Abstract: The key character of dispersive clay is to be dispersed and wrecked by water flows,causing serious threats to engineering safety.But those studies on dispersive clay,such as the methods of identification,dispersive mechanism and their applications,are still not deep enough and satisfactory due to their trivial discovery and few engineering practices.Based on the analysis of physical and chemical characters and mineralogical composition,the dispersivity and dispersive mechanism of the dam soil samples in Wenjiagou Reservoir are studied by double-hydrometer tests,pore water salts tests,pinhole tests,crumb tests and exchangeable sodium percentage tests,and further investigations on the impermeability of soil samples are also carried out by itself and under the protection of inverted filter materials.The results show that soil samples are transitional and dispersive soil because of rich montmorillonoid,plenty of sodium-ion and strong alkalescence.The soil samples have weak self-impermeability,but favorable impermeability under the protection of the inverted filter materials.The studies indicate that soil becomes dispersive clay only under the conditions of rich montmorillonoid contents,more sodium-ion and strong alkalescence,and the dispersive clay presents a better seepage prevention under the protection of appropriate inverted filter materials.