Mole drainage is a system in which excess water gets discharged through artificially constructed mole corridors. Instead of going directly to the ditch or an above-ground drain, a mole drain is an underground drain. The mole corridor connects directly to the ditch. If, as a result of a lot of precipitation, the groundwater level becomes higher than the artificial molar corridor (‘mole drain”), the water will start to drain. The mole drainage will therefore only function if the groundwater rises above the mole drain. In between the mole drains, the water will rise higher. The degree to which the water rises is very dependent on the type of soil, amount of water, the distance between the mole drains, and the quality of the drains. In contrast to pipe drainage, little theoretical research has been done about mole drainage. The construction is based on trial and error throughout practice, with quite variable results. Mole drainage is a vulnerability and therefore reliability cannot be determined with certainty.
The purpose of mole drainage is to drain water through the subsoil and to prevent puddles. This form of water drainage shows the advantages of good drainage, without the presence of trenches that are inconvenient for the treatment.
Mole drainage can be installed in a simple manner with the help of a vole with a spherical body on the underside, which pulls the artificial mole tube into the soil. The corridor will be at a depth of approximately 50 to 75 cm below ground level. The space between both ends may not be more than 5 to 6 meters. (see figure). After the corridor has been drawn, it can drain water from the surrounding soil and everything that enters through the drawn cut. The shape of the corridor must remain in place for proper functioning and the drainage must also be undisturbed. With heavy clay soil, the outside of the drawn corridor becomes very smeared. This inhibits the water that goes into it. In such a case, most of the water is supplied through the cut. Sometimes a mole drain is pulled into an existing trench, this way the trench bottom is more or less lowered. This too remains a vulnerable system.
Because no material is applied in the drawn corridors, the risk of disturbance is high. Due to activities of the soil life, but also due to driving over the soil, the corridor created collapses fairly easily, causing loss of function. Heavy clay soil retains its shape the most, especially under wet conditions. The risk of disturbance is, therefore, lowest on heavier soil. This is why mole drainage is almost exclusively applied on clay soil. However, the duration of action is also limited on this ground, it is, therefore, wise to only expect one season.
Pros and cons
Mole drainage has the great advantage that the investment costs are very low. However, these costs recur every year. The disadvantages of mole drainage are the vulnerability, which means that it can only be used on clay grassland, and is only expected to work for one season, and in the absence of ditches, the risk of flooding remains high. In addition, the dewatering depth remains limited. The groundwater will often rise to groundwater between the mole drains, while the corridors have to be placed at a small distance from each other. Mole drainage must therefore be seen as a measure whereby temporary puddles are prevented on clay grassland.