مطالعه‌ي ميداني مقايسه‌ي رفتار پايه‌هاي سنگريزه‌يي كوبشي با پايه‌‌هاي بتني درجاريز تحت بارگذاري فشاري

A Comparative Field Study of the Behavior of Rammed Aggregate Piers and Concrete Piers under Compressive Loading

در اين مطالعه، رفتار پايه‌هاي بتني درجا و پايه‌هاي سنگريزه‌يي كوبشي كوتاه به عنوان دو نوع پي نيمه‌عميق با مقياس كوچك در محل به صورت مقايسه‌يي بررسي شده‌اند. به همين منظور، دو گروه پايه‌هاي آزمايشي منفرد با قطر ثابت و طول‌هاي مختلف در محل، ساخته و آزمايش شده‌اند. محل انجام آزمايش‌ها، منطقه‌ي ويژه‌ي اقتصادي بوشهر و ناحيه‌ي آزمايشي شامل يك لايه لاي مرطوب به ضخامت 1 متر بر روي رس آبرفتي اشباع بوده است. نتايج نشان داده‌اند تا مادامي كه انتهاي پايه‌هاي بتني به لايه‌ي نرم‌تر زيرين نزديك يا وارد نشده‌اند، بار حد طراحي و مدول سختي بيشتري در مقايسه با پايه‌هاي سنگريزه‌يي كوبشي دارند. همچنين براساس اين نتايج، نشست بالاي پايه‌هاي بتني در حد طراحي بيشتر از مقادير نظير در پايه‌هاي سنگريزه‌يي كوبشي است.

Cast-in-place concrete piers and short rammed aggregate piers (RAPs) are suitable semi-deep foundations for improving some urban areas. However, fewer studies have been undertaken on this topic compared to other foundations commonly used in professional cases. The present research offers a comparative in situ study of the behaviors of these two types of foundation on a small scale. For this purpose, two groups of single trial piers were made and tested (on the site), which consisted of rammed aggregate piers and cast-in-place concrete piers with a constant diameter of 135 mm and variable lengths of 400, 600, 750, 900 and 1100 mm. The test site was Bushehr Special Economic Zone, where the testing area was made up of a uniform two-layered soil: a moist 1 meter-thick stiff silt layer over a layer of saturated alluvial clay, with medium strength, 1.4 meters thick. The trial piers were constructed and loaded in a linear path at a sufficient distance from each other. For reaction loading, a mobile reaction beam system, made up of a heavy cart and modular rails, was utilized. Results reveal that as long as the tips of the concrete piers have not entered or approached the soft lower layer of soil, they have a higher design limit load and stiffness modulus compared to the RAPs. The results also show that the design limit load and top settlement of the concrete piers are, on average, 1.1 and 1.4 times the corresponding values for the RAPs. This assessment was carried out on trial piers only, disregarding the effects of implementing RAPs on the surrounding soil. The results also suggest that there is higher agreement between the top settlement values, and the load transferred to the piers’ tips at the design limit, with the calculated values, based on the Randolph and Wroth method (1978), in the concrete piers than in the RAPs.