Pick drainage system based on soil type in area
Knowledge of local drainage schemes, and their effectiveness, will provide insight.
Test pits of at least 2.5 metres deep should be excavated. Soil type should be established and the rate and depth of water seepage into the test pits recorded. Visible cracking, areas of looser soil, and rooting depth can convey important information regarding the drainage status of the layers.
Either a ground-water or shallow drainage system can be used, farmers were told at the recent open day about land drainage and farming wet soils, at the Solohead Research Farm, Tipperary.
The system you choose is based on whether or not a layer is present at a workable depth that will allow the flow of water with relative ease in a ground-water, piped drain system.
Strong inflow of water or seepages from the faces of the pit walls indicate that layers of high-hydraulic conductivity are present.
Under these circumstances, the use of a piped drainage system is advised, at the depth of inflow.
This will remove ground-water, if a suitable outfall is available.
Conventional, piped drains, at depths of 0.8 to 1.5m, have been successful, but if there is deeper water flow, deep, piped drains are usually installed at a depth of 1.5 to 2.5m, and at spacings of 15 to 50m, depending on the slope of the land, hydraulic conductivity, and extent of the drainage layer.
Piped drains should be installed across slopes to intercept as much ground-water as possible, with open drains and main, piped drains running in the direction of maximum slope.
Due to the risk of drain collapse, deep drains are normally excavated with a tracked digger with a special, deep-drain, trapezoidal bucket with a bottom width of about 200mm.
For small jobs, a 300mm or similar-sized bucket may be used, but the side walls must be well-battered (sloped) to avoid cave-ins. While these drains are more difficult to install, they are very cost-effective, because so few are required.
Where ground-water seepage and springs are identified, drains which are 2m to 3m deep can be used to intercept flow.
Pipe drains are most effective located in, or on, the aquifer. Clean aggregate should be used to surround the land-drain pipe in conventional and deep drains.
The gravel should be filled to at least 300mm from the bottom of the drain, to cover the pipe. It should provide maximum connectivity to a layer of high-hydraulic conductivity. In drain lengths greater than 30m, stone backfill alone is unlikely to have sufficient flow capacity to cater for the water volume collected. Only short drains can operate at full efficiency without a pipe.
