# Recent Developments on Biomineralization for Erosion Control
## 3 Multi-Investigations of Biomineralization Erosion Control in Ocean Engineering
### Erosion Control of Coastline
Penetration resistance (up to 60 kN/m\({}^{2}\)) and SEM analyses revealed calcium carbonate's dual role: nano-crystals filled pores, while macroscale aggregates (100-240 \(\upmu\)m) formed load-bearing networks. Notably, higher treatment cycles (e.g., four cycles) yielded 30.1% CaCO\({}_{3}\) content, reducing erosion rates to <1 mm/h under wave velocities of 0.4 m/s. The nonlinear relationship between treatment cycles and erosion resistance highlights a threshold (beyond \(\sim\)0.08 M Ca\({}^{2+}\)) where excessive crystal size may paradoxically weaken cohesion. The erosion rate of bio-cemented sandy slope \(R_{\rm e}\), whichrepresents the erosion resistance, also shows a linearly increasing trend with the increase in the treatment cycle N because of the CaCO\({}_{3}\) formed. In order to more accurately simulate wave breaking on the coastline and analyze the erosion resistance of MICP-reinforced slopes, a series of large-section flume tests were carried out by [PERSON] et al. (2020a) [123]. The tested Fujian sand is characterized by a mean particle diameter (\(d_{5}0\)) of 0.17 mm and a specific gravity of 2.633. Its gradation parameters include a uniformity coefficient of 1.57 and a curvature coefficient of 0.96, indicating relatively poor sorting. The material exhibits a range of void ratios, with minimum and maximum values recorded at 0.607 and 0.952, respectively. The experiments evaluated erosion patterns, penetration resistance, CaCO\({}_{3}\) distribution, and pore water pressure under varying MICP treatment cycles (zero, two, four) and wave heights (8, 10, 12 cm). The results show that four MICP treatments formed a robust cementation layer on the sandy slope, eliminating visible erosion after 60 min of wave action (Figure 1). The maximum erosion depths on the slope under the untreated slope, the two-times MICP-treated slope, and the four-times MICP-treated slope are 6 cm, 5 cm, and 0 cm, respectively. Penetration resistance increased dramatically from 0.14 MPa (untreated) to 2.04 MPa (four-times treated), linked to surface CaCO\({}_{3}\) content reaching 7% via acid washing tests. The calcium carbonate content of the MICP-treated area decreases from the toe to the top of the slope because the bacterial and cementation solutions flow from the top to the toe due to gravity. With the increase in MICP treatment times, the permeability of the slope is gradually decreased, resulting in difficult infiltration of the bacterial solution, cementation solution, and oxygen. Therefore, the difference in the bacterial solution, cementation solution, and oxygen content between the deep and the shallow layer of the slope is increased, leading to the increased difference in calcium carbonate content. MICP treatments form CaCO\({}_{3}\) crystals between sand particles, reducing permeability and restricting water pressure transfer. Consequently, Figure 1c shows thickened excess pore water pressure gradients in the surface layers of treated slopes compared to untreated ones, highlighting limited fluid mobility. Some field implementations were also conducted to evaluate the erosion resistance of coastline slopes treated by MICP (Table 2). The Ahoskie study focused on a sandy slope with poor grading, employing three MICP delivery methods: surface spraying, prefabricated vertical drains (PVDs), and shallow gravel-packed trenches [124]. Surface spraying applied urea-CaCl\({}_{2}\) solutions twice daily via an irrigation system with 25 nozzles, forming a uniform surface crust through sequential bacterial solution and cementation solution cycles over 14 days. PVDs (30 cm depth) facilitated vertical seepage, leveraging gravity to  
Figure 1: Large-scale flume test: (**a**) experimental design, (**b**) erosion degree under 10 cm wave height actions, (**c**) CaCO\({}_{3}\) distribution of MICP-treated sandy slope, and (**d**) normalized pore water pressure distribution along slope depth under 12 cm wave height [123].