Utilizing the fly ash (FA) for environmental applications, such as for hazardous waste barrier materials, is one of the common alternative of FA utilization due to its several advantages, both on the chemical and the geotechnical view. However, the FA has a range variety of characteristics and tend to be site specific which depends on the coal source quality and the coal burning condition. Hence, an evaluation of the geotechnical properties of FA is needed to be conducted prior to its utilization in a specific application such as an additional mixture material in a barrier material to encapsulate the sulfide containing rock in an overburden dump, in which the stability and the integrity of the dump are the main issues that have to be maintained from a geotechnical point of view. The objectives of this research are to (i) characterize the engineering properties of FA and two types of overburden rock as well as its several mixture compositions; (ii) evaluate the properties of the FA-overburden mixtures as a barrier material candidate; and (iii) determine the most appropriate composition of material mixtures that provides a high degree of confidence in achieving an acceptable design criterion as a barrier layer. In order to address these objectives, several laboratory tests such as the consistency limit test, compaction test, and hydraulic conductivity test have been done on several compositions of FA-overburden rock mixture as well as the oxygen diffusion potency. The study concludes; is was that the 10 % w/w content of the FA in the material mixture is the maximum FA addition to achieve the recommended properties of the material mixture for an encapsulation purpose of the sulfide-containing overburden rock.