Hybrid bacteria mediated cemented sand: Microcharacterization, permeability, strength, shear wave velocity, stress-strain, and durability

Microbially induced calcite precipitation (MICP), a sustainable approach for sand biocementation, was investigated in previous studies based on metabolic activity of individual microorganisms. The individual bacteria, specifically Sporosarcina pasteurii (SP), Bacillus subtilis (BS), and Lysinibacillus sphaericus (LS), were found capable enough for sand biocementation. However, present study investigates synergistic effects of using bacterial-hybrids on cementation and consequent improvement in sand properties. The SP, BS, and LS strains were used in different combinations to create bacterial-hybrids and applied under simulated non-sterile field conditions. Initially, sand biotreatment was carried out in plastic tubes up to 14 days, using bacterial mixtures and 0.5 M cementation solution. Biocemented specimens were tested for calcite precipitation, XRD, FTIR, and SEM. The SP and LS combination (SPLS hybrid) showed maximum calcite precipitation, which is further used for biotreatment to create cylindrical sand samples for testing improved engineering properties. These samples were prepared using 0.5 M cementation solution in three pore volumes (1, 0.75, and 0.5 PV) and treatment cycles (12, 24, and 48 hrs TC) up to 18 days. Biocemented samples were tested for permeability (6th, 12th, and 18th days of biotreatment), unconfined compressive strength (UCS), split tensile strength (STS), ultrasonic pulse velocity (UPV), and consolidated undrained stress-strain response. Durability of biocementation was also investigated by determining reduction in strength and UPV subjected to freeze-thaw (FT) cycles (5, 10, 15, and 20). The results showed maximum UCS of 1902 kPa, STS of 356 kPa, UPV of 2408 m/s, and coefficient of permeability reduction up to 91%. The higher results were achieved with 11.11% calcite content in 1PV-12TC treated samples. The 1PV-12TC treated samples resulted in 4.2%, 8.3%, 17%, and 35% reduction of strength after 5, 10, 15, and 20 FT cycles, respectively. Overall, biocementation using hybrid bacteria is shown significant to improve sand's engineering properties, including potential to mitigate liquefaction.