WaiChing Sun

Connecting microstructural attributes and permeability from 3D tomographic images of in situ shear-enhanced compaction bands using multiscale computations

Tomographic images taken inside and outside a compaction band in a field specimen of Aztec sandstone are analyzed by using numerical methods such as graph theory, level sets, and hybrid lattice Boltzmann/finite element techniques. The results reveal approximately an order of magnitude permeability reduction within the compaction band. This is less than the several orders of magnitude reduction measured from hydraulic experiments on compaction bands formed in laboratory experiments and about one order of magnitude less than inferences from two-dimensional images of Aztec sandstone. Geometrical analysis concludes that the elimination of connected pore space and increased tortuosities due to the porosity decrease are the major factors contributing to the permeability reduction. In addition, the multiscale flow simulations also indicate that permeability is fairly isotropic inside and outside the compaction band.

Stress-induced anisotropy in granular materials: fabric, stiffness, and permeability

The loading of a granular material induces anisotropies of the particle arrangement (fabric) and of the material’s strength, incremental stiffness, and permeability. Thirteen measures of fabric anisotropy are developed, which are arranged in four categories: as preferred orientations of the particle bodies, the particle surfaces, the contact normals, and the void space. Anisotropy of the voids is described through image analysis and with Minkowski tensors. The thirteen measures of anisotropy change during loading, as determined with three-dimensional discrete element simulations of biaxial plane strain compression with constant mean stress. Assemblies with four different particle shapes were simulated. The measures of contact orientation are the most responsive to loading, and they change greatly at small strains, whereas the other measures lag the loading process and continue to change beyond the state of peak stress and even after the deviatoric stress has nearly reached a steady state. The paper implements a methodology for characterizing the incremental stiffness of a granular assembly during biaxial loading, with orthotropic loading increments that preserve the principal axes of the fabric and stiffness tensors. The linear part of the hypoplastic tangential stiffness is monitored with oedometric loading increments. This stiffness increases in the direction of the initial compressive loading but decreases in the direction of extension. Anisotropy of this stiffness is closely correlated with a particular measure of the contact fabric. Permeabilities are measured in three directions with lattice Boltzmann methods at various stages of loading and for assemblies with four particle shapes. Effective permeability is negatively correlated with the directional mean free path and is positively correlated with pore width, indicating that the anisotropy of effective permeability induced by loading is produced by changes in the directional hydraulic radius.

A unified method to predict diffuse and localized instabilities in sands

A simplified method to analyse diffuse and localized bifurcations of sand under drained and undrained conditions is presented in this paper. This method utilizes results from bifurcation analysis and critical state plasticity theory to detect the onset of pure and dilatant shear band formation, static liquefaction and drained shear failures systematically. To capture the soil collapse observed in experiments, the instability state line concept originated by Chu, Lo and Lee in 1993 is adopted. Emphasis is given to examine how the presence of pore-fluid may facilitate or delay instability after yielding occurs. The predictions of instabilities are compared with experimental data from triaxial compression tests on Toyoura and Changi sands.

Determining Material Parameters for Critical State Plasticity Models Based on Multilevel Extended Digital Database

This work presents a new staggered multilevel material identification procedure for phenomenological critical state plasticity models. The emphasis is placed on cases in which available experimental data and constraints are insufficient for calibration. The key idea is to create a secondary virtual experimental database from high-fidelity models, such as discrete element simulations, then merge both the actual experimental data and secondary database as an extended digital database (EDD) to determine material parameters for the phenomenological macroscopic critical state plasticity model. The calibration procedure therefore consists of two steps. First, the material parameters of the discrete (distinct) element method (DEM) simulations are identified via the standard optimization procedure. Then, the calibrated DEM simulations are used to expand the experimental database with new simulated loading histories. This expansion of database provides additional constraints necessary for calibration of the phenomenological critical state plasticity models. The robustness of the proposed material identification framework is demonstrated in the context of the Dafalias–Manzari plasticity model. [DOI: 10.1115/1.4031619]

Effects of spatial heterogeneity and material anisotropy on the fracture pattern and macroscopic effective toughness of Mancos Shale in Brazilian tests

For assessing energy-related activities in the subsurface, it is important to investigate the impact of the spatial variability and anisotropy on the geomechanical behavior of shale. The Brazilian test, an indirect tensile-splitting method, is performed in this work, and the evolution of strain field is obtained using digital image correlation. Experimental results show the significant impact of local heterogeneity and lamination on the crack pattern characteristics. For numerical simulations, a phase field method is used to simulate the brittle fracture behavior under various Brazilian test conditions. In this study, shale is assumed to consist of two constituents including the stiff and soft layers to which the same toughness but different elastic moduli are assigned. Microstructural heterogeneity is simplified to represent mesoscale (e.g., millimeter scale) features such as layer orientation, thickness, volume fraction, and defects. The effect of these structural attributes on the onset, propagation, and coalescence of cracks is explored. The simulation results show that spatial heterogeneity and material anisotropy highly affect crack patterns and effective fracture toughness, and the elastic contrast of two constituents significantly alters the effective toughness. However, the complex crack patterns observed in the experiments cannot completely be accounted for by either an isotropic or transversely isotropic effective medium approach. This implies that cracks developed in the layered system may coalesce in complicated ways depending on the local heterogeneity, and the interaction mechanisms between the cracks using two-constituent systems may explain the wide range of effective toughness of shale reported in the literature.

Anisotropy of a Tensorial Bishop’s Coefficient for Wetted Granular Materials

AbstractThe objective of this research is to use grain-scale numerical simulations to analyze the evolution of stress anisotropy exhibited in wetted granular matter. Multiphysical particulate simulations of unsaturated granular materials were conducted to analyze how the interactions of contact force chains and liquid bridges affect macroscopic responses under various suction pressure and loading histories. To study how the formation and rupture of liquid bridges affect the mechanical responses of wetted granular materials, a series of suction-controlled triaxial tests were conducted with two grain assemblies, one composed of large particles of similar sizes, the other composed of a mixture of large particles with significant amount of fines. The results indicate that capillary stresses are anisotropic in both sets of specimens, and that stress anisotropy is more significant in granular assemblies filled with fine particles. A generalized tensorial Bishop’s coefficient is introduced to analyze the connect...

Micropolar effect on the cataclastic flow and brittle-ductile transition in high-porosity rocks

A micromechanical distinct element method (DEM) model is adopted to analyze the grain-scale mechanism that leads to the brittle-ductile transition in cohesive-frictional materials. The cohesive-frictional materials are idealized as particulate assemblies of circular disks. While the frictional sliding of disks is sensitive to the normal compressive stress exerted on contacts, normal force can be both caused by interpenetration and long-range cohesive bonding between two particles. Our numerical simulations indicate that the proposed DEM model is able to replicate the gradual shift of porosity change from dilation to compaction and failure pattern from localized failures to cataclastic flow upon rising confining pressure in 2-D biaxial tests. More importantly, the micropolar effect is examined by tracking couple stress and microcrack initiation to interpret the transition mechanism. Numerical results indicate that the first invariant of the couple stress remains small for specimen sheared under low confining pressure but increases rapidly when subjected to higher confining pressure. The micropolar responses inferred from DEM simulations reveal that microcracking may occur in a more diffuse and stable manner when the first invariant of the macroscopic couple stress are of higher magnitudes.

Capturing the effective permeability of field compaction band using hybrid lattice Boltzmann/Finite element simulations

As observed in reconstructed 3D images of the field compaction bands (Lenoir et al 2010), their formation may cause a significant reduction in porosity and permeability. This paper describes essential a lattice Boltzmann/finite element model that may be advantageous for calculating effective permeability of compaction bands from obscured structure in a cost-efficient way. The pressing issues include the homogenization technique and the scale effect of the effective permeability.

Finite Element Analysis of Hydro-mechanical Coupling Effects on Shear Failures of Fully Saturated Collapsible Geomaterials

The fully coupled diffusion-deformation processes occurring within porous geomaterials, such as sand, clay and rock, are of interest to numerous geotechnical engineering applications. In this work, a stabilized enhanced strain finite element procedure for poromechanics is integrated with an elasto-plastic cap model to simulate the associative and non-associative hydro-mechanical responses of fluid- infiltrating biphasic collapsible porous geomaterials. We present a quantitative analysis on how macroscopic plastic response caused by pore collapse and grain rearrangement affects the seepage of pore fluid, and vice versa. Finite element simulations of shear failure problems will be presented to study the effect of pore pressure dissipation on the stress path and plastic response of the porous geomaterials.

A Multi-Phase-Field Anisotropic Damage-Plasticity Model for Crystalline Rocks

Many engineering applications, such as geological disposal of nuclear waste, require reliable predictions on the thermo-hydro-mechanical responses of porous media exposed to extreme environments. This presentation will discuss the relevant modeling techniques designed specifically for such environmental conditions. In particular, we will provide an overview of the coupling method of crystal plasticity and multi-phase-field model designed to replicate the thermal- and rate-dependent damage-plasticity of crystalline rock. Special emphasis is placed on capturing the intrinsic anisotropy of salt grain in 3D with respect to damage behavior and plastic flow by incorporating the crystallographic information of salt.