Impact of surface irregularities on coal wall stability and support mechanisms: Insights from physical and numerical experiments

• Undulation height and period significantly affect coal wall mechanical parameters, with height having a more pronounced effect; undulating surfaces reduce strength to 50-60% of flat surfaces. • Failure under uniaxial compression is predominantly tensile, producing long slender fragments with a characteristic 'III'-shaped fracture pattern. • Energy dissipation occurs at all loading stages for undulating surfaces, with a higher proportion in the early stage due to structural damage and internal crack formation. • Reducing the elastic modulus of the support plate material alleviates stress concentration at convex locations and increases peak strength.