Meter-scale MICP improvement of medium graded very gravelly sands: lab measurement, transport modelling, mechanical and microstructural analysis

Microbially induced carbonate precipitation (MICP) is a promising biogrouting method for ground improvement. Most studies to date have focused on MICP treatment of uniform clean sands, with few studies having been conducted at large-scale on well-graded field soils more representative of in situ deposits. This study presents a laboratory meter-scale MICP test on medium-graded very gravelly sands consisting of 3.9% fines collected from a quarry. The MICP treatment was conducted in a radial flow cell (diameter: ~1m; thickness: ~0.15 m) with an injection well located at the centre and a constant hydraulic head at the outer boundary to replicate field conditions. Aqueous chemistry of the effluent samples inside the flow cell was continuously monitored, and transport of tracer and bacteria breakthrough in the flow cell, and in separate 1-dimensional columns, was modelled and simulated for a better understanding of the MICP process. The MICP-treated soil was subjected to a series of hydraulic and mechanical tests and microstructural analysis. Transport modelling and effluent sampling monitoring of the electrical conductivity and pH show that there was an overall good delivery and reaction of the bacteria and chemicals in the radial flow cell, but there also existed preferential flow paths due to soil heterogeneity and fines migration, which caused variations in permeability. Interestingly, compared to previous studies, the biocemented core samples with medium-graded angular particles in this study had higher strengths (2.6-7.4 MPa) for a given calcite content (9.2-15.1%) than those in comparable studies treating uniform soils. Scanning Electron Microscopy (SEM) and X-ray computed tomography scans show that this can be attributed to the higher initial density of grain-to-grain contact points in medium-graded sands, and a high grain angularity which resulted in particle interlocking and longer grain-to-grain contact surfaces. Consolidated-drained triaxial compression tests on two samples cored from near the injection well showed peak deviatoric strengths of ~5 MPa under an effective confining stress of 500 kPa, with the clear formation of shear bands during loading. Comparison with the untreated soil showed that MICP treatment tripled the peak deviatoric strength achieved, as well as increasing the stiffness of the material. The study highlights that the formation of preferential flow paths may be a challenge for producing uniform biocementation in field applications of MICP. We propose that successful MICP treatment in heterogeneous soils will require a well-designed and well-executed site investigation programme that can identify, a priori, the geometry of any significant high or low permeability features within the soil body to inform the final MICP treatment strategy.