The water acceptance of wrapped subsurface drains

Abstract

The water acceptance of subsurface, agricultural pipe drains is largely determined by the hydraulic conductivity of the surrounding zone. If this zone consists of soil with a poor structural stability, such drains must be wrapped with an envelope to control the rate of pipe sedimentation while safeguarding easy access of water. The studies were made to elicidate the effects of envelope specifications on these requirements.Envelope response was observed in analogue models, for cohesionless, and weakly cohesive, very fine sandy soils. Cohesionless soils were stabilised best by "thin" envelopes. Water access was easy and was not a factor of importance in design. In weakly-cohesive soils, the capability of envelopes to meet the requirements was quantified using an "Envelope Suitability Index" (ESI). Both soil type and envelope type had a significant effect on ESI. Nevertheless, analogue model tests were of limited value because the findings could not be compared with field observations.A field survey was made of grade lines of 184 drains and of soil invasion and sedimentation patterns, root penetration and other phenomena in these drains. They were wrapped with various envelope types and installed in weakly-cohesive, very fine sandy soils in three experimental fields in The Netherlands. Over 9600 m of drain length were inspected. The rate of pipe sedimentation differed significantly between the experimental fields. The particle retention capability of envelopes was associated with the effective opening size of their pores, "O 90 ". The mechanisms of soil invasion into drains and the observed sedimentation rates differed from those predicted in analogue models. Generally, envelope specification had no significant effect on drainage resistance; only in cases where drains were also used for subirrigation did "voluminous" envelopes have significantly lower drainage resistances than "thin" ones.Cores, containing wrapped drain sections with the surrounding soil were sampled at 45 locations. All sections had been functioning in weakly-cohesive, fine-sandy soils for a period of 5 years. The effect of soil and envelope specification on the flow of soil particles near the drains was investigated by microgranulometric analysis. Generally, the finest soil particles were found to be concentrated near the soil/envelope interface. This tendency was largely accounted for by the particle size distribution of the soil. A "natural soil filter" had only developed in a few instances. The envelopes improve stability through supporting the soils rather than through acting as filters. The cores were also examined by x-ray computerised tomography (CT) through 50 adjacent slices. This yielded three dimensional (3D) mappings of the most permeable areas inside the drain envelopes and surrounding soils that convey most of the water to the drains. A finite element model was used to study the effect of radial soil heterogeneity around a subsurface drain on the water table height. Water flow and envelope clogging were found to be quite heterogeneous and were mainly determined by soil structural features. Soil structural stability is therefore the main determinant of the service life of wrapped drains. The physical effect of an envelope on physical soil/envelope interactions is less important than is generally assumed. On the contrary, soil properties are crucial