Rinsing of soil particles
Casing material aims to facilitate the entrance of water by increasing the "wet perimeter." As the soil in the area surrounding the drain is poorly permeable, the function of the encasing material becomes more important. Thus, the casing material not only increases the outer diameter but also lowers the entry resistance.
However, the influence of the actual drain distance as a result of these factors is manageable, and besides, the encapsulating material must repel the soil particles. Counteracting soil particles increases the risk of entry resistance. These conflicting factors make it difficult to choose the right encasement material in the various soil types and soils. This often results in a compromise between the choices.
Rinsing soil parts in the drain does not necessarily have to be undesirable. When the fine soil particles are discharged through the drain, the permeability of the total profile can increase, because the larger parts remain behind. If the fine bottom parts remain in the enclosure, it can become clogged, causing the entrance resistance to increase considerably. An important note is that the drain must be able to rinse out permitted soil parts.
[ table 4]
Drainage through natural drainage
The washed-in parts will also be discharged during the natural drain discharge. The speed of the flowing water in the drains plays an important role in this. The speed of the water and the falling speed of the soil particles are important here. The flow rate in a drain can vary greatly. The speed at the drainage of 7 mm per day is approximately four centimeters per second (0.04 m per second). The average drainage is approximately one to two mm per day.
The speed at which the soil particles are falling is shown in table 4.
If a soil part ends up in the pipe while the flow velocity is greater than the falling velocity, this part will be drained. If the flow velocity is less than the falling velocity, the part will remain in the tube. With a discharge of 1 mm per day, the flow velocity is high enough to discharge particles smaller than 85 µm and with a discharge of 2 mm per day, particles smaller than 120 µm.
In periods without drainage, the soil particles settle in the drain and attach themselves. Force must be applied to detach these parts. Certainly, after a prolonged dry period, a greater force than usual is required to move soil particles. When flushing the drain a flow velocity of five to six centimeters per second is created. At this speed, soil parts of approximately 300 µm can be moved. Due to the loosening of soil parts and the water loss through the pores, quite a lot of water loss occurs during flushing. The transport of the larger bottom parts is therefore disappointing. However, flushing does ‘smooth out’ the local contamination over a greater length of the pipe. Little research data exists regarding the proportion and size of the soil parts that can be washed out. For the time being, we assume that bottom parts smaller than 50 µm can be removed by rinsing. The parts in class 50 to 150 µm will only be rinsed to a limited extent. For the time being, we can assume that approximately half of this can be removed during flushing.
[ table 5]
Composition of washed-in parts
Research regarding the size of washed-in soil particles, the following compositions were found, as seen in table 5. This means that all particles with a grain size of 0 -2, 2 - 16 and 16 - 50 µm and half of the particles between 50 - 105 µm can be rinsed. Table 6 shows the parts that can be rinsed. The percentages are rounded because it is not exactly known which part can be rinsed out.
Irrigation and digestion of casing material
It was investigated whether there is a relationship between the degree of digestion of the (coconut) coating materials and the degree of wash-in. The result of this research is shown in table 7. This table shows that there is a clear relationship between the degree of digestion and the number of millimeters of dirt in the drain. Although the number of observations is limited, this confirms the function of the encapsulating material as a filter against washing up soil parts.
Chance of flushing
The likelihood of flushing increases as the soil becomes ‘lighter’ since the soil contains less sedimentable parts. This applies especially to the soil layer in which the drains are located. This is because the danger of loosening and flushing is greatest for this soil layer. However, a light layer can also occur elsewhere in the profile, these parts can flush to the drain via the coarser (sand) parts. When mixing the soil, for instance with the construction of a drain in a dug-up drain trench, soil particles can be transported from all layers of the soil. This is something that mainly happens in the period immediately after construction. After some time, the drain trench will stabilize. If drainage is constructed in a period with a high groundwater level, the chance of sedimentation is greatest and a high level of transportation of soil parts will take place. In (light) sandy soil the chance of sedimentation is present since the binding through natural forces is small. Humus and iron compounds can, however, be a binding factor. In peat soil, few parts of the soil will move. In a peat soil, which morphs into a sandy soil at drain depth, fine humus particles can compact and smear the drainage. Flushed-in parts, however, are easier to rinse out again, even with a normal discharge from the drain itself.