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Study on undrained shear strength of seabed by free fall penetration test
Linsen Wang, Xuezhi Feng, Chaoqi Zhu, Lu Dequan，Zhu Xianming, Zhu Na and Yonggang Jia

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.

Keywords: free fall penetration, undrained shear strength, in situ testing, seabed