numerical modeling

Today, tunnels are one of the main vital arteries for a city, which are developing significantly. The development of today's societies has caused the need for tunnels in various sectors, including urban public transportation, intercity transportation, and water and sewage collection and transmission networks, etc. It goes without saying that for underground structures such as tunnels, the existence of faults and consequently earthquakes are serious threats among natural hazards. Although tunnels should not be located near active faults, sometimes passing through them is unavoidable. Sometimes, after the construction of the tunnel, the existence of the fault is known. In such cases, the deformation caused by the fault is considered a big concern and has a significant effect on the behavior of the tunnels. Meanwhile, due to the fact that most urban shallow tunnels are built in loose ground, the necessity of studying the behavior of tunnels and ensuring the safety of these structures against failure is of great importance. The advantage of numerical methods, in this research, the effect of different parameters such as the thickness of the piece, the depth of the tunnel placement and the fault angle on the behavior of piece tunnels has been investigated using FLAC 3D software. The main goal of the research was to know the possible failure mechanism of segmental tunnels due to faulting. In this research, 24 numerical models were built to understand the behavior of segmental tunnels under the effect of reverse faulting. The validation of this numerical modeling is with the physical model of Kiani et al. The ways to improve the performance of segmental tunnels when faced with reverse fault movement is to increase the rigidity of the tunnel. In this research, the effect of the depth of the tunnel placement due to the reverse fault was also investigated. It was observed that increasing the depth of the tunnel placement when facing reverse faulting leads to a decrease in the deformation of the tunnel diameter. Also, the effect of changing the fault angle for the tunnel was investigated. Increasing the angle of the fault causes the change of places created on the ground surface and the displacement of the tunnel roof to decrease.

Excavation of underground spaces provides locality for military and defence application. The stability of the excavation wall is one of the important issues in the field of geotechnics. The use of steel anchor elements is one of the sustainable methods for stability of excavation walls. These anchors pin the surface of the wall to the back soil, providing the wall's stability. In this research, the behavior of soil nailing, anchored walls and their combination against the blast load has been modeled by Abaqus finite element software. Also reinforcing elements are placed in different walls so that the safety factor becomes 1.5. Different numerical models are chosen to determine the best plan for dealing with the dynamic load caused by the explosion. The explosive charge in software is equivalent to the 120 kg TNT explosion, and this load is applied at a distance of three times of the depth to excavation. Also the modeling of full anchored wall has been verified in static mode. Based on the results, the deformation of the top of the wall after the explosion increased by 5 to 65 times of those before the explosion in various models and the primitive deformation caused by static load increased after excavation. In a wall that was completely reinforced by nails, static analysis showed the highest amount of deformation, but after the explosion, the wall reinforced by the combination of anchors and nails showed the most deformation. The safety factor of the nailing wall before the explosion was 1.5 and then reached to 1.01. The explosion also caused a 110% increase in tensile stress of the Anchors.

An accurate and safer analysis for structure is possible if we can identify factors in the analysis more accurately. One of the important factors in the analysis is the identification the kind and intensity of structural loading. Among the types of loading, especially dynamic loads and impact loads resulting from the explosion, is far more complicated and difficult to determine. According to lab explosion modeling problems, using numerical modeling to analyze these phenomena can be reasonable. Uses grid methods like Finite element method caused a numerical error due to intense deformation and high velocity of blast. In this paper, underwater explosion was modeled and programmed with numerical mesh-less method, Smooth Particle Hydrodynamics (SPH), using Fortran programming language and explosion pressure and water level changes in the blast were studied. Finally the solving problem is compared with empirical relation. The results of this model are very similar to empirical relationship. This program can be used for underwater explosions modeling and the results can be used to determine pressures and impacts on marine structures.

There are major differences between the behavior of explosive and another loads due to the nature of the explosive charges and the small duration of shock loads. Study on underwater explosion effects is important for evaluation of damages caused by the terrorist activities such as hydraulic structures and marine docks, dams, transmission lines in the seabed and etc. The high cost of laboratory modeling and explosion risks due to blast loads and sudden and severe deformations caused by an explosive charges in a short time, justify the use of numerical methods. In this paper the experimental results were compared with grid base numerical modeling (Eulerian and Lagrangian nature) and a meshless method (smooth particles hydrodynamics, called SPH). The numerical simulations are carried out based on the Ls-Dyna software and the maximum distance from the center of the explosion pressure, pressure time history curves at different points, and changes in water level are compared in different numerical methods. The results show that, Lagrangian approach and meshless method presents higher explosion pressure than Eulerian approach. Also, using the meshless methods, the explosion pressure can be determined in closer distance to a charge center, compared to another grid base methods.

Bearing capacity of shallow foundations under blast loading depends on various factors such as the severity of the explosion, buried depth of the foundation and its materials, but comprehensive study hasn’t been done on the impact of these factors on the bearing capacity of this kind of foundations. The method of calculating bearing capacity of foundations under blast loading follows that after applying initial stress due to the soil gravity, during different loading in each step, the foundation will be placed by uniform stress. After analysis of the model in the static mode, the model will be analyzed under the blast loading again, and then the average settlement of points below the foundation is calculated. This procedure is performed for various stress levels and finally the settlement-stress graphs are plotted. This stress increasing continues until the soil failed under the blast loading. Then bearing capacity of the foundation under the blast loading is estimated by the cross-tangent method on the settlement-stress graph. The results indicate reduction of the bearing capacity and the asymmetrical settlement of the foundation under blast loading. Also under the blast loading like the static condition, bearing capacity is increased by increasing the buried depth.