A finite strain theory for gassy sludge

In sludge, which can be considered as a soft soil, biogenic gas occurs mostly in the form of discrete gas bubbles that are much larger than the surrounding void spaces. In this article, one-dimensional finite strain consolidation theory has been extended for gassy, soft soils and has been implemented numerically in an existing finite strain consolidation program. This theory has been verified experimentally by means of specific laboratory tests. A sensitivity analysis has been performed for a typical disposal site, using different assumptions of the distribution of gas versus sludge height. Compressibility and solubility of the gas, consisting mainly of methane, have been included as well as advection of the dissolved gas with the pore water outflow. It is shown that gas causes a significant retardation of consolidation. In addition, the final degree of consolidation is less, due to the presence of the gas. Practical consequences are a loss of disposal capacity and a change in the outflow of contaminated pore water.