بررسي رفتار تنش- اتساعي ماسه‌ي انزلي

A Study on the Stress-Dilatancy Behavior of Anzali Sand

از اواخر دهه‌ي 60 ميلادي به بعد، مدل‌هاي رفتاري متعددي براي ماسه‌ها ارايه شده است. اغلب اين مدل‌هاي رفتاري بر اساس سطوح تسليم متعدد و استفاده از تعريف كار خميري بنا نهاده شده‌اند. در اواسط دهه‌ي 80، رفتار ماسه‌ها بيش از پيش با درنظرگرفتن مفهوم سطح تنش مورد بررسي قرار گرفت. اثر سطح تنش در رفتار مقاومتي- اتساعي ماسه‌ها در آزمايش‌هاي مختلف ارزيابي و روابطي براي بيان مقاومت ماسه بر اساس سطح تنش اعمال‌شده و نيز نوع آزمايش ارايه شد. هدف اصلي در اين نوشتار، بررسي پارامترهاي مقاومت برشي ماسه‌ي شهر ساحلي انزلي، واقع در استان گيلان،‌ در ارتباط با رفتار تنش- اتساعي آن است. اين رفتار با انجام آزمايش‌هاي مقاومت برشي، از نوع برش مستقيم بر روي نمونه هايي از اين خاك سنجيده شده است. سپس بر مبناي روش‌هاي موجود در ارزيابي اتساع ماسه‌ها براي اين نوع ماسه تعميم داده شده است.

Since the late 1960s, several constitutive models have been developed for sands describing their stress-strain behavior. Most of these models were established based on a measure of the plastic work and some yield criteria, among them being the Lade-Duncan model. It was in the mid 1980s when the stress level concept was found to be an appreciable factor controlling the behavior of sands. The stress level is defined as the confining pressure or the vertical stress on a shear plane, which differs from point to point in a granular medium under a system of external loads. Based on this concept, a number of strength – dilatancy models were developed for sands. The shear strength of sand is related to the level of stress and the type of test in which the shear strength parameters are measured. In this research, the geotechnical properties of Anzali Sand (located in Guilan province, northern Iran, on the coastal line of the Caspian Sea) are investigated, in accordance with its strength –dilatancy behavior. As this particular type of sand is abundant in northern Iran, such research appears to be worthwhile, as most seismic geotechnical investigations, as well as other conventional studies, deal with the strength of this sand. For example, the liquefaction potential and the post – liquefied behavior of Anzali Sand are all based on a strength – dilatancy model, the behavior of which was measured by a number of different direct shear tests, as the most popular and available tests for sand. The dilative behavior of Anzali Sand was generalized to capture its relevant properties. Findings indicate that Anzali Sand, at least in the direct shear tests; does not show a very interpretable behavior, possibly due to inherent errors within the direct shear test, although a rather reasonable relationship between the peak and critical state friction angles was found as functions of the stress level and the density index.