# Recent Developments on Biomineralization for Erosion Control
## 3 Multi-Investigations of Biomineralization Erosion Control in Ocean Engineering  
### Erosion Control of Coastline  
MICP technology has gradually been applied in the field of erosion control of coastline. Many scholars have systematically evaluated the erosion resistance of MICP-treated sand through unit tests, model tests, and field verifications. According to [PERSON] et al. (2020) [120], the erosion resistance of two-layer sand (a filter-soil layer consisting of 25 mm of silica sand (\(d_{50}\) = 1.61 mm) overlying a base soil layer consisting of 25 mm of silica sand (\(d_{50}\) = 0.21 mm)) treated with MICP was systematically evaluated using an erosion function apparatus in Cambridge. Untreated sand exhibited rapid surface erosion under tangential flow, with cumulative height loss escalating linearly under incremental shear stress. In contrast, MICP-treated sand (0.02 M urea-calcium solution) demonstrated reduced cumulative erosion by up to 90% due to calcium carbonate crystal cementation. X-ray computed tomography revealed that calcium carbonate filled pore space and formed inter-particle bridges, diminishing erodibility by altering the failure mechanism from particulate detachment to block erosion. Higher cementation concentrations (0.04-0.1 M urea-calcium solution) promoted larger, lognormally distributed crystals (up to 210 \(\upmu\)m diameter), which enhanced aggregate stability. This microstructural evolution--linked to microbial metabolism and size-dependent crystal growth--significantly improved macro-scale resistance. The non-monotonic trend declines in erodibility (\(\kappa_{\rm d}\)) with increased calcium carbonate content, underscoring MICP's potential in stabilizing vulnerable interfaces in dams or offshore reservoirs. Future optimizations could target crystal uniformity and injection protocols to maximize erosion control while balancing permeability retention. At the model scales, slope erosion tests during tidal actions were conducted by [PERSON] et al. (2016) [121]. Sandy soil was sourced from Troon Beach in Ayrshire, UK and air dried in the laboratory. Untreated natural sandy slopes subjected to tidal cycles rapidly collapse to their angle of repose (35\({}^{\circ}\)), losing \(\sim\)0.2\({}^{\circ}\) per tidal event. However, MICP-treated slopes (using _Sporosarcina pasteurii_ and 0.7 M CaCl\({}_{2}\)-urea) resisted collapse by forming a bio-cemented crust (118-155 kg CaCO\({}_{3}\)/m\({}^{3}\)). At 35\({}^{\circ}\) inclinations, treatment reduced erosion significantly, while steeper 53\({}^{\circ}\) slopes showed negligible erosion despite high shear stresses. The mechanism stems from urea hydrolysis-driven calcite precipitation, forming a bio-cemented crust that withstood tidal shear stresses. SEM and penetration tests confirmed that calcite bridges increased interparticle friction and cohesion. Afterwards, slope erosion tests during wave actions were also conducted by [PERSON] et al. (2020) [122]. The sand used in the experiments featured a median particle size (D\({}_{5}\)0) of 0.71 mm and a specific gravity of 2.65. The physical model of the coastal sandy slope measured 60 cm in width and 20 cm in length, with three different heights tested--3.5 cm, 7.4 cm, and 14 cm--to simulate various slope conditions. Corresponding slope angles examined in the tests were 10\({}^{\circ}\), 20\({}^{\circ}\), and 35\({}^{\circ}\). The sand slopes were characterized by an initial void ratio of 0.82, a relative density of approximately 51.4%, and an effective internal friction angle of 34\({}^{\circ}\). Wave actions on untreated sandy slopes induced rapid particle detachment, forming bilinear erosion profiles. MICP treatment (four cycles of 0.5 mL/cm\({}^{2}\) bacterial and cementation solutions) reduced erosion rates by up to 97%, with calcium carbonate content correlating inversely with mass loss.