Richard Wan

Richard Wan
The University of Calgary | HBI · Department of Civil Engineering

About

187
Publications
34,401
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1,859
Citations
Additional affiliations
July 1993 - present
The University of Calgary
Position
  • Professor (Full)

Publications

Publications (187)
Article
Full-text available
This paper presents a micromechanical description of the swelling behavior of partially saturated clays following a two-stage homogenization procedure on a three-dimensional Representative Elementary Volume (REV) of clay. At the nano-scale, the REV of clay particles includes idealized parallel clay platelets and oblate spheroidal pores in between t...
Article
Full-text available
This paper presents a micromechanical description of the stress-deformation in partially saturated swelling clays, which accounts for the plastic dissipation induced due to the coupled action of mechanical, swelling and capillary forces that operate at the underlying small scales. This is achieved via a two-stage homogenization procedure based on t...
Preprint
Full-text available
In this work, we present a deep neural network architecture that can efficiently approximate classical elasto-plastic constitutive relations. The network is enriched with crucial physics aspects of classical elasto-plasticity, including additive decomposition of strains into elastic and plastic parts, and nonlinear incremental elasticity. This lead...
Article
Full-text available
The microstructure of granular materials has a significant influence on their macroscopic quasi-static strength and deformational behaviour. This microstructure is often quantified by a second-order fabric tensor that describes the primary orientational statistics of interparticle contacts. Here, it is investigated how the fabric tensor changes whe...
Conference Paper
Full-text available
The current study presents a micromechanical approach to investigate the aspects of failure in partially saturated soils. In a series of triaxial Discrete Element Modeling (DEM) simulations, it is demonstrated how capillarity, as the dominant mechanism arising from multiphasic interactions in partially saturated condition, affects the distribution...
Article
Full-text available
The aim of this paper is to investigate the effect of an adsorbed water layer on the mechanical behavior of fine-grained wet granular materials in the pendular regime with isolated capillary liquid bridges. The adsorbed water forms a thin liquid film tightly bound to a particle’s surface equilibrated by a so-called “disjoining pressure”. In a stres...
Article
The behavior of granular media under quasistatic loading has recently been shown to attain a stable evolution state corresponding to a manifold in the space of micromechanical variables. This state is characterized by sudden transitions between metastable jammed states, involving the partial micromechanical rearrangement of the granular medium. Usi...
Article
The proper solution of geotechnical boundary value problems requires robust constitutive models that can describe the mechanical behavior of geomaterials under various loading conditions, while also accounting as closely as possible for the different material scales of interest. This is even more relevant to granular media where the complexity of t...
Article
Buried pipeline systems form a key part of effective and safe infrastructures for the transportation of natural resources such as natural gas and liquid. However, they may be constructed in areas prone to landslides where ground movement may exert excessive strains on the pipe sections causing rupture. This paper presents a methodology for estimati...
Preprint
Full-text available
The behavior of granular media under quasi-static loading has recently been shown to attain a stable evolution state corresponding to a manifold in the space of micromechanical variables. This state is characterized by sudden transitions between metastable jammed states, involving the partial micromechanical rearrangement of the granular medium. Us...
Article
Full-text available
Water retention of clayey soils with wide particle size distributions involves a combination of capillary and adsorbed layers effects that result into suction–saturation relations spanning over multiple decades of matric suction values. The present study provides a physics-based analysis to reproduce the water retention curve of such soils based so...
Article
Full-text available
The current study investigates the non-coaxial plastic flow in granular media where the principal directions of incremental plastic strain differ from those of the current stress (and of the current fabric tensor). The stress probing technique is used with Discrete Element Method simulations to capture the plastic response of a 2D granular assembly...
Article
Full-text available
The paper describes a micromechanical approach that explores the anisotropic nature of the capillary stress tensor and its evolution in pendular granular materials via Discrete Element Modeling (DEM) simulations. Dimensionless parameters are used to address the conditions under which the contribution of capillarity (or cohesive interparticle forces...
Article
Full-text available
The paper investigates the failure of brittle rocks within a multiscale/multiphysics computational modeling framework so as to explicitly incorporate both microstructural and hydromechanical aspects in their overall (nonlinear) fracture behavior. Herein, the rock is idealized as a microfissured porous medium via a representative elementary volume (...
Preprint
Full-text available
In this paper, we investigate some micromechanical aspects of elasto-plasticity in heterogeneous geomaterials. The aim is to upscale the elasto-plastic behavior for a representative volume of the material which is indeed a very challenging task due to the irreversible deformations involved. Considering the plastic strains as eigen-strains allows us...
Article
Full-text available
The paper deals with numerical computations of hydraulic variations and groundwater flow changes in continuous permafrost due to climate change and open-pit mining in the cold regions of Northern Canada. The work is a case study in connection with the proposed Kiggavik project in Nunavut, Canada. A major challenge in simulating fluid flow through p...
Article
Full-text available
The paper examines the mechanics and physics of granular material responses at the macroscopic and microscopic levels during both monotonic and cyclic loadings. A numerical analysis referring to a long retaining wall is conducted using a two-dimensional discrete element model representing a granular system with a free top surface. On one of the lat...
Article
Full-text available
The study sets forth a method upon which the strength anisotropy of rocks can be estimated from particular characteristics of the elasticity tensor. By depicting rocks as assemblies of bonded particles, micromechanical formulation of force distribution and contact deformation in conjunction with a local strength criterion eventually lead to macrosc...
Article
Full-text available
With the ever accelerating spread of artificial intelligence (AI) in virtually all disciplines of science and engineering, the geotechnical studies and practices have also adopted these approaches for exploring and modeling of complex problems whose thorough understanding often falls beyond the reach of analytical and even numerical methods. In the...
Article
Full-text available
The paper is concerned with thermal disturbances in continuous permafrost due to open pit mining and tailings impoundment in the cold regions of Northern Canada. Numerical simulations were conducted to investigate issues of thermal regime changes and permafrost degradation in both the short term and the long term in connection with the proposed Kig...
Conference Paper
Pipelines for the transport essential natural resources are often constructed at shallow depths through different geologic terrains. Permanent ground deformations can occur in actively moving slopes, landslides, surface faulting and ground subsidence, which impose external load on buried pipes. Large strains may be accumulated in the buried pipes a...
Conference Paper
In this paper, unconfined compression strength (UCS) as obtained from an unconsolidated-undrained test is used as a consistency index for classification of natural clay. For saturated clay, the UCS or undrained shear strength is independent of the confining pressure and is an indicator of consistency in stiffness. However, the structure of compacte...
Article
Full-text available
This paper deals with a coherent analytical interpretation of the anisotropic strength of rocks as derived from standard laboratory experimental compression tests on rock samples under confinement. The critical plane approach is herein revisited by reformulating the Mohr–Coulomb failure criterion on a sliding plane when the operative strength param...
Article
Full-text available
The paper studies the evolution of contact forces in granular materials as a preferential growth process whereby the incremental change in forces is proportional to their current values. Preferential growth of forces is depicted herein as a positive feedback process that enhances the heterogeneity in granular media over the course of loading. As an...
Article
Full-text available
The paper presents a micromechanical approach to describe the failure of low‐permeability brittle rocks as a multiscale fracturing process based on a poroelastic microcrack‐damage model. Failure is formulated deep down at the fine pore scale as a material degradation phenomenon driven by microcrack growth that also impacts upon hydromechanical prop...
Conference Paper
A multiscale fracture model for low-permeability brittle rocks which accounts for their microstructure is presented. The work hinges on a microcrack-damage model within a poroelasticity and multiscale framework. A set of damage tensors describes the effect of dual-scale porosities (nanopores and microcracks) on both the hydraulic and poroelasticroc...
Article
Full-text available
The paper offers an analytical determination of the hydraulic properties of an unsaturated soil with reference to its retention curve, which describes the relationship between the volumetric water content and capillarity through matric suction. The analysis combines a particulate approach focused on the physics at the pore scale, including microstr...
Article
A detailed multiscale analysis is presented of the swelling phenomenon in unsaturated clay‐rich materials in the linear regime through homogenization. Herein, the structural complexity of the material is formulated as a three‐scale, triple porosity medium within which microstructural information is transmitted across the various scales, leading ult...
Article
Full-text available
The definition of an effective stress variable for idealized triphasic granular media as the contact stress arising from interparticle forces is examined through discrete element modelling computations in concert with appropriately derived analytical stress expressions based on homogenization. The latter take a more practical importance, in that th...
Article
Full-text available
The existence of structuration in natural clays and shales is believed to change their stiffness, yielding, dilatancy and strength characteristics. These constitutive features are widely known to ultimately reunite with those of the reconstituted parent soil upon large straining. However, some experimental results show that such reunification may n...
Article
Full-text available
A strictly micromechanical framework is proposed for the constitutive modelling of granular materials as a variant to existing micromechanically-enriched continuum models. The theory hinges on the fact that the constitutive behaviour of granular materials can be formally written via an incrementally non-linear expression of stress and strain derive...
Article
Full-text available
The stress-deformation behaviour of granular media is known to be directly linked to details of the underlying microstructure of contacts, or fabric. The notion of contact fabric and its role in defining stress and strain motivate the present study to explore the evolution of fabric in response to small strain probes applied to initially isotropic...
Article
Full-text available
The micromechanics of wet granular materials encompasses complex microstructural and capillary interconnects that can be readily described through a formal derivation of stress transmission in such a 3‐phase medium. In the quest for defining an appropriate stress measure, the stress tensor expression that results from homogenization [Duriez et al....
Article
Full-text available
The present paper investigates the mechanical behaviour of oil sand specimens in triaxial compression tests at both ambient and elevated temperatures. The emphasis is particularly on core sample disturbance and on the multiphase/strongly heterogeneous nature of the material that introduces difficulties in achieving an objective characterization of...
Article
In geotechnical engineering, characterization of time-dependent behavior of clayey soils under one-dimensional (1D) constant stress has been controversial. In some special cases, the creep deformation is insignificant during primary consolidation, and is assumed to start at the end of primary consolidation. In the general case, the total deformatio...
Article
A possible effective stress variable for wet granular materials is numerically investigated based on an adapted discrete element method (DEM) model for an ideal three-phase system. The DEM simulations consider granular materials made of nearly monodisperse spherical particles, in the pendular regime with the pore fluid mixture consisting of distinc...
Article
Full-text available
The origin of global deformations in granular media stems from various concurrent mechanisms at the microscopic scale. Recent micromechanical studies have pointed out inadequacies of traditional elasto-plastic theories with elastic nucleus to describe such materials. In addition, the fundamentals of additive decomposition of global strain into mech...
Article
Tensile and shear behaviour of unsaturated soil have been investigated separately in the past. However, there is no comprehensive study on the transition from tensile to shear failure in compacted clayey soils. A series of uniaxial tensile, direct shear and triaxial compression tests were conducted on compacted Regina clay specimens to investigate...
Conference Paper
We discuss the application of DEM to two major issues in Geomechanics: failure description and stresses in partially saturated granular materials. As for failure description, a significant theoretical complexity arises from the non-associatedness of geomaterials and DEM has been instrumental in the understanding of failure before the plastic limit...
Chapter
This chapter aims to establish the main reasons why the classical expressions of Bishop's type are not appropriate for describing the capillary stress in triphasic granular media. Generally speaking, the discrete element method (DEM) workflow introduces interaction forces between the solid particles and then applies the equations for rigid bodies m...
Book
Failure in Geomaterials offers a unified view of material failure as an instability of deformation modes framed within the theory of bifurcation. Using mathematical rigor, logic, physical reasoning and basic principles of mechanics, the authors develop the fundamentals of failure in geomaterials based on the second-order work criterion. Various for...
Conference Paper
Buried pipeline systems may traverse sections in moving soil masses. Large strains may be accumulated in buried pipes under long-term ground movements, and it may affect the performance of the pipes. It is a common practice in that a stress relief procedure is applied to the pipe by removing the soil around the pipe to allow the pipe to spring back...
Conference Paper
This paper investigates whether an effective stress possibly exists in partially saturated idealized granular media, based on a detailed micro-mechanics computation of spherical particle assemblies wetted at low moisture. Attention is focused on the contact stress tensor, the part of the total stresses of wet granular materials that stems from inte...
Conference Paper
The homogenized stress-strain behavior of partially saturated expansive clays is mathematically derived as a multi-phase system by highlighting the underlying physics at the nanoscale. The condition of partial saturation in the clay material results into a complex three-phase system involving three scales. At the finest scale, the pore space betwee...
Conference Paper
In this paper, we develop a rate independent, incrementally linear constitutive model for 2D granular materials within a robust multiscale framework where micromechanical interactions and microstructural rearrangements between particles are being described statistically. The model when implemented into a finite element code allows us to explore the...
Conference Paper
In this paper, the microphysical aspects leading to the occurrence of a localized failure were investigated by examining the initial emergence of ubiquitous potential slip planes and their link to the shear band development. A Discrete Element Model (DEM) representing a dense granular assembly was considered and subjected to a biaxial compression s...
Conference Paper
The paper deals with a mathematical description of instability and localization in rocks through a multi-scale modeling strategy with coupled multiphysics. We adopt a homogenization approach to work out an effective description of microscopic phenomena at the continuum through defining a Representative Volume Element (RVE). The role of hydro-mechan...
Article
Full-text available
We investigate the macroscopic mechanical influence of the local liquid–solid contact angle that governs the fluid distribution in granular soils under unsaturated conditions. To this end, a discrete element method (DEM)-based implementation that accommodates for any contact angle is proposed and applied to an idealized granular material in the pen...
Article
Full-text available
Microscale mechanisms involving the loss and gain of contacts in granular materials are non-dissipative in nature, but yet irreversible. This is because the creation or disintegration of contacts as an instantaneous event does not pertain to any global deformation of the granular system. Such a contact loss and gain regime is intriguing and has als...
Conference Paper
The study explores the feasibility of fracturing clays during fast electromagnetic (EM) heating. An oedometer setup customized with an EM wave source is simulated numerically at the continuum level. Solid matrix equilibrium, fluid flow and heat transfer equations are solved together with the laws of electromagnetism, i.e. Maxwell’s equations, using...
Conference Paper
The paper outlines the multiscale mathematical formulation of clay-rich shales as a swelling capillary porous medium with a resolution as fine as the nanoscale. The starting point is the description of the physicochemical interactions between elementary crystalline units–the so-called clay sheets or platelets. By way of homogenization, the clay pla...
Article
Full-text available
The structure of the contact network in granular assemblies can evolve due to either dissipative mechanisms such as sliding at contact points, or non-dissipative mechanisms through the phenomenon of contact gain and loss. Being associated with negligible deformations, non-dissipative mechanisms is actually active even in the small strain range of ~...
Article
Full-text available
The total stress tensor as the average stress within a triphasic granular medium is formally derived from micromechanics where internal forces associated with the solid phase, the two immiscible fluid phases and the associated three interfaces are explicitly accounted for. It is demonstrated that for rigid solid particles, the contributions of all...
Article
Full-text available
The stress description provided by the discrete-element method when modelling wet granular materials is investigated for the case of low degrees of saturation within the pendular regime. The stress tensor as computed solely from contact and resultant capillary forces between individual pairs of particles is analysed in depth, being compared with an...
Conference Paper
Buried pipeline systems may traverse sections of unstable soil masses. Long-term ground movement may induce large strains on the pipe over time. To maintain the integrity of the pipeline, pipeline engineers and designers need to assess the frequency of critical ground movements to perform necessary remediation such as a stress relief procedure to p...
Article
The nature of the stress tensor for an unsaturated pendular-state granular medium is investigated by following two micromechanical approaches. Firstly, a stress tensor is analytically derived through stress-homogenization of the medium with internal surfaces being explicitly incorporated in addition to the solid, liquid, and gaseous volumes. As suc...
Article
The present paper examines failure in discrete granular media with respect to its mode and nature based on energetic considerations through a comprehensive discrete element modelling computational analysis. The mode of failure refers to whether deformations will localize into an intense shear band or be diffuse. More subtly, a given failure mode ca...
Article
Full-text available
The present paper examines failure in discrete granular media with respect to its mode and nature based on energetic considerations through a comprehensive discrete element modelling computational analysis. The mode of failure refers to whether deformations will localize into an intense shear band or be diffuse. More subtly, a given failure mode ca...
Article
The fracture behaviour of cemented materials such as rock consisting of randomly distributed microcracks is addressed within a coupled physics and Hydro-Mechanical (HM) framework. Average-field theory is used to formulate a continuum description of such heterogeneous material based on observed microscale physical mechanisms in the fully saturated c...
Article
Full-text available
The present paper is concerned with the redundancy of equations describing the static equilibrium of a granular assembly in relation to emergent behavioural features in granular materials such as critical state, jamming transition, instabilities and yielding. It is proposed to link the concept of jamming to critical state phenomena by introducing a...
Article
Full-text available
The paper deals with a closed form continuum description of coupled fluid flow-deformation behaviour of porous media with distributed strong discontinuities. Based on the underlying physics of the solid and fluid phases at the microscale, the macroscopic hydro-mechanical (HM) behaviour of a representative elementary volume is eventually retrieved i...