Transversely Isotropic Research Articles

Processing the Artificial Edge-Effects for Finite-Difference Frequency-Domain in Viscoelastic Anisotropic Formations

Real sedimentary media can usually be characterized as transverse isotropy. To reveal wave propagation in the true models and improve the accuracy of migrations and evaluations, we investigated the algorithm of wavefield simulations in an anisotropic viscoelastic medium. The finite difference in the frequency domain (FDFD) has several advantages compared with that in the time domain, e.g., implementing multiple sources, multi-scaled inversion, and introducing attenuation. However, medium anisotropy will lead to the complexity of the wavefield in the calculation. The damping profile of the conventional absorption boundary is only defined in one single direction, which produces instability when the wavefields of strong anisotropy are reflected on that truncated boundary. We applied the multi-axis perfectly matched layer (M-PML) to the wavefield simulations in anisotropic viscoelastic media to overcome this issue, which defines the damping profiles along different axes. In the numerical examples, we simulated seismic wave propagation in three viscous anisotropic media and focused on the wave attenuation in the absorbing layers using time domain snapshots. The M-PML was more effective for wave absorption compared to the conventional perfectly matched layer (PML). In strongly anisotropic media, the PML became unstable, and prominent reflections appeared at truncated boundaries. In contrast, the M-PML remained stable and efficient in the same model. Finally, the modeling of the stratified cross-well model showed the applicability of this proposed algorithm to heterogeneous viscous anisotropic media. The numerical algorithm can analyze wave propagation in viscoelastic anisotropic media. It also provides a reliable forward operator for waveform inversion, wave equation travel-time inversion, and seismic migration in anisotropic viscoelastic media.

Dynamic analysis of eccentrically loaded rigid strip foundation on layered transversely isotropic poroelastic media

This paper presents an analytical method for analyzing vertical and rocking motions of a rigid strip foundation on layered transversely isotropic poroelastic media. The extended precise integration method is employed to obtain the plain strain dynamic responses of layered poroelastic media due to the reaction force from the rigid strip. Then, the contact stresses for dynamic soil-structure interactions (SSI) are modelled by the Bessel series expansions, and a set of linear equations can be developed from the dual integral equations of mixed boundary value problems. The explicit expressions for the contact stresses are further obtained by solving these linear equations. Finally, the presented method is verified, and numerical examples are provided to demonstrate the effects of stratifications, transverse isotropy and permeability of media on the dynamic responses of eccentrically loaded rigid strip foundation.

Study of the anisotropic elastic response of a foliated phyllite

Anisotropy is a crucial characteristic of metamorphic rocks whereby minerals oriented in a preferential direction can originate mineral foliation. Inherent anisotropy can affect the rock behaviour significantly. Transverse isotropy is a particular case of anisotropy where foliation planes are distributed in the rock mass. Therefore, in this study the anisotropy effect on the elastic properties of a foliated phyllite is considered. To this end, a series of laboratory tests was programmed. Triaxial tests were carried out under confining pressures up to 20 MPa. Elastic parameters such as elasticity modulus, E, and Poisson's ratio, ν, are assessed from triaxial test results as well as from ultrasonic tests, where compression and shear wave velocities are determined. Empirical relationships for elastic parameters are suggested as a function of foliation angle, β, and confinement. In addition, anisotropy indexes are adopted to evaluate results, comparing when possible with those of previous works. It was found that E increases with confinement; however, the effect of anisotropy on E reduces with confinement. It was also found that ν is affected by β but not by confinement.

Axis-symmetric analysis of layered transversely isotropic saturated elastic soils containing a monopile under time-harmonic vibration

The axis-symmetric dynamic soil-pile interaction analysis is of fundamental significance in geotechnical, seismic and transportation engineering. In addition, soils tend to show an obvious stratification after the long-term deposition. Physical and mechanical properties of soils are generally transversely isotropic due to the orientation of soil particles in deposition. Therefore, the axis-symmetric analysis of layered transversely isotropic saturated elastic soils containing a monopile under time-harmonic vibration is carried out in this study. Firstly, the extended precise integration method (EPIM) is employed to obtain the dynamic fundamental solution for layered transversely isotropic saturated soils, which would be utilized as the kernel function of the boundary element method (BEM). Moreover, by modeling the pile as a one-dimensional bar, the motion equation of the monopile under steady-state vibration is acquired based on the finite element method (FEM). Finally, the coupled BEM-FEM formulation representing the soil-pile interaction is developed and the axis-symmetric solution for layered soils containing a monopile is acquired. Compared with existing results, the accuracy of the proposed method is verified. Numerical examples are conducted to analyze the influence of frequency, transverse isotropy, the stratification of soils and the length-radius ratio of piles on soil-pile interaction behaviors.

Validation and Practical Application of a Data Reduction Software for the Analysis of Data from Stress Relief Tests

In his work "Rock Anisotropy and the Theory of Stress Measurements", Amadei (1983) highlights the importance of the correct geological-technical characterisation of the rocks involved in open pit or underground excavation works. At the laboratory of Geomechanics and Geotechnology (IGG-CNR, Polytechnic of Turin), a data reduction software has been developed which, based on the theory developed by Amadei, allows the estimation of the main stresses acting in selected volumes of rock, on the basis of strains resulting from over-coring, possibly taking into account the transverse isotropy of the material. This paper, after listing the most popular techniques for assessing the state of stress in on-site rocks, focuses on the stress relief method. In particular, the validation tests of the software, called BM2000, are presented, comparing the calculated results with those published by Amadei and with those provided by two software packages from the literature (Smith82, STRESsOUT). After having outlined the input data and the necessary adjustments for their application in the aforementioned software, the results obtained showed excellent correspondence, for the cases of both isotropic and transversely isotropic rock.

A trishear model with self-adaptive propagation-to-slip ratio

Natural beds at different stratigraphic levels display different degrees of folding. Variable propagation-to-slip ratios in the trishear models are more appropriate for the reproduction of natural folds. An improved trishear model with self-adaptive propagation-to-slip ratio, called TMSP, is proposed in this paper. Transverse isotropy elastic constitutive relationships are utilized to formulate the mechanic behaviors of the overlying sediments. The von Mises failure criterion is introduced to determine whether the material points are destroyed in TMSP. Compared with previous trishear models, TMSP has the advantage that propagation-to-slip ratio can be updated with the proceeding of fault rupture propagation and is not required to be predetermined. A natural fault-propagation fold, Tejerina fold, can be reproduced well by TMSP. Finally, a series of simulations are conducted to investigate the influences of three key parameters of TMSP, including apical angle, yield stress, and elastic modulus, on faulting process and folding growth.

A computationally efficient approach for generating RVEs of various inclusion/fibre shapes

A computationally efficient method for generating virtual periodic representative volume element (RVE), capable of handling arbitrary inclusion shapes, is developed. A universal collision/overlap detection and repair method is proposed, where each inclusion shape is represented as a union of n-Spheres (UnS). A constrained optimization problem is formulated and solved to remove inclusion overlaps; a closed-form solution is derived for calculating the degree of inclusions overlap and its gradient vector with respect to inclusion position. RVE generation is illustrated with circular, spherical, four non-circular and four non-spherical inclusion shapes. Computational efficiency is demonstrated using an elaborate RVE generation time study. The generated RVEs are evaluated using various statistical metrics; results confirm the random distribution of inclusions. Effective properties of RVEs, representing unidirectional composites, are determined using homogenization with various fibre cross-section shapes; obtained mechanical properties have shown transverse isotropy.

Quantifying stability of parameter estimates for in vivo nearly incompressible transversely-isotropic brain MR elastography

Easily computable quality metrics for measured medical data at point-of-care are important for imaging technologies involving offline reconstruction. Accordingly, we developed a new data quality metric for in vivo transversely-isotropic (TI) magnetic resonance elastography (MRE) based on a generalization of the widely accepted octahedral shear-strain calculation. The metric uses MRE displacement data and an estimate of the TI property field to yield a ‘stability map’ which predicts regions of low versus high accuracy in the resulting material property reconstructions. We can also calculate an average TI parameter stability (TIPS) score over all voxels in a region of interest for a given measurement to indicate how reliable the recovered mechanical property estimate for the region is expected to be. The calculation is rapid and places little demand on computing resources compared to the computationally intensive material property reconstruction from non-linear inversion (TI-NLI) of displacement fields, making it ideal for point-of-care evaluation of data quality. We test the predictions of the stability map for both simulated phantoms and in vivo human brain data. We used a range of different displacement datasets from vibrations applied in the anterior-posterior (AP), left-right (LR) and combined AP + LR directions. The TIPS and variability maps (noise sensitivity or variation from the mean of repeated MRE scans) were consistently anti-correlated. Notably, Spearman correlation coefficients ∣R∣>0.6 were found between variability and TIPS score for individual white matter tracts with in vivo data. These observations demonstrate the reliability and promise of this data quality metric to screen data rapidly in realistic clinical MRE applications.

Harmonic generation in a shear wave beam with spatially varying polarization in transversely isotropic soft tissue

Shear wave beams with spatially varying polarization are characterized here by stretching the medium in the plane perpendicular to the propagation direction. Such a beam is produced in a soft tissue by applying motion perpendicular to the source plane, as with a piston source, due to the quasi-incompressible nature of the medium. In the transversely isotropic (TI) soft tissue, such as muscle, this polarization causes extension and contraction in the fiber direction, resulting in nonlinear propagation effects that are unique to the TI tissue compared to propagation in an isotropic tissue [Cormack, J. Acoust. Soc. Am. 150, 2566 (2021)]. A KZ-type equation is presented that describes a special case of propagation of a shear wave beam with spatially varying polarization in the TI soft tissue, accounting for material anisotropy along with leading-order effects of nonlinearity and diffraction. Solutions for harmonic generation from a time-harmonic source are presented for focused and unfocused Gaussian-derivative distributions of transverse displacement in the source plane. Of practical interest is the longitudinal displacement in the shear wave field, which possesses a second-harmonic component on the axis that results from stretching in the fiber direction. This second harmonic, which is absent in isotropic tissues, may be measured using existing elastography techniques.

Stochastic aspects of crack deflection and crack path prediction in short fiber reinforced polymer matrix composites

Owing to the production process, short fiber reinforced composites exhibit a pronounced anisotropy of both elastic properties and crack growth resistance. In particular the latter issue has a major impact on crack deflection and inevitably has to be taken into account for the sake of an accurate prediction of crack paths. The perpendicular axes of transverse isotropy are associated with the fiber orientations, whereupon a crack in transverse direction encounters the largest fracture toughness. While the local mean fiber orientations in polymer matrix composites are determined by the injection molding process, their statistical fluctuations can approximately be described in terms of Gaussian random field models. Furthermore, statistical variations of the volume fraction of fibers and the fiber–matrix adhesion give rise to stochasticity of the local ratio of fracture toughness anisotropy. Influencing factors of prediction regions of crack paths obtained from finite element simulations with an adapted J-integral deflection criterion are investigated, just as stochastic aspects of bifurcation phenomena of crack deflection observed under mode-I loading.

On the coupled thermo–electro–chemo–mechanical performance of structural batteries with emphasis on thermal effects

Carbon fibre (CF) based structural batteries is a type of battery designed to sustain mechanical loads. In this paper, a fully coupled thermo–electro–chemo–mechanical computational modelling framework for CF-based structural batteries is presented. We consider the combined effects of lithium insertion in the carbon fibres leading to insertion strains, and thermal expansion/shrinkage of the constituents leading to thermal (free) strains, while assuming transverse isotropy. The numerical studies show that the developed framework is able to capture the coupled thermo–electro–chemo–mechanical behaviour. Moreover, it is found that the dominating source for heat generation during galvanostatic cycling is associated with discontinuities in the electrical and chemical potentials at the fibre/electrolyte interface. Further, a limited parameter study shows that the temperature change during electrochemical cycling is significantly influenced by the applied current, thermal properties of the constituents and heat exchange with the surroundings. Finally, for large temperature variations, e.g. as identified during relevant (dis)charge conditions, the magnitude of the thermal strains in the structural battery electrolyte (SBE) are found to be similar to the insertion induced strains.

Pressure-dependent elastic anisotropy: A Bakken petroleum system case study

The complexity of the micropore fabric and organic texture in tight unconventional reservoirs often complicates their correlations with physical rock property parameters. Anisotropy, which is a common physical property of tight oil reservoirs, can affect directional deformation and permeability during the reservoir development and pressure depletion period. Thus, accurately measuring seismic anisotropy and its relationships with petrophysical properties can refine our understanding of vertical transverse isotropy rocks and ultimately improve fracturing design as well as production forecasting. We have developed a measurement technique of ultrasonic velocities (P and S) from multiple directions, that is, bedding-parallel, bedding-perpendicular, and at 45° to bedding under simulated in situ reservoir pressure. Following these measurements, we also correlate these findings to pressure-dependent porosity and permeability, in addition to geologic attributes and textural anisotropy. The samples that we investigate are tight carbonaceous siltstone and organic-rich mudrock cores originating from the Bakken petroleum system, which include Lodgepole, upper Bakken shale, middle Bakken, lower Bakken shale, and Three Forks. Depending on the rock fabric and mineral constituents, each formation exhibits distinctive anisotropic behavior in response to pressure changes. The results indicate that the elastic parameters and petrophysical properties are inversely related under pressure for all samples. We also find that P-wave anisotropy and SH-wave anisotropy parameters decrease gradually with increasing confining pressure. The anisotropic behavior can be related to volumetric concentration and directional connectivity of clay layers, kerogen veins, and subparallel microfractures.

Revealing the nonlinear mechanical behavior of white matter brain tissue by analyzing the asynchronous deformation and damage of matrix and axonal fibers

The mechanical behavior of the brain plays a vital role in regulating brain morphology and brain function. The experimental results show that the brain tissue behaves with strong nonlinearity, heterogeneity, loading rate and direction dependence, and complicated damage behavior. However, the underlying mechanism attributed to the interlacement of extremely soft matrix and complex axonal fibers network in white matter brain tissue is still not well understood. In this work, the nonlinear mechanical behavior of white matter brain tissue is analyzed by developing a mesoscale visco-hyperelastic constitutive model that considers isotropic matrix and anisotropic axonal fibers separately. Based on the diffusion tensor imaging data, the generalized structure tensor is introduced to reflect axonal fiber's orientation and distribution characteristics in different parts of white matter. Furthermore, the damage models of matrix and axonal fibers are established to characterize their damage evolution differences. In particular, a damage initiation criterion based on the equivalent strain of axonal fibers stretch is proposed by analogy with the Tsai-Hill criterion, according to the distribution characteristics of axonal fibers. Then the mesoscale constitutive model is applied to reveal the effects of matrix and axonal fibers on the deformation and damage of white matter. On the one hand, it is found that the “S-shaped” nonlinear stress curve of white matter in uniaxial tension is the superposition of the convex stress curve of matrix and the “J-shaped” stress curve of axonal fibers, and the nonlinearity is stronger with the increase of strain rate. On the other hand, with the change of the distribution of axonal fibers from uniaxially oriented to dispersed, the anisotropy of white matter caused by the stress reinforcement of axonal fibers gradually evolves from transverse isotropy to isotropy. With the increase of strain rate, the reinforcement effect of axonal fibers tends to be more isotropic, and the proportion of isotropic matrix stress is increased, resulting in the isotropic tendency of white matter. Moreover, the asynchrony of matrix and axonal fibers damage also results in the complex transition of white matter deformation behavior. Along with the increase of deformation, the anisotropic mechanical behavior of white matter rapidly degenerates to isotropy with the damage of axonal fibers. Overall, the strong coupling of nonlinear, anisotropic, and strain rate-dependent mechanical behaviors of white matter is well explained by combining the asynchronous deformation and damage of matrix and axonal fibers in this study. It will help understand brain injury, brain growth and development, physiological brain diseases, and other relevant problems.

Noninvasive Assessment of In Vivo Passive Skeletal Muscle Mechanics as a Composite Material Using Biomedical Ultrasound.

This study develops a biomedical ultrasound imaging method to infer microstructural information (i.e., tissue level) from imaging mechanical behavior of skeletal muscle (i.e., organ level). We first reviewed the constitutive model of skeletal muscle by regarding it as a transversely isotropic (TI) hyperelastic composite material, for which a theoretical formula was established among shear wave speed, deformation, and material parameters (MPs) using the acoustoelasticity theory. The formula was evaluated by finite element (FE) simulations and experimentally examined using ultrasound shear wave imaging (SWI) and strain imaging (SI) on in vivo passive biceps brachii muscles of two healthy volunteers. The imaging sequence included 1) generation of SW in multiple propagation directions while resting the muscle at an elbow angle of 90°; 2) generation of SW propagating along the myofiber direction during continuous uniaxial muscle extension by passively changing the elbow angle from 90° to 120°. Ultrasound-quantified SW speeds and muscle deformations were fitted by the theoretical formula to estimate MPs of in vivo passive muscle. Estimated myofiber stiffness, stiffness ratio of myofiber to extracellular matrix (ECM), and ECM volume ratio all agreed with literature findings. The proposed mathematical formula together with our in-house ultrasound imaging method enabled assessment of microstructural material properties of in vivo passive skeletal muscle from organ-level mechanical behavior in an entirely noninvasive way. Noninvasive assessment of both micro and macro properties of in vivo skeletal muscle will advance our understanding of complex muscle dynamics and facilitate treatment and rehabilitation planning.

Method to account for natural fracture induced elastic anisotropy in geomechanical characterization of shale gas reservoirs

Shale has been usually recognized as a transverse isotropic (TI) medium in conventional geomechanical log interpretation due to its laminated nature. However, when natural fractures exist in the shale rock, additional elastic anisotropy is introduced, converting laminated Shale to an orthorhombic (OB) medium. Previous studies illustrate that neglecting the natural fracture induced anisotropy in shale geomechanical log interpretation could lead to inaccurate evaluations of elastic moduli and in-situ stresses.In this paper, a new method is developed to account for the natural fracture induced anisotropy in geomechanical log interpretation based upon the TI acoustic model developed by the author and a characterization technique of elastic wave anisotropy (Sayers, 1991). The new OB model incorporates the four acoustic log data inputs and five modeling constraints in a nonlinear optimization algorithm to solve for the nine independent stiffness coefficients of an OB rock, and further to solve for the geomechanical properties and in-situ stress profiles in an OB formation.The new method was validated with a Marcellus Gas Shale field case. Both the new OB model and the conventional TI model were applied to interpret the minimum horizontal stress profile for the same formation. By comparing the results, the OB model is more robust than the TI from two aspects. First, the average stress magnitude predicted by the OB model is closer to the one measured by the Diagnostic Fracture Injection Test (DFIT). Second, the OB model predicts a more obvious stress barrier between the lower Marcellus and upper Onondaga Limestone than the TI model does. The predicted stress barrier is consistent with the observation of the microseismic events of a horizontal well drilled and completed nearby, which reveals that no hydraulic fracture propagates downward through the bottom boundary of Marcellus Shale into the underlying Onondaga Limestone.

Impact of Stress-Induced Rock Damage on Elastic Symmetry: From Transverse Isotropy to Orthotropy

In this article we explore the change in elastic symmetry and anisotropy of two geological materials with stress-induced damage. Two independent uniaxial deformation experiments on two layered and natural geomaterials, a shale and a sandstone, support this analysis. Both samples were loaded along their bedding planes at a constant strain rate up to mechanical failure. During deformation, an array of ultrasonic P- and S-wave transducers were employed to monitor the evolution of the ultrasonic velocities up to and beyond sample failure. We report here the impact on elastic symmetry and anisotropy of a uniaxial load applied parallel to the bedding plane of the transversely isotropic shale and sandstone samples. Based on symmetry considerations we analyse whether this load and the resulting stress-induced damage preserve the original transverse isotropy of the rock prior to loading, or lead to orthotropy (orthorhombic symmetry). Our results show that the two transverse isotropic samples retained their transverse isotropy in the initial stages of loading/damage. However for the more anisotropic shale sample the main failure mode was splitting of the bedding planes. In contrast the sandstone sample failed along a shear plane inclined to the bedding plane. In addition, the experimental data are further analysed using continuum damage mechanics to identify the evolution of the general fourth order anisotropic damage tensor during loading. We found that the highest damage variable of the damage tensor was D_{11} for the shale and D_{55} for the sandstone sample. The next highest damage variables for the shale sample were related to the bedding plane (x_2–x_3 plane) symmetry: D_{23}, D_{32}, D_{44}. However the damage variables obtained for the less anisotropic and more brittle sandstone sample were harder to interpret due to the mixed mode of fracturing present.

Simultaneous Bayesian inversion for effective anisotropy parameters and source locations: a physical modelling study

SUMMARY Estimating microseismic event locations is important for applications of geophysical monitoring, including hydraulic fracturing and carbon-capture and storage. Field sites for these applications are typically located in sedimentary basins that include finely stratified sediments, particularly around the target depth of the application. The fine stratification causes vertical transverse isotropy (VTI) for seismic wave propagation. In addition, such sediments often exhibit a vertically fractured rock mass that can cause horizontal transverse isotropy (HTI). Therefore, geophysical monitoring can be strongly affected by the occurrence of anisotropy caused by sets of aligned vertical fractures in finely horizontally layered media. While both HTI and VTI theories exist, a more efficient approximation to include both effects is by effective orthorhombic (ORT) models. To account for such anisotropy in microseismic monitoring, we simultaneously estimate ORT parameters, perforation shot locations, and microseismic event locations with Bayesian methods based on direct P-wave arrival times. A comparison to a HTI parametrization is carried out to examine anisotropy-model choice. The quasi-P-wave group velocities in HTI and ORT media are approximated by linearization. Anisotropy parameters are estimated with Markov chain Monte Carlo sampling that includes parallel tempering and principal-component diminishing adaptation to ensure efficient sampling of the parameter space. In contrast to deterministic inversion, our probabilistic non-linear approach includes uncertainty quantification by approximating the posterior probability density with an ensemble of model-parameter sets for effective anisotropy parameters, microseismic event locations, and horizontal locations of perforation shots. The noise standard deviation of P-arrival times is also treated as unknown. The inversion is carried out for simulated data, and for data from a physical laboratory model. In the latter case, an anisotropic layer is represented by a phenolic canvas electric material, and a star-shaped surface-receiver configuration is used to record microseismic signals. Results show that obtaining unbiased event locations requires an appropriate choice of anisotropy model and the ability to resolve anisotropy parameters. The resolution of anisotropy parameters requires significantly more data information from microseismic acquisition than required for isotropic models. Therefore, we study several acquisition scenarios for simulated and laboratory data. Assuming an HTI model in the inversion when data originate from an ORT medium causes systematic errors in event locations. However, appropriate resolution of ORT parameters requires a large acquisition aperture, an accurate perforation-shot timing, and the combination of surface acquisition with a vertical downhole array. These scenarios provide new knowledge about field requirements to produce sufficient information for the resolution of microseismic event locations in the presence of ORT effects in the data.

HPC Geophysical Electromagnetics: A Synthetic VTI Model with Complex Bathymetry

We introduce a new synthetic marine model for 3D controlled-source electromagnetic method (CSEM) surveys. The proposed model includes relevant features for the electromagnetic geophysical community such as large conductivity contrast with vertical transverse isotropy and a complex bathymetry profile. In this paper, we present the experimental setup and several 3D CSEM simulations in the presence of a resistivity unit denoting a hydrocarbon reservoir. We employ a parallel and high-order vector finite element routine to perform the CSEM simulations. By using tailored meshes, several scenarios are simulated to assess the influence of the reservoir unit presence on the electromagnetic responses. Our numerical assessment confirms that resistivity unit strongly influences the amplitude and phase of the electromagnetic measurements. We investigate the code performance for the solution of fundamental frequencies on high-performance computing architectures. Here, excellent performance ratios are obtained. Our benchmark model and its modeling results are developed under an open-source scheme that promotes easy access to data and reproducible solutions.

Three‐dimensional dynamic response of transversely isotropic saturated media subjected to transient loads

AbstractThis paper investigates three‐dimensional (3D) transient responses of stratified transversely isotropic (TI) saturated media. Firstly, we transform the partial differential equations in Cartesian coordinates into ordinary differential equations (ODEs) for the convenience of solving. Then, we consider the transient load forms and apply the extended precise integration method (EPIM) to study the response of stratified TI saturated media. This paper involves the transverse isotropy of solid constituent, the permeable anisotropy of pore fluid, and various load forms including concentrated loadings and uniform rectangular loadings. The solution in the physical domain can be obtained through the numerical inversion of Laplace‐Fourier transform. Several examples are set up to verify the proposed theory and to investigate the influences of the transverse isotropy of solid constituent, the permeable anisotropy of pore fluid and the stratification of media.

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.