Experimental study on optimization of pressure grouting MICP solidification for sandy clayey purple soil

Technology of microbially induced calcite precipitation (MICP) has a wide application prospect in geotechnical material reinforcement. The method of pressure grouting is suitable for the test of geomaterials with low permeability. The effects of grouting pressure, concentration of the cementation reagent, grouting flow rate and cycle condition are measured through L₉(3⁴) orthogonal test of Chongqing sandy clayey purple soil. Cooperated with the micro-structure analysis of SEM, the influence of pressure grouting approaches on sample's micro-structure, strength, secant modulus and uniformity distribution of calcite distribution are unveiled. The results show that the reagent concentration and flow rate are important factors to determine the deposition location of calcite. The calcite can be transported to the interior of sample before a large amount of flocculation precipitated on upper layer by properly increasing the flow rate. After 1, 3 and 5 grouting cycles, the utilization rate of reagent decreases successively, which is 67%, 48%, and 36%, respectively. Under the influence of yield calcite, grouting pressure and periodicity, the permeability coefficient of the sample decreased by 1-2 orders of magnitude from an initial value of 2.5×10^-4 cm/s. Calcite production rate for peak unconfined compressive strength increment (ΔUCS) expresses an approximately linear relationship, the influence of more than 1.1% of calcite increment can make the purple soil ΔUCS obtain visible improvement. After 5 times of pressure grouting, the strength is maximumly increased by 2 times; The secant modulus is negatively correlated with the dispersion and fluctuates with the increase of calcium carbonate. The secant modulus E₅₀ of cured samples by pressure grouting is up to 301 MPa, 3.2 times that of uncured samples. The optimal grouting Conditions for the treatment are: a grouting pressure of 150 kPa, 28 mL/h, a flow rate of a cementation reagent concentration of 1 mol/L, and 5 grouting cycles. Designed pressure grouting system with controllable flow rate and grouting pressure can measure the permeability coefficient after each round of treatment, and the test results can provide reference for the application of microbially pressure grouting in low permeable soil.