Mechanical Properties Test and Microscopic Mechanism of Lignin Combined with EICP to Improve Silty Clay

To enhance the improvement effect of Enzyme-Induced Carbonate Precipitation (EICP) technology more effectively, an abundant renewable resource—lignin—was introduced as an additive during the EICP modification process of silty clay. The mechanical properties of the improved soil specimens were analyzed from a macroscopic point of view by using unconsolidated undrained (UU) triaxial tests and unconfined compressive strength (UCS) tests to determine the optimal lignin content and curing time. The micro-mechanism of the improved soil specimens was elucidated from the microscopic point of view by combining scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The experimental results showed that lignin synergized with EICP could effectively improve the mechanical properties of the soil, and the mechanical properties of the co-consolidated soil specimens were better than those of the single consolidated and untreated soil specimens as a whole. The single EICP-consolidated soil specimen had undergone brittle damage; lignin could enhance the toughness of the soil and weaken its brittle characteristics. With the increase of lignin content, the mechanical indicators of co-consolidated soil specimens showed the trend of increasing and then decreasing, and reached the optimum at 0.75%. Moreover, the addition of lignin significantly increased the cohesive force, while the friction angle was less affected. With extended curing time, the mechanical indicators of the co-consolidated soil specimens increased overall, and tended to stabilize after 7 days of curing, hence selecting 7 days as the optimal curing time. From the microscopic point of view, lignin provides nucleation sites for the calcium carbonate precipitates generated by EICP, and the joint action of the two can fill the soil pores and cement the soil particles, thereby improving the overall strength of the soil. The results of the study can provide a theoretical basis and practical reference for the construction of foundation projects in silty clay areas.