Аннотация:A geomechanical model is developed to simulate the formation and propagation of a defect meant as a crack (fracture) with a nonzero thickness. The modelling is performed with the axiomatics of deformable solid mechanics, using the concept of a prefracture zone. The prefracture zone is a part of a solid where the properties of material change under external loads. The behaviour of the prefracture zone is explained in terms of repeated superposition of large deformations when non-local strength criteria apply. The model is tested through finiteelement analysis of fracture growth.