Mechanistic Comparisons of MICP-treated Residual Soil and Sand Part I – Microstructural Formation and Deformation Behaviour of Soils

The application of MICP in improving fine-grained soil or soil containing fines, such as clay or residual soil has increasingly drawn attentions in recent years. However, there are still very limited studies focusing on the comparison between the behaviour of MICP-treated soil containing fines and the more established MICP-treated sand. This study investigates the differences of microstructural formation and deformation behaviour between (untreated and MICP) residual soil and sand through a series of monotonic undrained triaxial tests. The untreated and MICP-treated soils were prepared at a slightly dense state (nearly 80 % of relative density) and subjected to three levels of consolidation pressure (i.e. 40 kPa, 120 kPa, and 220 kPa), before being sheared under undrained condition. A significant improvement in the mechanical behaviour was observed in the bio-treated residual soil despite a low concentration of calcium carbonate content was precipitated (< 5.0 %). This finding was encouraging for field applications of the MICP ground improvement technology in residual soils that were typically known for containing appreciable fines content and being less permeable than sand. From the results, the isotropic compression behaviour and the undrained compressive strength of the MICP-treated residual soil were enhanced significantly. The MICP-treated residual soils tended to manifest a more isotropic deformation behaviour and possessed an elastic soil fabric, as compared to the untreated counterpart which showed anisotropic behaviour. In practical sense, the lightly treated residual soil behaved like a dense soil on account of the densification effect within the pore structure and enhanced interparticle strength. Despite there was almost no alteration of mechanical behaviours in the dense sand, certain implications were gained and will be useful in the comparison with microstructure of residual soil.