ارزيابي رفتار ديناميكي ماسه‌ هاي كربناتي

Dynamic Behavior of Calcareous Sands

ارزيابي دقيق منحني­ هاي رفتار ديناميكي خاك­ ها از مراحل مهم و اساسي در حل مسائل ژئوتكنيك لرزه‌ اي از جمله تحليل پاسخ زمين مي­ باشند. مطالعات بسياري جهت ارزيابي اين منحني­ ها كه شامل منحني‌ هاي سختي برشي و نسبت ميرايي هستند، صورت گرفته است. اكثر اين مطالعات بر روي خاك­ هاي سيليكاتي انجام پذيرفته است. مطالعات زمين ­شناسي حاكي از آن است كه بخش وسيعي از سطح زمين در نواحي گرمسيري از جمله نواحي جنوبي كشور ايران پوشيده از خاك­ هاي كربناتي است. از اين‌ رو بررسي رفتار خاك ­هاي كربناتي تحت بارهاي ديناميكي از اهميت بالايي برخوردار است. در اين مقاله مطالعات تحليلي، تجربي و آماري انجام شده بر روي منحني­ هاي رفتار ديناميكي خاك­ ها بررسي و اثر پارامترهاي مختلف بر اين منحني­ ها شرح داده مي­ شود. در ادامه مطالعات آزمايشگاهي چرخه ­اي و ديناميكي انجام شده در فشارهاي محدودكننده‌ ي ميانگين 40، 200 و 400 كيلو پاسكال و دانسيته­ ي نسبي 50 و 80 درصد بر روي ماسه كربناتي بوشهر گزارش مي­ شوند. در نهايت پارامترهاي ديناميكي حاصل از آزمون‌ هاي آزمايشگاهي اين تحقيق با مطالعات گذشته مقايسه شد كه نتايج حاصل نشان از تفاوت رفتاري ماسه ­ي كربناتي بوشهر با الگوهاي مبتني بر ماسه­ هاي سيليكاتي داشته است.

Reliable and accurate assessment of dynamic soil behavior curves is necessary for the solution of many soil dynamic problems, such as the site response analysis. The shear modulus and damping curves of silicate soils under different conditions have been investigated by many geotechnical researchers. Carbonate sediments are located in temperate and tropical areas and cover approximately 40% of the ocean surface. This type of soil is typically observed near offshore hydrocarbon industries, such as the Persian Gulf. Carbonate sand is the accumulation of pieces of carbonate materials; it usually originates from reworked shell fragments and skeletal debris of marine organism. Foundation problems associated with carbonate soil deposits, particularly as experienced by the offshore hydrocarbon industry have led to significant research focused on understanding the behavior of these soils. This study focuses on evaluation of dynamic properties of Bushehr calcareous sand. The shear modulus and damping ratio of the tested sand was measured at small to large shear strains using resonant column and cyclic triaxial tests. The calcareous sand specimens were tested by a fixed-free type of resonant column apparatus (SEIKEN model). By using the resonant column apparatus, shear modulus and damping ratio of the calcareous sand for the shear strain amplitude ranging from about 10-4 % to 10-2 % were measured. The cyclic triaxial tests were conducted using a fully automated GDS triaxial testing apparatus. The cyclic tests were done on samples with shear strain amplitudes ranging from about 10-2 % to 1 %. The procedure used to perform the dynamic and cyclic tests was the multi-stage strain-controlled loading under undrained condition. The tests were conducted in three levels of initial effective mean confining pressure equal to 40, 200, and 400 kPa. The sand specimens were constructed in relative densities lower than 50 or 80 percent, depends on the initial effective stress, in order to acquire the target relative densities (i.e. 50 o The experimental results indicate that with an increased shear strain amplitude, shear modulus decreases and damping ratio increases. This trend, which was observed for all the tests, is typical behavior of soils under dynamic loading, as observed in the previous studies. The effect of mean effective confining pressure and relative density on the normalized shear modulus (G/Gmax) curves of the Bushehr calcareous sand was investigated. Gmax is the small-strain shear modulus measured at shear strain amplitude about 10-4 %. The increase in mean effective confining pressure causes the normalized shear modulus to increase; however, it is more pronounced in low effective confining pressure. Changes of the normalized shear modulus curves (G/Gmax-γ) at the range of σ'm=200-400 kPa is less than that at the range of σ'm=40-200 kPa. Normalized shear modulus curves are almost independent of the changes in relative density of the sand. The results also indicate that the increased amount of initial mean effective confining pressure leads to the smaller damping ratio for the tested sand. It is also observed that damping ratio curves (D-γ) are not affected strongly by relative density changes. Comparison of the tests results with the ranges and models recommended by the previous researchers reveals that the normalized shear modulus and damping ratios of the studied calcareous sand are somewhat inconsistent. In fact, there might be a necessity to modify the previous recommendations for accurate prediction of the dynamic behavior curves of calcareous sands.