Triaxial Compression Conditions Research Articles

Study on the Compressive and Tensile Properties of Latex-Modified Cement Stone.

The integrity of wellbores is essential for the safe and efficient operation of drilling activities. Cement plays a critical role in this process, serving as a primary barrier that isolates the casing from the surrounding formation. To ensure the proper application of cement in wells, a thorough understanding of its mechanical properties is essential. Latex-modified cement stone (LMCS) offers significant advantages due to its anti-channeling, anti-corrosion, and mechanical characteristics. This study examined the mechanical properties of LMCS through uniaxial and triaxial compression and Brazilian splitting tests. Under uniaxial compression, the elastic modulus, Poisson's ratio, and compressive strength of LMCS were found to range from 4.08 to 8.29 GPa, 0.05 to 0.46, and 15.82 to 22.21 MPa, respectively. In triaxial compression tests with confining pressures of 2 MPa, 4 MPa, 6 MPa, 8 MPa, and 10 MPa, the elastic modulus ranged from 4.48 to 6.87 GPa, Poisson's ratio from 0.05 to 0.16, and compressive strength from 27.38 to 39.58 MPa. The tensile strength of LMCS ranged from 2.34 to 3.72 MPa. Moreover, the compressive strength of LMCS increased with confining pressure, showing enhanced resistance to failure due to the confining effect. However, the rate of increase gradually diminished. Strength criteria for LMCS, including Mohr-Coulomb and Drucker-Prager parameters, were derived from the triaxial compression tests. These strength criteria parameters provide a useful reference for developing the constitutive model of LMCS and for simulating triaxial compression conditions. The findings of this research offer valuable insights that can guide the construction of oil and gas wells.

Calculation method and evolution rule of the strain energy density of sandstone under true triaxial compression

Deep rock masses are typically in complex stress states, and research on the evolution of their strain energy density is of highly important for understanding their failure characteristics. In this work, a true triaxial stress‒balanced unloading test is designed to analyze the ud and ue evolution of sandstone under true triaxial compression conditions. The study results indicate that as σ1 increases, the elastic strain decreases in the σ2 and σ3 directions, whereas the residual strain progressively increases, and the magnitude of decrease in elastic strain exceeds the magnitude of increase in residual strain. Throughout the loading process of σ1, ue progressively decreases in the σ2 and σ3 directions, whereas ud gradually increases, and the magnitude of decrease in ue surpasses the magnitude of increase in ud. The ud and ue of sandstone under different stress levels were calculated via true triaxial stress‒balanced unloading tests, and the evolution of ud and ue in the three principal stress directions and the overall strain energy density of sandstone followed a linear energy storage law. On the basis of this law and the true triaxial stress‒balanced unloading test, a new method for calculating the true triaxial ud and ue was proposed. A study on the σ1 unloading stress path revealed that the σ1 unloading stress path significantly affects the storage and dissipation of the strain energy density in the three principal stress directions of sandstone. On the basis of the research results, the criteria for determining rockbursts were discussed.

STUDIES OF THE EFFECT OF REINFORCEMENT WITH GEOSYNTHETIC MATERIALS ON THE STRENGTH OF SOILS UNDER CONDITIONS OF TRIAXIAL COMPRESSION AND SINGLE-PLANE SECTION

The study is a comprehensive analysis of the effect of geosynthetic materials on the strength characteristics of soils under triaxial compression and single plane shear conditions. The triaxial compression method is used to investigate the complex effects on soil specimens, which reflects real conditions and ensures the reliability of the results.The aim of the study is to evaluate the effectiveness of geosynthetic materials as a means of soil reinforcement. Particular attention is paid to analysing changes in the mechanical properties of soils reinforced with different types of geosynthetics compared to unreinforced samples. The study includes laboratory tests, during which the parameters of strength, deformability and stability of soils under different loads and variations of test conditions are evaluated.The results of the study provide important information for specialists in geotechnical design and construction. They help to evaluate the effectiveness of geosynthetic materials in strengthening soil structures under high loads and complex conditions.Additionally, the study includes an analysis of the effect of geogrid on the strength of weak soils, which allows to identify optimal strengthening options and compare their effectiveness.Thus, the study is a relevant contribution to the development of knowledge on the interaction between geosynthetic materials and soil structures, contributes to the optimisation of design and increases the durability of soil structures

Numerical simulation of mechanical properties and damage process of carbon nanotube concrete under triaxial compression and splitting conditions

To investigate the mechanical properties and failure processes of carbon nanotube concrete, a three-dimensional meso-scale finite element model of concrete was constructed. Using ABAQUS software, numerical simulations were conducted on concrete samples with different carbon nanotube concentrations under triaxial compression and splitting conditions. The results indicate that under splitting conditions, the failure of the specimens initially occurs along the loading direction from both ends, with few cracks and slow propagation. Subsequently, the cracks rapidly expand, penetrating the entire specimen, leading to the formation of a macro-fracture surface with a crack aligned with the loading direction. This failure mode manifests as a straight crack. At the same loading rate, the tensile strength of the specimens with different carbon nanotube concentrations increases to varying degrees during splitting failure, with a maximum increase of 45.10% to 4.15 MPa. nder triaxial compression conditions, the main failure mode of the specimens exhibits shear failure characteristics. As the confining pressure gradually increases, the failure angle of the specimens enlarges. Under low confining pressure, there are fewer wing-shaped tensile cracks at the shear failure surface of the specimens. As the confining pressure increases, irregular shear failures and multiple shear planes appear in the specimens, resulting in more complex failure modes. For specimens with the same carbon nanotube concentration, the triaxial compressive strength of carbon nanotube concrete specimens increases with increasing confining pressure. The triaxial compressive strengths of the specimens under confining pressures of 5 MPa, 10 MPa, 15 MPa, and 20 MPa are 90.48 MPa, 122.72 MPa, 137.22 MPa, and 172.65 MPa, with strength increases of 35.63%, 51.66%, and 90.81%, respectively.

Energy evolution and damage deformation behavior of cemented broken coal specimen under triaxial compression condition

Coal is a major energy source, and deep coal mining often leads to fractured coal around roadways. Grouting is a common method used to reinforce fractured rock masses, where the slurry encases coal particles, forming a cemented broken coal (CBC) body. The content of coal particles (CP) significantly affects the energy characteristics of CBC specimens. To investigate this, a series of triaxial compression tests and triaxial unloading tests were conducted on CBC specimens with varying CP contents. The results indicate that as CP content increases, both peak strength and cohesion decrease, while the internal friction angle increases in both tests. In the triaxial unloading tests, specimens with higher CP content showed tensile-shear failure. Additionally, energy trends during damage were similar in both tests. In triaxial compression tests, dissipated energy increased with CP content, while in the triaxial unloading test, it initially rose and then fell. During the unloading phase of the triaxial unloading tests, the CBC specimen with 40 % CP content exhibited the most significant change in energy increment and its rate. Analysis of damage and deformation characteristics indicated that damage, deformation, and dilation were effectively restrained in CBC specimens with 40 % CP content during unloading.

Исследование физико-механических свойств ийолит-уртита в условиях одноосного и трехосного сжатия

The study of deformation and failure processes in rock masses is often carried out in laboratory conditions on samples. Widely used test equipment is used for this purpose, which makes it possible to create loading conditions similar to those inside the rock mass. Basically, the loading mode is implemented when forces are applied only to one side of the sample (along one axis), that is, uniaxial compression or tension. Test equipment that allows creation of loading along three axes, i.e. triaxial compression, is used less frequently. Such equipment is often very expensive, and testing requires a lot of time and resources. Nevertheless, this loading mode is one of the most interesting, since it is close to real conditions in the rock mass. This paper presents the results of experimental studies of ijolite-urtite samples (rocks from the Khibiny massif, the Kola Region) in conditions of uniaxial and triaxial compression. The purpose of the research is to establish how the values of the compressive strength of this rock change, to assess the critical values of the specific strain energy, as well as the tendency to dynamic destruction during the transition from the uniaxial loading mode to the triaxial one. The experimental studies revealed that there exists a sharper increase in the values of compressive strength and the critical values of specific strain energy of the ijolite-urtite samples in conditions of triaxial compression than under uniaxial compression. It has been established that the studied rock is prone to dynamic failure under uniaxial compression and preserve its tendency to this type of failure under triaxial compression.

Compaction localization in geomaterials: A mechanically consistent failure criterion

Compaction bands play a key role in the deformation processes of porous rocks and explain different aspects of physical processes in geological formations. The state-of-the-art description of the localized strains that lead to compaction banding has limitations from the mechanical point of view. Thus, we describe the phenomenon using a consistent axiomatic formulation. We build a viscoplastic model using minimal assumptions; we base our model on six principles to study compaction band strain localization triggered by viscous effects. We analyze different stress states to determine the conditions that trigger compaction bands. Laboratory experiments show that a material undergoes different strain localizations depending on the confinement pressure; thus, we perform a series of numerical experiments that reproduce these phenomena under varying triaxial compression conditions. These simulations use a simple viscoplastic constitutive model for creep based on Perzyna’s viscoplasticity and show how confinement changes the strain localization type for different triaxial tests.

Expanded pore-emanated cracking model for brittle failure of sedimentary rocks under triaxial compression

Brittle fracture and its relationship to deformation and strength have been a fundamental area of research in rock mechanics. This paper presents an expanded pore-emanated cracking model to better understand the fracture behaviors and predict the compressive strength of sedimentary rocks. This proposed model is developed to account for a triaxial compression condition using the triaxial compression test results on sandstone, limestone and siltstone samples from Wyoming, USA and experimental data on sedimentary rocks collected from published literature. The normalized critical crack length is determined from the proposed model through which the peak compressive strength is estimated when the stress intensity at the crack tip reaches a critical value called the fracture toughness. Results indicate that the rock porosity and pore radius have an inverse relationship with the compressive strength. Adopting the porosity-permeability relationship, the pore radius is calculated in terms of porosity and grain size. Next, the effect of grain size is implicitly included in the model and negatively correlated with the compressive strength. Moreover, a new approach is proposed for the estimation of fracture toughness based on the pore radius and confining pressure. The predicted compressive strengths from the proposed model show a good agreement with the measured strengths with a mean bias (i.e. average ratio of the measured to predicted strengths) of 1.014. The influence of ϕ and KIC on σ1 was thoroughly studied using parametric study. The study concludes that the effect of ϕ is more prominent than KIC on σ1. At a constant porosity of 0.1, the stress ratio decreases from 0.0082 to 0.0078 when KIC increases from 0.1 to 0.2, indicating a 5% decrease in stress ratio. Whereas, at a constant KIC of 0.1, the stress ratio increases from 0.0082 to 0.014 when the porosity increases from 0.1 to 0.2, indicating 71% increases in stress ratio and therefore compressive strength.

Study on failure characteristics and evaluation index of aquifer shale based on energy evolution

AbstractThe presence of abundant clay components and microporous structure in shale results in its high hydrophilicity, making a water-rich environment inevitable in petroleum exploration projects. Therefore, it is crucial to consider the influence of bedding structure, moisture content, confining pressure, and their combined effects on the geomechanical properties of shale. This article aims to investigate the mechanical properties of deep shale under varying water content conditions, elucidate the failure mode and failure mechanism of shale in actual engineering scenarios, and explores the interplay between stress, structure, moisture content, and other factors on its mechanical properties. The evaluation of wellbore stability and fracture propagation effects is proposed based on laboratory experiments using triaxial stress and strain data, along with the application of energy evolution theory. The experimental procedures encompass an analysis of shale's microscopic components and structure, as well as anisotropic shale triaxial compression tests conducted under different moisture contents and confining pressures. The results demonstrate that shale exhibits dense pores in its microstructure and displays pronounced anisotropic characteristics in its macrostructure. The presence of water within these pores, combined with the in situ stress within the formation, significantly influences the mechanical properties of shale. This anisotropy decreases with increasing moisture content, but the mechanical performance still decreases. Under triaxial compression conditions, the increase in confining pressure to some extent enhances the anisotropy of shale's deformation characteristics, which is related to the failure modes of shale. However, the detrimental effect of moisture content on shale's mechanical properties still persists. In order to quantify the impact of these factors, this study utilizes the elastic modulus as an indicator of the coupling effect. It combines the triaxial strain curve obtained from laboratory tests and proposes an evaluation index for shale mechanical properties based on the energy evolution theory. This index is suitable for assessing wellbore stability (the stability index called SIr) and crack expansion (the brittleness index called BIr). The calculation results reveal that, during the wellbore drilling process, excavating parallel to the direction of shale bedding while maintaining low moisture content and high confining pressure yields a higher SIr value, indicating better wellbore stability. On the other hand, during reservoir fracturing, fracturing perpendicular to the shale bedding direction and maintaining low confining pressure and moisture content result in a smaller BIr value. This approach is more beneficial for the expansion of shale fracture network in engineering.

Numerical investigation on the fracture and mechanical behaviors of marble containing two parallel fissures under triaxial compression conditions using FDEM

This paper aims to investigate the fracture and mechanical behaviors of fissured marble under triaxial compression. The two-dimensional models of marble with two parallel fissures and various rock bridge angles were established, and then the numerical simulations considering the effect of confining pressure were performed with the finite-discrete element method. The brittle-ductile transition and stress multi-peak characteristics are presented with the increase of the confining pressure. The characteristic strain, stress, and elastic strain energy exhibit various trends with the increment of the rock bridge angle, whilst increasing trend with the increment of the confining pressure. The cohesion presents the variation of first increasing and then decreasing with the increase of the rock bridge angle, and the friction angle presents the opposite trend to the cohesion. The proposed strength model can reflect the evolutions of the cohesion and the friction angle considering the effect of confining pressure and rock bridge angle. The elastic strain energy has the trends of nonlinear increase and fluctuation in the pre-peak and post-peak phases, respectively. The variation of kinetic energy is induced by the internal local fracture during the progressive failure. The failure pattern transforms from the tensile failure to the shear failure with the increase of the confining pressure. The coalescence behavior of cracks at the rock bridge can be classified into four modes.

A generalized nonlinear three-dimensional Hoek‒Brown failure criterion

To understand the strengths of rocks under complex stress states, a generalized nonlinear three-dimensional (3D) Hoek‒Brown failure (NGHB) criterion was proposed in this study. This criterion shares the same parameters with the generalized HB (GHB) criterion and inherits the parameter advantages of GHB. Two new parameters, β, and n, were introduced into the NGHB criterion that primarily controls the deviatoric plane shape of the NGHB criterion under triaxial tension and compression, respectively. The NGHB criterion can consider the influence of intermediate principal stress (IPS), where the deviatoric plane shape satisfies the smoothness requirements, while the HB criterion not. This criterion can degenerate into the two modified 3D HB criteria, the Priest criterion under triaxial compression condition and the HB criterion under triaxial compression and tension condition. This criterion was verified using true triaxial test data for different parameters, six types of rocks, and two kinds of in situ rock masses. For comparison, three existing 3D HB criteria were selected for performance comparison research. The result showed that the NGHB criterion gave better prediction performance than other criteria. The prediction errors of the strength of six types of rocks and two kinds of in situ rock masses were in the range of 2.0724%–3.5091% and 1.0144%–3.2321%, respectively. The proposed criterion lays a preliminary theoretical foundation for prediction of engineering rock mass strength under complex in situ stress conditions.

Fracture propagation and failure mode characteristics of lamellar lacustrine shale under true triaxial compression conditions

Fracture propagation and failure mode characteristics of lamellar lacustrine shale under true triaxial compression conditions

Brittleness index evaluation of gas-bearing sandstone under triaxial compression conditions

Brittleness is a critical property in gas-bearing rock excavation and reservoir fracturing. This paper analyzes the three types of brittleness indices in detail, proposes new indices, and conducts relevant conventional triaxial tests for analysis. A stress–strain curve-based brittleness index would be appropriate for the evaluation of brittleness if the parameters represented unique and certain stress–strain curves. Therefore, a new brittleness index is proposed. It consists of two components: the stress variation index and the strain variation index, which reflects the strain ratio of the stress drop and the relationship between elastic and post-peak strains, combining the stress variation and strain variation in the pre- and post-peak phases. Triaxial compression tests of sandstone under different confining pressures and gas pressures were conducted, and specific stress and strain parameters were collected. Based on the comparison and analysis of various brittleness indices, it is found that the newly proposed brittleness index can be a good measure of the brittle evolution of sandstones under different confining pressures and gas pressures, and it can provide a more accurate estimate of rock brittleness than the existing indices. The index is also used to further investigate the relationship between rock brittleness and confining and gas pressures. This has important implications for the assessment of rock brittleness in practical engineering.

Space-time evolution characteristics of loaded gas-bearing coal fractures based on industrial μCT

Abstract The space-time evolution characteristics of fractures in coal seams under external loads are of great significance for the exploration and development of coalbed methane and the safe mining of coal. In this study, the in situ industrial μCT scanning experiments of gas-bearing coal under triaxial compression conditions was carried out using a loaded coal rock industrial μCT scanning system. The computed tomography (CT) scan images of different deformation stages were obtained. The temporal characteristics and spatial evolution patterns of fractures during gas-bearing coal rupture were investigated by combining image retrieval techniques and fractal theory. The results show that (1) as the axial load increases, the maximum similarity of the CT images showed a gradually decreasing trend. (2) Under the triaxial compression conditions, the number of two-dimensional cracks in a coal-containing gas at different spatial positions exhibited a changing pattern in which it slowly decreased before rapidly increasing. (3) The patterns of change in the three-dimensional fracture rate, fracture density, and fractal dimension were quite similar, showing four stages of slow decline, slow growth, sharp growth, and slow growth again. In contrast, the Euler number exhibited the exact opposite pattern of change. (4) The coal sample space showed prominent zonal failure characteristics. The bottom of the coal sample space position (Area C) near the loading indenter had the most developed cracks and serious damage, the middle of the coal sample space position (Area B) was the second, and the upper of the coal sample space position (Area A) near the fixed indenter had slow fracture development and minimal damage.

Viscoelastic Strains of Palaeozoic Shales under the Burger’s Model Description

This article presents the results of creep studies of Palaeozoic shales from the Baltic Basin in which the exploitation of shale gas in Poland was planned. Knowledge of instantaneous and long-term properties investigated in triaxial stress conditions is important from the point of view of exploitation techniques related to hydraulic fracturing. Rheological phenomena also play an important role in the analysis of the initial stress in shales, the knowledge of which is indispensable in the hydraulic fracturing process. The tests were carried out on samples representing four siltstone–claystone lithostratigraphic units occurring in the Baltic Basin. The studies and analyses were aimed at determining the character of creep in shales, selection of the appropriate rheological model for the analyzed rocks, and determination of the threshold of the linear creep under triaxial compression conditions. An original approach together with analysis results are presented here, which enable the separation and monitoring of shear and volume creep effects, and on this basis, the determination of the significance of the contribution of volume creep in the entire creep process. A relatively simple methodology for determination of the parameters of the Burgers model using this division is presented. The original value of the article is also due to the test results themselves and the parameter values of the analyzed model for triaxial creep of shales, which are not numerous in the literature. The investigations were performed at various loading levels in relation to the triaxial strength of the shales. Depending on the load, at its low values up to 0.7 (σ1 − σ3)max, creep had a determined character and did not show features of progressive creep. The linear creep threshold was also analyzed in this range. The loading level of 0.7 (σ1 − σ3)max was the limit of linear creep. Exceeding this load resulted in the loss of the linear character of creep, which in consequence lead to the subsequent third creep phase ending with rock damage. Parameters of the Burger’s model for gas shales from the Baltic Basin (northern Poland) were identified. There are significant differences in the behavior of shales depending on the lithostratigraphic unit from which the samples were collected. The mineral composition of the shales also influenced their behavior.

An Experimental Investigation on the Foliation Strike-Angle Effect of Layered Hard Rock under Engineering Triaxial Stress Path.

Deep underground engineering encounters substantial layered hard rock formations, and the engineering triaxial stress path involves an increase in maximum principal stress, constant intermediate principal stress, and a decrease in minimum principal stress. However, previous research has focused on rock layer angles under conventional triaxial stress conditions, disregarding the influence of foliation strike angles in engineering triaxial stress scenarios. This study experimentally investigates the effects of foliation strike angles on layered hard rock under an engineering triaxial stress path. To account for the brittleness of layered hard rock, we propose a specific small sample-processing method tailored to the foliation strike angle. True triaxial loading tests are conducted on steep, thin slate samples with two different loading orientations, accompanied by acoustic emission monitoring. Results indicate that the strength under a traditional true triaxial compression condition is similar for specimens with 90° and 0° strike angles. Stress-strain curves show that larger deformations occur perpendicular to bedding planes, while surface fractures propagate exclusively along the bedding planes. Mechanical responses differ significantly between specimens subjected to the engineering triaxial stress path at 0° and 90° strike angles compared to conventional true triaxial loading tests, with a lower bearing capacity and differentiated intermediate and minimum principal strains in the 0° case. Conversely, the 90° case exhibits a higher bearing capacity, consistent deformation, and more acoustic emission events. Numerical simulations comparing plastic zone sizes during actual underground excavation support these conclusions. These findings highlight the effects of foliation strike angles, favoring the 90° strike-angle configuration for excavation activities and providing enhanced stability in the surrounding rock mass.

Critical State Characterization of New Zealand East Coast Sand for Numerical Modeling

This paper outlines the findings of a laboratory-based and numerical study to investigate the undrained flow failure behavior of East Coast Sand (ECS). ECS is a commonly encountered coastal deposit from the upper North Island of New Zealand. The study focused on establishing the undrained strength characteristics of ECS under static, triaxial compressive loading conditions, and at confining pressures in the range of typical engineering interest and for a range of soil densities considered in loosely deposited sands. The research objectives of establishing the basic soil properties and the intrinsic advanced geomechanical properties specific to ECS from Auckland were achieved through laboratory experiments and matching numerical simulations with an advanced critical-state compatible soil constitutive model (Norsand). The current work examined five different aspects of the ECS undrained behavior under static loads. It was shown that loosely deposited ECS within mean effective stresses ranging between 50 and 200 kPa was highly susceptible to expensive flow failures of structures built on or with them. The obtained approximate peak undrained shear strengths before failure and critical states were 29 kPa, 84 kPa, 130 kPa, and 200 kPa for test confining stresses of 50 kPa, 100 kPa, 200 kPa, and 300 kPa, respectively. Similarly, the corresponding excess pore water pressures were 48 kPa, 98 kPa, 200 kPa, and 240 kPa, respectively. The above results proved that the soil’s effective and confining stress are key determinants of the soil’s undrained shear strength characteristics which was consistent with the existing literature.

Investigation of Damage and Creep for Bedding's Carbonaceous Slate with Chemical Erosion Effect.

Tunnel projects in the southwestern mountainous area of China are in full swing. According to the tunnel burial depth, structural characteristics, and chemical erosion environments of the Lixiang railway tunnel, carbonaceous slate specimens obtained in the field were taken to experimentally investigate the physical, mechanical, and creep characteristics of the bedding's slate specimens after chemical erosion. The results of scanning electron microscopy (SEM) indicate that chemical erosion leads to internal damage in the carbonaceous slate specimens, and the internal damages are increasing with the increase of erosion days. Moreover, the specimens' ultrasonic test (UT) results prove that specimens with smaller bedding angles suffer a more serious erosion and induce more internal cracks. Under conventional triaxial compression conditions with 40 MPa of confining pressures, the conventional triaxial compressive strength (σs) decreases with the decrease of the bedding angle and the increase of erosion days, and the failure modes of the specimens are mainly controlled by oblique shear fractures and accompanied by the occurrence of slip dislocation fractures between the bedding inclination. Under creep conditions with 40 MPa of confining pressures, the final deformations of specimens are increasing with the increase of erosion days, which means the longer the erosion days, the greater the deformations. The failure modes of the specimens under creep conditions are controlled by shear fractures, and for the specimen with a 60° bedding angle and long-term erosion, there are block separations and many cavities along the shear planes. Therefore, more attention should be paid to prevent serious failure of the surrounding rock if the surrounding rock has a bedding angle of 60° or suffers long-term erosion.

Cracking property and brittleness evaluation of granite under high-temperature true triaxial compression in geothermal systems

Cracking property and brittleness are critically important to the drillability of injection and production wells of enhanced geothermal systems. This paper is devoted to evaluating the cracking property and brittleness of the Gonghe granite under high-temperature true triaxial compression conditions through a series of laboratory tests. Thermal–mechanical coupled true triaxial compression tests were conducted on Gonghe granite samples under four representative temperatures (20 °C, 120 °C, 180 °C, and 240 °C) and three different minimum (5, 10, and 30 MPa) and intermediate (40, 60, and 75 MPa) principal stresses that correspond to the in-situ conditions of Gonghe geothermal reservoir. The strength, deformation, and cracking characteristics of the Gonghe granite are quantitatively evaluated from macro- to micro-scales based on the experimental results. Meanwhile, a novel brittleness index evaluation method considering both energy and failure surface roughness is proposed to accurately assess the brittleness strength of the Gonghe granite. We found that the strength of Gonghe granite is reduced by more than 20% when the temperature increases from room temperature to 240 °C. Higher temperature contributes to a smoother fracture surface and reduced brittleness of the Gonghe granite under thermal–mechanical coupled true triaxial compression. Our findings provide new insights for brittleness evaluation of the Gonghe granite formation and assist in efficient wells drilling design of enhanced geothermal systems.

A Hybrid Anisotropic Elastoplastic Model for Layered Rock Mass and Numerical Implementation

An elastoplastic model that exhibits hybrid initial and stress-induced anisotropy is developed for layered rock masses. The formulation considers the thermodynamic aspect, and is consistent and rigorous. Both initial anisotropy and stress-induced deformation anisotropy are reflected by a hybrid anisotropic stiffness matrix influenced by the deterioration development degree (DDD) in different directions. By employing a combination of the strength criterion of rock material and rock bedding plane, an anisotropic failure formulation for layered rock mass has been established. The hybrid anisotropic model has been implemented in cellular automata software for the engineering rock mass fracturing process (CASRock). The performance of the anisotropic part is demonstrated by reproducing the deformation and failure characteristics of initial or stress-induced anisotropic behaviors for layered rocks under uniaxial, conventional triaxial, and true triaxial compression and Brazilian splitting conditions. Important features, such as the strength, mechanism, deformation, DDD, and fracturing process variation, can be captured by the proposed model. In addition, a numerical simulation of tunnel excavation in a layered rock mass is performed to study the anisotropic excavation-induced damage zone (EDZ) distribution in the field. The results indicate that the model is able to reproduce the observed failure mode satisfactorily.