The Protodyakonov strength index method is regarded as one of the most efficient index methods for assessing the risk of coal gas outbursts. It is also considered effective for obtaining an overview of the uniaxial compressive strength properties of coal, especially when acquiring intact coal samples is challenging due to adverse geological conditions (Brook and Misra, 1970). This study provides a comprehensive correlation between the Uniaxial Compressive Strength (UCS) and the toughness index (f), along with evaluating the influence of inherent moisture and volatile content. In this study, a diverse set of coal specimens from different seam locations was collected, considering variations in coal lithology. A modified coal toughness rig was developed for this study, which improved the accuracy and efficiency of the results. The compressive strength tests were conducted using the uniaxial loading machine (Instron), and the failed samples were then used for the toughness index measurements using the modified toughness rig. The inherent moisture and volatile matter content data were obtained using proximity analysis.

The analysis demonstrated a strong correlation between UCS, and toughness index f, emphasizing the interconnectedness of these parameters in assessing coal strength. The study also explored the influence of inherent moisture, which showed varying degrees of correlation across different sections of the coal seam. The Pearson correlation coefficient was used to measure the linear correlation. Additionally, the influence of volatile components on the toughness index f showed a moderate negative correlation, indicating that the toughness index tends to decrease as the volatile content increases.

Overall, this study provides valuable insights into the relationship between coal strength properties, moisture content, and volatile content, which will contribute to the assessment of coal outburst risk and the current gas outburst threshold limits.