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Potyondy D.O.,Itasca Consulting Group Inc.
46th US Rock Mechanics / Geomechanics Symposium 2012 | Year: 2012

The bonded-particle model (BPM) consisting of parallel-bonded disks or spheres suffers from the limitation that if one matches the unconfined- compressive strength (qu) of a typical hard rock, then the direct-tension strength (σt) of the model will be too large. This limitation can be overcome in two dimensions by introducing a polygonal grain structure to provide rotational restraint arising from inter-granular interlock. The flat-jointed BPM (in which each disk-disk contact simulates the behavior of a finite-length interface between two disks with locally flat notional surfaces such that even a fully broken interface continues to resist relative rotation) provides such a structure and supersedes the parallel-bonded BPM by mimicking more of the micro- and macro-mechanisms associated with rock damage. Copyright 2012 ARMA, American Rock Mechanics Association.


Zhang F.,Georgia Institute of Technology | Damjanac B.,Itasca Consulting Group Inc. | Huang H.,Georgia Institute of Technology
Journal of Geophysical Research: Solid Earth | Year: 2013

The coupled displacement process of fluid injection into a dense granular medium is investigated numerically using a discrete element method (DEM) code PFC2D® coupled with a pore network fluid flow scheme. How a dense granular medium behaves in response to fluid injection is a subject of fundamental and applied research interests to better understand subsurface processes such as fluid or gas migration and formation of intrusive features as well as engineering applications such as hydraulic fracturing and geological storage in unconsolidated formations. The numerical analysis is performed with DEM executing the mechanical calculation and the network model solving the Hagen-Poiseuille equation between the pore spaces enclosed by chains of particles and contacts. Hydromechanical coupling is realized by data exchanging at predetermined time steps. The numerical results show that increase in the injection rate and the invading fluid viscosity and decrease in the modulus and permeability of the medium result in fluid flow behaviors displaying a transition from infiltration-governed to infiltration-limited and the granular medium responses evolving from that of a rigid porous medium to localized failure leading to the development of preferential paths. The transition in the fluid flow and granular medium behaviors is governed by the ratio between the characteristic times associated with fluid injection and hydromechanical coupling. The peak pressures at large injection rates when fluid leakoff is limited compare well with those from the injection experiments in triaxial cells in the literature. The numerical analysis also reveals intriguing tip kinematics field for the growth of a fluid channel, which may shed light on the occurrence of the apical inverted-conical features in sandstone and magma intrusion in unconsolidated formations. © 2013. American Geophysical Union. All Rights Reserved.


Kwok C.-Y.,Itasca Consulting Group Inc. | Kwok C.-Y.,University of Cambridge | Bolton M.D.,University of Cambridge
Geotechnique | Year: 2010

Discrete element modelling (DEM) has been used to simulate creep in assemblies of spherical grains possessing an interfacial coefficient of friction that varies with sliding velocity according to rate process theory. Soil stiffness is represented by a pair of values of linear spring stiffness normal and tangential to each intergranular contact, and the limiting coefficient of contact friction is described as varying linearly with the logarithm of sliding velocity. DEM simulations of an assembly of 3451 spheres reproduce a number of significant phenomena including: creep rate as a function of the mobilisation of deviatoric stress; initially linear decay of creep strain rate with time plotted on log-log axes and with a slope m in the range 20.8 to 21; and ultimate creep failure in triaxial simulations at high deviatoric stress ratios. Creep-induced failure is shown to occur at a unique axial strain for a given state of initial packing, and to be linked with dilatancy. The numerical results are compared quantitatively with the test data of soils from the literature. The effects of activation energy are considered in relation to the different magnitudes of creep encountered in sands and clays.


Potyondy D.O.,Itasca Consulting Group Inc.
Geosystem Engineering | Year: 2015

We generalize our view of a bonded-particle model (BPM) to consist of a base material (that is a packed assembly of rigid grains joined by deformable and breakable cement at grain–grain contacts) to which larger-scale joints can be added and whose mechanical behavior is simulated by the distinct-element method using the two- and three-dimensional discontinuum programs PFC2D and PFC3D. The micromechanical processes that control brittle fracture, and thus, should inform any micromechanical theory or model, are summarized. The rich variety of microstructural models that can be produced by the bonded-particle modeling methodology are described and classified with respect to their microstructural and larger-scale features. These models provide a wide range of rock behaviors that encompass both compact and porous rock at both an intact and rock-mass scale, and examples are provided of how BPMs are being used to model rock at these scales. The examples include an intact anisotropic material that may swell and contract in response to changes in saturation, the behavior of two alternative BPMs that can match both the uniaxial and tensile strengths of compact rock and the embedding of an intact BPM within a larger continuum model to study fracturing around a gold-mine stope in quartzite. © 2014 Taylor & Francis.


Varun,Itasca Consulting Group Inc. | Assimaki D.,Georgia Institute of Technology | Shafieezadeh A.,Georgia Institute of Technology
Soil Dynamics and Earthquake Engineering | Year: 2013

We present a macroelement for soil-structure interaction analyses of piles in liquefiable soils, which captures efficiently the fundamental mechanisms of saturated granular soil behavior. The mechanical model comprises a nonlinear Winkler-type model that accounts for soil resistance acting along the circumference of the pile, and a coupled viscous damper that simulates changes in radiation damping with increasing material nonlinearity. The formulation for a gap element is also proposed to account for formation of gap at pile-soil interface. Validation of the macroelement is conducted using full-scale forced vibration test data from a blast-induced liquefaction test bed, and centrifuge data for seismic loading of piles with superstructure. The macroelement parameters are estimated as a function of the measured soil properties and the level of effective stress. Predictions of bending moments and acceleration time histories at the top of pile and the superstructure are found to be in good agreement with both the full and the model scale data. A comparison with predictions from alternative established methodologies is also presented. © 2012 Elsevier Ltd.

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