The spiral shear fracture networks are commonly found during the cavity contraction process of geomaterials, particularly in sedimentary rocks. This type of failure was often analyzed and simulated by adopting an elastoplastic constitutive model in the rock material, where the ring region near the borehole dominated by spiral shear fractures deforms as plastic behavior. However, fracture mechanics based analysis is a better approach to this problem since explicit cracks have been observed both in the field and in laboratory tests. This study proposes a novel 2D plane strain model, in which a group of evenly distributed spiral fractures are initiated and propagated around a cavity. The Mohr-Coulomb criterion and dilatancy rule are imposed on the shear fractures to constrain the stresses and the displacement discontinuities. The shear fracture propagation direction is then determined by minimizing plastic dissipation. To determine the proper number of shear fractures, the slip-weakening softening model is introduced with a variant shear cohesion in the Mohr-Coulomb criterion. The cohesion decreases as the shear displacement discontinuity increases, which allows the model to simulate the localization behavior of geomaterials. The displacement discontinuity method is used to numerically solve the discretized problem, in which a general optimization algorithm is adopted to both minimize the plastic dissipation and constrain the non-linear Mohr-Coulomb criterion. Simulation results reveal an unstable shear crack propagation pattern in the cavity contraction problem, which is unobtainable without the slip-weakening softening model. Furthermore, this approach also leads to an alternative interpretation of the density of the spiral shear fracture networks.