Cementation Stress Characteristic Curve for Sands Treated by Microbially Induced Carbonate Precipitation

Microbially induced carbonate precipitation (MICP) has been widely studied as an innovative method for ground improvement applications by taking advantage of the cementation effect of the porous geomaterial and thus enhancing its stiffness and strength. Yet, a quantitative and precise description of the interparticle stresses induced by ${\mathrm{CaCO}}_3$ cementation, and the effective stress of MICP-treated sands remain elusive and obscure. Note that bonding among particles induced by the calcium carbonate bridging is ubiquitous in a more general term of cementing granular materials commonly seen by capillary water, ice or gas hydrates, polymers, etc. In this work, the concept of cementation stress for MICP-treated sands was proposed, which was found to be equal to the isotropic tensile strength of MICP-treated sands, to unify the effective stress in uncemented and MICP-treated sands. The cementation stress characteristic curve can be defined as the relationship between the cementation stress and ${\mathrm{CaCO}}_3$ content to describe how ${\mathrm{CaCO}}_3$ cementation develops for sands as ${\mathrm{CaCO}}_3$ precipitates. Cementation stress characteristic curves were used to predict the shear strength of MICP-treated sands at different ${\mathrm{CaCO}}_3$ contents and the contour of shear strength distribution of a MICP-treated sand box, which compared favorably with the measured shear strengths from triaxial, direct shear, and unconfined compression tests. The validity of the cementation stress characteristic curve was confirmed for several sands and test conditions, considering factors of ${\mathrm{CaCO}}_3$ contents (up to 25%), poorly graded sands with particle size ranging from 0.1 to 2 mm, confining pressure (up to 400 kPa), and concentrations of urea and calcium chloride between 50 mM and 1 M. The cementation stress characteristic curve provides a potentially simple and practical way to describe the state of stress in MICP-treated sands and contributes a unified method to quantify the ${\mathrm{CaCO}}_3$ cementation effects and predict the shear strength of MICP-treated sands.