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Test pits can help determine water seepage

If there is money to do new drainage work, the next step in the process is to determine what drainage system is required on the farm.

The combination of high rainfall, low evapo-transpiration rate, and a low rate of percolation of water through the soil causes wet soils.

The last 18 months of wet weather has impeded much of the lands natural drainage system, and a change in the weather to a natural cycle, including summer drying, would actually aid the soil structure.

However, where soil conditions are poor, implementing the right drainage systems when ground conditions are right will lower the water table and improve soil drainage.

It is important to realise that poor drainage is a result of three main causes.

These are seepage and springs, a high water table or soils which are impervious.

The latter feature is often referred to as soils with a low hydraulic conductivity, which basically means that the water struggles to percolate through the soil.

To determine which drainage system is required, rather than a series of costly trial and error approaches, it's important to establish the soil type and the cause of the poor drainage.


This is done by opening a number of test pits.

Local knowledge of the effectiveness of previous drainage schemes can also be a good insight.

Tests pits are often dug to 2.5m deep, and as they are being done, it's important to examine the face of the pits to determine the rate and depth of water seepage.

Essentially, you are trying to assess at what level the water is flowing into the pit, and also trying to see which layers are permeable.

Here you are looking at the flow of surface water, for springs that are bubbling up, or seepage at a certain layer.

Also note areas with visible cracks, looser soil and the rooting depth.

Test pits can collapse so don't enter the pit. Instead make your observations from a safe distance and inspect the excavated material.

If you are dealing with an impermeable soil, the movement of water is impeded at all levels, with no distinct layer with a higher permeability.

Once these tests pits have been dug, you will be more informed on whether you need either a groundwater system of drainage or a shallow drainage system.

A ground water system is a network of piped drains to establish a deep drainage base in the soil.

These drains are used where test pits indicate a strong inflow of water or seepages from the face of the pit walls.

Conventional piped drains at 0.8m to 1.5m below ground level have been successful where there is a suitable layer of soil that has high water conductivity. These drains cost around €2,500-€3,000/ac.


However, deep drains may also be required at 1.5m to 2.5m using spacing of 15m to 50m, depending on the slope of the land and the depth and thickness of the drainage layer.

Make sure that open drains and the main deep drains also run in the direction of maximum slope.

While deep drains are difficult to install and may need doing in a series of stages to reduce the risk of collapse, they are very cost effective as so few are required. The usual cost range is €1,500-€2,500/ac.

Clean stone should be filled to a minimum depth of 300mm from the bottom of the drain and should surround the drainage pipe.

While farmers have differing views on this topic, it's important to realise that the drain pipe is vital to allow a high flow rate, and that stone alone is unlikely to have sufficient flow capacity to be effective, except for very short drain lengths (less than 30m).

Irish Independent