Orientation Of Foliation Research Articles

Rotation of crustal domains related to Gondwanide deformation in the Neoproterozoic–Cambrian Sauce Chico Complex (Argentina), southwestern margin of the Río de la Plata Craton: Insights from AMS data

The tectonic evolution of the Ventania System fold and thrust belt and the origin of its curvature have been the subject of debates for decades. The Gondwanide Orogeny generated the mylonitization of its Tonian–middle Cambrian basement (i.e., Sauce Chico Complex), accompanied by an intense hydrolysis which produced secondary muscovite (i.e., sericite), and the folding of the overlying Paleozoic sedimentary cover. To evaluate the structural and tectonic evolution of the Ventania System as a result of the interaction of its basement with the Río de la Plata Craton, we studied the anisotropy of magnetic susceptibility (AMS) and rock magnetism on samples of the Sauce Chico Complex. Rock magnetic studies suggest that paramagnetic minerals are the main carriers in the entire Sauce Chico Complex. We suggest that our results reflect secondary muscovite generation due to mylonitization. AMS data reveal that the orientation of the magnetic foliation coincides to a large extent with the rock foliation. However, magnetic and rock foliation orientations are not uniform among the different sampled units, displaying strikes between N 320° (NW) and N 345° (NNW) on average. Localities with different orientations are separated by regional strike-slip structures, referred to as tear faults, which have also been reported and interpreted by previous authors. According to this, we suggest that the variations detected between the orientation of the magnetic and rock foliations of the different basement units may be the result of the rotation of crustal blocks bounded by tear faults. We also propose the existence of a new tear fault not previously recognized. Furthermore, a tear fault of regional character, known as the Sauce Chico Tear Fault, juxtaposes two distinct crustal domains. The differential displacement of such domains could have contributed not only to different structural styles between blocks on both sides of the Sauce Chico Tear Fault, but also to the curved shape of the Ventania System during the progression of the Gondwanide deformation.

Discrete element analysis of hydraulic stimulation in a slate geothermal reservoir using the ubiquitous foliation model

This study employs the discrete element method for a series of coupled hydro-mechanical analyses aimed at investigating the hydraulic stimulation effect on a slate geothermal reservoir. Initially, a ubiquitous foliation model (UFM) is devised to characterize the mechanical properties of slate. The UFM consists of a foliation model for near-field discontinuity features and a ubiquitous model for far-field anisotropic rock behavior. The devised model undergoes validation through an established benchmark analysis considering isotropic conditions. The analysis is subsequently extended to investigate anisotropic shear response influenced by the foliation. In cases where the anisotropic angle equals 90°, new fractures initiate along the foliation, which leads to a concentrated and severe damage pattern. In contrast, models with the anisotropic angle of 45° and 135° exhibit minor failures due to foliation orientation hindering fracture propagation. Lastly, an anisotropic hydraulic-mechanical analysis is undertaken to assess the hydraulic stimulation influence and its consequential effects on a slate geothermal reservoir. The results indicate that higher injection rates amplify damage to the rock matrix and the propagation of fractures, which are concentrated between open fractures and adjacent foliations.

Structural and Metamorphic History of the Leech River Shear Zone, Vancouver Island, British Columbia

AbstractThe Leech River Shear Zone (LRSZ) on southern Vancouver Island separates the metasedimentary schists of the Leech River Complex (LRC) from the accreted oceanic plateau of the Siletz‐Crescent terrane. The juxtaposition of these rock units suggests a possible origin as a subduction plate boundary but tectonic context of the LRSZ has yet to be fully established. We present field, microstructural, petrological, and geochronological observations that constrain the structural and metamorphic history of LRSZ. The mylonite zone of the LRSZ straddles the lithologic contact between the schists of the LRC and basalts of the Siletz‐Crescent terrane. Foliation orientations, a steeply plunging stretching lineation, and kinematic indicators all suggest reverse‐sinistral motion. Compositions of garnet in the schist and amphibole in the metabasalt record synkinematic growth at temperature and pressure conditions of 550°C–570°C and 450–490 MPa. These metamorphic conditions require elevated geotherms that are consistent with plate models that position the Kula‐Farallon Ridge and Yellowstone Hotspot in the region in the Eocene (∼50 Ma). Detrital zircon U‐Pb age distributions for the Leech River Schist have Paleocene maximum depositional ages and are similar to the Upper Nanaimo Group that unconformably overlies the Wrangellia terrane. Our ages support early Paleocene deposition of the schist in a subduction trench/slope environment followed by underthrusting and underplating. These results establish the exhumed mylonite zone of the LRSZ as a paleo‐plate interface that was active during Eocene subduction.

Influences of foliation orientation and lateral stress difference on the deformation and fracturing of a thin-layered rock around underground excavations: insight from multi-axial loading tests

Influences of foliation orientation and lateral stress difference on the deformation and fracturing of a thin-layered rock around underground excavations: insight from multi-axial loading tests

Structural Setting of the Sixtymile Gold District, Yukon, Canada: Insights into Regional Deformation and Mineralization from Field Mapping and 3D Magnetic Inversion

The Sixtymile gold district, Yukon, Canada has been mined for placer gold since the late 19th century. However, increasing demand for gold has prompted exploration of new lode deposits. Previous studies in the nearby Klondike gold district have shown correlation between placer deposits and bedrock occurrences. Poor bedrock exposure and a complex deformation history, however, make it difficult to determine structural controls on gold mineralization. Through structural analysis involving mesoscopic-scale field observations of fractures, faults, foliation, and folds, and 3D geophysical inversion, the goal of this study was to determine the structural setting of the Sixtymile district to enhance discovery success. Structural measurements in the Glacier Creek, Miller Creek, Bedrock Creek, and Sixtymile River areas show the relationships among the orientations of foliation, fractures, and veins. In most localities, veins are found both parallel and at high angles to foliation, and there is generally a weaker correlation between fractures and veins compared to between foliation and veins. This correlation between foliation and veins is corroborated by inferred gold-bearing horizons from gold assay data. Outcrops of oblique reverse and strike-slip faults, possibly related to a larger-scale thrust-zone, and to the left-lateral Sixtymile-Pika Fault, respectively, were documented for the first time in this study. The results of the 3D probabilistic inversion of total magnetic intensity data for magnetic susceptibility show that magnetic susceptibility highs are preferentially associated with volcanics, but also point to possible intrusive bodies or hydrothermal alteration zones associated with mineralization. A geologic cross-section through the lithologies demonstrates highly variable deformation styles, including extensive folding, possibly indicative of a multiphase deformational history necessitating further, more detailed investigations of the area.

Magnetic fabric and geomorphic characteristic of Neotectonic activity along strike direction of North Almora Thrust, Kumaun Lesser Himalaya, India

Structurally, the North Almora Thrust (NAT), is placed in the northern proximity of the Almora Nappe (AN) in the Kumaun Lesser Himalaya. The NAT is characterized by the presence of a zone of mylonitic rocks of basal Saryu Formation (1800 ± 100 Ma) in the hanging wall and the metasedimentaries of Inner Lesser Himalayan rocks as the footwall block. The NAT has been dextrally offset by two estabilished major faults i.e. Saryu River Fault (SRF) in Saryu Valley and Dwarahat-Chaukhutia Fault (DCF) in Ramganga valley. In present study, we identified four new faults, based on field study, geomorphic landform and magnetic fabric analysis. These faults are N–S trending Pancheshwar Fault, NE–SW Rameshwer Fault, NNE-SSW trending Kosi Fault, and NNE-SSW trending Gagas Fault (GF). Our results show that the zone is bound by cross cutting relation with NAT and these fault zones are comparatively more active than other regions. Furthermore, we suggest that the steep and NW–SE orientation of magnetic foliation within the NAT zone is a result of NE–SW oriented progressive regional compression. The magnetic foliations represent the unseen internal foliations in the rocks developed due to preferred alignment of magnetic minerals and can be found through the Anisotropy of magnetic susceptibility (AMS) study. Variation in the alignment of the field and magnetic foliations are developed due to superimposed brittle deformation along the faults over the pre-existing field foliations. Magnetic foliations represent the impact of last stage deformation and significant to find brittle deformation and finite strain in the rocks of the study area. The lowering of anisotropy (Pj) away from the fault zone represents distribution of strain across the NAT zone. AMS fabric confirms the presence of faults developed across the NAT zone and also explains the deformation pattern along these faults. The geomorphic anomalies and steepness changes across the NAT zone are correlated with active deformation along the NAT and associated transverse faults.

Study of the anisotropic tensile and compressive strength of a foliated phyllite

Anisotropy is an important characteristic of rocks, especially distinguishable in metamorphic rocks. Transverse isotropy is a particular case of anisotropy where foliation planes are distributed in the rock mass. Anisotropy can also originate from mineral foliation where minerals are oriented in a preferential direction. This inherent anisotropy can affect the rock strength significantly. Here the effect of foliation on the anisotropic strength of a phyllite is experimentally investigated. Phyllite specimens with defined foliation orientations are prepared and a series of laboratory tests carried out. Results for tensile strength and unconfined compressive strength are analysed. Maximum values of tensile strength and unconfined compressive strength are found for β = 0 and 90° and a significant reduction of strength is found for β = 45°. Compressive triaxial tests are conducted under confining pressures up to 20 MPa. The Hoek–Brown failure criterion is found to capture the experimental results in a better form than the Mohr–Coulomb criterion. Anisotropy indexes are adopted to evaluate the anisotropy effect on strength. Confinement is found to reduce the effect of anisotropy on phyllite strength. Cohesion and angle of shearing resistance were also found to be affected by the stress level, and further influenced by β.

Role of stress, slenderness and foliation on large anisotropic deformations at deep underground excavations

High stress concentrations around underground excavations can result in significant damage to deep hard-rock mines. These conditions can be the result of stopping activities, blasting, seismicity, or other mining activities. Large anisotropic deformation and excavation closure, especially under high-stress conditions, are expected if the excavation is located in a foliated or thin-bedded rock mass. In this research, the behaviour of excavations under deep and high-stress conditions was investigated and categorised. The main purpose was to enhance the existing knowledge of managing large anisotropic deformations and to help prepare suitable measures for handling such contingencies. Numerical simulations using the distinct element method (DEM) and model calibration were performed to reproduce the anisotropic deformation of an ore drive based on the collected field data. Then, the roles of key factors (i.e. stress ratio, slenderness ratio, foliation orientation, and foliation considering excavation orientation) on the large deformation and damage depth of the excavations were investigated. This study found that increasing both the stress ratio and slenderness ratio induced linear increases in wall closure and damage depth, whereas increasing the foliation angle first increases the deformation and damage depth and then reduces them both before and after 45°. The wall closure and damage thickness decreased with increasing orientation intercept. The deformation and damage levels were classified based on these factors.

Assessment of structural, magnetic, and P-wave velocity anisotropy of two biotite gneisses from X-ray and neutron tomography

Anisotropy of the physical properties of rocks is directly related to their internal structure, which is often exhibited by lineation and foliation fabrics as anisotropy markers. Conventional methods of diffraction measurements only provide a part of the structural information that is related to the crystallographic preferred orientation of minerals. In this work, we show the contribution of imaging methods such as X-ray and neutron tomography in studying rock structure that exhibits anisotropic character through lineation and foliation fabrics. In our tomography studies, spherical samples of two different biotite gneisses were tested. Virtual 3D structure models of samples were obtained by both neutron and X-ray tomography, which depict the spatial distribution of mica minerals (biotite and muscovite) - the major carriers of rock anisotropy in the studied samples. An application of scanning method to the 3D data of mica's spatial distribution allowed us to reveal the presence, orientation and strength of foliation and lineation fabrics. We have also performed grain shape analysis using the approximation of the individual elements of segmented mica phase by the equivalent (Legendre's) ellipsoids. As a result, we determined the shape preferred orientations of mica grains and calculated corresponding shape orientation distribution functions. The comparison of structural properties obtained from tomography studies with experimentally determined P-wave, and magnetic anisotropy demonstrated their mutual correlation, providing a basis for the quantitative interpretation of anisotropic rock fabric in relation to the physical properties of rock. The effective elastic and magnetic properties were calculated based on the shape orientation distribution functions determined from tomography data. The good agreement between calculated and measured effective properties have shown the potential of X-ray and neutron tomography for the prediction of magnetic and seismic anisotropy using a 3D model of a rock sample.

Structural diversity within a thrust complex reflecting progressive overprinting during a protracted orogenic process of terrane accretion, Natal belt, South Africa

The Tugela Terrane, forming the northern segment of the Mesoproterozoic Natal belt, South Africa, represents an example of accretionary tectonics that were initiated by closure of the Tugela Ocean basin, and culminated in accretion of magmatic arcs against the southern margin of the Archaean Kaapvaal Craton during the assembly of Rodinia. A detailed structural analysis of the Madidima thrust sheet, part of the Tugela Terrane, reveals five distinct deformation events, D1 to D5. Deformation varied between dominantly non-coaxial during D2, D3 and D5, and dominantly coaxial during D4, the latter being interpreted as a phase of gravitational collapse. While D2 ductile shearing characterizes the most pervasive event that defines the tectonic grain of the sheet, most, if not all, of the structural imprints can be understood as representing consecutive stages of deformation overprinting during a protracted orogenic process of arc-continent convergence. The observed structural complexity is partly due to this multiple overprinting in the course of orogeny, but also due to the broadly synchronous formation of different types of folds within the thrust sheet, and the corresponding variation of foliation orientations. The persistently north-east-directed tectonic transport during at least three of the deformation events is accompanied by intense transposition of early-formed structures, multiple foliation reactivation and formation of transpression-related structures.

Anisotropic Failure in Foliated Rock Tunnels: Comparison between Deformational Behavior and Confined Pressure

Foliated rock mass, such as slate, schist, and phyllite, is widely distributed in Northwest China. During tunnel excavation, some geotechnical problems are encountered in the surrounding rock, such as excessive over break and intense deformation by asymmetric squeezing, because of low strength, poor self-stationary, and intense anisotropy of structural strength. The large deformation results in intense damage of the primary support and splitting of the secondary support, which can significantly influence the construction and security of tunnels. Anisotropic failure in foliated rock tunnels under high stress conditions has become an engineering challenge. The deformational anisotropy of both rock and underground structures has drawn increasing attention in the field. The relative orientation of foliation plays an essential role on the deformational behavior of layered surrounding rocks by laboratory experiments and numerical simulations. Meanwhile, the symmetrical rigid support structure cannot effectively resist the asymmetric deformation exhibited by the excavation in foliated rock mass. Consequently, the excessive confined pressure generated between surrounding rock and support is essential for anisotropic failure in tunnels. In-situ monitoring tests of deformation and contact pressure were conducted in different positions of sections within a deep-buried tunnel in the Lanzhou–Chongqing railway. The comparison results indicated that the contact pressure of the support system showed irregular space distribution, which agreed with the concentrated deformation position of the surrounding rock. However, the monitoring data showed that the occurrence of failure obviously lagged behind the stabilization of convergence deformation. The internal confined pressure could last for a long time period than the deformation monitored outside. The failure of the support takes place in the weak axis plane because of its small lateral anti-bending rigidity. The authors believe that the proposed findings will be a step further in supporting the design of tunnels passing through foliated rock mass under high in-situ stress conditions.

Low Static Shear Modulus Along Foliation and Its Influence on the Elastic and Strength Anisotropy of Poorman Schist Rocks, Homestake Mine, South Dakota

We investigate the influence of foliation orientation and fine-scale folding on the static and dynamic elastic properties and unconfined strength of the Poorman schist. Measurements from triaxial and uniaxial laboratory experiments reveal a significant amount of variability in the static and dynamic Young’s modulus depending on the sample orientation relative to the foliation plane. Dynamic P-wave modulus and S-wave modulus are stiffer in the direction parallel to the foliation plane as expected for transversely isotropic mediums with average Thomsen parameters values 0.133 and 0.119 for epsilon and gamma, respectively. Static Young’s modulus varies significantly between 21 and 117 GPa, and a peculiar trend is observed where some foliated sample groups show an anomalous decrease in the static Young’s modulus when the symmetry axis (x3-axis) is oriented obliquely to the direction of loading. Utilizing stress and strain relationships for transversely isotropic medium, we derive the analytical expression for Young’s modulus as a function of the elastic moduli E1, E3, ν31, and G13 and sample orientation to fit the static Young’s modulus measurements. Regression of the equation to the Young’s modulus data reveals that the decrease in static Young’s modulus at oblique symmetry axis orientations is directly influenced by a low shear modulus, G13, which we attribute to shear sliding along foliation planes during static deformation that occurs as soon as the foliation is subject to shear stress. We argue that such difference between dynamic and static anisotropy is a characteristic of near-zero porosity anisotropic rocks. The uniaxial compressive strength also shows significant variability ranging from 21.9 to 194.6 MPa across the five sample locations and is the lowest when the symmetry axis is oriented 45° or 60° from the direction of loading, also a result of shear sliding along foliation planes during static deformation.

Three‐dimensional discrete element simulation of indirect tensile behaviour of a transversely isotropic rock

SummaryThis paper presents the development of a three‐dimensional discrete element model using flat‐joint and smooth‐joint contact models to investigate the effect of anisotropy on the tensile behaviour of slate, a transversely isotropic rock, under Brazilian testing from both macro and microscales. The effect of anisotropy is further realised by exploring the influence of foliation orientations (β and ψ) on the tensile strength, fracture pattern, microcracking and stress distribution of the transversely isotropic rock. The variation of tensile strength with foliation orientation is presented. The cross‐weak‐plane fracture growth observed in laboratory is reproduced, and the criterion for which to form is also given from the aspect of foliation orientation. Furthermore, the proportional variations of microcracks well account for the effects of foliation orientation on the tensile strength and failure pattern. Finally, it is found that the existence of weak planes increases both the heterogeneity and the anisotropy of stress distributions within the transversely isotropic rock, with the degree of influence varying with the foliation orientation.

Mechanical and hydraulic transport properties of transverse-isotropic Gneiss deformed under deep reservoir stress and pressure conditions

In central Europe, many geo-energy reservoirs have revealed to be hosted in transverse isotropic crystalline rock, where the rock's mechanical and hydraulic transport properties are poorly constrained. Here, we performed triaxial experiments on Cresciano Gneiss samples under realistic stress (25–40 MPa) and fluid pressure (5 MPa) conditions. We tested 5 different foliation orientations towards the major principal stress (0, 30, 45, 60, 90°). During deformation, we measured the porosity evolution and acoustic emission activity of the samples. In addition, we measured the axial permeability and P-wave velocity of the samples both during isostatic confinement and after sample failure. Our results show that the mechanical and hydraulic transport properties of transverse isotropic tight crystalline rocks can be separated into two classes. First, the mechanical properties such as onset of dilatancy, yield stress, peak strength and residual strength, follow a “U-type” anisotropy towards foliation angle, with maximum values at 0 and 90° and minima between 30 and 45°. These properties, as well as the porosity variation during deformation which follows an inversed “U-type” shape can be explained by anisotropic wing crack models. Second, the volumetric physical properties (permeability and P-wave velocity) follow a “decreasing order” shape towards foliation angle, with maximum values at 0° decreasing to the minimum at 90°. These properties show a high dependence on the stress state and the wave path. We discuss the implications of these results for deep geothermal energy prospection, and for reservoir stimulation and operation.

Integrated multi-temporal analysis of the displacement behaviour and morphology of a deep-seated compound landslide (Cerentino, Switzerland)

Deep-seated gravitational slope deformations (DSGSDs) are complex slope instabilities commonly involving long-term evolution with significant spatial and temporal variations in slope kinematics. The Cerentino compound landslide in southern Switzerland exemplifies the long-term evolution of DSGSDs as well as short-term movements causing substantial damage to infrastructure. Using an integrated analysis of surface geomorphology, displacement monitoring data, borehole core and cosmogenic nuclide dating, we discuss the preconditioning and driving factors, kinematic behaviour, and long-term evolution of the instability. We demonstrate that foliation orientation and lithological boundaries, together with slope geometry and alpine faults, likely played significant roles in preconditioning the slope for failure. Based on morphology mapping and hierarchical clustering analysis of 2007–2018 detrended surface displacement data, we show that the landslide is separated into up to eight compartments at surface, revealing differences in kinematic behaviour from very active at the toe to insignificant movements in the upper region of the landslide above the village of Cerentino. These surficial compartments likely correspond to secondary sliding zones within the landslide mass. The analysis of data from inclinometer and fibre optic strain sensors installed in a new deep borehole (230 m) provided insight into movement within the landslide mass. Results show vertical heterogeneity in total displacement, which appears to be controlled by an active discrete shear zone located at 107 m, by shallower secondary sliding surfaces, and by distributed movement, particularly in the upper 20 m of the instability. Extrapolation of cosmogenic nuclide dating results from five samples taken on the exposed upper sliding scar reveals a possible age of failure initiation of 5–6 ka, with long-term decelerating movement trends. These generally slow averaged movements show multi-annual responses to long-term changes in precipitation and locally rapid responses to strong recharge events. The rare combination of dating and detailed analysis of displacement allows us to demonstrate the behaviour of DSGSDs at long and short timescales, with implications for hazard assessment and mitigation of these complex failures.

Geology and geothermal potential in the eastern flank of Eger Rift (Litoměřice area, Czech Republic)

Utilization of geothermal energy in low enthalpy regions is made possible with the use of EGS (Enhanced Geothermal Systems), which utilize a system of two boreholes connected by artificially fractured rock volume providing permeability for heating the cold water injected in one of the boreholes and produced in the other one. We focus on the area of Litoměřice in the NW part of Bohemian Massif, where a pilot 2.1 km deep geothermal borehole was drilled in 2007. We combine existing geological knowledge with newly acquired data to build a conceptual geological model of the area. Temperature logging and thermal characteristics of the collected rock samples provided data for thermal modelling. The lower part of the borehole samples relatively homogeneous mica-schists, which are overlain by rhyolite ignimbrites, upper-Carboniferous continental siliciclastic sediments and sedimentary cover of Bohemian Cretaceous Basin on the top. Several sets of brittle structures of various ages were identified in the area, however only some of them are prone for reactivation in the present stress field corresponding to NNW-SSE compression. The prevailing orientation of metamorphic foliation and missing signs for contact metamorphism appear to preclude the possible occurrence of granites in the deeper horizons. Equilibrium temperature in 1.8 km is 57.5 °C, which combined with the thermal characteristics of the rocks gives the heat flux of 78 mW/m2 and temperature prediction of about 140 °C in 5 km depth, which ranks the area to typical geothermal provinces of the NW part of the Bohemian Massif. Further research will be focused on seismic surveys and stress measurements to assess the seismicity potential of the formation.

Flow plane orientation in the upper mantle under the Western/Central United States from SKS shear-wave splitting observations

SUMMARY The causes of seismic anisotropy are still under debate. In particular, it is important to understand the extent to which seismic anisotropy is due to more recent geodynamic activities in the asthenosphere, or to frozen-in deformation in the lithosphere. We show that these two endmember cases can in principle be distinguished using shear-wave splitting observations from SKS waves. This is illustrated by the simple example of pure olivine with horizontal a-axis, and differing orientations of the other two axes, namely, vertical b and vertical c. The azimuthal dependence of shear-wave splitting measurements is described by two parameters, which can provide additional information about subsurface deformation. In particular, the oscillation parameter d1 constrains the orientation of foliation. We demonstrate that shear-wave splitting in the Western and Central United States indeed shows the predicted azimuthal dependence, related to a mainly subhorizontally oriented flow plane of deformation in the upper mantle. This has important implications for asthenospheric flow.

Investigations of P-Wave velocity, mechanical behavior and thermal properties of anisotropic slate

Rock anisotropies are an important indicator for design and construction of various engineering projects across the world. To investigate the anisotropic characteristics of P-wave velocity, mechanical behavior and thermal properties, laboratory tests were performed on slate samples with various foliation orientations with respect to horizontal direction. The results indicate that the uniaxial compression strength (UCS) exhibits a typical U-type trend, the Young's modulus first decreases and then increases, and the variation of Poisson's ratio shows an opposite trend with the variation in Young's modulus. The anisotropy in strength is greater than that in deformation. Three typical failure modes, i.e. splitting across the foliation planes, shearing and sliding along the foliation planes, and axial splitting failure along the foliation planes, were observed. Scanning electron microscope (SEM) was also applied to analyze the microscopic failure mechanisms. It shows that the slate anisotropies are intrinsically attributed to the directional arrangement of mineral compositions observed by polarizing microscope. P-wave velocity and thermal conductivity both increase with increasing foliation angle, but the linear coefficients of thermal expansion show a decreasing trend. A new empirical formula was then proposed for describing the P-wave velocity and thermal parameters with respect to foliation angle of slate. It displays the empirical predictions match the laboratory measurements. The obtained results can better facilitate our understanding of mechanical behavior and thermal properties of anisotropic rocks, which should be considered in underground engineering applications in layered strata.

Ground support performance in deep underground mine with large anisotropic deformation using calibrated numerical simulation (case of mine-H)

High-stress and complex geological conditions impose great challenges to maintain excavation stability during deep underground mining. In this research, large anisotropic deformation and its management by support system at a deep underground mine in Western Australia were simulated through three-dimensional finite-difference model. The ubiquitous-joint model was used and calibrated in FLAC3D to reproduce the deformation and failure characteristics of the excavation based on the field monitoring results. After modeling verification, the roles of mining depth also the intercept angle between excavation axis and foliation orientation on the deformation and damage were studied. Based on the results, quantitative relationships between key factors and damage classifications were presented, which can be used as an engineering tool. Subsequently, the performance of support system installation sequences was simulated and compared at four different scenarios. The results show that, first surface support and then reinforcement installation can obtain a better controlling effect. Finally, the influence of bolt spacing and ring spacing were also discussed. The outcomes obtained in this research may play a meaningful reference for facing the challenges in thin-bedded or foliated ground conditions.

Numerical Investigation of the Mechanical and Damage Behaviors of Veined Gneiss During True-Triaxial Stress Path Loading by Simulation of In Situ Conditions

Investigations on the Posiva Olkiluoto spalling experiment (POSE) at the nuclear waste disposal site in Olkiluoto, western Finland, have revealed that the presence of anisotropic veined gneiss (VGN) could affect the evaluation of spalling/damage strength of the host rock. In this study, the mechanical and damage behaviors of anisotropic VGN during complex true-triaxial in situ stress path loading is numerically investigated, considering the lifetime stress evolution of POSE. Overall, the discrete element numerical results show reasonable agreement with those of laboratory experiments for both strength and damage development. Numerical simulations at the microscale showed that the damage is concentrated within the discontinuous foliation bands and at the interfaces between weak and strong bonds when the mineral composition of the foliation bands is heterogeneous. It is possible that the varying stiffnesses of the two materials cause strain concentration at strong segments within the weak foliation bands or high strain gradients at the interfaces between the weak and strong bands, leading to tensile failure of the strong bonds. However, if continuous foliation bonds are employed, fractures mainly propagate within the weak bonds, indicating that the heterogeneity of the foliation bonds is a predominant factor affecting the failure of VGN. After weak bonds and interfaces failure, shear movements occur along the weak bonds, forcing the strong bonds to bear most of the load. The simulations also show that the unloading of σ2 and σ3 triggers microcracks in the early stages, revealing the role of unloading in rock damage. The change in the secant modulus in relation to the foliation angle is generally found to follow a U-shaped anisotropy and obtains the lowest value at 45°. For each test, the modulus gets the minimum value in the damage stage and recovers in the unloading stage, but it cannot reach its original value. Sample strength increases as the foliation orientation angle increases from 0° to 45° in relation to σ2, because the increase in normal stress from σ2 causes the role of the strong foliation bonds to become significant as the bonds strengthen. The damage to the strong bonds leads to a larger reduction in the modulus at the damage stage.