Study On Small Strain Dynamic Characteristics Of Microbial Solidification Of Calcareous Sand Considering Particle Size Distributio

Calcareous sand is a common geotechnical medium used in the construction of islands and reefs in the South China Sea.It was often subjected to vibrational loads such as earthquakes,wind and tides.The mechanical properties of calcareous sands are different from those of ordinary sand of terrestrial origin due to their special genesis,and they need to be reinforced before being used in marine engineering.A promising and environmentally friendly consolidation method is Microbial Induced Calcium carbonate Precipitation（MICP）technique.In addition,the particle grading parameters are important factors in the mechanical properties of calcareous sands,and there was disagreement in established studies regarding the effect of particle grading on the small-strain dynamic properties of calcareous sands.There is therefore a need for a more systematic and comprehensive study of it.In this thesis,to study the effect of particle size on the dynamic properties of microbially cured calcareous sand in the small strain range,Sporosarcina pasteurii was selected to reinforce the calcareous sand,and the GDS-RCA resonant column apparatus was used to simulate the forces on the calcareous sand.Three sets of working conditions are set up to consider the effects of the mean particle size d50,the uniformity coefficient C_u and the curvature coefficient C_c on the dynamic shear modulus,the damping ratio,the dynamic elastic modulus,the maximum dynamic shear modulus and maximum dynamic elastic modulus of the cured calcareous sand.The empirical equations for the maximum dynamic shear modulus and maximum dynamic elastic modulus were further derived to provide theoretical and experimental bases for subsequent practical applications.The key findings of the thesis are as follows:（1）The calcium carbonate content and dry density tests revealed that the calcium carbonate content and dry density of the MICP-cured calcareous sand samples tended to decrease as the mean particle size d50 increased,with the maximum calcium carbonate content and dry density at d50=0.20mm.The calcium carbonate content and dry density tended to decrease as the uniformity coefficient C_u increased,with the maximum calcium carbonate content and dry density at C_u=1.31.As the curvature coefficient C_c increases,the calcium carbonate content and dry density tend to increase and then decrease.The sand sample is well graded and the calcium carbonate content and dry density are maximum at C_c=2.14.（2）The dynamic shear modulus G,dynamic shear modulus ratio G/G_max,dynamic young’s modulus E,maximum dynamic shear modulus G_max and maximum dynamic young’s modulus E_max of the calcareous sand samples before and after curing were found to increase with increasing mean particle size d50,decrease with increasing uniformity coefficient C_u and decrease then increase with increasing curvature coefficient C_c.The damping ratio D of the calcareous sand samples before and after curing decreases with increasing mean particle size d50,increases with increasing uniformity coefficient C_u,and increases then decreases with increasing curvature coefficient C_c.（3）The fitting parameter A_G was found to be more significantly affected by the mean particle size d50 and the uniformity coefficient C_u in the empirical formula for the maximum dynamic shear modulus..The fitted parameter n _G was more significantly influenced by the uniformity coefficient C_u.In the empirical formula for the maximum dynamic modulus of elasticity,the fitting parameter A_E was significantly influenced by the mean particle size d50and the uniformity coefficient C_u.The fitting parameter n _E was more significantly influenced by the mean particle size d50.Empirical equations for maximum dynamic shear modulus G_maxand maximum dynamic elastic modulus E_max of microbially cured calcareous sand with consolidation pressure,pore ratio,mean particle size and inhomogeneity coefficient as variables were established as follows:（?）After data analysis,the prediction error of the modified G_max empirical formula is less than 15%,and the prediction error of the modified E_max empirical formula is basically less than 25%,which are within the engineering acceptance range.