Temperature evolution and freeze–thaw characteristics of water-saturated-dried coal samples during low-temperature freeze–thaw process

Abstract

In order to study the frost heaving deformation characteristics of coal under the action of liquid nitrogen (LN2), this paper takes bituminite under saturated and dry conditions as the research object. The temperature-strain monitoring test was used to explore the evolution law of coal body strain during LN2 freezing and thawing. NMR was used to study the effect of free water space on pore structure. Uniaxial compressive strength was also used to evaluate mechanical properties. The results show that the temperature drop rate of the saturated coal sample is higher than that of the dry sample during the freezing process. During the thawing process, the time required for the core temperature of the dry coal sample to reach equilibrium is shorter. Based on the volume deformation of coal samples during the freezing and thawing process of LN2, the coal samples were divided into eight stages: freezing contraction zone I-freezing expansion zone II-freezing contraction zone III-equilibrium zone IV-thawing expansion zone V-thawing contraction zone VI-thawing expansion zone VII-equilibrium VIII zone. The thermal expansion coefficient κ is defined to analyze the relationship between the volume strain and the core temperature of the saturated and dry coal samples, and it is found that the saturated coal samples have stronger sensitivity to temperature during the freezing and thawing process. After the freezing and thawing of LN2, the internal pore damage of saturated coal sample is more serious. In addition, the uniaxial compressive strength of saturated and dry coal samples decreased, and the deterioration degree of saturated coal sample was greater than that of dry coal sample.