Evaluation report depot modelling

Presents the study 'Depot Modelling' which has been carried out by wL Delfthydraulics in the period 1996-1998. This study was commissioned by the Dutch Ministry of Public Works in order to determine the impact, both qualitatively and quantitatively, of gas production in dredging sludge on the storage capacity of an artificial depot. Gas is produced as a result of the decomposition of organic material, which is normally 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 diffusivetransport proceed extremely slowly. Nucleation of gas bubbles occurs already at a limited oversaturation of pore water. Methane and carbon dioxide are then transferred from the liquid phase into the gas phase, which is coupled with a volume expansion. 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: bubble accumulation therefore continues below a level of about 10 m from the surface. 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. Alarge expansion may occur in sludge with a lowstrength, however. 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 to 37%, depending on the initial water content of the sludge. Only then gas can 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.