Compressive strength of thermally treated Barakar sandstone in Jharia coal mine fire region: Interplay of strength, strain rate, and temperature

This comprehensive study investigates the effects of water evaporation and thermally induced micro-cracks on the compressive strength of Barakar sandstone, subjected to temperatures ranging from 25 °C to 800 °C under both quasi-static and dynamic loading conditions. The quasi-static and dynamic compressive strengths of thermally treated Barakar sandstone were measured using a Universal Testing Machine (UTM) and Split Hopkinson Pressure Bar (SHPB), respectively. Our findings reveal that up to 400 °C, water evaporation significantly enhances sandstone strength in both loading scenarios due to increased friction between mineral grains. Beyond 400 °C, the formation of micro-cracks and micropores degrades the rock's strength, leading to noticeable reductions under both loading conditions. Unexpectedly, the Dynamic Increase Factor (DIF) remained unchanged up to 400 °C but increased beyond this threshold. The study also validates the Kimberley universal rate-dependent scaling law, indicating that the characteristic strain rate, at which strength doubles compared to quasi-static strength, decreases beyond 400 °C, with some fluctuations observed up to this temperature. Fracture patterns consistently showed longitudinal splitting at all temperatures, with increasing severity beyond 400 °C due to extensive thermal cracking, as evidenced by microscopic and image analyses of the samples. This study provides crucial insights into the thermal and mechanical behavior of Barakar sandstone, highlighting the critical 400 °C threshold beyond which significant changes in strength and fracture characteristics occur. These findings have important implications for geological and geotechnical applications involving high-temperature conditions and dynamic loading of sandstone.