Increasing Shear Displacement Research Articles

岩盤不連続面内の接触変化を考慮した流動機構数値解析手法の提案と検証

The fluid flow in a single rock fracture during shear processes has been an important issue in rock mechanics and is investigated in this study using the Finite Element Method (FEM), considering evolutions of aperture and transmissivity with shear displacement histories under different normal loading conditions. The distributions of fracture aperture and its evolution during shear were calculated from the initial aperture, based on the laser-scanned sample surface roughness features, and shear dilations measured in the laboratory coupled shear-flow tests. Three rock fractures with different roughness were used as parent samples from which nine replicas were made and tested under three normal loading conditions. For flow simulations, a special algorithm for treating the contact areas as zero-aperture elements was used to produce more accurate flow field simulations, which provide good agreement with the flow rate data obtained from the laboratory tests, showing that complex histories of fracture aperture and tortuous flow fields with changing normal stress and increasing shear displacements.

Rock Joint Surfaces Measurement and Analysis of Aperture Distribution under Different Normal and Shear Loading using GIS

The geometry of the rock joint is a governing factor for joint mechanical and hydraulic behaviour. A new method for evaluating the aperture distribution, based on measurement of joint surfaces and three dimensional characteristics of each surface, is developed. This method allows one to determine and visualize the aperture distribution under different normal stresses and shear displacements, which is difficult to observe experimentally. A new laser scanner system is designed and developed for joint surface measurements. Special attention is paid to both surfaces’ data gained by measurements and processing, such as x-y coordinate table modification, data referencing, and matching between upper and lower surfaces. The surfaces of an artificial joint in granite are measured, processed, analyzed and three dimensional approaches are carried out for surface characterization. Parameters such as “asperity’s heights”, “slope angles”, and “aspects” distribution at micro scale, local concentration of elements and their spatial localization at local scale are determined by Geographic Information System (GIS). These parameters are used for joint surfaces matching and its real behavior quantitative analysis. The upper surface is brought down to make contact with the lower surface and the distance between the two surfaces is evaluated from the joint mean experimental aperture, which is obtained from normal and shear tests. Changes of aperture distribution at different normal stresses and various shear displacements are visualized and interpreted. Increasing normal load causes negative changes in aperture frequency distribution which indicates high joint matching. However, increasing shear displacement causes a rapid increase in the aperture and positive changes in the aperture frequency distribution, which could be due to un-matching, surface anisotropy and spatial localization of contact points with proceeding shear.

Discussion of “Analysis of a Large Database of GCL Internal Shear Strength Results” by Jorge G. Zornberg, John S. McCartney, and Robert H. Swan, Jr.

The discusser cites Chiu and Fox (2004) regarding a large published database containing analysis of internal and interface geosynthetic clay liners (GCL) shear strengths, including those of the discusser. One item found in this database conflicts with the authors' results ? the failure envelope. For hydrated needle-punched GCLs, the non-linear regression analysis produced a zero intercept instead of the permitted non-zero intercept. This contradicts the authors' test results. In shear-induced excess pore pressures, the discusser states that the hypothesis of the authors -- that pore water suction causes progressively larger shear strengths with increasing shear displacement rate (SDR) at low normal stresses -- is not borne out in the data from Fox et al. (1998). As for SDR effects at high normal stress conditions, the discusser notes that more research is needed.

Effect of Shear Stress and Displacement on the Hydraulic Properties of a Marble Fracture with Sand

A test apparatus was developed to study shear-flow coupling properties for a marble fracture with sand. Experimental tests include shear stress-flow coupling process and shear displacement-flow coupling process. The shear stress-flow coupling test results, revealed that mechanical aperture keeps constant during shear, and that hydraulic aperture and conductivity decrease almost linearly with increasing shear stress. The shear displacement-flow coupling test results showed that hydraulic behavior under shear displacement includes two phases. During the first about 0.05mm shear displacement, hydraulic aperture and conductivity decrease with increasing shear displacement, and mechanical aperture remains constant or increases only by several microns of magnitude. After shear displacement is up to about 0.05mm, the fracture dilates remarkably, and hydraulic aperture and conductivity increase with shear displacement increasing.

Mechanisms of crack bridging by composite and metallic rods

Experiments are reported that explore the mechanics of single pins or rods that bridge a delamination crack in a miniature model specimen. The effects of material and geometrical parameters are determined by varying the angle of the rod, its material, and the material in which it is embedded. Different tests represent mode I and mode II loading, with respect to a pre-existing delamination crack. Many observed mechanisms are similar to those previously reported for stitches and in limited studies on rods. They include debonding and sliding of the rod relative to the substrate, lateral deflection of the rod into the substrate (when mode II is present), rod pullout, and rod rupture. Sliding in mode I can be explained by assuming that pullout is resisted by uniform friction, which has a modest value (, 1 – 20 MPa). However, when mode II loading is present and the rod deflects laterally, a more complicated friction behaviour is suggested. Peak load occurs well after the whole rod has begun to slide out of the specimen, implying that the pullout process is stable in mode II to large displacements. This, together with the high values observed for the peak loads and displacements suggest the presence of an enhanced friction zone (snubbing effect) extending over the segment of the rod that has been laterally deflected by mode II loading. This zone can grow under increasing shear displacements even after the whole rod begins to slide, leading to increasing shear loads (stable pullout). Various characteristics of the pullout experiments are consistent with this model. q 2004 Published by Elsevier Ltd.

Shear induced anisotropy and heterogeneity of fluid flow in a single rock fracture with translational and rotary shear displacements ¿ a numerical study

The fluid flow and tracer transport in a single rock fracture during shear processes has been an important issue in rock mechanics and is investigated in this thesis using Finite Element Method (FEM) and streamline particle tracking method, considering evolutions of aperture and transmissivity with shear displacement histories under different normal stresses, based on laboratory tests. The distributions of fracture aperture and its evolution during shear were calculated from the initial aperture fields, based on the laser-scanned surface roughness features of replicas of rock fracture specimens, and shear dilations measured during the coupled shear-flow-tracer tests in laboratory performed using a newly developed testing apparatus in Nagasaki University, Nagasaki, Japan. Three rock fractures of granite with different roughness characteristics were used as parent samples from which nine plaster replicas were made and coupled shear-flow tests was performed under three normal loading conditions (two levels of constant normal loading (CNL) and one constant normal stiffness (CNS) conditions). In order to visualize the tracer transport, transparent acrylic upper parts and plaster lower parts of the fracture specimens were manufactured from an artificially created tensile fracture of sandstone and the coupled shear-flow tests with fluid visualization was performed using a dye tracer injected from upstream and a CCD camera to record the dye movement. A special algorithm for treating the contact areas as zero-aperture elements was used to produce more accurate flow field simulations by using FEM, which is important for continued simulations of particle transport, but was often not properly treated in literature. The simulation results agreed well with the flow rate data obtained from the laboratory tests, showing that complex histories of fracture aperture and tortuous flow channels with changing normal stresses and increasing shear displacements, which were also captured by the coupled shear-flow tests of fracture specimens with visualization of the fluid flow. From the obtained flow velocity fields, the particle transport was predicted by the streamline particle tracking method with calculated flow velocity fields (vectors) from the flow simulations, obtaining results such as flow velocity profiles, total flow rates, particle travel time, breakthrough curves and the Peclet number, Pe, respectively. The fluid flow in the vertical 2-D cross-sections of a rock fracture was also simulated by solving both Navier-Stokes (NS) and Reynolds equations, and the particle transport was predicted by streamline particle tracking method. The results obtained using NS and Reynolds equations were compared to illustrate the degree of the validity of the Reynolds equation for general applications in practice since the later is mush more computationally efficient for large scale problems. The flow simulation results show that the total flow rate and the flow velocity predicted by NS equations are quite different from that as predicted by the Reynolds equation. The results show that a roughly 5-10 % overestimation on the flow rate is produced when the Reynolds equation is used, and the ideal parabolic velocity profiles defined by the local cubic law, when Reynolds equation is used, is no longer valid, especially when the roughness feature of the fracture surfaces changes with shear. These deviations of flow rate and flow velocity profiles across the fracture aperture have a significant impact on the particle transport behavior and the associated properties, such as the travel time and Peclet number. The deviations increase with increasing flow velocity and become more significant when fracture aperture geometry changes with shear. The scientific findings from these studies provided new insights to the physical behavior of fluid flow and mass transport in rock fractures which is the scientific basis for many rock mechanics problems at the fundamental level, and with special importance to rock engineering problems such as geothermal energy extraction (where flow rate in fractures dominates the productivity of a geothermal energy reservoir) and nuclear waste repositories (where radioactive nuclides transport through fractures dominates the final safety evaluations) in fractured rocks.

Analytical approach for anisotropic permeability through a single rough rock joint under shear deformation

In this study, anisotropic permeability of a single rock joint with rough surfaces and its change due to shear displacement are investigated using the latest analytical approximation for permeability. The permeability approximation is a function of the spatial correlation of the aperture as well as the statistical parameters (mean and standard deviation) of the aperture distribution. The present approach could well explain the significant increase of permeability due to shear displacement and its gradual approach to an asymptotic upper bound of the permeability. The asymptotic permeability with increasing shear displacement is dependent on the standard deviation of surface roughness. The anisotropy in the permeability was also induced by the shear displacement. Far from the expectation, the degree of anisotropy in rough rock joints with random surface model was more or less negligible, compared with the remarkable increase due to the shear dilation. It was also noticable that the shear‐induced anisotropic permeability became more isotropic as the shear displacement exceeded the characteristic correlation length of surface asperities. All these results were compared and verified by the numerical shear flow experiments using the randomly generated rock surfaces. However, it should be noted that the understanding in the study was based on initially isotropic surfaces, and under absence of normal load.

Implications of permeability and its anisotropy in a mica gouge for pore pressures in fault zones

Large permeability anisotropy due to anisotropic alignment of clay shape fabric may play an important role in focusing fluid flow along fault zones in clay rich rocks. The degree of permeability anisotropy depends on the differences in tortuosity of the fluid flow path and mean pore aperture, which are of a function of effective stress. We have conducted hydrostatic compaction and frictional sliding experiments on 1 mm thick artificial muscovite gouge layers with grain size <90 μm, at normal stresses on the layer ranging from 25 to 125 MPa. Permeability was measured both perpendicular and parallel to the gouge layer using water as pore fluid. Shear displacement up to 200 mm was achieved. During initial packing and subsequent hydrostatic compaction, muscovite basal planes were aligned parallel to the fault plane, leading to the largest possible contrast in tortuosity between flow path perpendicular and parallel to the fault zone. We found that fluid permeability parallel to the fault zone is about one order of magnitude higher than perpendicular to it and that the magnitude of permeability anisotropy does not change with increasing normal stress. During shear deformation, the permeability anisotropy decreased with increasing shear displacement, an effect interpreted as being due to progressive rotation of muscovite basal planes away from the fault plane. Previous hydrogeological modelling has shown that permeability anisotropy on the order of 3–5 orders of magnitude is needed to effectively focus fluid flow along fault zones. Thus, our results seem to indicate that fabric anisotropy alone is not enough to cause focused fluid flow along natural fault zones. During hydrostatic compaction of muscovite gouge, a linear relationship holds between ln permeability and normal stress and the slope is about 0.03 MPa −1, which is higher than for most porous sandstones. The value is reduced to about 0.006 MPa −1 after shear deformation. The values are lower by 1–2 orders of magnitude than the value used in Rice's fault model, suggesting that maintaining high pore pressures would be harder than he assumed.

Enhancement of fluid permeability during shear deformation of a synthetic mud

This study concerns the effect of stress paths on permeability and permeability anisotropy in a synthetic mud at conditions where cataclastic flow is dominant. The synthetic mud is composed of 10 wt.% montmorillonite, 40 wt.% illite, and 50 wt.% silt-sized quartz. In the experiments where the mud was first consolidated at 90 MPa effective pressure ( p e) and then also sheared at 90 MPa p e, permeability decreased continuously with increasing shear displacement; subsequent shear deformation at 30 MPa and 20 MPa effective pressure resulted in permeability increases. The permeabilities parallel to and across the mud layer are similar during shear deformation at 90 MPa p e and remain so during shear deformation at lower p e. In the experiments where shear deformation commenced at 30 MPa p e after an initial consolidation at 90 MPa, permeability increased significantly with increasing displacement. Combined effects of reducing mean effective stress and shear sliding result in permeability enhancement up to two orders of magnitude. The permeability parallel to the shear direction is one order of magnitude higher than that across the shear plane. Our shear deformation experiments at high pressures confirm the importance of stress path on the evolution of fluid permeability during shear deformation. The experimental results clearly indicate that to effectively enhance permeability and to channel fluids along shear zones in wet sediments, mean effective stress needs to be less than several tens of MPa.

Development of a shear-flow test apparatus and determination of coupled properties for a single rock joint

Abstract A new laboratory technique for coupled shear-flow tests of rock joints was developed and used to investigate the coupled effect of joint shear deformation and dilatancy on hydraulic conductivity of rock joints. This technique was used to carry out coupled shear-flow tests with an artificial created granite joint sample under constant normal loads and up to residual shear deformations of 20 mm. The hydraulic conductivity was estimated by using a finite difference method and an approximate equation assuming the cubic law. The shear-flow testing results revealed that the change of hydraulic conductivity is approximately similar to that of the dilatancy of a joint. The hydraulic conductivity increases rapidly, by about 1.2–1.6 orders of magnitude for the first 5 mm of shear displacement. After passing the residual shear stress, the hydraulic conductivity becomes gradually a constant value with increasing shear displacement. On the other hand, the hydraulic conductivity after shearing is about one order of magnitude larger than that prior to shearing. Shear-flow coupling characteristics obtained from these tests have a consistent trend with Barton's model prediction during the initial shear process. However, increasing deviation between measured and predicted hydraulic conductivity of rock joint samples has been observed with increasing shear displacement.

Permeability anisotropy and pressure dependency of permeability in experimentally sheared gouge materials

The effects of fault slip and effective mean stress on permeability and permeability anisotropy have been investigated during long displacement shearing of synthetic quartz, feldspar, muscovite and granitic gouges under high pressures. In quartzo-feldspathic gouges, permeability anisotropies up to one-and-a-half orders of magnitude were developed by heterogeneous grain crushing associated with shear sliding. In muscovite gouge, a permeability anisotropy of about one order of magnitude was first developed during initial pressurization of gouge materials due to a preferred alignment of mica basal planes. During shear sliding, rearrangement of packing and rotation of mica basal planes lead to a slight decrease in permeability anisotropy. The pressure dependency of permeability of gouge materials changes systematically with shear displacements. In quartzo-feldspathic gouges, the pressure dependency of permeability first decreased with shear sliding and then increased to a value typical for rocks containing microcracks. In contrast, the pressure dependency of permeability in muscovite gouge decreases continuously with increasing shear displacement. The evolution of permeability anisotropy and pressure dependency of permeability for different gouge materials during shear sliding implies that the distribution of fluid pressure in natural fault zones could be very heterogeneous.

Effect of shear displacement on the aperture and permeability of a rock fracture

The results of experiments using radial and unidirectional flow in a carefully described single rough aperture are reported and compared with numerical predictions. Aperture replicas of a natural sandstone fracture were made at 0, 1 and 2 mm shear displacements using silicone rubber, with a reproducibility of better than 2%. The experimental arrangement permitted shear displacement to be obtained without causing damage to the two displaced surfaces, which thus retained their original (and essentially matching) geometry. Both the number of contact points and the fractional contact area decreased with increasing shear displacement. With increasing shear displacement, mean aperture and standard deviation increased and the ratio of standard deviation to mean aperture increased slightly. Semivariogram studies indicated that as shear displacement increased in the direction normal to the roughness ridges, the aperture distribution became more closely correlated in the direction parallel to the roughness ridges than in the shear direction. Flow tests showed that with increasing shear displacement, the fracture became heterogeneous and anisotropic and became more permeable in the direction perpendicular to the shear displacement than in the direction parallel to the displacement. Simulations were made of flow through the fracture using the Reynolds equation and the actual aperture distribution; the measured hydraulic apertures were generally about 20% lower than those measured numerically.

The effect of fault slip on permeability and permeability anisotropy in quartz gouge

The effects of fault slip on permeability and permeability anisotropy have been investigated during large displacement shearing of quartz bare surfaces and 1 mm thick artificial quartz gouge at a normal stress of 25 MPa. Within the first 10 mm of shear displacement, grain size reduction occurred nearly uniformly within the fault zone. Permeability decreased by 2–3 orders of magnitude and the permeabilities perpendicular and parallel to the fault plane remained approximately equal. With further increasing shear displacement up to 200 mm, deformation was localized within a band of Y shears where strong grain comminution occurred. Permeability decreased further by 1–2 orders of magnitude and the permeability perpendicular to the fault showed larger reduction than parallel to it. A permeability anisotropy of about one order of magnitude developed. The permeability anisotropy is a result of heterogeneous deformation within the fault zone; flow perpendicular to the fault zone is impeded by the fine-grained band of Y shears, but flow parallel to the fault zone can occur in the less-deformed adjacent gouge. Our results suggest that fluid flow in natural faults depends on the degree of shear localization and the permeability contrast between the localized zone and the rest of the fault zone.

Modeling of Neogene crustal block rotation: Case study of southeastern California

Numerical modeling using a two‐dimensional distinct element method (DEM) was performed to give an additional understanding of the mechanism of crustal block rotation in a highly active portion of the Pacific‐North America transform plate boundary. Paleomagnetic and structural investigations have shown that large (30°–50°) horizontal rotations of crustal blocks occurred in the Eastern Transverse Zone and the Northeast Mojave Block (southeastern California) since the end of early Miocene. The main aspect of the modeling was to investigate whether an anisotropic horizontal stress state can be responsible for the rotation of crustal blocks with a length of 100 km or not. The simulations were performed using different stress boundary conditions and different parameters of the fault strength. Applying an assumed ratio of 2 for the horizontal stress components as boundary condition, block rotations in the order of 40° only occur if (1) the major horizontal stress SH is oriented N20°E with a maximal deviation of 15° to the east or west, and (2) the fault strength described by the friction coefficient is smaller than 0.12, or 14% only of the value 0.85 for intact rocks (Byerlee's law). For a weakening model of the fault strength an initial friction coefficient of 0.85 allows also rotations of about 40° if the friction coefficient decreases rapidly due to an increasing shear displacement to a basic friction coefficient of 0.12 or smaller. For the same amount of rotation (40°) a decreasing stress ratio requires a lowering of the friction coefficient used, whereas an increase of the ratio allows a higher friction coefficient. The simulation also show that the modeling with elastic blocks can adequately describe the observed main features of the crustal deformation, especially the cumulative slip values. Discrepancies in the observations and modeling results for the western parts of the Mojave Desert are the result of a neglect of the three‐dimensional deformation in our two‐dimensional modeling.

886141 Permeability changes during shear deformation of fractured rock : Teufel, L W Proc 28th US Symposium on Rock Mechanics, Tucson, 29 June–1 July 1987P473–480. Publ Rotterdam: A A Balkema, 1987

The effect of shear deformation on the permeability of fractured rock has been investigated in drained triaxial compression experiments on prefractured samples of Coconino sandstone. These experiments show that permeability across a fracture decreases with increasing shear deformation because of localized deformation along the fractures and the evolution of a gouge zone. Petrographic observations show a progressive decrease in grain size and porosity of the gouge zone with increasing shear displacement. With increasing normal stress there is a more rapid reduction in grain size and a higher degree of compaction of the gouge per increment of shear displacement, which results in a faster decline in permeability during sliding. Although not tested in this study, the observed localized microfracture zone immediately adjacent to the sliding surface suggests that a narrow channel of high permeability parallel to the fracture might also develop during shear deformation.

Continuous failure state direct shear tests

The paper deals with a new testing procedure aimed at determining the failure envelopes for peak and residual strength in direct shear tests. In a “continuous failure state” direct shear test the specimen is maintained in a state of permanent sliding while the shear and normal stress are being steadily changed. First of all, the specimen is brought to a state of failure in the conventional manner at a chosen constant normal stress. Then under monotonously increasing shear displacement the normal stress is continuously adjusted so that a straight line is produced in the shear stress-displacement plane. Both an increase in the stresses and a decrease is possible. The proper selection of the inclination of the straight line may involve a stress path which corresponds closely to the failure envelope of the specimen. In the case of smooth joint surfaces or in the residual strength state of rough surfaces it is possible to determine exactly the failure envelope with the aid of a single test specimen. The paper also describes a newly developed shear test apparatus suitable for combination with sophisticated servo-controlled loading machines generally available in rock mechanics laboratories.

The influence of shear velocity on the frictional resistance of rock discontinuities

A study has been made into the nature of the rate-dependent behaviour of rock joints. An experimental investigation using a dynamic direct shear machine showed the frictional resistance of rock joints to be dependent on the rate of shear displacement and that the magnitude of this effect was quite variable, depending mainly on the rock type and normal stress level. In general, for harder rocks, the frictional resistance was found to decrease with increasing shear displacement rates greater than a variable critical velocity. Conversely, for softer rocks, the resistance was observed to increase with increasing shear velocity, up to a critical shear displacement rate, and thereafter remain essentially constant.