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This study introduces a novel large three-dimensional physical model experimental system (PMES) for studying the geomechanical problems related to deep underground engineering. The apparatus is mainly composed of a counterforce frame, a hydraulic loading subsystem, a sample installation and transportation subsystem, an excavation subsystem, an integrated monitoring subsystem, and auxiliary facilities. During the design and development of the PMES apparatus, key technological issues were addressed. A high-stiffness frame structure with opening and closing function was developed. Uniform, gradient, and designed non-uniform stresses can be applied with high accuracy. An independent long-term load-holding unit and a surface dynamic disturbance unit were designed. The basic and innovative functions of the apparatus were verified through calibration experiments. A large physical model experiment was conducted to simulate the excavation of a deep-buried drainage tunnel in China's Jinping-II hydropower station. The experiment obtained the failure processes of the surrounding rock masses induced by high in-situ stress, specific engineering geological structures and excavation disturbance. The internal fracturing and strain evolution within the sample were successfully captured by the integrated monitoring subsystem. The results also validated the reliability of this novel apparatus. The development of this novel apparatus provides a new tool for the study of geomechanical problems in deep underground engineering. Highlights: A novel large three-dimensional physical model experimental system for studying the geomechanical problems related to deep underground engineering was established. The basic and innovative functions of the apparatus were verified through calibration experiments. A large physical model experiment was conducted to simulate the excavation of a deep tunnel in China's Jinping-II hydropower station. [ABSTRACT FROM AUTHOR]

Heat transfer in jointed rock masses exhibits characteristics of discontinuous, heterogeneous, and anisotropic due to the effects of rock discontinuities. In this work, thermal contact resistance (TCR) evolution of granite joints under normal stress was investigated. A novel method was proposed for the measurement of TCR for rough rock joints. A numerical calculation program was established for the determination of contact area based on the initial morphology of the joint surfaces and the monitored joint closure during the experiment. An estimation model of TCR for granite joints based on the equivalent joint aperture was established. It is found that TCR of granite joints shows decreasing trend with normal stress. The decreasing rate is relatively large at the initial loading stage and gradually reduces with normal stress. The values of TCR eventually stabilize. TCR of granite joint has no obvious correlation with joint roughness coefficient. It is more significantly affected by the contact area. Plastic deformation and rupture of the micro-convex body on the joint surfaces occurred due to the application of normal stress, which improve the heat transfer efficiency and lead to the decrease in TCR. In general, predication results of the TCR estimation model show good consistency with the experimental data. The model is more suitable for the cases with small joint apertures. This study can provide knowledge to the better understanding of heat transfer in rock masses containing different kinds of discontinuities. Highlights: A novel method was proposed for the measurement of thermal contact resistance for rough rock joints. A numerical calculation program was established for the determination of contact area for rough rock joints. Evolution of thermal contact resistance with normal stress and joint contact area was obtained and analyzed. An estimation model of thermal contact resistance for granite joints based on the equivalent joint aperture was established. [ABSTRACT FROM AUTHOR]

In this work, effective thermal conductivity (λEff) of fractured rock masses was numerically investigated. A two-dimensional Discrete Fracture Network (DFN) model of the fractured rock masses was established based on the statistic results of natural fracture development in a potential area for high level radioactive waste disposal in China. Steady state heat transfer processes in the fractured granite rock masses were numerically simulated using finite element method (FEM). The calculated λEff values of the fractured granite rock masses in dry and saturated conditions are 1.99 W/(m K) and 2.31 W/(m K), respectively. Compared with the thermal conductivity of intact granite [λIntact, 2.5 W/(m K)], the drop rates are 20.4% and 7.6%, respectively. Sensitivity analysis was conducted on the main model parameters including fracture density (FDensity), trace length (FLength), thermal contact resistance (FTCR), and λIntact. The results indicate the relation between λEff and three fracture parameters (FDensity, Flength and FTCR) can be fitted using power law or negative exponent functions with good consistency. When fracture network parameters remain unchanged, λEff is in linear positive correlation to λIntact. The slop of the fitted line is determined by the fracture network parameters. Due to the fact that distribution of generated fractures in different directions are quite uniform, λEff did not show significant difference in different directions. On the basis of the above-mentioned results, an estimation model was proposed for the determination of λEff of fractured rock masses using P21 (total length of fracture traces per unit area), FTCR, and λIntact. The proposed estimation model shows good consistency to the calculated results of FEM model. [ABSTRACT FROM AUTHOR]

In this work, the effect of mechanical damage on the thermal conductivity of granite was experimentally investigated. Granite samples were artificially damaged through cyclic loading and unloading tests, with the strain evolution and acoustic emission activity monitored simultaneously. P-Wave velocity of the damaged samples was tested to evaluate the damage effect. Thermal conductivity of the undamaged and damaged granite samples was tested using optical scanning method. The influence of mechanical damage on thermal conductivity was analyzed. Thermal conductivity difference in different testing directions was analyzed based on the experiment results in six scanning lines on the two surfaces of each disk sample. The results indicate that thermal conductivity of damaged samples reduced compared with that of the undamaged samples, while the thermal inhomogeneity factor increased. Thermal conductivity increased in water-saturated condition for both undamaged and damaged samples. The increased rate of damaged samples was bigger than that of the undamaged samples. The mechanical damage treatment caused the increase in the thermal conductivity difference in different testing directions. The difference decreased when samples were water saturated. Research results in this work can provide better knowledge to the evolution mechanism of thermal conductivity of engineering rock masses. [ABSTRACT FROM AUTHOR]

Based on the frequent occurrence of stress-induced hazards in deep rock tunnels, a physical model experimental study of the development process, failure mechanism, and onset conditions of deep hard rock spalling in horseshoe-shaped tunnels is carried out. The test process mainly includes model preparation, loading, excavation, and monitoring. A high-speed camera, distributed fiber optic sensing, acoustic emission (AE) system, and visual imaging correlation-2D (VIC-2D) techniques are integrated to monitor the mechanical response of the surrounding rocks. The test results indicate that the physical model test successfully simulates the spalling phenomena of the deep hard rock. The integrated monitoring system successfully captures the information on deformation, AE signals, and crack propagation in the instantaneous process of the surrounding rock spalling failure, which provides a sound basis for a more comprehensive understanding of the mechanism. Microcracks and dilation appear first on the sidewall surface; the cracks then continue to propagate and coalesce, forming thin rock flakes and spalling in a lamellar manner from shallow to deep. The quantitative relationships between the onset stresses, the critical strains, and the initial boundary stresses of the spalling failure are established. At the same depth from the excavation boundary, the radial strain of the sidewall is larger than that of the crown, and it decreases away from the excavation boundary to the interior. The increasing loading in vertical direction leads to the stress concentration in the shallow part at the sidewall and a sharp increase of deformation. There is a positive correlation between the spalling failure degree and the initial boundary stresses. [ABSTRACT FROM AUTHOR]

The common practice in understanding some puzzling geotechnical and geomechanical issues by large-scale three-dimensional physical model tests is recently much improved by the introduction of 3D-printing technology which can realize the reconstruction of complex geological structures. However, during 3D printing process, the regular rock-like material suffers from some general problems such as short initial setting time, water segregation, decreasing fluidity induced by chemical reaction, and some other unclear influencing factors. To promote further development of physical model via 3D printing method and to fabricate large-scale high-precision 3D geological models, in this paper, the flow characteristics of rock-like materials were first investigated using a new fluidity testing apparatus. Based on the test results, a novel 3D-printing technology of geological material was formulated. Then, the technologies of fabricating the desired structural specimens, acquiring samples with mechanical properties and cracking behaviors similar to natural rock, printing large-scale complex geological models were formulated. The results show that the 3D-printing technology of geological materials had a principle: no losing fluidity during printing time. The parameters of print head diameter, line width, line span, and line slump were the four key factors affecting desired structural samples. From the enlightenment of printing small-scale sample, the printing methods of complex large-scale geological models including decreasing printing time, acquiring heterogeneous geological model with inner structures, desired density, and surrounding smooth surface were proposed. [ABSTRACT FROM AUTHOR]

The rockburst and spalling problems encountered in the Jinping Underground Laboratory are to be investigated using 3-D physical model testing. New similarity relationships for the brittleness, brittle–ductile transition and ductility characteristics of deep hard rock are first established based on the characteristics of the deep rock mass, thereby providing guidance for the development of analog materials with new characteristics that are appropriate for investigating deep rock engineering disasters via large-scale 3-D physical model tests. Achieving similarity simultaneously among the multiple physical–mechanical properties of deep hard rock constitutes the major challenge of research on analog materials. Consequently, a new method for designing analog materials by controlling the aggregate characteristics is proposed. The test results indicate that the aggregates of the analog materials can be controlled to achieve properties that are similar to those of the original rock. The newly developed analog material exhibits basic mechanical parameters and properties that are similar to those of Jinping marble, including high strength, high brittleness, and high unit weight in addition to brittle–ductile transition and ductility characteristics and deformation and failure modes. The new analog material successfully simulates the major physical–mechanical properties of Jinping marble and overcomes the limitations of preexisting methods, which develop analog materials via cementing agents, in which the achievement of similarity simultaneously among multiple properties is impossible. [ABSTRACT FROM AUTHOR]