مطالعه‌ي آزمايشگاهي بر مدول برشي ديناميكي بيشينه‌ي ماسه‌ي كربناتي بوشهر

LABORATORY STUDY ON THE MAXIMUM SHEAR MODULUS OF BOUSHEHR CALCAREOUS SAND

رفتار ديناميكي مصالح كربناتي جنوب ايران علي‌رغم خطر بالاي لرزه خيزي و موقعيت استراتژيك آن مورد توجه چنداني قرار نگرفته است. در پژوهش حاضر، مدول برشي بيشينه ي ماسه ي كربناتي بوشهر با انجام آزمون هاي آزمايشگاهي ستون تشديد مورد مطالعه قرار گرفته است. مدول برشي بيشينه، يكي از مهم‌ترين كميت هاي ژيوتكنيكي در تحليل هاي ديناميكي است. اثر فشار محدودكننده ي ميانگين و نسبت تخلخل در روند تغييرات اين كميت بررسي شده است. آزمايش ها در دو حالت همسان و غيرهمسان انجام شده‌اند، تا اثر ناهمساني تنش در رفتار خاك مشخص شود. نتايج نشان داده‌اند كه افزايش فشار ميانگين و كاهش نسبت تخلخل، افزايش مدول برشي ماسه را به دنبال دارد. ناهمساني تنش تحكيم اوليه نيز موجب افزايش مدول برشي بيشينه شده است. براساس نتايج پژوهش حاضر، رابطه‌يي براي مدول برشي بيشينه ي ماسه ي كربناتي بوشهر ارايه و سپس با مدل هاي پيشنهادي پژوهشگران مقايسه شده است. درنهايت، اثر ناهمساني تنش در تغييرات مدول برشي بيشينه لحاظ شده است.

Geological studies indicate that many tropical regions of the planet, including the southern part of Iran, are covered by calcareous sediments. Despite the high risk of seismicity and strategic position as well as the enormous resources of oil and gas in southern Iran, dynamic behavior of calcareous soils of these regions has not been much considered. Therefore, assessing dynamic behavior of calcareous soils is a vital step for engineering projects located in these regions. Maximum shear modulus is one of the most important properties of soil deposit used in the dynamic analysis. In this study, shear modulus of Boushehr calcareous sand is investigated in the range of small strains using resonant column tests. Bulk samples of Boushehr sand were collected from the North bank of the Persian Gulf near the Boushehr port. The effects of mean confining pressure and void ratio on the maximum shear modulus of the calcareous sand are investigated. Moreover, for evaluating the effect of stress anisotropy on the small-strain shear modulus of the calcareous sand, resonant column tests were conducted under both isotropic and anisotropic conditions. The experimental results confirm that the increase of the mean confining pressure and decrease of the void ratio increases the maximum shear modulus of the sand, as previously reported for the other sands. Also, the results of resonant column tests indicated that the increase of stress anisotropy leads to the increase of maximum shear modulus. By increasing mean confining pressure, the effect of initial stress anisotropy on the maximum shear modulus increases. Based on the tests results, predictive equation for estimating the maximum shear modulus of calcareous sand is presented in this study. It is demonstrated that the proposed model has great capability for prediction of the experimental shear modulus at small strains, compared with the models recommended for silicate soils. Finally, to capture the influence of stress anisotropy on the maximum shear modulus equation, a relationship is presented as a function of initial shear stress ratio. The relationship presented in this study can be employed to evaluate the maximum shear modulus of Boushehr calcareous sand under both isotropic and anisotropic conditions.