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Mechanism and evaluation of the sinking of small heavy objects caused by wave-induced liquefaction of seabed
Jingyi Zhang, Hu Wang, Yuru Yan, Xuefeng Zhang and Zhe Huang

In this paper, the physical mechanism and evaluation methods of the sinking of small heavy objects into a wave-induced liquefied seabed are particularly investigated. First, the sinking patterns and mechanisms of an object under different forms of liquefaction (i.e., residual liquefaction and momentary liquefaction) are analyzed. Then, a viscous approach for estimating the sinking of the object into a wave-induced liquefied seabed is proposed. The framework, the procedures, and the influencing factors of the approach are discussed in detail. Finally, cases are discussed to assess the performances of the models. Results and analysis indicate that the object sinks continuously to the rigid bottom of the residual liquefied seabed at a constant velocity, while the object alternately sinks and stops sinking in a momentarily liquefied seabed because the seabed alternates between liquefied and non-liquefied states during every wave period. The process of objects sinking into a wave-induced liquefied seabed can be well represented by solving the mechanical equilibrium equations for objects sinking in an isotropic dense viscous fluid. For the residual liquefaction case, the final sinking depth of object can be obtained by multiplying the sinking velocity by the liquefaction duration, with its value not exceeding the liquefaction depth. While for wave-induced momentary liquefaction case, the possible sinking depth in a single wave period is calculated by multiplying the sinking velocity by the liquefaction duration, if the initial sinking position is shallower than the maximum liquefaction depth during the wave period, this product is added to the total sinking depth, the final sinking depth is obtained by adding the sinking depths during each wave period together. Two viscosity regions of the liquefied soil are delineated as 1–10 kPa·s and 100–5000 kPa·s based on the comprehensive summary of the experimental data and the case analysis of objects sinking in a wave-induced liquefied seabed. The viscosity can be approximately taken as 10 kPa·s in Region I and 1000 kPa·s in Region II for regular-shaped objects sinking in wave-induced residual and momentarily liquefied seabed, respectively. Equations for a spherical object with an equivalent diameter and a more concentrated viscosity in the vicinity of 10 kPa·s are suggested to enable better evaluation of the sinking of irregularly shaped munitions in a wave-induced momentarily liquefied seabed.

Keywords: small heavy object, object sinking, wave-induced seabed liquefaction, viscosity approach, quantitative evaluation