Abstract:Compared with the general columnar jointed basalt,the one at Baihetan Hydropower Station on the Jinsha River has significant characteristics as follows:undulating and irregular columnar joints,irregular and incomplete cutting columniation,microfissures enriched in the inner of columniation,gently dipping structural surface developed comparatively in basaltic mass,poor integrity but still intermittent mosaic structure in basaltic mass. Based on the engineering geological investigation and field experiments,the essential character and the deformation mechanism under different experimental conditions of loading on columnar basalt at Baihetan Hydropower Station are systematically analysed. Columnar joint,microfissure and gently dipping structural surface are the three fundamental factors which lead to the small deformation modulus of basaltic mass. The wide disparities in the horizontal and vertical direction of columnar jointed basalt¢s deformation modulus at Baihetan are determined by the structural surface and stress condition. Since columnar joints and microfissures are hard structural surface,they are closed under confining stress,but relaxed under unloading. Columnar basaltic mass maintains large deformation modulus in confining state.