Study On Physical And Mechanical Properties Of Expansive Soil Improved By Soybean Leaching Solution Induced Calcium Carbonate (SUICP)

Microbial induced calcium carbonate technology（MICP）has shown good results in rock and soil reinforcement,anti-liquefaction treatment,seepage prevention,plugging,pollution control and so on.The latest research found that plant urease can also induce the calcium carbonate precipitation,which provides a new idea for the improvement of soil.Expansive soil is a kind of special clay soil,which has the engineering characteristics of swelling and water loss shrinking,and has great harm to all kinds of projects.In this paper,soybean leaching solution induced calcium carbonate precipitation（SUICP）was selected to study expansive soil improvement.A large number of beans and other related plants were searched,the best source of the leaching solution was selected,the mechanism and properties of the plant leaching solution precipitated calcium carbonate were explored.And the effects of substrate concentration and moisture content in SUICP technology on the swelling-shrinkage and mechanical properties of improved expansive soil were studied.The main conclusions are as follows:（1）Soybean leaching solution and black bean leaching solution had higher enzyme activity in different beans and soybean related parts,which were 3.78 and 4.49 mol/min,respectively.From the perspective of economic benefits,soybean was the best source of urease.The hydrolysis activity of urea increased linearly with the increase of the concentration of soybean leaching solution,and the formula was U=0.04051c_b.The concentration of 120 g/L was selected for the subsequent test.Soybean leaching solution was heated at 25～65℃,showing good enzyme activity,which proved that the effect of subsequent experiments was better at room temperature.The calcium carbonate crystal induced by soybean leaching solution was a kind of porous spherical calcite with nanoscale pores on the surface,which belongs to a new type of calcite crystal.The particle size was between 4～6 um,the specific surface area was 138.5 m²/kg,and the thermal stability was at the medium level.（2）Compared with the expansive soil without SUICP treatment,the free expansion rate,loaded expansion rate,expansion force and shrinkage rate of the treated expansive soil were reduced.The free expansion rate decreased with the increase of substrate concentration.The substrate concentration was greater than 1 mol/L and the moisture content had no significant effect on the free expansion rate.When the substrate concentration was 1 mol/L,the water content was 23%～26%,and the improvement effect of loaded expansion rate was the best.The expansion force showed a continuous decreasing trend with the substrate concentration.The expansion force was the smallest when the substrate concentration was 1 mol/L and the water content was 26%.When the substrate concentration was 2 mol/L and the moisture content was 26%,the shrinkage of each sample was the smallest.The results had shown that the swelling index of expansive soil can be significantly reduced by SUICP treatment.The substrate concentration of 1mol/L,water content of 23%～26%is the best improvement effect.（3）Through unconfined compressive strength test,compression test and direct shear test,the mechanical properties of expansive soil improved by calcium carbonate precipitation induced by soybean leaching solution were studied.After treatment,the maximum unconfined compressive strength of the sample increased to 270.35 k Pa,and the soil stiffness also increased.SUICP treatment could reduce the compressibility of expansive soil,with compressibility coefficient from 0.7 Mpa^-1 to 0.2 Mpa^-1,from high compressibility soil to medium compressibility soil.The shear strength of the samples increased linearly with the increase of vertical pressureσ,and the cohesion and internal friction angle were improved by the SUICP treatment.When the substrate concentration was 0 mol/L,the cohesion and internal friction angle were 20 k Pa and 9.79°,respectively.When the substrate concentration increased to 1 mol/L,the cohesion and internal friction angle increased to74.81 k Pa and 27.32°,respectively.The ratio of water content of 23%and substrate concentration of 1 mol/L was the best for improving the mechanical properties of expansive soil.An improved green economic method was proposed in the experimental study of SUICP treatment expansive soil,in which the ratio of water content of 23%～26%and substrate concentration of 1mol/L could improve the swelling-shrinkage properties and mechanical properties of expansive soil.Meanwhile,a green and efficient method for the synthesis of spherical calcite was provided.