Study On Hydraulic-mechanical Properties And Strengthening Mechanism Of Purple Soil Solidified By Microbial Grouting

Under the influence of geology,climate and human activities,soil loss,landslides and other disasters occur frequently in purple soil mountainous areas,and most of them are fundamentally initiated by soil erosion.In order to study and control the erosion of purple soil,it is necessary that the mechanism,the internal characteristics of purple soil and the impact of external environment?climate and geographical conditions?are comprehensively considered.The internal cause of purple soil erosion is mainly due to the low content of cementitious material between soil particles and loose soil.The external cause is closely related to the humid climate of the reservoir area,abundant rainfall conditions,rolling terrain of hills and mountains,and other external force.The periodic fluctuation of reservoir water level,runoff formed by rainfall and undercurrent in fissures in the Three Gorges Reservoir area will intensify the erosion and loss of cementitious materials that cause the change of particle size dist and soil loss.When the soil layer within the influence range of the water dissipation zone is immersed and the water loss is alternately dry and wet,the internal water state of the soil will also transform between saturation and unsaturation.In the mean time,the soil hydraulic and mechanical properties change with the internal structure and the degree of water saturation,the soil integrity is gradually damaged,the anti-erosion ability and strength of the soil are reduced,which result in the soil erosion and the slope sliding instability.Furthermore,rural road and soil bank are destructed and collapsed.Considering production and construction,social and economic development,and the increasingly fragile ecological environment,it is necessary that the characteristics of applicability,efficiency,and environmental protection are provided with the soil reinforcement engineering and methods.Microbially induced calcite precipitation?MICP?exactly meets the requirement.As a kind of biomineralization process widely existing in nature,it is often used to accelerate the mineralization of calcite type calcium carbonate crystal by specific microbial induction and providing sufficient Ca²⁺and nitrogen source nutrients.Compared with the traditional chemical cementitious materials,it has more advantages in controllability and environmental friendliness.It contains huge economic and environmental benefits.Based on evaluating the applicability of MICP technology to purple soil,reasonable grouting materials and pouring process are determined.Meanwhile,a number of hydraulic-mechanical tests including triaxial,infiltration,dry-wet cycle,water erosion and durability,as well as SEM,XRD and other material analysis tests are measured.Subsequently,micro and macro test data are combining analysized by the way of testing,statistical analysis and theoretical modeling.Hence,the interaction among microorganisms,nutrients and purple soil are explored.The hydraulic mechanical properties of purple soil are determined.The evolution law of microstructure,mechanical strength and erosion resistance of loose purple soil particles are revealed.It provides a scientific basis for the systematic recognition of microbial induced deposition of calcium carbonate for soil consolidation,and for the exploration of new methods for the prevention and control of shallow landslides and soil erosion in the purple soil area of the Three Gorges Reservoir area.It is of great theoretical significance and practical value to study the mechanism of micro structure cementation and hydro mechanical properties of sandy clayey purple soil by microbial grouting and solidification,and to put forward reasonable engineering measures for soil erosion restoration and landslide disaster prevention in the purple soil area.The main research work and conclusions are as follows:?1?Based on the theory of mass conservation and seepage flow,the permeability conditions of MICP technology under the control of flow velocity are deduced.Furthermore,the permeability conditions could be judged by the formula of permeability coefficient of Terzaghi and China Academy of Water Resources and Hydropower.There is a good correlation between the solidification strength and the amount of calcium carbonate.Moreover,permeability is the key factor.Based on the effective porosity,three kinds of calculation formulas of seepage conditions can be used to judge the applicability of various soils.At the same time,the formula of total grouting amount and grouting time can provide the basis for the design calculation and scheme selection of large-scale outdoor simulation test and microbial reinforcement engineering.The judgment standard,test method and empirical formula of microbial induced calcium carbonate applied to engineering soil are put forward.Through a number of tests,statistics and analysis,the empirical formulas and conditions of the applicability,curing effect and inspection standard of MICP curing technology for purple soil are obtained,which show that the applicability of MICP curing technology for poorly graded purple soil is better.It can be widely used in purple soil erosion control projects;the particle size conditions of purple soil suitable for MICP curing technology are d₅₀?0.22mm or d₆₀?0.37mm,the effective porosity condition is n_0u?33.7%.It can be used as an empirical formula to analyze and judge the applicability of MICP technology.The soil particle composition has a certain effect on the improvement of solidification effect.When the sand content D_s?[84.4%,91.9%],the enhanced effect of microbial grouting on purple soil is the best.The mechanical enhanced effect of microbial grouting on purple soil is difficult to achieve in the short term when the sand content D_s?80.8.The mechanical strengthening effect of microbial grouting is not good when the sand content D_s?95.According to the empirical formula of the growth rate of unconfined compressive strength characterized by the number of cyclic grouting solidification N and the average density growth rate of calcium carbonate ME,the obvious effect of microbial grouting solidification on purple soil is givenQ_u=1.The threshold values of 1,i.e.the unconfined compressive strength increases by 1 time,are as follows:the curing times of cyclic grouting N>3,ME>1.514%,which can be used as the empirical formula of curing effect for testing.?2?Based on the analysis of urease activity of bacteria,supernatant and bacterial solution,there is a time difference in its action.The urease activity of the bacterial group changed most dramatically,while the urease activity of the supernatant group had long-term characteristics.The initial performance of the bacterial solution is slightly worse than that of the bacterial group.However,the viscosity is smaller than that of the bacterial group,which could significantly improve the uniformity of curing.The concentration and dosage of cementitious fluid play an important role in the production and yield of calcium carbonate.Meanwhile,sufficient reaction time must be guaranteed,and the interval time of cementitious fluid must be considered in the design of MICP construction.Multiple step grouting can improve the solidification efficiency and effect.The strength of soil solidified by MICP technology has the mechanism of continuous growth.24 to 48 hours curing is conductive to the improvement of the curing effect.0.5mol/L cementitious liquid has the lowest solidification strength,but its properties are relatively smooth.2mol/L cementitious liquid has the highest solidification strength,but0.75mol/L cementitious liquid and 2mol/L cementitious liquid have similar solidification strength.It is suggested to choose 0.75mol/L cementitious liquid concentration based on the consideration of both solidification effect and efficiency.?3?The calculation formula of effective calcium carbonate considering the distribution of calcium carbonate is established by introducing the permeability parameter.The relationship between soil strength index and water content is analyzed through the solidification test and erosion resistance test of purple soil.The influence of hydraulic erosion on soil erosion is studied.There is a quantitative relationship between soil strength index and moisture content,dry and wet cycles,hydraulic erosion and other factors.The moisture change and dry and wet cycles are based on the interaction of soil microstructure and water film.Through the dry and wet cycle test,scour test,28-days durability test and plant compatibility test,the anti erosion ability of soil solidified by MICP technology is verified.?4?The crystal form,distribution quantity and location of the microbial induced calcium carbonate under the micro scale,as well as the correlation with the soil particle in the soil,combining with the macro scale are studied based on the comprehensive analysis of the micro test results and the macro mechanical test results of the microbial solidified purple soil with different cementation times and different soil particle composition through the X-ray diffractometer and scanning electron microscope tests.The physical and mechanical properties of purple soil solidified by microbial grouting,such as compressive strength,calcite content and distribution,porosity and so on,are analyzed.Moreover,the mechanism of strengthening effect of purple soil solidified by microbial induced deposition of calcium carbonate is studied.Based on the state parameters of disturbance dynamics and the characteristics of MICP curing,two parameters are defined.The evaluation of curing effect is demonstrated by experiments and statistical analysis,which provides guidance for the establishment of related constitutive model,the further development of curing engineering and the detection and evaluation of curing effect.?5?Based on the concept of disturbed state,the mechanical enhancement model of purple soil is established by introducing the disturbance variable of microbial solidification and cementation,and the mechanism of microbial induced deposition and cementation enhancement of purple soil is revealed.Based on the quantitative characterization of the effects of wetting,drying and wetting cycles and scour on the erosion of undisturbed soil and solidified soil,an enhanced mechanical model of purple soil solidified by MICP technology based on the concept of disturbance is established.The test results show that it can be further extended to the general state model of solidified soil in erosion environment.The solidification of purple soil by MICP technology is a very complicated process,and it is necessary to carry out multi-scale,long-term test,practice and more in-depth analysis and research.