Review on engineering properties of MICP-treated soils

Microbial induced calcium carbonate precipitation (MICP), a sustainable and effective soil improvement method, has experienced a burgeoning development in recent years. It is a bio-mediated method that uses the metabolic process of bacteria to cause CaCO3 precipitation in the pore space of the soil. This technique has a large potential in the geotechnical engineering field to enhance soil properties, including mitigation of liquefaction, control of suffusion, etc. Multi-scale studies, from microstructure investigations (microscopic imaging and related rising techniques at micron-scale), to macroscopic tests (lab-based physical, chemical and mechanical tests from centimeter to meter), to in-situ trials (kilometers), have been done to study the mechanisms and efficiency of MICP. In this article, results obtained in recent years from various testing methods (conventional tests including unconfined compression tests, triaxial and oedometric tests, centrifuge tests, shear wave velocity and permeability measurements, as well as microscopic imaging) were selected, presented, analyzed and summarized, in order to be used as reference for future studies. Though results obtained in various studies are rather scattered, owning to the different experimental conditions, general conclusions can be given: when the CaCO3 content (CCC) increases, the unconfined compression strength increases (up to 1.4 MPa for CCC=5%) as well as the shear wave velocity (more than 1-fold increase in V_s for each 1% CaCO3 precipitated), and the permeability decreases (with a drop limited to less than 3 orders of magnitude). Concerning the mechanical behavior of MICP treated soil, an increase in the peak properties, an indefinite increase in friction angle and a large increase in cohesion were obtained. When the soil was subjected to cyclic/dynamic loadings, lower pore pressure generation, reduced strains, and increasing number of cycles to reach liquefaction were concluded. It is important to note that the formation of CaCO3 results in an increase in the dry density of the samples, which adds to the bonding of particles and may play a major part in the improvement of the mechanical properties of soil, such as peak maximum deviator, resistance to liquefaction, etc.