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The hydrothermal field status of soil–rock mixture–concrete interface changes significantly under freeze–thaw cycles, which causes pore structure changes, interface debonding, and cracking; therefore, the shear strength deteriorates and the safe operation of buildings in cold regions is affected. Indoor direct shear and nuclear magnetic resonance (NMR) tests and numerical simulations were conducted to explore the strength deterioration mechanism at the interface between the soil–rock mixture and concrete with different roughness degrees under freeze–thaw cycles in this study. The results suggest that freeze–thaw cycles have little effect on the rotation and displacement of particles, and roughness has a considerable effect. There are primarily three types of pores in the interface layer: (1) micropores; (2) mesopores; and (3) macropores. By increasing the number of freeze–thaw cycles, the T2 spectral peaks of the pore structures shifted to the upright side and the total T2 spectral area increases. The increase in the roughness degrees tends to increase the proportion of mesopores and enhances the interfacial strength. The results could benefit research on soil–rock and concrete interface systems and provide effective guidance for the construction, operation, and maintenance of pile foundation engineering in cold regions. [ABSTRACT FROM AUTHOR]

Understanding the capillary imbibition laws of brackish water in rocks is necessary to reveal the mechanism of fluid, salt, and ion transport. In this study, we investigated the capillary imbibition laws of a Na2SO4 solution of different concentrations in sandstone by measuring the parameters of water absorption mass, water migration front height, nuclear magnetic resonance (NMR) T2 spectra, and stratified moisture distribution. The results indicate the following: (1) With an increase in the salt solution concentration, the water absorption rate of samples increases, specifically manifested in an increase in the rate of absorption mass and a rising rate of the absorption front. (2) With an increase in the salt solution concentration, the total NMR signals in samples measured at the end of water absorption decreases; that is, the total amount of water absorption decreases. (3) When the solution concentration exceeds 0.50 g/L, variations in the NMR signal of samples and the absorbed water mass over time are not synchronic and are even opposite at some stages. Based on the capillary dynamic theories of liquid, the influence of salts on solution properties and the modification of the pore structure by crystallization are considered when discussing the underlying mechanism of capillary imbibition in sandstone. By calculating the physical properties such as the density, viscosity, surface tension, and contact angle of solutions with different concentrations, the imbibition process does not exhibit any significant variation with the difference in the properties of the liquid. The equivalent capillary radii of the samples at varying salt concentrations are obtained by fitting the capillary dynamics curves with the theoretically calculated values. The equivalent capillary radii of samples in higher salt concentrations are larger, i.e., the difference in capillary imbibition laws introduced by the salt concentration should be attributed to modifications to the pore structure caused by salt crystallization. [ABSTRACT FROM AUTHOR]

The mechanical properties of frozen fissured rock masses are crucial considerations for engineering in frozen earth. However, there has been little research on the mechanical properties of frozen fissured sandstone, including its strength, deformation, and geometric parameters. In this study, sandstone samples with three open en echelon fissures were observed using high-speed photography and acoustic emissions during uniaxial compression tests. The aim was to investigate sandstone's strength, deformability, and failure process in order to elucidate the effects of freezing on its mechanical properties. In the frozen-saturated and dried states, the uniaxial compression strength (UCS) initially decreases and then increases with an increase in fissure inclination angle. Conversely, the UCS of samples in the saturated state continuously increases. The UCS follows a decreasing trend, as follows: frozen-saturated state > dried state > saturated state. The initial crack angle decreases as the fissure inclination increases in all states, irrespective of temperature and moisture conditions. However, the initial crack stress and time show an increasing trend. The uniaxial compression strength (UCS) of frozen fissured sandstone is influenced by four mechanisms: (1) ice provides support to the rock under compression, (2) ice fills microcracks, (3) unfrozen water films act as a cementing agent under tension or shearing loads, and (4) frost damage leads to softening of the rock. [ABSTRACT FROM AUTHOR]

Liquid nitrogen (LN2) fracturing is beneficial to the development and utilization of geothermal energy. In this paper, red sandstone was heated from room temperature to different temperatures (25°C–800°C) and then cooled with LN2. After attaining room temperature, nuclear magnetic resonance (NMR), uniaxial compression, and acoustic emission (AE) tests were conducted, and results were compared for different samples. The results showed that with an increase in quenching temperature difference, the volume of micropores decreased gradually, while the volume of fine pores, mesopores, macropores, and total porosity increased, resulting in the reduction of the compressive strength. A higher quenching temperature difference also reduced the sample's total time to fail, and the failure mode was transformed from single inclined shear failure to conical failure. This is because the thermal stress (caused by the rapid cooling of LN2) expanded the original cracks of the sandstone, leading to high porosity and low compressive strength of the heat-treated and quenched samples. [ABSTRACT FROM AUTHOR]

For the particular engineering-geological conditions and natural environment in permafrost regions, the highway has a high damage ratio during the operation. In warm and ice-rich permafrost regions, highway pavement and embankment have poor serviceability, which threatens driving the vehicle and safe operation. Traditional field surveys on highway damage cannot entirely, rapidly and precisely obtain the distress information along the whole road. In this paper, the Qinghai-Tibet Highway (QTH) pavement distress was investigated in 550-km-long permafrost regions based on the field surveys in 2014 and 2019 and the unmanned aerial vehicle (UAV) imagery. The type and damage ratio of distress were extracted by the remote sensing image classification method. The pavement roughness and embankment height were acquired by the spatial analysis of geographical information system. This paper provides a novel insight and efficient method for the distress investigation and exploration of distress formation in permafrost region, which can be applied in other similar engineering projects in the cold and inclement permafrost region. Furthermore, this paper also presents valuable and first-hand field data for evaluating highway serviceability and prevention of road damage during operation and maintenance stages. [ABSTRACT FROM AUTHOR]