Geotechnical characterization of Estonian oil shale semi-coke deposits with prime emphasis on their shear strength

Oil shales are considered as an alternative energy resource because they contain bituminous organic matter that can be extracted by retorting. However, oil shales have typically a high mineral content (up to 80%), thus an inevitable issue is the formation of environmentally hazardous solid waste that is piled into large depositories next to oil extraction plants. In Estonia more than 100 Mt of oil shale retorting waste (called semi-coke) is deposited in open dumps. Mechanical stability of these deposits as well as optimal landfill design depend on the geotechnical properties of the waste. The aim of the study was to examine the geotechnical properties of semi-coke waste dumps with the focus on shear strength and cementation. The results from this study indicate that shear strength is controlled not only by the frictional component but also by the cohesive component. Cohesion is mainly developed due to the secondary cementing mineral ettringite, which gives the deposits additional mechanical stability by forming monolithic zones. It is found that this cementation is not permanent due to the lowering of pH and mechanical crushing at high normal stress. Also slight hydrophobicity of semi-coke was recorded, which may influence the behavior of shear strength and saturation together with the complex inner structure.