Microbial Solidification Technology Of Coral Sand In High Salt Environment In South Island Reef

The South China Sea is an important part of China’s territory,occupying an important geographical position and military strategic significance.Coral sand is widely distributed in the South China Sea reefs.It has many characteristics,such as multi-angular,porous,irregular shape,low strength and fragile.Traditional mechanical method,physical action and chemical cementation reinforcement of foundation will lead to shortcomings such as high transportation cost of equipment,poor construction environment and pollution of marine environment.In recent years,MICP（microbial induced calcite precipitation）technology has sprung up.Using this method to reinforce coral sands in the continental area of South China Sea islands and reefs can increase its strength and compactness,make it a qualified foundation,and provide research theory for the future construction of infrastructure on islands and reefs.However,the freshwater resources in the South China Sea are scarce.If we can use the seawater resources to cultivate microorganisms,it will bring enormous economic and environmental benefits.In this thesis,we selected Bacillus pasteurius to activate and cultivate in the laboratory,studied the effects of different salinity water sources and environmental factors（pH and temperature）on the growth curve and urease activity of Bacillus pasteurium,grouting test on coral sand of Xisha Islands,and studied the effects of different bacterial concentration,cementing fluid concentration and grouting speed on the grouting effect after reinforcement.The permeability test,uniaxial compression test,scanning electron microscopy（SEM）test and X-ray diffraction（XRD）test were carried out to test the effect of microbial reinforcing coral sand.The following conclusions were drawn.（1）The growth curves of Bacillus pasteurius cultured in seawater and freshwater were consistent,but the former was lower than the latter in bacterial concentration(OD₆₀₀).In the bacterial growth curve,the bacteria in seawater culture medium had one more environmental adaptation period than those in freshwater culture medium.（2）The OD₆₀₀00 value and urease activity of Bacillus pasteurius cultured in fresh water and sea water were affected by temperature and pH.When the temperature is lower than 30°C,the OD₆₀₀00 value of bacteria increases rapidly with the increase of temperature;when the temperature is between 30°C and 60°C,the OD₆₀₀00 of bacteria increases slowly with the increase of temperature;at around 30°C,the OD₆₀₀00 reaches its maximum value,which is 1.537.The urease activity first increases and then decreases with increasing temperature,reaching a maximum at around 30°C.When the pH is equal to 6,the bacteria are difficult to survive,and the OD₆₀₀00 value and urease activity are in a very low state.When the pH is between 7 and 10,the bacterial concentration OD₆₀₀00 and urease activity are in the state of first rising and then falling.At the vicinity of pH=9,the bacterial OD₆₀₀00 and urease activity reach the maximum..（3）The grouting parameters such as bacterial concentration,cementing fluid concentration and grouting speed have influence on the reinforcement of coral mortar by grouting.The higher the OD₆₀₀00 value,the higher the cementing fluid concentration and the lower the grouting speed,the stronger the strength of reinforced coral sand column,the lower the permeability coefficient,the more calcium carbonate crystals produced,and the better the reinforcement effect.（4）The composition of seawater is complex.There are many kinds of crystals produced by coral mortar grouting with bacteria cultured in seawater medium.The two main types are calcium carbonate and calcium orthosilicate.Calcium carbonate is mainly calcite,followed by columbite.The crystalline form of calcium carbonate is influenced by reaction conditions and environment.Calcite is the crystal form of calcium carbonate formed between the samples with slow urea hydrolysis rate,and columbite is the main form among the samples with fast urea hydrolysis rate.