Experimental study on mechanical properties of sandy clayey purple soil cemented by a new EICP grouting method

The purple soil is widely distributed in the Three Gorges Reservoir area of China. Influenced by the extreme environmental and climatic conditions, its structure and mechanical properties deteriorate significantly. To address this issue, the use of the biomineralization technology (MICP and EICP) for reinforcement is considered. However, due to its sandy-clay nature and weak permeability, the conventional methods have limited effectiveness in cementation. Thus, a new EICP technology for one-phase-low-pH negative pressure grouting is introduced. Through various means such as surface hardness, unconfined compressive strength (UCS), calcium carbonate content, SEM and XRD tests, the solidification effects of pre-mixing, one-phase-low-pH grouting, two-phase-low-pH negative pressure grouting and one-phase-low-pH negative pressure grouting methods are compared and analyzed. The results show that the optimal concentrations for EICP grouting to solidify the purple soil are 50 g/L urease concentration and 1.2 mol/L cementitious solution concentration. The one-phase-low-pH negative pressure grouting method increases the surface hardness by 9.1%, achieving enhancements of 84.44% and 144.37% in the UCS and stiffness, respectively. The calcium carbonate content after one round of one-phase-low-pH negative pressure grouting is 3.09%, which is the highest among the 4 solidification methods. The UCS increases exponentially with the increase in the calcium carbonate content. The SEM analysis shows that the calcium carbonate crystals obtained by the one-phase-low-pH negative pressure grouting method have the most uniform distribution, with a substantial amount of contact cementation. The crystal types are calcite and vaterite. The results demonstrate that the new EICP technology for the one-phase-low-pH negative pressure grouting method effectively improves the mechanical properties of the sandy clayey purple soil, showing potential application value in the reinforcement of fine-grained soil.