Large-scale testing of free-ended piles in unsaturated expansive clays under cyclic and monotonic lateral loading

Abstract

This article presents an investigation into the influence of swell on the behaviour of large-scale free-ended piles installed in an unsaturated expansive clay deposit when subjected to lateral loading. A ‘dry’ pile was founded in a profile kept at natural water content conditions, and a ‘wet’ pile was founded in a profile which was flooded for six months to allow swelling of the clay prior to testing. Load cycles under two different load magnitudes were applied to the piles, followed by monotonic loading to failure. The applied load, pile head displacement and bending strain distributions with depth (determined using fibre Bragg gratings) were recorded during these tests. The behaviour under 113 kN load cycles was similar for the two piles, with the ‘wet’ pile exhibiting marginally stiffer response. This was attributed to a swell-induced increase in lateral stress acting against the pile shaft. Under load cycles of 145 kN and during the monotonic test, the ‘wet’ pile exhibited a substantially softer response and significantly lower ultimate capacity than that of the ‘dry’ pile. This was attributed to the effects of swell-induced softening and the reduced yield stresses in the swelled clay. These phenomena were interpreted with reference to a site investigation programme including seismic continuous surface wave tests and standard penetration tests, as well as some laboratory testing. Effects of local yielding of the bonded material at fissure interfaces upon pile ratcheting during cyclic loading have also been discussed.