Study On The Hydraulic And Mechanical Properties Of Soil Reinforced By Microbial Induced Calcium Carbonate Precipitation

At present,the construction quantity and demand of infrastructure projects at home and abroad,such as water conservancy projects,transportation infrastructure,urban buildings,etc.,continue to grow.Engineering geological disasters such as dam collapse,slope instability,road settlement and other engineering geological disasters caused by poor engineering soil occur frequently.Therefore,improving the hydraulic and mechanical properties of bad soil is an important safety guarantee for the construction of infrastructure projects.Microbial induced calcium carbonate precipitation(MICP)technology has become a research hotspot in the field of soil improvement and reinforcement due to its advantages of environmental protection,economy and sustainability.Soil in practical engineering is often in the unsaturated state,and the theoretical research of soil reinforcement by unsaturated microbial grouting is not enough.Therefore,the in-depth study of microbial soil reinforcement technology in unsaturated state and the revelation of micromacroscopic action mechanism of microbial reinforcement technology can improve the practical application value of MICP technology,promote technological innovation,and realize the green and efficient development of the industry.In this paper,the soil reinforcement test of MICP technology with different grouting saturation factors has successfully realized the effective reinforcement of soil under the condition of low grouting saturation.The influence mechanism of grouting saturation on the application of MICP technology to soil was analyzed,and the action mode of microbial grouting solidification was defined.The research contents and achievements of this paper are as follows:(1)The characteristics of different bacterial species in MICP technology were analyzed by literature comparison,and the bacteria most suitable for MICP technology were selected;In order to determine the optimal concentration and activity level of bacteria in MICP test,the concentration and activity of bacteria were measured.The experimental equipment and scheme of microbial solidification under full saturation and unsaturated conditions are developed,and the soil reinforcement using MICP technology under low grouting saturation is realized.(2)Based on different grouting saturation conditions,the physical and hydraulic characteristics of MICP cemented sand and Yellow River silt were studied,and the relationship between reinforcement times and calcium carbonate production was analyzed,as well as their effects on water absorption and soil-water characteristic curve.The results show that the mass ratio of calcium carbonate increases with the increase of reinforcement times and grouting saturation,and the increasing trend becomes slow.The higher the calcium carbonate content,the lower the water absorption rate,the higher the air intake value,the higher the dehumidification rate,and the higher the water retention ability.It is revealed that the mechanism of MICP reinforcement times affects the physical and hydraulic properties of the reinforced soil through the calcium carbonate content.(3)The mechanical properties of MICP consolidated sand and Yellow River silt were studied.Through unconfined compressive strength test,the strength characteristics and failure forms of the samples were analyzed,and the relationship between the calcium carbonate content and unconfined compressive strength of the samples with different grouting saturation was studied.When the grouting saturation of MICP reinforced sand samples was greater than 30%,under the condition of the same calcium carbonate content,The unconfined compressive strength of the samples with low grouting saturation is higher than that of the samples with high saturation,and the unconfined compressive strength increases faster with the increase of calcium carbonate content.For MICP reinforced silty soil samples,the strength of low grouting saturation samples is higher than that of high grouting saturation samples when the content of calcium carbonate is the same.(4)Triaxial shear tests were conducted on samples with different grouting saturation under specific reinforcement times.The stress-strain curve and peak deviator stress were analyzed based on two factors of confining pressure and grouting saturation,and the influence of grouting saturation on mechanical strength indexes such as cohesion and internal friction Angle of reinforced soil was explored.In unsaturated samples,The sand sample with low grouting saturation has better strength characteristics,and the internal friction Angle of soil can still be effectively improved when the content of calcium carbonate is low,which indicates that calcium carbonate precipitation is effectively consolidated under the condition of unsaturated grouting,and proves the effectiveness of using MICP technology to strengthen soil under low grouting saturation.The influence of different grouting saturation on the strength characteristics of soil reinforced by microorganisms was investigated.(5)The microscopical results of microscopically solidified soil with different grouting saturation were comprehensively analyzed by X-ray difftometry,scanning electron microscopy and computed tomography tests,etc.,as well as the results of macroscopic mechanical tests,to study the morphology,distribution quantity and location of calcium carbonate crystals precipitated by microbe and the contact surface between particles at microscopic scale.The unconfined compressive strength and mass ratio of calcium carbonate were analyzed in combination with the physical and mechanical properties of microbially solidified soil at macro scale.The solidification mechanism of calcium carbonate soil induced by microorganisms was analyzed.The results show that calcite of low grouting saturation samples is mainly distributed between particles,and that of high grouting saturation samples is distributed between particles and on the surface of particles.Macroscopically,the unconfined compressive strength of low grouting saturation samples is higher when the content of calcium carbonate is the same.At the same time,the reinforcement effect of microbially induced calcium carbonate precipitated soil is mainly based on the cementing contact between particles,namely the number of calcium carbonate Bridges.In terms of macroscopic mechanics,it is mainly manifested as the increase of internal friction Angle,namely the increase of friction between particles,and the increase of shear strength,which illustrates the microscopic mechanism and mode of action of MICP reinforcement soil.