Experimental Study On Mechanical Properties And Microstructure Of Microbial-solidified Calcareous Sand Based On Particle Gradation

Microbial-induced calcium carbonate precipitation（MICP）is a new soil reinforcement technology.Urea is hydrolyzed by urease produced by bacteria to produce CO₃^2-,which is combined with Ca²⁺in the surrounding environment to form calcium carbonate crystals.Calcium carbonate crystals fill the pores between sand particles,making the sand sample bond together as a whole,thus improving the mechanical properties of the sand.The particle size distribution of sand is an important factor affecting the mechanical properties of sand,but the influence of particle size on the mechanical properties of solidified sand is rarely studied by scholars at home and abroad.Therefore,it is necessary to carry out a more systematic and comprehensive experimental and theoretical research.In this paper,two large working conditions were set:changing the non-uniformity coefficient Cu and keeping the mean particle size d₅₀ unchanged(Cu=1.31,1.92,3.03 and 6.12;d₅₀=0.20),changing the Curvature coefficient Cc and keeping the non-uniformity coefficient Cu unchanged（Cc=0.21,0.31,0.70,2.14 and 4.08;Cu=6.12）.The effects of particle size pairing on the mechanical properties of solidified calcareous sand were investigated from the macro and micro perspectives by physical properties test,unconfined compressive strength test,dynamic triaxial test and SEM microstructure test.The research results of this paper are as follows:（1）Under the condition of changing the non-uniformity coefficient Cu and keeping the mean particle size d₅₀ unchanged,with the increase of Cu,the calcium carbonate content and dry density showed a trend of gradual decline,and water absorption showed a trend of rise.When Cu is 1.31,the calcium carbonate production and dry density are the highest and water absorption is the lowest.When Cu is 6.12,the calcium carbonate production and dry density are the lowest and water absorption is the highest.Under the condition of changing the Curvature coefficient Cc and keeping the non-uniformity coefficient Cu unchanged,with the increase of Cc,the calcium carbonate production and dry density increased first and then decreased,while water absorption decreased first and then increased.When Cc is 2.14,the calcareous sand particles have good gradation,the calcium carbonate production and dry density of solidified calcareous sand are the largest,but the water absorption rate is the lowest.（2）ACcording to the unconfined compressive strength test,under the condition of changing Cu and keeping d₅₀ unchanged,the unconfined compressive strength（UCS）of solidified sand decreases gradually with the increase of Cu.UCS is the highest of 2.32MPa when Cu is 1.31,and the lowest of 1.28MPa when Cu is 6.12.Under the condition of changing Cc and keeping Cu unchanged,UCS of samples increased firstly and then decreased with the increase of Cc.The solidified sand with good particle size distribution（Cc=2.14）had the highest UCS of 2.08 MPa and the fastest rising rate.（3）Under cyclic loading,the cyclic vibration order N required for liquefaction failure of both dispersed loose sand and solidified sand decreases with the increase of cyclic stress ratio CSR.The dynamic strength of both loose sand and solidified sand decreases with the increase of Cu.The dynamic strength of loose sand and solidified sand increases first and then decreases with the increase of Cc.Through dynamic strength line,the CRR₁₅（the cyclic stress ratio of the15th cyclic vibration）of solidified sand is much larger than that of loose sand.It can be concluded from the variation Curves of pore water pressure and dynamic strain that the development trend of solidified sand is slower than that of loose sand.It shows that the anti-liquefaction property of calcareous sand treated by MICP can be significantly improved.（4）The results of SEM shows that calcium carbonate precipitated crystals can be observed in the solidified sand and mainly exist in rhomboid calcium carbonate crystals.Under the condition of changing Cu and keeping d₅₀ unchanged,with the increase of Cu,more calcium carbonate crystals attached to the surface of coarse particles are formed,while less calcium carbonate crystals（effective calcium carbonate）are formed between sand particles.Effective calcium carbonate is the main reason for the improvement of mechanical properties of sand,which explains the weaker compressive strength and liquefaction resistance of solidified sand with higher Cu.In the condition of changing Cc and keeping Cu unchanged,the effective calcium carbonate increased first and then decreased with the increase of Cc.The calcinous sand with good grain size can produce the most effective calcium carbonate,which can better bond the adjacent sand particles together,thus improving the compressive strength and liquefaction resistance.