This paper introduces the Enhanced Anisotropic Damage Plasticity (eADP) model, a novel engineering constitutive approach tailored for tunnel analyses in Opalinus Clay – the designated host rock for the Swiss radioactive waste repository. The model's key innovations lie in its ability to comprehensively capture the complex behaviour of Opalinus Clay within an extremely efficient computational framework, making it suitable for routine engineering calculations and performance assessments. The eADP model adeptly reproduces Opalinus Clay's highly nonlinear stress-strain responses, accounting for anisotropic characteristics for stiffness, strength, and hardening. Honouring such complexity in material behaviour while keeping an efficient numerical performance was a key aspect of the eADP implementation. The model calibration relies on an extensive dataset derived from undrained triaxial tests performed on Opalinus Clay samples sourced recently from the Swiss candidate repository sites. The enhanced formulation and the multi-level optimisation scheme developed and applied in this work ensures a simple and robust parameter identification, showcasing the model's adaptability across varying loading conditions and confining pressures. The verification through hydromechanical computations at repository depths underscores the model's efficacy in realistic tunnel excavation scenarios.