Analysis of the Behaviour of Very Slender Piles: Focus on the Ultimate Load
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RESEARCH PAPER
Analysis of the Behaviour of Very Slender Piles: Focus on the Ultimate Load Michele Placido Antonio Gatto1 · Lorella Montrasio1 Received: 9 December 2019 / Revised: 10 June 2020 / Accepted: 9 July 2020 © The Author(s) 2020
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. Keywords Slender piles · Buckling · Ultimate load · FE analysis · Pile–soil interface
1 Introduction 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. These techniques can be grouped into interventions based on soil replacement [1] or providing reinforcement in different ways, such as sand compaction piles [2], stone columns for slope stability [3] or lightweight fill [4], through sawdust, tire-derived aggregate and geofoam, for consolidation and differential settlements minimisation. Among the soil reinforcement methods, a common solution is based on the use of micropiles [5, 6]; case studies and numerical simulations have * Lorella Montrasio [email protected] Michele Placido Antonio Gatto [email protected] 1
Department of Engineering and Architecture, University of Parma, 43124 Parma, Italy
demonstrated the increase of the soil stiffness and bearing capacity due to the use of micropiles [7–11]. “Most micropiles are 100–250 mm in diameter, 20–30 m long” [6]: due to their small diameter d, when micropiles of considerable length L are axially loaded, they may experience the
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