Research On Improvement Effect And Mechanism Of MICP For Red Layer Mudstone Replacement Layer Of High Speed Rail Roadbed

It is unavoidable to cross soft rock sections such as red mudstone in high-speed railway construction.Base replacement layer of high-speed railway is the key position connecting the upper structure with the basement mudstone.Red mudstone material needs to be used for replacement layer filling under economic conditions.Red mudstone has poor engineering performance,low strength and is easy to soften and disintegrate in water.Therefore,it needs to be improved to improve its physical and mechanical properties.In the past,additives such as cement,lime,fly ash and curing agent are usually mixed into the materials to be improved in common ways,which have achieved good results.It is urgent to develop new green ecological improvement technology under the national"3060 double carbon"strategy.Based on this,this paper applies MICP technology to the material improvement of replacing layer of subgrade foundation.At first,the material proportion of microbial improvement is optimized through response test.Then,the microbial improvement test of red mudstone is carried out by using metakaolin as modified material design.By combining macro physical and mechanical tests and micro-examination,the strengthening effect of metakaolin on microbial modified red mudstone is analyzed and its strengthening mechanism is revealed.Finally,Based on the results of the final test,a constitutive model of the modified body was established.The main research results are as follows:（1）The total amount of microbial modified materials is limited by the optimal moisture content conditions of the soil.Based on this,considering the main influencing factors such as the ratio of cementitious bacteria,the concentration of consolidating fluid,and the concentration of bacterial fluid,single factor experiments and response surface experiments were designed and conducted using the amount of calcium carbonate deposition as the evaluation index.Firstly,from the single factor experiment,it can be seen that the ratio of cement to bacteria,the concentration of cement solution,and the concentration of bacterial solution all have a significant impact on the deposition amount of calcium carbonate,and there is an optimized interval range;Then,through response surface test analysis,the optimal microbial material ratio under the optimal moisture content was determined:consolidating fluid to bacterial fluid ratio of 4.16:1,consolidating fluid concentration of 3.21mol/L,and bacterial fluid concentration of 1.78.Under the optimized plan,improvement experiments were conducted on the red mudstone cushion material,and the calcium carbonate deposition amount could reach 4.77%,which better met the compressive performance requirements of the roadbed cushion material.（2）In order to promote the effect of red mudstone subgrade cushion material,combined with the growth characteristics of microorganisms and the characteristics of metakaolin,metakaolin was used as the modified material to design and carry out the microbial improvement test of red mudstone.The results show that after adding metakaolin,the compactness,compression resistance,shear resistance and impermeability of the ameliorated body were improved,but the effect on improving water stability was not obvious,The softening rating of the sample is Grade II,with a significant degree of influence from water.The disintegration rating is Grade I,with a strong degree of disintegration;Further,alkali activator was used to activate metakaolin,and then mixed into red mudstone for MICP improvement.All physical and mechanical properties were greatly improved.Among them,cement production increased by 60.80%,longitudinal wave velocity increased by 6.63%,porosity of each section decreased by 2.44%～25.98%,unconfined compressive strength increased by 250.00%,cohesion increased by 302.72%,and internal friction angle increased by 168.13%,The permeability coefficient decreased by two orders of magnitude,the softening coefficient increased by 48.19%,the softening coefficient was rated as Grade IV,the no-load expansion rate decreased by 97.57%,the disintegration was Grade IV,and the sample did not disintegrate,indicating that metakaolin has a better strengthening effect on MICP modified red mudstone after activation,and has better improved the compactness,compression resistance,shear resistance,impermeability,and water stability of the improved body.（3）The pore structure of the red mudstone improved sample has a better optimization effect after being strengthened by metakaolin,which is also reflected in the improvement of soil strength and soil structural stability in the macro.After being mixed with metakaolin,due to its small particle size,it can play a good filling effect between soil particles.At the same time,metakaolin has a large specific surface area,which has strong adsorption activity,which is conducive to the colonization of microorganisms in the sample,and thus increases the deposition of calcium carbonate;After the metakaolin is activated with alkali activator,the metakaolin undergoes pozzolanic reaction and polymerization,generating a large number of silica alumina gel phase materials,and finally forming a composite cementation structure of hydrated silica aluminate polymer calcium carbonate,which effectively improves the compactness and bonding strength between soil particles.（4）According to the characteristics of the microbial modified red mudstone,this article divides it into three parts:the well cemented part,the poorly cemented part,and the non cemented part.Based on the Mohr-Coulomb failure criterion,the micro element strength of the material is expressed,and the Weibull statistical distribution theory is used to establish a constitutive mechanical model of the improved red mudstone.Establish the statistical constitutive equation for damage of its red mudstone sample and influence coefficientξ.It is used to characterize the proportion of micro-defects to the area of damaged material.Through analysis,it can be seen that the influence coefficient is related to mechanical parameters E,c,?,and external load parametersσ₁,ε₁.There is a good correlation.It is proved that the calculated value of the model is in good agreement with the experimental value and the model is reasonable.