Coupled acoustic emission and petrographical insights into thermal degradation and fracture dynamics of an Indian sandstone

Abstract

Land subsidence induced by thermal stress degradation of rocks surrounding coal seams during the underground coal gasification (UCG) and underground coal-mine fire (UCF) constitutes a significant engineering geological hazard. The Jharia basin of India represents a critical case, as it is both a potential site for syngas production using the underground coal gasification techniques and is extensively affected by underground coal fire. To better understand the response of Jharia sandstone in extreme thermal environments, this research examines the physico-mechanical behaviour and fracture dynamics at various treatment temperatures between 25 °C and 800 °C. The research examines how grain size affects the strength and acoustic emission properties of sandstone at elevated temperatures. Crack initiation and damage thresholds were determined from linear-to-exponential trends in cumulative acoustic emission (AE) counts that marks the onset of stable microcrack nucleation. In contrast, crack damage stress threshold ratio indicates accelerated crack coalescence, evidenced by sharp increases in acoustic emission counts and energy. Microcracking initiated at 400 °C, and the strength parameters peaked when transgranular cracks were dominant. However, the strength parameters decreased significantly when the intergranular cracks were dominant at 600 °C. The grain-size and acoustic emission analyses revealed that the cumulative count and cumulative energy increased with increasing grain size. In contrast, the crack damage stress threshold showed an inverse relationship with the grain size.