| With the major strategic deployment of building a maritime power and the "One Belt,One Road" development initiative,the reclamation and land reclamation projects and the construction and development of islands and reefs in the South China Sea have entered a period of vigorous development.In the infrastructure construction on the islands and reefs of the South China Sea,it is inevitable to encounter the problem of calcareous sand foundation,and the poor engineering characteristics of calcareous sand have brought many challenges and difficulties to these engineering constructions.Microbial reinforcement technology,as a new foundation reinforcement technology,has been widely studied in recent years and has achieved certain results in laboratory tests,model tests and field applications.However,the current research on the fracture mechanism of microbial-reinforced calcareous sand and its foundation is still focused on the purely mechanical level,which has certain limitations in reflecting the fracture mechanism of microbial-reinforced calcareous sand,a complex synthetic material.Therefore,this paper uses the mechanical test and acoustic emission synchronous acquisition test,and adopts the "avalanche" dynamic analysis method to deeply explore the unconfined compression and compression resistance of calcareous sand particle fracture,particle friction,and microbial reinforcement from multiple scales.Triaxial shear fracture and fracture mechanism of microbe-reinforced calcareous sand foundation under static penetration conditions.The main work and research results obtained in this thesis are as follows:(1)In the "grain" scale.The "avalanche" dynamic characteristics of calcareous sand particle rupture and particle friction mechanism were studied through the acoustic emission synchronous acquisition experiment of single particle fracture and group particle friction of calcareous sand.The research results show that the acoustic emission signals of single particle fracture and group particle friction of calcareous sand all satisfy the "avalanche" dynamic law.Close to the predicted values of mean field theory and force integral mean field theory respectively.(2)In the uncompressed "unit" scale.Through the unconfined compression and acoustic emission acquisition experiments of microbially strengthened calcareous sand,it is found that the "avalanche" critical exponent of the unconfined compressive fracture of microbially strengthened calcareous sand is between the calcium between critical exponents for the mechanisms of sand particle fracture and particle friction.The critical exponent of the "avalanche" during fracture fluctuates between the critical exponents of particle fracture and particle friction mechanisms.The changing trend of the fracture mechanism is: significant force drop and acoustic emission signal are generate by fracture mechanism firstly,and then it is transformed into a mixed mechanism dominated by the friction mechanism or the combined action of particle fracture and particle friction.(3)In the scale of pressured "units".Through the simultaneous acquisition test of triaxial shear fracture and acoustic emission of microbially strengthened calcareous sand at different confining pressure levels,the confinement effect of confining pressure on the microbially strengthened calcareous sand triaxial was explored.The research results show that the acoustic emission energy power-law distribution is transformed from a pure power-law distribution to a damped power-law distribution under the constraint of confining pressure.The energy power-law exponents are close to each other,while the energy damping scale increases with increasing confining pressure.From the change of the distribution of waiting time,it was found that the fracture of microbially strengthened calcareous sand was transformed from brittle failure to ductile failure under the action of confining pressure.(4)In the "model" scale.The calcareous sand foundation model was reinforced with the self-designed microbial two-phase low-pressure grouting method,and the static penetration test during the reinforcement process and the sampling test after the reinforcement was completed.The transformation of the "avalanche" mechanism corresponding to the strength change of the calcareous sand foundation model during the reinforcement process and the reinforcement effect and "avalanche" characteristics of the samples were analyzed.The research results show that the strength of the calcareous sand foundation is significantly improved after four times of reinforcement,while the foundation strength has no significant change after eight times of reinforcement and basically reaches the reinforcement limit of the reinforcement method used.With the progress of reinforcement,the "avalanche" mechanism of acoustic emission energy of calcareous sand foundation changes from a pure power-law distribution dominated by friction mechanism to a mixed power distribution dominated by friction mechanism but mixed with fracture mechanism when reinforcement is twice.The "avalanche" mechanism of calcareous sand foundation failure is transformed into a damping powerlaw distribution affected by restraint after four reinforcements until the reinforcement is completed. |
