Centrifuge modeling of rocking foundations on improved soil

The nonlinear response of shallow foundations when subjected to combined loading has attracted the attention of the research engineering community over the last few decades, providing promising evidence for incorporation of such response in design provisions. Failure in the form of soil yielding or foundation uplifting may accommodate high ductility demand and increase the safety margins of the whole structure. However, increased permanent displacement and rotation may occur. This paper explores the concept of shallow soil improvement as a means to locally increase soil strength and thus limit rocking-induced settlement. Bearing in mind that the rocking mechanism is relatively shallow, failure may be contained in a soil layer of known properties that extends to a shallow depth beneath the foundation. The performance of a system in poor soil conditions, on an ideal soil profile, and on improved soil profiles was explored through a series of centrifuge tests at the Center for Earthquake Engineering Simulation at Rensselaer Polytechnic Institute. The shallow soil improvement, evaluated in terms of moment-rotation and settlement-rotation response, proved quite effective even when its thickness was limited to 25% of the footing width. Some interesting similarities between poor, improved and ideal systems were observed in terms of cyclic moment capacity, contact pressure at the soil-foundation interface, and soil-foundation effective contact area. It is shown that even when uplifting is substantial and only 25% of the footing surface remains in contact with the soil, behavior in all four examined cases is quite stable-an important finding in seismic design of rocking foundations.