FEM analysis of deformation localization mechanisms in a 3-D fractured medium under rotating compressive stress orientations

摘要

Stress distributions and deformation patterns in a medium with a pre-existing fracture set are analyzed as a function of the remote compressive stress orientation (σ_H) using finite element models with increasingly complex fracture configurations. Slip along the fractures causes deformation localization at the tips as wing cracks or shear zones. The deformation intensity is proportional to the amount of slip, attaining a peak value for α=45° (α: angle between the fracture strike and σ_H) and slip is linearly proportional with fracture length. Wing cracks develop for high deformation intensities for 30°<α<60°, whereas primary plastic shear zones develop for low deformation intensities. Additionally, two types of secondary shear zones develop for α<30° and α>60°, with constant angles of 135° and -60° with σ_H, respectively.Mechanical interaction between fractures in a fracture zone, quantified as change in slip compared to an isolated fracture, decreases with increasing fracture separation. Fracture underlap elongates the fracture length and therefore increases the amount of slip, while fracture overlap exhibits the opposite effect. Fracture slip decreases with an increasing amount of directly adjacent fractures. Mechanical interaction becomes negligible for fracture configurations with spacing-to-length and spacing-to-overlap ratios exceeding 0.5 and that in this case fractures are decoupled.Independent of the pre-existing fracture configuration, the development of a secondary systematic fracture set driven by a remote stress rotation is dominated by σ_H; development of wing cracks or shear zones is restricted to the fracture tips. Blocks with tapered geometries are present in models with a variable fracture strike, where the maximum principal stress (σ_1, applying the geological convention that compressive stresses are positive) trajectories consistently deviate from σ_H; the presence of two systematic σ_1 trajectory orientations suggests that two types of secondary features could develop in one re-activation phase.