• Geomechanics and Engineering
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• v.29 no.6
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• pp.627-643
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• 2022

Water-rock interactions have a significant influence on the mechanical behavior of rocks. In this study, uniaxial compression and tension tests on different water-treated sandstone samples were conducted. Acoustic emission (AE) monitoring and micro-pore structure detection were carried out. Water-rock interactions and their effects on rock mechanical behavior were discussed. The results indicate that water content significantly weakens rock mechanical strength. The sensitivity of the mechanical parameters to water treatment, from high to low, are Poisson ratio (𝜇), uniaxial tensile strength (UTS), uniaxial compressive strength (UCS), elastic modulus (E), and peak strain (𝜀). After water treatment, AE activities and the shear crack percentage are reduced, the angles between macro fractures and loading direction are minimized, the dynamic phenomenon during loading is weakened, and the failure mode changes from a mixed tensile-shear type to a tensile one. Due to the softening, lubrication, and water wedge effects in water-rock interactions, water content increases pore size, promotes crack development, and weakens micro-pore structures. Further damage of rocks in fractured and caved zones due to the water-rock interactions leads to an extra load on the adjoining coal and rock masses, which will increase the risk of dynamic disasters.

• Geomechanics and Engineering
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• v.39 no.4
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• pp.357-368
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• 2024

The pressurized water conditions of goafs weaken the support of remaining coal and rocks, which causes instability, failure, and sudden ground collapse. The impact of pressure-bearing water and CO₂ on the tensile properties of residual coal pillars was explored in old goafs. Coal was analyzed using a pressure-water soaking device, electronic scanning microscope, and 3D full-field strain measurement system. Besides, Brazilian splitting tests were performed. The failure characteristics and energy evolution law of the macro-microscopic structure of coal specimens were analyzed under different soaking conditions-desiccation (DC), CO₂ soaking (CS), water-CO₂ soaking (WCS), and water soaking (WS). The peak stress of coal specimens and time to reach the peak decreased with varying soaking environments. Stress concentration initially occurred at the water end under the WCS condition, indicating that coal specimens deteriorated more under the pressure-bearing WCS condition compared with the CS condition. Fractures of coal specimens exhibited the highest development under the WS condition. Besides, dissolution was observed at the fractures of coal specimens, with severe failure to their internal microstructures. In conclusion, the instability failure of residual coal pillars is significant in studying the old goafs.

• Geomechanics and Engineering
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• v.24 no.4
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• pp.349-358
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• 2021

The Brazilian test has been widely used to determine the indirect tensile strength of rock, concrete and other brittle materials. The basic assumption for the calculation formula of Brazilian tensile strength is that the elastic moduli of rock are the same both in tension and compression. However, the fact is that the elastic moduli in tension and compression of most rocks are different. Thus, the formula of Brazilian tensile strength under the assumption of isotropy is unreasonable. In the present study, we conducted Brazilian tests on flat disk-shaped rock specimens and attached strain gauges at the center of the disc to measure the strains of rock. A tension-compression bi-modular model is proposed to interpret the data of the Brazilian test. The relations between the principal strains, principal stresses and the ratio of the compressive modulus to tensile modulus at the disc center are established. Thus, the tensile and compressive moduli as well as the correct tensile strength can be estimated simultaneously by the new formulas. It is found that the tensile and compressive moduli obtained using these formulas were in well agreement with the values obtained from the direct tension and compression tests. The formulas deduced from the Brazilian test based on the assumption of isotropy overestimated the tensile strength and tensile modulus and underestimated the compressive modulus. This work provides a new methodology to estimate tensile strength and moduli of rock simultaneously considering tension-compression bi-modularity.