Analysis and Research on Experimental Process of Water Thermal Migration of Freeze–Thaw Cracked Rock Based on Particle Tracking Method and Thermal Imaging Technology

In high-altitude and cold regions, external dynamic geological processes, such as glacial melting and other processes are intense, which frequently results in surface dynamic geological processes, such as slope collapse, landslides, debris flows, and ice avalanches along the route. For high and steep slopes in high-altitude regions containing controlled fractures, the key is to grasp the water-heat process and the evolution of the frost heaving force induced by it within the fractures. This can then lead to the exploration of the multi-phase and multi-field damage propagation, and a disaster mechanism within the fractures under repeated freezing and thawing. The visual tracking of the water-heat migration process within the fractured rock mass is helpful in observing the evolution process of frost-heaving force and providing a theoretical basis for the frost-heaving mechanism. First, research on particle tracking, thermal imaging tests, and image processing technology was conducted to demonstrate that particle tracking and thermal imaging techniques can track the freezing front within the rock fractures and the migration of liquid water inside the rock. Then, by selecting fluorescent particles and improving the observation window and using a waterproof insulation cardboard, the development of a visualization device system for the water-heat migration process was achieved, allowing the tracking of the water-heat migration process. The results of the verification test showed that under freezing and thawing conditions, the experimental device could effectively track the temporal and spatial changes of water-heat migration inside and outside the rock fractures and monitor the real-time changes of the freezing front. Reliable experimental results were obtained, which provided a visual record of the water-heat migration and water-ice phase transition within the fractured rock mass during the freezing and thawing process. Combining thermal imaging technology with the real-time recording of the motion rate of fluorescent particles, this experiment described the movement speed of the freezing front and the convection of free water within the fractures in rock water-heat migration, which is of significant importance for the study of the frost-heaving force under the influence of water-heat migration

Mechanical Characteristics of Frozen Sandstone under Lateral Unloading: An Experimental Study

The lateral unloading strength and deformation of surrounding frozen rock are the key parameters for safety evaluation of frozen shaft construction. A low-temperature and high-pressure rock triaxial test system was used to simulate freezing construction, and a constant axial pressure unloading confining pressure test was carried out on frozen sandstone. The effects of freezing temperature, initial confining pressure, and unloading rate on the strength, deformation, and failure modes of frozen sandstone are studied. The main results of the study are as follows: (1) under the initial confining pressure of 20 MPa, the temperature of the sandstone decreases from 20°C to –5°C, and the peak stress and elastic modulus of triaxial compression increase by approximately 3 times. Under lateral unloading conditions, the peak stress of frozen sandstone is about 2∼3 times that of 20°C sandstone, and the peak strain of 20°C sandstone is smaller than that of frozen sandstone. The temperature of frozen sandstone decreases and the rate of increase in the peak stress of triaxial compression is slightly less than the rate of increase in the peak stress of lateral unloading. (2) The initial confining pressure of frozen sandstone increases, the growth rate of axial and radial strain increases, the radial strain dominates the failure process, and the lateral unloading strength decreases significantly. (3) The lateral unloading rate of frozen sandstone increases, the peak strength increases, and the axial and radial strain decrease. At a low unloading rate, partial creep deformation occurs. (4) The frozen rock sample undergoes tensile splitting failure under lateral unloading. According to the stress-strain curve of the frozen rock sample, the relationship between changes in the deformation modulus and changes in the confining pressure unloading amount during the unloading process of the rock sample is obtained

Dynamic Splitting Behavior and the Constitutive Relationship of Frozen Sandstone Containing a Single Fissure

Fractured sandstone is widely distributed in mining areas throughout western China where the artificial freezing method is extensively adopted to construct vertical shafts. Blasting and excavation generate stress waves and break frozen fractured sandstone. Among the failure modes of frozen fractured rocks, tensile failure is very common. In this study, the dynamic tensile strength of fractured sandstone samples with four crack inclination angles (0°, 30°, 60°, and 90°) is tested by using a split Hopkinson pressure bar at four subzero temperatures (−5, −10, −15, and −20°C). Accordingly, a damage constitutive relationship that considers the effect of fissure angle and freezing temperature is established. The results show the following: (1) the fissure angle does not significantly affect the dynamic tensile strength of frozen fractured sandstone but mainly affects the failure mode of the sample. (2) The dynamic tensile strength of fractured sandstone has a negative linear correlation with the freezing temperature. (3) When the fissure angle is small, only tensile cracking occurs; when the fissure angle is large, tensile cracking occurs along both the loading direction and the fissure; and shear cracking occurs along the fissure as well. (4) Regardless of the fissure angle, tensile cracking is initiated at the stress-concentration zone and then propagates towards the loading end. Fissure ice provides both resistance to deformation and resistance to crack propagation which affects the crack propagation and coalescence mode. A dynamic constitutive relationship is established by considering the effects of fissure angle and freezing temperature on the dynamic properties of frozen fractured sandstone, which is proven to be highly reliable and provides a reference and basis to study the dynamic mechanical properties of similar rock types

Study on the Acoustic Emission Characteristics and Failure Precursors of Water-Rich Frozen Sandstone under Different Lateral Unloading Rates

The artificial freezing method is used to cross the water-rich soft rock strata in order to exploit deep coal resources. At present, studies that consider both freezing effect and unloading rate are insufficient. To study the influences of the excavation rate using the artificial freezing method on the unloading deformation and failure of the water-rich surrounding rock, we carry out mechanical and synchronous acoustic emission (AE) tests on frozen (−10 °C) sandstone samples under different lateral unloading rates. Combined with the AE signals, the stress, strain and failure process are analysed to determine the mechanical behaviours of frozen rock samples under different lateral unloading rates. The damage difference between normal temperature rock and frozen rock during lateral unloading is studied. According to acoustic emission signals, the damage relationships among acoustic emission amplitude, energy, cumulative acoustic emission energy (CAEE), stress and strain were compared and analyzed. In this paper, acoustic emission 3D positioning system is used to monitor the fracture propagation trajectory in the process of unloading confining pressure of frozen sandstone. The results show that the peak stress of frozen sandstone during lateral unloading is about 2.5 times of that at 20 °C. More than 2 AE amplitudes per second are regarded as the precursor of failure (FP), and point FP is taken as the first level warning. The CAEE of rock samples at 20 °C and frozen rock samples shows the same change law over time, increasing slowly before the FP point and exponentially after the FP point. Peak stress increases and axial strain decreases with the increase of unloading rate of frozen rock sample. The CAEE at point FP and the peak acoustic emission energy (AEE) and the CAEE at the time of failure increase when the unloading rate of frozen rock sample increases. Principal component analysis method was used to extract key characteristic energy to obtain a clearer AEE concentration area, which was defined as second-level early warning. The research results can provide guidance for freezing shaft construction to reduce the occurrence of disasters

Investigation of Sandstone Mesostructure Damage Caused by Freeze-Thaw Cycles via CT Image Enhancement Technology

The mesostructures of rocks determine their macromechanical properties. These rock mesostructures may be altered by the freeze-thaw cycles in cold regions. In this regard, this paper proposes a quantitative evaluation method based on computed tomography (CT) scanning technology for investigating the mesostructure and damage characteristics of sandstone subjected to freeze-thaw conditions. CT scan images of two sandstones with different grain sizes were obtained after 0, 20, 40, 60, 80, and 100 freeze-thaw cycles, using a high-precision CT scanner. Based on the microphysical information contained in these CT images, pseudo-color-enhancement of the CT images of rocks subjected to freeze-thaw cycles was realized. The use of such a pseudo-color-enhancement technique can improve the resolution of CT images. Thus, particle detachment, crack initiation, crack propagation, and increased porosity due to the volumetric expansion of water inside the rock could be detected and clearly observed. Furthermore, a numerical expression for the mesostructure and damage information contained in the pseudo-color-enhanced images is presented herein; this serves as a convenient method for quantitative analyses of sandstone damage under freeze-thaw cycles. An analysis of the pseudo-color-enhanced images shows that, under freeze-thaw cycles, damage propagation in sandstone originates from existing damage or defect sites. After the stages of crack (pore) formation, penetration, and propagation, the freeze-thaw cycle-induced damage increases gradually, while the effective bearing area of the rock decreases continuously. Herein, a schematic of a conceptual model for the freeze-thaw cycle-induced deterioration in sandstone mesostructures is presented. Damage propagation models for sandstones with two different grain sizes subjected to freeze-thaw cycles were also developed. Based on the damage mechanics theory, a damage variable expressed in terms of the pore area was defined. Moreover, the relationship between this damage variable and the freeze-thaw cycles was established