Mechanical properties and damage constitutive model of coal specimen with different moisture under uniaxial loading

The moisture content has an important influence on the strength and stability of coal pillar dams in underground reservoirs. In this paper, the uniaxial compression test was performed on coal samples with three different moisture contents, to study the mechanical properties of them under water intrusion condition. The results show that, water has a significant damaging and softening effect on the mechanical properties of the samples. As the moisture content grows of coal samples, the peak strength and elastic modulus fall exponentially while the peak strain increases exponentially. In addition, the energy characteristics of coal samples with different moisture contents under uniaxial compression were analyzed. The releasable elastic strain energy, dissipated energy and total input energy all experience a decline with water content, and the proportion of the releasable strain energy in the total energy drops. The capacities of energy storage and strain energy release of coal samples both fall under influence of moisture, and more energy was transformed. Given this result, the damage variable on the basis of energy dissipation was introduced to explore the influence of moisture contents on the damage variable of coal samples and the damage evolution equations of coal samples with three different moisture contents were calculated from the results of uniaxial loading tests. Finally, the compaction coefficient considering the influence of immersion time was proposed to describe the compaction degree and being used to amend the damage constitutive model obtained, and the fitted results of the models were compared to experimental data. Results show that the values of damage variables increase exponentially with strains, and the modified damage constitutive equation can give a more accurate description of the compressive deformation of coal samples that contain varying contents of moisture.