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In het Nederlandse oppervlaktewater zijn de fosfaatconcentraties vaak te hoog en vormt eutrofiëring een groot probleem. Fosfaatuitspoeling uit landbouwgronden draagt flink bij aan de totale fosfaatbelasting van het oppervlaktewater. Met brongerichte maatregelen, zoals evenwichtsbemesting, zal in sommige delen van het landelijk gebied de hoofddoelstelling van de Kaderrichtlijn Water niet worden gerealiseerd. Hiervoor zijn aanvullende maatregelen nodig, die kunnen bijdragen aan het verbeteren van de chemische waterkwaliteit. We hebben de effectiviteit getest van een drainagebuis omhuld met ijzerzand om fosfaatuitspoeling te verminderen. Deze veldproef is uitgevoerd op een duinzandgrond in het bloembollengebied. Het gemiddelde zuiveringsrendement van de omhulde drainagebuis bedraagt 94 procent. Het omhullen van drainagebuizen met ijzerzand lijkt een veelbelovende maatregel voor het verminderen van fosfaatuitspoeling

In het Nederlandse oppervlaktewater zijn de fosfaatconcentraties vaak te hoog en vormt eutrofiëring een groot probleem. Fosfaatuitspoeling uit landbouwgronden draagt flink bij aan de totale fosfaatbelasting van het oppervlaktewater. Met brongerichte maatregelen, zoals evenwichtsbemesting, zal in sommige delen van het landelijk gebied de hoofddoelstelling van de Kaderrichtlijn Water niet worden gerealiseerd. Hiervoor zijn aanvullende maatregelen nodig, die kunnen bijdragen aan het verbeteren van de chemische waterkwaliteit. We hebben de effectiviteit getest van een drainagebuis omhuld met ijzerzand om fosfaatuitspoeling te verminderen. Deze veldproef is uitgevoerd op een duinzandgrond in het bloembollengebied. Het gemiddelde zuiveringsrendement van de omhulde drainagebuis bedraagt 94 procent. Het omhullen van drainagebuizen met ijzerzand lijkt een veelbelovende maatregel voor het verminderen van fosfaatuitspoeling

The purpose of this Paper is to provide this practical information to drainage engineers and contractors. The Paper is based on the current knowledge of water flow into drain pipes and envelopes, their properties and applicability. It also contains guidelines for the selection of drainage materials and practical guidelines for the implementation of laboratory and field investigations to evaluate the performance of drainage materials. The selection of the materials must also consider the required specifications, their availability, durability and cost. Guideline for selection of the most appropriate envelope material, as related to local conditions

The purpose of this Paper is to provide this practical information to drainage engineers and contractors. The Paper is based on the current knowledge of water flow into drain pipes and envelopes, their properties and applicability. It also contains guidelines for the selection of drainage materials and practical guidelines for the implementation of laboratory and field investigations to evaluate the performance of drainage materials. The selection of the materials must also consider the required specifications, their availability, durability and cost. Guideline for selection of the most appropriate envelope material, as related to local conditions

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.

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.

In order to check the state of subsurface drain/envelope combinations, several dig-ups were made in the State of Michigan (United States), the Province of Ontario (Canada) and the southwestern part of the Netherlands, These opportunities were used to sample soil material in the vicinity of the pipe as well as sediment inside. No preparatory moves were made as regards sample containers, sample preservation etc., so all sampling activit1es have been fully incidental in their nature.

Met een hier beschreven afleiding wordt een formule verkregen voor de grootte van de weerstand, welke de stroming in de omgeving van een drainbuis ondervindt. Hoewel zekere benaderende veronderstellingen worden ingevoerd en de invloed hiervan niet zorgvuldig is nagegaan mag worden verwacht dat deze formule voldoende nauwkeurig is voor de praktijk. Hieruit zal blijken dat de toelaatbaarheid van de veronderstelling van een homogeen doorlatende drainbuiswand van meerdere omstandigheden afhankelijk is.