جستجوی مقالات مرتبط با کلیدواژه "buried pipelines" در نشریات گروه "عمران"

Buried pipelines are used to transport water, liquid fuel, gas, oil, etc and they must remain in service in all circumstances such as permanent transverse ground deformation caused by slope instability. In the literature, modeling of soil-pipe interaction is carried out by using two methods of soil-equivalent springs and continuum. In this paper, numerical simulation in domain of continuum is applied to predict the behavior of the pipe embedded inside the slope. Problem modeling has been done in the FLAC 3D software by using finite difference method. The effect of parameters such as pipe diameter and thickness, width of the slope, soil adhesion and internal friction angle of soil on soil and pipe interactions were investigated. Comparison of the present numerical model with other numerical, analytical and physical models, indicates that the maximum displacement of the transverse ground and pipe occurs in the center of the area and reaches zero in the sides. Also, the anchor and force in the pipe are symmetric to the center of the model and reach a maximum value in the center. By taking steps such as increasing the diameter of the pipe, increasing the thickness of the pipe wall and reducing the angle of the slope, the displacements, tensions and strains created in the pipe can be reduced to some extent. On the other hand, the internal angle of friction of the soil and cohesion are effective in increasing the soil's resistance and also reducing the deformations in the pipe.

Reinforcing soils with geosynthetics is an effective method for improving the uplift capacity of buried pipelines.The pullout resistance of the reinforcing elements is one of the most notable factors in increasing the uplift capacity. In this paper, a new reinforcing element including the anchors attached to the ordinary geogrid, namely grid-anchors, was used for increasing the pullout resistance of the geogrid. An experimental study was performed to investigate the uplift resistance of the pipelines buried in sand reinforced with this system. Then, 37 experimental models and 33 numerical models were conducted to investigate the effect of pipe diameter, soil moisture content, soil type, pipe burial depth, the location of reinforcement layers installation, as well as the number of reinforcement layers and the width of reinforcement layers on the uplift resistance of granular soils. The PIV method reveals that due to a developed longer failure surface, inclusion of grid-anchor system in a soil deposit significantly increases the uplift capacity. Therefore the grid-anchor system of reinforcing can increase the uplift capacity 2.5 times greater than that for an ordinary geogrid and 4 times greater than that for non-reinforced sand. Compared to the multilayer reinforcement, the single layer reinforcement was more effective in enhancing the uplift capacity. Besides, the efficiency of reinforcement layer inclusion for uplift resistance in lower embedment ratios is higher. In addition, by increasing the pipe diameter, the efficiency of the reinforcement layer inclusion will be lower. There appears the efficiency of reinforcement layer inclusion for uplift resistance in the single layer reinforcement with higher width and installed in the pipe bottom; lower embedment ratios and lower pipe diameter are higher. To verify the experimental results and completion of research, the finite deference software FLAC-3D was used. It was found that experimental and numerical results were in good agreement.

Saturated granular soils are possible liquefied when subjected to earthquake loading. This phenomenon is result from generation of excess water pore pressure because of non enough time to water drainage and govern non- Consolidated Condition.
When liquefaction is occurred, many forces are generated and undergrounds structures are affected. In this research numerical analysis on buried pipelines in FLAC 2D software are performed and verified duration a comparative process with experimental result from ASCE organization. In present research surveyed effects of various parameters on liquefaction occurrence and probable damages to buried pipelines as dilatancy and friction angle of soil, relative density of back fill around the pipe, diameter and buried depth of pipe and underground water level.
Results indicated that uplift of pipe decrease when dilatancy and friction angle of soil increased in constant relative density condition. This result is different for varied relative density. In low and medium relative density by increasing of dilatancy angle, uplift of pipe increase, reach to pick and decrease. But floating decrease with increasing dilatancy angle for high relative density always. Buried pipe in depth trench and increase of dead load result from back fill on pipeline and usage of pipes with small diameter, decrease uplift the pipe in liquefaction occurrence too. Of course don’t expect perform this subjects in all conditions. for example conflict ion to other underground installation, necessary hydraulic gradient for fluids flow or excavation in region with up underground level, don’t make to excavation of deep conduits. The analysis demonstrate that vertical displacement and damages to pipe is decrease if around installed pipe in conduit back fill with non- liquefied soils. In this new analysis all physical properties of soil and pipe in model are without any change except the cohesion and friction angle of soil around the pipe. Cohesion soils are low potential to liquefaction. For this reason we increase this coefficient from zero to 30 kpa and reach the friction angle to 30 degree. Results are demonstrated in a graph that show uplift versus thickness of non- liquefied soil normalized with diameter of pipe.
Final parameter that surveyed in this research is effect of underground water level on floating buried pipeline. Results show decrease of underground water level cause to decrease of floating and damages to pipeline. For this purpose add a new water level to base model and run the analysis. In next steppes the underground water level is lesser and results are show in a graph that explain variation of vertical displacement versus water level normalized by thickness of soil model. This work possible by excavation of drainage shaft and drop down water level nearby the pipeline. Of course, look this work isn’t economical proposal for long transmission pipelines as petroleum or water conveyance. But in limit industrial sites as refineries this proposal is an improvement work to prevent any damage and and continual service of lifelines duration of unpredictable phenomenon.