Drainage systems carefully chosen for heavy soil farms

Deep drains have been installed on three farms, tapping into permeable soil layers. On the other four farms, no permeable soil layers were found so various systems of shallow drainage were installed.

Evaluation of the effectiveness of this work will begin this year and that will guide further work. Here, we outline the types of drainage installed, why it was chosen and what it cost.

Cork – Macroom

Deep drains, reseed: €6,043/ha

Deep or ground water type drainage has been carried out on this farm to improve drainage on 1.5ha. The work was carried out last June. This type of drainage was chosen because the soil had a reasonably permeable layer of between 1.5m and 1.7m in depth.

Permeability was observed at 1.5m. To exploit this permeable layer, drains were installed at 1.7m with an 80mm corrugated drainage pipe at the bottom and backfilled with 0.4m to 0.6m (15 to 24in) of clean 10mm to 40mm clean stone. The pipe drains were installed at 20m spacing.

In total, 900m of pipe was used and 180 tonnes of stone.

The costs of the work were:

Cork – Boherbue

Ditch clean, shallow drain system, reseed - €3,060/ha.

The first drainage job on this farm focused on a 2ha block. After investigation, it was decided to use a sub-soiler to open a network of shallow drainage channels which would intersect and empty into new collector pipe drains installed at a depth of 1m. The pipe drains would empty into an open watercourse, which was to be cleaned out.

This system was chosen because the test holes revealed peaty topsoil with silty, clay loam subsoil which would benefit hugely from the disturbance caused by the sub-soiler.

In this system, the deeper collector drains must be installed first. This work was carried out last June. The collector drains are 80mm corrugated drainage pipe. They were installed at 1m depth and covered with 0.6m (24in) of clean stone.

The stone used was 10mm to 40mm in size (0.5 to 1.5in). The pipe drains were installed at 15m spacing.

With the pipe drains installed, the shallow drainage system could then be installed. The ground was sub-soiled to a depth of 0.65m (24in) at 1.5m spacing. This work was followed by reseeding.

The costs of the work were:

Limerick – Athea

Shallow drain system, reseed: €8,129/ha

Again, trials holes to a depth of 3m found high levels of dense clay and no layers with good permeability. It was decided that gravel mole drainage was the most appropriate solution.

The work was carried out on 1.65ha last June/July.

Tipperary –Rossmore

Deep drains, reseed - €5,020/ha

On this farm, a block of 2.6ha was targeted for improvement by drainage and reseeding.

After investigation, it was decided to clean out the existing open drain and install pipe drains at a depth of 1.6m and with varying spacing, emptying into this drain.

This type of drainage was chosen because the soil had a reasonably permeable layer at 1.3m to 1.6m.

Seepage or permeability was observed at 1.3m.

As the area to be drained is flat, with only a little fall, it was decided to install drains at 1.6m (5ft 3in) deep with an 80mm corrugated drainage pipe and 0.6m (24in) of 10mm to 40mm clean stone.

The work was carried out last June. In the lower, wetter area, the drains were installed at every 15m point. In the higher, drier area, they were spaced 30m apart, keeping down costs. The costs broke down as:

Kerry – Listowel

Deep drains, reseed: €6,547/ha

Deep drainage and reseeding has been carried out on 2ha of this farm. The work was carried out last August. Trial holes to a depth of 3m found peat 70cm deep, followed by shallow, impermeable layers of silt to a depth of 110cm. The first permeable layer was between 110cm and 150cm and, at the time of the investigation, it showed evidence of seepage.

The decision was made to install deep drains in the permeable layer emptying into the open watercourse, thus lowering the water table. This will promote cracking of the soil during dry weather, helping root penetration.

The drains were put in at 1.6m depth and at a spacing of 15m. A total of 1,400m of 80mm pipe was installed, emptying into an open watercourse. The pipes were covered with a 24in layer of 10mm to 40mm (0.5in to 1.5in) stone – Costs broke down as follows:

Kerry –Castleisland

Shallow drain system, reseed: €7,337/ha

When 3m deep investigatory holes were dug on this site, no layer with good permeability was found. Therefore, it was decided that a shallow drainage system emptying into new collector drains would be needed.

The first phase was carried out on 1.25ha last June. A main drain was installed along the length of the field, emptying into a river. Collector drains were then placed at a depth of 1m and 20m apart, emptying into the main drain.

Clare – Doonbeg

Shallow drain system, reseed - €6,793/ha

Trials holes to a depth of 3m found no worthwhile permeable layer. The lower layers were poorly structured and dense. Therefore, mole drainage over new collector drains was identified as the best option and this was carried out last July on 1.6ha. The dense nature of soil layers means naked moles (not filled with pea gravel) should have an acceptably long working life.

The work and cost breakdown as:

NOTE: Cost of piped drains varied from €5/m to €10/m across the farms, with the amount of stone cover used and distance of haulage being the main variation in cost.