Effect of freeze-thaw cycles on engineering properties of biocemented sand under different treatment conditions

The recently developed microbially induced calcite precipitation (MICP) method is a potentially effective and environmentally friendly technique to improve the unfavorable soil conditions. However, the durability of biotreated sand under freeze-thaw cycles has not been investigated. This study investigates the effects of freeze-thaw (FT) cycles on shear strength and shear modulus of Narmada river sand (a poorly graded liquefiable sand) treated with MICP under different conditions. The bacterial growth and biotreatment were conducted under non-sterile and environmentally uncontrolled conditions that are typically encountered in the field conditions. First, the effects of main control variables involved in the MICP treatment, specifically two bacteria strains i.e., Sporosarcina pasteurii (S. pasteurii) and Bacillus sphaericus (B. sphaericus), two treatment cycles viz. 12 h and 24 h, and different pore volumes (1 PV, 0.75 PV, and 0.5 PV) of cementation solution injection, were investigated. After 18 days of continuous treatment, the biocemented specimens were tested for unconfined compressive strength (UCS), split tensile strength (STS), hydraulic conductivity, ultrasonic pulse velocity (UPV), and shear modulus after 0, 5, 10, 15 and 20 freeze-thaw (FT) cycles. The calcite content distribution in all of the biotreated samples was also analyzed. The results demonstrated that the maximum UCS, STS, UPV, shear modulus, and reduction in hydraulic conductivity were 2333.2 kPa, 437.7 kPa, 2670 m/s, 13 GPa, and 96.6%, respectively, at 12% calcite content using S. pasteurii, 12 h treatment cycle, and injection of 1 PV cementation solution in every cycle up to 18 days. The biocemented specimens maintained almost 90% shear strength and 77% shear modulus after 10 FT cycles in 9–12% calcite precipitated samples augmented with S. pasteurii. The similar biocemented samples resulted in 14–17% and 31–35% decrease in shear strength, and 41–44% and 54–57% decrease in shear modulus after 15 and 20 FT cycles, respectively.