| Biogeotechnology is a novel ground improvement technology developed in recent decades.This technology utilizes the metabolism of microorganisms to produce mineral precipitation with cementation to achieve the purpose of improving soil mechanics and engineering properties.Due to the multiple conditions of in-situ construction,the complex soil composition and properties,and the difficulties in guaranteeing the implementation and effectiveness of the in-situ injection strategies,current research of this technology mainly focuses on laboratory tests,and the applications of field-scale experiment or practical engineering are limited.In this dissertation,based on the microbially induced carbonate precipitation(MICP)via urea hydrolysis,the laboratory and field experiments were carried out to investigate the feasibility,environmental impact factors,in-situ monitoring and testing methods of MICP.A field-scale reactive-transport model of MICP was established to optimize the design of injection strategies and evaluate the bio-cementation performance of MICP application.The main research contents and results are listed below.(1)Aiming at the potential environmental influence factors in engineering,the lab experiments were conducted on the bio-cemented sand columns.Combined with the tests of unconfined compression strength,calcium carbonate content and distribution,water content and distribution and scanning electron microscope,the influences of water content,temperature,salt type and content on the mechanical behavior of bio-cemented sand were investigated and the combined impacts of water,salt and calcium carbonate on the biocemented sand were analyzed from a mesoscopic view.The results indicate that the combined influence of environmental water,salt and calcium carbonate leads to the difference in the morphology and structure of calcium carbonate crystals.The strength of bio-cemented sand with 15% calcium chloride is 4.9 times higher than that without salt,and more effective cementation is formed within the micro-structure which improves the mechanical behaviors of bio-cemented soil significantly.(2)The liquid test was carried out to investigate the decay of urease bacteria with time under standing and shaking conditions.The sand column test was used to investigate the adsorption and desorption of bacteria in sand with different injection rates.Then,the numerical simulation was conducted to fit parameters and equations of bacterial adsorption and desorption.The results present a linear decline with time of the unit urease’s activity under standing condition.The adsorption and desorption of bacteria in soil were mainly affected by the concentration of bacteria in the environment,rather than the injection rate.(3)A field-scale reactive-transport model of MICP was established which considered the adsorption,desorption and fixation of bacteria,decay of urease activity,reaction rate of urea hydrolysis,precipitation of calcium carbonate,reduction of porosity and permeability during the injection process.The model was verified by the simulation and analysis of the reaction and transport of solutes during injection in a sand column test.Combined with the typical injection strategies and wells implementation methods,the model adopted the injection of bacteria followed the injection of cementation solution to analyze the effects of the concentration of fixative solution,injection rate,injection strategies of cementation solution and wells implementations on the treatment performance of MICP.The results show that the well implementation strategy which has a central extraction well surrounding by six injection wells and the cyclic injection method with an increased cementation solution to 2 mol/L can greatly improve the transport efficiency of bacteria and reactants,reducing the differences of precipitation amount within the target area and enhancing the uniformity of treatment effectively.(4)The field experiment of biogrouting via MICP was carried out.The feasibility and injection strategies of the extraction and the enrichment of indigenous urease bacteria and the feasibility of in-situ bio-cementation were determined through preliminary field surveys and pre-laboratory tests.Real-time monitoring and on-site testing were conducted to investigate the concentration of solutes,electrical conductivity and p H of groundwater,cementation level of treatment and stability improvement of soil.The methods of performance evaluation,the experience,difficulties and solutions of MICP field application were drawn.Furthermore,a two-dimensional reactive-transport model was established based on the field experiment to analyze the concentration and distribution of bacteria and reagents,the effective thickness of treated soil layers,the actual reaction rate,the dilution of groundwater and treatment performance during the MICP process.The results show that the injection of indigenous urease bacteria induces the formation and attachment of calcium carbonate precipitates successfully,and the stability of the treated area is improved;the thickness of the effective cemented soil layer is only 5% of the target area due to the heterogeneity of the soil layers.The actual urea hydrolysis reaction is only 1% of the expected.This model can reflect the reaction process of MICP in the field application quickly and effectively,and evaluate the actual treatment performance reasonably,providing a reference for designing the cost-effective injection strategies in-situ. |
