Measurement of hydraulic conductivity in oil sand tailings slurries: Reply

Abstract

The authors thank the discusser for his interest in the paper. The authors agree with the discusser that the flow pump technique is an excellent method to determine hydraulic conductivity and have used this technique to measure hydraulic conductivity in conventional soils. The fine tails, however, are slurries with void ratios up to 10. At these high void ratios, the initial effective stress, at which hydraulic conductivity had to be measured, was only 0.2 kPa. Only very small head differences of approximately 0.02 kPa could be allowed to prevent consolidation during the test, and it was easier to control this with the constant head horizontal burette technique. The discusser suggests measuring the inflow and outflow to determine the volume change of the specimen. The authors did not want the specimen to change volume during flow, as this would have changed the hydraulic conductivity in these high void ratio materials. To ensure that there was no volume change, the inflow and outflow were measured and compared. Figure 1 indicates that there was no volume change during the entire hydraulic conductivity test as the inflow and outflow flow velocities are always equal. The suggestion to plot the ratio of flows with time is interesting and Fig. 2 plots the data from Fig. 1 in this manner. As the ratio is always one, it does show that no volume change was occurring. However, it fails to show an interesting property of high void ratio slurries: the almost two orders of magnitude drop in hydraulic conductivity at the beginning of the test. The reasons for this behavior are given in the paper. The authors did not have any problem distinguishing the steady state from transient state in the constant head tests. The constant head tests were performed for about 10 to 15 h, which is not dissimilar to the 26 h time for the flow pump test shown in the discussion. The authors agree that long-term tests are uneconomical, and the test results may be susceptible to physical and chemical changes in the specimen. In the flow pump test, a known quantity of flow is forced thorough the specimen and the corresponding pressure difference is measured, which is used to determine the hydraulic gradient. This is exactly the opposite of the constant head test. Aiban and Znidarcic (1989) reported that the constant head test and flow pump test yield the same hydraulic conductivity values when the specimenis tested under similar conditions. References