Micro mechanics of critical states for isotropically overconsolidated sand

Abstract

A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription.For more information, please contact eprints@nottingham.ac.uk AbstractThe discrete element method has been used to investigate the micro mechanics of shearing to a critical state on the loose and dense sides of critical. Isotropic compression has previously been modelled in 3D using a large number of particles and without the use of agglomerates. The same procedure is used here. Particle fracture is governed by the octahedral shear stress within the particle due to the multiple contacts and a Weibull distribution of strengths. Isotropic compression of a silica sand has been simulated to 20 MPa and followed by unloading to a range of stresses before shearing to a critical state, using micro parameters which relate to the silica sand particle strengths. The samples at the lowest stress levels exhibit peak strength and dilation. The sample at the highest stress exhibits contraction and ductile yielding to a critical state. A critical state line is established, which appears to become parallel to the isotropic line in log e-log p space at high stress levels. This paper shows that it is the evolving fractal particle size distribution during isotropic normal compression which governs the behaviour on unloading to different overconsolidation ratios. The micro mechanics of the critical state line are shown to be in the evolving particle size distribution during normal compression, and how such an aggregate behaves when it is unloaded.

Investigating the effects of particle shape on normal compression and overconsolidation using DEM

“…Recent work by McDowell et al [19], who performed one-dimensional compression simulations using the same model but with a different aspect ratio, reported a K0,nc value of approximately 0.7 for spheres, slightly lower than the value found in the present simulations for the same material. This is almost certainly due to the different shape of the samples, their sample [19] had a different aspect ratio more suitable for triaxial shearing (it was taller than it was wide), which reduced the influence of the boundaries, and resulted in a larger volume of dense random packing. McDowell et al [19] used their value of K0,nc to predict a critical state friction angle using the Jâky equation [20]:…”

“…This is almost certainly due to the different shape of the samples, their sample [19] had a different aspect ratio more suitable for triaxial shearing (it was taller than it was wide), which reduced the influence of the boundaries, and resulted in a larger volume of dense random packing. McDowell et al [19] used their value of K0,nc to predict a critical state friction angle using the Jâky equation [20]:…”

“…The value of a is typically around 0.5 [e.g. 23,25] for clays; however, for sands, one can expect a similar relationship to exist, considering both granular soils and clays exhibit the same behaviour during normal compression, and Jâky's equation applies to both [19]. Feda [21] noted that values as high as 0.6 are suitable for dense sands.…”

Micro mechanics of drained and undrained shearing of compacted and overconsolidated crushable sand

“…In particular, a number of recent attempts have been made to investigate the three-dimensional triaxial behaviour of crushable soil (e.g. Bolton et al, 2008;de Bono & McDowell, 2014;Hanley et al, 2015;McDowell et al, 2015). However, in general the majority of published studies of soil behaviour using DEM neglect particle crushing (Thornton, 2000;Sitharam et al, 2002;Cui & O'Sullivan, 2006;Minh & Cheng, 2013).…”

“…The authors have published several recent studies using a simple crushing model, and have previously attempted to establish a CSL (de Bono & McDowell, 2014;McDowell et al, 2015). This paper follows on from that work by aiming to establish a full CSL over a wide range of stresses for a simulated silica sand and provide an in-depth fundamental analysis of the micro-scale behaviour of a crushable soil during and after being sheared to a critical state, from initial states looser and denser than critical, involving a range of different stress paths, including constant-volume conditions.…”