Influence patterns of confining pressure and burning temperature on mechanical properties and permeability of sandstone

Abstract

[Objective] High temperatures generated by coal seam spontaneous combustion cause thermal damage to surrounding rock, leading to significant changes in its mechanical properties and structure and potentially inducing geological disasters such as water inrushes and roof collapses. The evolution patterns of mechanical properties, permeability, and fracture characteristics of rock under different burning temperatures and burial depths are investigated in this study. [Methods] Sandstone specimens subjected to different high-temperature pretreatments were selected as the research objects. Triaxial compression seepage tests were conducted under constant seepage pressure and varying confining pressures, with synchronous acoustic emission monitoring. [Results] The result showed that the mechanical properties of sandstone progressively strengthened with increasing confining pressure, while they initially exhibited slight enhancement and then gradual deterioration as the burning temperature rose. Under varying confining pressures and burning temperatures, the permeability followed a trend of initial reduction followed by a subsequent increase. [Conclusion] The result indicate that:(1) during triaxial compression, the peak strength and elastic modulus of sandstone initially increase and then decrease with rising burning temperature, while both parameters increase with higher confining pressure. Axial peak strain increases continuously with elevated burning temperature and also rises gradually with increasing confining pressure. Closure stress, crack initiation stress, and yield stress exhibit a slight initial increase followed by a progressive decline as burning temperature increases, with a temperature threshold observed near 200 ℃.(2) The permeability evolution patterns of burnt sandstone are classified into three types: “√”-shaped, “U”-shaped, and “”-shaped. Both confining pressure and burning temperature promote a transition from the “√”-shaped pattern to the “U”-shaped and “”-shaped patterns.(3) Under seepage pressure conditions, acoustic emission ring-down counts remain low before rock failure but exhibit a sharp surge immediately prior to peak failure.(4) Burning temperature has no significant effect on the variation characteristics of acoustic emission ring-down counts, although acoustic emission signals are notably enhanced during the compaction stage from 400 ℃ to 600 ℃, and become intense during the yielding stage after 800 ℃.(5) The triaxial failure modes of sandstone are consistent with shear failure. Both confining pressure and burning temperature exert an inhibitory effect on the permeability of dominant fractures.

Keywords

sandstone / deep mining / coal seam spontaneous combustion / mechanical properties / permeability evolution / rock mechanics / thermal mechanical coupling / influencing factors