مطالعه آزمایشگاهی اثرات تنش سیکلی و ریزدانه غیر پلاستیک بر پتانسیل روانگرایی خاک غیر چسبنده

در این پژوهش با استفاده از آزمایش سه‌محوری تناوبی تاثیر عواملی مانند نسبت تنش سیکلی (csr) و درصد ریزدانه غیر خمیری در فشار همه‌جانبه ثابت بر پتانسیل روانگرایی ماسه لای‌دار و لای‌ها مورد بررسی قرار گرفته است. همچنین در این پژوهش تاثیر فشار وارده بر اسکلت خاک در مرحله اشباع‌سازی، بر نتایج روانگرایی مورد بررسی قرار خواهد گرفت. نتایج بیانگر آن است که با افزایش لای در ماسه به میزان 30 درصد، کاهش مقاومت روانگرایی رخ داده و سپس با افزایش بیشتر لای، افزایش مقاومت مشاهده می‌شود. مشاهدات نشان می‌دهد که با افزایش ریزدانه تا 30 درصد، رفتار ماسه حاکم بوده و در مقدار ریزدانه‌های بیشتر از 30 درصد، رفتار خاک ریزدانه و لای حاکم شده است. همین‌طور نتایج نمایانگر آن است که با تغییرات نسبت تنش سیکلی، میزان درصد ریزدانه‌هایی که به ترتیب بیشترین فشار آب حفره‌ای را ایجاد می‌کنند تغییر خواهند کرد. در همین راستا معادله منحنی روانگرایی محدوده کاملی از خاک‌ها ارائه شده است. از سویی دیگر، تاثیر فشار وارده بر اسکلت خاک در مرحله اشباع‌سازی (bvalue) نمونه‌ها، بر روی نتایج روانگرایی و کرنش‌های حاصل از آن، در لای خالص ناچیز بوده ولی در  ماسه لای‌دار و تا حدودی ماسه آشکارتر به نظر می‌رسد.

Laboratory Study of the Effects of Cyclic Stress Ratio and Non-Plastic Fine Content on the Liquefaction Potential of Non-Cohesive Soil

Liquefaction is the primary cause of most earthquakeinduced damages in saturated loose and mediumdense deposits. The literature on liquefaction potential has mainly focused on clean sand, and it has been assumed that liquefaction can take place only in sand, and fine or coarsegrained soils cannot generate waterpore overpressure. However, after the occurrence of multiple earthquakes, it was found that nonplastic fine soils also have a significant liquefaction potential in addition to sands. A review of earlier literature showed no study on the effect of the cyclic stress ratio (CSR) on the critical silt content in the study of the liquefaction potential of sand with a fines content of equal to greater than 40% to 100% pure silt content. In other words, the mutual effects of CSR and nonplastic fine aggregates on liquefaction potential of sands with high fine aggregate content, especially pure silt, has not been studied up to this moment. Moreover, discrepancies exist between the results reported on the liquefaction potential of sand and silty sand containing 40% fines. This study investigates the effects of CSR and nonplastic fines on the liquefaction potential of silt and silty sand at a constant confining pressure through undrained cyclic triaxial tests. The test was repeated under the same conditions and steps at different cell pressures (CP) and bottom back pressures (BBP) to investigate the effects of pressure application steps in the saturation phase (Bvalue) of different soil types, from sand to slit, on the liquefaction potential and resulting strains, which had not been investigated before. The cyclic triaxial test was performed on nonplastic slit and Firuzkuh sand 161 according to ASTM D5311. The specimens with a diameter of 5 cm, a height of 10 cm, and a density (Dr) of 30% were prepared by wet tamping According to the results, the liquefaction resistance of sand decreased as the silt content increased up to 30% and then increased when the silt content increased to 60%. A relatively uniform behavior was observed when the slit content increased to 100% (pure slit); because, when the slit content of clean sand is increased to 30%, the fine slit particles fill the empty space between the coarsegrained sand particles, leading to a decrease in the soil drainage capacity during earthquakeinduced vibrations or cyclic loading. Therefore, the liquefaction potential increased under these conditions; but the sand behavior was dominant until the particle content was increased to 30%. The soil behavior changed when the slit content exceeded 30%, and the finegrained soil behavior was observed, declining the liquefaction potential. On the other hand, the CSR affected the liquefaction behavior of all soil specimens so that the fines content generating the maximum porewater pressure (PWP) varied by increasing the CSR. The liquefaction curves were presented for different soil types. Four specimens with different silt contents showed different strain behaviors at different CSRs so that the strain path taken by the four soil types to reach ru = 1 is more uniform at CSR = 0.15 than CSR = 0.2. The effect of pressure applied to the soil structure in the saturation phase (Bvalue) on the liquefaction results and respective strains was insignificant for pure slit; but more tangible in silty sand and sand.