Rate effects on the vertical uplift capacity of footings founded in clay

Abstract

This paper describes results from a series of uplift tests performed in a drum centrifuge on shallow square footings founded on clay and sand, with an overlying sand backfill. The test series investigated footings of various widths loaded at displacement rates extending over five orders of magnitude. Additional tests were performed to examine the influence on uplift capacity of the direction of load inclination. The uplift tests performed on footings founded on sand enabled estimations to be made of the suction that developed at the base of the footings founded on clay. It is shown that the suction mobilised at very fast rates may be assessed using existing undrained bearing capacity approaches and a (rate corrected) operational undrained strength similar to that measured in triaxial extension. The uplift rates required to reach fully undrained conditions are almost two orders of magnitude higher than those for footings loaded in compression, and the upward footing displacements required to generate maximum resistance can be up to 10% of the footing width. These findings have important implications for shallow foundations such as those used to support high-voltage electricity transmission line support structures.