Seminar - Chunshun Zhang - An analysis of grain size reduction during penetration of piles into granular media

Wednesday 4 October 2009, 1.10 pm - 1.55 pm
Civil Engineering Lecture Theatre 3

Abstract:
Previous research on penetration of piles into crushable granular media was mainly based on experiments (centrifuge tests [1] and calibration chamber tests [2]) or numerical simulations in the form of the Discrete Element Method [3]. This paper presents a continuum model that describes the penetration of piles into crushable media based on the breakage mechanics theory [4-5]. Our model provides a solution that enables to track how the gsd (grain size distribution) evolves near and surrounding the pile. The significance of the gsd in governing the mechanical behavior of granular materials during pile penetration is well appreciated [1]. The process involves the production of fine particles in the vicinity of the pile (especially along the interface and surrounding the tip). As a result, the surrounding granular layer to the pile contracts, which causes stress relaxation in the vicinity of the pile. Furthermore, during the penetration the finer crushed particles offer the shear zone more mobility by their organization as a rolling-element bearing. By tracking the evolution of gsd, we quantify and analyze the shaft friction and tip resistance. Moreover, the changes of permeability in the vicinity of the pile are directly linked to the changes in porosity and tortuosity from the increasing specific surface area.

The breakage constitutive model is implemented into a FE program for the analysis of pile penetration. We construct an Eulerian FEM model of pile foundation with a flat-toe, where the material flows through a fixed FE mesh. Such a solution has the advantage of overcoming typical large distortion problems that can arise when applying a standard large-strain Lagrangian method.

References:
[1] McDowell, G.R., Bolton, M.D. (2000), Effect of particle size distribution on pile tip resistance in calcareous sand in the geotechnical centrifuge. Granular Matter 2, 79–187
[2] White, D.J., Bolton, M.D. (2004), Displacement and strain paths during plane-strain model pile installation in sand. Geotechnique 54(6), 375–397
[3] Lobo-Guerrero, S., Vallejo, L.E. (2005), DEM analysis of crushing around driven piles in granular materials. Geotechnique 55(8), 617–623
[4] Einav I. (2007a), Breakage mechanics–Part I: Theory, Journal of the Mechanics and Physics of Solids 55(6), 1274-1297
[5] Einav I. (2007b), Breakage mechanics–Part II: Modelling granular materials, Journal of the Mechanics and Physics of Solids 55(6), 1298-1320