Spiral shear fracture networks are frequently observed during the cavity contraction of geomaterials, particularly in sedimentary rocks. Traditionally, this failure mode has been analysed using elastoplastic constitutive models, which homogenize the annular region dominated by spiral shears into a continuous plastic zone. However, a fracture mechanics-based approach offers a more physically representative alternative, given that explicit cracks are clearly observed in both field data and laboratory tests. This study proposes a novel 2D plane strain model wherein a set of uniformly distributed spiral fractures initiates and propagates around a cavity. The mechanical behaviour of the shear fractures is governed by the Mohr-Coulomb criterion and a dilatancy rule, which constrain the relationship between stresses and displacement discontinuities. The direction of fracture propagation is determined by minimizing plastic dissipation. To determine the density of shear fractures, a slip-weakening softening model is introduced, characterized by variable shear cohesion within the Mohr-Coulomb criterion. Specifically, cohesion decreases as the shear displacement discontinuity increases, enabling the model to capture the strain localization behaviour of geomaterials. The problem is discretized using the Displacement Discontinuity Method (DDM) and solved via a general optimization algorithm that simultaneously minimizes plastic dissipation and enforces the non-linear Mohr-Coulomb constraints. Simulation results reveal an unstable shear crack propagation pattern during cavity contraction, a phenomenon that cannot be captured without the slip-weakening model. Furthermore, this approach provides an alternative theoretical interpretation for the density of spiral shear fracture networks.