Comparison of improved shear strength of biotreated sand using different ureolytic strains and sterile conditions

The present study aims to improve shear strength of poorly graded sand using microbially induced calcite precipitation (MICP) technique. Two soil‐based bacteria, namely Sporosarcina pasteurii (SP) and Bacillus sphaericus (BS), were bioaugmented in sand. The cementation solutions of 0.50 M concentration were prepared using distilled water (DWCS) and tap water (TWCS), maintaining sterile and non‐sterile conditions, respectively. The biotreatment was carried out in direct shear test (DST) moulds using 12‐hr treatment cycles up to 14 days. The biotreated sand samples were subjected to DST to determine shear strength parameters, and results were statistically explored using analysis of variance (ANOVA). The amount of calcite precipitation, XRD and SEM was used to interpret biocementation. The non‐sterile treatment using TWCS showed significant strength. Additional biotreatment samples were prepared in DST and cylindrical PVC moulds using TWCS for 18 days but adopting different treatment cycles of 12, 24, 48 and 72 hr to explore inexpensive treatment option. All of these biotreated samples were tested for hydraulic conductivity, shear strength (based on DST and triaxial consolidated undrained (CU) tests) and three‐layer calcite contents. In CU tests, stress‐strain response of biocemented sands showed initial brittle behaviour followed by strain‐softening, analogous to rock‐like behaviour similar to sandstone. The maximum increase in strength was achieved with 12‐hr treatment cycle. The 24‐hr treatment cycle showed 1.7–1.8 times lesser strength than 12‐hr treatment cycle specimens. However, a significant amount of calcite precipitation and strength in 24‐hr treatment cycle showed its effectiveness in achieving cost reduction for large‐scale field applications.