Macro-mesoscopic strength features and fissure extension of X-shaped marble exposed to uniaxial compression conditions

In room and pillar mining, rooms cause stress redistribution, and pillars are prone to stress concentration and repetitive incoming pressures. Hence, pillars may spall along some key joints/fissures to form X-shaped broken pillars under high-stress conditions. This study uses mechanical tests and numerical simulations to reproduce strength characteristics and fissure evolution of X-shaped cracked rock pillars (X-CRP) covering different center diameters (D50) under uniaxial compression conditions from macro-mesoscopic perspectives. The research's conclusions are as follows: X-CRP shows obvious brittle characteristics under axial pressure, and the peak stresses of X-CRP-50 and X-CRP-20 correspond to maximum and minimum values of 95.2 MPa and 21.42 MPa, respectively, and peak stress drops by a decrease of D50; Variation of dissipated energy (ud) of X-CRP is related to stress-strain response, by high and low energy storage rates for X-CRP-20 and X-CRP-40, respectively. Numerical model outcomes specify that the force chain's strength and the particle vector velocity's magnitude are linked with D 50 and stress level, and the smaller the D50, the earlier the stress concentration appears and the larger the value. X-CRP's macroscopic failure style is allied by D50, X-CRP-50 exhibits tension-shear damage, X-CRP-40 and X-CRP-30 mainly reflect shear-tension damage, the upper part of which both produce large shear slip, and X-CRP- 20 produces transverse shear at D50, with a small area of shear surface, and the sample undergoes integral structural The results of the study can make up the blank of the complex mechanical properties and rupture behavior of X-CRP, and offer a basis for guessing service life and bearing capacity of underground X-shaped broken pillars, the selection of reinforcement method and support timing.