Effect of particle size distribution on pile tip resistance in calcareous sand in the geotechnical centrifuge

Until recently, the micro mechanical origins of soil behaviour have remained illusive, but it is now known that that the constitutive behaviour of a soil is largely determined by its particle size distribution. This paper examines the specific boundary problem associated with the penetration of a model pile into two different gradings of dry calcareous sand in a geotechnical centrifuge, in order to establish the effect of the inclusion of fine particles on the pile end bearing resistance. The first grading of sand comprised particles smaller than 0.5 mm; the second grading contained particles of nominal size d such that 0.15 mm < d < 0.5 mm. Each test was performed on each of two samples of each grading. Tip resistance was observed to rise to a peak at shallow depths, and then fall; a micro mechanical explanation is presented for this instability. Following the centrifuge tests, particles were retrieved from the centres of the soil samples, where the pile had previously been driven, for subsequent particle size analysis. It was found that insignificant crushing had occurred in the sand retrieved from depths less than the depth of peak resistance, but that significant crushing had occurred in the sand retrieved from greater depths. The peak in tip resistance was a factor of two larger for the well-graded sand, but the ultimate tip resistance at greater depths was found to be approximately independent of the initial particle size distribution for all four tests. A micro mechanical explanation is also proposed for this observation. Received: 11 November 1999