Discharge capacity of prefabricated vertical drains confined in clay

This paper reports the long-term discharge capacities of four types of prefabricated vertical drains (PVDs) confined in clay. The results indicate that under clay confinement, the PVD discharge capacities significantly reduced with time. The discharge capacity reduction may be caused by creep deformation of the PVD filter or clogging of the drainage channels. PVD filter creep test results indicated that, for the conditions considered, creep deformation was limited, which implies that clogging is the main cause of the discharge capacity reduction. Hydraulic shocks were applied, which partially unclogged the PVD, recovering a significant amount of the discharge capacity. The higher the PVD hydraulic gradient and the larger the shape factor of the drainage channel (defined as the drainage area per channel divided by its perimeter length), the less discharge capacity reduction with time. However, there was no clear trend regarding the effect of the apparent opening size, O-95, of the filter on long-term discharge capacity. Based on the test results, an empirical equation was proposed for estimating the long-term clay-confined discharge capacity by using short-term rubber membrane-confined test results. Regarding long-term discharge capacity, it is recommended that a PVD with a larger drainage area per channel (approximately 3 mm(2)) and a larger drainage channel shape factor (larger than 0.4 mm) should be selected.