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Boxboro, MA, United States

Joseph P.G.,Engineering Solutions
Geotechnique | Year: 2010

This note models monotonic soil shear as a dynamical system. It provides additional information to support the hypothesis that for monotonic soil shear, the rates of change of the shear stress, effective normal stress and void ratio are proportional to the applied shear and effective normal stresses with the proportionality values decaying with strain to become zero at the steady-state condition. This hypothesis provides close fits to stress-strain-void ratio curves from undrained shear tests of uncemented, resedimented clays and drained shear tests of uncemented sands and silts, using triaxial and truetriaxial equipment, and various stress paths. For undrained shear, model parameters vary smoothly with over-consolidation ratio; for drained shear, they vary in an orderly way with relative density. The model's value lies in that a well-formed hypothesis, derived from the steady-state condition, provides a simple, alternative approach to current complex elastoplastic models based on critical state theory. Source


Joseph P.G.,Engineering Solutions
International Journal of Geomechanics | Year: 2013

Previous work indicated that rates of change of shear stress, effective normal stress, and void ratio of a sheared soil are proportional to applied values of shear and effective normal stress; initial proportionality values decay exponentially with strain to become zero at the steadystate condition. This paper proposes that the physical basis for this behavior is an underlying stochastic process in which particles move at random shear strains into the steady-state flow structure under the action of shear stress, countered by frictional resistance generated by the effective normal stress. The resulting dynamical systems model with physical properties closely fits 130 undrained and drained triaxial and true-triaxial shear tests, exhibiting strain softening or strain hardening, using various stress paths, conducted on uncemented, resedimented clays at various overconsolidation ratios (OCRs) and uncemented sands and silts at various relative densities. Parameters varied orderly with OCRs (clays) and confining pressure (silts and sands). The model's value is that based on a simple hypothesis of particles moving into the steady state at random shear strains, it closely matches data from a variety of tests. Present limitations of the model are that it only applies to static loading and not yet to generalized stress paths found in field situations. © 2013 American Society of Civil Engineers. Source


Joseph P.G.,Engineering Solutions | Graham-Eagle J.,University of Massachusetts Lowell
International Journal of Geomechanics | Year: 2014

Soil shear can be described as a dynamical system in which particles move at random shear strains into the steady-state flow structure in a process governed by simple friction. This paper studies strain-rate effects in soil shear in the context of dynamical systems soil-shear theory. The theory highlights the fact that conditions at the start of plastic deformation are strain-rate dependent and that much of the initial linear variation of stresses with strain is not attributable to the elasticity behavior of the soil but rather is the expected small-strain behavior of a nonlinear process of plastic deformation. The static coefficient of friction dominates at very small strains, after which the friction coefficient reduces to its dynamic value. Variations with strain rate in the stress-strain and void ratio-strain curves are small because of the correspondingly small dependence of the friction coefficients on strain rate. © 2014 American Society of Civil Engineers. Source


Joseph P.G.,Engineering Solutions | Graham-Eagle J.,University of Massachusetts Lowell
Computer Methods and Recent Advances in Geomechanics - Proceedings of the 14th Int. Conference of International Association for Computer Methods and Recent Advances in Geomechanics, IACMAG 2014 | Year: 2015

Soil deformation under the action of shear and normal stresses can be described as a dynamical system in which particles move at random shear strains into the steady-state flow-structure under the action of simple friction. A differential equation describes the resultant dynamical systems model. Consequently, an analytical solution for this differential equation would be most useful. However, this differential equation is of a type for which no exact analytical solution presently exists. An analytical solution is available for an approximation of the dynamical systems model. This approximation is to require that the rates at which particles move to the steady-state are the same for both shear and confining stress. While a numerical solution that assumes this equality provides fits to within 20%, the resulting analytical solution however fits test data poorly because both initial and final conditions are not well known. Consequently, an accurate solution to the soil deformation dynamical systems model presently must remain numerical. © 2015 Taylor & Francis Group, London. Source


Joseph P.G.,Engineering Solutions
Geotechnical Special Publication | Year: 2013

Dynamical systems-based soil shear holds that rates of change of shear stress, effective normal stress of a sheared mass of particles are proportional to applied values of shear and effective normal stress; initial proportionality values decay exponentially with strain to become zero at the steady-state condition. This behavior results from an underlying stochastic process in which load-carrying particles in the shear zone move into the steady-state at random shear strains, causing the number of load carrying particles in the shear zone which are not in the steady-state to decay exponentially with shear strain. This note proposes a reason for why load-carrying particles move into the steady-state configuration at random shear strains, and for why - as the number of particles in the shear zone that are not in the steady-state decay exponentially - so too does their total inter-particle contact area. The reason is that all load-carrying particles in the shear zone move at random during shear. If this is the case, then further entropic considerations show that the percent number of load-carrying particles must distribute exponentially against inter-particle contact area for load-carrying particles not in the steady-state and uniformly for those in the steady-state. © 2013 American Society of Civil Engineers. Source

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