Investigations of mechanical and failure properties of 3D printed columnar jointed rock mass under true triaxial compression with one free face

The failure and mechanical properties of columnar jointed rock mass (CJRM) after excavation are of great significance to the stability evaluation of rock mass engineering. In this investigation, for understanding the mechanical properties and excavation responses of CJRM in the Baihetan Hydropower Station, the CJRM specimens (100 mm x 100 mm x 200 mm) containing internal implicit joints (IIJs) within the columns and irregular joints between the columns were created via 3D printing. After that, the tests of true triaxial compression with one free face were carried out. The laboratory tests results show that the exfoliation of the 3D printed columns at the tip of CJRM specimens occurred in the loading stage, which is in accordance with the failure pattern of natural CJRM. In addition, when the inclination angle of columns (alpha) is 0 degrees, the 3D printed rock specimen undergoes splitting failure along with the structure of columnar joints. When the inclination angle (alpha) is 30 degrees or 60 degrees, the shear failure of the structure of columnar joints is predominated in the 3D printed rock specimen. When the inclination angle (alpha) of columns is 90 degrees, the splitting failure of the structure of columnar joints is the main failure pattern of the 3DP CJRM sample. The mechanical properties of peak strength and tangential modulus of 3D printed columnar jointed rock specimens show obvious anisotropic characteristics. the acoustic emission (AE) characteristics of the 3D printed specimens varied with different inclination angles as well. When the inclination angle of columns is low (alpha = 0 degrees or 30 degrees), the acoustic emission mode of the 3D printed columnar jointed rock specimen is foreshock-principal shock-aftershock type. When the inclination angle (alpha) is 60 degrees, the acoustic emission mode is multi-peak type, which corresponds to the shear failure of the 3D printed rock specimens. When the inclination angle (alpha) is 90 degrees, the acoustic emission events are the most numerous and the energy released is the highest. In addition, under true triaxial compression with one free face, the scanning electron microscopy (SEM) images of the failure surfaces of 3D printed specimen exhibit directionality, and the brittleness of the failure surface is lower than that of natural rocks.