Mechanism and engineering application of HCL-induced weakening of coal-measure sandy mudstone

The sandy mudstone within collapse columns is characterized by its density and high mechanical strength, posing significant challenges for direct excavation by coal shearers. Consequently, coal mining operations are impeded by the inability to traverse collapse columns directly. This investigation represents the inaugural elucidation of the chemical weakening mechanism of sandy mudstone subjected to hydrochloric acid (HCl) etching, and it substantiates the practicality of this method within engineering contexts. The employed research methodologies encompass HCl acid etching experiments, rock mechanical property assessments, X-ray diffraction (XRD) analyses, and scanning electron microscopy (SEM) examinations. The findings demonstrate that: The uniaxial compressive strength of sandy mudstone exhibits a negative logarithmic correlation with the volume fraction of the HCl solution; Post-etching, the concentrations of muscovite and kaolinite within the sandy mudstone diminish by 44.74% and 62.96%, respectively, with extensive dissolution of framework particles leading to interconnected pores; The HCl induces a synergistic weakening effect via H
2
O intrusion and H
+
dissolution, substantially degrading cohesion and structural integrity; The engineering applicability indicates that prior to mining, the injection of a 2% HCl solution into the collapse column weakens the rock for over 10 days, enabling the mining face to traverse it directly. A subsequent injection of an optimal amount of Ca(OH)
2
solution into the goaf then neutralizes any residual HCl and mitigates acid corrosion in the mined-out area. This study furnishes a technical strategy for managing complex geological formations, such as waterless collapse columns, in the context of coal mining engineering.