A comprehensive analysis of coal toughness index: Relationships among coal toughness index, uniaxial compressive strength and proximity parameters

Coal and gas outbursts are significant safety hazards in underground coal mining, particularly in deep Australian coal seams. While the traditional outburst risk management relies on threshold limit values of seam gas content, recent experiences have shown that this approach overlooks important factors such as coal toughness and strength. This study evaluates the effectiveness of the coal toughness test, also known as the Protodyakonov strength test, as a predictor of outburst risk. After upgrading the test rig and updating testing procedures, the toughness index f was compared with conventional strength parameters, revealing a strong positive correlation with uniaxial compressive strength (UCS) and drop hammer impact strength. An analysis of 98 coal samples taken from seven seams at three Australian mines revealed that lower moisture content and higher apparent relative density are associated with increased coal toughness. Specifically, the Pearson and Spearman correlation coefficients of -0.79 and -0.78 were observed, respectively, indicated a strong negative relationship between coal moisture content and toughness. Additionally, coal seams with lower volatile matter content exhibited higher toughness values. The study proposes adopting the coal toughness index as a robust alternative to the conventional indicators such as UCS, especially when obtaining intact coal samples is difficult. Incorporating the toughness index into the outburst risk assessment can improve safety by adopting a more comprehensive and reliable approach.