Discharge capacity of prefabricated vertical drains confined in clay

This paper reports the long-term discharge capacities of four types of pref abricated vertical drains (PVDs) confined in clay. The results indicate tha t under clay confinement, the PVD discharge capacities significantly reduce d with time. The discharge capacity reduction may be caused by creep deform ation of the PVD filter or clogging of the drainage channels. PVD filter cr eep test results indicated that, for the conditions considered, creep defor mation was limited, which implies that clogging is the main cause of the di scharge capacity reduction. Hydraulic shocks were applied, which partially unclogged the PVD, recovering a significant amount of the discharge capacit y. 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. How ever, 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 te st results, an empirical equation was proposed for estimating the long-term clay-confined discharge capacity by using short-term rubber membrane-confi ned 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.