Land drainage divides up into two broad types: deep and shallow. A decision on which is used in any field comes down to soil type. Here I outline how each system works and typical cost.
What is deep drainage?
Deep drainage, also known as ground water drainage, involves placing pipes at a depth of about 1m or more, at regular intervals. This is to accept rainwater that has soaked down through the soil and allow it to flow away quickly to an open ditch. The pipes may also be installed to accept water that has seeped across land, under the topsoil, from higher fields. This water may bubble up as springs or just seep into the wet area. It is very common to see a wet field at the bottom of higher, drier fields.
When is it best used?
Deep drains work best when the soil layers are relatively permeable and allow rainfall to soak down into the ground and across into the pipes. Where the soil is permeable, each pipe will drain the surrounding area – water can flow sidewards and enter the pipe. Lowering the water table can allow more cracking and opening of soil to take place in subsequent dry spells and allow deeper penetration of plant roots. These will help drainage of surface water down to the drains.
How far apart are drains placed?
In practice, drains are placed as far apart as possible to reduce cost. They are typically placed anywhere from 8m to 30m apart. The more permeable the soil, the wider the spacing that is acceptable. Conversely, where soils are less permeable, the drainage pipes are best kept closer together. In favourable conditions, pipes can be spaced up to 30m and 35m apart. This means that each pipe is removing water from 15m of ground on each side. Where such wide spacing is used, the cost of land improvement is reduced.
What about annual rainfall?
In areas of high rainfall, more water must drain into the soil – drains may need to be kept closer together. Annual rainfall in farming areas can vary from 1m to over 1.5m.
How deep are they placed?
Drainage pipes are routinely installed at depths of 1.4m to 1.8m but sometimes as deep as 2m. If the soil layers are permeable, placing the pipe a bit deeper will increase the area that each pipe can drain. It will also allow for wider spacing. Often, the limit on depth is the availability of a good outlet. I have featured drainage jobs where the contractor has lowered the floor of the existing open ditch by two or three feet to be able to install pipes at 2m to 2.5m across the field and lower the water table.
What pipe is used?
Most field drains are made using plastic corrugated pipe placed in the bottom of a clean trench and covered with a layer of stone. This type of pipe is low-cost and, if placed deep enough, won’t be crushed by machinery. Some contractors use rigid plastic pipe. It can be easier to handle and easier to keep straight and level at the bottom of the trench. Concrete pipes are still used where load bearing strength is required in roadside drainage and bridges, for example.
Why is stone used?
Stone has a number of functions. It acts as a filter to keep out silt and other particles that might otherwise enter the pipe and block it. Stone is generally effective as a filter when it is in the size range of 10mm to 40mm. Larger stone will not be as effective a filter, although on some soils this may not be a problem. In practice, drainage contractors tend to use whatever type of stone is available locally as this keeps down costs. The stone layer also increases the effective size of the drain. This may be an advantage if water volumes are high. While the plastic pipe in the bottom of the cut may be 80mm in diameter, the stone layer is typically four or five times this size, increasing the area available for water flow. Placing a layer of stone under the pipe can help keep the pipe clean and help let in water. But it adds to the cost and is often not done.
Should pipes be stoned to the top?
This is done on football pitches and golf courses where budgets are high. An advantage is that rainwater can get down to the drainage pipes below more quickly. It would not be economical in farm drainage. In any case, given that deep drains are used in permeable soils, the rainwater will generally be able to percolate into the soil, and into the drainage pipes, at an acceptable rate. Some farmers will do so where a drainage pipe is being installed to tackle one isolated wet spot.
How long will deep drains work for?
If installed correctly and where the outlet watercourse is kept clean and flowing, piped drains will have a long working life. The pipe is out of the way of tractor wheels and other heavy equipment. In favourable soil conditions, a working life of over 100 years could be expected.
What does it cost?
The cost of deep drains varies from €4 to €10/m depending on depth, size of pipe, depth of stone cover and local availability/price of suitable stone.
Drainage pipe costs about 80c to €3/m depending on size and type. Pebble costs from €180 to €350 per 20t load. Excavator hire costs about €35 to €50 per hour, all plus VAT.
Where are deep drains not suited?
They won’t be used if the soil is impermeable. Water is unable to move sidewards from the surrounding soil into the pipe. Rainwater cannot get down into the soil quickly enough. It is then necessary to place drains closer together, at every 8m or 10m, for example. Drainage then becomes more expensive. At a certain point, a shallow drainage system will become more cost effective.
What is shallow drainage?
This usually involves installing relatively shallow carrier pipes and ripping open cuts in the soil above to allow rainwater flow down into the pipes.
The carrier pipes are typically placed at a depth of about 1m and are covered with a layer of stone. The drainage cuts are opened at the surface using a subsoiler or a similar soil-ripping machine. These run across the piped drain and intersect the stone cover, giving water access to the pipes below.
Where is it used?
Shallow drainage is the preferred option where soils are impermeable, where water is unable to flow down from the surface or sidewards within the soil. The soil openings must be made closely enough together to allow surface water find its way down into the collector drains. The main rationale behind this system is cost. Making openings in soil with a machine like a subsoiler or mole plough is far cheaper than installing pipes at close intervals. A downside is that the openings made by a subsoiler or mole plough will close up again under trafficking. It must also be carried out in the correct conditions.
What size and type of collector pipes are used?
Typically, 80mm to 120mm corrugated plastic pipes.
Does stone have to be used?
Stone must be used over collector pipes for the system to be effective. The teeth of the ripping machine must be able to cut into the stone layer – without coming too close to the plastic pipes. If you have collector pipes installed on your land, record how deep they are placed and what cover of stone is put over them.
Are collector pipes always used?
No. If there are permeable layers of soil below the topsoil, opening the top surface by ripping can be effective in letting water down. This will also help if there is an iron pan. However, installing collector drains will usually result in a longer-lasting drainage system.
How is soil above the pipes opened?
This can be done using a subsoiler, a panbuster, a deep plough, a mole plough or a gravel mole plough. Mole ploughs open a channel of about four inches and these will be most persistent in soils with high clay content. Filling moles with pebble will extend their life.
Does it matter when this work is done?
Subsoiling and ripping is most effective when soil is dry. The disturbing action of the implement creates many small cracks which allow surface water to drain down to the collector pipes below.
What does it cost?
Collector drains will typically cost €5 to €8 per metre. Subsoiling is usually charged by the hour and typically works out at €80 to €150 per hectare, moling costs €100-200 per hectare.
This article was written by Paul Mooney and was first published on 20 February 2014.