Abnormalities in Hydraulic Flow Through Fine-Grained Soils

Evidence for deviations from Darcy's law during hydraulic flow through fine-grained soils is reviewed. Abnormal water properties, electrokinetic coupling, fabric changes under the action of seepage forces, and experimental errors are considered as possible causes for the observed behavior. Data are presented to suggest deviations from Darcy's law in saturated kaolinite and in saturated, compacted silty clay. No evidence for a threshold gradient was found for these materials. The behavior of the compacted specimens was such as to indicate that particle migrations are more likely causes for non-Darcy flow than are abnormal water properties. Support for this conclusion was provided by the results of tests wherein pore pressure distributions along the length of specimens were determined during flow. In this way it was possible to determine the variations in hydraulic gradient with time at different points in the sample. The effects of non-Darcy flow behavior in soil mechanics problems are discussed. If high gradient tests are used in the laboratory, the results may bear little relationship to the behavior in the field where gradients seldom exceed unity. Data are presented illustrating the variation of excess pore pressure with time at several points throughout the height of a consolidating clay specimen. The results indicate, contrary to other evidence in the literature, that there could have not been a threshold gradient for flow. Because of the considerable experimental uncertainty associated with permeability measurements on fine-grained soils, laboratory test results must be interpreted with caution.