The casing of drainage

Casing is not always necessary, sometimes there is an alternative. 


Casing has two functions:

  • It serves as a filter and ensures that soil particles aren't able to flow inside.
  • It makes the water flow easier by increasing the so-called “wet perimeter”. The wet perimeter is the circumference of the drain pipe including the casing material present. (see figure 18). 


We can classify the casing materials into:

  • Thickness: thick and thin
  • Origin of the material: organic or plastic


The advantage of thick material is that a better water flow is possible due to a larger wet perimeter. In addition, the layer in which the soil part can deposit is thicker, this means that the filter becomes clogged less quickly.

Thin material is often more limited in weight, but only serves as a filter. A layer can deposit on this filter, and this results in a clogged filter. 

A filter consisting of organic material is often thick and can function well. The disadvantage of this material is that it is easily digested at a high pH and with the presence of free carbonic acid lime. The function is thereby lost. 

Plastic material has been used for a long time; this makes it easier to set requirements for the prescribed quality. A wide variety of materials exist, both as a thin filter and in the form of a thick casing. 

When selecting casing material, characteristics of the ground at drain depth play an important role. 

Stability and clay content

Soils containing more than 25% clay are so stable that little to no leaching of soil particles is to be expected. If particles do wash in, they are often so small that they can easily be washed out. Soil with more than 25% lutum generally occurs on black sandy clay and clay soil. However, many of these soils have a lighter soil precisely at the depth of the drain, i.e., a lower clay content. It is easier to wash in here, even if in practice this is referred to as “heavy soil”. Where more than 25% clay occurs at drain depth, the casing is not required for a filter. Please note; This must be the case everywhere in the plot. In practice, the composition and weight change regularly as a result of the natural deposition. Another given is that this concerns the lutum content and not a percentage that can be sludged. At a lutum content of 25%, the sludge-able content is often between 35 and 40%. Drainpipe without casing can therefore not be used without a thorough preliminary investigation.  



In addition to the clay content, the degree of maturing of the soil also influences the choice of the casing material. Especially on clay soils, maturations play a role. Heavy casing material is necessary here. Table 9 lists the maturation classes of clay soil. 


Loam content

The loam content is the content of fine soil parts smaller than 0.05 millimeters. Thin encasement material can be used in low-clay and weak loamy sand (up to 17.5 percent loam). With a higher clay content, the risk of clogging of thin casing material increases. This is why heavy casing material must be used. 


A lot of our soil contains iron. In some areas, there isn’t a lot of iron in the soil, but the iron is supplied with water from elsewhere; “seepage water”. Iron is dissolved in the groundwater and is present as a bivalent ion. As soon as this ion comes into contact with oxygen, it precipitates in the form of rust (iron oxide). With better drainage, more oxygen can enter the soil. This causes the iron to precipitate as rust and discolorization of the bottom will occur. Especially in the summertime, a lot of iron precipitates. The supply of oxygen is greatest around the drain and iron can easily precipitate. This could result in clogging because of encapsulating material. The holes (perforations) in the drainpipe can easily clog. Microorganisms also play a role in this process. These are active in an oxygen-rich environment. By the presence of iron in the soil, or through the seepage elsewhere, the decrease in iron can be limited through;

  • Using casing material where microorganisms feel uncomfortable, such as coconut, peat-coconut, peat, or straw.
  • Built drains underwater, this way they are completely filled with water, and no oxygen can enter. 
  • Use drain pipes with relatively large perforations.
  • Use heavy casing material.
  • Choose a larger drain diameter.


If drains are installed underwater, the following conditions apply:

  • The drains must be installed horizontally, this prevents parts from running dry, for example in summer.
  • The ditch level must be constant so that the drains do not occasionally discharge above water. Not only does this allow oxygen to enter, but it also increases the chance of blockage by dirt. 
  • The power tubes must be marked very thoroughly so that they are not damaged while cleaning the locks. 


In an area without seepage, the iron deposition will be temporary. After some time, iron has deposited in the power tube, a new equilibrium is reached. Depending on the amount of locally occurring iron, additional maintenance is required one or more times a year. After this, a constant state will arise. The problems are often greater in seepage areas. Here, drainage requires extra maintenance and depreciation must be accelerated. Clearly, when making a drainage plan, the potential for an iron deposition must be taken into account. There are a number of field features that indicate the presence of iron. They are listed in Table 9.

The description in Table 9 only provides an indication of the extent to which iron occurs. It is difficult to estimate in advance whether there is a possible increased supply of iron as a result of an increase in seepage. There are a number of methods for determining the iron content. The scale for these methods is often quite broad. They only give an indication and the field characteristics remain just as important. 

Organic material or plastic

Casing materials can be organic material or plastic. A distinction can also be made between grains, fibers, fleece, and stocking. 


Organic material 

Examples of organic material are; coconut, peat, peat-coconut, and straw. Organic materials are sensitive to digestion. In soil with active soil life, the organic material will break down. Preventing micro-organisms plays an important role in this. Its presence can be demonstrated by means of the carbon/nitrogen ratio in the soil; the C/N quotient. This ratio can be determined in the laboratory. If a trencher is used for the construction of the drain, extra care is required. With the first return of the abutment, a lot of active soil life is applied directly into the drain. When using organic casing material this must be avoided. 


Grains, fibers, fleece, or stocking

Another difference is the structure. Materials as coconut, peat-coconut (-fiber), and straw have the structure of fiber. The plastic materials Polypropylene (PP) and Polyethylene (PE) have a similar structure. Polystyrene and peat are in the shape of grains and chunks. Materials such as glass fiber, typar, and cerex consist of a membrane and lie in a thin layer around the drain pipe. This also applies to the stocking shape. The advantages and disadvantages of a thick or thin filter have been described in an earlier chapter. 


Carbonic lime content and acidity 

Another reason for the rapid digestion of the organic casing material is the presence of free carbonic acid lime and a high pH. The carbonic lime content can be demonstrated by treating a quantity of soil originating from the drain depth with hydrochloric acid (10 normal). In the presence of free carbonic acid lime, effervescence occurs. From the degree of effervescence, an estimate can be made of the speed of digestion. With a visible and audible effervescence, the casing is digested within a few years. 


On peat soil, the pH is often low enough to prevent the risk of digestion. For example, studies show that after 30 years the laying of the coco’s in peat soil is still in excellent condition. On clay soil, the number of digestion cases on examination increases as the pH rises. On sandy soil, digestion has also been found a number of times in places with a high pH in the subsoil. In general, the chance of digestion on clay soil is greatest. Plastic material is therefore preferred as casing on this soil type. 

Tables 10 and 11 show the aspects that influence the choice of casing material.

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