Falling Head Permeameter Test and Darcy's Equation

Abstract A new permeameter was proposed for field falling head test, and evaluated under laboratory and field conditions. The device, an open-ended standpipe permeameter connected to two manometers by flexible tubes, was designed to measure heads at double ends of the sediment column inside the permeameter during the falling head tests. The heads were used to estimate water flux (q) and vertical hydraulic gradient (i) for determination of vertical hydraulic conductivity (K) using Darcy equation. Water flux was calculated by taking the first derivative of the best fit equation for the water level change above the sediment column. Differences in heads measured simultaneously at the two points were used to calculate hydraulic gradients. Tests under both laboratory and field conditions showed strong linear relationships between q and i (R2 > 0.999), indicating that Darcy’s equation could be directly applied for determination of K from the falling head tests. The laboratory test data was also analyzed using the Hvorslev falling head permeameter test for comparison, and the results showed a good agreement with those of the proposed method. The mean with 95% confidence interval for K values by the proposed (0.0478\(\pm\)4.1E-4 cm/s) is similar to that by the Hvorslev falling head permeameter test (0.048\(\pm\)8.5E-4 cm/s). Kruskal-Wallis ANOVA showed that the two groups are not significantly different at the 0.05 level. Field tests in a streambed also showed the relationships between water flux and hydraulic gradient by the proposed method were highly linear (R2 > 0.999). The ratios of mean K values by the two methods at each point, Kproposed / KHvorslev, were closer to 1 (0.91 at point A, 1.06 at point B, 1.07 at point C, and 1.03 at point D), indicating the agreement between the two methods was good.