روش جديد پيش‌بيني نشست آني شالوده‌هاي سطحي با استفاده از سختي حداكثر خاك‌ها

A New Method to Estimate Immediate Settlement of a Shallow Foundation using Soil Maximum Stiffness

بيشتر روش‌هاي موجود جهت تخمين نشست شالوده‌هاي سطحي مستقر بر روي خاك‌هاي غيرچسبنده براساس همبستگي‌هاي ميان نتايج آزمايش‌هاي برجا، همچون نفوذ استاندارد با مدول يانگ هستند. نشست پيش‌بيني‌شده بر اساس نتايج اين‌گونه آزمايش‌ها، پراكنده و اغلب غير واقع‌بينانه است. در اين نوشتار سعي شده است با ارايه‌ي يك رويكرد جديد، تخمين نشست شالوده‌هاي سطحي بر روي خاك‌هاي غيرچسبنده مورد توجه قرار گيرد، كه در آن از مدل رفتاري كشساني- غيرخطي و مشخصات ديناميكي خاك‌ها استفاده شده است. نتايج اين روش با مقادير نشست حاصل از آزمون‌هاي بارگذاري بر روي صفحه با ابعاد مختلف مقايسه شده است. اين روش در مقايسه با روش‌هاي مبتني بر همبستگي نتايج آزمايش‌هاي برجا، نتايجي واقع‌بينانه و نزديك‌تر به مقادير اندازه‌گيري‌شده از خود نشان مي‌دهد. علاوه بر آن، سرعت عمل اين روش و هزينه‌ي نسبتاً كم آن، مزيت ديگري است كه آن را نسبت به ساير روش‌هاي متداول، متمايز ساخته است.

In the design of shallow foundations, permissible settlement is often the controlling design criterion. Numerous methods have been developed over the years to estimate the elastic settlement of footings over cohesionless soils. The conventional methods utilize correlations between measured settlements and some parameters from reasonably simple field tests. These include, in particular, standard penetration tests (SPT) and cone penetration tests (CPT). Often, the mentioned correlations overpredict settlements. In-situ direct estimation of the maximum or small-strain stiffness (Gmax or Emax) of soil is more effective and reliable than those derived from resistance-based correlation or laboratory testing. In addition to a geophysical refraction seismic survey, there are several other techniques for measuring maximum stiffness at the site, such as the cross-hole, the down-hole, spectral analysis of surface wave (SASW) and the continuous surface wave method (CSW). In these methods, an electromechanical vibrator or impact source is used to generate surface waves. Then, geophones receive ground responses to measure up Rayleigh wave velocity and maximum stiffness. In this paper, a new method is provided in order to determine the settlement of a shallow foundation based on small-strain stiffness. The suggested relationship in this study will be the modified small-strain stiffness of the soil layer, according to the level of foundation pressure. The elastic settlement is obtained using the relationships of the elasticity theory, based on foundation width, stress field and modified stiffness. In order to validate the proposed method, the results of the survey of loading tests and seismic geophysical tests at several sites were evaluated and compared. Appropriate coincidence between the results of the loading test and the predicted settlement shows the accuracy of the proposed method in comparison to other methods, which were more accurate than the SPT or CPT based predictions. In general, predictions based on in situ parameters from seismic measurements are closer to the measured settlement under service loads.