Wall Effects in Permeameters

The effect of a permeameter wall on the mean flow velocity measured in a permeameter containing granular material is investigated. The difference between the measured mean flow velocity and a true one-dimensional velocity in an infinite medium is attributed to a much higher than average flow rate in a narrow zone of high porosity immediately adjacent to the wall. An equation for the ratio of measured to true velocity is derived. Data from an experimental investigation into the velocity-hydraulic gradient relationship for coarse granular media at various porosities are combined with the results of wall-zone porosity measurements to predict values of the ratio. Measured values of this ratio for 16-mm diameter glass marbles and 3 / 4-in. river gravel in a 22.5-in diameter permeameter are compared with calculated values.