Rock fracture toughness is a critical parameter for optimizing reservoir stimulation during deep resource extraction. This index characterizes the in situ resistance of rocks to fracture and is affected by high temperature, in situ stress, thermal shock, and chemical corrosion, etc. This review comprehensively examines research on rock fracture properties in situ environments over the past 20 years, analyses the influences of various environmental factors on rock fracture, and draws the following conclusions: (i) Environmental factors can significantly affect rock fracture toughness through changing the internal microstructure and grain composition of rocks; (ii) While high temperature is believed to reduce the rock strength, several studies have observed an increase in rock fracture toughness with increasing temperature, particularly in the range between room temperature and 200 degrees C; (iii) In addition to a synergistic increase in fracture toughness induced by both high temperature and high in situ stress, there is still a competing effect between the increase induced by high in situ stress and the decrease induced by high temperature; (iv) Thermal shock from liquid nitrogen cooling, producing high temperature gradients, can surprisingly increase the fracture toughness of some rocks, especially at initial temperatures between room temperature and 200 degrees C; and (v) Deterioration of rock fracture toughness occurs more rapidly in acidic environments than that in alkaline environments. In addition, this review identified current research trends and suggested some potential directions to provide suggestions for deep subsurface resource extraction. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/

Rapid and partial acquisition are features of rock drilling for obtaining rock properties. Most previous research has primarily concentrated on how to quickly obtain rock mechanics parameters, with limited emphasis on extracting rock parameter fields, particularly in three dimensions. This study attempted to develop a numerical integrated method to extract 3D parameter fields of rocks based on a newly developed digital-controlled drilling platform. The importance of incorporating a damage model for accurate simulations of rock drilling through finite element analysis (FEA) was investigated. By calibrating damage parameters through uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) tests, these parameters can be considered constants in rock drilling simulations across various rock types. The accuracy of rock parameters estimated by the proposed method and the derived analytical model were further demonstrated through comparison with the corresponding standard tests. Furthermore, the 3D parameter field of rocks was obtained by integrating a deep learning method and micro-CT technology. The numerical prediction illustrated the advantages of acquiring a rock parameter field in achieving more accurate simulations of the rock failure process. Besides, our solution can also provide support for the parameter selection of numerical models considering spatial variability for natural rocks. A digital-controlled equipment for rock drilling was developed.Equations were derived for extracting rock parameters through the drilling data.A deep learning method was developed to reproduce the three-dimensional parameter field of rocks.The significance of considering the realistic rock parameter field was numerically demonstrated.

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, b, 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. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V.

To study the microscopic structure, thermal and mechanical properties of sandstones under the influence of temperature, coal measure sandstones from Southwest China are adopted as the research object to carry out high-temperature tests at 25 degrees C-1000 degrees C. The microscopic images of sandstone after thermal treatment are obtained by means of polarizing microscopy and scanning electron microscopy (SEM). Based on thermogravimetric (TG) analysis and differential scanning calorimetric (DSC) analysis, the model function of coal measure sandstone is explored through thermal analysis kinetics (TAK) theory, and the kinetic parameters of thermal decomposition and the thermal decomposition reaction rate of rock are studied. Through the uniaxial compression experiments, the stress-strain curves and strength characteristics of sandstone under the influence of temperature are obtained. The results show that the temperature has a significant effect on the microstructure, mineral composition and mechanical properties of sandstone. In particular, when the temperature exceeds 400 degrees C, the thermal fracture phenomenon of rock is obvious, the activity of activated molecules is significantly enhanced, and the kinetic phenomenon of the thermal decomposition reaction of rock appears rapidly. The mechanical properties of rock are weakened under the influence of rock thermal fracture and mineral thermal decomposition. These research results can provide a reference for the analysis of surrounding rock stability and the control of disasters caused by thermal damage in areas such as underground coal gasification (UCG) channels and rock masses subjected to mine fires. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Strength theory is the basic theory for calculating and designing the strength of engineering materials in civil, hydraulic, mechanical, aerospace, military, and other engineering disciplines. Therefore, the comprehensive study of the generalized nonlinear strength theory (GNST) of geomaterials has significance for the construction of engineering rock strength. This paper reviews the GNST of geomaterials to demonstrate the research status of nonlinear strength characteristics of geomaterials under complex stress paths. First, it systematically summarizes the research progress of GNST (classical and empirical criteria). Then, the latest research the authors conducted over the past five years on the GNST is introduced, and a generalized three-dimensional (3D) nonlinear Hoek-Brown (HB) criterion (NGHB criterion) is proposed for practical applications. This criterion can be degenerated into the existing three modified HB criteria and has a better prediction performance. The strength prediction errors for six rocks and two in-situ rock masses are 2.0724%-3.5091% and 1.0144%-3.2321%, respectively. Finally, the development and outlook of the GNST are expounded, and a new topic about the building strength index of rock mass and determining the strength of in-situ engineering rock mass is proposed. The summarization of the GNST provides theoretical traceability and optimization for constructing in-situ engineering rock mass strength.

Surrounding rocks of underground engineering are subjected to long-term seepage pressure, which can deteriorate the mechanical properties and cause serious disasters. In order to understand the impact of seepage pressure on the mechanical property of sandstone, uniaxial compression tests, P-wave velocity measurements, and nuclear magnetic resonance (NMR) tests were conducted on saturated sandstone samples with varied seepage pressures (i.e. 0 MPa, 3 MPa, 4 MPa, 5 MPa, 6 MPa, 7 MPa). The results demonstrate that the mechanical parameters (uniaxial compressive strength, peak strain, elastic modulus, and brittleness index), total energy, elastic strain energy, as well as elastic strain energy ratio, decrease with increasing seepage pressure, while the dissipation energy and dissipation energy ratio increase. Moreover, as seepage pressure increases, the micro-pores gradually transform into meso-pores and macro-pores. This increases the cumulative porosity of sandstone and decreases P-wave velocity. The numerical results indicate that as seepage pressure rises, the number of tensile cracks increases progressively, the angle range of microcracks is basically from 50 degrees-120 degrees to 80 degrees-100 degrees, and as a result, the failure mode transforms to the tensile-shear mixed failure mode. Finally, the effects of seepage pressure on mechanical properties were discussed. The results show that decrease in the effective stress and cohesion under the action of seepage pressure could lead to deterioration of strength behaviors of sandstone. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

The anisotropic mechanical behavior of rocks under high-stress and high-temperature coupled conditions is crucial for analyzing the stability of surrounding rocks in deep underground engineering. This paper is devoted to studying the anisotropic strength, deformation and failure behavior of gneiss granite from the deep boreholes of a railway tunnel that suffers from high tectonic stress and ground temperature in the eastern tectonic knot in the Tibet Plateau. High-temperature true triaxial compression tests are performed on the samples using a self-developed testing device with five different loading directions and three temperature values that are representative of the geological conditions of the deep underground tunnels in the region. Effect of temperature and loading direction on the strength, elastic modulus, Poisson 's ratio, and failure mode are analyzed. The method for quantitative identi fication of anisotropic failure is also proposed. The anisotropic mechanical behaviors of the gneiss granite are very sensitive to the changes in loading direction and temperature under true triaxial compression, and the high temperature seems to weaken the inherent anisotropy and stress-induced deformation anisotropy. The strength and deformation show obvious thermal degradation at 200 degrees C due to the weakening of friction between failure surfaces and the transition of the failure pattern in rock grains. In the range of 25 degrees C-20 0 degrees C, the failure is mainly governed by the loading direction due to the inherent anisotropy. This study is helpful to the in-depth understanding of the thermal-mechanical behavior of anisotropic rocks in deep underground projects. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

To reveal the dynamic mechanical characteristics of deep rocks, a series of impact tests under triaxial static stress states corresponding to depths of 300-2400 m were conducted. The results showed that both the strain rates and the stress environments in depth significantly affect the mechanical characteristics of rocks. The sensitivity of strain rate to the dynamic strength and deformation modulus shows a negative correlation with depth, indicating that producing penetrative cracks in deep environments is more difficult when damage occurs. The dynamic strength shows a tendency to decrease and then increase slightly, but decreases sharply finally. Transmissivity demonstrates a similar trend as that of strength, whereas reflectivity indicates the opposite trend. Furthermore, two critical depths with high dynamically induced hazard possibilities based on the China Jinping Underground Laboratory (CJPL) were proposed for deep engineering. The first critical depth is 600-900 m, beyond which the sensitivity of rock dynamic characteristics to the strain rate and restraint of circumferential stress decrease, causing instability of surrounding rocks under axial stress condition. The second one lies at 1500-1800 m, where the wave impedance and dynamic strength of deep surrounding rocks drop sharply, and the dissipation energy presents a negative value. It suggests that the dynamic instability of deep surrounding rocks can be divided into dynamic load dominant and dynamic load induced types, depending on the second critical depth. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).