بر‌رسي اثر اندازه‌ي پي گسترده‌ي دايره‌يي در متوسط ضريب عكس‌العمل بستر خاك‌هاي سيمانته‌شده‌ي تهران با استفاده از نرم‌افزار

Evaluation of Circular Foundation Size Effect on Calculation of Average Coefficient of Subgrade Reaction Using Numerical Modeling

تخمين ضريب عكس‌العمل بستر (ks) براي پي‌هاي گسترده معمولاً براي طراحان سازه حايز اهميت است. يكي از روش‌هاي تعيين ks در محل، آزمايش بار‌گذاري صفحه است. با استفاده از اين آزمايش، ks براي محدوده‌ي تاثير كوچك صفحه‌ي بار‌گذاري تعيين مي‌شود. از جمله روابطي كه به‌ كمك آنها مي‌توان ks را براي اندازه‌هاي بزرگ‌تر پي تخمين زد، روابط ترزاقي (1955) است كه با وجود برخي محدوديت‌ها، همچنان مورد استفاده قرار مي‌گيرند.‌ در اين نوشتار به‌كمك مدل‌سازي عددي براي خاك‌هاي سيمانته، اثر اندازه‌‌ي پي در ks بررسي و نتايج آن با روابط ترزاقي (1955) مقايسه شده است. به منظور درستي‌سنجي، نتايج آزمايش بار‌گذاري صفحه‌ي واقعي با مدل عددي مقايسه شده است. همچنين اثر تغيير ضخامت پي در تغيير ks مختصراً بررسي شده است. نتايج مدل‌سازي عددي حاكي از آن است كه ks در خاك‌هاي "سيمانته"، تقريباً نزديك مقاديري است كه روابط ترزاقي (1955) براي خاك‌هاي "رسي" تخمين زده است.

The "Coefficient of Subgrade Reaction" was introduced in the form of "Set of springs" model by Winkler during 19th century. In this model, the subgrade is modeled as a set of springs with a stiffness of ks per unit area of the foundation carrying the structural loads. In other words, the "Contact pressure to Settlement" ratio for every point located in contact zone of soil and foundation is called “Coefficient of Subgrade Reaction”, or ks. This ratio explains the interaction of soil and foundation, and it can be used for calculation of shear and moment distributions under the foundation. The model has been frequently used by structural designers for decades, and they have improved it to calculate stresses under flexible foundations like continuous footings or raft foundations as well as the concrete pavements subjected to wheel loads. Therefore, proper estimation of ks for raft foundations is essential for structural engineers. A commonly used method for in situ direct measurement of ks is Plate Load Test (PLT). As the PLT is carried out at small scale, therefore the influence zone is small. The results cannot then be used directly for large scale calculations. Among the commonly used equations for correlating the PLT results to ks for large raft foundations are those proposed by Terzaghi (1955). The main objective of this paper is investigating the ks for cemented granular materials using a numerical model. The results of three plate load tests are used for verification purposes and calibrating the FEM numerical model used for predictions. The effects of the change of foundation size and thickness on the value of ks are briefly discussed. The results indicate that Terzaghi (1955) equation for "Clay" estimates an acceptable value for ks in the cemented granular soils similar to Tehran alluviums.