Mesocracking and Macrofailure Mechanisms: Excavation Surface Geometrical Parameters

Excavation-induced damage of rock masses may cause substantial deterioration in underground engineering. Inappropriate expansion of the excavation damage zone (EDZ) in areas without support during roadway excavation is a critical technical issue, especially for rock masses with poor self-stability. The geometrical parameters of excavation surfaces around underground roadways are important for preventing the failure of excavation surfaces. However, understanding the role of geometrical parameters in the failure of rock masses is challenging because of the difficulty of obtaining real-time monitoring information in situ during excavation. To reveal the mesocracking and macrofailure mechanisms of shallow surrounding rock masses during the initial stage of roadway excavation, a series of numerical simulations that consider different excavation surface geometrical parameters were performed by coupling the commercial software FLAC3D and PFC3D. The results indicate that the three-dimensional crack distribution characteristics of the shallow and internal parts of the roadway roof are similar during the progressive failure process. The rock fracture boundary is covered with interconnected shear cracks, which are the main cause of surrounding rock fragmentation. The impact of roadway excavation on the damage to the shallow surrounding rock shows significant differences with the variation in the length of cycle excavation (LCE). A high LCE (1.5–3.0 m) results in roadway boundaries that are overall connected and highly damaged, while a low LCE (0.5 m, 1.0 m) only leads to local fracture of a small portion of the rock mass. The average destabilization volume rate of the unit block mass of the roadway roof decreases as the LCE decreases, and an LCE ≤ 1.0 m can reduce the scope and extent of the highly damaged zone (HDZ). To minimize damage to shallow surrounding rock, the precut contour excavation method was proposed. Compared with those after performing the conventional excavation method, the local damage to the roof and the stress concentration factor are significantly lower after precut contour excavation. This research provides a basis and reference for evaluating and controlling the stability of rock masses that are not supported after excavation.