# An Investigation of the Effect of Utilizing Solidified Soil as Scour Protection for Offshore Wind Turbine Foundations via a Simplified Scour Resistance Test

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## Abstract

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## 1. Introduction

_{c}, is an important parameter that can be used to judge the occurrence of erosion. When the shear stress caused by the flow is lower than the critical shear stress, erosion will not occur. Briaud [30] tested the scour resistance of sand using an erosion function apparatus (EFA) and suggested that the critical shear stress of the sand is related to and numerically equal to the median grain size of the sand. In a flume experiment, Maniatis et al. [32] investigated the pebble transport mechanism at a fine scale by implanting a micro-electromechanical system in the pebbles to monitor the acceleration and the force to which the pebbles are subjected during erosion. Li et al. [33] used a computational fluid dynamics/discrete element method (CFD-DEM) coupled numerical model to analyze the erosion forces and trajectories of soil particles around the foundation in the local scour of a bridge. The authors have proposed using the simplified scour resistance test (SSRT) to evaluate the scour resistance of the soil samples. From an engineering perspective, however, it is still not easy to use these test methods due to their complicated operation and high costs. At the same time, an unconfined compression test is relatively simple and is commonly used in engineering practice. Once the relationship between the geotechnical indices (namely the unconfined compressive strength, UCS), and the scour resistance of the materials has been established, it can be convenient for engineers to evaluate the scour characteristics of cement-solidified soils for the purpose of using them as scour countermeasures.

## 2. Mechanism of Scour and Scour Protection

#### 2.1. Scour around Underwater Foundations

_{w}) and the foundation parameters (pile radius D), while the scour resistance of the soil (critical flow velocity V

_{c}or critical shear stress τ

_{c}) is influenced by the geotechnical parameters of the soil itself (the median particle size d

_{50}, cohesion c, etc.). The occurrence of the scour phenomenon can be determined using the following equation:

_{m}is the mean combined bed shear stress, τ

_{w}is the wave-induced bed shear stress, τ

_{c}is the current-induced bed shear stress, and ϕ is the angle between the wave and the current.

_{c}or critical shear stress τ

_{c}. Briaud [35] analyzed the results of EFA tests on sand and combined several tests with field data [36,37,38] to calculate critical shear stress based on the median grain size of sand. For cohesive soils, Briaud [30] recommended using the critical shear stress and the scour velocity obtained from EFA tests to predict the depth of erosion. However, the discrete nature of the soil parameters leads to different scour phenomena at different locations, and the above equation is empirical in nature.

#### 2.2. Use of Solidified Soils as Scour Countermeasure

## 3. Experimental Setup and Procedure

#### 3.1. Soil Samples and Testing Groups

_{2}generated during the production of sulfate aluminate is 40% less than that for ordinary silicate cement [43], which is more advantageous under current carbon emission policies. Considering the above factors, No. 425 sulphate aluminum cement was selected as the solidification material for the tests in the present study; the basic parameters of this cement are shown in Table 2. The cement was kept in a cool and dry place to prevent moisture from affecting the performance of the cement.

#### 3.2. Unconfined Compression Test

#### 3.3. Simplified Scour Resistance Test

_{c}is the critical shear velocity, l is the radius of the non-erosion area, and n is the rotation speed. It is worth noting that SSRT does not involve the simulation of complex hydraulic conditions (e.g., currents, waves, tides or their combinations) as is usually considered in macroscopic studies. The generated flow field is used to create a linear distribution of the flow velocity at the water–soil interface as well as to evaluate the scour resistance of soil samples when considering different stabilization conditions.

## 4. Results and Discussion

#### 4.1. Unconfined Compression Test Results

_{c}is the cement content (%). The calculated coefficient of determination R

^{2}is 0.9670; as this coefficient is greater than 0.95, the reliability of the regression results is considered to be high.

#### 4.2. Simplified Scour Resistance Test Results

#### 4.3. Relationship between Scour Resistance and Solidified Soil UCS

_{cr}of the scour resistance of the solidified soil should be generated by gravity W and the cohesion F between solidified soil clumps, as shown in Figure 16. According to the force analysis, the following equation can be obtained:

_{R}is the shear stress of the soil, c is the cohesion, φ is the internal angle of friction, W is the gravity of the soil clump, F is the cohesion between soil clumps, and A is the effective surface area of the soil cluster.

_{1}, confining pressure σ

_{3}, and the shear strength index in triaxial test can be expressed by Equation (8). For the UCT, the ultimate positive stress q

_{u}is σ

_{1}and σ

_{3}is equal to 0. Therefore, Equation (8) can be converted to Equation (9).

_{1}is the positive stress, σ

_{3}is the confining pressure, c is the cohesion, φ is the internal angle of friction, and q

_{u}is the UCS.

^{3}), h(x,y) is the topographic function after erosion (cm), S is the erosion area (cm

^{2}), D is the soil sample box radius (cm), D

_{cr}is the erosion critical radius (cm), and h

_{max}is the maximum erosion depth (cm).

^{2}, are all above 0.95, indicating that the fitted formulas have high reliability for the experimental results.

## 5. Conclusions

- The development pattern for the UCS of solidified soil containing a cement admixture can be approximated by fitting a power function. The fitted results can be used to estimate the UCS of solidified soils at the corresponding admixture levels;
- The direct solidification of natural soil with a high moisture content can ensure compatibility during construction, but the UCS is small. In engineering practice, for the solidification of silt with a high moisture content, water reduction measures should be considered to reduce the moisture content of the silt soil and to reduce the required amount of cement;
- The critical flow velocity, equilibrium erosion depth, and equilibrium erosion volume of the solidified soil are several parameters used to measure the scour resistance of the solidified soil, and these can be determined by conducting SSRTs. The fitted relationship between the parameters for scour resistance and the UCS can be obtained as shown in Table 5. In the initial setting state, the critical scour velocity of the solidified soil grows with the UCS, and the growth slows when the UCS is above 300 kPa. In the final setting state, the critical flow velocity of scouring of the solidified soil increases rapidly with the growth in UCS, and the critical flow velocity is above 3.14 m/s when the UCS is above 300 kPa;
- In the strength range of the test design, the critical scour velocity of the initial solidified soil tends to level off with increasing strength, while the critical scour velocity of the final solidified soil increases significantly with increasing strength. The test results show that the solidification state of the solidified soil has a great impact on its scour resistance, and the critical scour velocity of the final solidified soil increases by 80% to 150% as compared to the initial solidified soil at the same strength.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Annual installed offshore wind capacity (left axis) and cumulative capacity in GW (right axis) by country from 2010 to 2021.

**Figure 3.**Schematic diagram of solidified soil protection. (

**a**) In situ solidification (

**b**) Ex situ solidification.

**Figure 5.**Preparation of specimens for unconfined compression tests. (

**a**) Cylindrical moulds (

**b**) Specimens in the curing room.

**Figure 8.**Photographs of simplified scour resistance tests. (

**a**) Top view of specimen (

**b**) Side view of specimen (

**c**) Activation of erosion simulator.

**Figure 12.**Typical test results obtained from simplified scour resistance tests. (

**a**) Low strength solidified soil (

**b**) High strength solidified soil.

**Figure 13.**Erosion topography map for each test condition (initial setting). (

**a**) 50 kPa (w = 60 rad/s) (

**b**) 100 kPa (w = 50 rad/s) (

**c**) 150 kPa (w = 90 rad/s) (

**d**) 200 kPa (w = 100 rad/s) (

**e**) 300 kPa (w = 131 rad/s) (

**f**) 400 kPa (w = 153 rad/s).

**Figure 14.**Erosion topography map for each test condition (final setting). (

**a**) 50 kPa (w = 124 rad/s) (

**b**) 100 kPa (w = 123 rad/s) (

**c**) 150 kPa (w = 156 rad/s) (

**d**) 200 kPa (w = 201 rad/s).

**Figure 15.**Evolution of erosion of solidified soil (initial setting). (

**a**) Erosion line formation (

**b**) Erosion line extension/penetration (

**c**) Local intense erosion (

**d**) Erosion equilibrium.

**Figure 16.**Schematic diagram of the water flow and the scour resistance of solidified soil during erosion.

**Table 1.**Basic properties of silty clay in the ④ layer in Shanghai [33].

Moisture Content (%) | Density (g/cm ^{3}) | Pore-Solid Ratio | Liquid Limit | Plastic Limit | UCS (kPa) |
---|---|---|---|---|---|

36.0~49.7 | 1.64~1.79 | 1.12~1.67 | 34.4~50.2 | 19.0~26.0 | 42~77 |

Main Components | Rank | Specific Surface Area (m ^{2}/kg) | UCS (MPa) | Solidified Age (min.) | ||
---|---|---|---|---|---|---|

1-Day | 3-Day | Initial | Final | |||

Sulphate, aluminate | 425 | ≥350 | 30 | 42.5 | 15 | 30 |

Group * | Soil-to-Cement Ratio | Moisture Content of Solidified Soil | Solidified Age |
---|---|---|---|

U1 | 5% | 75% | 7 days |

U2 | 10% | 75% | 7 days |

U3 | 15% | 75% | 7 days |

U4 | 20% | 75% | 7 days |

U5 | 25% | 75% | 7 days |

U6 | 30% | 75% | 7 days |

Group | Targeted UCS (kPa) | Solidification States |
---|---|---|

S0 | control group | no curing |

S1, S2 | 50 | initial setting, final setting |

S3, S4 | 100 | initial setting, final setting |

S5, S6 | 150 | initial setting, final setting |

S7, S8 | 200 | initial setting, final setting |

S9, S10 | 300 | initial setting, final setting |

S11, S12 | 400 | initial setting, final setting |

Relationship | Solidified State | Formula (Units) | R^{2} Value |
---|---|---|---|

UCS vs. critical flow velocity | Initial | ${v}_{c}=4.359{q}_{u}{}^{0.5407}\begin{array}{cc}& (\mathrm{cm}/\mathrm{s})\end{array}$ | 0.9822 |

Final | ${v}_{c}^{t}=0.0003{q}_{u}{}^{2.37}+65.72\begin{array}{cc}& (\mathrm{cm}/\mathrm{s})\end{array}$ | 0.9991 | |

UCS vs. equilibrium erosion depth | Initial | $d=618.8{q}_{u}{}^{-0.9489}\begin{array}{cc}& (\mathrm{mm})\end{array}$ | 0.9854 |

Final | ${d}^{t}=-0.1775{q}_{u}{}^{0.5428}+4.334\begin{array}{cc}& (\mathrm{mm})\end{array}$ | 0.9504 | |

UCS vs. erosion volume | Initial | $V=5526{q}_{u}{}^{-1.097}\begin{array}{cc}& ({\mathrm{cm}}^{3})\end{array}$ | 0.9842 |

Final | ${V}^{t}=-1934{q}_{u}{}^{-0.0040}+1981\begin{array}{cc}& ({\mathrm{cm}}^{3})\end{array}$ | 0.9770 |

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**MDPI and ACS Style**

Wang, J.; Xie, J.; Wu, Y.; Wang, C.; Liang, F.
An Investigation of the Effect of Utilizing Solidified Soil as Scour Protection for Offshore Wind Turbine Foundations via a Simplified Scour Resistance Test. *J. Mar. Sci. Eng.* **2022**, *10*, 1317.
https://doi.org/10.3390/jmse10091317

**AMA Style**

Wang J, Xie J, Wu Y, Wang C, Liang F.
An Investigation of the Effect of Utilizing Solidified Soil as Scour Protection for Offshore Wind Turbine Foundations via a Simplified Scour Resistance Test. *Journal of Marine Science and Engineering*. 2022; 10(9):1317.
https://doi.org/10.3390/jmse10091317

**Chicago/Turabian Style**

Wang, Jing, Jinbo Xie, Yingjie Wu, Chen Wang, and Fayun Liang.
2022. "An Investigation of the Effect of Utilizing Solidified Soil as Scour Protection for Offshore Wind Turbine Foundations via a Simplified Scour Resistance Test" *Journal of Marine Science and Engineering* 10, no. 9: 1317.
https://doi.org/10.3390/jmse10091317