Volume 25, Issue 3 (September 2002)
A Cyclic Gradient Ratio Test Device
Gradient ratio test results are reported from the commissioning of a device that was modified to impose dynamic cyclic flow. The flow conditions, using head control, are similar to those in marine protection works. Each test included a stage of unidirectional flow, followed by stages of cyclic flow at a frequency of 0.2 Hz and 0.02 Hz. Flow reversal invokes an initial transient state of water head distribution in the sample, attributed to changes in hydraulic gradient to accommodate the new values of constant head at the inflow and outflow. Consequently, tests at the higher frequency yielded unsteady flow in the sample, while tests at the lower frequency resulted in steady flow during each half cycle. In unsteady flow, the transient state of water head distribution (and hence gradient ratio) cannot serve as an index of soil-geotextile compatibility. Therefore, a recommendation is made that testing of filtration compatibility in dynamic cyclic flow include a stage of slow flow reversal (or alternatively unidirectional flow) to permit a quantification of water head distribution under steady flow.