Study On Dynamic And Liquefaction Charcteristics Of Temperature Controlled MICP-treated Calcareous Sand

For the national strategic goal of building maritime power and meet the needs of exploiting and utilizing ocean resources,developing the tourism and maintaining national security,our country is vigorously carrying out the construction on the islands and reefs in the South China Sea.Calcareous sands are broadly-distributed along the coasts of the South China Sea,its engineering mechanical characteristics are different from terrestrial sands.Meanwhile,the South China Sea is characterized with harsh natural environment and fragile reef ecological environment.The construction in this area often faces the problem of guarantying the construction quality,environmental pollution and cost control.Therefore,there is an urgent need for proposing a new reinforcement technology,which is eco-friendly,suitable for the marine environment and calcareous sand site.Microbialinduced calcium carbonate precipitation(MICP),as a new green geotechnical reinforcement technology,brings new opportunities for the construction on islands and reefs in the South China Sea.In this study,a series laboratory tests including microbiological tests,cyclic triaxial test and shaking table test were conducted to systematically study the MICP technology and the main factors,analyze the dynamic and liquefaction characteristics of MICP-treated calcareous sand in unit scale and model scale.The main scopes and results of this study are given as follows:(1)The reaction mechanism of microbially induced calcium carbonate precipitation(MICP)was comprehensively elaborated in this dissertation.A sample preparation device was designed and fabricated to produce low-strength specimens.The process of bacteria selection and cultivation was carried out,and laboratory tests were conducted to investigate the factors which affects MICP reaction and efficiency.The changes of urease activity with cell concentration,p H value,ambient temperature and the concentration of reaction solution were analyzed,and an optimal MICP recipe for S.pasteurii bacteria under the present experimental conditions was found.According to the study of bacterial urease activity,a temperature-controlled MICP treatment(TCMT)technique based on low temperature and one-phase grouting was proposed.The sand column improvement result using two-phase method,p H controlled MICP treatment method and TCMT method were compared,the result shows TCMT method has simple process and the best treatment effect.Thus,TCMT method provides technical support for subsequent dynamic tests on biocemented calcareous sands.(2)A series cyclic triaxial tests of biocemented calcareous sands were carried out to explore the influence of biocementation level,relative density,effective confining pressure and dynamic shear stress ratio on the liquefaction behaviors of calcareous sand.Meanwhile,the anti-liquefaction index,improvement factor If,of the calcareous sand was proposed,and found that the biocementation level and relative density have a significant effect on If.Through the study of the changes between anti-liquefaction index If and sample mass increment,and the microanalysis of MICP-treated calcareous sand,the antiliquefaction performance improvement mechanism of MICP-treated calcareous sand and the influence of the change of the soil particle structure characteristics on the dynamic characteristics and anti-liquefaction performance were revealed.(3)A systematic study of the dynamic strength characteristics,dynamic pore pressure characteristics,and dynamic deformation characteristics of MICP-treated calcareous sand was carried out based on cyclic triaxial test,and the effects of biocementation level,relative density,effective confining pressure and dynamic stress amplitude on dynamic strength,pore pressure development and deformation development characteristics were discussed.The empirical formula of dynamic strength was optimized,and a uniform dynamic strength criterion for MICP-treated calcareous sand was proposed.Three pore pressure development patterns for MICP-treated calcareous sand were summarized and a new uniform pore pressure stress model for MICP-treated calcareous sand was proposed.The relationship between the liquefaction cyclic number and 5%strain number for MICP-treated calcareous sand was analyzed,and found the selection of the failure criterion has a certain effect on the deformation development and dynamic strength of the sample for MICP-treated calcareous sand.(4)The TCMT calcareous sand model test and MICP-treated calcareous sand shaking table test were carried out,and the effects of the biocementation level and the acceleration amplitude on the dynamic responses of the biocemented calcareous sand model were discussed from four aspects: the macroscopic liquefaction phenomenon,the development of acceleration response,the development of pore pressure,and the settlement after shaking.The study found that the acceleration amplification coefficient,pore pressure ratio,and deformation increased at various degrees when acceleration amplitude increases.The increment in biocementation level reduce the acceleration amplification effect,and restrain the development of pore pressure and deformation.Under the same acceleration,when the MICP treatment level increases,the dilatancy of the soil becomes more significant,and the heavily biocemented soil foundation soil exhibits obvious phase change transition under cyclic loading.(5)Based on acceleration time history response,the inversion analysis of dynamic shear stress strain,equivalent shear modulus and equivalent shear wave velocity is carried out.The dynamic response characteristics and dynamic strength development of TCMT calcareous sand model foundations with different biocementation level and acceleration amplitude were studied.It was found that the biocementation level and acceleration amplitude has significantly effects on the dynamic characteristics.The study results demonstrated the overall improvement effect of TCMT calcareous sand model foundation.