Abstract: The dam site of a hydropower station  planned to be built in a gorge area has developed inactive and regional fault passing through the river bankside dam foundation. The fault  can be divided into cataclasite rock belt and F41，F42  weak tectonic fault zones, which, known as fault muddy zones, are the weakest part in the dam foundation of the above hydropower station whose maximum dam height is more than 200 m. To provide geological data for the hydropower project design, we studied the engineering characteristics of the F41，F42  fault muddy zones on the basis of their geometrical features by means of engineering investigation, indoor test on the physical and mechanical properties of samples, in-situ adit test on the mechanical properties, and seepage deformation test on  undisturbed samples. According to  the test results, we proposed the recommended values of main physico-mechanical properties and permeability parameters of the fault muddy zones. The research  results indicate that treatments must be taken on the fault muddy zones owing to its weak engineering properties. It would provide reference for similar faults of other projects.