Mechanical Characteristics of Similar Weakly Cemented Soft Rock under Directional Shear Stress Path and Modified Lade–Duncan Criterion

Abstract

To avoid engineering challenges during roadway excavation or workface mining in weakly cemented soft rock, the characteristics of this rock under the influence of associated stress paths must be explored. Therefore, this study investigates the principal strain and generalized shear strain of weakly cemented soft rock under a directional shear stress path, through a series of principal-stress directional shear tests conducted using a hollow cylinder apparatus. The damage morphology characteristics for different major principal-stress directions are also discussed. The results show that the major principal strain (ɛ1) gradually increased with increasing major principal-stress direction angle (α), when α ranges from 0° to 45°, whereas the minor principal strain (ɛ3) and intermediate principal strain (ɛ2) gradually decreased. The peak stress of the soft rock gradually decreased and a main slip-damage crack developed in the major principal-stress direction. The peak generalized shear strain increased slowly in the first compression–torsion stage and rapidly in the second. As α ranges from 45° to 90°, ɛ1 and ɛ2 decreased gradually whereas ɛ3 increased gradually. The peak stress also increased slowly, with the main crack developing in the minor principal-stress direction. Additionally, the peak generalized shear strain rapidly decreased in the first tension–torsion stage and then slowly decreased in the second. The principal-stress rotation space transformation matrix was introduced to modify the Lade–Duncan (L-D) criterion, and the criterion reliability was verified using the peak value of the soft rock deviatoric stress under different principal-stress directions. The modified L-D criterion considered the effects of both the principal-stress magnitude and direction. The results are of great significance for studying the influence of principal-stress direction and provide an important method to investigate the mechanical properties of rocks with complex stress paths comprehensively.