Abstract: The F22 fault is developed on the right bank slope of Nanyo 4 hydropower station in Laos, it passes through JHJ10 section of the dam foundation.  The lithology is divided into cataclastic rock belt and soft tectonic rock belt (debris and mud type). To provide the engineering geological data necessary for the engineering design of hydropower stations and to quantify the geometric characteristics of F22 fault development, geological investigation exploration, in-situ and laboratory physical and mechanical properties tests, water pressure test were conducted. Characteristics of the fracture zone, including distribution and size, mineral composition, physical and mechanical properties and engineering characteristics, are also investigated. The results show that there are no high expansive minerals such as montmorillonite exist in the argillaceous slate. The disintegration test indicates that the argillaceous slate softs and disperses quickly after soaking in water, which proves that the argillaceous slate has poor hydrologic properties, and the silty slate has good hydrologic properties. There are grooves on the upstream side of F22 fault zone in the dam foundation, which can solve the seepage problem in dam foundation. The deformation of the dam foundation can be mitigated by “bridging” the fault with concrete pad near the fault zone.

Key words: F22 Fault, Slope, Dam Foundation, Engineering Characteristic, Treatment Measure