Changes in Entrance Resistance of Subsurface Drains

Abstract

This study was conducted to examine the entrance resistance and head losses in the immediate vicinity of subsurface drains and to measure the change with time just after drain installation. Laboratory experiments were conducted on drain tubes and sandy loam soil excavated from the drain depth. Tubes were tested both with and without a thin fabric envelope. Theoretical and approximate methods of quantifying entrance resistance and head losses were compared to measured results. Changes in hydraulic conductivity, head loss, and entrance resistance in the immediate vicinity of the drains were examined using laboratory soil tanks. The hydraulic conductivity decreased over time. The effective radius \Ir\de\N, for the tube without a fabric envelope increased over the test period. The two drains equipped with fabric envelopes had fairly constant \Ir\de\N values throughout the test period. The resistance factor, \IR\N, for the tube with no envelope was not constant. The value of \IR\N increased throughout the test period for the tubes with fabric envelopes. Experimentally determined entrance constant values were initially higher than the theoretical values and then decreased below the theoretical values with time.