Effect of weak inclusions on the fracture toughness of interfaces between various rocks

•The effect of weak inclusions on the fracture toughness of rock interfaces is described.•The fracture toughness coefficient varies from 0.832 to 1.089 for gypsum stone.•The fracture toughness coefficient varies from 2.447 to 2.461 for flint layer.•Digital image processing of clays on the fractures between rock layers was applied.•Layers of carbonaceous clays weaken the interfaces between gypsum stone and flint. Fracture toughness (FT) is one of the parameters that characterise the tendency of materials to crack and fracture. FT is usable for individual materials (such as metals, concrete, and composites) and the interfaces between them. The fracture toughnesses of interfaces between different rocks or minerals are also of interest when assessing the stability of rock mass in mining and building construction. The novelty of this work is regarding the fracture toughness of interfaces between various types of rocks. Standardized techniques exist for metals, concrete, and composite materials. For rocks, the investigation uses the methodology recommended by the International Society of Rock Mechanics (ISRM). The paper contains the FT values for gypsum stone, flint, as well as for the interfaces between them, which also include interlayers of carbonaceous clays. The regression equation describes dependence of the fracturetoughness on the relative area of carbonaceous clays interlayers, with the correlation factor R = –0.714.