Centrifuge modeling of jacked pile in clay

Notable contributions to our improved understanding of jacked pile behavior in sand have been achieved through instrumented model pile tests in laboratory test chambers, at elevated g-levels in the centrifuge, and in the field. In recent years, research focusing on pile behavior in clay has declined. Consequently, predictive methods for pile capacity have not advanced beyond those provided in American Petroleum Institute (API 2000) recommendations, which were based on research conducted in the early 1980s. This paper re-focuses attention on the shaft capacity of jacked piles in clay. Three centrifuge scale pile tests were performed in kaolin clay in the drum centrifuge at the University of Western Australia. The tests were performed in pre-consolidated blocks of kaolin, and were subsequently spun in the centrifuge at three different g-levels of 50g, 125g and 250g respectively. The piles were equipped with total pressure sensors located at different depths and were installed by jacking into samples of reconstituted kaolin clay. The kaolin clay samples were prepared to measure the range of the cone penetration test end resistance (q_t), undrained strengths (s_u-Tbar), and overconsolidation ratios (OCRs). These pile tests were used to investigate the lateral stress changes (σ_r) developed along the pile shafts during pile installation and equalization. In addition, the change in the value of lateral stress (Δσ_r) and the changes in pile shaft resistance during the pile tension test were discussed. The characteristics of the jacked pile in the clay with different over-consolidation ratios (OCRs) were revealed. Furthermore, the centrifuge data were subsequently used to examine the current design methods for the evaluation of the shaft capacity of displacement piles in clay. The centrifuge test results show that during the pile penetration, a strong dependence of lateral stress on the relative depth of the pile tip (h/B) develops, and the total radial stress, as measured in a particular soil horizon, is observed to decrease as the relative depth of the pile tip (h/B) increases (where h is the height of the sensor above the pile tip, and B is the diameter of the pile). Based on the cone penetration test during the investigation, it is observed that the lateral stress developed on a displacement pile is strongly depended on the cone penetration test end resistance (q_t) and the relative depth of the pile tip (h/B). It is shown that the empirical method allowing for a dependence of shaft friction on q_t, and h provides good estimates of the shaft capacities measured in centrifuge experiments. The research results have certain theoretical and engineering significance for pile construction and bearing capacity design in the soft soil region.