Deep drainage involves placing pipes at 1m or deeper to accept ground water and carry it to an open ditch. This water may have soaked down through the soil from above, after rain.

Alternately, some of the water may have seeped horizontally across land, under the topsoil, from surrounding, higher fields. It’s common to see a wet field at the bottom of higher, drier fields.

Deep drains work best when the soil layers are relatively permeable – water can flow sidewards and enter the pipe. Each pipe will drain the surrounding area. In practice, to reduce cost, drains are placed as far apart as possible, typically 8 metres to 30 metres.

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 – increasing cost.

In areas of high rainfall, significantly more water must drain into the soil – drains may need to be kept closer together.

Rainfall in farming areas of Ireland can vary from 1.0 metres to well over 1.5 metres per year. Drainage pipes are routinely installed at depths of 1.4 metres to 1.8 metres, sometimes as deep as 2 metres.

If the soil layers are permeable, then placing the pipe a bit deeper will tend to increase the area that each pipe can effectively drain – and allow for wider spacing.

Often, the limit on depth is the availability of a good outlet. Sometimes a drainage contractor will lower the floor of the existing open ditch by two or three feet in order to be able to install pipes at 2 metres to 2.5 metres depth across the field and significantly lower the water table.

Pebble is placed over the pipe to keep out silt. While the plastic pipe may be 80mm in diameter, the stone layer is typically four or five times this size, so also increasing the area available for water flow.

Pipes are generally covered with a 4in-12in bed of pebble and the trench backfilled with 2ft-3ft of soil.

On permeable soils, the rainwater will generally be able to percolate down into the drainage pipes at an acceptable rate. Some farmers will bring pebble to the top for a wet hollow, particularly on grassland.

The cost of deep drainage varies from €4 to €10/metre depending on depth, size of pipe, depth of stone cover and local availability/price of suitable stone.