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The depth and distance of drainage pipes

The purpose of drainage is to lower the groundwater level to such an extent that it won’t rise to ground level as often. This has already been described in more detail in an earlier chapter. The dewatering requirements indicate how high the groundwater level may rise at the individual starting points.

Drainage depth

The bulging of the groundwater between the drains is, therefore, an important factor in determining the drainage depth. The bulging is very dependent on the soil type and the permeability of the soil. These differences are mainly manifested by a difference in drain distance. Too small a drain distance or too great a drain depth can also increase the risk of drought damage. This is particularly the case when dry spells occur in early spring. The optimal drain depth is therefore a compromise between the prevention of flooding and damage from drought.

Grassland
The optimal drain depth for grassland is, depending on the type of soil, approximately 80 cm below ground level. This minimizes the risk of drought damage as much as possible.

Maize and arable land
Stricter dewatering standards apply to maize and building plots. This means that depending on the soil type, different drain depths apply. These standards are shown in table 5. They also apply to horticulture.

Drain depth = trench bottom depth
The drainage depths mentioned for grassland, arable land, or maize land indicate the depth of the trench bottom. It is assumed here that the drains are laid horizontally.

Ground-level drop
The drainage depth should be based on a possible drop in the ground level. This is due to the heavy leveling of the plot. Peat soil may have a drop that is greater than the natural drop as a result of additional oxidation or shrinkage. It is recommended to take this extra drop into account during construction by laying the drains a little deeper.

Permeability
The stated drain depth should be seen as a guideline. And should only be used when the permeability at the stated depth does not cause any problems. If there is a poorly permeable layer at the optimal drainage depth, you must deviate from the indicated depth. If this causes the drains to be higher, the drains will also be placed a little closer together. However, the height is also limited. The drains should preferably remain covered with a layer of at least 70 centimeters of soil. This is mainly to prevent damage during the cultivation of the field. Experience has shown that a poorly permeable layer becomes more permeable through better drainage. Sometimes a layer can be broken by profiling. This mainly happens with a sandy surface. As a result, the drain distance can sometimes become significantly larger which reduces the number of drains required per hectare, which means lower investment in drainage.

Starting points of drainage depth

The aforementioned drain depth does not guarantee that the groundwater will never rise to ground levels. The following points were taken into account when determining the optimal drainage depth:

The amount of water to be drained
The drainage depth
The number of times the drainage depth did not meet the requirements

Amount of water

The amount of water that needs to be discharged is converted into millimeters of water per ha per 24-hour period. This quantity is directly related to the associated groundwater level, measured in the middle of the drains. This is called the desired dewatering depth. This is 30 cm for grassland and 50 cm for arable land a maize land below ground level. When this groundwater level occurs, the drains must discharge a certain amount of water in a certain period. In reality, the amount disposed of can be quite different. With a lot of rainfall in a short time the groundwater level will be high and therefore the discharge will be greater. In practice, for example, a discharge of 20 mm per 24 hours has been measured, whereas 7 mm per 24 hours was the norm. The quantities that we assume in the calculations are stated in table 6. When the groundwater sinks deeper, the discharge will of course be less. The drainage is easy to check. This is further indicated in chapter ‘check and maintenance’.

Dewatering depth

The permitted height of the groundwater level, or the dewatering depth, applies in the first instance to prevent disadvantages being experienced during the cultivation of crops. These disadvantages include poor grassland carrying capacity, yield losses due to late grazing and trampling of the grass, late mowing and poor drying of the crops, and problems with sowing and harvesting crops. When the desired drainage depth increases, higher demands are placed on the drainage, and the drainage depth will increase and will come closer together. A greater drainage depth must be possible in practice in connection with the existing ditch level or the depth of the existing ditch bottom.

Exceeding drainage depth

Table 6 shows the standards for the drainage and drainage depth that must be considered in the construction of drainage. It doesn’t mean that if these standards are met, that the groundwater level will never exceed the desired drainage depth. For example, if a lot of precipitation falls in a short time. The drains will then discharge more water than the standard, but the groundwater level will still rise above the desired drainage depth. This will not take long if there is good permeability. If the permeability is bad, it can take much longer. In both cases, we speak of only one violation of the standard, although the period varies widely. For soils with poor permeability, the number of times that the desired drainage depth is exceeded is, therefore, more important than on well-drained soils. On well-drained soils, the dewatering depth may be exceeded about five times. When calculating the desired drain distance, the number of times that the standards are not met can also be calculated.

Drain distance

The distance between the drains can be determined based on theoretical formulas as well as based on knowledge and experience. In both cases, help must be sought by an expert. When determining the drain distance, this expert is based on several starting points. The requirements can be determined by yourself; the standards are given in table 6. Other starting points have been established. The soil type is a given. Only the structure can be changed by deep tillage. Soil build-up can greatly affect drainage capability. The drain depth can also be a fixed given by, for example, the height of a ditch spike that cannot or may not be changed, or a layer with poor permeability at a certain depth in the profile. The permeability of the entire soil profile is important for determining the drain distance. It has a major influence on the speed at which the water is discharged through the soil to the drains and/or ditches. The permeability of the soil is important for both the theoretical and practical calculation of the drain distance.

Permeability

The permeability of the soil is the ability of the soil to allow gas or liquid to pass through. With drainage, the saturated permeability is measured. This means that in this case all pores are filled with water. A measure of the permeability is the transmission factor, which we display with the letter K. Table 7 gives an overview of the valuation of the transmission factor.

The permeability factor can differ per soil type and soil layer. And it is not always constant. It can vary widely by compaction or by loosening of solid layers, swelling, and shrinkage. The temperature also has some influence. Usually, it is calculated that the temperature would be 10 degrees Celsius. In special cases, the transmission factor is determined in the laboratory or with the help of calculations. Usually, however, people go into the field. The most important field method is the borehole method. In addition, there are also the reverse borehole method, the cube method, and some other methods. All these methods give an impression of the permeability, that is to say, the possibility with which the water can flow through the soil at a certain speed in a certain time. If the soil consists of layers, the permeability per layer must be determined, after which an average permeability of the entire profile can be calculated.

Current

The current of groundwater in the soil influences the resistance that the water encounters. We distinguish a stationary flow and a non-stationary flow. A stationary flow occurs with an even discharge of excess precipitation. During this flow, the water moves in different directions and encounters a corresponding resistance (figure 14). The resistance the water encounters to the drain can be distinguished into;

The vertical resistance in layers in which the water mainly moves vertically downwards
The horizontal resistance in layers in which the water mainly moves in the horizontal direction to the drainpipe
The radial resistance because of the water that flows together near the drain
The entry resistance to enter the drainpipe; the resistance of the wrapping material can also be included in this

The discharge of precipitation will often involve a non-stationary flow. The pressure and speed of groundwater will change. This is because the amount and intensity of precipitation often vary greatly. This non-stationary flow can be calculated using formulas. For example, it can be used to calculate how often the desired drainage depth is exceeded in time.

Theoretical calculation of drain distance

Various formulas have been developed for calculating the drain distance. Among these, the Hooghoudt and Ernst formulas should be mentioned in particular. In both formulas, the horizontal and radial resistance is used for calculations. Vertical resistance is only used in the formula of Ernst. The entry resistance is being neglected because it is normally quite small. However, the entry resistance can become very high in case the casing material and/or the holes in the drainpipe are blocked. We will not elaborate on these formulas because in a practical business situation it is almost impossible to collect the various data and perform the calculations. When carrying out large drainage assignments, it may be sensible to have the drain distance calculated by a cultural engineering bureau using these formulas.

Drain distance in practice

In practice, highly variable variations occur in profile structure and soil type. This change often occurs within the same parcel. After this it is difficult to determine the correct drain distance based on the theoretical calculations alone. Especially when the permeability also changes over time. This occurs, for example, on clay soil, as a result of swelling and shrinkage or through ripening. The permeability also increases through improved structure due to air entry, profiling, rooting activities, or soil life. In practice, these situations are very common. Experts can then determine the drain distance through knowledge and experience. These experts must regularly check this knowledge against the results in practice.

In addition, incidental observations and possible trial field data can support these experiences. A lot of drainages are installed based on the method. Experiences and the practical assessment of the soil also play their part in a theoretical calculation.

Commonly used drain distance

The drain depth generally used for grassland is at least 80 cm below ground level. For arable land and maize, the optimum depth is at least 1 meter or more. The distance can vary widely at these depths. On some soil a distance of less than 10 meters is required, in another case, more than 20 meters can give an excellent result. In practice, most drains are located at a distance of 10 to 15 meters from each other.

Drainage research and a drainage plan

If considering the construction of a drain, a plan must be made first. In addition to theoretical knowledge, preliminary research in the field must be included. Only then can a final decision be made about the location, depth, distance, direction, and choice of material. The points that must be included in the drainage research are mentioned below.

Before a drainage plan is drawn up, it is useful to obtain information about the possible restrictions that may be commissioned by the government. This information can be found through the water authority, the polder, and/or the municipality. Information can also be obtained within the boundaries of a land development plan. Possibilities for obtaining a subsidy can also be explored. It is also necessary to know whether the level of the ditch will be changed over time. Perhaps even an occasional lowering is possible, which makes drainage possible on plots where the ditch level is currently too high. A permit from the water Authorities or the polder is often required to discharge into water. Checking these things in advance prevents disappointments and unnecessary costs.

The ditch water level

The height of the ditch water level is often a given.

For grassland, this should be at least 90 cm below ground level. This is called the ‘reclamation standard’. The drains then discharge into the water 10 cm above ditch level. This normal ditch level applies with a half design discharge, which means that this level may be exceeded for a maximum of 10 to 20 days per year. At a higher ditch level, the drains will regularly disappear underwater, this is at the expense of the quality of drainage.

Soil type and permeability

The type of soil has a major influence on the permeability of the soil and thus the distance and the drainage depth. As you may have read in the chapter ‘drain distance’, the permeability in the field is usually determined by drilling holes. In addition, a soil map can provide additional information. An expert’s experience in drainage of similar soil types can also play a role. A good impression of the natural condition of the soil at different depts is obtained by digging holes to at least drainage level. The natural state in boreholes is very disturbed and this makes it a good measure for assessing the profile. A good impression of the permeability can be obtained through theoretical data and observation in the field.

Layering in the soil

In addition to the soil type, the structure of the soil is also important. In the field, the presence and the nature of the various layers can be determined by drilling holes in the soil. Besides, holes can be dug to assess whether any layers can be improved by performing deep tillage. It is also possible that the permeability of the soil is poor as a result of disturbing layers in the profile. This has a major influence on the drain depth and the drain distance. Sometimes a disturbing layer can mean that the drainage of a plot must be advised against. For example, when there is a very poorly permeable layer of boulder clay within 60 to 70 cm below ground level. Also, untouched clay layers, sometimes completely enclosed by peat layers, can be so poorly permeable that drainage should also be discouraged. Poorly permeable layers that occur below the drain level can also influence the drain distance. This is included in the calculations of drainage formulas.

Variation within the plot

Within the same parcel, there can be quite a lot of variation in ditch level, soil type, stratification, and/or permeability. This should be taken into account when constructing drains, for example by placing the local drains higher or lower, closer together or further apart. Sometimes drainage is not necessary on certain parts of a plot. In another case drainage on a part is pointless.

Lot pattern

The plot pattern and the plot shape present within the plot usually indicates in which direction the drains will be located. It can also influence the choice of which parcels are drained first and in what order this takes place over several years. It is also an important part of the total plan for grassland improvement and parcel layout. The plot size and the construction of a possible plot path can also play a role. This is shown in figure 14. Figure 15 shows how the drain pipe can run on a plot that is enclosed on two sides.

Attenuations

When constructing a drainage system, some ditches can sometimes be filled. These damping influence the chosen drain direction. The drain pipe should preferably not cross damped ditches. Ditches that have been filled in, in the past should also be avoided as much as possible. Lastly, it is important to determine whether ditches will be filled in the future. This is important for several reasons. In the first place, the drain cannot end in such future damping. The water discharge from the ditch to which the drains that need to be constructed should not be disrupted by future damping. When crossing a cushioning cannot be avoided, the following points should be taken into account.

The ditch that needs to be filled must carefully be cleaned before the filling is carried out. This means free from dredge.

The material used to fill the ditch must, as far as possible, consist of material comparable to the immediate surroundings and the soil of the adjacent parcels. This will prevent a permanent height or low difference due to sagging. There is also less chance of flooding or additional drying out at the location of the damping.

Provisions must be made to support the drain where it intersects the damping. This can be done, for example, by applying a quantity of sand at the location of the intersection. So-called bridges can also be installed or a perforated smooth tube can be slid around the drain. When installing a bridge or smooth tube, a slot is of course required at the location of the damping.

The material that is being used for the damping should preferably be settled or pressed before the drainage is applied.

Ditch distance

The presence of a different ditch level, a parcel path, or a public road sometimes makes it impossible to fill ditches. And the question of whether a permit is granted by the competent authority for filling in a ditch or changing a plot pattern remains. In addition to the height of the ditch level, the existing ditch distance is an important factor for the drainage status of a plot. After all, even if the desired drain distance is calculated scientifically and reliably based on observations in the field, the ditch distance must be divisible by the calculated drain distance. If this calculation is incorrect, then a narrower or wider drain distance must be chosen.

Example

The calculated drain distance is 12 meters and the ditch distance is 100 meters. The calculation shows that there are 8 drain distances of 12 meters, with 4 meters remaining. You can also choose 8 distances of 11 meters and 1 x 12 meters. The number of drains always remains 8. If a closer distance of, for example, 10 meters is chosen, 1 extra drain is required. If, for example, 6 distances of 13 meters and 2 distances of 11 meters are chosen, 1 less drain will suffice and 7 are needed.

Underground pipes

There are many underground pipes in The Netherlands. Such as telephone and electricity cables, and gas and water pipes. Pipes for, drinking water, manure, or irrigation may also be installed by the company itself. The depth of these pipes is rarely deep enough to be able to intersect with a drain that needs to be installed. In the overall plan for the site layout, the presence of these pipes must therefore be taken into account. Until October 1ste 2008, the presence of these pipes could be requested from the Cables and Pipelines Information Centre (CPIC).

A new law

Please note that this paragraph was written according to the Dutch law in 2008. For current legislation, always gain the latest information from your local government.

As of July 1, 2008, the Underground Networks Information Exchange act, better known as the earthmoving scheme, has been adopted. The purpose of this act is to prevent agitation incidents near cables and pipelines. The law mainly regulates the exchange of information regarding the location of cables and pipelines between network operators and soil agitators. The law also includes careful decisions about careful digging and commissioning and taking precautions for dangerous pipes. As of October 1, 2008, soil agitators are obliged to submit their agitation reports to the Land Registry. The Land Registry knows through network operators which network operators it must forward agitation reports. The law contains an obligation to perform agitation work with care. This applies to all parties in the chain, so both the client and the soil agitator. This means, for example, that the client should provide enough time and opportunity for the soil agitator to dig carefully. The soil agitator is legally responsible for the agitation. This does not always have to be the digger.

For the soil agitator, careful digging means that he is legally obliged to request the data of the layout and to further investigate the exact location of cables and pipes. The maps must be present at the digging location. This also means that the digger should have knowledge of the location of the cables and pipes. You may only dig after a digging report has been made. The parties from the sector have further completed the understanding of ‘carefully digging’. The CROW has set up a directive "carefully digging process" on behalf of the cables and pipes consultations (KLO).

The soil agitator

The law shows the most important obligations for the person who agitates the ground:

Report the mechanical agitation work intended;
Carefully carry out the agitation work;
Report a deviating location to the Land Registry, including networks that are not on the map;
Determine precautions for dangerous pipes;
An agitation ban until a report has been made (with the exception of calamities);
Report damages to the network operators

Process of information exchange

According to the law, the soil agitator (or contractor) is obligated to request the cable and pipe information at the Land Registry. He reports the intended agitation work at the Land Registry. This concerns mechanical work (on land and in water). This request cannot be made 20 working days before the digging activities take place, this way the soil agitator has up-to-date material with the start of agitation work. A request can be made on the internet 24 hours a day.

Forward drawings

According to the law, the soil agitator (or contractor) is obligated to request the cable and pipe information at the Land Registry. The Land Registry forwards the request of the soil agitator to the network operators. The network operators remain responsible for registering the location of their cables or pipes. The Land Registry becomes a source of information, and therefore not a manager of a public central register. The network operator sends the drawings directly to the soil agitator (transition phase) or through the Land Registry to the soil agitator (final phase).

Receiving the drawing

The soil agitator will receive the drawing digitally in the final phase, as quickly as possible, but within two days after his request. This means that it is also possible to receive the drawings within the hour, and then print them yourself. It is then possible to view the cables and pipelines of different network operators on one map. But certain data can also be shielded so that a map remains uncluttered and more useful, this can be handy in urban areas. This makes the data quickly available and user-friendly.

Keep in mind that damage to pipes can have drastic (financial) consequences.