Study On Creep Characteristics Of Microbial Solidified Sand

In the research of geotechnical engineering,rock and soil mass has been regarded as a kind of special material with complex static mechanical properties and inanimate,but the microorganism and its function in the soil have been neglected.Microorganisms are ubiquitous in rock and soil,and during the long process of geological formation,microbial mineralization plays an important role in the formation of sedimentary minerals such as limestone.Microbial Induced Carbonate Precipitation is a process of biomineralization using biochemical reaction of microorganisms to produce calcium carbonate crystals to consolidate loose sand particles as a whole.This technology has a good application prospect in soil reinforcement,improvement of soil liquefaction resistance and seepage prevention.The creep behavior of sand after solidification by microbe induced calcium carbonate precipitation?MICP?was studied.Pasteurized Bacillus octadecane was used for extended culture to obtain bacterial solution and mix nutrient solution.The standard sand and calcareous sand were treated with 3times,6 times and 9 times of grouting to prepare the samples.The strength characteristic and treating effect of sand after treating were studied by strain-type direct shear instrument,and then the creep characteristics of two kinds of sand with different grouting times are studied by direct shear creep meter.Finally,the microstructure of solidified sand is analyzed by SEM.Through the combination of macroscopic direct shear creep deformation and microscopic scanning,the creep characteristics and deformation mechanism of solidified sand samples are studied,and the creep constitutive model of microbial solidified sand is constructed.The main research results of this paper are as follows:?1?The surface of standard sand particles is smooth,while the surface of calcareous sand is rough and porous,and the inner pores are rich in development.After microbial treating,the shear strength of both sand was greatly improved.Cohesive force of standard sand samples and calcareous sand c increase greatly with the increase of grouting times.The surface of calcareous sand is porous,and the larger space makes it retain more bacterial and nutrient solution in the process of nine-time grouting,which induces the formation of more calcium carbonate precipitation,so the increase of cohesion of nine-time grouting sample is greater than that of six-time grouting sample than that of standard sand.For internal friction angle?,the internal friction angle is basically unchanged after six times of standard sand treatment.The internal friction angle of calcareous sand after three-time treatment is basically unchanged,and the friction angle of calcareous sand after stabilization is less than that of standard sand.An increase in the?value of microbe-treated sand indicates that the friction or occlusal force between sand particles is increased by precipitation of calcium carbonate induced by microbe.?2?After microbial treating,the sand is cemented into a whole,and the sand is more difficult to deform.It mainly shows that with the increase of grouting times,under the same consolidation pressure,the sand needs greater shear stress to reach the same deformation amount.In general,the creep characteristics of calcareous sand samples after microbial curing are still stronger than that of standard sand samples,and the creep characteristics of standard sand and calcareous sand samples are weakened with the increase of grouting times.Different from the creep characteristics of natural sand soil,the creep characteristics of sand soil under various stages of consolidation pressure,there are three with the increase of shear stress,first strengthened and then weakened,first strengthened and then weakened.Sand after treating creep characteristics mainly depend on shear stress and cementation effect between sand particles after treating.?3?The calcium carbonate covering the surface of sand can be clearly seen by SEM.The surface of calcareous sand is porous after microbial treating,and it is easier to attach after the formation of calcium carbonate.However,the distribution of calcium carbonate on the surface of standard sand is not uniform,the adhesion of calcium carbonate on the smooth surface is less,and the distribution at the depression is more concentrated.When the amount of calcium carbonate is less,it is mainly attached around the contact point or the surface of the particles.With the increase of the number of grouting times,the amount of calcium carbonate production increases,gradually filling the pores of sand particles and cementing the sand particles into a whole,which makes the overall strength of sand increase.Calcium carbonate in microbially solidified sand consists of chalazal?globular,flake?calcium carbonate and oblique Square hexahedron calcite type calcium carbonate.It is easier to have mother liquor retention at the depression of sand particles and pores,which accelerates the transformation process of spherulite,and calcite crystals appear.?4?Generalized Kelvin five-element model fits the characteristic of decaying creep curve,and can fit the law of straight shear creep well.With the increase of grouting times,the elastic modulus E_Hof fitting parameters is increasing and the viscosity coefficient?_1??₂decreasing.The nonlinear creep model is constructed by combining the parameters obtained from creep deformation and straight shear test.The nonlinear creep model is used to calculate the strain when the microorganism solidified sand reaches the creep stable state.