A trenchless plough is equipped with a V-shaped cutting blade. With this system, also known as a Delta plough, the blade lifts a V-shaped part of the soil. The pipe is laid through a hollow space at the side of the V-shaped blade. After that the V-shaped part of the soil will fall back into the original position. As a result of this system, there is little chance that the trench will settle further. The installation of drainage on grassland using a trenchless plough will leave few ‘scars’, especially on peaty soil and clay on top of peat.
Laying drains in clay can cause ridges in the soil. How much of this will occur depends on a number of factors:
a. The density of the clay. Clods will form in the soil. These will be bigger in heavy clay than in light clay soil. When the soil is replaced after laying the pipes, cavities can occur due to the large clods. These temporary spaces cause ridges to appear. After a period of swelling and shrinking (a wet and a dry period) these ridges will almost entirely disappear. However, if the ground is levelled immediately following installation of the field drains, it is likely that troughs will form after a similar period of swelling and shrinking.
b. Moisture content. The moisture content determines how easily the soil can be worked and how easily it can be cut through. When dry, the structural elements of the soil will be more clearly defined. This will cause difficulties when the soil is replaced resulting in a greater chance of a temporary ridge being formed above the field drain.
c. V-shaped. A narrower angle of the V-blades means that the soil will be further compacted as it is lifted. This too can result in a temporary ridge above the drain.
In sandy soil ridge-forming causes the most nuisance. In firm sandy soil the churning can loosen up the sand in such a way that a very good soil structure will be created. However, this will result in ridges which, in contrast to clay soil, will not disappear following swelling and shrinking. Additionally, no decomposition of soil elements will take place. Only the natural sinking process may cause the ridge to flatten slightly. The density of soil profile and the firmness and the coarseness of the sand will determine if, and for how long, the ridge will remain.
Trenchless ploughing will result in fewer after effects in comparison to using a trencher. In both cases, the field drains should be laid in the direction of cultivation of the plot. This will prevent possible problems when cultivating the land in both the short- and the long-term. Immediately flattening the ridges may damage the pipes. This does not only depend on the soil structure but also on the machine being used. Allowing the trench to sink naturally will cause the fewest problems. For some soil profiles the lifting of the soil might even mean a lasting improvement in the flow of ground water to the drain.