Abstract: The knee-shaped deflection fractured zone located in the dam-foundation of a hydropower station is a main engineering geological problem of the project. The lithology is soft sandstone with poor integrity and pore cementation contact, which composed by cataclastic rock and clastic rock. The engineering mechanical properties of sandstone may have a significant impact on the deformation and stability of the dam. Based on the characteristics of loose organizational structure, high moisture content and poor physical mechanical properties, the basic physical properties tests are carried out to fractured zone sandstone first. It's considered the sandstone which is extremely complex microstructure and good permeability as small porosity sandstone. Secondly, the conventional and field tests are carried out to study mechanical properties. Finally, the influences of the fractured zone are discussed based on a certain macromechanical parameter. The results show that, the internal structure of sandstone is damage seriously; and main mineral compositions are quartz, feldspar, sericite and so on; the main chemical constituent is SiO2; but the chemical erosion is not significant. The shear curve shows the characteristics of four distinct stages. The failure mechanism of triaxial compression test is plastic shearing; and the curve shows a low strength, significantly ductility dilatancy; and yield platform is formed. Also the cataclastic rock strength is slightly higher than clastic rock. The field tests show that the deformation of rock mass is uniform, but the deformation modulus is low of which the range is 60-630 MPa. And the foundation bearing capacity ranges from 0.8-2.3 MPa. The research results provide greatly referential value for dam foundation of hydropower station.

Key words: rock mechanics, hydropower station, deflection fractured zone, cataclastic rock, clastic rock, physical mechanical properties, experimental investigation