تحليل عددي عوامل موثر بر روانگرايي ناشي از زلزله بر ساختگاه لوله هاي مدفون

Numerical analysis of effective parameters on liquefaction occurance result from earthquake on site of buried pipelines

خاك هاي ماسه اي اشباع در حين زلزله به دليل بارگذاري سريع و بدون مهلت زهكشي به دليل تشكيل اضافه فشار آب حفره اي ممكن است دچار روانگرايي گردند. در اثر وقوع روانگرايي در خاك، نيروهايي به وجود مي آيد كه سازه هاي مدفون ازجمله لوله ها را تحت تاثير قرار مي دهد. در تحقيق حاضر مدل سازي لوله مدفون در فضاي نرم افزار FLAC 2D، صحت سنجي مدل طي فرآيند مقايسه اي با تحقيق آزمايشگاهي انجام گرفته از سوي سازمان ASCE و بررسي پارامترهاي مختلف شامل تاثير زاويه اتساع و اصطكاك خاك، ميزان تراكم نسبي خاكريز اطراف و روي لوله، حضور خاك هاي غير روانگرا در اطراف لوله مدفون، قطر و عمق دفن لوله و نيز تراز آب زير زميني پيراموني انجام گرفته است. نتايج نشان مي دهد كه افزايش زاويه اصطكاك و اتساع در تراكم نسبي ثابت از آسيب هاي وارده بر لوله مي كاهد. در تراكم نسبي نسبي بيش از 70% با افزايش زاويه اصطكاك، ميزان برخاستگي و تبعاً آسيب پذيري لوله در حين بارگذاري زلزله، همواره سير نزولي دارد. همچنين دفن لوله در عمق زياد و نيز استفاده از لوله هاي با قطر كمتر، ميزان آسيب پذيري لوله ها در حين وقوع روانگرايي را به طور چشمگيري كاهش مي دهد. از ديگر نتايج اين تحقيق، كاهش ميزان خطرپذيري لوله هاي مدفون با استفاده از خاكريزهايي با پتانسيل روانگرايي پايين و نيز پايين انداختن تراز آب زير زميني مي باشد.

Abstract: 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. Keywords: Liquefaction, buried pipelines, FLAC, finite difference method, Finn’s model. Liquefaction, buried pipelines, FLAC, finite