The uplift of high voltage transmission tower foundations

Abstract

The in-service performance of transmission tower foundation systems is poorly understood. This knowledge deficiency is particularly acute with regard to the dynamic and transient loading of these foundations in uplift. There is also uncertainty surrounding the integrity of existing assets as design practice appears to overestimate the capacity of the foundations when they are subject to testing. A significant component of cost of new high voltage overhead line route construction or uprating involves the maintenance or reinforcement of the individual transmission tower foundation systems. Therefore, a more developed understanding of the foundation system behaviour is required to facilitate these works in a cost-effective and timely manner. To gain a better understanding of foundation system performance, a series of full scale rapid uplift tests were carried out in July 2012. The tests bridged understanding of the load-displacement, load-rate and rate effects of soils from previous experimental research to field scale, with associated construction and in situ soil nonlinearities. The tests made use of modern instrumentation and monitoring techniques in combination with rigorous numerical finite element back analysis to update understanding of in situ failure mechanisms and capture uplift capacity enhancements due to the application of rapid loading. The field tests and numerical back analysis results highlighted significant limitations in current design practice particularly the reliance on an outdated failure mechanism and ultimate limit state criterion. The results of the rapid uplift tests compared to standard industry practice suggested that the latter method may be unduly conservative leading to an underestimation of in-service capacities. The results presented will lead to a better understanding of foundation system performance and more legitimate design and testing practice technical specifications.

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