Abstract: Columnar jointed basalt in Baihetan hydropower station is jointed rock mass formed by the combined cutting effect of original joint and aphanitic micro-cracks. Due to the difference of born modes， structures， attitudes and mechanical properties， the failure mode of columnar jointed basalt under various stress states is diverse. In this paper， D-CRDM method was used to simulate unloading process of columnar jointed basalt， and to analyze the failure mechanism and mode of columnar jointed basalt combining with in-situ AE test. This study indicates that in the unloading process， original joints are easy to cause surface spalling in tension state whereas aphanitic micro-cracks were inclined to induce internal cracks of surrounding rock in tension or compression and overall collapse of rock mass. The research results are justified by in-situ observations， and meanwhile， it provides a reference in the selection of supporting methods for underground cavern.

Key words: geotechnical mechanics, columnar joint, original joint, aphanitic micro-cracks, failure mode