# Recent Developments on Biomineralization for Erosion Control
## 3 Multi-Investigations of Biomineralization Erosion Control in Ocean Engineering
### Local Scour Protection Around Monopile  
This section summarizes the local scour protection around a monopile treated by MICP. [PERSON] et al. (2022b) investigated the efficacy of MICP for mitigating local scour around monopile foundations under unidirectional currents with an average velocity of 0.1 m/s, 0.2 m/s, and 0.3 m/s [126]. The experiments were conducted in a large-scale flume using silica sand (\(d_{50}=0.16\) mm) treated with MICP technology. The height and diameter of the model pile in the test are 0.84 m and 0.11 m. MICP treatments involved sequential spraying of bacterial solution (OD600 = 2.0, urease activity =v20 U/mL) and cementation media (1M urea/1M CaCl\({}_{2}\)) 1-4 times over 4 days. For comparison, traditional riprap protection (5-10 mm gravel) was also tested. A non-contact camera system and handheld 3D scanners monitored scour evolution and post-test topography. Sediment transport rates were calculated using acid-washed CaCO\({}_{3}\) content and SEM analysis. As shown in Figures 3 and 4, two-times MICP treatments reduced maximum scour depth (\(S_{\text{end}}/D_{\text{p}}\)) by 84% (from 1.14 \(D_{\text{p}}\) in untreated sand to 0.18 \(D_{\text{p}}\)), four-times treatments eliminated visible scour holes under 0.3 m/s live-bed conditions, while riprap protection achieved only 0.38 \(D_{\text{p}}\). The \(D_{\text{p}}\) represents the diameter of the monopile. Penetration resistance increased exponentially with treatment cycles \(N\). Surface CaCO\({}_{3}\) content reached 3.8%, forming an inverted cone-shaped cemented layer (height = 4-9 cm) that assisted downflow and horseshoe vortices. Sediment transport rate dropped by 53% (from 19.4 kg/m/h to 9 kg/m/h at pile proximity), attributed to CaCO\({}_{3}\) bridging effects (spherical crystals, 5-20 \(\upmu\)m). However, the maximum edge scour depth (length) increases from 0.19 (3.18) \(D_{\text{p}}\) to 0.29 (4.36) \(D_{\text{p}}\) after two-times to four-times MICP treatments. It is necessary to consider the balance of MICP protective times and edge scour. To mitigate this, ensuring the treated area remains flush with the surrounding seabed helps reduce flow disturbance. Additionally, expanding the treatment area provides a buffer zone that dissipates shear forces. Incorporating gradual transitions or overlapping treatments further minimizes hydraulic gradients at the edges. These strategies enhance interface stability and reduce localized erosion. [PERSON] et al. (2024) investigated the lateral response of offshore  
Figure 2: Field implementation of MICP and EICP for erosion reduction of coastline sandy slope in Sanya, Hainan: (**a**) overall view of field design, (**b**) double liquid spray equipment, (**c**) visual observation of reinforcement effect, (**d**) evolution of the penetration resistance in atmospheric region, (**e**) evolution of the penetration resistance in tidal region, and (**f**) erosion volume in tidal region at 20 days after treatments [125]. monopiles reinforced by MICP through static and cyclic loading tests [127]. Key findings reveal that precast bio-reinforcement (4 \(D_{\text{p}}\) width and 1 \(D_{\text{p}}\) depth) enhances the lateral bearing capacity by 50% and reduces maximum bending moments by 25%, transitioning failure modes from localized to global overturning. Under cyclic loading, accumulated deformation decreased by 30-60% for one-way loading, with secant stiffness ratios(MICP-treated vs. untreated) ranging from 1.65-2.82 (higher under smaller loads). Mechanistically, MICP-treated sand improved shallow soil resistance, but stiffness evolution was governed by three competing factors: untreated sand compaction, bio-cementation degradation at low strains, and soil subsidence around the reinforced zone. The low-pH grouting method demonstrated eco-friendly advantages over traditional cementation, offering a sustainable solution to optimize monopile dimensions for offshore wind turbines while balancing cyclic durability and environmental impact. Separately, [PERSON] et al.