Abstract
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 strength cu = 15 kPa). The comparison between the critical load and the bearing capacity shows that their ratio becomes less than 1 for critical slenderness LCR 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, LCR 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 because LCR is so big that the common slenderness does not exceed it.
Highlights
Buildings founded on soil of poor properties may experience excessive settlements or give rise to the failure of soil, due to its low bearing capacity; the soil improvement techniques are finalised to reduce these phenomena
Very slender piles are axially loaded in order to be driven into the soil and it is not so obvious that the ultimate conditions are reached for bearing capacity loss: because of high slenderness, they can buckle
It is investigated whether there is a threshold above which the pile ceases to behave as “a pile” and the buckling effects predominate
Summary
Buildings founded on soil of poor properties may experience excessive settlements or give rise to the failure of soil, due to its low bearing capacity; the soil improvement techniques are finalised to reduce these phenomena. Semi-empirical formulae have been provided based on full-scale load tests considering soil schematised into springs [19,20,21]; other literature analytical formulations model the soil contribution with a subgrade reaction modulus and a spring approach [22,23,24,25,26,27,28] In all these cases, buckling has been mainly studied for piles, in general; only a few literature examples reserve specific attention to the case of micropiles [29, 30]. Once the model is calibrated, it is applied for the evaluation of the PCR in different pile geometries and slenderness; the critical loads are compared to the Qlim values, derived through the classical formula of bearing capacity of deep foundations on soft clay, in order to establish how the slenderness affects the ultimate behaviour
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