Analysis of the Behaviour of Very Slender Piles: Focus on the Ultimate Load

The paper aims to analyse the influence of slenderness on the ultimate behaviour of piles with a very small diameter (less than 10 cm) that are often employed in soil reinforcement and for which the slenderness can significatively influence the failure behaviour, reducing the ultimate load. The aim is reached by means of numerical analyses on small-diameter piles of different geometries, embedded in clayey soil. The critical load is evaluated numerically in undrained conditions and then compared to the bearing capacity estimated by the classical approaches based on limit equilibrium method. The numerical model is first calibrated on the basis of the results of experimental laboratory tests on bored piles of a small diameter in a cohesive soft soil (average undrained shear strengthc(u) = 15 kPa). The comparison between the critical load and the bearing capacity shows that their ratio becomes less than 1 for critical slendernessL(CR)that decreases, nonlinearly, with the decreasing of the pile diameter. The results of the analysis show that varying the diameter of the pile from 0.06 to 0.18 m,L(CR)varies from 65 to 200. The aforementioned evidence suggests that the evaluation of the ultimate load of piles of very small diameter has to follow the considerations on the critical load of the pile, especially if it is embedded in soft soil; on the contrary for piles of greater diameters (bigger than 20 cm) the buckling is not meaningful becauseL(CR)is so big that the common slenderness does not exceed it.

Automated summary

The study investigates the influence of slenderness on ultimate behavior of very small diameter piles through numerical analysis and experimental tests. It shows that as pile diameter decreases, the critical slenderness also decreases. Ultimate load evaluation for very small diameter piles should consider critical load, while for larger diameter piles, buckling is not a significant factor.