Rheology and kinematics of dense granular fault gouges with DEM: shear bands formation and evolution 

2021 ◽  
Author(s):  
Nathalie Casas ◽  
Guilhem Mollon ◽  
Ali Daouadji
Keyword(s):  
Surrounding Medium ◽  
Shear Bands ◽  
Confining Pressure ◽  
Fault Gouge ◽  
Initial Porosity ◽  
Discrete Element Modelling ◽  
Ductile Material ◽  
Third Body ◽  
Fluid Displacement ◽  
Multiple Parameters

<p>Earthquakes happen with frictional sliding, by releasing all the stresses accumulated in the pre-stressed surrounding medium. The geological third body (i.e. fault gouge), coming from the wear of previous slips, acts on friction stability and plays a key role in this sudden energy release. A large part of slip mechanisms is influenced, if not controlled, by fault gouge characteristics and environment. We aim to link third body properties (geological, mechanical, physical…) to its rheological behavior by testing numerically different types of dense geological third body (% of porosity, % of cohesion, grains shapes…) with distinct contact laws. Different granular samples are generated to simulate a mature fault gouge with mineral cementation between particles. The gouge is then inserted between two rock walls to realize direct shear experiments with Discrete Element Modelling in the software MELODY2D (Mollon, 2016). A dry contact model is considered to investigate mechanisms without fluid (displacement-driven and under constant confining pressure). Researches are based here on a millimeter-scale portion of gouge, considering that the output values could be used in another model at larger scale.</p><p>The peak strength can be sharp, short, and intense for dense and highly cohesive cases (angular particles, 15% initial porosity) and relatively low for ultra-dense samples (polygonal particles, 0% initial porosity). The observed regimes also correspond to an evolution of the amount of ductility within the sample. A very dense or highly cohesive sample behaves as a brittle material, whereas a typical cohesionless and porous geological layer tends to behave as a ductile material. The evolution of gouge characteristics truly influences the shape and formation time of Riedel shear bands. A change in contact laws between particles (%cohesion, friction) modifies the entire kinematics of Riedel bands formation. Indeed, with cohesion between particles, Riedel bands are directly linked to the importance of the dilation phase, depending itself on the initial porosity present within the sample (Casas et al., 2020). Then, increasing friction not only changes the principal orientation or Riedel bands but makes them more numerous within the gouge. It also leads to a more sudden post-peak weakening, which is prone to switch the fault behavior from a ductile aseismic response to a brittle seismic slip, depending on the stiffness of the surrounding medium. Global stiffness of the gouge also has an important role to play on Riedel bands formation, and it can be defined as a combination of multiple parameters such as initial porosity, shape and size of particles, numerical stiffness, gouge thickness… The local Breakdown energy, or energy needed to weaken the fault, is also calculated to be connected to Riedel bands formation.</p>

scholarly journals Shear bands in dense fault gouge

2021 ◽  
Vol 249 ◽  
pp. 11006
Author(s):  
Nathalie Casas ◽  
Guilhem Mollon ◽  
Ali Daouadji
Keyword(s):  
Granular Media ◽  
Surrounding Medium ◽  
Shear Bands ◽  
Orientation Angle ◽  
Fault Gouge ◽  
Discrete Element Modelling ◽  
Third Body ◽  
Slip Direction ◽  
Riedel Shear ◽  
Dry Contact

Earthquakes happen with frictional sliding, by releasing all the stresses accumulated in the prestressed surrounding medium. The geological fault gouge, coming from the wear of previous slips, acts on friction stability and plays a key role in this sudden energy release. A large part of slip mechanisms are influenced, if not controlled, by the characteristics and environment of this tribological “third body”. A 2D granular fault (mm scale) is implemented with Discrete Element Modelling (DEM). A displacement-driven model with dry contact is studied to observe kinematics and properties of the slipping zone. Increasing the length of the granular media increases the slip needed to weaken the friction from friction peak to steadystate. Low-angle Riedel shear bands are mostly observed. Their number increases with the inter-particle friction coefficient, which also influences shear bands formation in their orientation angle (higher friction leads to higher angle with the main slip direction).

A small-scale numerical study of fault slip mechanisms using DEM

2020 ◽  
Author(s):  
Nathalie Casas ◽  
Guilhem Mollon ◽  
Ali Daouadji
Keyword(s):  
Numerical Study ◽  
Surrounding Medium ◽  
Computational Cost ◽  
Static Friction ◽  
Discrete Element Modelling ◽  
Small Scale ◽  
In Situ Experiments ◽  
Fault Gouges ◽  
Fault Network

<p>How do earthquakes start? What are the parameters influencing fault evolutions? What are the local parameters controlling the seismic or aseismic character of slip?</p><p>To predict the dynamic behaviour of faults, it is important to understand slip mechanisms and their source. Lab or in-situ experiments can be very helpful, but tribological experience has shown that it is complicated to install local sensors inside a mechanical contact, and that they could disturb the behaviour of the sheared medium. Even with technical improvements on lab tools, some interesting data regarding gouge kinematics and rheology remains very difficult or impossible to obtain. Numerical modelling seems to be another way of understanding physics of earthquakes.</p><p>Fault zone usually present a granular gouge, coming from the wear material of previous slips. That is why, in this study, we present a numerical model to observe the evolution and behaviours of fault gouges. We chose to focus on physics of contacts inside a granular gouge at a millimetre-scale, studying contact interactions and friction coefficient between the different bodies. In order to get access to this kind of information, we implement a 2D granular fault gouge with Discrete Element Modelling in the software MELODY (Mollon, 2016). The gouge model involves two rough surfaces representing the rock walls separated by the granular gouge.</p><p>One of the interests of this code is its ability to represent realistic non-circular grain shapes with a Fourier-Voronoï method (Mollon et al., 2012). As most of the simulations reported in the literature use circular (2D)/spherical (3D) grains, we wanted to analyse numerically the contribution of angular grains. We confirm that they lead to higher friction coefficients and different global behaviours (Mair et al., 2002), (Guo et al., 2004).</p><p>In a first model, we investigate dry contacts to spotlight the influence of inter-particular cohesion and small particles on slip behaviour and static friction. A second model is carried out to observe aseismic and seismic slips occurring within the gouge. As stability depends on the interplay between the peak of static friction and the stiffness of the surrounding medium, the model includes the stiffness of the loading apparatus on the rock walls.</p><p>The work presented here focuses on millimetre-scale phenomena, but the employed model cannot be extended to the scale of the entire fault network, for computational cost reasons. It is expected, however, that it will lead to a better understanding of local behaviours that may be injected as simplified interface laws in larger-scale simulations.</p>

scholarly journals Relationship between microstructures and resistance in mafic assemblages that deform and transform

Solid Earth ◽  
2020 ◽  
Vol 11 (6) ◽  
pp. 2141-2167
Author(s):  
Nicolas Mansard ◽  
Holger Stünitz ◽  
Hugues Raimbourg ◽  
Jacques Précigout ◽  
Alexis Plunder ◽  
...  
Keyword(s):  
Grain Size ◽  
Shear Strain ◽  
Phase Distribution ◽  
Shear Bands ◽  
Constant Strain Rate ◽  
Peak Stress ◽  
Confining Pressure ◽  
Shear Zones ◽  
Reaction Products ◽  
Fine Grained

Abstract. Syn-kinematic mineral reactions play an important role for the mechanical properties of polymineralic rocks. Mineral reactions (i.e., nucleation of new phases) may lead to grain size reduction, producing fine-grained polymineralic mixtures, which have a strongly reduced viscosity because of the activation of grain-size-sensitive deformation processes. In order to study the effect of deformation–reaction feedback(s) on sample strength, we performed rock deformation experiments on “wet” assemblages of mafic compositions in a Griggs-type solid-medium deformation apparatus. Shear strain was applied at constant strain rate (10−5 s−1) and constant confining pressure (1 GPa) with temperatures ranging from 800 to 900 ∘C. At low shear strain, the assemblages that react faster are significantly weaker than the ones that react more slowly, demonstrating that reaction progress has a first-order control on rock strength. With increasing strain, we document two contrasting microstructural scenarios: (1) the development of a single throughgoing high-strain zone of well-mixed, fine-grained aggregates, associated with a significant weakening after peak stress, and (2) the development of partially connected, nearly monomineralic shear bands without major weakening. The lack of weakening is caused by the absence of interconnected well-mixed aggregates of fine-grained reaction products. The nature of the reaction products, and hence the intensity of the mechanical weakening, is controlled by the microstructures of the reaction products to a large extent, e.g., the amount of amphibole and the phase distribution of reaction products. The samples with the largest amount of amphibole exhibit a larger grain size and show less weakening. In addition to their implications for the deformation of natural shear zones, our findings demonstrate that the feedback between deformation and mineral reactions can lead to large differences in mechanical strength, even at relatively small initial differences in mineral composition.

A Comparative Study Between Lateral and Upward Anchor-Soil and Pipe-Soil Interaction in Dense Sand

2016 ◽  
Author(s):  
Kshama Roy ◽  
Bipul Hawlader ◽  
Shawn Kenny ◽  
Ian Moore
Keyword(s):  
Shear Bands ◽  
Experimental Studies ◽  
Confining Pressure ◽  
Design Guidelines ◽  
Vertical Strip ◽  
Dense Sand ◽  
Progressive Development ◽  
Lateral Resistance ◽  
Strain Behaviour ◽  
Uplift Forces

Buried pipelines are extensively used in onshore and offshore environments for transportation of hydrocarbons. On the other hand, buried anchors have been used for many years to stabilize various structures. In the development of design guidelines for pipelines, theoretical and experimental studies on buried anchors are sometimes used assuming that pipeline-soil and anchor-soil interaction are similar. In the present study, finite element (FE) modeling is performed to simulate the response of pipeline and anchor buried in dense sand subjected to lateral and uplift forces. The similarities and differences between the responses of these two types of structures are examined to justify the application of anchor theory to pipeline behaviour. The stress-strain behaviour of dense sand is modeled using a Modified Mohr-Coulomb (MMC) model, which considers the pre-peak hardening, post-peak softening, density and confining pressure dependent friction and dilation angles. A considerable difference is found between the lateral resistance of pipeline and vertical strip anchor of similar size. Progressive development of shear bands (shear strain concentrated zone) can explain the load-displacement behaviour for both lateral and upward loading.

Investigating fault sinuosity using discrete element modelling in 3D

2020 ◽  
Author(s):  
Janis Aleksans ◽  
Conrad Childs ◽  
Martin Schöpfer
Keyword(s):  
Numerical Models ◽  
Distinct Element ◽  
Normal Fault ◽  
Confining Pressure ◽  
Three Dimensions ◽  
Spherical Particles ◽  
Particle Flow Code ◽  
Discrete Element Modelling ◽  
Normal Faults ◽  
Fault Systems

<p>Scaled numerical models of faults are useful complements to geological data and by providing insights into fault dynamics they can improve our understanding of the different stages of development of normal fault systems, from nucleation through to localisation and maturity.</p><p>In this work, we use Particle Flow Code in three dimensions, which implements the Distinct Element Method (DEM), to study the development of systems of normal faults. The modelling is based on spherical particles that interact via a linear force-displacement law. Cohesion is modelled by adding linear elastic bonds to particle-particle contacts. These bonds break if the critical normal or shear strength is exceeded, thus creating a fracture surface within the rock volume. Model boundaries are represented by rigid and frictionless walls enclosing the modelled volume vertically and at the ends, with periodic lateral boundaries. Extension is replicated by slowly moving the end walls away from the centre while maintaining a constant confining pressure.</p><p>The DEM models replicate many aspects of the geometry and dynamics of natural fault systems with stages of fault nucleation, propagation, interaction and linkage. Here we focus on the sinuosity of model fault map traces which show a similar variability to that seen in nature. In the models, fault trace sinuosity is negatively correlated with the Young’s modulus of the rock, so that faults become less sinuous as the stiffness of the solid medium increases. This relationship supports a model in which the lengths of fault segments formed at the early stages of extension are smaller in rocks with lower Young’s modulus than in rocks with higher Young’s modulus. Longer initial fault segments become connected as displacement increases, to give lower sinuosity faults.</p>

The Critical State Constitutive Model and Strain Localization for Hostun Sand

2014 ◽  
Vol 525 ◽  
pp. 552-555
Author(s):  
Ping Hu ◽  
Mao Song Huang ◽  
Deng Gao Wu
Keyword(s):  
Inclination Angle ◽  
Strain Localization ◽  
Critical State ◽  
Shear Bands ◽  
Confining Pressure ◽  
Critical State Model ◽  
State Model ◽  
Model Parameters ◽  
Constitutive Theories ◽  
State Dependent

Aiming at the limitation of traditional constitutive theories which can not explain the decrease of inclination angle of shear bands correctly when confining pressure increases, a state-dependent critical state model is employed to analyze strain localization of Hostun sands. Firstly, the critical state formulation and model parameters are calibrated through available drained triaxial test results of Hostun sand. Then an analysis of strain localization is performed on the drained plane strain tests. The results show that the state-dependent critical state model is capable of simulating the effect of initial effective confining pressure on inclination angle of shear bands.

The Shear Dilation and Shear Band of Coarse Grained Soil Based on Discrete Element Method

2015 ◽  
Vol 744-746 ◽  
pp. 679-685 ◽  
Author(s):  
Zhi Hua Zhang ◽  
Guo Dong Zhang ◽  
Xue Liang Li ◽  
Zhi Hua Xu
Keyword(s):  
Discrete Element Method ◽  
Shear Bands ◽  
Confining Pressure ◽  
Discrete Element ◽  
Coarse Grained ◽  
Biaxial Test ◽  
Confining Stress ◽  
Shear Dilation ◽  
Coarse Grained Soil ◽  
Element Method

Based on the triaxial test of coarse grained soil, using the discrete element method to simulate the biaxial test by using PFC2D as the discrete element method (DEM) tools, and generate particles by a gradation which is similar with that in laboratory. Through self-programming, depends on the macro-reaction in lab to find out the mesoscopic parameters and deformation characteristics of coarse grained soil, then calibrate the value of friction coefficient correspond to that in laboratory, and at last find out the shear bands. The results show that the value of sample volumetric increment ratio (VIR)dεv/dεis positive (shear shrinkage) at first and then negative (shear dilation) when confining stress is 300kPa, when the confining pressure is greater than 300kPa, samples have been on the shear shrinkage.

scholarly journals Shearing of granular materials in a confined split-bottom Couette cell

2021 ◽  
Vol 249 ◽  
pp. 03004
Author(s):  
Mahnoush Madani ◽  
Maniya Maleki ◽  
M. Reza Shaebani
Keyword(s):  
Shear Band ◽  
Granular Matter ◽  
Applied Pressure ◽  
Shear Bands ◽  
Confining Pressure ◽  
External Pressure ◽  
Free Surfaces ◽  
Filling Height ◽  
Image Velocimetry ◽  
Couette Cell

Formation of shear bands is one of the most remarkable phenomena in the dynamics of granular matter. Several parameters have been so far identified to influence the behavior of the shear bands. We carried out experiments to investigate the evolution of the shear bands in the split-bottom Couette cell in the presence of confining pressure. We employed the Particle Image Velocimetry (PIV) to characterize the shear band both in the absence and presence of external pressure. Our results show that the location and width of the shear band are affected by both the confining pressure and the filling height. The shear zone evolves towards the middle of the cylinder and expands to a broader region with increasing applied pressure or filling height; also the angular velocity decreases relative to the rotation rate of the bottom disk. Our findings are consistent with prior empirical observations on the formation of wide shear bands at free surfaces.

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