Experimental Study On Erosion Resistance Of Granite Residual Soil Treated Using Microbial Induced Calcite Precipitation (MICP)

In the subtropical and tropical areas south of the Yangtze River in China,granite remnants are widely distributed.This kind of soil has problems such as loose soil,crack development,easy disintegration,etc.,and is very easy to occur collapse erosion under rainfall conditions,causing serious soil erosion,which brings great threats to the local natural environment and residents’production and life.MICP technology is a new environmentally friendly bioreinforcement technology that has shown its unique potential in the field of slope reinforcement.In this paper,the growth curve and optimal culture conditions of relevant microorganisms were cultured,and the granite residual soil in southern Jiangxi Province was reinforced by MICP technology,and the disintegration,one-dimensional soil column infiltration and simulated rainfall erosion tests were carried out to study the effects of different reinforcement conditions on soil disintegration,water infiltration and slope erosion after reinforcement,and the following conclusions were mainly drawn:1.By laboratory culture of Bacillus pasteurii and measurement of its growth curve,the overall growth of the bacterial solution is divided into four stages:retardation phase,logarithmic phase,stable phase and death phase.Among them,the bacterial liquid reached a stable period at about 24h,and the OD₆₀₀ value was about 1.6.By monitoring the bacterial concentration and urease activity of the bacterial solution,it was found that the OD₆₀₀ value and the unit urease activity had a linear relationship before the stabilization period.The optimal culture temperature and p H of Bacillus pasteurii are 30℃and p H=9,respectively.2.Through the disintegration test of samples with different reinforcement combinations,the disintegration forms of the reinforced specimens are divided into three types:complete disintegration type,incomplete disintegration type and non-disintegration type.Among them,the concentration of bacterial liquid OD₆₀₀=0.75,the concentration of cementation solution 1.2mol/L and five rounds of reinforcement can obtain better reinforcement effect.SEM test found that Ca CO₃ particles formed around the soil particles after MICP treatment,and it was speculated that the effect of MICP treatment samples came from the filling of soil pores by Ca CO₃ particles.3.Through the one-dimensional soil column infiltration test on the samples with different reinforcement combinations,it was concluded that the concentration of bacteria liquid OD₆₀₀=0.75 and the concentration of cementation solution was 1.2mol/L,and the wetting front movement speed and cumulative infiltration of the specimen after five rounds of reinforcement were the smallest.The migration speed of the wet front after treatment varies with the treatment method:the samples treated with lower concentration bacteria and cementation solution cannot form a dense reinforcement layer to prevent water infiltration;The sample treated with higher concentration of bacteria solution forms a reinforcement layer prematurely,which makes its surface dense very high,but at the same time prevents the infiltration of subsequent bacterial liquid cementation solution,so that the overall reinforcement depth of the sample is insufficient,once the water breaks through the reinforcement layer,the soil below absorbs water and expands,the damage to the reinforcement layer is more serious,and the subsequent infiltration rate is significantly accelerated;Higher concentration cementation solution has a significant inhibitory effect on the activity of bacterial solution,so that the sample treated with higher concentration cement solution cannot form a dense reinforcement layer to prevent water infiltration.The cumulative infiltration after treatment is approximately the same.The distance-time curves of wet front migration before and after reinforcement can be fitted by power function,and the depth of wet front and soil cumulative infiltration are linear relations.4.By simulating rainfall on samples with different reinforcement combinations,different reinforcement methods have a greater impact on the overall failure mode of the sample.The sample treated with low bacterial concentration and cementation solution concentration cannot form a reinforced layer of sufficient strength,and the reinforcement layer is easily damaged under heavy rainfall conditions,resulting in serious slope erosion.The higher concentration of bacteria will increase the speed of the MICP reaction and cause the surface to form a reinforcement layer prematurely,so that the subsequent rounds of bacterial liquid cementation solution have poor infiltration effect and insufficient reinforcement depth.In addition,higher concentrations of cementation liquid have a significant inhibitory effect on the activity of bacterial liquid,so that the strength of the reinforcement layer formed by it is very poor,and it is also easy to be destroyed in heavy rainfall.With the increase of treatment rounds,the erosion resistance of the specimen gradually increased,but after five rounds of treatment,sufficient reinforcement layer has been formed on the surface of the specimen,and the effect of continuing to reinforce the erosion resistance is small.The test found that the samples with OD₆₀₀=0.75 and cementation concentration of1.2mol/L were less eroded in the test,and the sand yield and mud content were significantly reduced.