by using linear models can be efficiently reduced. Subsequently, the model was utilized to investigate Ardak dam-break as a case study. In this study, three equivalent-roughnesses (Horton, Lotter and Pawłowski) were used for the numerical simulation of the flow induced by dam-break and to calculate water depths and velocities at different times and places through the river. The maximum average-depth throughout the river was 24.78m with using Pawłowski roughness, and the maximum average-velocity was calculated 15.05m/s by using Lotter roughness at t=100s.In this research, the shallow-water equations are described and the governing equations are discretized using Rayleigh–Ritz method in order to assemble the required local and global stiffness matrices. The depths and the velocities of fluid flow throughout the channel and at different times are acquired by solving the governing equations using non-explicit methods. The obtained results from the numerical model with three, five and seven node elements were compared to the pre-existing results of other researchers, and it was observed that they are in a reasonably good agreement. It was concluded that employing the non-linear elements, oscillations induced by using linear models can be efficiently reduced. Subsequently, the model was utilized to investigate Ardak dam-break as a case study. In this study, three equivalent-roughnesses (Horton, Lotter and Pawłowski) were used for the numerical simulation of the flow induced by dam-break and to calculate water depths and velocities at different times and places through the river. The maximum average-depth throughout the river was 24.78m with using Pawłowski roughness, and the maximum average-velocity was calculated 15.05m/s by using Lotter roughness at t=100s.