Investigation On The Mechanical Characteristics And Reinforced Mechanism Of Desert Aeolian Sand Solidified With Urease Microorganisms And Basalt Fiber

Aeolian sand was widely distributed in the severe desert environment,and it can be used as a typical roadbed filling.With the improvement of transportation capacity,the roadbed settlement,roadbed liquefaction,and roadbed coverage caused by traffic loadings have a serious impact on the safe operation.To construct the ecological civilization and ensure traffic safety operation,it is imperative to solidify the aeolian sand.As a new efficient,green,and durable method,the brittle failure of soil solidified by microbially induced calcite precipitation（MICP）has become a bottleneck limiting its development.However,the toughening mechanism of fiber-reinforced method can help to improve its erosion resistance.Based on the unit,model,and micro experiments,the mechanical characteristics and mechanism of aeolian sand solidified by MICP combined with basalt fiber reinforced（BFR）were investigated from macroscopic and microscopic scales.The aim is to achieve the solidified of aeolian sand via combination of microbial bonding and fiber reinforcement,which is an interdisciplinary field of microbiology,chemistry,and geotechnical engineering,and it has important theoretical value and scientific significance for guiding the traffic engineering construction in desert areas.The main research results are as follows:（1）Based on response surface method,the optimal condition for bacterial growth was determined and the conditions with temperature of 35^oC,p H of 9,and shaking frequency of170 rpm.Based on the orthogonal experimental method,the optimal conditions for MICP reaction was obtained and the condition with bacterial solution OD₆₀₀ of 1.5,concentration of cement solution of 1 mol/L,reaction time of 16 hours,p H of 9,and temperature of 25^oC.（2）Based on permeability and unconfined compressive strength（UCS）experiments,it was found that the permeability coefficient of MICP solidified sand decreased with increasing of initial dry density and cementation times,while the Ca CO₃ content increased with increasing of cementation times.With increasing of cementation times,the UCS showed first increasing,then stabilizing,and subsequently increasing.When the initial dry density is 1.5g/cm³,the optimal reinforcement condition was determined with fiber length of 9 mm and fiber content of 0.6%.When the initial dry density is 1.6 g/cm³,the optimal reinforcement condition was determined with fiber length of 12 mm and fiber content of 1.0%.（3）Based on consolidated undrained triaxial experiments,it was found that the stress-strain curve exhibited strain softening characteristics.Fibers can effectively reduce the brittleness index of MICP solidified aeolian sand,which is beneficial for improving the ductility of the samples.With increasing of fiber content and fiber length,the cohesion of aeolian sand with an initial dry density of 1.5 g/cm³ showed first increasing and then decreasing,while the cohesion with an initial dry density of 1.6 g/cm³ showed an increasing trend.Meanwhile,the internal friction angle changed slightly.Based on the test results,a shear strength model was established and verified considering the effects of fiber length and fiber content.（4）Based on consolidation undrained dynamic triaxial experiments,it was found that the growth rate of dynamic strain gradually decreased and approached a stable state as the vibrations number increased.The dynamic strain increased with increasing of dynamic stress amplitude and frequency,while decreased with increasing of cell pressure.The dynamic elastic modulus decreased with increasing of dynamic stress amplitude and frequency,and increased with increasing of cell pressure.The damping ratio increased with increasing of dynamic stress amplitude and frequency,and decreased with increasing of cell pressure.Based on the test results,a dynamic strain model was established and verified considering the effects of dynamic stress amplitude and cell pressure.（5）Based on wind tunnel experiments,it was found that the threshold friction velocity decreased with increasing of wind erosion angle.MICP-BFR solidified sand has the highest threshold friction velocity and the best wind erosion resistance.The wind erosion modulus showed a linear increasing trend with increasing of wind speed and wind erosion angle.The order of wind erosion modulus follows:loose sand>MICP solidified sand>MICP-BFR solidified sand.Based on the test results,a regression analysis method was used to establish a prediction model of wind erosion modulus considering the effects of wind speed and wind erosion angle.The predicted results agreed well with the measured results,indicating that the model is suitable for predicting the wind erosion modulus.（6）Based on microscopic experiments,it was found that the Ca CO₃ crystals played the role of covering,bonding,and filling,while the fibers played the role of one-dimensional reinforcement and spatial mesh structure.When a fiber subject to tension,it pull other fibers to form a three-dimensional network,allowing local loadings to be transmitted to a wider area,enhancing the overall stress performance of composites.The pore size,surface area,and equivalent diameter of MICP and MICP-BFR solidified sand decreased with increasing of initial dry density.The T₂ spectrum showed multi peaks with discontinuities feature,indicating that no through pores formed inside the aeolian sand.At initial dry densities of 1.5g/cm³ and 1.6 g/cm³,the cumulative porosity of MICP solidified sand was 24.13%and21.62%,respectively,while the cumulative porosity of MICP-BFR solidified sand decreased by 18.94%and 14.25%,respectively.The microscopic test results confirmed well with the macroscopic test results.