The influence of footing stiffness on the behaviour of surface strip foundations on sand

Abstract

Foundation systems are some of the most widely used structures in civil engineering. The design thereof is governed by two failure mechanisms, either bearing capacity failure or settlement failure. Generally, the governing mechanism is settlement failure. In order to accurately predict the settlement of a foundation system, the exact material properties of all the various constituents of the system are required. However, due to the complexity of the interaction problem, assumptions have been introduced in the design of foundations to simplify the process. To govern against shear failure of a footing, structural engineers increase the thickness thereof. However, there is no limit to the extent with which this increase can be made. In order to obtain an economic design, the optimum thickness of a footing for a certain set of soil parameters should be obtained. The main objectives of this study is to determine whether the settlement, deflection and contact stress distribution of a footing on cohesionless sand can be predicted as a function of the stiffness’s of the footing and sand. Centrifuge model tests were conducted on seven aluminium footings and one reinforced concrete footing, all with an equivalent 4.5 m × 4.5 m plan area. The tests were conducted on dense, cohesionless sand. The settlement and deflection data were captured with the use of particle image velocimetry, whereas the stress distribution was captured with the use of TekscanTM pressure sensors. The main conclusions drawn from the study are as follows. The relative stiffness of the foundation system affects the deflection and contact stress distribution beneath the footing. The contact stress distribution beneath stiff footings are uniform. A decrease in the flexibility of a footing results in curvature of the footing during loading, which reduces the contact stress at the footing edges. The behaviour of the initially stiff concrete footings change to flexible at the onset of cracking, with the load distribution changing from uniform to a distribution reducing linearly towards the edge. Finally it was concluded that the results obtained from the aluminium footings could be used to predict the settlement, deflection and contact stress distribution beneath a footing.