Sublevel open stoping is widely used in many underground metal mines both in India and abroad. The major risk associated with this method, in terms of both stability and economy, is overbreak. For assessing stability, numerical analysis is adopted in many mines. The significance of varying geological and geotechnical parameters in geotechnical design, especially numerical analysis is often ignored or underestimated. Geological information about the host rock and other concomitant geological disturbances are the major factors to assess any potential failure. A significant challenge in geotechnical analysis arises from the fact that most existing design approaches—be they empirical, analytical, or numerical—are mainly developed for isotropic rock mass conditions. As a result, the influence of anisotropy is often underestimated or entirely neglected to simplify the design process and conform to conventional practices. Nevertheless, anisotropy can have a profound impact on the stability of underground excavations and must be carefully considered during geotechnical design. In many instances, practical observations indicate that anisotropy may exert a greater influence on failure mechanisms than stress regime. This paper demonstrates the importance of lithological contacts in stope design through numerical modelling in FLAC3D, using the Improved Unified Constitutive Model (IUCM) as the constitutive model. By comparing the results of an isotropic model with one that considers weak lithological contacts, it is found that accurate geological modelling and geotechnical characterization are crucial for any critical stope design. The paper also illustrates the adoption of IUCM as a constitutive model and validates its effectiveness with field results, highlighting the benefits of using IUCM in daily geotechnical model preparation.