Casing of drain pipes
In the Netherlands 90% of all field drain pipes have a casing.
The casing has two functions:
• it acts like a filter and prevents inflow of soil particles;
• the enlarged ‘wet circumference’ will result in easier influx of water.
Casing materials are classified as thick or thin and according to the nature of the material – organic or synthetic.
- Thick or voluminous material has the advantage of a better influx of water due to a larger wet circumference. Furthermore, the layer for the sediment to settle in is thicker so the filter will not become blocked so easily;
- Thin materialis less heavy but serves only as a filter. A layer of deposit can more easily build up on the filter and can sooner result in blockage;
- Organic material is voluminous by itself. The disadvantage is that this material will decompose easily at a higher pH and oxygen level. It therefore loses its function;
- Synthetic material this casing can meet the quality requirements. There are a great number of materials available, many of which are made of recycled fibres.
When choosing the casing material, the characteristics of the soil at the depth of the drains will play an important role. Soils with more than 25% lutum (particle size > 2 µm) are stable enough such that little or no inflow of soil particles is expected. Therefore, casing material will not be necessary. However, it is vital to check that on the entire plot the soil is of the same quality at the depth of the drain. Besides the proportion of lutum which is present, the stage of the maturity of the land is another factor which influences the choice of which casing material should be used.
Iron is to be found in much of the land in the Netherlands, both in the soil and in the seepage. Better drainage allows oxygen in the soil. This causes iron sediment to form on the pipes and the casing material. This can cause blockages. When there is a risk of iron sediment, voluminous and open casing materials, greater than 700 µm, are advised. Much used casing materials are PP450, PP700, PE1,000, PS, artificial grass and coconut fibre. Thin materials can be nonwoven or woven or PES circular knitting. The casing materials are specified by their raw material as well as the size of the perforations, known as a O90 number. The higher the value, the more perforations in the casing material. The O90 number indicates the density of sand. The O90 value is determined by a sieve in which the sand of a known fraction is applied to the tested material. The objective is that 90% of the sand is stopped by the casing material. This test is repeated with different fractions to determine the correct perforation size.