Investigation into the mechanical behaviour of bio-cemented sands using the discrete element method

Abstract

Bio-mediated methods, such as microbially induced carbonate precipitation (MICP) and enzyme induced carbonate precipitation (EICP), have gained considerable attention as alternatives to invasive ground improvement techniques. MICP and EICP use biogeochemical processes to drive carbonate precipitation and bind soil grains, thereby improving the mechanical performance of soils. While MICP and EICP have the potential to transform geotechnical engineering, challenges persist, particularly in understanding the spatial distribution of the precipitated minerals and predicting the associated mechanical improvement. This thesis addresses these uncertainties and sheds light on the role of the microstructure on the mechanical behaviour of bio-cemented sands and the effectiveness of the bio-cementation treatment.

To this end, experimental observations on the microstructure of bio-cemented sands are first reviewed. Four typical carbonate distribution patterns are identified depending on the location of the precipitated carbonates with respect to the granular assembly: grain bridging, contact cementing, grain coating, and pore filling. The discrete element method (DEM) is then used to investigate the effect of the aforementioned carbonate distribution patterns on the mechanical behaviour of bio-cemented sands. A toolbox based on the open-source DEM platform YADE, called Cementor, is developed for modelling various crystal distribution patterns and contents.

DEM models of bio-cemented sand samples with different properties, i.e. distribution pattern and mass content of carbonates (up to 3\%), are subjected to drained triaxial compression tests under various confinements. It is found that carbonates in the pattern of coating and pore filling have a negligible influence on the mechanical response of the material. In contrast, grain bridging and contact cementing lead to a noticeable improvement in stiffness, peak strength and dilatancy. The difference in the macroscopic behaviour of the cemented samples is explained by microscopic indicators such as the effective coordination number and bond breakage evolution.

Some experimental studies surprisingly show that MICP-treated sands can exhibit a lower residual strength than uncemented sands. Such a phenomenon has been observed experimentally but not explained. The presented DEM model offers insights into this behaviour. It was found that carbonates precipitated in the bridging distribution pattern are more likely to form a metastable structure. This metastable structure is prone to relative movement of particles, which in turn leads to the development of shear bands and, overall, a lower residual strength than the uncemented sample.

The variation in the microscopic properties of bio-cemented sands leads to uncertainty of mechanical improvements gained from bio-mediated treatment. It is crucial to develop a probing method to gain confidence in the treatment. Seismic measurements can be used to probe the treated soil. To evaluate the seismic response of bio-cemented sands, DEM samples with different characteristics, including properties of the host sand (void ratio, uniformity of particle size distribution) and properties of the precipitated minerals (distribution pattern, content, Young’s modulus), are modelled and subjected to static probing to examine the small-strain stiffness. These factors are all found to affect the small-strain stiffness of bio-cemented sands, hence, the seismic response. Microscopic analysis indicates that there are two mechanisms which, together, determine the overall efficiency in improving the small-strain stiffness of bio-cemented sands: the number of effective bonds and the ability of a single bond to improve stiffness.

This thesis contributes to the understanding of the macroscopic mechanical behaviour of bio-cemented sands from the microscopic point of view. In particular, the role of crystal distribution patterns is highlighted by explicitly modelling the precipitated carbonates in pre-defined locations. The findings of this thesis can support the prediction of the mechanical behaviour of bio-cemented soils and guide the design of MICP/ EICP treatment.