Strength degradation and fracture evolution characteristics of pre-damaged coal samples under unilateral soaking condition

Abstract

With the wide application of the narrow-coal-pillar gob-side entry retaining technology in underground coal mines, the narrow coal pillars adjacent to the goaf under such conditions are affected by goaf ponding for a long time, resulting in significant changes in their physical and mechanical properties. In this paper, the narrow coal pillar adjacent to the goaf of gob-side entry retaining is taken as the research object. Coal samples with different pre-damage levels are prepared, and unilateral water immersion tests are carried out. Combined with acoustic emission technology and fractal theory, the strength and crack evolution of coal samples are analyzed. The results show that: (1) The average initial damage variables of the five groups of coal samples calculated by the revised damage variable calculation method are 0, 0.135, 0.141, 0.157 and 0.169; (2) With the increase of pre-damage level, the strength and elastic modulus of coal samples continuously degrade. The maximum strength reduction of highly damaged coal samples reaches 39.6%, and the degradation range of elastic modulus is 37.4%; (3) The peak value of acoustic emission and the cumulative ringing count gradually decrease with the increase of damage, and the damage changes from brittle tensile damage to ductile tensile-shear composite damage. Under the condition of high pre-damage, it changes to tensile peeling failure; (4) The fracture fractal dimension presents a variation trend of "decline—fluctuating minimum value—rise", which fully reflects the three-stage evolution law of primary fracture development, directional penetration of damaged fracture surfaces and scattered cracking of coal blocks. The research results can provide a reference for the long-term stability evaluation of gob-side coal pillars and the prevention and control of local underground water hazards. ( date of received paper: )