Uplift performance of transmission tower foundations embedded in clay

The contribution to the uplift stiffness and capacity provided by the clay beneath the base of shallow footings typical in configuration to those employed to support high voltage electricity transmission towers is examined. Pore pressures developed at the base of appropriately scaled footings founded on clay were measured over a wide range of uplift rates in a geotechnical centrifuge. These measurements, coupled with data from tests on identical footings founded on sand, are used to provide insights into the influence of uplift rate on the failure mechanism and footing capacity. Data from a series of undrained triaxial extension tests, conducted over a range of strain rates, are presented and these data combined with finite element back-analyses of the centrifuge uplift tests are used to provide designers with a means of assessing the capacity and load–displacement response of footings on clay subjected to high rates of uplift in service

Uplift performance of shallow foundations with cement-stabilised backfill

This paper describes the results of a series of centrifuge model tests undertaken to investigate the effects of modifying a loose granular backfill using cement on the uplift performance of shallow anchors. These model tests, which involved a range of cement contents, are supported using a series of laboratory element tests and finite-element analyses. The study indicates that significant increases in uplift stiffness and peak capacity can be achieved by the addition of relatively small quantities of cement. Such increases are, however, limited to relatively low uplift displacements because of the brittle nature of the improved backfill shear strength characteristics

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

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 structure

An experimental study and numerical study of rate effects for plate anchors in clay

Existing design charts for evaluation of the uplift capacity of plate anchors are difficult to apply in practice as it is often unclear if the applied loading will lead to an undrained response and, if it does, how the average/operational undrained strength of the clay should be assessed. This paper examines explicitly the effects of partial drainage during load application in a series of centrifuge model tests in overconsolidated kaolin and reports predictions from complimentary coupled-consolidation finite element analyses. Excellent agreement between the experimental results and numerical predictions is demonstrated, and inferred normalised velocities marking the uplift rates at which drained and undrained behaviour may be assumed are shown to be in agreement with those inferred from variable rate T-bar penetration tests