Abstract: The fracture propagation characteristics of weak rock during split grouting reinforcement remain unclear, and empirically designed grouting parameters fail to effectively reinforce the surrounding rock. To investigate these characteristics in in-situ soft rock during split grouting, true triaxial split grouting tests were conducted on soft rock-like specimens. By analyzing acoustic emission parameters, we obtained the influence laws of varying principal stress differences and pre-existing crack angles on fracture propagation. Theoretical analysis and numerical simulation methods were employed to discuss the differences in splitting pressure and the mechanism of fracture propagation turning under triaxial stress conditions in weak rock specimens. The results reveal that as the principal stress difference increases, the angle between the fracture plane and the horizontal direction decreases, and the proportion of shear cracks in the specimen initially decreases and then stabilizes with increasing vertical pressure. As the angle between the pre-existing crack and the vertical principal stress increases, the initial splitting pressure first rises and then falls. When the pre-existing crack angles are 0º and 90º, they have a minimal impact on the direction of fracture propagation. At a 60º angle, the fracture propagates through the pre-existing crack. At a 30º angle, the fracture turns and intersects under high grout pressure. Combining the bare hole splitting theory, the analysis of splitting pressure differences and turning mechanisms under triaxial stress indicates that the higher the ratio of the maximum to minimum principal stresses, the lower the ratio of the tensile strength in bare holes under triaxial stress to the tensile strength measured by the Brazilian test. The initial direction of fracture propagation aligns with the direction of the minimum hoop stress in the bare hole, and it deflects towards areas of stress concentration at both ends of the pre-existing crack during propagation. Changes in the stress distribution within the rock mass are the primary cause of the turning in fracture propagation.

Key words: splitting grouting, soft rock-like material, true triaxial, principal stress difference, prefabricated cracks