Estimating undrained shear strength in very-soft marine clays is challenging with traditional tests. This study uses shallow penetration tests with circular plate penetrometers on lime-treated soft clays. The tests yield consistent penetration force profiles corresponding to undrained strengths from 2 to 34 kPa. A simple depth-dependent empirical formula is proposed for the bearing capacity factor, validated against laboratory triaxial test data showing strong agreement. Although direct visual observation of cavity closure in clay is limited by soil opacity, indirect evidence suggests that soil flow and partial cavity closure influence shallow penetration resistance. The novelty of this work lies in adapting numerical solutions originally devised for different geotechnical problems into a practical, systematic, and user-friendly formulation tailored for field penetration testing of surficial ultra-soft clays. This adaptation bridges the gap between complex numerical models and onsite applicability. The validated closed-form N_c relationship highlights the importance of cavity closure mechanisms in interpreting shallow penetration resistance in ultra-soft, lime-treated clays. These findings facilitate rapid, reliable laboratory estimation of undrained shear strength in challenging soft clay environments.