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Columnar jointed basalt,with a lot of small-spacing structural planes and poor integrity,is a kind of fractured rock mass.Through comprehensive study of columnar joints shape,roughness of fracture surface and chemical composition of basalt,it is known that columnar joints of Baihetan dam area were formed as a result of cooling and shrinkage effects of magma.The columnar jointed basalt is mainly formed through chemical reaction of chlorite,kaolinite,epidote and tremolite,and the columnar joints mainly consist of chlorite according to slice identification and chemical analysis.Test results show that the columnar jointed basalt has high uniaxial compressive strength,low friction coefficient,and high cohesion,shear strength and deformation index.Meanwhile,the columnar jointed basalt is closely locked,and joint surfaces are well closed.The permeability of the rock is quite weak,and the P-wave velocity in the rock could get up to 5 000 m/s.All these show good rock properties.The columnar joints develop regularly,different from the general fractured rock masses.In summary,the columnar jointed basalt can be used directly as a foundation of dam.