The roughness of the fracture surface directly affects the strength, deformation, and permeability of the surrounding rock in deep underground engineering. Understanding the effect of high temperature and thermal cycle on the fracture surface roughness plays an important role in estimating the damage degree and stability of deep rock mass. In this paper, the variations of fracture surface roughness of granite after different heating and thermal cycles were investigated using the joint roughness coefficient method (JRC), three-dimensional (3D) roughness parameters, and fractal dimension (D), and the mechanism of damage and deterioration of granite were revealed. The experimental results show an increase in the roughness of the granite fracture surface as temperature and cycle number were incremented. The variations of JRC, height parameter, inclination parameter and area parameter with the temperature conformed to the Boltzmann's functional distribution, while the D decreased linearly as the temperature increased. Besides, the anisotropy index (I-p) of the granite fracture surface increased as the temperature increased, and the larger parameter values of roughness characterization at different temperatures were attained mainly in directions of 20 degrees-40 degrees, 60 degrees-100 degrees and 140 degrees-160 degrees. The fracture aperture of granite after fracture followed the Gauss distribution and the average aperture increased with increasing temperature, which increased from 0.665 mm at 25 degrees C to 1.058 mm at 800 degrees C. High temperature caused an uneven thermal expansion, water evaporation, and oxidation of minerals within the granite, which promoted the growth and expansion of microfractures, and reduced interparticle bonding strength. In particular, the damage was exacerbated by the expansion and cracking of the quartz phase transition after T > 500 degrees C. Thermal cycles contributed to the accumulation of this damage and further weakened the interparticle bonding forces, resulting in a significant increase in the roughness, anisotropy, and aperture of the fracture surface after five cycles. (c) 2025 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/ 4.0/).

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/).

The influence of thermal damage on macroscopic and microscopic characteristics of different rocks has received much attention in the field of rock engineering. When the rocks are subjected to thermal treatment, the change of macroscopic characteristics and evolution of micro-structure would be induced, ultimately resulting in different degrees of thermal damage in rocks. To better understand the thermal damage mechanism of different rocks and its effect on the rock performance, this study reviews a large number of test results of rock specimens experiencing heating and cooling treatment in the laboratory. Firstly, the variations of macroscopic behaviors, including physical parameters, mechanical parameters, thermal conductivity and permeability, are examined. The variations of mechanical parameters with thermal treatment variables (i.e. temperature or the number of thermal cycles) are divided into four types. Secondly, several measuring methods for microstructure, such as polarizing microscopy, fluorescent method, scanning electron microscopy (SEM), X-ray computerized tomography (CT), acoustic emission (AE) and ultrasonic technique, are introduced. Furthermore, the effect of thermal damage on the mechanical parameters of rocks in response to different thermal treatments, involving temperature magnitude, cooling method and thermal cycle, are discussed. Finally, the limitations and prospects for the research of rock thermal damage are proposed. (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/).

In this paper, numerical simulations of a special energy pile, which constitutes a spiral-injected pipe and one straight discharge pile for Geothermal Heat Pump Systems (SGHEs-P(parallel)), were conducted by Fluent software. The effects of the spiral pitches on the heat transfer rate based on the G-function method and peripheral soil temperature of the pile were investigated under continuous and intermittent operation strategies. The impact of spiral tube sizing on the surface heat transfer coefficients was studied. The results indicated that SGHEs-P may be preferred for office buildings under intermittent operation conditions. For a short period, the temperature profiles and heat transfer efficiency of SGHEs-P were mainly influenced by the fluid type, length of the spiral tube, and spiral pitch. The smaller the spiral pitch, the more uniform the temperature distribution, and the better the heat transfer effect, but the heat transfer per unit depth of pile decreased. The average temperature variation curve of the soil around the energy pile with different spiral pitches was simulated and obtained over time. Meanwhile, the impact of spiral radius, spiral pitch, and spiral tube radius on the convective heat transfer coefficient was also presented. Through data fitting, the formulas for the correction coefficients of spiral radius, spiral pitch, and spiral tube radius on convective heat transfer coefficient were obtained, respectively.