Application and implications of thick base layer support technology with gradient coordination in soft coal roadways under coal pillar stress concentrations

In closely spaced coal seam mining, the stress concentration of overlying remnant coal pillars is the dominant trigger for surrounding rock failure in lower coal roadways, seriously hindering the safe and efficient extraction of coal. Here, the Wangwa No. 2 Mine in China is taken as a case study, and the failure characteristics of the surrounding rock and the fracture development patterns are first elucidated. Furthermore, the instability mechanism of a thick-layered coal‒mudstone composite roof in a complex stress environment is analysed in terms of stress distribution, geological structure, and support system design. In addition, a thick base layer support technology with gradient coordination is proposed based on five support principles: persistent tensile–shear resistance, stable high-stress zone formation, self-stabilizing base layer construction, coordinated support between primary and secondary members, and precise spatiotemporal grouting. In this approach, the support technology is implemented using support materials such as anchor-grouting casings and high-strength cables. The effectiveness of the technique is demonstrated through comparative block modelling using 3DEC-BBM. A full-scale field test under typical roadway conditions is carried out, and the monitoring results confirm its effectiveness. Relative to the original support system, the new approach reduces roof settlement and side displacement by 52.7% and 40.27%, respectively, and significantly reduces layer separation. The axial force of cables is highly sensitive to surrounding rock deformation, and borehole imaging reveals that the average crack evolution depth in the roof rock decreases from 4.7 to 1.55 m, a 67% decrease. Furthermore, limitations in practical engineering applications are discussed, and directions for optimizing this new technology are suggested. These findings provide a valuable reference for the control of roadway stability under similar geological and mining conditions.