بررسي افت تنش در مصالح ماسه‌اي هنگام اشباع شدن

Investigation of stress reduction in sandy soils during saturation

درصورتي‌كه مصالح دانه‌اي خشك، تحت شرايط محيطي، اشباع شوند كاهش تنش در توده خاك اتفاق افتاده و همراه آن نشست‌هايي در مصالح به وجود مي‌آيد كه نشست ناگهاني ناميده مي‌شود. در اين مقاله تاثير اشباع شدن مصالح ماسه‌اي روي پارامترهاي مقاومت برشي اين خاك و نيز عوامل موثر بر ميزان افت تنش اين مصالح به وسيله‌ي دستگاه آزمايش سه محوري بررسي شده است. از اين‌رو در ابتدا، آزمايش‌هاي اوليه براي تعيين خصوصيات فيزيكي خاك ماسه‌اي از جمله آزمايش‌ دانه‌بندي، تعيين چگالي ويژه و دانسيته‌هاي كمينه و بيشينه صورت گرفت و پس از تهيه نمونه‌هايي با دانسيته نسبي مورد نظر، براي ارزيابي پارامترهاي مقاومت برشي، آزمايش‌هاي خشك و اشباع در تنش‌هاي همه‌جانبه مختلف انجام شدند. براي بررسي افت تنش به دست آمده از اشباع شدن مصالح، ابتدا آزمايش سه محوري تحت شرايط خشك انجام شده سپس با ثابت نگه داشتن كرنش محوري، مصالح اشباع شد و پس از گذشت زمان كافي، بارگذاري در آزمايش سه‌محوري ادامه داده شد تا نمونه‌ها به گسيختگي برسند. بدين ترتيب اثر عواملي مانند ميزان تنش همه جانبه، ميزان سطح تنش برشي، دانسيته‌ي نسبي، درصد رطوبت اوليه و اثر ريزي و درشتي دانه‌ها بر افت تنش مصالح ماسه‌اي بررسي شدند

Saturation of dry cohesion-less geotechnical materials causes a great stress release in the whole mass, which leads to sudden settlement in the environment, called collapse settlement. Observational investigations in laboratory tests and field monitoring have indicated the possibility of collapse behavior in a wide range of geotechnical materials and it has been vastly investigated by many researchers. This phenomenon occurs in upstream shell of embankment dams during the first reservoir filling and even in downstream in rainy seasons. Sudden reduction in volume upon wetting leads to collapse settlement in soil mass which is of paramount importance, especially in dry and loose soil deposits. In fact some factors are needed in order to produce the collapse in a soil mass such as: an open, partially unstable unsaturated fabric in the soil mass, a high enough net total stress that will cause the structure to be unstable, a bonding or cementing agent that stabilizes the soil in the unsaturated condition, and finally the addition of water to the soil which causes the bonding or cementing agent to be reduced and the inter-aggregate or inter-granular contacts to fail in shear, resulting in a reduction in total volume of the soil mass. In this research, collapse phenomenon in some sandy soils is investigated via triaxial shear strength tests. Most experiments in this project were conducted on materials prepared from sandy beach of northern part of Iran (Chaboksar Beach). This material had a uniformity coefficient of (Cu=1.5) and was prone to collapse after saturation. Another type of material with greater size, prepared from the city of Zanjan in western part of Iran, was used to evaluate the effect of particle size on the collapse settlement. The index and density tests including: grain-size analysis, specific gravity, maximum and minimum densities were performed on the samples. Then soil samples were prepared with specific relative density and triaxial shear strength apparatus was used in dry, saturated and collapse states to determine shear strength parameters and stress relaxation of the samples. Collapse tests were done on the initially dry samples which were flooded during shearing in some specific stress states. Different parameters can influence the collapse settlement of the soil mass; As a result, majority of this research was devoted to study the effect of diverse factors on the collapse behavior of soil samples. In order to recognize the effect of these factors, the specific parameters such as, confining stress, relative density, initial moisture content, shear stress level during saturation and size of particles were considered. A number of triaxial shear strength tests were performed to investigate the effect of important factors on the collapse behavior of the sand samples on the presence of water, which are summarized below: It was determined that, the amount of collapse settlement increases by increasing the confining pressure and shear strength level of saturation, and decreases by increasing the initial moisture content and relative density. Also, in specimens with the same uniformity coefficient but different maximum particle sizes, the amount of collapse coefficient increases as the maximum particle sizes increases