Experimental Study On The Physical And Mechanical Properties Of Microbial Modified Solidified Sand

With the economy developing rapidly and the urbanization accelerating,the consumption of land has become more and more serious.Poor soil conditions and regional environment are important factors that restrict my country’s urban modernization.In bad land,sand occupies a large part.Sandy soil is not only widely distributed in our country,but also exists in all kinds of soils.（MICP）technology is a new technology for treating poor foundations in the field of geotechnical engineering.Although the MICP-treated sand can reach a high level of strength,it is a typical brittle material to a large extent and has certain hidden dangers in terms of safety.This article is based on the research of microbial solidified sand,and considers volcanic ash and basalt fiber to improve the microbial solidified sand.Which choose 3 kinds of volcanic ash with different particle size ranges（0～0.25mm,0.25～0.5mm,0.5～0.75mm）,and 4 kinds of basalt fibers with different content（0.15%,0.25%,0.35%,0.45%）for microbial solidification test.Based on the results of triaxial tests,seepage tests,and nuclear magnetic resonance tests,the optimal particle size of volcanic ash and the optimal amount of basalt fiber in the research range of this paper are selected for microbial reinforcement tests.Based on relevant physical and mechanical test results,analyze the influence of volcanic ash and basalt fiber on the effect of microbial solidification from a macro perspective.Finally,based on the SEM scanning test results and the corresponding microbial solidification model,the mechanism of the incorporation of volcanic ash and basalt fiber on the microbial solidification of sand was explored from a microscopic point of view:Research indicates:（1）The incorporation of volcanic ash can significantly improve the mechanical properties and impermeability of microbes and solids.From the point of view of the failure characteristics of the samples,the improvement of the brittle failure characteristics of the samples reinforced with pozzolan microbes is not obvious.Based on the results of triaxial test,seepage test and nuclear magnetic resonance test,it is concluded that the optimal particle size of pozzolan is 0～0.25mm,when the confining pressure is 100k Pa～300k Pa,the compressive strength can reach 1.51MPa～2.3MPa,the cohesive force is 252.22k Pa,the internal friction angle is 41.67°,the average permeability coefficient is 4.31×10^-6cm/s,and the porosity is13.31%.Compared with the sample without pozzolan,the compressive strength,cohesion and internal friction angle are increased by 53.42%～55.3%,45.21%,28.25%,the permeability coefficient and porosity are reduced by 60.28%,44.14%,respectively.（2）The incorporation of basalt fiber mainly improves the brittle failure characteristics of the soil.The effect of fiber reinforcement is obvious,and the stress decreases in a"step"type after reaching the peak.With the increase of fiber content,in the residual strength stage,the curve shows a slight strain hardening characteristic.According to the analysis of relevant test results,the optimal content of basalt fiber is 0.35%,when the confining pressure is100k Pa～300k Pa,the compressive strength can reach 1.04MPa～1.6MPa,the cohesive force is187.46k Pa,the internal friction angle is 36.04.°,the average permeability coefficient is5.56×10-6cm/s,and the porosity is 18.87%.Compared with the sample without fiber,the compressive strength,cohesion and internal friction angle are increased by 30.97%～35.22%,26.28%,and 17.14%,the permeability coefficient and porosity are reduced by 48.76%and30.7%,respectively.（3）Based on the separate action of volcanic ash and basalt fiber,the combined effect of volcanic ash and basalt fiber combines the advantages of both.While effectively improving the mechanical properties and impermeability of the soil,it also improves the brittle failure characteristics and increases the toughness and deformation resistance of the soil.For specimens reinforced with pozzolan and basalt fiber at the same time,when the confining pressure is 100k Pa～300k Pa,the compressive strength can reach 1.71MPa～2.76MPa,the cohesive force is 273.13k Pa,the internal friction angle is 39.6°,and the average permeability coefficient is 4.66×10-6cm/s,porosity is 15.21%.Compared with the samples without pozzolan and fiber,the compressive strength,cohesion and internal friction angle are increased by58.38%～61.17%,38.69%,and 35.84%,the average permeability coefficient and porosity decreased by 57.05%,44.14%,respectively.（4）The main role of calcium carbonate crystals in the solidification of loose sand with MICP technology is cementation and filling.The reinforcement of volcanic ash and basalt fiber and the improvement of soil by MICP technology mutually promote:The incorporation of basalt fiber and volcanic ash can improve the colonization of bacteria,increase the deposition efficiency and yield of calcium carbonate,and enhance the skeleton effect of sand,thereby improving the overall mechanical properties and impermeability of the sample.The cementation effect of calcium carbonate can improve the effect of the fiber network and the force of the fiber-soil particle interface,thereby improving the reinforcement effect of the basalt fiber.