Bio-based ground improvement through Microbial Induced Desaturation and Precipitation (MIDP)

Abstract

Improving and alternating soil foundation conditions is a common task in construction and civil engineering. Besides conventional ground improvement methods, there are several biological processes that can improve ground properties by precipitating calcium carbonate. Denitrification is one of the biological processes that can be used. The process generates besides carbonate precipitation also a gas phase in the soil. Therefore, the denitrification based method, or Microbially Induced Desaturation and Precipitation, MIDP, expands the potential of biological processes to improve the ground conditions for different applications.
Liquid batch experiments show that denitrification based MICP is a coupled process, in which denitrification and calcium carbonate precipitation processes influence and are beneficial for each other. To minimize accumulation of the denitrification intermediates, both the substrate ratio and concentration to be used are important. When using a relatively low substrate concentration at the ratio that favours microbial growth, there was no nitrite at the end of the experiments, precipitation rate was up to 0.26 w%/day. This value is higher than the observed values in literature and improves the potential of using this process for ground improvement applications. After one batch treatment, sufficient gas was produced within 1 or 2 days to desaturate the sand to the gas percolation threshold, which ranged from 21 to 50% depending on pore size. The gas stability appeared to be dependent on the relative proportion of the produced gas volume with the gas percolation threshold of the soil. When the treated sand was subjected to monotonic loading in triaxial tests, the soil stiffness and dilatancy were increased, and the pore pressure response in undrained loading was dampened. The treatments also decrease the soil hydraulic conductivity, and even led to clogging in the experiment using low substrate concentration whereby the microbial growth was favoured.
Overall in the thesis, MIDP has shown its capability to alter hydro-mechanical behaviour of sandy soils at laboratory scale, and can be applied for a wide range of ground improvement applications. Upscaling the investigation and optimizing it toward different specific applications are required for future work.