جستجوی مقالات مرتبط با کلیدواژه « اجزای محدود » در نشریات گروه « زمین شناسی »

The extensive knowledge of groundwater resources' status has double importance for the planning of the sustainable future in the arid regions. Strictly, without an efficient model with accurate simulation, it is impossible to have a good understanding of groundwater behavior. Since groundwater modeling in real problems comprises irregular boundaries, heterogeneity, anisotropy, and conceptualization, there exist many challenges in the simulation process. Due to proper acceptable performance, this study aims to present a proper framework to simulate groundwater flow based on Finite Difference (FD), Finite Element (FE), and Meshfree (Mfree) methods. The governing equation of groundwater flow was discretized through FD, FE, and Mfree approximations, and their performance was examined in two case studies: hypothetical case and field study. The findings of this study revealed that the Mfree has more applicability than others because some noticeable challenges were found in the simulation process of the FD and FE. Comparison of analytical solutions and simulated values in the hypothetical aquifer indicated that all methods are compliant. In terms of the RMSE criterion, the Mfree, FE, and FD were evaluated respectively, 0.005, 0.016, and 0.018 m. However, the numerical methods' act, especially the mesh-based methods (FD and FE) in the field study, becomes less accurate. Performance assessment of the FD indicated that this method has good ability in a simple test case with regular geometry, while in an aquifer with irregular geometry associated with isotropy and heterogeneity, its accuracy is greatly diminished. So that, the RMSE for FD methods in field study was evaluated 1.77 m. The same results may be obtained from the FE method, such that its RMSE was obtained 0.36 m. Investigations of this study showed that the FE needs time-consuming preprocessing practices to implement in the different aquifer. The results of the Mfree application indicated that this method is more efficient than others due to its very high flexibility in irregular geometry with isotropy and heterogeneity issues. The RMSE for the Mfree in the field study obtained 0.26 m.

Hydraulic fracture is known as one of the effective methods for producing or being caused a change in the physical structure of a reservoir rock. In oil and gas reservoirs which have been fractured by a hydraulic approach, this method causes that a production well and the time of reservoir production increase. In this research, fracture analysis have been comprised with the outputs produced by previous models. Its results show that the opening intersection produced by hydraulic fracture has been being changed in various time during pumpage, therefore, the fracture should be kept open by propanent substances. In the next stage, the amount of porosity pressure in the trend of the fracture is considered. In this survey, two parameters, i.e. the fracture measure and the change of pore pressure have been obtained using the hydraulic fracture modeled process under the actual condition of pay zone and its confining layers, and by the finite element approach. In this method, pumping processing has been assigned for fluid and propanent. At the end, by making a comparison between these result and the results obtained from previous studies, it can be found out that this approach is applicable and efficient.

Concrete faced rockfill dams have been considered in recent years more than other types of dams due to their low dependency on the bed and the shape of the valley, as well as the simpler construction technology. In this regard, rockfill dams are a suitable substitute for embankment dams because of higher stability of the body and the availability of rock aggregates. On the other hand, because the permeability of rock aggregates is much higher than other materials, different methods are used to seal these types of dams. One of these methods is the use of non-impermeable concrete facing in the upstream of these dams. This particular type of gravel dams is called Concrete-Faced Rockfill Dams (CRFD). In this study, a contact element with a definition of elastic-plastic failure in the modeling process is proposed to simulate the surface separation and re-contact of the concrete face with the rockfill surface of the dam.

In this paper, behavior of a concrete faced rockfill dam under earthquake loads is investigated. For this purpose, near-field earthquake records with focal depth lower than 15 km (for example Tabas earthquake 1978, M=7.4, and San Fernando earthquake 1970, M=6.6) are used. Moreover, to study the dam behavior under dynamic loads, interaction between concrete face and rockfill part of the dam is investigated and finally, some parameters including displacement, absorbed energy and base shear are evaluated. So, finite element method and Abaqus software is used for the study. Verification of the models is carried out using the results of previous researches by conducting modal analysis and determining natural vibration period. Then, the interaction between the concrete face and rockfill part as well as the effect of water level changes in stability of dam under dynamic load is investigated. Concrete behavior is simulated using concrete damaged plasticity. Therefore, concrete density, compressive strength and tensile strength and elasticity modulus are 2350 kg/m3, 25 MPa, 3 MPa and 29 GPa, respectively. Poisson’s ratio is assumed to be 0.2. Furthermore, 4-node shell elements are used to simulate concrete face and Drucker-Prager constitutive model is used to define rockfill material behavior. The density and Poisson’s ratio for 2B, 3C and 3B layers are 2150 kg/m3 and 0.35, respectively. The shear modulus values for these layers are respectively 8.93, 2.89, and 3.85 GPa. In order to perform the simulation, the part of the dam structure beside the bed rock and the surrounding rock is considered as fixed bearing, and only the rockfill part and concrete face of the dam is simulated. Based on this assumption that the bed is rigid, there is no need to consider the dam foundation. This method is frequently used in literature review. All the surfaces of the dam and bed rock are considered as fixed bearing to simulate the real condition where the dam is attached to bed rock and the surrounding rock. The interaction between dam layers is defined as tie. For defining the interaction between rockfill body and concrete face, tangential and normal contacts are defined using penalty method with friction coefficient equal to 0.5. In the next step, the model is meshed using 4-node shell elements for concrete face, 8-node brick and 4-node pyramid solid elements for rockfill body. Rayleigh damping is used to simulate the structure damping. The effective length of the dam reservoir has been determined by conducting several analyzes, so that the minimum required length for reservoir is reached in order to decrease the number of elements of the model.

1. Interaction between concrete face and rockfill body  The results show that the increase of friction coefficient between concrete face and rockfill part from 0.5 to 0.7 has not affect the displacement of dam crown along the earthquake direction. However, when the concrete face is fixed to the rockfill part, significant changes are induced in dam crown displacement time history. In all cases, the deflection due to the dam weight is increased when the concrete face is attached to the rockfill body. The reason can be attributed to the tied interaction between these layers which results in similar deflection of concrete face with rockfill body and higher deflection of concrete dam crown. However, after the application of earthquake load, the displacement of the dam crown decreased in both analyses when tie interaction is defined between concrete face and rockfill body. In this study, due to the very high volume of analysis and its timeliness, it was not possible to examine the dam behavior in the free vibration regime, and therefore, it is not possible to assume the last displacement values at the end of analyses as the permanent displacement of dam. Figure 1 shows the relative displacement of the dam for the two selected earthquakes with a friction coefficient equal to 0.5 between the concrete face and the gravel body. According to Figure 1, the maximum displacement induced by the earthquake is related to Tabas and then, San Francisco earthquake. Furthermore, the high energy content of the Tabas record has been more effective in inducing greater displacement than the other record. Figure 1. Lateral displacement of dam crown relative to the dam base for the selected earthquakes; Tabas and San Fernando. The results also indicate that when the friction coefficient between concrete face and rockfill body is 0.5, the lowest damage occurs in the dam compared to that happens when friction coefficient is 0.7 or when the surfaces are tied. When the tied surfaces are used, the most damages takes place in concrete face, since all rockfill body displacement transmits to concrete face which results in much more concrete damages compared to the other interaction cases. 2. Effect of water level in reservoir on dam behavior In this section, the effect of water level on seismic behavior of dam is investigated. For this purpose, the dam reservoir is analyzed in three cases including empty, half full and full (90% of dam height). Each study cases are examined under San Fernando and Tabas earthquakes. Figure 2 shows the relative displacement of dam crown in the three water level case for San Fernando and Tabas earthquakes. Figure 2. Relative displacement of dam crown in three water level cases of empty, half and full for (a) Tabas and (b) San Fernando earthquakes According to Figure 2, for both earthquakes, the dam crown displacement along the earthquake direction is significantly increased by increasing the water level, so that the maximum displacement in full case is 50% higher than empty case.

In this study, using the finite element method and simulation by Abaqus, the seismic behavior of concrete face rockfill dams has been investigated. For this purpose, the verification is firstly carried out using previous research results in literature. In the next step, nonlinear dynamical analysis is carried out, taking into account large displacements for the models under the earthquake record acceleration. The results illustrate that increasing the friction coefficient between the concrete face and the rockfill body from 0.5 to 0.7 has no significant effect on the displacement of the dam crown under earthquake load. Moreover, by using tie interaction between the concrete layer and the rockfill body, there is a substantial difference in the history of the relative displacement of the dam, and the displacement of the dam due to its weight has been increased. Furthermore, the results of this study exhibit that, with increasing the water level in dam reservoir, the deformation of the crown of the dam along the earthquake application direction has had a relatively significant increase, such that in the full state, the maximum displacement is increased by about 50% compared to that of the empty case. This is while the most damage of concrete is observed in the case when half height of dam in filled by water. Due to the more destructive power of near-field earthquakes and their impact nature, only near-fault earthquakes have been used in this research. Therefore, the results of this study are valid only for the behavior of dam under near-field earthquakes.

Rectangle array is widely used in resistivity and induced polarization (IP) studies. The purpose of this array is to restrict the wide areas especially in the exploration of sulfide minerals. On the contrary to the wide application of this array, less attention has been paid to the results of modelling and true estimates. The interpretations are normally qualitative. A 3D resistivity and IP model was developed for the geoelectric surveys with a rectangle array. We used the COMSOL environment to solve the DC-resistivity and Maxwell’s equations by the finite element method. Codes were programmed in Matlab language. A common geometry of the model space was used for both resistivity and IP modelling. In the rectangle array, two current electrodes were located in a large distance and different potentials were measured on the profiles parallel to the current electrodes. Our model was formed by a homogeneous half space (a large block with dimensions 800 × 800 × 500 m3, with a resistivity of 400 ohm.m). Two current electrodes with a 200-m distance were located on the surface. Non-polarizing electrodes were located in a 5-m distance. The two measuring electrodes were moved on the profiles (parallel to the current electrode direction). Nine parallel profiles were located symmetrically on each side of the current electrode direction. Each profile had a 40-m length. The distance between the profiles was 5 m. The electrode configuration could be changed in the model. IP and resistivity anomalies could be created from different blocked locations in the subsurface (into the half space). The blocks near the potential profiles had small dimensions. The block sizes increased as the depth increased. We calculated the geometrical factor for the half-space. Apparent resistivity for each dual potential electrode was calculated from different potentials measured during the code execution and its geometry factors. We compared the results from different anomalies by sensitivity Δρa/Δρi, where Δρa is the difference between the apparent resistivity of the anomaly and the homogeneous half-space (400 ohm.m) and Δρi is the difference between the resistivity value of the half-space and the anomaly in block number i. Frequency domain IP was calculated directly from Maxwell's equations. Block scheme of the model done in the modelling space resistivity were used here. There was a resistivity value for each subsurface block in the resistivity model while there were a resistivity and a dielectric value for each block in an IP model. Resistivity and dielectric values of each block are functions of the frequency. We used the Cole-Cole model in order to calculate the resistivity and dielectric values in each frequency. Four intrinsic Cole-Cole parameters (DC-resistivity, chargeability, time constant and frequency relaxation) were considered for each block. During the frequency changes, these parameters were constant. Finally, apparent resistivity and percentage frequency effect (PFE) maps were calculated in a frequency range of 0.1 to 12000 Hz. In this research, we studied the effect of size, depth and overburden thickness of the subsurface anomalies. The geoelectrical effects of vertical and horizontal anomalies were investigated. The impact of the potential electrode separation was also verified. The results showed that the qualitative interpretation using the apparent resistivity and appearent percentage frequency effect (PFE) maps was correct when anomaly had remarkable dimensions, a small depth and a high conductivity. The apparent-resistivity map reflected the effect of conductive and polarisable anomalies better than the PFE map.

Underground excavations provoke in their vicinity a region where the rock is disturbed, i.e., loosened due to micro as well as macro fractures. The shapes, dimensions, and properties of such so-called excavation damaged zones (EDZ) are of increasing importance for the planning and construction of geotechnical barriers in underground repositories for toxic and problematic wastes. Dynamic stability assessment of rocks after underground excavation is important. Mechanical changes related to an excavation damage zone (EDZs) leads to changes the physical properties of rocks. In fractured and unsaturated materials, resistivity is sensitive not only to the matrix electrical properties but also to the saturation of the water phase and to the density and orientation of cracks. Recent studies show that electrical resistivity tomography (ERT) and induced polarization (IP) methods are capable of monitoring the mechanical behaviour of EDZs. ERT and IP methods are performed in the galleries which are excavated in clay-rocks of the Tournemire test site located in the south of France. The aim of these geophysical investigations is the characterization of EDZ zones and hydro-mechanical behaviour. ERT is performed for an arc profile on the walls of the gallery, with 43 electrodes arranged on the floor and wall with a distance of 20 cm between them. Non-polarizing electrodes of Cu/CuSO4 were used. Interpretation of the ERT section on the straight line profile, carried out on the floor of the tunnel, confirmed the existence of a high electrical resistivity zone near the surface (a fractured and partially saturated zone with a depth of 50 cm). A 2D electrical resistivity model was developed to perform a tomography survey for the arc profiles on the walls of the underground excavations (horseshoe section). All the Wenner, dipole-dipole, and Schlumberger electrode arrays can be used for ERT surveying. Current and potential electrodes can be arranged on the floor, walls and ceiling of the tunnel, with equal intervals. A finite element method was performed in order to solve the Poisson equation for all points of the model space with respect to boundary conditions. The finite element approach involves solving a discretized form of the weak formulation of the Poison equation. For each quadripole (two current and two measured electrodes), the code is run once and the electrical resistance can be calculated. The geometric factor of electrical array can be calculated when the code is run for a homogeneous electrical conductivity earth around the tunnel. The space model surrounding the gallery (up to 10 times the distance to the tunnel diameter) is divided into quadrilaterals whose conductivity can be changed. The Neumann boundary condition is considered for the inner and outer surface of the tunnel wall. The outer surface is far from the walls of the gallery.

In this research, two-dimensional analysis is applied to surrounding underground excavations that behave in nonlinear elasto-plastic manner. A computer program (FEP: Finite Element Program) was developed based on finite element method. The main output of applying the FEP is the calculation of the value of stress and strain in elasto-plastic conditions. The principles of FEM in stress analysis are first described, then the principles of solid mechanics explained, and consequently the final formulas of finite element analysis are derived. Main formulas of plasticity are defined and then combined with elastic formulas to acquire the FEM elasto-plastic formulas. The details of subroutines can be found after the theoretical discussion. Finally, the ability of software was examined by execution of the FEP with theoretical and practical examples. A good agreement is observed when the FEP is compared with other program (Ansys) and empirical methods. Criteria definition of rock mechanics is the main advantage of the FEP with respect to other softwares.