*3.3. Pore Pressure*

The pore pressure is an important indicator of the properties of soil masses [27,28]. A total of seven wave application processes each with a duration of approximately 1 hour were performed. It was found from Figure 10 that the pore pressure in the seabed soil body significantly accumulated

*3.3. Pore Pressure* 

and could reach the maximum cumulative pore pressure within a short time (approximately 5 min) and remained stable. The fine-grained seabed had low permeability, and it was difficult to quickly dissipate the excess pore water pressure caused by the wave. was found from Figure 10 that the pore pressure in the seabed soil body significantly accumulated and could reach the maximum cumulative pore pressure within a short time (approximately 5 minutes) and remained stable. The fine-grained seabed had low permeability, and it was difficult to quickly dissipate the excess pore water pressure caused by the wave.

seven wave application processes each with a duration of approximately 1 hour were performed. It

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**Figure 10.** Excess pore pressure during the wave action. The curves thorough from (**a**) to (**g**) represent the excess pore pressure curves for the first to seventh wave application process; (**h**) excess pore pressure of the seabed for different wave application processes at a depth of 20 cm; (**i**) excess pore pressure of the seabed at a depth of 35 cm from different wave application processes. **Figure 10.** Excess pore pressure during the wave action. The curves thorough from (**a**–**g**) represent the excess pore pressure curves for the first to seventh wave application process; (**h**) excess pore pressure of the seabed for different wave application processes at a depth of 20 cm; (**i**) excess pore pressure of the seabed at a depth of 35 cm from different wave application processes.

Figure 10 shows the excess pore pressure curve at a depth of 20 cm and 35 cm in the seabed during seven wave application processes. The figures from 10a to 10g show that the excess pore pressure at a depth of 35 cm was slightly greater than that at a depth of 20 cm, indicating that the greater the depth was, the harder it was for the excess pore pressure to dissipate. The excess pore pressure data from the seven wave applications at the same depth (Figure 10h,i) indicated that the maximum excess pore pressure formed from the process of the last wave application was lower than that formed from previous wave applications. For example, the maximum pore pressure at a depth of 20 cm of the seabed hit approximately 1.4 kPa in the first wave application, approximately 1.0 kPa in the third wave application, and approximately 0.2 kPa in the seventh wave application. The maximum excess pore pressure inside the soil mass showed a decrease trend with the increase in the Figure 10 shows the excess pore pressure curve at a depth of 20 cm and 35 cm in the seabed during seven wave application processes. The figures from 10a to 10g show that the excess pore pressure at a depth of 35 cm was slightly greater than that at a depth of 20 cm, indicating that the greater the depth was, the harder it was for the excess pore pressure to dissipate. The excess pore pressure data from the seven wave applications at the same depth (Figure 10h,i) indicated that the maximum excess pore pressure formed from the process of the last wave application was lower than that formed from previous wave applications. For example, the maximum pore pressure at a depth of 20 cm of the seabed hit approximately 1.4 kPa in the first wave application, approximately 1.0 kPa in the third wave application, and approximately 0.2 kPa in the seventh wave application. The maximum excess pore pressure inside the soil mass showed a decrease trend with the increase in the number of wave actions, indicating that the whole soil mass tended to be stable. It was consistent with the experimental phenomenon that the sliding surface gradually moved up and became shallow with wave application process, and that the thickness of the oscillation layer reduced. Each wave application process might lead to an increase in the excess pore pressure of the seabed sediment. However, the liquefaction

resistance of the sediment improved upon each wave action process, resulting in a decrease in the excess pore pressure caused by the next process of wave action. This might be due to the fact that the wave action can promote the rearrangement of sediment particles. The experiment results show that the seabed is the most unstable in the initial stage of rapid deposition of sediment, and it is most likely to be destroyed under wave action. Due to the limitations of laboratory experiments [29,30], more field work is required to confirm these experimental results. However, the liquefaction resistance of the sediment improved upon each wave action process, resulting in a decrease in the excess pore pressure caused by the next process of wave action. This might be due to the fact that the wave action can promote the rearrangement of sediment particles. The experiment results show that the seabed is the most unstable in the initial stage of rapid deposition of sediment, and it is most likely to be destroyed under wave action. Due to the limitations of laboratory experiments [29,30], more field work is required to confirm these experimental results.

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number of wave actions, indicating that the whole soil mass tended to be stable. It was consistent with the experimental phenomenon that the sliding surface gradually moved up and became shallow

application process might lead to an increase in the excess pore pressure of the seabed sediment.

### *3.4. Sediment Strength 3.4. Sediment Strength*

The overlying water mass was discharged after the seventh wave action, and the entire soil sample was subject to a penetration strength experiment with a micro penetration instrument. The diameter of the mini penetrometer was 3 cm, the horizontal separation distance between test points was 15 cm, and the vertical depth interval was 5 cm. As shown in Figure 11, the penetration strength of the soil mass was substantially greater than 50 kN in the depth range from 0 cm to 20 cm, and the penetration strength of the soil mass ranging from 8 cm to 13 cm was greater, with a mean value greater than 80 kN, indicating there was a high-strength formation at this depth. The soil mass had a relatively large variation in strength gradient, with the strength of the soil mass decreasing from 60 kN to 30 kN in this 7-cm-thick range. The penetration strengths of the soil mass with the depth ranging from 20 cm to 40 cm were less than 30 kN, and most of the values were approximately 10 kN, being less than the penetration strength in the hard formation of the surface strength. By comparing the strength distribution law for sliding surface and soil penetration, it was found that the depth of the high-strength formation was slightly less than the depth (15 cm) of maximum sliding surface, that is, the high-strength formation was located above the sliding surface. This indicates that exiting surface sediment undergoes multiple wave actions, and its strength is improved in the absence of new deposition conditions. The overlying water mass was discharged after the seventh wave action, and the entire soil sample was subject to a penetration strength experiment with a micro penetration instrument. The diameter of the mini penetrometer was 3 cm, the horizontal separation distance between test points was 15 cm, and the vertical depth interval was 5 cm. As shown in Figure 11, the penetration strength of the soil mass was substantially greater than 50 kN in the depth range from 0 cm to 20 cm, and the penetration strength of the soil mass ranging from 8 cm to 13 cm was greater, with a mean value greater than 80 kN, indicating there was a high-strength formation at this depth. The soil mass had a relatively large variation in strength gradient, with the strength of the soil mass decreasing from 60 kN to 30 kN in this 7-cm-thick range. The penetration strengths of the soil mass with the depth ranging from 20 cm to 40 cm were less than 30 kN, and most of the values were approximately 10 kN, being less than the penetration strength in the hard formation of the surface strength. By comparing the strength distribution law for sliding surface and soil penetration, it was found that the depth of the high-strength formation was slightly less than the depth (15 cm) of maximum sliding surface, that is, the high-strength formation was located above the sliding surface. This indicates that exiting surface sediment undergoes multiple wave actions, and its strength is improved in the absence of new deposition conditions.

**Figure 11.** Sediment penetration strength profile after the experiment. **Figure 11.** Sediment penetration strength profile after the experiment.

#### **4. Conclusions 4. Conclusions**

The instability behavior of the fine-grained seabed under wave action was simulated in in-lab flume experiments in this work. The main experimental results are summarized as follows: The instability behavior of the fine-grained seabed under wave action was simulated in in-lab flume experiments in this work. The main experimental results are summarized as follows:

	- (2) We found the first evidence for possible gas migration due to wave action. The presence of gas may serve as a primer for submarine slope failures, which suggests a new mechanism for seafloor instability in the Yellow River Delta.

**Author Contributions:** Conceptualization, H.L.; Data curation, X.W. and C.Z.; Formal analysis, X.W. and C.Z.; Funding acquisition, H.L.; Writing—original draft, X.W. and C.Z.; Writing—review & editing, X.W., C.Z., and H.L. The final manuscript has been approved by all the authors.

**Funding:** This research was funded by the National Natural Science Foundation of China (grant number 41877223) and Shandong Provincial Key Laboratory of Marine Ecology and Environment & Disaster Prevention and Mitigation (grant number 201801).

**Acknowledgments:** Authors acknowledge the experimental and technical support provided by Minsheng Zhang.

**Conflicts of Interest:** The authors declare no conflict of interest.
