Mineral and ochre clogging of subsurface land drainage systems in the Netherlands

Abstract Dutch research into mineral and ochre clogging of subsurface land drainage pipes and envelopes aims at developing design criteria for drainage envelopes. Research outcomes are often conflicting and tend to have only regional validity. The present recommendations for envelope applications are comparatively reliable but they are based on the results of contemporary empirical research techniques. Still, little is known about the underlying soil movement mechanisms which appear to be more complicated than has been assumed for a long time. Current empirical research methods are not suited to solve the problem because they are inadequate in monitoring these mechanisms. During a nationwide field inspection programme, 1200 drain sections were dug out revealing a substantial variability of soil/envelope performance. Many soil/envelope combinations are being subjected to empirical permeameter flow tests. Simultaneously, field data are being collected at three pilot areas. The results indicate that soil structure, which is not included in traditional filter rules, has a defective impact on the clogging risk. The existing knowledge of clogging phenomena is insufficient. Many drainage systems function well, but too many do not. The Computer Assisted Tomography (CT) X-ray scanner appears to be a promising new research tool for determining soil structure.