مدل‌سازي سربار ناشي از احداث سازه بر روي يك خاك روانگرا

Modeling of surcharge due to building on a liquefaction soil

تاثير سازه بر استعداد روانگرايي خاك بسيار با اهميت است، به اين دليل كه احداث سازه ممكن است از رخداد پديده روانگرايي جلوگيري كند و يا عاملي برابر افزايش پتانسيل و شدت روانگرايي در لايه‌هاي زيرين شود. بنابراين در اين پژوهش به منظور مدل‌سازي سربار ناشي از احداث سازه روي يك خاك روانگرا، ابتدا روش تحليل عددي تفاضل محدود معرفي شده و در ادامه با به كارگيري نرم‌افزار FLAC 2D تحليل ديناميكي غير خطي سربار انجام شده است. فرآيند اعتبار سنجي و حصول اطمينان از نتايج تحليل عددي، با مدل‌سازي يك آزمايش سانتريفيوژ و مقايسه نتايج آن با مدل عددي انجام گرفته است. نتايج مدل‌سازي نشان داد كه تاثير سربار در وقوع روانگرايي به تراكم خاك، نوع زلزله، شكل سازه و عمق اشباع بستگي داشته و بسته به شرايط زمين و زلزله تاثير متفاوتي در وقوع روانگرايي دارد.

Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid. If the pressure of the water in the pores is great enough to carry all the load, it will have the effect of holding the particles apart and of producing a condition that is practically equivalent to that of quicksand the initial movement of some part of the material might result in accumulating pressure, first on one point, and then on another, successively, as the early points of concentration were liquefied.The phenomenon is most often observed in saturated, loose (low density or uncompacted), sandy soils. This is because a loose sand has a tendency to compress when a load is applied; dense sands by contrast tend to expand in volume or ʹdilateʹ. If the soil is saturated by water, a condition that often exists when the soil is below the ground water table or sea level, then water fills the gaps between soil grains (ʹpore spacesʹ). In response to the soil compressing, this water increases in pressure and attempts to flow out from the soil to zones of low pressure (usually upward towards the ground surface). However, if the loading is rapidly applied and large enough, or is repeated many times (e.g. earthquake shaking, storm wave loading) such that it does not flow out in time before the next cycle of load is applied, the water pressures may build to an extent where they exceed the contact stresses between the grains of soil that keep them in contact with each other. These contacts between grains are the means by which the weight from buildings and overlying soil layers are transferred from the ground surface to layers of soil or rock at greater depths. This loss of soil structure causes it to lose all of its strength (the ability to transfer shear stress) and it may be observed to flow like a liquid (hence ʹliquefactionʹ). The effect of structure on liquefaction potential of soil is very important, because it may prevent the occurrence of liquefaction phenomena or may to increase the intensity of liquefaction in the lower layers; hence the surcharge due to structures can be an important factor in the occurrence of liquefaction. Therefore in this study to model the surcharge of constructing a structure on liquefiable soil, first introduced the finite difference numerical analysis, then using FLAC 2D nonlinear dynamic analysis modeling of surcharge is carried. In this analysis, the modeling of surcharge due to building on a liquefaction soil and the effect of liquefaction potential of the economy has been studied. Also, the validation process and ensure the results of numerical analysis, modeling and comparing the results with a numerical model of centrifuge tests have been conducted. Results showed a significant decrease in the use of numerical modeling cost structures will be studied.