مطالعه ي اثر ناهمساني ذاتي در رفتار خاك هاي ريزدانه ي منطقه ي شمال تبريز

STUDY ON THE EFFECT OF INHERENT ANISOTROPY ON THE BEHAVIOR OF FINE- GRAINED SOILS AT THE NORTH OF TABRIZ

جهت طراحي دقيق پي سازه ها و همچنين آناليز پايداري سازه هاي خاكي، مطالعه ي مقاومت برشي خاك ها با توجه به اثر ناهمساني اهميت زيادي دارد. از طرف ديگر، در سال هاي اخير ساخت و ساز و بلندمرتبه سازي در نوار شمالي و شمال شرق شهر تبريز رواج پيدا كرده است. مصالح سطحي مناطق مذكور عمدتا از مارن هاي زرد و زيتوني رنگ تشكيل يافته است. با توجه به اجراي پي ساختمان ها بر روي مصالح ذكر شده و همچنين لزوم پايداري گودبرداري هاي موجود در منطقه و عدم لحاظ اثر ناهمساني در خاك مناطق نوار شمالي و شمال شرق، ويژگي هاي رفتاري خاك مناطق اخير اهميت زيادي پيدا كرده است. در پژوهش حاضر، رفتار مكانيكي خاك هاي مارن مناطق مرزداران و نگين پارك با انجام آزمايش سه محوري و با لحاظ ناهمساني ذاتي مطالعه شده و نتايج نشان داده است كه ناهمساني در مقاومت برشي و فشار آب حفره يي هر دو خاك تاثيرگذار بوده و باعث شده است كه در يك زاويه ي خاص، مقاومت برشي كمترين و فشار آب حفره يي بيشترين مقدار را داشته باشد. ميزان بيشينه ي كاهش مقاومت برشي در فشارهاي محصوركننده ي 200، 300 و 500 كيلوپاسكال براي مارن زرد به ترتيب 32٪، 31٪ و 20٫4٪ و براي مارن زيتوني به ترتيب 30٪، 29٫7٪ و 21٫8٪ بوده است. روند مشابهي براي تغييرات مقاومت در سطوح كرنش مختلف مشاهده شده است.

A study of shear strength and compressibility characteristics of soils considering anisotropy phenomenon is essential to the accurate design of foundations and stability analysis of earth structures. The directional dependency of physical and mechanical behavior of soil is referred as anisotropy phenomenon. The sedimentation process in soil formation arranges the soil particles in a special direction, known as the bedding direction. Due to this pattern, changing loading direction relative to stratification changes the soil behavior, which is known as inherent anisotropy. On other hand, the construction of high-rise buildings has developed at the north and northwest of Tabriz city. Subsurface materials of this area have formed from yellow and olivaceous marl. With regard to the construction of building foundations and the necessity of stability of excavations in this area, it is the most important characteristics of these soils with considering anisotropic behavior. In this research, a series of triaxial tests were conducted to evaluate the mechanical behavior of a fine-grained soil and the effect of inherent anisotropy on the behavior of fine-grained soils located in Negin-Park and Marzdaran regions in Tabriz city. To this end, a special soil sampler was designed to coring soil with capability in different directions. Then, cylindrical triaxial samples were prepared with 38 mm in diameter and 76 mm in height and saturated by passing distilled water and applying back pressure up to the B-value of 0.97. The samples finally were isotropically consolidated under effective stresses of 200, 300 and 500 kPa, and loaded at a rate of 0.05 mm/min. The results showed that the shear strength and compressibility of the samples intensively depend on the sampling direction so that the shear strength and pore water pressure reach the minimum and maximum values at a given anisotropy angle, respectively. The reason for this behavior can be related to change failure surface as compared with bedding orientation.