Experimental Study On Reinforcement Of Silt Surface By Microbial-induced Calcium Carbonate Precipitation Based On Viscous Material Modification

Microbial-induced calcium carbonate precipitation（MICP）is a new soil improvement technology that combines CO₃²⁺generated by microbially promoted urea hydrolysis with artificially provided Ca²⁺to produce Ca CO₃ by mineralization around the microorganism,thus achieving the effect of cementing soil particles and solidifying the soil,which is biocompatible,sustainable and significant for environmental protection.It has been widely used in many fields,including soil improvement.However,the MICP technique is currently used in sandy soil consolidation,and relatively little research has been conducted on fine-grained soils such as Yellow River silt.Based on MICP technology,this paper introduced three kinds of viscous materials（hydroxypropyl methyl cellulose,hydroxyethyl cellulose,sodium alginate）,and conducted relevant tests and analyses on the surface layer of silt reinforced by MICP combined viscous materials under different dosage gradients of bacterial and cementation solutions and different treatment times.The main contents and research conclusions are as follows:（1）Based on the MICP technology combined with hydroxypropyl methylcellulose（HPMC）for the reinforcement of powder soil surface layer,different dose gradients of bacterial solution and cementation solution and different treatment times were set,and the performance of the HPMC-MICP reinforced specimens was tested by macroscopic tests（mechanical properties of surface layer,wind and rain erosion resistance of specimen surface layer）and microscopic tests（Ca CO₃ content test,ammonia retention rate test,and SEM and XRD tests）.The performance of the specimens after reinforcement with HPMC-MICP was tested and compared with the specimens after reinforcement with conventional MICP and HPMC aqueous solutions under the same treatment conditions.The results showed that the HPMC-MICP-treated specimens had better erosion resistance,surface strength and ammonia retention rate than those treated with conventional MICP and HPMC aqueous solutions,with the highest surface strength of401.95 k Pa,the lowest wind erosion rate of 0.083%,the lowest mass loss of 9.28 g in the rainfall erosion test,and the highest ammonia retention rate of 73.53%.In addition,HPMC can effectively increase the proportion of Ca CO₃ in surface crust.Meanwhile,SEM and XRD showed that the surface microstructure of HPMC combined with MICP treatment was dense and the mineral composition of the product Ca CO₃ did not change.（2）In this study,hydroxyethyl cellulose（HEC）was added to the conventional MICP cured silt surface treatment to immobilize the calcium carbonate nucleation sites to promote more calcium carbonate production in the surface layer and form a silt crust.The effect of HEC-MICP on reinforcing the surface layer of silt was investigated by varying the amount of cementation solution and the number of MICP treatments.The tests showed that HEC can work synergistically with MICP to provide a versatile and high performing method of curing powdered soil surface layers.The HEC-MICP treatment has many advantages,including the localization of the promoted cementation reaction allowing more Ca CO₃ production in the powdered soil surface layer,the reduction of harmful by-product ammonium,and the achievement of better soil strength and erosion resistance.The test obtained topsoil crusts of silt can withstand strengths up to 474.69 k Pa,ammonium retention up to 83.21%,loss rate of0.018%at 14 m/s wind speed,and minimum mass loss of 7.13 g at 2.5 mm/min rain intensity.In addition,microscopic results showed that the main crystal form of Ca CO₃ minerals did not change after HEC-MICP treatment and the particle gap was effectively reduced.These results demonstrate the potential of HEC-assisted MICP to provide effective soil improvement and ammonia mitigation for surface erosion control and consolidation of silt.（3）In this study,a series of small-scale laboratory experiments were conducted to investigate the feasibility of using a mixture of MICP and sodium alginate（SA）solutions in the surface curing of silt to reinforce the silt surface layer.The powdered soil surface treatment was achieved by spraying SA and applying MICP on the soil surface.The properties such as erosion resistance were evaluated by macroscopic tests and microphysical tests using different methods.Surface strength tests,small-scale simulated wind tunnel tests and rainfall tests were conducted to investigate the mechanical properties of the MICP combined with SA-treated silt.The wind erosion resistance,rainfall erosion resistance and surface crust strength were measured and presented.Ammonia retention rate in the treated soil was also investigated,as well as the amount of calcium carbonate production after the technique was used to treat the top layer of the silt.The experimental results showed that the specimens after the addition of SA had better performance compared with those treated with conventional MICP and aqueous SA solution,and that although the cross-linking of SA and Ca²⁺after SA-MICP treatment reduced the Ca CO₃production,the soil crust of calcium alginate-Ca CO₃-soil particles formed after the treatment effectively improved the properties of the surface layer of the silt specimens:the SA-MICP treated specimens showed a minimum wind erosion resistance of 0.016%,a minimum rainfall mass loss of 9.83 g,a maximum ammonia retention of 67.19%,and a maximum surface strength of 342.83 k Pa.In addition,scanning electron microscopy and X-ray diffraction analysis techniques were used to conclude that the primary crystal form of Ca CO₃ minerals would not be altered by the addition of SA.（4）The macroscopic properties（surface strength,wind and rain erosion resistance）and microscopic properties（Ca CO₃ production and ammonia retention）of HPMC-MICP-treated,HEC-MICP-treated,and SA-MICP-treated specimens were compared and summarized at the optimal MICP treatment solution dose and optimal number of treatments,and the results showed that:When the number of treatments was 3 and the dose gradient of MICP treatment solution was IV,the HEC-MICP treated specimens had superior surface strength and rainfall erosion resistance compared to the other two viscous materials in the application of enhancing soil strength and improving rainfall erosion,with a surface strength of 474.69 k Pa and a minimum rainfall mass loss of 7.13 g.The SA-MICP treated specimens showed the lowest Ca CO₃ production due to SA cross-linking reaction,and the total Ca CO₃ production of HEC-MICP and HPMC-MICP treated specimens was almost the same.In terms of reducing NH₃emissions during MICP,HEC-MICP had better ammonia retention compared to HPMC and SA,with 9.68%and 16.02%higher ammonia retention compared to HPMC and SA,respectively.In terms of dust control,the HPMC,HEC,and SA combined with MICP treated specimens all had excellent wind erosion resistance,with mass loss rates below 0.1%at 14 m/s wind speed.