Micromechanical modelling of jacked piles in sands
Matteo O. Ciantia, University of Dundee, School of Science and Engineering, Dundee, UK
Driven piles provide the most common foundation system for fixed oil and gas production platforms (Jardine, 2019).They are also employed in deep-water applications including tension leg platforms. The design of such systems involves the determination of diameter, wall thickness and embedded length. Like for any other foundation type in sand, the capacity and deformability of the foundation depends on the stress state of the corresponding soil. Despitethe improvements achieved by the most recent design codes the stress profile around the shaft is often based on empirical correlations (Randolph, 2003). These are more unreliable when dealing with complex soil profiles such ascrushable calcareous sands. Calibration chamber experiments (Yang et al., 2014) and centrifuge tests (Klotz and Coop, 2001) have highlighted the intense stress concentrations that develop below the pile tips, and also shown that pile geometry and driving cycles affect the final stress regime. Particle breakage, which is known to affect sands’ mechanical behaviour significantly(Ciantia et al., 2019a) also occurs during driving. Physical tests with highly instrumented piles and calibration chambers have identified aspects of the evolution of ground displacements and stresses around penetrating piles (White and Lehane, 2004). These programmes have provided benchmarks to test numerical modelling approaches for penetration problems. The discrete element model (DEM), which considers individual soil particles and their interactions explicitly, is a numerical tool which is very well suited to study large displacement contact problems such as pile penetration. Its discrete nature also provides fundamental insights into the mechanisms that underlie macroscopic behaviour (Ciantia et al., 2019b). In this work a 3D DEM is used to simulate highly instrumented calibration chamber experiments of a cone shaped tip pile penetrating into crushable granular media. Particle breakage is simulated by employing a rigorous failure criterion applied to elasto-brittle spheres. Particle scaling is used to limit the number of particles considered and it is shown that the penetration curves become scale independent above a threshold limit provided a scalable crushing model is used. The particle crushing model parameters are calibrated by matching triaxial and one-dimensional compression tests. Both monotonic and cyclic jacking are modelled and it is shown that the DEM model captures very well the experimental stress measurements during and after its penetration. The particle-scale mechanics that underlie the observed macroscopic responses are analysed, placing emphasis on the distribution of crushing events around the pile tip and distributions of particle stresses and forces around the shaft. By comparing simulations run with crushable and uncrushable grains the physical mechanisms for the well-known, yet not fully understood, marked shaft capacity increases developed over time by piles driven in sands is proposed.
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