Experimental And Simulation Research On Microbial Flower-tube Grouting To Solidify Sand

Microbial grouting is a kind of new soil improvement technology based on Microbial Induced Carbonate Precipitate(MICP)with the advantage of long transport distance,reaction controllable and environmental friendly,which has broad application prospects.At present,longitudinal grouting method was widely adopted in microbial grouting,easily causing the blockage of CaCO₃ crystals at the grouting point.Thus,the grouting cannot continue because the decrease in permeability of sand.Therefore,microbial flower-tube grouting technology is proposed in this paper to improve the uniformity of the solidified sand,in consideration of the transverse permeability of natural soil is greater than its longitudinal permeability.Four aspects about microbial flower-tube grouting were researched,including optimization of the grouting factors,quantization of CaCO₃ crystals,exploration of a large-scale grouting experiment and two-dimensional simulation calculation.The detailed research contents and conclusions are as follows.(1)Twenty-five sand column tests were carried out by a orthogonal design with six test factors were coupled.The influences of the test factors on Ca²⁺utilization and CaCO₃ distribution were analyzed.The microscopic morphology of the crystals was observed by SEM method.Moreover,the unconfined compressive strengths of the solidified sand columns were detected.The results show that,?1 Higher concentration of bacterial suspension,lower concentration of cementing solution and longer reaction time result in higher Ca²⁺utilization.?2 Bacterial cells can sink during the retention time of bacterial suspension in sand.?3 A higher flow rate of grouting solutions can result in a higher average CaCO₃ content and a more uniform crystals distribution.?4 The size,shape characteristics,growth mechanism and surface morphology of the crystals in the sand are all affected by the test factors.?5The unconfined compressive strengths of the MICP-solidified sand columns grow with the increase of CaCO₃ content,but are also influenced by other factors,such as the distribution and size of the crystals.?6 The optimization results of the six test factors are the OD₆₀₀ value of bacterial suspension is 0.5?1.0,the retention time of bacterial suspension is three hours,the concentration of cementing solution is lower than 500m M,the flow rate of the solutions is related to the its transport distance,the retention time of cementing is to completely consume all the Ca²⁺.?7 The uniformity of CaCO₃crystals in solidified sand columns obtained by microbial flower-tube grouting method is better than that of traditional longitudinal grouting method.(2)Based on the mechanism of MICP,the influences of substance concentrations,sandy environment and reaction level on the formation of CaCO₃ in the process of microbial grouting were researched.Then a model to quantify the CaCO₃ amount was established and a series of time-dependent equations,evaluating the relationship between the CaCO₃ amount produced with bacterial concentration,cementing solution's concentration and reaction time,was proposed,which can predict the amount of CaCO₃ produced in microbial grouting with a deviation less than 5%.During the process of constructing the CaCO₃ quantification model,it was found that bacteria and urea promoted the urea hydrolyzed rate,Ca²⁺inhibited the urea hydrolyzed rate,NH₄⁺and Cl^-had on effect on urea hydrolyzed rate.The sandy environment promoted the urea hydrolyzed rate compared with the liquid environment for that the surfaces of sand particles could adsorb bacterial cells to provide larger area for the fixation of bacteria.This promotion gradually weakened as the bacterial concentration increased.Moreover,the heterogeneity of reaction substances can affect the reaction degree in different positions in sand,so that some Ca²⁺ions in the sand still exist in ionic state.(3)A large-scale sand with a scale of 1.2m×0.6m×0.3m was solidified by microbial grouting method.According to the CaCO₃ distribution in the sand,it was found that the solidification radius around the grouting tubes is 5?10 cm.Moreover,the irrigation method can supplement the inadequate solidification of surface sand in microbial flower-tube grouting.The strength tests results show that the compressive strength of the solidified sand was 0?2 MPa,and the strength of the upper layer was higher than that of the lower layer.Based on the calculation of the bearing capacity of solidified sand after grouting,it was found that the improvement effect of microbial grouting on the bearing capacity of foundation increased with the grow of the ratio of solidified thickness to load width.When the solidified thickness reaches 0.4 times of the load width,the bearing capacity of foundation can be increased by more than two times.(4)A two-dimensional simulation calculation model for microbial flower-tube grouting was carried out based on the advection-diffusion-reaction differential equations in saturated sand,followed by the calculation of the formation of CaCO₃crystals and the change of sand porosity during grouting.Then the influences of bacterial distribution and solution's injection rate were researched and the results show that a uniform bacterial distribution are beneficial to the uniformity of the produced CaCO₃ in sand.In the process of microbial center grouting,downward density flow appears,causing the solidification radius of the lower part of the sand higher than that of the upper part.This influence can be reduced by increasing the injection rate of the solution.In a typical case,0.3 m-thickness of sand is solidified by microbial flower-tube grouting with the bacterial OD₆₀₀ of 0.25,cementing solution's concentration of0.25 mol/L and injection rate of 1×10^-5 m/s.The calculated solidification radius of sand is 0.15?0.4 m.