Coal bursting tendency evaluation by needle penetration test

The bursting tendency is the inherent mechanical property of coal, and the important basis for evaluating the degree of coal bursting risk in underground coal mine. In this paper, an experimental method based on a needle penetration test for a rapid determination of coal bursting tendency was proposed. Combined with the needle penetration test, longitudinal wave velocity test and uniaxial compression experiment, the differences in the physical and mechanical properties of coal samples with different needle penetration indexes ( NPI ) and the energy evolution law during compression failure were explored. The residual elastic energy release rate index K_RE was used to quantitatively characterize the accumulation and release characteristics of elastic strain energy and evaluate the coal bursting tendency. Through data fitting and regression analysis, the corresponding relationships between NPI and the physical and mechanical parameters such as longitudinal wave velocity, uniaxial compressive strength, elastic modulus were obtained. The correlation expressions between NPI and the coal bursting tendency indexes such as bursting energy index, elastic strain energy index, dynamic failure time, residual elastic energy release rate index were established. The classification of coal bursting tendency based on NPI was proposed and validated using the ejection mass ratio of coal cuttings F. The results show that there is a power function positive correlation between the NPI and the physical and mechanical parameters such as longitudinal wave velocity, uniaxial compressive strength, elastic modulus, and the coal bursting tendency indexes such as bursting energy index, elastic strain energy index, dynamic failure time, residual elastic energy release rate index. An exponential function negative correlation was found between the NPI and the dynamic failure time. The larger the NPI is, the stronger the elastic deformation and energy storage performance of coal. When the NPI is 85 N/mm, there is a sudden change in the uniaxial compressive strength of coal. The needle penetration test is feasible and reliable in predicting the physical and mechanical properties, elastic strain energy storage and release characteristics and the bursting tendency of coal.