Gouge Material Research Articles

Semi-brittle deformation of granitoid gouges in shear experiments at elevated pressures and temperatures

The physical and chemical processes acting in the ‘brittle-to-plastic’ transition are of great interest for a better understanding of fault rheology. We performed a series of experiments on granitoid gouge material under high confining pressures (Pc = 500–1500 MPa), temperatures (T = 300 °C and 500 °C) and fast shear strain rates (∼1.8 × 10−4 s−1) where the material deforms by semi-brittle flow. Samples deformed at 500 °C are systematically weaker than samples deformed at 300 °C over the whole examined confining pressure range indicating a non-frictional component of the deformation. All samples develop an S–C′ fabrics and deformation localizes in slip zones containing ‘amorphous’ feldspar material with an intermediate composition (Na, Ca and K-rich). Further, we observe changes in composition of feldspars (enrichment in the albite component) in the highly fragmented – but crystalline – regions with increasing finite shear strain. Our results indicate that mass-transfer processes keep pace with frictional deformation even at high strain rates and together with viscous flow of the ‘amorphous’ material are responsible for the observed strength difference.

Movement behaviour of the Little Chief Slide

The Little Chief Slide has been extremely slow moving at a rate of 4–14 mm/year since the late 1960s. The 800 million m3slide lies about 3 km upstream of Mica Dam in British Columbia. The extremely slow rate was not affected by the Mica Dam reservoir filling in the 1970s. Although the Little Chief Slide is classified as a rock slide, the extremely slow movements take place on seams of soft materials composed mainly of micaceous clay gouge. The characteristics of the movement trends with time were investigated at six zones using in-place inclinometers (IPIs). The trends of the movement with the reservoir level fluctuations and pore pressure changes were investigated to quantify the contribution of steady-state creep to the total in situ movements. The creep behaviour of the clay gouge materials forming the movement zones was also investigated in the laboratory. The laboratory and field creep behaviours were then compared. The movement could be partitioned into steady-state creep and seasonally induced creep movements. This partitioning identified the locations where the seasonal movements are the highest, and, hence, where drainage measures could have the biggest impact.

Analysis of loading effects on reinforced shotcrete ribs caused by weakness zone containing swelling clay

Tunneling in weakness zones containing swelling clay represents one of the most difficult conditions in hard rock tunneling. Evaluation of rock support requirements in such zones is normally empirical, i.e. mainly based on experience. In this paper, rock supports in a major weakness zone containing swelling clay is analysed based on monitoring results and numerical analysis in order to enrich the experience of support evaluation under such conditions. The Finnfast subsea road tunnel is used as a case. Laboratory tests of the gouge material from the actual zone indicate that the swelling pressure is moderate. All instrumentation data and simulation results show that the loading on the sprayed concrete is far less than its compressive strength, even for the rock support without the reinforced sprayed concrete rib. This suggests that such zones may often be excessively supported when the swelling clay is not very active. It is also found that for reliable stability control through such zones the instrumentation should be installed as close to the excavation face as possible.

Temporal evolution of weathered cataclastic material in gravitational faults of the La Clapiere deep-seated landslide by mechanical approach

After a few years of research, the observation and the analysis of the deep-seated landslides suggest that these are mainly controlled by tectonic structures, which play a dominant role in the deformation of massif slopes. The La Clapiere deep-seated landslide (Argentera Mercantour massif) is embedded in a deep-seated gravitational slope deformation affecting the entire slope, and characterized by specific landforms (trenches, scarps…). Onsite, the tangential displacement direction of the trenches and the scarps are controlled by the tectonic structures. The reactivation of the inherited fault in gravitational faults create a gouge material exposed to an additional mechanical and chemical weathering as well as an increased of leaching. The displacement of these reactivated faults gets increasingly important around the area of the La Clapiere landslide and this since 3.6 ka BP. In this study, mechanical analysis and grain size distributions were performed and these data were analysed according to their proximity the La Clapiere landslide and times of initiation of the landslide by 10Be dating. Triaxial test results show that the effective cohesion decreases and the effective angle of internal friction increases from the unweathered area to the weathered area. The whole distribution of the grain size indicates that the further the shear zone is open or developed, the further the residual material loses its finest particles. This paper suggests that the mechanical evolution along the reactivated fault is influenced by the leaching processes. For the first time, we can extract from these data temporal behaviour of the two main mechanical parameters (cohesion and angle of internal friction) from the beginning of the La Clapiere landslide initiation (3.6 ka BP) to now.

Traces from the past: the Cenozoic regolith and intraplate neotectonic history of the Gun Emplacement, a ferricreted bench on the western margin of the Mt Lofty Ranges, South Australia

The Gun Emplacement is a small but distinctive bench on the Eden–Burnside Fault Escarpment near Anstey Hill, in the northeastern suburbs of Adelaide, South Australia, occurring at an elevation of ∼210–220 m asl. It is underlain by Middle Eocene North Maslin Sand and is capped by resistant, ferricreted colluvium. Paleomagnetic dating of hematitic mottles in the ferricreted colluvium, immediately underlying the emplacement, returned a Pliocene/Early Pleistocene age. This age is equivalent to that obtained for summit surface weathering. Fault scarps and exposures, including slickensides and fault gouge material, suggest that the Eden–Burnside Fault at this location has a strong en échelon pattern developed in response to reverse-sinistral oblique-slip faulting, reflecting continental stress fields. Remnants of ferricrete cappings forming stranded benches on the Eden–Burnside Fault Escarpment at elevations up to 25 m above the Gun Emplacement demonstrate recurrent tectonism of the South Mt Lofty Ranges related to intraplate deformation. There are at least four distinct ferricrete benches preserved on the eastern side of the active fault leading up from the Gun Emplacement surface. These benches demonstrate alternating periods of stability and tectonic activity disrupting and uplifting the ferricreted surfaces. A fresh surface rupture occurs and may be related to a recent seismic event.

The mechanical behavior of anhydrite and the effect of CO2 injection

Abstract Maintaining caprock integrity is important to long-term geological CO2 storage. Worldwide, many depleted oil and gas reservoirs, and several current injection sites, are capped by anhydrite caprock. We investigated the effect of CO2 on the mechanical strength of Zechstein anhydrite, which caps many potential CO2 storage sites in the Netherlands. No short-term chemical effects of CO2 and pore fluid on the strength of anhydrite were observed. Reaction of bulk anhydrite with CO2-saturated solution was slow, though the relative fast reaction of fault gouge material (anhydrite+CO2+H2O→carbonates) could affect long-term mechanical behavior and transport properties.

高速すべり時における岩石の摩擦強度弱化の機構

Previous laboratory studies on rock friction at slow slip rate ( < 1 mm/s) contribute to form the basis of rate- and state-dependent frictional constitutive laws. However, the frictional parameters determined in conventional frictional experiments at the slow slip rates are probably no longer applicable when fault slip approaches to the average slip rate of order of 1 m/s. Recent progress in experimental studies on rock friction reveals that laboratory simulated faults weaken dramatically at fast slip rate conditions ( > 0.1 m/s) with a large slip-weakening distance ( > 1 m). We focus here on the recently reported weakening processes of simulated faults during high-velocity friction from the cases without frictional melting. Several mechanisms has been proposed so far for the cause of the weakening processes of rock friction; thixotropy of a silica gel formed on quartz-rich rocks, formation of a weak material due to thermal decomposition, and drain-off process of adsorbed moisture from the fault due to frictional heating. High-velocity friction of natural fault gouge samples also showed slip weakening with a large weakening distance. Importantly, the laboratory derived weakening distance of the gouge friction decreased with an increase of normal stress. The exact mechanism that leads to the slip-weakening of the gouge samples is unknown, but it may involve a process in which the cause of the weakening is related to the increase of temperature of the gouge material during the high-velocity slip.

Shear strength estimation for Åknes sliding area in western Norway

This paper deals with the unstable rock-slope at Åknes sliding area, located in the county of Møre and Romsdal in western part of Norway. The sliding body has a complex geometry with several sliding planes at different levels, involving unfilled joints, gouge material/brecciated material as well as bridges of intact rock. Stability of the rock slope strongly depends on the shear strength of the sliding plane(s) and this paper discusses the shear strength of the materials present, ranging between the strength of intact rock and crushed, clay containing gouge material. Estimation of the in situ shear strength at the Åknes sliding area is discussed on the basis of triaxial test results and the Barton–Bandis empirical method. Triaxial tests have been carried out on samples of gouge material from a potential sliding plane and intact rock specimens, while the empirical method has been applied for rock joints in the area. Comparison and correlation with experimental results found in literature have also been made. The resultant shear strength range for the zone of sliding has been found to be in the range of 0.5–1.3 MPa, depending on the normal stress and the composition of the sliding zone.

Evaluation of tar deposits using neutron tomography, Canning Basin, Western Australia

SUMMARY Tar or bitumen deposits in the Canning Basin, Western Australia, indicate possible paths of petroleum migration. Neutron and X-ray tomography is applied to rocks containing biodegraded tar in order to investigate the distribution of petroleum within the pore system of the samples. Neutron tomography allows the imaging of hydrogen-rich components, such as tar and bitumen, within a sample, whereas X-ray tomography assists density analysis of the grains and pore space. The results indicate that the methodology discriminates excellently between tar and the other rock matrix and open void components. The images permit analysis of the connectivity of pore paths and enable conclusions to be drawn about petroleum migration. In the case history presented, the images demonstrate the injection of petroleum into a dilated fault zone, and the interaction of the petroleum with the fault gouge material. The images also demonstrate the nature of petroleum movement into sandstone beds, which were in communication with this fault zone.

Nature of stress accommodation in sheared granular material: Insights from 3D numerical modeling

Active faults often contain distinct accumulations of granular wear material. During shear, this granular material accommodates stress and strain in a heterogeneous manner that may influence fault stability. We present new work to visualize the nature of contact force distributions during 3D granular shear. Our 3D discrete numerical models consist of granular layers subjected to normal loading and direct shear, where gouge particles are simulated by individual spheres interacting at points of contact according to simple laws. During shear, we observe the transient microscopic processes and resulting macroscopic mechanical behavior that emerge from interactions of thousands of particles. We track particle translations and contact forces to determine the nature of internal stress accommodation with accumulated slip for different initial configurations. We view model outputs using novel 3D visualization techniques. Our results highlight the prevalence of transient directed contact force networks that preferentially transmit enhanced stresses across our granular layers. We demonstrate that particle size distribution (psd) controls the nature of the force networks. Models having a narrow (i.e. relatively uniform) psd exhibit discrete pipe-like force clusters with a dominant and focussed orientation oblique to but in the plane of shear. Wider psd models (e.g. power law size distributions D = 2.6) also show a directed contact force network oblique to shear but enjoy a wider range of orientations and show more out-of-plane linkages perpendicular to shear. Macroscopic friction level, is insensitive to these distinct force network morphologies, however, force network evolution appears to be linked to fluctuations in macroscopic friction. Our results are consistent with predictions, based on recent laboratory observations, that force network morphologies are sensitive to grain characteristics such as particle size distribution of a sheared granular layer. Our numerical approach offers the potential to investigate correlations between contact force geometry, evolution and resulting macroscopic friction, thus allowing us to explore ideas that heterogeneous force distributions in gouge material may exert an important control on fault stability and hence the seismic potential of active faults.

Permeability of fault rocks from the Median Tectonic Line in Ohshika-mura, Nagano, Japan as studied by pressure-cycling tests

Abstract We describe laboratory measurements of the permeability of fault rocks and their host protoliths obtained from the Median Tectonic Line (MTL), the largest strike-slip fault in Japan. The measurements are made using a gas-medium apparatus that simulates in situ conditions. Samples of fault gouge, cataclastic mylonite, and protoliths were collected from the Kitagawa and Ankoh outcrops of the MTL and adjacent areas in Ohshika-mura, Nagano Prefecture, central Japan. Permeabilities of these samples were measured at room temperature under dry conditions, with nitrogen as the pore fluid. Most samples from the incohesive fault zone have a permeability ranging between 10 −13 and 10 −17 m 2 (100–0.01 mD). These permeabilities are greater than those of cemented cataclasites and mylonites by more than two orders of magnitude at all effective pressures ( P e ) up to 180 MPa. Clayey fault gouge material has a permeability as low as 10 −19 m 2 (0.1 µD) at high effective pressures, but such impermeable fault gouge does not constitute a continuous zone on the two outcrops we studied. Permeability of the incohesive fault rocks exhibits large hysteresis upon P e cycling, compared with cataclasite and mylonite, because those cemented, cohesive fault rocks undergo much less inelastic deformation during the pressure cycling.

Grain size and grain shape analysis of fault rocks

New methods for microstructural analysis of fault rocks (new shape descriptors for convexity/concavity and angularity) and visualization (D-mapping) were developed and tested on experimentally deformed granites. The samples were deformed at 300 °C, 500 MPa confining pressure, and 10 − 4 s − 1 strain rate. SEM micrographs of the resulting fault rocks were used for digital image analysis. Cracked fragments and mature gouges can be differentiated on the basis of the slope D on a log–log plot of the grain size distribution. Both types of fault rocks exhibit two slopes: for grain sizes < 2 μm, D ≈ 1.0 for both types; for grain sizes > 2 μm, cracked material shows D ≈ 1.6 while gouge has D ≥ 2.0. In the case of the gouge, the fractal nature of the grain size distribution is questioned. The D-mapping technique was introduced to visualize the spatial distribution and connectivity of gouge and cracked material in fault rocks. Grain shape analyses show clear differences between cracked minerals and fault gouges and between quartz and K-feldspar grains. The aspect ratio is measured as L/ S (longest/shortest diameter): L/ S of cracked quartz (range: 1.0–8.0, average 2.9) is higher than that of K-feldspar (range: 1.0–4.0, average 2.1). L/ S of gouge is always low (range: 1.0–3.0, average 1.5). From the difference between a shape and its convex envelope, two shape descriptors are derived: the paris factor and the area difference deltA. Both show decreasing values from cracked to gouge material, and the values of cracked K-feldspar (range: 0–100%, average: 15%) are higher than those for quartz (range: 0–100%, average: 5%). Gouge always shows low paris factors (range: 0–20%, average: 2.5%). From the histogram of vertex angles, the Ω-value (fraction of angles < 0°) is derived: Ω of cracked material is significantly higher (30–40%) than that of gouge (10–20%). Automatic digitization enhances the distinction between cracked and gouge material.

Finite deformation model of simple shear of fault with microrotations: apparent strain localisation and en-echelon fracture pattern

Strain localisation is a widespread phenomenon often observed in shear and compressive loading of geomaterials, for example, the fault gouge. It is believed that the main mechanisms of strain localisation are strain softening and mismatch between dilatancy and pressure sensitivity. Observations show that gouge deformation is accompanied by considerable rotations of grains. In our previous work as a model for gouge material, we proposed a continuum description for an assembly of particles of equal radius in which the particle rotation is treated as an independent degree of freedom. We showed that there exist critical values of the model parameters for which the displacement gradient exhibits a pronounced localisation at the mid-surface layers of the fault, even in the absence of inelasticity. Here, we generalise the model to the case of finite deformations characteristic for the gouge deformation. We derive objective constitutive relationships relating the Jaumann rates of stress and moment stress to the relative strain and curvature rates, respectively. The model suggests that the pattern of localisation remains the same as in the linear case. However, the presence of the Jaumann terms leads to the emergence of non-zero normal stresses acting along and perpendicular to the shear layer (with zero hydrostatic pressure), and localised along the mid-line of the gouge; these stress components are absent in the linear model of simple shear. These additional normal stresses, albeit small, cause a change in the direction in which the maximal normal stresses act and in which en-echelon fracturing is formed.

Modeling Granular Crushing in Ring Shear Tests: Experimental and Numerical Analyses

A fault consists of a zone of heavily fragmented granular rock (gouge), which is confined between two rough walls made of fractured rock. The granular gouge is the result of previous fracturing of the wall rock by the combined effect of compressive and shear stresses. Through time, the granular fault gouge will experience various episodes of further fragmentation (crushing) as a result of the mobilization by shear of the fault walls. The evolution of crushing in a simulated gouge material was studied using laboratory ring shear tests and DEM ring shear simulations. The laboratory ring shear tests were developed using sugar as a weak granular material. It was found that the residual friction coefficient of this material maintained a constant value regardless of the severe degradation of the particles. This degradation was induced by increasing the angular deformation or increasing the applied vertical stress. Moreover, it was found that the grain size distribution of the original uniform material evolved toward a fractal distribution of sizes. The results from the DEM simulations confirmed those from the laboratory tests and provided also a visualization of the evolution of crushing. Event though originally DEM does not consider particle breakage, this was allowed by replacing particles fulfilling a predefined tensile failure criterion with an equivalent group of smaller particles.

Influence of particle characteristics on granular friction

We report on laboratory experiments designed to illuminate grain‐scale deformation mechanisms within fault gouge. We vary particle size distribution, grain and surface roughness, and gouge layer thickness to better understand how grain sliding, rolling, dilation, and compaction affect the strength and stability of granular fault gouge. The experiments employed the double direct shear testing geometry and were run at room temperature, controlled humidity, and shearing rates from 0.1 to 3000 μm/s. Experiments were carried out under constant normal stress of 5 and 10 MPa and thus within a nonfracture loading regime where sliding friction for smooth, spherical particles is measurably lower than for rough, angular particles. We compare results from shear between smooth boundaries, where we hypothesize that grain boundary sliding is the dominant deformation mechanism, and roughened surfaces, where rolling and granular dilation contribute to shear deformation. We find that particle angularity and bounding surface roughness increase the frictional strength within sheared layers, indicating differences in particle reorganization due to these factors. In gouge material composed of &lt;30% angular grains we observe repetitive stick‐slip sliding where stress drop decreases while preinstability creep increases with increasing gouge layer thickness. Our data show significant differences in stick‐slip characteristics as a function of gouge layer thickness and particle size, which we interpret in terms of the mechanics of grain bridges that support forces on the layers. We suggest that force chains exhibit qualitative differences as a function of grain angularity and bounding surface roughness.

Dominant coaxial deformation of veins during the interseismic stage of the fault-valve cycle: microfabrics of laminated quartz veins of the Hutti gold mine, India

Auriferous shear veins formed during greenschist facies reactivation of amphibolite facies shear zones of the Hutti mine. The laminated vein fabric, hydraulic breccias and fault gouge material formed by failure during the fault-valve cycle. Microstructural observations in the laminated quartz veins indicate that porphyroclasts of quartz 1, scheelite 1, and pyrrhotite have an oblate shape. Quartz 2 grains with a width of 0.03–0.05 mm result from predominant subgrain rotation, while predominant bulging recrystallization formed quartz 2 grains with a width of 0.01–0.03 mm. The pole figures of the quartz texture are symmetrical with respect to a shortening direction perpendicular to the vein walls. Strain partitioning of shear stress into the rheologically weaker altered host rocks preserved these fabrics in the stronger quartz veins. A non-coaxial overprint of the veins is only weakly recorded by the microfabrics and quartz textures. The study of microfabrics and quartz textures helps to understand processes of seismic activity in shear zones during fault-valve behavior. In the case of Hutti, coaxial deformation normal to the shear zone is indicated during the initial stages of shear stress and fluid pressure build-up, following the crack-seal stages.

On the Possibility of Elastic Strain Localisation in a Fault

The phenomenon of strain localisation is often observed in shear deformation of particulate materials, e.g., fault gouge. This phenomenon is usually attributed to special types of plastic behaviour of the material (e.g., strain softening or mismatch between dilatancy and pressure sensitivity or both). Observations of strain localisation in situ or in experiments are usually based on displacement measurements and subsequent computation of the displacement gradient. While in conventional continua the symmetric part of the displacement gradient is equal to the strain, it is no longer the case in the more realistic descriptions within the framework of generalised continua. In such models the rotations of the gouge particles are considered as independent degrees of freedom the values of which usually differ from the rotation of an infinitesimal volume element of the continuum, the latter being described for infinitesimal deformations by the non-symmetric part of the displacement gradient. As a model for gouge material we propose a continuum description for an assembly of spherical particles of equal radius in which the particle rotation is treated as an independent degree of freedom. Based on this model we consider simple shear deformations of the fault gouge. We show that there exist values of the model parameters for which the displacement gradient exhibits a pronounced localisation at the mid-layers of the fault, even in the absence of inelasticity. Inelastic effects are neglected in order to highlight the role of the independent rotations and the associated additional parameters. The localisation-like behaviour occurs if (a) the particle rotations on the boundary of the shear layer are constrained (this type of boundary condition does not exist in a standard continuum) and (b) the contact moment-or bending stiffness is much smaller than the product of the effective shear modulus of the granulate and the square of the width of the gouge layer. It should be noted however that the virtual work functional is positive definite over the range of physically meaningful parameters (here: contact stiffnesses, solid volume fraction and coordination number) so that strictly speaking we are not dealing with a material instability.

FE analysis of time-dependent behavior of tunneling in squeezing ground using two different creep models

Ground movement and contact pressure on the lining of Stillwater Tunnel (Utah, USA) were investigated. Axisymmetirc finite element analysis was used in the analysis. Power law and hyperbolic creep models were used to model ground squeezing and to show the differences in the results between the two models. Creep parameters for the two models were evaluated based on the experimental creep and strength tests that were performed by other investigators on the gouge materials encountered along the tunnel axis through the heavily sheared and fault zones of the Red Pine shale. The results of the analysis which include normalized inward movement at the tunnel crown, normalized radial ground convergence with depth, and lining-ground contact pressure were compared with the results that were measured along the tunnel axis by other investigators. The results of the analysis show that lining pressure and deformation can be predicted well from the use of power law creep model if the delay time before lining erection is considered.

A particle‐tracking approach to simulating transport in a complex fracture

Nearly all of the studies of flow and transport in fractured rocks have assumed that each fracture can be modeled as an open space between two surfaces with constant or variable separation. Field observations of rock fractures have shown that a fracture in the field can be rather more complex. Recent studies indicate that a complex fracture can be characterized as a thin fracture zone having several interconnected subfractures, which can contain mechanically dislodged and chemically altered materials with an enhanced porosity. This paper proposes a particle‐tracking approach to calculate solute transport in a complex fracture, with structures in the fracture thickness normal to the fracture plane. These structures include subfractures, dead‐end pores, gouge materials, small matrix blocks, and the adjacent rock matrix, all providing material in which solute diffusion and sorption can occur. The method is described; an example is provided to show its feasibility and the reasonableness of its parameter dependence; and, finally, direction for its further development is discussed.

Effects of pore volume–transmissivity correlation on transport phenomena

The relevant velocity that describes transport phenomena in a porous medium is the pore velocity. For this reason, one needs not only to describe the variability of transmissivity, which fully determines the Darcy velocity field for given source terms and boundary conditions, but also any variability of the pore volume. We demonstrate that hydraulically equivalent media with exactly the same transmissivity field can produce dramatic differences in the displacement of a solute if they have different pore volume distributions. In particular, we demonstrate that correlation between pore volume and transmissivity leads to a much smoother and more homogeneous solute distribution. This was observed in a laboratory experiment performed in artificial fractures made of two plexiglass plates into which a space-dependent aperture distribution was milled. Using visualization by a light transmission technique, we observe that the solute behaviour is much smoother and more regular after the fractures are filled with glass powder, which plays the role of a homogeneous fault gouge material. This is due to a perfect correlation between pore volume and transmissivity that causes pore velocity to be not directly dependent on the transmissivity, but only indirectly through the hydraulic gradient, which is a much smoother function due to the diffusive behaviour of the flow equation acting as a filter. This smoothing property of the pore volume–transmissivity correlation is also supported by numerical simulations of tracer tests in a dipole flow field. Three different conceptual models are used: an empty fracture, a rough-walled fracture filled with a homogeneous material and a parallel-plate fracture with a heterogeneous fault gouge. All three models are hydraulically equivalent, yet they have a different pore volume distribution. Even if piezometric heads and specific flow rates are exactly the same at any point of the domain, the transport process differs dramatically. These differences make it important to discriminate in situ among different conceptual models in order to simulate correctly the transport phenomena. For this reason, we study the solute breakthrough and recovery curves at the extraction wells. Our numerical case studies show that discrimination on the basis of such data might be impossible except under very favourable conditions, i.e. the integral scale of the transmissivity field has to be known and small compared to the dipole size. If the latter conditions are satisfied, discrimination between the rough-walled fracture filled with a homogeneous material and the other two models becomes possible, whereas the parallel-plate fracture with a heterogeneous fault gouge and the empty fracture still show identifiability problems. The latter may be solved by inspection of aperture and pressure testing.