Abstract: Primary microfractures develop in the columnar jointed basaltic rock mass of Baihetan hydropower station, with implicit joints that can hardly be observed by naked eye. The joints are flat and closed in an intermittent mosaic structure; but the rock strength is high with large longitudinal wave velocity, resulting in the inconsistency between the tested physical and mechanical properties and the mass of rock mass as well as its actual structural characteristics. In view of this, in-situ core drilling, sound wave test, and point load test were conducted on columnar jointed basalt as a representative rock mass containing primary implicit joints. Several commonly used methods were employed to classify the quality of the rock mass, and the results were compared with actual engineering results.Furthermore, an improved classification method for engineering rock mass with primary implicit joints was proposed based on the BQ classification method to modify the K_v value by incorporating RQD under specific conditions.The proposed method was applied to the engineering classification of columnar jointed basaltic rock mass at Baihetan hydropower station, and the classification result agreed well with engineering practice.Finally, the method was verified through in-situ rock deformation test, and the results demonstrated that the classification result of the improved BQ method consisted well with the measured deformation parameters.

Key words: rock mechanics, primary joint, implicit joint, quality classification of rock mass, classification method