Construction and verification of strength prediction model for frozen coal with coexisting water and gas based on ultrasonic wave velocity

The freezing method is a new technique for preventing coal and gas outbursts during rock cross-cut coal uncovering. It works by rapidly freezing the gas–water–coal mixture inside the coal body, forming a high-strength complex that enhances coal-rock stability and reduces outburst risks. However, there is currently no efficient way to quickly evaluate the strength of frozen coal with coexisting water and gas. To address the limitation, with a self-developed comprehensive test platform for the freezing mechanical properties and frost damage characteristics of gas-containing coal, tests were conducted on the f-value, compressive strength, and ultrasonic wave velocity of frozen coal with coexisting water and gas. By establishing the qualitative and quantitative relationships among various parameters of frozen coal, a strength prediction model for frozen coal based on ultrasonic wave velocity was proposed. The results showed that after freezing treatment, the f-value of coal with coexisting water and gas was significantly improved, with an increased range of 23.94%–56.52%, and there was a significant positive correlation between the f-value and compressive strength. Static elastic modulus showed a strong positive correlation with compressive strength, while static Poisson's ratio exhibited weak correlation and minimal impact on coal strength. Frozen coal exhibited significantly higher dynamic elastic modulus compared to static values, whereas its dynamic Poisson's ratio was consistently lower than the static counterpart. Strong correlation was observed between dynamic and static elastic modulus, while Poisson's ratios showed weak inter-dependence. The ultrasonic wave velocity-based model demonstrated strong predictive capability for frozen coal's f-value, showing less than 15% relative error between predicted and measured values. This prediction accuracy satisfies engineering requirements, enabling real-time strength evaluation with both rapid response and high precision. The research results provide technical support for revealing the mechanical mechanism of the freezing method for preventing and controlling coal and gas outbursts.