# Model Tests of Soil Reinforcement Inside the Bucket Foundation with Vacuum Electroosmosis Method
## 1 Introduction  
Offshore wind farms have been widely developed in recent years due to abundant offshore wind, lack of occupied land, and lack of pollution. Common types of offshore wind power generator foundations include single pile foundations, gravity foundations, jacket foundations, and floating foundations [1; 2]; however, bucket foundations have been widely examined and employed in recent years due to their simple construction, low cost, and recyclability [3; 4; 5; 6; 7; 8]. In 2018, the world's largest single-capacity 8.8 MW wind turbine was successfully installed in the Aberdeen offshore wind farm in Scotland, and its foundation consists of four bucket foundations with a diameter of 9.5 m and height of 15 m [9]. The composite bucket foundation (CBF) is a new type of bucket foundation proposed by Tianjin University and Daoda Company, which adds bulkheads to the bucket and the transition section above the bucket foundation. The foundation can be prefabricated on land and transported and installedusing a one-step installation vessel, which significantly reduces cost [10; 11; 12]. In November 2018, two wind turbines with a CBF foundation were installed in the Sanxia Dafeng Sea in Jiangsu, China. The prefabrication, transportation and installing processes are shown in Figure 1.  
The seabed is commonly composed of silt, silty clay, and other marine soft soils with high water content, high compressibility, and low permeability, which commonly results in insufficient foundation bearing capacity and excessive settlement after foundation installation. In addition, during the installation of the bucket foundation, hydraulic fracturing of the soil in the bucket is caused by the large seepage force, which reduces the bearing capacity of the top cover of the bucket foundation [13]. Furthermore, cracks may form in the bucket wall, and the soil and foundation may separate during leveling of the bucket foundation [14; 15; 16]. Fracturing will destabilize and reduce the bearing capacity of the bucket foundation. Consequently, the soil inside the bucket foundation needs to be reinforced to improve the bearing capacity of the foundation. Soil failures during the installation of a bucket foundation are shown in Figure 2.  
[PERSON] et al. [17] applied a vacuum preloading method combined with electroosmosis reinforcement to the internal soil of the bucket foundation. The effectiveness of soil reinforcement was confirmed through a series of reinforcement model tests [18; 19; 20]. The vacuum-electroosmosis method for bucket foundation can be studied using existing vacuum preloading and electroosmosis methods. Vacuum preloading is performed by forming a vacuum under a sealing membrane covering the ground, and the pressure difference across the membrane causes the soil layer to consolidate, which increases the soil effective stress by reducing the pore water pressure without changing the total stress. The vacuum preloading method was first proposed by the Swedish civil engineer [PERSON] in 1952 and has been widely used in soft soil foundation treatment [21; 22; 23; 24; 25; 26; 27; 28; 29]. Electroosmotic method is accomplished by applying a direct current at both ends of a soil deposit. The free water and weakly absorbed water in the soil move toward the cathode together with the cations, thereby draining and consolidating the soil. In 1939, Cassagrande in Germany used the electro-osmotic soil treatment method for the first time  
Figure 1: The composite bucket foundation (CBF) with offshore wind turbine: (a) Prefabrication onshore; (b) One-step transportation; (c) Integrated installation. Figure 2: Soil failure during the installation of a bucket foundation: (a) Soil plug during installation (b) Fractures beside the bucket wall [16]. in an excavation of railway subgrade [30; 31]. Electroosomosis improves the permeability coefficient of the soil and reduces the amount of weakly absorbed water regardless of grain assize [32]. In recent years, electroosmotic strengthening of soil has been widely studied [33; 34; 35; 36].