Study on undrained shear strength of seabed by free fall penetration test

Understanding the physical properties of the seabed is crucial for effective marine engineering. The free-fall penetrometer, driven by gravity, offers a cost-effective and convenient method for assessing the undrained shear strength of the seabed in situ. However, due to the rapid penetration rate and intense interaction between the penetrometer and seabed, the penetration mechanism becomes complex, leading to varying results with different data interpretation methods. This study consolidates calculation approaches for undrained shear strength using acceleration and dynamic cone penetration resistance data. Through in-situ experiments in the Yellow Sea, a systematic quantitative analysis of penetration behavior was conducted. Comparative analysis of computed undrained shear strengths using acceleration and dynamic cone penetration resistance data reveals the strengths and weaknesses of both methods, prompting refinement. Findings indicate that in soft sediments, dynamic cone resistance increases significantly while acceleration remains relatively constant. Conversely, in consolidated sediments, acceleration sharply increases while dynamic cone resistance rises slowly. Thus, the acceleration-based method is unsuitable for ultra-soft sediments like seabed silt or unconsolidated mud. However, in higher seabed strength conditions, it aligns with dynamic cone resistance results. Acceleration data uniquely aids in identifying seabed layering characteristics, while dynamic cone resistance helps determine mud or silt strength, albeit with less sensitivity to sediment layering characteristics.