Unsaturated soil properties of MICP treated granitic residual soil of Shantou region of China

Unsaturated soil properties such as soil–water characteristic curve (SWCC) and shear strength are required for seepage and stability flow analyses in various geo-engineering infrastructures. Microbial-induced calcite precipitation (MICP) has been recently adopted for enhancing strength of soils however, with rare focus on improvement in unsaturated soil properties of granitic residual soil. It is known that granite residual soil exhibits unique disintegration properties upon interaction with water. The objective of this study is to investigate the unsaturated properties under different vertical stresses (0, 100, 200 and 300 kPa) for MICP treated granitic residual soils. Further, microstructural characterization of MICP treated soil was conducted to analyse its water retention and shear strength, so as to provide theoretical basis for engineering application of MICP in strengthening granite residual soil. Pressure plate apparatus and FDJ-20 quadruple shear strength apparatus were utilized to obtain SWCCs and shear strength, respectively. Based on the result, it can be concluded that the treatment by MICP is found to enhance the air entry value of granitic residual soil. In addition, MICP treated soils possess higher water content than untreated soil at near-saturated condition. This is due to calcite precipitation on surface of grains and carbonate formation at contact points, which in turn reduces void ratio. However, the difference in water retention reduces with an increase in suction and also confining stress. It is possibly due to breakage of carbonate bonds at contact points at higher stresses. After five times grouting, the effective cohesion, internal friction angle and matric suction angle is found to increase very significantly.