The uplift behaviour of shallow foundations

A series of uplift tests have been undertaken in a geotechnical centrifuge to investigate the behaviour of shallow plate anchors embedded in sand and clay. Uplift response and capacity has been studied at a range of displacement rates. The results of the centrifuge study have been examined in relation to the limits of fully drained and fully undrained behaviour and the failure mechanisms developed at uplift velocities leading to partially drained response are discussed. In addition numerical solutions for anchor uplift capacity over the range of velocities are compared to the experimental results obtained. The contribution to uplift stiffness and capacity provided by the clay at the base of transmission tower footings has also been investigated in the centrifuge. Negative pore pressures developed at the base of such foundations are measured directly across a range of uplift velocities and are used to provide insight into the influence of uplift rate on the failure mechanism at the footing base. Data from a series of triaxial extension tests, conducted at high axial strain rates, highlight the rate dependence of the stiffness and undrained strength of the underlying clay. These results are combined with finite element back analyses of the centrifuge tests to predict the uplift capacity and load displacement response of footings on clay subject to high rate of uplift such as those which may be experienced by transmission tower foundations in service. Finally, the effect of modifying a loose granular backfill using cement on the uplift performance of shallow anchors is examined. Conclusions are drawn in relation to peak foundation capacity and load displacement response.