To improve the mechanical properties of calcareous sand, it is proposed that microbial induced calcium carbonate precipitation (MICP) technology be used. A series of solidification tests were conducted in natural seawater and freshwater environments. The standard stress path static triaxial apparatus was used to conduct shear tests on calcareous sand and solids under varying reinforcement conditions. The composite power-exponential (CPE) model is proposed to describe the stress–strain relationship curve of the solid, and the method for determining model parameters is presented. The experimental results showed that the strength of calcareous sand with solids increased with the increase in number of reinforcement times for both test environments. Owing to the high salinity of seawater, which inhibits the activity of urease in bacterial solutions, the reinforcement strength in the seawater environment was generally lower than that in the freshwater environment. The compactness had an evident effect on the strength of the added solids. With the increase in compactness, the strength of the sample also increased, but the rate of increase was reduced. The simulation results showed that the established constitutive model can accurately describe the stress–strain relationship of microbial-reinforced calcareous sand and verified the applicability of the model.

Microbial-Induced Calcium Carbonate Precipitation (MICP) is an environmentally friendly, efficient, and sustainable new soil reinforcement technology. For this study, Bacillus pasteurii were domesticated and cultured in a natural seawater environment with multiple gradients and used for coral reef calcareous sand reinforcement, comparing the mineral composition of the generated precipitates and the reinforcement strength under different domestication gradient conditions. The results revealed that, while the natural seawater environment inhibits the growth of Bacillus pasteurii, the gradient domestication method allows the bacteria to gradually adapt to the natural seawater environment. Notably, their shape becomes thin and long under the seawater environment. Furthermore, the MICP mineralisation reaction rate is faster in the natural seawater environment and, with an increase in the domestication gradient, the mineralisation reaction precipitates increased. At the same time, in the seawater environment, a small amount of mineral components were generated in addition to ${\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3$, such as ${\mathrm{M}\mathrm{g}}_{\mathrm{x}}{\mathrm{C}\mathrm{a}}_{\mathrm{y}}({\mathrm{C}\mathrm{O}}_3{)}_{\mathrm{z}}$, and the ${\mathrm{M}\mathrm{g}}^{2+}$ mineral content increased with an increase in the domestication gradient. When comparing the curing effect under different gradients in the natural seawater environment, it was found that the Bacillus pasteurii can effectively enhance the curing effect of the calcareous sand after multi-gradient domestication in the seawater environment, with the curing effect increasing with an increase in the domestication gradient. The results of this study provide new ideas for the application of MICP technology in seawater environments for the reinforcement of calcareous sand in the construction of South China Sea islands and reefs.

Coastal erosion is increasing worldwide due to the increasing frequency of extreme natural phenomena and excessive human exploitation. In this study, a small model experiment was conducted to investigate the solidification effects of three enzyme sources—soybean urease, freshwater Bacillus pasteurella, and seawater domesticated Bacillus pasteurella—on coastal sediments and their impacts in a seawater environment. The solidifying effect of different enzyme sources was determined by measuring the mechanical properties and corrosion resistance of the cured specimen model. The influence of solidified seawater in a seawater environment was obtained by measuring the changes in the pH value, calcium ion concentration, and ammonia nitrogen content of solidified seawater. The results show that different enzyme sources have a certain strengthening effect on coastal sediments. The mechanical properties of coastal sediments can be enhanced by increasing the amount of enzyme solution or level of solidification and can effectively resist simulated flow erosion. Comparing the reinforcement effects of different enzyme sources, it can be seen. It was observed that Bacillus pasteurella acclimated in seawater had better reinforcement effects than Bacillus pasteurella fresh water, and Bacillus pasteurella fresh water had better reinforcement effects than soybean urease. In the seawater measurement tests, the solidification of coastal sediments using different enzyme sources led to a decrease in the seawater pH value, and the acidification of seawater dissolved the generated calcium carbonate, increased the concentration of calcium ions in seawater, and produced ammonia nitrogen as a byproduct in the seawater. It was observed that, compared with the other two enzyme source solutions, the seawater-domesticated Bacillus pasteurella can better adapt to the high-salt environment of seawater, microbial metabolism is not inhibited, urea decomposition ability is improved, and calcium carbonate production is higher, which can effectively improve the engineering characteristics of coastal sediments and play a positive role in coastal protection and development.