Microbially induced carbonate precipitation (MICP) through denitrification can potentially be applied as a bio-based ground improvement technique. Two strategies involving multiple batch treatments in a modified triaxial test setup were used to study the process efficiency. Both strategies aim to achieve 1 weight percentage (% by weight) of precipitated calcium carbonate (CaCO₃) and differ in number of flushes, hydraulic residence time, and substrate concentrations. In the experiment with few flushes and high substrate concentrations the microbial process was inhibited, only 0.28% by weight CaCO₃ was measured in the sand after 5 weeks of treatment. The regime with many flushes and low substrate concentrations stimulated microbial growth resulting in 0.65% by weight CaCO₃ within the same time period. Biomass growth and nitrogen gas production were stable throughout the experiment at low concentration, reducing the hydraulic conductivity of the sand, which eventually led to clogging. Precipitation rates up to 0.26% by weight/day CaCO₃ were observed. Applying a suitable substrate regime and residence time is important to limit inhibition and maintain the cell activity, allow for an efficient conversion, and generate a good precipitation rate. ... Read more

Desaturation by biogenic gas formation can significantly affect the hydro-mechanical behaviour of soil. The high compressibility of the gas dampens pore pressure build up during both monotonic and cyclic undrained loading. Stimulating biogenic gas production therefore has potential as a ground improvement method to mitigate the risk of both static liquefaction and earthquake induced liquefaction. However, gas generated below the ground water table at shallow depth may also constitute a hazard for offshore foundations and terrestrial deposits, as a sudden release of trapped gas may cause instability. In order to evaluate the potential use of biogenic gas for geotechnical applications it is essential to be able to predict gas production and assess its effect on the hydro-mechanical behaviour of a soil. A basic theoretical framework to estimate the volume of gas produced by a biogenic process and the related degree of saturation, experimental results on the rate of gas generation, and its impact on soil behavior are presented herein. ... Read more

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. ... Read more

The process of microbially induced carbonate precipitation (MICP) by denitrification was investigated in relation to its potential use as a ground improvement method. Liquid batch experiments indicated that the substrate solution had an optimum carbon–nitrogen ratio of 1·6 and confirmed that combining nitrate reduction and calcium carbonate precipitation leads to an efficient conversion, at which the pH is buffered slightly below 7 and the accumulation of toxic intermediate nitrogen compounds is limited. Sand column experiments confirmed that the volume and distribution of the gas phase strongly depend on the stress conditions. The produced gas volume is inversely related to the pore pressure and can be predicted based on a mass balance analysis, assuming conservation of mass and using theoretical laws of physics. At low pore pressure, the gas formed and accumulated at the top of the column, whereas calcium carbonate precipitation occurred mostly at the bottom near the substrate inlet; an excess amount of gas was produced, which vented from the sand columns and induced cracks in the sand at low confining pressures, which negatively affected the sand-stabilising effect of the calcium carbonate minerals. ... Read more

One of the major challenges of in situ applied ground improvement techniques like Microbial and Chemical Induced Calcite Precipitation (MICP and CICP) is the homogeneous or at least spatially controlled, distribution of the desired reaction products in order to obtain controlled improvement of geotechnical bulk soil properties (Redding, 2007; Van Paassen, 2009 Numerical modelling simulations at continuum (Darcy) scale are performed to study the spatial distribution of reaction product(s) as a function of the injection strategy. An example of the simulations is shown in the figure below. The models are validated with laboratory experiments using a quasi two dimensional flow box with a large number of ports which could be either used as injection/extraction well or as a sensor port. This article presents the results obtained when multiple injection wells are operated simultaneously in which two reactive solutions (calcium chloride and sodium (bi-) carbonate) are injected separately in alternating wells located perpendicular to the background flow. Water is used as a non-reactive spacer. Hydraulic pressures and electrical resistivity tomography (ERT) are used in the laboratory set-up to monitor the spatial distribution of reactants and products. The shape and location of the mixing zone during treatment, and the spatial distribution of calcium carbonate after treatement are evaluated for different injection strategies. ... Read more

Microbial induced carbonate precipitation (MICP) has been confirmed to effectively reinforce granular soil by employing biological processes. Biological denitrification is one of these processes, in which the microbes use nitrate to oxidize organic matter and produce nitrogen gas and inorganic carbon, which results in precipitation of calcium carbonate in the presence of calcium ions Accordingly, MICP via denitrification may stabilize the soil in multiple ways. Besides the strengthening effect of calcium carbonate precipitate crystals, the induced gas phase may enhance the undrained response as the compressible gas may dampen dynamic loads on the soil. The effectiveness of this method has been investigated on different sand types using the triaxial test set-up, at different pressure conditions. During the reaction phase, the water displacement into the back pressure controller was used to monitor gas production. After reaction samples were flushed from bottom to top at a constant head difference. The flow rate was measured to determine permeability. Results of these experiments showed that the volume of produced gas strongly depend on the pressure conditions. The gas storage capacity ranged from 22% to 40% of the total pore volume, depending on grain size. At a pore pressure of 1 bar, the gas storage capacity was exceeded before the reaction was completed and excess gas escaped to the back pressure controller. Gas which remained inside the sample was easily removed during flushing, and the permeability was hardly affected. At higher pore pressure or smaller grain size, the gas phase was well distributed and resulted in a decrease of the permeability after the treatment by 50 to 300%. The drained shear strength of a single batch treatment was not significantly improved, indicating that the amount of calcium carbonate precipitation was not sufficient and suggesting multiple treatments are required to bear an increase of strength. Upon undranied loading the small strain stiffness was significantly improved, but the peak undrained strength was reduced. The results indicate that controlling the gas formation is important in order to optimize this method for field applications ... Read more

Rice husk ash is a promising pozzolanic material produced from rice husk burning and has significant potential a sustainable replacement for cement in construction and ground improvement applications. In this study the effect of burning conditions on the ash reactivity and its potential for soil stabilization applications have been investigated Three different burning procedures were applied: 1) controlled burning at 500°C followed by rapid cooling, which was according to literature considered to produce the highest reactivity, 2) burning at 700°C with slow heating and cooling, which was expected to result in lower reactivity due to die crystallization of the silica and 3) uncontrolled burning in open-fire. The third procedure unexpectedly produced the most reactive and still some carbon. Adding rice husk ash and lime to wet clay significantly increased the strength of the clay after curing. Again, the carbon-containing ash showed the largest strength improvement Incomplete combustion seems therefore preferable for soil stabilization applications with rice husk ash. ... Read more