A model for rockfill compressibility

Material behaviour that exhibits characteristics of creep induced by a spontaneous mineral dissolution enhanced by material damage is studied. It is believed that the characteristic rates of the chemical processes involved determine the time-rate dependence of the resulting strain. A basic model of a combined chemo-plastic softening and chemically enhanced deviatoric strain hardening for saturated geomaterials is presented. Chemical softening is postulated to occur as a consequence of the net mass removal resulting from dissolution and precipitation of specific minerals occurring at the damage-generated inter-phase interfaces. Closed and open systems are discussed. In the former case, deformation at constant stress results entirely from a local compensation mechanism between the chemical softening and strain hardening. The classical three stages of creep are interpreted in terms of mechanisms of dissolution and precipitation, as well as the variation in the reaction surface areas involved in the mass exchange. In an open system, the above local mechanism is enhanced by the removal of mass via diffusion of species affecting the mass balance. Such a system is addressed via a boundary value problem as shown in an example. scheme of inelastic stress-strain laws. Hence, numerous attempts in various contexts and for various materials were undertaken, first, to frame creep within visco-plasticity, rate-dependent plasticity or endochronic theory, and second, to attribute particular mechanisms to creep (e.g. dislocation motion, inter-grain diffusion (Nabarro [3]), or intra-grain diffusion (Coble [4]) resulting in macroscopically observed strains at constant stress). Intrinsic time or similar variables were introduced to link the phenomenology to the actual progress of the involved processes, see e.g. Valanis [5]. For soils, early efforts in this direction were undertaken by Murayama and Shibata [6], Christensen and Wu [7], Mitchell [8] and Singh and Mitchell [9] or Kuhn and Mitchell [10].In this paper, we advance a hypothesis that one of the mechanisms of creep in earthen materials is a chemical process triggered by a prior mechanical damage. The general idea comes from the observation that what is perceived as creep occurs only when yielding, and hence some damage involving formation of new water/mineral interfaces is inflicted to the material. The slow pace of the deformation triggered by that damage suggests a coincidence with a simultaneous chemical process. In this specific case, a time-dependent chemical enhancement occurs through dissolution of minerals from a zone of dilatant damage and subsequent mass removal via diffusion. The damage is associated with microcracking (see e.g. Brace et al. [11]), which generates a new free surface area at the crack walls. When in contact with interstitial fluid, these walls constitute new solid/fluid interface and hence new dissolution sites. Consequently, the removal of the mineral mass via dissolution leads to an ulterior material softening and further deformation at constant s...

“…This includes the sub-critical crack propagation into a chemically weakened crack tip environment when flooded with water (see e.g. Wiederhorn et al [38], Oldecop and Alonso [39]). …”

Section: Mass Removal As a Results Of Chemical Reaction In Geological mentioning

confidence: 99%

mentioning

confidence: 99%

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Creep of saturated materials as a chemically enhanced rate‐dependent damage process

Hueckel

2007

“…This is well known in rockfill dam engineering. Oldecop and Alonso summarizes early experimental work in this regard, and Alonso et al describes the relevant effect of rockfill partial and full wetting to explain the long‐term recorded behaviour of Beliche dam.…”

Section: Introductionmentioning

confidence: 99%

“…Relative humidity (RH) controlled tests reported by Oldecop and Alonso, Chavez and Alonso and Ortega indicate that RH controls the volumetric and deviatoric stiffness of rockfill Wetting (RH increase) of stressed rockfill results in compressive volumetric deformations (collapse) Rockfill, under constant stress, deforms progressively in time. The intensity of this creeping or secondary behaviour increases as RH increases (decreasing suction).…”

Section: Introductionmentioning

confidence: 99%

Suction and time effects in rockfill deformation

Alonso

Tapias

2019

Summary Ambient relative humidity controls the stress‐strain‐time behaviour of granular media exhibiting particle breakage. Breakage is the result of crack propagation inside loaded particles. The DEM model developed uses the results of subcritical crack propagation in brittle rock to predict breakage and the subsequent structural rearrangement. Two closed‐form solutions at grain level, stress distribution under diametrically opposed forces and the mode I crack propagation velocity, were included in the DEM formulation. Crack propagation velocity depends explicitly on relative humidity. The resulting model incorporates comminution and splitting modes of particle breakage as well as arbitrary grain shapes by means of aggregation of spherical microparticles. The model was first validated against a large diameter suction‐controlled oedometer test on hard limestone crushed gravel. Direct tests on contact properties (local stiffness and friction) helped to validate the model. The model was then used to predict the response of large diameter suction‐controlled triaxial tests on gravels having different (uniform) initial sizes. Time delayed deformations and wetting‐induced collapse deformations under maintained load are also a natural outcome of the model. They were shown to follow quite accurately the experimental results.

A Generalized Backward Euler algorithm for the numerical integration of a viscous breakage model

Marinelli

Buscarnera

2018

Summary This paper discusses the formulation and the numerical performance of a fully implicit algorithm used to integrate a rate‐dependent model defined within a breakage mechanics framework. For this purpose, a Generalized Backward Euler (GBE) algorithm has been implemented according to two different linearization strategies: The former is derived by a direct linearization of the constitutive equations, while the latter introduces rate effects through a consistency parameter. The accuracy and efficiency of the GBE algorithm have been investigated by (1) performing material point analyses and (2) solving initial boundary value problems. In both cases, the overall performance of the underlying algorithm is inspected for a range of loading rates, thus simulating comminution from slow to fast dynamic problems. As the viscous response of the breakage model can be recast through a viscous nucleus function, the presented algorithm can be considered as a general framework to integrate constitutive equations relying on the overstress approach typical of Perzyna‐like viscoplastic models.