بررسي آزمايشگاهي آثار چگالي نسبي و ميزان ريزدانه ي غيرخميري در پتانسيل روانگرايي و نشست ماسه ي اشباع

LABORATORY STUDY OF THE EFFECTS OF RELATIVE DENSITY an‎d NON-PLASTIC FINES ON LIQUEFACTION POTENTIAL an‎d SETTLEMENT OF SATURATED SAND

روانگرايي پديده يي است كه طي آن رسوب هاي سست اشباع,هنگامي كه تحت بارهاي ديناميكي قرار مي گيرند,مقاومت شان به طور چشمگيري كاهش مي يابد و يا از بين مي رود,لذا شناخت هر چه بهتر روانگرايي و عوامل موثر در آن اهميت زيادي دارد. در پژوهش حاضر,تاثير چگالي نسبيو ميزان ريزدانه ي غيرخميري در پتانسيل روانگرايي و نشست ماسه ي اشباع با استفاده از دستگاه ميزلرزه بررسي شده است. نتايج حاصل نشان مي دهند كه با افزايش تراكم نسبي,حتي در تكان هاي نسبتا شديد,فشار آب حفره يي به طور محسوسي كاهش مي يابد و در چگالي هاي بالانيز احتمال روانگرايي كاملا از بين مي رود. به علاوه مقدار نشست سطحي خاك نيز به صورت قابل توجهي كاهش خواهد يافت. همچنين با زياد شدن مقدار ريزدانه ي غيرخميري,فشار آب حفره يي افزايش و در مقابل ميزان نشست خاك,كاهش خواهد يافت. به علاوه با افزايش ميزان چگالي نسبي و در درصدهاي سيلت بالا,حالت روانگرايي سريع تر از بين مي رود.

In liquefaction occurrence, shear strength of loose cohesionless deposits of saturated soil is considerably reduced or even is lost when subjected to dynamic or seismic loads. The earthquake-induced liquefaction can cause significant damage to existing facilities and structures.It is, therefore, of prime importance to deeply understand the liquefaction mechanism and the related and affecting parameters on this phenomenon. In the current study, using a semi- automated shaking table, the effect of the relative density and the percentage of non-plastic fine content of soil material on the liquefaction potential and the settlement of saturated sand layers were investigated. Acceleration of the shaking table in all tests was adopted to be constant and equal to 0.3g having a frequency of approximately 4.7 Hz. In contrast to many other researchers conducted in recent years, loose and very loose sands are considered for experimental studies. Further, shaking table studies conducted here on these materials are considered as relatively large scale tests which notably differ from those conducted mostly by common triaxial tests in the past. The results obtained showed that the higher the relative density, the lesser is the pore water pressure even if the shaking level is high and that the probability of liquefaction is reduced to the lowest level. In addition, the surface settlement of soil layer is significantly reduced. It was also observed that, despite the existing challenges and even opposing point of views among researchers, an increase in the percent of non-plastic fine content in the soil material would lead to a reduction in settlement of soil layer and ironically, to an increase in pore water pressure. Other observations on the experiments conducted in this research verified that the time during which soil remains in liquefied state is reduced if and/or percentage of fine content of soil material is high.