In situ test of excavation damaged zone of columnar jointed rock masses under different borehole conditions

The depths of excavation damaged zone (EDZ) and highly damaged zone (HDZ) in rock masses around a tunnel are generally measured by borehole testing. The test results of boreholes with different angles and diameters varied due to the complex structure and anisotropy of such jointed rock masses, therefore, accurately determining the depth of EDZ and HDZ in jointed rock masses is essential to their engineering safety. By investigating columnar jointed basalt in Baihetan Hydropower Station (Sichuan Province, China), a comprehensive method based on the substantial test results has been proposed for determining the depth of EDZ according to HDZ in jointed rock masses. The method is applicable in rock masses where the acoustic wave testing cannot be conducted. The borehole test results in both EDZ and original rock under different drilling conditions were obtained, and revealed that the depth of EDZ was related to borehole angle rather than diameter, while the degree of damage to the original rock in the borehole was more likely to be influenced by the borehole diameter than the angle. The proposed method for determining the depth of EDZ provides a new idea for determining the depth of EDZ in surrounding rocks. Additionally, the data provide an insight into the borehole testing method and a basis for supporting design and stability control of columnar jointed rock masses.