# Experimental Study on Silty Seabed Liquefaction and Its Impact on Sediment Resuspension by Random Waves

^{1}

^{2}

^{3}

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## Abstract

**:**

## 1. Introduction

## 2. Experimental Design and Data Processing

#### 2.1. Experimental Flume and Instruments

#### 2.2. Soil Parameters

_{50}) of 46.5 μm. This shows that there was a large number of fine particles in the experiment soil, meaning it had low permeability and was apt to liquefaction [16,17].

#### 2.3. Experimental Wave Conditions and Procedures

#### 2.4. Data Processing

#### 2.4.1. Soil Liquefaction Criteria

^{2}) [18]:

_{0}is the static lateral earth pressure coefficient, representing the ratio between horizontal and vertical effective stress (k

_{0}is not a constant value for silt) which was set at 0.57 for these experiments; $z$ is the depth from the soil surface; and ${\mathsf{\u03d3}}^{\prime}={\mathsf{\u03d3}}_{s}-{\mathsf{\u03d3}}_{w}={\rho}^{\prime}g$ is the submerged unit weight of the test soil, $g$ is the gravitational acceleration, ${\rho}^{\prime}$ is the effective density of soil, which is defined as the density of soil minus the density of water. So, ${\mathsf{\u03d3}}^{\prime}$ is related to water content and saturation degree [19]. The measured initial submerged unit weight ${\mathsf{\u03d3}}^{\prime}$ of the test soil in experiments I, II, III, and IV before the wave action is 8.85, 8.46, 8.32, and 5.46 kN/m

^{3}, respectively. It should be mentioned that the submerged unit weight of the test soil changed during the wave action process, but we did not sample the test soil and measure the ${\mathsf{\u03d3}}^{\prime}$ value during the experiments to avoid the artificial disturbance to the test soil, so we used the initial measured ${\mathsf{\u03d3}}^{\prime}$ values for calculating the initial mean normal effective stress ${\sigma}_{0}$.

#### 2.4.2. Method for CALCULATING SSC

^{2}of 0.9986 indicates that the conversion was reliable enough to accurately estimate the SSC.

#### 2.4.3. Method for Calculating Wave Shear Stress

^{2}) is described in Equations (6)–(8) [23]:

_{w}is the wave friction factor, $\omega =\frac{2\pi}{T}$ is the angular or radian frequency,$k=\frac{2\pi}{L}$ is the wave number, ${k}_{s}=\frac{2.5{d}_{50}}{30}$ is the bottom physical roughness, and ${A}_{\delta}=\frac{{U}_{w}T}{2\pi}$ is the near-bottom excursion amplitude.

#### 2.4.4. Method for Calculating Turbulent Kinetic Energy (TKE)

^{2}/s

^{2}) is described by Equation (10) [26]:

## 3. Experimental Results

#### 3.1. Excess Pore Pressure Response to Random Waves in Nonliquefied Soil

#### 3.2. Excess Pore Pressure Response to Random Waves in Liquefied Soil

#### 3.3. Sediment Resuspension Induced by Random Waves

## 4. Discussion

_{t}and v’

_{t}were similar, with a maximum of approximately 0.04 m/s; and the value of w’

_{t}was small, with a maximum of approximately 0.01 m/s. TKE was also small, with a maximum of 0.0008 m

^{2}/s

^{2}, which owed to the seabed being flat at the initial stage of wave impact. This planar surface was not conducive to the generation of turbulence, so the effect of turbulence on the seabed was small (shaded area in Figure 12a). With the continuous impact of waves, the seabed gradually liquefied, and its interface began to fluctuate with random waves. This was conducive to the generation of turbulence, and TKE gradually increased. When varying degrees of liquefaction occurred in all parts of the seabed (t = 60 s), u’

_{t}and v’ increased to 0.16 m/s, w’

_{t}increased to 0.03 m/s, and TKE increased to 0.015 m

^{2}/s

^{2}. Owing to the increase of TKE, the turbulent shear of the seabed was enhanced, increasing the SSC to 10.9 g/L at 2 cm above the seabed (Figure 12a,b). As seabed liquefaction continued to increase, the vibration amplitude of the seabed interface also increased, leading to higher turbulent velocity and TKE. The values of u’

_{t}and v’

_{t}increased to 0.35 m/s (9 times that at the nonliquefied state), w’

_{t}increased to 0.06 m/s (6 times that at the nonliquefied state), and TKE increased to 0.18 m

^{2}/s

^{2}(200 times that at the nonliquefied state). In this stage, turbulent waves had a more drastic impact on the seabed, increasing the SSC at 2 cm above the seabed to 11.9 g/L (Figure 12c,d).

## 5. Conclusions

- (1)
- The excess pore pressure in the nonliquefied seabed oscillated with wave fluctuations, but there was a net upward pressure gradient, which possibly promoted sediment resuspension.
- (2)
- After seabed liquefaction, there were abrupt changes in the waveforms of excess pore pressure, generating asymmetric crests and troughs with relatively flat crests and sharp troughs. Seabed liquefaction first occurred in the shallow layers, and expanded downward. Large-amplitude waves dissipated excess pore pressure and small-amplitude waves accumulated it. This response differed to that of the nonliquefaction state.
- (3)
- Seabed liquefaction accelerates sediment resuspension in four ways: by reducing the critical shear stress of the soil, by forming seepage channels inside the seabed soil, by forming mud waves and leading to an increase in TKE, and by dissipating excess pore pressure, resulting in the porewater carrying fine-grained sediment upward into the water body, causing an increase in SSC.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Layout of the experimental flume and particle size distribution curve of the experimental soil.: (

**a**) layout of the experimental flume; (

**b**) layout of measuring instruments and sensors; (

**c**) particle size distribution curve of the experimental soil. Note: d

_{50}is the average particle size of the soil and CC is the percentage of clayey particles in the soil.

**Figure 3.**Time-series of random waves, excess pore pressure, and cumulative pore pressure during the experiment process: (

**a**) time-series of random waves (blue curve) and excess pore pressure at a depth of 10 cm (red curve); (

**b**) time-series of random waves (blue curve) and excess pore pressure at different depths (other colors, detailed view); (

**c**) cumulative pore pressure at different depths over time, with the dashed line indicating the time taken to reach the first crest at distinct depths.

**Figure 4.**Time-series of random waves and excess pore pressure at various depths in experiment IV-2: the shaded bar shows ${\sigma}_{0},\text{}\mathrm{the}$initial mean normal effective stress of the seabed; and the white curve shows cumulative pore pressure.

**Figure 5.**Excess pore pressure response for liquefied seabed impacted by random waves in Experiment IV-2 (detailed view): blue curve—random waves; orange curve—excess pore pressure response at 5 cm below the seabed; black curve—excess pore pressure response at 30 cm below the seabed;${\sigma}_{0}$—initial mean normal effective stress of the seabed.

**Figure 6.**Variation on the water–soil interface during a single wave cycle. (

**a**–

**d**) is the photograph at 512, 512.5, 513 and 513.5 s after the wave action in experiments IV-2, respectively.

**Figure 7.**Fluctuation of mud waves along with water waves through time: blue curve—random waves; black curve—seabed interface.

**Figure 8.**Response of SSC impacted by the waves: (

**a**,

**b**) represent experiments II-3 and II-4, with effective wave heights of 16 cm and 18 cm, respectively, depicting the SSC response of nonliquefied seabed; (

**c**,

**d**) represent experiments IV-2 and IV-3, with effective wave heights of 16 cm and 18 cm, respectively, depicting the SSC response of liquefied seabed.

**Figure 9.**Change in SSC with time (at 5 cm above the seabed): red line—liquefied seabed (Experiment IV-2); and black line—non-liquefied seabed (Experiment II-3).

**Figure 10.**Relationship between SSC (at 5 cm above the seabed) and wave-induced shear stress in the (

**a**) non-liquefied and (

**b**) liquefied states; the red curve is SSC and the black curve is wave-induced shear stress, ${\tau}_{w}$.

**Figure 11.**Changes through time in response to wave condition IV-2: (

**a**) random wave elevation, (

**b**) excess pore pressure (z = 5 cm), (

**c**) SSC (at 5 cm above the seabed), and (

**d**) the measured water flow velocity at 5 cm above the seabed.

**Figure 12.**Time-variation of the turbulent velocity, TKE, and SSC (at 2 cm above the seabed): (

**a**,

**b**) are the nonliquefied state and the initial stage of the liquefied state, respectively, where 𝑢’

_{t}, v’

_{t}, and w’

_{t}represent the longitudinal, transverse, and vertical components of the turbulence, and TKE is the turbulent kinetic energy; (

**c**,

**d**) represent the later stages of liquefaction.

Instrument | Model | Sampling Rate | Precision | Range of Measurement |
---|---|---|---|---|

Pore pressure sensor | CYY2 piezoresistive sensor | 16 Hz | 0.5% | 0–10 kPa |

Wave gauge | Rod-shaped capacitive wave height gauge | 50 Hz | 0.2% | 0.5–50.0 cm |

Current meter | HR-ADCP | 1 Hz | 1% | 1–25 cm |

ADV | 32 Hz | 0.5% | 2 cm above the seabed | |

Turbidity profiler | ASM-IV | 1 Hz | 1% | 1–96 cm |

Wave Condition | H (cm) | T (s) | D (cm) | Seabed Response |
---|---|---|---|---|

I-1 | 14 | 1.5 | 50 | No liquefaction |

I-2 | 14 | 2.0 | 50 | No liquefaction |

I-3 | 14 | 2.2 | 50 | No liquefaction |

I-4 | 14 | 2.5 | 50 | No liquefaction |

II-1 | 10 | 2.0 | 50 | No liquefaction |

II-2 | 14 | 2.0 | 50 | No liquefaction |

II-3 | 16 | 2.0 | 50 | No liquefaction |

II-4 | 18 | 2.0 | 50 | No liquefaction |

III-1 | 14 | 2.0 | 55 | No liquefaction |

III-2 | 14 | 2.0 | 50 | No liquefaction |

III-3 | 14 | 2.0 | 45 | No liquefaction |

III-4 | 14 | 2.0 | 40 | No liquefaction |

IV-1 | 14 | 2.0 | 50 | Liquefaction |

IV-2 | 16 | 2.0 | 50 | Liquefaction |

IV-3 | 18 | 2.0 | 50 | Liquefaction |

IV-4 | 18 | 2.0 | 50 | Liquefaction |

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## Share and Cite

**MDPI and ACS Style**

Dong, J.; Xu, J.; Li, G.; Li, A.; Zhang, S.; Niu, J.; Xu, X.; Wu, L.
Experimental Study on Silty Seabed Liquefaction and Its Impact on Sediment Resuspension by Random Waves. *J. Mar. Sci. Eng.* **2022**, *10*, 437.
https://doi.org/10.3390/jmse10030437

**AMA Style**

Dong J, Xu J, Li G, Li A, Zhang S, Niu J, Xu X, Wu L.
Experimental Study on Silty Seabed Liquefaction and Its Impact on Sediment Resuspension by Random Waves. *Journal of Marine Science and Engineering*. 2022; 10(3):437.
https://doi.org/10.3390/jmse10030437

**Chicago/Turabian Style**

Dong, Jiangfeng, Jishang Xu, Guangxue Li, Anlong Li, Shaotong Zhang, Jianwei Niu, Xingyu Xu, and Lindong Wu.
2022. "Experimental Study on Silty Seabed Liquefaction and Its Impact on Sediment Resuspension by Random Waves" *Journal of Marine Science and Engineering* 10, no. 3: 437.
https://doi.org/10.3390/jmse10030437