A stability criterion for elasto‐viscoplastic constitutive relationships

Summary This paper reappraises Perzyna‐type viscoplasticity for the constitutive modelling of granular geomaterials, with emphasis on the simulation of rate/time effects of different magnitude. An existing elasto‐plastic model for sands is first recast into a Perzyna viscoplastic formulation and then calibrated/validated against laboratory test results on Hostun sand from the literature. Notable model features include (1) enhanced definition of the viscous nucleus function and (2) void ratio dependence of stiffness and viscous parameters, to model the pycnotropic behaviour of granular materials with a single set of parameters, uniquely identified against standard creep and triaxial test results. The comparison between experimental data and numerical simulations points out the predicative capability of the developed model and the complexity of defining a unique viscous nucleus function to capture sand behaviour under different loading/initial/boundary and drainage conditions. It is concluded that the unified viscoplastic simulation of both drained and undrained response is particularly challenging within Perzyna's framework and opens to future research in the area. The discussion presented is relevant, for instance, to the simulation of multiphase strain localisation phenomena, such as those associated to slope stability problems in variably saturated soils.

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Predictive potential of Perzyna viscoplastic modelling for granular geomaterials

Lazari

Sanavia

Prisco

et al. 2018

“…() Some authors have explained the temporal evolution of the failure process by remarking the role of the instability line during the initiation of an accelerating response. () This concept has been recently incorporated by Pisanò and di Prisco within a rigorous theoretical framework able to interpret unstable phenomena in viscous solids. The performance of this theory has been explored with reference to the undrained creep of loose sands by Marinelli et al, who showed that these particular class of instability criteria is able to detect the accelerating trends of behavior characterizing delayed liquefaction.…”

Section: Delayed Liquefaction For Quasi‐saturated Soilsmentioning

confidence: 99%

“…The performance of this theory has been explored with reference to the undrained creep of loose sands by Marinelli et al, who showed that these particular class of instability criteria is able to detect the accelerating trends of behavior characterizing delayed liquefaction. Here, the theory proposed by Pisanò and di Prisco is specialized to quasi‐saturated soils by incorporating the role of occluded gas bubbles. For this purpose, the viscoplastic response is rearranged by means of a new constitutive operator relating accelerations and rates of the response variables.…”

Section: Delayed Liquefaction For Quasi‐saturated Soilsmentioning

confidence: 99%

“…A rate‐dependent constitutive framework based on Perzyna viscoplasticity() is used to define instability criteria enabling the identification of accelerations leading to diffuse failure. For this purpose, the theory recently proposed by Pisanò and di Prisco is recast in a form applicable to quasi‐saturated soils. In particular, a new mathematical operator relating the first‐ and second‐order derivatives of the response variables of quasi‐saturated porous media will be defined.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Instability criteria for quasi‐saturated viscous soils

Marinelli

Buscarnera

2017

Summary This paper presents a theoretical framework to interpret the inception of unstable undrained creep in quasi‐saturated soils. For this purpose, the effect of gas bubbles occluded in the fluid phase is embedded into an augmented compressibility of the fluid mixture, while the mechanical characteristics of the solid skeleton have been simulated through a viscoplastic strain‐hardening model. This constitutive framework has been been used to formulate a theoretical platform able to detect runaway failures resulting from extended stages of undrained creep. It is shown that the conditions identifying the onset of spontaneous accelerations are governed by the same stability index associated with the initiation of static liquefaction. At variance with soils saturated by incompressible fluids, the conditions for undrained instability are altered by the appearance of the Skempton coefficient B, thus reflecting the beneficial effect of the fluid compressibility and its ability to decrease the liquefaction potential. The capabilities of the theory are verified through a sequence of undrained creep simulations showing the transition from stable to unstable behavior resulting from an increase of the degree of saturation. The proposed findings provide a conceptual framework to interpret the effects of gas bubbles in loose soils, as well as to assess effectiveness and longevity of liquefaction mitigation strategies based on desaturation technologies.

Numerical simulation of unstable suction transients in unsaturated soils: the role of wetting collapse

Chen

Buscarnera

2021

Fluid injection is one of the major triggering factors of instability in unsaturated soils. Robust simulation tools are therefore required to examine hydrologic transients in such a class of inelastic materials. In this paper, the governing equations imposing the balance of mass and momentum in deformable unsaturated porous media are inspected from an analytical perspective. Our main goal is to examine the role of volume collapse on the stability of suction transients. Distinct scenarios are considered in terms of material behavior, distinguishing the case of elastoplastic and viscoplastic materials. It shows that in elastoplastic materials the mathematical ill-posedness of the partial differential equations leads to a diffusive instability of the pore pressure field, signaled by the loss of stress controllability under water injection. By contrast, the introduction of viscosity restores the mathematical well-posedness and ensures the positive diffusivity. Numerical implications of these findings have been explored through simulations of water injection with a 1D finite element solver. It shows that an increase of the soil hydraulic sensitivity (i.e., a marked deterioration of the yield stress upon suction removal) can deteriorate the diffusive stability and prevent the computation of suction changes across a collapsible soil layer. While the incorporation of minor amounts of viscosity can restore numerical stability by suppressing the constitutive singularity. Such numerical results have been corroborated by the computation of local stability indicators based on controllability theory, which proved to be robust diagnostic tools to identify the culprit of runaway instability emerging from coupled hydro-mechanical processes.