Time-dependent uplift capacity of driven piles in low to medium density chalk

Abstract

A series of load tests have been performed on instrumented 762 mm dia. tubular steel piles driven into low to medium density grade A/B chalk at St Nicholas at Wade, Kent, UK. This paper presents the results from the static axial uplift tests, which were performed on two piles 7, 50 and 120 days after installation in order to investigate the time-dependent variations in shaft resistance. The results show that the static ultimate shaft resistance of this type of chalk can increase by up to a factor of seven over this time period, as a consequence of ‘set-up’ effects. The test results also show that the ‘set-up’ effect is reduced if the pile is subject to lateral loads up to 50% of the ultimate lateral capacity before uplift loading, while the application of lateral loading up to 10% of ultimate lateral capacity had negligible influence on axial capacity. The measured load distribution from strain gauges suggests a mobilisation of larger unit shaft resistance in the lower half of the pile. This paper also describes the geotechnical site conditions, the pile instrumentation and the effects of pile driving on the chalk.