جستجوی مقالات مرتبط با کلیدواژه « روش عددی » در نشریات گروه « زمین شناسی »

Nowadays, according to the increase in fuel demand, the oil wells drilling process has increased to exploit the reservoirs. The most important issue in oil and gas wells drilling topic, is the appropriate drilling rate and reducing the cost. Studies and the usage of geomechanical methods can play an important role in this matter. One of the studies regarded to the topic of reducing drilling costs that has received a lot of attention today is the well wall stability and determining the appropriate mud weight to avoid instability in the well wall. In recent years, a lot of research has been done in this field. In the present study, the well wall stability status of one of the excavated wells in Iran’s southwest oil fields, in 88.3 inches, in the upper section of Sarvak Formation was analyzed by using analytical and numerical methods through FLAC3D software. The input parameters in this research are obtained by interpreting the data from Dipole Shear Sonic Imager (DSI), the logs from Formation Micro Imager (FMI), density, caliper and gamma, MDT tools and also from the daily excavation and geology reports. Also, the window for the required mud weight to meet the dissipation situations, fracture opening, well wall symmetric downfalls, and well eruption was determined. The beneficial results of this study can be used to reduce the time spent on excavating the wells in the investigated field in the future, which results in saving drilling costs.

Estimation of Liquefaction is one of the main objectives in geotechnical engineering. For this purpose, several numerical and experimental methods have been proposed. An important stage to predict the liquefaction is the prediction of excess pore water pressure at a given point. In general, there are two important methods for soil dynamics analyses, fully coupled effective stress and uncoupled total stress analysis. The main purpose of this study is to evaluate the model capacity of the finite difference software, FLAC, based on effective stress analysis methods to predict the excess pore water pressure during seismic loading. A level ground centrifuge test conducted during the VELACS project on the Nevada sand with a density of 40%, was utilized to calibrate the numerical model. After the validation of the numerical model, a model was conducted to predict excess pore pressure and consequently the liquefaction for the site of Bandar Abbas Mosque. 

  A fully coupled u–P formulation, where pore pressures and displacements are computed simultaneously and interactively at each time step, is used in FLAC software. This feature is used to simulate the excess pore water pressure time histories during cyclic loading. The finite difference based software, FLAC, used the Finn model that incorporates two equations correlating the volumetric strain induced by the cyclic shear strain and excess pore water pressure produced during cyclic loading. As mentioned above, the pore water pressure generation can be computed from two sets of equations: Martin et al. (1975) and the Byrne (1991) formulations in which the volumetric strain that was produced in any cycle of loading is depended on the shear strain that was formed during that cycle as well as the previously accumulated volumetric strain.

The VELACS model # 1 centrifuge test representing a level ground site constituted of the Nevada sand at 40% relative density has been numerically simulated in the current study to validate the numerical model. The centrifuge model contains a laminar box with slipping “rings” that allows differential horizontal displacements. This was simulated in the FLAC model by free-field boundary conditions which prevent reflection of the waves in the side walls. Figure 1 shows comparison of EPWP time histories ratio of numerical modeling and centrifuge test. Static analysis was carried out before dynamic analysis in order to find initial stress and strain state. At the next stage, the dynamic loads were applied at the base of the model and dynamic analysis was performed.The Bandar Abbas mosque project is located approximately 500 meters from the coast. In the project, due to the groundwater level and the existence of loose layers of silt, investigating the potential of liquefaction is necessary. For numerical modeling the results of the general soil mechanics test on soil samples and standard penetration test performed on the site were used to calibrate the parameters and select the model constants.

The results of numerical modeling have been matched to experimental results of the centrifuge test using both Martin and Byrne formulations, except for the case of 5 m the numerical model has predicted lower excess pore water pressure values than the experimental values. This may be originated from the fundamental assumption of the Martin et al. (1975) EPWP theory, in which excess pore water pressure is directly related to the relevant volume changes. On the other hand, the Martin et al. (1975) model was adopted for one-dimensional measures of shear strain, while, in a 2D analysis under both horizontal and vertical shakings, there are three strain rate measures. FLAC uses some assumptions to solve this problem and it can affect the results.The results of the numerical model showed liquefaction to a depth of about 5 meters that is almost compatible with the results from the lab, which has declared that the depth 2 to 5 m is liquefiable. With careful selection of numerical model parameters one can generally use the simulation results to have a general sense on the pore water pressure generation and liquefaction prediction.

The study of variations of damage intensity for different earthquakes indicates the importance of the site effects on earthquake induced damage and ground motion parameters such as peak ground acceleration and induced amplification. In the occurrence of an earthquake, local site conditions such as soil characteristics, dimension of topographic irregularities, seismic bedrock depth, etc. as well as characteristics of incident wave have important e ects on seismic ground responseLocal site effects play an important role in earthquake-resistant designs and should be assessed carefully. By assessment of induced damages to structures and major infrastructures, seismic geotechnical researchers have concluded that the site conditions significantly influence on the failure distribution in urban and rural areas. Consequently, to determine the characteristics of seismic motions of the ground, it is essential to study the effective geotechnical factors. Consideration of the effects of the site response in the design of civil structures systems is of important to mitigate the damages to a certain extent on structures and the environment. Hence, it is relatively crucial to reliably attain the dynamic soil parameters of an earthquake-prone city/state. The nature of local site effects can be explained by using different methods such as simple theoretical analysis of ground response, measuring of surface and subsurface actual motion at the same site, and measuring the movements of the ground in sites with different conditions in comparison with the proposed site. In this paper, the effective factors on ground motion and site response in the soil classes II and III for Tehran city under the influence of Tabas and Bam earthquake were reviewed with numerical method and by using the finite difference software (FLAC 2D). Some of the effective parameters of the site dynamic characteristics on the seismic response of the ground surface have been studied and the response of peak ground acceleration and the resonance function has been compared. The results of this study show that at both sites, the horizontal maximum acceleration of the ground surface increases with respect to the bedrock shear velocity, reflecting the influence of the site on ground movement. At both sites, with increasing soil internal friction angle, the maximum acceleration at ground level increases and the acceleration response decreases. Also, as the shear wave velocity increases, the horizontal maximum acceleration at ground level for both structures decreases. Increasing the shear wave velocity in the surface layer, assuming the thickness of the layer is constant, results in a decrease in the natural frequency, which is far from the normal frequency range of conventional structures in Tehran. This indicates a decrease in the earthquake's dynamic force on surface structures. The natural frequency of five to seven story buildings in Tehran is about 0.5 to 0.7 Hz. As the surface layer thickness and bedrock depth increase, the horizontal maximum acceleration of the ground surface also decreases, meaning that the structural response can also be reduced.

Retaining walls are designed to withstand lateral earth and water pressures, the effects of surcharge loads, and the self-weight of the wall and, in special cases, earthquake loads in accordance with the general principles specified in this section. Retaining walls are constructed for a certain service life based on consideration of the potential long-term effects of material deterioration on each of the material components comprising the wall. Permanent retaining walls are designed for a minimum service life of 50 years. Temporary retaining walls should be designed for a minimum service life of 5 years. Gravity retaining walls rely on their self-weight to resist lateral earth pressures. Analysis of the seismic behavior of gravity retaining walls during earthquake loading is a quite complex task. Seismic wall movements can occur as sliding or rotational displacements. In some cases, only one of these displacements can be dominant and for some of them, both sliding and rotation can occur. Foundation soil deformability, backfill, wall stiffness, and input record motion as the main variables used in the analysis of walls are subjected to a strong earthquake. The analysis of the seismic stability of walls retaining backfill soil is based on the following assumptions: (1) the wall–soil system is long enough for ignoring the end effects (plane strain condition); (2) the soil is homogeneous, dry, and cohesion-less; (3) the retaining wall is subjected only to horizontal displacements; (4) the seismic action is uniform horizontally distributed in the whole mass of the system; and (5) the failure wedge is a plain. Furthermore, the upper bound limit analysis is based on the assumption that soil will be deformed according to the associated flow rule and the convexity of the soil yield condition. In the following analysis, we assumed that these conditions are met. For many decades, the seismic analysis of retaining walls has been based on the simple extension of Coulomb’s limit equilibrium analysis, which has become widely known as the Mononobe-Okabe method. The method modified and simplified by Seed and Whitman has prevailed mainly because of its simplicity and the familiarity of engineers with the Coulomb method. Designing walls for stability against earthquake risks in seismic zones is done through the analysis of the seismic behavior of the soil-structure system. The methods established using newmark sliding block procedure are based on forces (pseudo-static and pseudo-dynamic) and allowable displacements. These methods are frequently used in the seismic design. Dynamic analysis of retaining walls can also be done by finite-element methods. ABAQUS is among the computer programs that suite for finite-element analysis. In this study, a series of finite elements were carried out in ABAQUS in order to find out typical wall movements including rotation and lateral top and base displacements. This research presents that many variables such as maximum acceleration, properties of foundation and backfill soils, and characteristics of the wall affect the seismic behavior. Design charts were derived from the numerical analyses to predict both lateral displacements at base and top. The proposed charts consider the most relevant factors in the system response. The result obtained can be used to develop an optimum design procedure for gravity retaining walls.

Lack of proper drinking water is a major problem in the Central and Eastern cities of the country. Therefore, the necessity of water transmission projects are posed and since meeting these needs using traditional methods are very difficult, time-consuming and costly; so the mechanized excavation of tunnels and segmental lining are used. The water transferred through the pipes has significant surface destruction. That would result in serious environmental problems. Also, transferring water through the pipes is more expensive than the tunnel. In this paper, after introducing all procedures of segments design, segmental lining of Golab water conveyance tunnel using finite element was modeled. Firstly, loads on segmental lining was determined by UDEC software after installation in tunnel. Then, these amounts of load on the segments used for modeling was applied in ABAQUS software. Results obtained by 3D modeling software showed that the greatest amount of displacement of segments under different stress conditions concerns to the bottom segment. Based on the performed modeling, due to the high compressive stress, the crack is only seen in segment concrete and these results are totally in compliance with Golab tunnel segments observations.

Nowadays urban transportation is one of the most important problem in the large cities. The best way to reduce this problem is to use the underground tunnels. The excavation of tunnel and other underground structures lead to remove a part of insitu rock and soil masses and the considerable changes in stress conditions around them and surface settlement has been occurred, consequently. Therefore, estimation of surface settlement due to the tunnel excavation, especially in urban areas, is of great importance. The Niayesh subway tunnel was built in the northern part of Tehran and it includes north and south tunnels. From a geological point of view, the Niayesh highway tunnels are mostly placed in the Hezar Dareh Formation (A) and also some parts especially the eastern and middle parts of north tunnel are placed in Kahrizak Formation (Bn). The tunnels have been excavated by the New Austrian Tunnelling Method (NATM). In this method, primary shotcrete applied as a short-term support and the final lining is used to satisfy final support. In this paper, surface settlement has been studied by empirical methods, numerical analysis and real settlements. For the empirical and numerical methods, O’Reilly and New (1982) method and finite element method by PLAXIS2D software have been used, respectively. The five sections (CS-1 to CS-5) have been selected in order to study the surface settlement during Niayesh tunnel excavation. On the basis of the achieved results, the numerical method in all sections (except section 3) is in agreement with the real settlements. However, empirical methods have estimated the settlements more than real settlements in these sections. Also, the achieved results from real settlements, empirical methods and numerical analysis show that the maximum settlement due to tunnel excavation is more than allowable settlement and it places in the warning range.

Tunneling in complex geological and geotecnical conditions is often inevitable، especially in urban areas. The stability analysis and the assessment of ground surface settlement of a shield tunneling are of major importance in real shield tunneling projects. The objective of this research is to determine the collapse pressure of a shallow circular tunnel driven by a Tunnel Boring Machine (TBM) of the Earth Pressure Balance (EPB) type. In this study، analytical methods and three-dimensional numerical modeling with ABAQUS software were implemented to examine the effect of face pressure on the behavior of tunnel. The parameters were calculated using data from Karaj subway- line 2 in a case study. The analytical method used in this study is: Leca-Dormiex which is based on limit analysis stress. The method is based on a translational multiblock failure mechanism. Also، elastic and Mohr-Colomb constitutive model have been used for soil behavior. The results of analytical method and numerical modeling were then compared. According to the findings of this study، face pressure assessed from the analytical method of Leca-Dormiex (upper bound) is the minimum pressure that can be implemented on the face tunnel. Also، analysis shows that with implemention of suggested pressure of analytical method، Karaj subway face tunnel is most probably stable and consequently execution of pre-consolidation methods in this section of the tunnel does not seem to be necessary.