The nitty-gritty from site inspection to excavation
Published 27/05/2015 | 02:30
The initial site inspection allowed the team to establish natural outfalls, slopes, areas of poor growth, existing drains and natural water courses. A detailed chat about the history of the field revealed that there had been a previous open drain that was covered in on the western side of the field.
Test pits were dug to examine the soil profile. These went deep enough to reveal the point where the water table started - in this case at about 5ft.
The team decided that the water table should be lowered, requiring a groundwater drainage system.
To open up drains at this depth, the channel was dug in two stages, with a wider tapered moulding bucket used for the first metre, before switching to a narrow tile drainage bucket.
While the narrow channel minimised the amount of stone required at 5ft, the wider section at the top of the drain ensured that the channel did not collapse during construction.
Pipes were laid at 5ft in the permeable layer where water was naturally able to travel. Large stones of up to 3ft in width had to be dug out at points to allow these to be laid, which added to the costs involved.
"We never had a shortage of stone here so we tended to do without pipes, so using them was new to us, but we can see the benefits in terms of the amount of water that is coming off the field," said Con's dad, Neilie.
Significantly, at 89c/m, the 80mm pipes did not add significantly to the overall cost of the project, coming in at €525/ha . While these will probably need to be jetted or rodded every two to three years, the Lehanes have the peace of mind of knowing that they can maintain their investment.
Approximately 40cm of 0.5-2 inch stone was used with the pipe. While there are 'no-stone' piping alternatives, where a fleece is used to filter the water entering the pipe, the Teagasc Heavy Soils team were not keen on these systems.
"The jury is still out on these but I'm not a fan," said Pat Tuohy. "Stone is proven. It doesn't have to be round but it does need to be clean. So watch out for chalky stone."
While the natural slope of the field was to the south, the field drains were routed west-east, at 20m spacings, in order to get the water to the deep open drain.
This meant that they were almost flat in some parts of the field, a situation that Lehane's digger operator was not too impressed with.
"Don't worry too much about the fall," Mr Tuohy assured farmers. "A one in 800 will actually be enough, which is very little really. If the water is able to move, then it will find its way out."
In addition to the significant deepening of the open drain on the eastern side of the field, the open-drain at the top of the field was also deepened in order to intercept the flow of water from uphill.