EN ECHELON CRACK-ARRAYS IN POTASH SALT ROCK

There are two types of fracture patterns in the yield pillars of the p otash mines of Saskatchewan. The individual members of both patterns a re tensile (extension) fractures that propagate parallel with the maxi mum principal stress trajectory (perpendicular to the minimum principa l stress). The difference between the two patterns lies in the arrange ment of the member fractures. In the en echelon tensile crack-array, t he macroscopic fracture consists of individual tensile cracks that are slightly offset from each other. They have only a small overlap and t he child crack seems to form randomly on either side of its parent. Co nsequently, the en echelon tensile crack-array inherits the axial orie ntation of its members. In contrast, the tensile cracks of an en echel on shear crack-array, have a larger overlap and their lateral displace ment from each other is biased in one direction. Therefore, the crack- array is no longer axial but inclined 20-25 degrees from the maximum p rincipal stress direction. With increasing stress, the shear crack-arr ay often collapses, forming the envelope or hourglass structures of th e potash mines.