1 Permeability of soft porous media under one-dimensional compaction

A novel experimental approach to measure permeability of porous material samples under variable longitudinal compression has been developed. The material has a non-linear behavior and exhibits a small hysteresis during loading and unloading experiments. The new permeameter includes a moving piston with controllable speed inside a plexiglas cylinder and a test section where the porous material sample is placed under compression by two grids with adjustable positions. Time-dependent pressure was recorded at four different locations along the sample together with the velocity of the piston. Experiments with two different sample lengths have been performed at three different Reynolds numbers based on the apparatus diameter. The results show that pressure gradient and permeability data do not depend on sample length. All experiments included measurements at various compression rates of the material followed by measurements during decompression/expansion of the material. No hysteresis was observed in the pressure gradient and permeability data during compression and expansion of the material. The effects of small velocity fluctuations due to variable friction of the moving piston with the cylinder’s wall were also considered. These velocity fluctuations result in pressure fluctuations within the first half of the sample which are damped completely towards the last part of the material sample. It was found that permeability, which is a material property, is drastically reduced with increased compression ratio of the material while its solid fraction changes substantially and its porosity remains practically unchanged. A comparison with the Carman-Kozeny expression for random porous media was also examined. The theory predicts qualitatively the reduction of permeability with compression. However, the predicted values of permeability are very sensitive to the initial value of porosity