Abstract: Columnar jointed basalts are a unique geological phenomenon in the volcanic region. Its engineering characteristics and excavation responding characteristics are very different from those of common rocks because of the unique geological origin. In order to evaluate the variation characteristics of relaxation depth of tunnel columnar jointed rock mass under different borehole properties, an in-situ test method, which combined digital borehole camera with acoustic velocity measurement, was proposed. The comprehensive evaluation method of relaxation depth was proposed as well. The unloading relaxation properties of columnar jointed basalts after excavation under different angles and diameters were obtained. Results showed that the diameter of borehole almost had no effect on the relaxation depth in the jointed rock mass, while the relaxation depths in borehole with different angles were significantly different. In addition, the borehole deviation angle, the angle between borehole axis and the jointed columnar axis had a substantial influence on the evaluation of relaxation test results of relaxation depth. Further discussion on the mechanism of relaxation depth variation under different borehole conditions was made. This study provides important references to the stability analysis of surrounding rock mass and support scheme optimization of tunnel project.

Key words: columnar jointed rock mass, relaxation depth, acoustic velocity, digital borehole camera, boreholes with different diameters and angles