Experimental Study on Mechanical and Structural Properties of Enzyme-Induced Carbonate Precipitation Solidified Soil Under Freeze–Thaw Cycles

Enzyme-induced carbonate precipitation (EICP), an environmentally friendly geotechnical reinforcement method, is commonly adopted in water conservancy infrastructure, like reservoir bank slopes. Currently, limited studies have been performed on the mechanical and structural properties of EICP-solidified soil (ES) under freeze–thaw (F-T) cycles. In this study, a series of unconfined compressive strength (UCS) tests were performed to investigate the strength degradation characteristics and failure modes of ES and untreated soil (US) under a various number of F-T cycles. The “freeze–thaw structural parameter Mσ” and “initial freeze–thaw structural parameter Mp” were established to study the structural evolution laws of ES with strain and number of F-T cycles. Finally, the effect of F-T cycles on the microscopic pore structure of soil was investigated. The results indicated that the ES exhibited good strength retention capabilities subjected to F-T cycles. After one F-T cycle, the strength loss rate of the US was as high as 69.33%, while that of the ES was only 64.69% after 15 F-T cycles. The “freeze–thaw structural parameter Mσ” and the “initial freeze–thaw structural parameter Mp” presented the enhancement degree of structural strength and stabilization of ES under F-T cycles. The Mσ with strain could be divided into three stages. The nonlinear fitting results regarding the Mp showed a negative logarithmic relationship with the number of F-T cycles. With various F-T cycles, the pore area ratio of ES increased by an average of 0.603%, lower than that of US, 1.19%. After 10 to 15 F-T cycles, the Feret diameter reduction in ES was only 0.015 μm, which was 7% of the US, verifying the macroscopic test results. In the design of the reservoir slope, Mp and Mσ can be used to evaluate the deterioration of mechanical and structural properties after freeze–thaw disturbance, and to predict the stress and deformation response.