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. @en

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@en

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. @en