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Geothermal energy and hot dry rock (HDR), as an important clean energy technology, have garnered widespread attention globally in recent years. Enhanced Geothermal Systems (EGS), a technology for extracting energy from low-permeability Hot Dry Rock (HDR) reservoirs, is crucial for the utilization of geothermal energy. Although interest in this area has significantly increased, a comprehensive and systematic analysis providing a clear understanding of the development context is still lacking. To this end, this paper presents a bibliometric analysis of 1764 relevant publications from 1996 to 2023, revealing research trends and hotspots in this field. Utilizing tools such as Bibliometrix (Version 4.2.3), CiteSpace (Version 6.2.R2), and VOSviewer (Version 1.6.19), the study analyzes publication trends, subject categories, journals, authors, institutions, and national contributions. The results indicate that EGS technology, rock mechanical behavior, and environmental impact assessment are the primary research hotspots, with China being the leading country in terms of publication volume. Future research directions include technological optimization, environmental sustainability, and the advancement of interdisciplinary collaboration. This study provides a valuable reference for further research and application in geothermal energy and HDR and offers a dynamic perspective on shifting research priorities.

Columnar jointed basalt is a composite rock mass consisting of many prismatic rock blocks with irregular polygons in the transverse section. Similar material was used to simulate a columnar jointed rock mass with different dip angles (β = 0°–90°), and uniaxial compression, triaxial compression and true triaxial unloading tests were conducted to determine the anisotropic properties, and unloading failure mechanism of jointed basalt rock. The results show that the dip angle has significant effects on the strength anisotropy of the columnar jointed rock mass. The stress–strain curves under uniaxial and true triaxial unloading indicate strain softening, and the stress–strain curves under triaxial compression reflect strain hardening. The specimen with β = 60°–75° has the lowest strength and is most sensitive to the confining pressure. The analysis of the failure mode and failure mechanism of the columnar jointed rock mass under different load conditions indicate that the failure mode can be divided into three types: stress-controlled failure, structure-controlled failure, and stress-structure-controlled failure. The peak stress growth coefficient is higher under triaxial compression than under true triaxial unloading. Finally, the applicability of different strength criteria to the columnar jointed rock mass is analyzed by evaluating the fitting accuracy and comparing the test results with measured data. Regression analysis demonstrates that the Mogi-Coulomb strength criterion is better suited to the study data than the Drucker-Prager strength criterion. The research results have reference significance for the stability analysis of excavated tunnels in columnar jointed rock masses.

In complex underground conditions, the excavation of deep foundation pits has a significant impact on the deformation of retaining structures and nearby subway stations. To investigate the influence of deep excavation on the deformation of adjacent structures, a three-dimensional numerical model of the foundation pit, existing subway station, and tunnel structure was established using FLAC 3D software, based on the Shenzhen Bay Super Headquarters C Tower foundation pit project. The study analyzed the deformation characteristics of retaining structures, adjacent subway stations, and tunnels during different stages of deep excavation, and the accuracy of the numerical simulation results was validated through field monitoring data. The results indicate that during the excavation process of the foundation pit, the lateral horizontal displacement of the retaining structure is generally small, with a typical “concave inward” lateral deformation curve; the horizontal displacement value of the contiguous wall section is less than that of the interlocking pile section. The bending moments of the retaining structure show a distribution pattern with larger values in the middle and smaller values at the top and bottom of the pit, with a relatively uniform distribution of internal support forces. The maximum displacement of the nearby subway station is 8.75 mm, and the maximum displacement of the subway tunnel is 2.29 mm. The research findings can provide references for evaluating the impact of newly built foundation pits near subway stations and contribute to the rational design and safe construction of new projects.

In view of the typical anisotropy of columnar jointed basalt in the dam area of the Baihetan Hydropower Station on the Jinsha River in China, columnar jointed rock mass models with different column angles ( α = 0°–90°) were constructed with similar materials, and three sets of intermittent joints are considered in the physical models. Uniaxial and equal-confining-pressure triaxial compression tests were carried out with a true triaxial testing machine, and the variations of the failure mechanisms, stress-strain relationships, and strength characteristics of the columnar jointed rock masses with the joint inclination angle were investigated under different stress conditions. The results revealed the following: 1) during triaxial compression, two types of stress-strain curves existed, namely the ideal plastic curve and the strain-hardening curve; 2) the strength and elastic modulus of the sample under uniaxial compression presented U-shaped curves with the variation of α . Moreover, the strength of the samples increased with the change of α under triaxial compression, and the elastic modulus E r increased significantly with the increase of the confining pressure. 3) There were four typical types of failure within the columnar joint samples under triaxial compression. In addition, the test data were used to verify that the Mogi strength criterion good applicability to the columnar jointed rock mass under three-dimensional stress. An empirical prediction model of the relationship between the elastic modulus and confining pressure of the columnar jointed rock mass was established, and its applicable conditions were analyzed. The empirical prediction model is verified by the test results of the columnar jointed rock mass, which indicates that the prediction model was in good agreement with the test data. Finally, the strength characteristics and failure modes of columnar jointed rock masses under different loading conditions were summarized.

The complex structure of the columnar jointed rock mass (CJRM) brings difficulty in determining its field mechanical parameters, and a reliable estimation of the strength and deformation of CJRM is crucial for engineering safety. Uniaxial compression tests were conducted on artificial CJRM specimens with columnar structures of quadrangular, pentagonal and hexagonal prisms to model the mechanical behaviour of the CJRM. The strength and deformation anisotropy were compared and analyzed on the basis of the results by combining the structural features of the three models. The failure modes and mechanisms were summarized in accordance with the failure processes and final appearances of artificial specimens. The empirical relations of predicting the field mechanical parameters were derived on the basis of the joint factor approach, the empirical gradient method and the rock quality designation (RQD). For the CJRM of the Baihetan Project, the proposed empirical correlations were used to estimate the field unaxial compression strength and elastic modulus, and the predicted values were compared with those obtained from the field tests and other empirical methods. Results revealed that the estimated values calculated by the proposed empirical equations are in good agreement with the field test results, and the three empirical relations can predict the mechanical parameters of the Baihetan CJRM effectively. The empirical relation based on RQD is extremely valuable when RQD is the only available information about discontinuities in site investigations.

The special structure of a columnar jointed rock mass (CJRM) leads to its complex mechanical properties, and accurately grasping the strength and failure behaviors of CJRMs under different stress conditions is crucial for engineering safety. According to the natural structure of the CJRM, the Voronoi diagram and three-dimensional (3D) printing technology were used to make irregular columnar jointed molds with different dip angles. Uniaxial, conventional triaxial, and true triaxial compression tests were conducted on the artificial irregular CJRM (ICJRM) specimens. The effects of the dip angle, intermediate principal stress and minimum principal stress on the stress–strain curve and failure strength of the ICJRM were analyzed. The failure modes and mechanisms of ICJRM specimens under different stress conditions were summarized. For three types of jointed rock masses, including ICJRM, the material parameters in the Mohr–Coulomb (MC) and Hoek–Brown (HB) criteria were calculated only from the conventional triaxial test results. Six 3D strength criteria related to MC and HB parameters were used to predict the polyaxial failure strengths of three jointed rock masses, and the prediction performances were compared. The results show that the Mogi criterion using MC parameters has the best overall prediction effect on the failure strengths of three jointed rock masses, while the modified Lade criterion is the best choice for the ICJRM.HighlightsPolyaxial compression tests, including uniaxial, conventional triaxial and true triaxial compression tests, are performed on the artificial irregular columnar jointed rock mass specimens.The effects of the dip angle, intermediate principal stress and minimum principal stress on the failure strength behavior of irregular columnar jointed rock mass are revealed.The failure modes and mechanisms of irregular columnar jointed rock mass specimens under different polyaxial stress conditions are summarized.The feasibilities of using the Mohr–Coulomb and Hoek–Brown parameters obtained from the conventional triaxial test results to solve the true triaxial failure strength of jointed rock masses, including the irregular columnar jointed rock mass, are verified and compared.

Columnar jointed rock mass (CJRM) is a highly symmetrical natural fractured structure. As the rock mass of the dam foundation of the Baihetan Hydropower Station, the study of its permeability anisotropy is of great significance to engineering safety. Based on the theory of composite mechanics and Goodman’s joint superposition principle, the constitutive model of joints of CJRM is derived according to the Quadrangular prism, the Pentagonal prism and the Hexagonal prism model; combined with Singh’s research results on intermittent joint stress concentration, considering column deflection angles, the joint constitutive model of CJRM in three-dimensional space is established. For the CJRM in the Baihetan dam site area, the Quadrangular prism, the Pentagonal prism and the Hexagonal prism constitutive models were used to calculate the permeability coefficients of CJRM under different deflection angles. The permeability anisotropy characteristics of the three models were compared and verified by numerical simulation results. The results show that the calculation results of the Pentagonal prism model are in good agreement with the numerical simulation results. The variation of permeability coefficient under different confining pressures is compared, and the relationship between permeability coefficient and confining pressure is obtained, which accords with the negative exponential function and conforms to the general rule of joint seepage.

Because of its special structure, the anisotropic properties of columnar jointed rock mass (CJRM) are complicated, which brings difficulty to engineering construction. To comprehensively study the anisotropic characteristics of CJRM, uniaxial compression tests were conducted on artificial CJRM specimens. Quadrangular, pentagonal and hexagonal prism CJRM models were introduced, and the dip direction of the columnar joints was considered. Based on the test results and the structural features of the three CJRM models, the deformation and strength characteristics of CJRM specimens were analyzed and compared. The failure modes and mechanisms of artificial specimens with different dip directions were summarized in accordance with the failure processes and final appearances. Subsequently, the anisotropic degrees of the three CJRM models in the horizontal plane were classified, and their anisotropic characteristics were described. Finally, a simple empirical expression was adopted to estimate the strength and deformation of the CJRM, and the derived equations were used in the Baihetan Hydropower Station project. The calculated values are in good agreement with the existing research results, which reflects the engineering application value of the derived empirical equations.

In this work, an anisotropic constitutive model of hexagonal columnar jointed rock masses is established to describe the distribution law of deformation and the failure of columnar joint caverns under anisotropic conditions, and is implemented to study the columnar jointed rock mass at the dam site of the Baihetan Hydropower Station on the Jinsha River. The model is based on the Cosserat theory and considers the mesoscopic bending effect on the macroscopic mean. The influences of joint plane inclination on equivalent anisotropic elastic parameters are discussed via the introduction of an off-axis transformation matrix and the analysis of an example. It is also pointed out that the six-prism columnar jointed rock mass changes from transverse isotropy to anisotropy under the influence of the angle. A numerical calculation program of the Cosserat constitutive model is developed and is applied to the simulation calculation of a Baihetan diversion tunnel to compare and analyze the respective plastic zones and stress distributions after tunnel excavation under both isotropic and anisotropic conditions. The results reveal that, compared with the isotropic model, the proposed Cosserat anisotropic model better reflects the state of stress and asymmetric distribution of the plastic zone after tunnel excavation, and the actual deformation of the surrounding rock of the tunnel is greater than that calculated by the isotropic method. The results aid in a better understanding of the mechanical properties of rock masses.

Infilling fractured rock masses are widely distributed in the deeply buried oil reservoirs and surrounding rocks of mine caves. The internal filling material has a great influence on the mechanical properties and seepage characteristics of fractured rock mass. In this paper, through theories and experiments, the mechanism of permeability changes of infilling fractured rock under a coupling condition is studied. In terms of theory, the fracture compaction effect coefficient δ is added to the classical matchstick model, and the volume strain principle is used to propose a permeability model for fractured rock. Furthermore, based on the Hertz contact theory, mineral particles are generalized into rigid spheres, and the mechanism of crack development between mineral particles under seepage pressure is analyzed. In terms of experiment, a true triaxial seepage test was carried out on rock-like specimens to obtain the change law of the permeability characteristics of fractured rock. The test results are largely consistent with the theoretical calculation results of the theoretical model, which verifies the applicability of the model proposed in this paper. After the loading failure of the specimen, the internal filling material was taken out and analyzed, and by observing the distribution of cracks on the surface, it is verified that the seepage pressure promotes the development of cracks in the filling fracture.