Mechanism And Method Of Microbial Induced Calcium Carbonate Precipitation In Reinforcing Water-Rich Sand Layer

Tunnel engineering is an important support for national strategies such as transportation,marine,energy and island defense projects.Tunnel construction often needs to pass through Quaternary water-rich sand layer.There is no bonding ability and poor self-stability between sand particles in water-rich sand layer(quicksand layer).Under the influence of disturbance of tunnel excavation project and groundwater,water inrush,sand inrush,mud inrush and even tunnel collapse are very likely to occur.Traditional reinforcement methods,such as freezing method,precipitation method and grouting method,can not meet the requirements of high efficiency and energy saving for water-rich sand layer,which is an unfavorable geology.This research innovatively and systematically carried out the research on microbial-induced calcium carbonate precipitation(MICP)technology to strengthen water-rich sand layer.Firstly,the research is to extract,isolate,culture and purify a kind of ureaseproducing bacteria from soil in target reinforcement areas,and named it as self-extracting urease-producing bacteria.For self-extracting urease-producing bacteria,orthogonal test was designed to explore the optimum culture conditions of the bacteria:urea content 20g/L,pH nature,temperature 25C,culture time 30h,NaCl addition 0g/L.Further,the mineralization mechanism was explored by expanding self-extracting urease-producing bacteria,the immersion reaction test was designed,the reaction kinetics of MICP process was studied based on PHREEQC reaction model and water chemistry(urea,calcium ion)monitoring test,and the final reactant(calcium carbonate precipitation)was analyzed by a series of micro-test methods(SEM,XRD).Calcium urea concentration and proportion,pH value,nutrients,and water chemistry were obtained.Mechanism of influence of biochemical factors such as bacterial concentration on MICP process(reaction rate,precipitation form).Finally,based on the best biochemical parameters of MICP process,the MICP grouting reinforcement sand column test device is independently designed,the indoor simulation test of MICP reinforcement for water-rich sand layer is carried out,and the freeze-thaw erosion test for the reinforced sand column is carried out to verify the reinforcement effect and anti-freeze-thaw characteristics of MICP reinforcement for water-rich sand layer.Based on self-extracted urease producing bacteria,this study proposes a different method from the previous physical reinforcement and chemical grouting methodmicrobial induced calcium carbonate precipitation(MICP)for reinforcing water-rich sand formations,which can also be called MICP biological grouting reinforcement.This method has significant advantages over traditional methods in reinforcing water-rich sand formations,such as low disturbance,low cost,controllable reaction,low carbon and environmental protection,etc.It is a significant guidance and reference for solving the problem of reinforcing water-rich sand formations.