Enzyme-induced Calcite Precipitation For The Reinforcement Of Earthen Sites

There are a large number of earthen sites in the northwestern part of China,and these earthen sites have a long history and are mostly listed as national key cultural relics protection units,with high historical,cultural,tourism,archaeological and scientific values.The urease-induced calcium carbonate precipitation technology provides a new idea for the anti-weathering protection of earthen sites.In order to study the reinforcement effect and reinforcement mechanism of enzyme-induced calcium carbonate deposition in the soil of earthen sites,the soil near Xiaguanying Ancient City on the bank of Wanchuan River in Yuzhong County of Gansu Province was used as the research object.The soil was reinforced by means of dripping reinforcement agent.According to the laboratory tests,with the different dripping sequence,curing conditions and concentration ratios of calcium chloride,urea and urease considered,the permeability test,durability test,wet and dry freeze-thaw cyclic test and mechanical strength test of the soil were carried out.The water permeability and air permeability,durability,thermal conductivity,mechanical strength and durability of the reinforced soil by enzyme-induced calcium carbonate precipitation?EICP?were studied.According to the protection rules of relevant laws and the principle of selection of protective materials,the reinforcement effect was evaluated.On the basis of the laboratory tests,the outdoor tests were carried out.By observing the change of the shape and surface hardness of the sample after dripping and combining the results of the laboratory tests,the reinforcement effect by EICP was further evaluated.The main results are as follows:?1?According to the experimental study on the color difference of the remolded soil samples after dripping,it was found that the color difference of the samples which were dripped by the commercially available pure urease was not changed greatly;the homemade crude urease could affect the color difference due to other impurities,however,after grinding the surface floating layer with sandpaper,the color difference was greatly reduced.In this regard,the enzyme-induced calcium carbonate precipitation technique is suitable for the reinforcement of soil sites.CaCl₂ and Urea drip in the soil after the soil color is qualified.The color difference between the commercially available urease and CaCl₂+Urea mixed drip was basically qualified,while the color difference of the self-made urease and CaCl₂+Urea mixed drip was changed greatly,but the color difference was greatly reduced after the surface layer was removed.Therefore,if the soil ruins need to be reinforced with crude urease in order to reduce costs in the future,the excess impurities must be removed by the column.?2?After EICP drip infiltration to reinforce the site soil,the air permeability is weakened,while the disintegration and frost resistance are improved.The permeability of each sample did not change much after one time of dripping;the air permeability of the samples decreased after multiple times of dripping;and the higher the solute concentration,the worse the gas permeability.From the test results of disintegration and frost resistance test after dripping,it can be seen that the average disintegration rate of the untreated samples was 0.8%/s;the disintegration rate of the samples with the solute concentration of 0.8 mol/l was reduced to about 0.42%/s after one time of dripping;the average disintegration rate of the samples with the solute concentration of 0.4 mol/l was reduced to 0.2.%/s after three times of dripping,which indicates that urease-induced calcium carbonate technology can effectively improve the disintegration characteristics of the site soil.The freeze-thaw coefficient of the untreated sample was 0.8;the freeze-thaw coefficient of the samples after the dripping is increased and the freeze-thaw coefficient of the samples with the solute concentration of 0.2mol/l can be increased to 1.16 or so after 3 times of dripping.?3?The enzyme-induced calcium carbonate precipitation technology can effectively improve the static and dynamic characteristics of the site soil.The uniaxial compressive strength of the samples without the drip-reinforcing treatment was about378.5 kPa.The most obvious increase in the uniaxial compressive strength of the samples with the solute concentration of 0.8 mol/l after one time of dripping was about 1800 kPa.The cohesive force and internal friction angle of the untreated samples were 25 kPa and 37°,and the cohesive force and internal friction angle of the samples with the solute concentration of 0.8 mol/l after two times of dripping were most obvious,which were 560kPa and 70°.?4?From the outdoor test,the site soil strengthened by drip infiltration is more durable than the untreated site soil sample.From the results of XRD analysis,the CaCO₃ minerals such as calcite and vaterite in each sample increased significantly after dripping.From the SEM test results,a large amount of CaCO₃ was formed on the surface and pores of the soil particles.This also confirms the previous conclusions.