Full-scale tests on laterally loaded railway overhead line equipment mast foundations

Abstract

Pile foundations for masts supporting overhead line equipment (OLE) on railways in Western Europe have traditionally been designed using empirical formulae derived from tests carried out in the 1950s under the auspices of the Union Internationale des Chemins de fer (UIC). Recent application in the UK of ostensibly more analytical approaches led to significantly increased pile lengths, which contributed to the high-profile cost over-run on the Great Western electrification programme. Further, the loads associated with some newer designs of mast are greater than those covered by the original field tests, and hence are outside the evidence base for the empirical approach. To address this, full-scale field tests were carried out on three, 610 mm dia. circular hollow-section steel piles installed in a railway embankment at the High Marnham test track (Nottinghamshire, UK). Each pile was designed according to the empirical OLE master index (OLEMI) method for a different form of modern OLE mast, and instrumented using Shape accelerometer arrays (SAAs) to determine pile deformations. The results provide valuable insights into the mechanisms of deformation of piles of different lengths, both at and beyond the expected in-service loads. p-y curves derived from the SAA measurements compare reasonably well with curves constructed using the American Petroleum Institute method for undrained clay soils. The tests also demonstrate the suitability of the OLEMI approach for large structures and loads; and that the Eurocode 7 partial factors on load and undrained shear strength applied to an undrained (total stress) limit equilibrium calculation are likely to be sufficient to meet serviceability requirements for standard single-and twin-track cantilever structures.