Construction of drainage
Different types of machines can be chosen for the construction of drainage. For instance, there are machines that dig a trench and machines that lay a drain in the soil without digging a trench: the so-called trenchless machines.
A trenching machine, also called a chain digger, digs a trench and places a drain pipe. The machine can also be provided with an installation that fills the trench with coarse materials. A trencher can be preferred over a trenchless machine. For example, in an area with (boulder) loam in the subsoil. If this loam occurs at varying depths, it means that the drains occasionally lie in boulder clay and remain above it in other places. Sometimes (depending on the type of boulder clay) this boulder clay can be drained. In these cases, the trencher is preferred. The trench dug in the boulder clay is refilled with well-mixed material. The permeability of this material is usually sufficient and fulfills an important function in the drainage of water. The trencher is also preferred when draining into soft, untouched soil. By exposing the excavated material to air for a few days, the material can dry. This also applies to the slot. The permeability of the trench plus the nature of the material to be returned is thereby greatly improved. The weather conditions play a role in this. The trencher can also be used well if the fields are leveled immediately and little nuisance is expected to form of the trench settling. This is the case when drainage takes place in the cultivation direction of the plot and on light clay or sandy soils. This also causes a few problems when used as arable farming or for maize cultivation.
In order to increase the drainage effect of the trench, it can be completely or partially filled. Gravel or gravel mixtures, but also shells, glass embers, lavalite (glass ash) and sand can be used for this. This will certainly increase the drainage. Due to the high supply costs of these materials, this method is economically only attractive near places where these materials are close by. They are often used in conjunction with an uncoated tube.
A disadvantage of the trenches is that incisions are left on the grassland. If these are located parallel to the ditches, or even in the ditches, this has little to no objection. However, if the drains are transverse to the ditches, a drain trench can be a problem, especially when no reseeding will take place in the short term. The soil will collapse in newly filled drainage trenches, creating difficult strips in the form of trenches. This is most obstructive on peat soil and on peat soil with a clay cover. Drain trenches perpendicular to the working direction are also very obstructive on clay soil. Leveling is often only done after several years on heavy clay soil. The trenches are necessary for the first years because the subsoil has not matured enough for smooth drainage. The duration of this transition depends on;
- The ratio between precipitation and evaporation in a particular season and the length of this period
- The quality of the existing turf
- The evenness of the plot and the nuisance encountered during processing
- The priority given to the need for equalization
On heavy clay soil or soil with strong loamy topsoil, ditches are often required in addition to drains. To prevent puddles from forming after heavy rainfall. The chance of puddles forming is greatest in the period after equalization and sowing when no closed sod has yet formed.
Drainage without trench
The trenchless draining machine was initially developed with a vertical agitating body. This method had the disadvantage that more sod damage occurred on grassland in comparison with using a trencher. As a result of lateral pressure, soil compaction was also regularly established. Which impeded the flow of water to the drain. The height of the drains often left something to be desired. In the 1970’s a trenchless alternative was therefore developed in the form of a V-shaped blade. With this system, also referred to as ‘Delta plow’, the knife lifts a V-shaped bottom part. The drain pipe is placed in the ground through a hollow space on one side of the V-shaped knife. After the V-shaped bottom part has been lifted, it will return to its original place, and in its almost original shape. The results of this system mean there is little chance of the drain trench sagging, this is the case with the trench digger. Especially on (clay) peat soil, the construction of drainage on grassland can be carried out this way with little damage.
After using this drainage system on clay soil, quite a lot of ridge formation can occur. The extend of this ridge formation depends on several things;
- The heaviness of the clay soil. Structure chunks can occur in the clay profile. These chunks are larger on heavy soil than on light clay soil. On heavy soil, sockets can occur as a result of the large chunks when the lifted soil mass is put back. These temporary sockets create high ridge formation. After a period of swelling and contraction (a wet and dry period) these formations will often return to their original shape. But if leveling is done immediately, then layers can form after a certain time.
- The amount of moisture of the soil. The amount of moisture mainly determines the extent to which clay soil is editable and can be cut through. In the dry state, the structural elements are more apparent, and vigorous replacement is more difficult. In this case, the chance of temporary ridge formation at the drain is greater.
- V-shape. The narrower the V-shape of the knife, the more the bottom is compressed when it is lifted. This can lead to a greater chance of a temporary ridge formation at the drain.
Ridge formation is especially annoying on sandy soil. Particularly in solid sandy soil, the soil body can loosen the sand which can result in a very favorable soil structure. However, this does result in a ridge that, unlike clay soil, does not recover by itself due to swelling and contraction. There is also no digestion of soil materials. Only through natural sagging ridge formation can flatten slightly. Depending on the density of the soil profiles, the consistency, and coarseness of the sand, a ridge will remain.
Trenchless drainage has fewer consequences than a trencher. Nevertheless, in both cases, laying the drains in the machining direction of the field is preferred. This can obstruct both the short and long term. The direct pressing of the back can damage the drain. This does not only depend on the soil structure, but also the construction of the machine. The natural settlement of the trench creates a risk of disruption. On some profiles, lifting the soil even means a permanent improvement of the water flow to the drain.
Moment of implementation and groundwater level
For proper operation, the drain trench and the soil directly around the drain must be well permeable. The chance of deterioration during drainage must be as small as possible. With the trencher, the risk of compaction of the drain trench is high if the drains are constructed below the groundwater level that is present at that moment. This is especially risky on light fine sandy soils. The chance that the material will be wasted and disposed of does not only apply to the removed material, but also to the material that is returned. The side of the trench can also be smoothed by the chain, creating compaction. Such smearing can also occur with trenchless machines. In this case, the compaction occurs on the cutting surface, or even close to the drain. Construction of drainage at a high groundwater level increases the risk of disturbances of the soil profile. If the groundwater levels are above the drain depth, drainage will start. Due to the soil parts that have been set in motion, the groundwater will easily be supplied into the drain and the casing. This is a major risk, especially on plots where the subsoil consists of light clay and sand. There is a risk of organic material being washed into peat soil, which gets worse if the groundwater level is high at the time of drainage. The greatest success and the smallest chance of a reduced permeability and/or wash in can be expected when the drainage is installed at the lowest possible groundwater level.
In addition to the drain pipes, various accessories are often required for drainage. There are click sleeves for connecting two drainage pipes, drain bridges (for supporting a drain when crossing damping), control wells, power pipes, and slope gutters.
End tube and slope gutter
Both the end pipe and the slope gutter are two important parts of a drainage system. The end pipe replaces the last part of the drainpipe at the end of the ditch. it can be made of polyvinyl chloride (p.v.c.), polyethylene (p.e.), or polypropylene (p.p.). It prevents the drain from sinking into the slope. The standard length of a power tube is one meter. The outlet of the end pipe usually sits on the slope of the ditch above the waterline. Due to water flow from the drain, soil easily flushes out from under the outlet. As a result, the power tube loses its support from the bottom and moves forward. The drainage is then disturbed and the end pipe can even come loose from the drain. This can be prevented by using a longer telescopic tube that opens directly above the ditch water or the ditch bottom. (Figure 21). A slope gutter can also be installed. A telescopic end tube can be retracted when cleaning the ditch. Slope gutters must be installed within the slope of the embankment. The latter are available in various designs. when choosing the gutter must be properly clamped in the slope. The chance that it will be damaged during the cleaning of the ditch is therefore small.
Outlet end tube and marking
The drain distance depends on many factors. As previously described. The exact location of the drain can still be influenced by drains already leading into the ditch. For example, if an adjacent plot has already been drained, it is recommended that the new drains discharge directly opposite the existing drains. This is useful when cleaning the locks. The drains are easier to recognize, which means a lot of advantages in an overgrown slope. The standard for calculating the distance should not be violated. Extra-long power tubes can also be used, for example of one and a half or two meters. These can protrude further from the slope. The pipe can also be marked by driving a pole into the slope. When installing a fence for livestock, the fence posts can also be placed at the location of an end pipe.
Requirements for the material
For the drainpipe, the casing material, and the fittings, there are requirements for which the products can be certified. In the Netherlands, this is the KOMO quality mark. (Figure 22). This quality mark with certificate numbers is issued by the KIWA. When asked to use KOMO-certified products, each material must be individually provided with such a quality mark. With this mark, the producer guarantees that the material meets the quality requirements as stated on the certificate. If there are complaints, please contact: KIWA nv, Sir Winston Churchillaan 273, 2281 EA in Rijswijk.
Control on the construction
The construction of drainage requires a substantial investment. An optimal return on an investment is not always certain. The drainage plan and material are visible and verifiable. The position of the drain, however, is not visible. Although almost all drains are laid using laser equipment, an ideal location cannot always be guaranteed. The height of the drain can be checked with the aid of advanced equipment. Measurements in the past sometimes showed a large deviation from the desired results. Certainty of the height is therefore desirable. To guarantee the location, the height can be measured randomly. For this purpose, a hose is placed in the drain with a measuring head at the end. An up and down movement of the head is registered and displayed through an ingenious measuring system. The exact height can be measured this way. This data can be compared with the desired location and checked against the permitted deviation. An inadmissible deviation in height can result in trapped air bubbles, which increases the flow resistance. An unwanted gutter can lead to an accumulation of sludge. This also increases the flow resistance and can block the drain.
With two deviations in succession, an airlock can form, as shown in figure 24. An airlock seriously hampers the drainage of water. Due to the upward pressure of the air, to overcome a lock, a water level equal to the height of the unwanted bulge is necessary. This pressure height is caused by the increase of the groundwater level around the drain. The air does not easily escape through the casing material because the pores are filled with water during a period of discharge. If several air ditches come into succession, the groundwater level will rise rapidly. This is shown in figure 26. The rise in the groundwater level is the sum of the heights of the individual air ditches.
Another important factor in the height of the drain is the chance of sludge deposits in the lower parts. With a partially filled drain, the water flows fastest in the place where there is the least amount of water. Where there is more water (the lower parts), the water flows more slowly. Most of the sludge and sand settle here. (figure 27).
To avoid the aforementioned problems, the following requirements can be imposed on the height. The deviation of the inner bottom edge of the drainpipe should not be more than half the inner diameter of the drain. This concerning the ideal line of the elevation. The deviation referred to above may not remain in the drain water above the axis as a result of ‘negative slope’. (figure 28b). The requirements mentioned before must be strictly adhered to during construction and testing during an inspection.
If the client and the drainer agree on the installation of drainage, it will be recorded in writing. Several provisions can be included in this. The map with the direction and number and height of drains plays an important role. The presence of cables and pipes must be checked. The contractor/drainer is responsible for determining the correct location of these underground works.