Experimental investigation of tunnel damage and spalling in brittle rock using a true-triaxial cell

Deep underground excavations in brittle rocks are subject to several ground stability hazards such as spalling and rockburst. These hazards are typically associated with brittle failure mechanisms for hard and massive rock mass. In this study, an experimental investigation has been carried out to evaluate the mechanisms underlying these induced hazards in deep underground excavations. The main objective of this study is to investigate the behavior of a freshly excavated circular unsupported tunnel in a brittle synthetic rock with a focus on induced damage and spalling response. The experiment used a miniature tunnel boring machine (TBM) to excavate a tunnel in a cubical specimen placed in a true-triaxial cell. The material selected for this experiment was a reproducible synthetic rock analogous to sandstone with brittle characteristics. In the experiment, the specimen was loaded with increasing confining stress under incremental isotropic conditions in the true-triaxial cell until the tunnel failed. Macro-photography was utilized to verify the excavation damage zones and failure mechanisms around the tunnel boundary. The main observed failure mechanism of the model tunnel was spalling, which occurred due to brittle failure and a sudden stress release at the tunnel excavation boundary. In addition to spalling, three zones of damage were identified by macro-photography in the tunnel due to damage from construction, fracturing, and plastic rock deformation. The outcomes point to a unique and in-depth comprehension of how damage and spalling failure during underground excavation develop and its impact on tunnel stability.