ارزيابي مدول برشي ماسه كربناتي بوشهر با استفاده از آزمايش هاي ستون تشديد و سه محوري سيكلي

Estimating the shearing modulus of Bushehr calcareous sand using resonant column and cyclic triaxial experiments

مدول برشي يكي از مهم‌ترين ويژگيهاي نهشته هاي خاكي است، كه براي انجام تحليل هاي ديناميكي بايد مورد ارزيابي دقيق قرار گيرد. از سويي ديگر مناطق وسيعي از كره زمين، از جمله بخش هاي وسيعي از مناطق جنوبي ايران، پوشيده از خاك هاي كربناتي است. اين خاك ها بسياري از ميدان هاي عظيم نفتي و گازي جهان را در خود جاي داده و همواره مورد ساخت و سازهاي گسترده اي نيز قرار دارند. از اين‌رو بررسي كامل رفتار اين خاك ها از اهميت بالايي برخوردار است. در اين پژوهش، مدول برشي ماسه ي كربناتي بوشهر با استفاده از آزمون هاي آزمايشگاهي ستون تشديد و سه محوري سيكلي مورد بررسي قرار گرفته است. اثر فشار محدود كننده و دانسيته ي نسبي بر تغييرات مدول برشي اين ماسه مطالعه شده است. مطالعات آزمايشگاهي اين پژوهش در دو حالت همسان و غيرهمسان انجام گرفته تا چگونگي اثر شرايط تنش حاكم اوليه بر المان خاك در رفتار آن مشخص شود. نتايج آزمايش ها نشان مي دهد كه افزايش فشار محدود كننده ي ميانگين و همچنين دانسيته ي نسبي، افزايش مدول برشي خاك را به دنبال دارد. همچنين نتايج بيانگر آن است كه ناهمساني تنش تحكيم اوليه در مقايسه با ساير پارامترها اثر كمتري بر رفتار خاك دارد. نتايج با بازه ها و مدل هاي پيشنهادي پژوهشگران مقايسه شده و در نهايت مدلي براي ارزيابي منحني تغييرات نسبت مدول برشي خاك كربناتي بوشهر در برابر كرنش برشي ارايه شده است.

Shear modulus is one of the most important properties of soil deposit that should be evaluated as a preliminary step for site response analysis. Although numerous studies have been conducted to evaluate this parameter for silicate soils, there are considerably less studies on calcareous soils. However, extensive regions of the earth is covered with calcareous soils. This type of soil is typically observed near offshore hydrocarbon industries, such as the Persian Gulf. Calcareous sand is the accumulation of pieces of carbonate materials, originated from reworked shell fragments and skeletal debris of marine organism. These soils typically include huge oil and gas reservoirs which are continuously under an extending construction. Therefore, assessment of dynamic behavior of calcareous soils is a vital step for engineering projects. In this study, shear modulus of calcareous sand are investigated in the range of small and large strains using resonant column and cyclic triaxial tests, respectively. Bulk samples of Bushehr sand were collected from the North bank of the Persian Gulf near the Bushehr port. Preliminary mineralogy tests were conducted in order to estimate carbonate content of the samples. The sand contains considerable level of carbonate content and skeletal structure of the soil can be observed easily. Remolded samples of this sand were prepared via dry deposition method for either triaxial or resonant column tests. The results are presented in terms of shear modulus versus shear strain. The effects of confining pressure and relative density on the shear modulus of the calcareous soil are investigated. Moreover, for evaluating the effect of stress anisotropy on the shear modulus of calcareous soil, dynamic and cyclic tests were conducted under both isotropic and anisotropic conditions. The experimental results confirm that confining pressure has an important influence on the shear modulus of the tested samples. Increase of the mean confining pressure and relative density increases the shear modulus of the sand, as previously reported for the other sands. The results indicate that the effect of stress anisotropy on dynamic properties of calcareous sand is less important than those of mean confining pressure and relative density. With increasing mean confining pressure, the effect of relative density and initial stress anisotropy on the shear modulus increases. The normalized shear modulus are compared with the G-reduction ranges proposed for silicate sand by the previous researchers. This comparison show the need for some modification of the previous proposed ranges for normalized shear modulus curves. Finally, a modified hyperbolic model is presented for estimating the normalized shear modulus of Bushehr calcareous sand. It is demonstrated that the proposed model has more capability for prediction of the experimental G-reduction curves, compared with the models recommended for silicate soils. One advantage of the proposed model is the simple correlation developed for the reference shear strain in terms of initial effective confining pressure. The modified hyperbolic model presented in this study can be employed for site response analysis of the calcareous deposits of the Bushehr city.