Gas production and transport in artificial sludge depots

Abstract

This paper presents a study to determine the impact of gas production in dredging sludge on the storage capacity of artificial sludge depots. Gas is produced as a result of the decomposition of organic material present in dredging spoil. This process, in which methane and carbon dioxide are formed, may lead to expansion of sludge layers, partly or even completely counterbalancing consolidation. The study shows that, even with a very conservative estimation of the rate of gas production, accumulation of gas occurs as convective and diffusive transport proceed very slowly. Nucleation of gas bubbles occurs already at a limited oversaturation of pore water. During their growth, bubbles push aside the surrounding grain matrix. Resulting stresses may initiate cracks around bubbles. If these cracks join, they may form channels stretching out to the depot surface and along which gas may escape. However, channels are only stable to a limited depth below which bubble accumulation may continue. The gas content at which sufficient cracks and channels are formed to balance the rate of gas production with the rate of outflow strongly depends on the constitutive properties of the dredging sludge considered. In sludge with a high shear strength (>10 kPa), stable channels are created already at low deformations. However, a large expansion may occur in sludge with a low strength. The present study shows that accumulation of gas may continue until a bulk density less than that of water is attained. This is equivalent to a gas fraction of about 25–37%, depending on the initial water content of the sludge. Only then can gas escape as a result of instabilities in the sediment matrix. This should be well taken into account during the design and management of artificial depots.