Abstract: Investigating the strength behavior of rock under the combined effects of bedding plane inclination and confining pressure is fundamental to understanding the strength behavior of gently inclined mine pillars constrained by infill. Numerical simulation was employed instead of inclined loading tests under confining pressure to study the shear failure laws and strength characteristics of rock under the combined effects of inclination and confining pressure. Red sandstone was selected as the research object, and uniaxial and shear tests were conducted to obtain its basic mechanical parameters. Based on the calibrated mechanical parameters of red sandstone, numerical simulations of inclined loading of rock were carried out for 7 dip angles and 6 confining pressure combinations, with the average compressive and shear loads of the specimen in the limit state captured using Fish language. The shear failure laws and strength characteristics of rock under the combined effects of dip angle and confining pressure were obtained. The results show that as the inclination angle increases, the angle between the shear band and the horizontal plane becomes larger, and as the confining pressure increases, the shear band thickens; increasing the confining pressure effectively reduces the influence of the inclination angle on rock strength. Then, an unconventional stress circle was used to characterize the stress state of the rock in the limit state and reveal the changing law of the stress path. Based on the Mohr-Coulomb strength theory, the gradient descent algorithm was used to connect the “points” representing the ultimate stress state on the stress circle for 7 inclination angles and 6 confining pressures, yielding 7 sets of strength envelope equations corresponding to the 7 inclination angles. By employing a polynomial approximation method and introducing the dip angle dimension, the 7 sets of “strength envelopes” were extended to a “strength surface”, realizing the transition from “points” to “lines” to “surfaces” and constructing a rock strength model that includes the dip angle factor. The research results have important scientific significance for revealing the coupling effect of dip angle and confining pressure on the strength of rock mass engineering, such as mine pillars.

Key words: strength behavior, inclination effect, confining pressure effect, Mohr-Coulomb criterion, gradient descent algorithm