Implementation of an Empirical Joint Constitutive Model into Finite-Discrete Element Analysis of the Geomechanical Behaviour of Fractured Rocks

Abstract

An empirical joint constitutive model (JCM) that captures the rough wall interaction behaviour of individual fractures associated with roughness characteristics observed in laboratory experiments is combined with the solid mechanical model of the finite-discrete element method (FEMDEM). The combined JCM-FEMDEM formulation gives realistic fracture behaviour with respect to shear strength, normal closure, and shear dilatancy and includes the recognition of fracture length influence as seen in experiments. The validity of the numerical model is demonstrated by a comparison with the experimentally established empirical solutions. A 2D plane strain geomechanical simulation is conducted using an outcrop-based naturally fractured rock model with far-field stresses loaded in two consecutive phases, i.e. take-up of isotropic stresses and imposition of two deviatoric stress conditions. The modelled behaviour of natural fractures in response to various stress conditions illustrates a range of realistic behaviour including closure, opening, shearing, dilatancy, and new crack propagation. With the increase in stress ratio, significant deformation enhancement occurs in the vicinity of fracture tips, intersections, and bends, where large apertures can be generated. The JCM-FEMDEM model is also compared with conventional approaches that neglect the scale dependency of joint properties or the roughness-induced additional frictional resistance. The results of this paper have important implications for understanding the geomechanical behaviour of fractured rocks in various engineering activities.

Polyaxial stress-dependent permeability of a three-dimensional fractured rock layer

“…The FEMDEM model that accommodates the finite strain elasticity coupled with a smeared crack model is able to capture the complex behaviour of fractured rocks involving deformation, displacement, rotation, interaction, fracturing and fragmentation. The principles of the 3D FEMDEM model for solving stress, deformation and interaction as well as fracture propagation are similar to those of the 2D model as presented in the literature (Latham et al 2013;Lisjak and Grasselli 2014;Lei et al 2016). Here, only some adaptations to 3D problems are described.…”

“…Extensive developments and applications of the FEMDEM method have been conducted in the past decade or so with different versions having emerged such as the code collaboratively developed by Queen Mary University of London (UK) and Los Alamos National Laboratory in the USA (Munjiza et al 2011(Munjiza et al , 2013Rougier et al 2014), the YGeo and Irazu by the University of Toronto, Canada (Mahabadi et al 2012;Lisjak and Grasselli 2014;Lisjak et al 2017), and the Solidity platform by Imperial College London (Xiang et al 2009a, b;Guo et al 2016;Lei 2016;Lei et al 2016). The FEMDEM model that accommodates the finite strain elasticity coupled with a smeared crack model is able to capture the complex behaviour of fractured rocks involving deformation, displacement, rotation, interaction, fracturing and fragmentation.…”

Stress‐Induced Anomalous Transport in Natural Fracture Networks

“…We explicitly resolve each fracture as a discrete entity, which permits analyzing the impact of geological stress and fracture geometrical properties on fluid flow and transport. The FDEM model represents a 2‐D fractured rock using a fully discontinuous mesh of three‐node triangular finite elements linked by four‐node broken (representing discontinuities) or unbroken (representing rock matrix) joint elements (Lei et al, ). The motions of linear‐elastic, constant‐strain finite elements are governed by Newton's second law where M is the lumped nodal mass matrix, x is the vector of nodal displacements, F int are the internal nodal forces induced by the deformation of finite elements, and F ext are the external nodal forces consisting of external loads F l contributed by boundary and body forces, cohesive bonding forces F b caused by the deformation of unbroken joint elements, and contact forces F c generated by the contact interaction via broken joint elements.…”

“…To capture the nonlinear deformation of natural rough fractures under normal and/or shear loadings, an empirical joint constitutive model has been implemented into the FDEM framework (Lei et al, ). The compression‐induced fracture closure is characterized by a hyperbolic relation (Bandis et al, ) where η n is the current compression‐induced closure (mm), σ n is the effective normal stress (MPa), k n 0 is the initial normal stiffness (MPa/mm), and η m is the maximum allowable closure (mm).…”

Modeling Simultaneous Multiple Fracturing Using the Combined Finite-Discrete Element Method

“…FDEM inherits the advantages of FEM in describing elastic deformations and the capabilities of DEM in capturing discontinuities. FDEM is widely used for model collision, fracturing, and fragmentation where both discontinuum and continuum are involved [31,32]. By coupling it with a fluid flow solver, FDEM has also been applied in handling hydromechanical (H-M) coupling problems [33][34][35][36].…”