Prefabricated vertical drains flow resistance under vacuum conditions

The results of experimental research are presented and discussed with focus on the internal well resistance of prefabricated vertical drains (PVD) under vacuum-induced water flow. Measured results included fluid flow rates for two different cross-sectional hydraulic profiles (Types I and II PVDs). Experimental results indicated linear relationships, independent of the PVD widths, between extracted fluid velocity and the applied hydraulic gradient. Data showed a laminar flow regime to predominate for test velocities corresponding to hydraulic gradients <0.5. The larger nominal hydraulic radius of the Type II PVD is credited with providing a flow rate equal to approximately 3.2 times that of the Type I PVD at approximately the same operating total head. There was no apparent dependency of the transmissivity theta on the width or lengths (3, 4, and 5 m) of PVDs tested. In the case of the 100-mm-wide Type I PVD, theta = 618 mm(2)/s was estimated from the measured data versus theta = 1,996 mm(2)/s for Type II PVD with the same dimensions.