Tips on constructing an Agricultural or Soakage Drain,


Tips on constructing an Agricultural or Soakage Drain, the graphics are hand drawn and as I am not an artist please accept my apologies for any lack of artistic quality.

If you are thinking of putting in an agricultural drain, you have probably by now determined where your water is coming from, so you will have some idea which direction the drain will flow and where to. So I will talk about actual construction as I will not be able to work out your site, or actual depth of drain.

You can either hire a ditch witch and then dispose of the earth that is removed from the trench, or you could employ the use of a local contractor with a dump truck and Bobcat or similar piece of equipment, this will save you a lot of extra toil, as he can load the earth into his truck and take it away for you, there may be an extra charge unless you find somewhere for him to dump the solid fill.

Ideally the trench should be 8 inches (200mm) wide and on the average around 2 foot to 3 foot (600mm to 900mm) deep, the length will be determined by the area you need to collect the water from.

As per the above diagram lay newspaper (about 6 to 10 pages thick) along the bottom of the trench, the paper acts as a barrier to collect water before it soaks through the bottom of the trench and it will take quite a long time to decompose that far down.

Lay in the 4 inch (100mm) PVC Agricultural pipe (the type with elongated slits along the length)[see bottom diagram].

Backfill with 3/4 inch (20mm) screened gravel fill to approximately 4 inches (100mm) from the top.

Lay a double layer of old shade cloth or shade cloth off cuts over the gravel the length of the trench, in some cases your landscape nursery may have old cloth for free or very cheap. The cloth is needed to prevent the drain from silting up too quickly.

Over the top of the shade cloth lay your new soil/sand and re-seed or turf as needed.

The reason that it is best to use PVC pipe is that in years to come when the pipe clogs, you are able to then use and electric drainer's eel to clean it out, and rejuvenate the drains capabilities. You have the option of using the black flexible pipe but it is pretty hard to ever clean it out, unless you dig up the drain.

If the lay of your ground does not allow you to run the water off into a culvert or storm water drain or open land. Then you will need to run it into a soakage trench or pit.

You will probably need to work out the soakage rate of the ground to determine the actual dimensions of this trench or pit.

After the trench/pit is dug back fill with the 20mm gravel top with old shade cloth, soil/sand and re-seed or turf as required.


How we determined the placement of the ag' drain we were involved in installing.

First up like I said I had done a couple or 3 or 4 years of observation, this water came travelled through our neighbours foundations and flooded our front yard in times of heavy rain and the water continued to flow through his foundations and across his underhouse storage garage area, so at the least inconvenient for him as it was not dry storage then, for some days week whatever after the rain event.

Anyhow after a period and as any good neighbour might I began to sell him on the idea of an ag' drain, yes i will be bitterly honest here if he didn't do it above his house we were going to have to do it along the bottom side of his boundry in his yard to benefit us alone, there was no option to do anything in our yard. but it would hardly be worth our effort for some front yard flooding.

We got on well and he could see the issue with probable failure of cement slab and cement stumps. so i talked him through how I saw it.

He was situated near the bottom of this app' 6% slope. Now at the top of the slope was another street that storm water went direct to the adjacent wetland, a couple houses right at the very top app' 200 meters up and away. so that meant abut 5 houses could not gravity run their storm water to the street or storm water drain as they too were below the slope.

So as our enquiries found, the builder developer was permitted to put in a large rubble drain down the inside of the back property lines of these affected house, this rubble drain was fed into a gravel sump (huge hole filled with gravel to allow the trapping of certain water and let it permiate into the clay sub soil) yes all soil was clay.

Needless to say of course these sorts of systems provide a cheap remedy and are council apporved but have serious limitations, you see clay makes for an ideal dam, farmers can't have dams without clay soil, once clay saturates no more water will flow into or through it, why dams work so well. Later on at another suburb our next door neighbour put in a large car apron creating a catchment of which I indicated to him he could not pass that water down to us, so his builder put in such a drain system, they made it obvious they did not like having to do this, and yes in heavy or continued rain it all flowed out the top of the sump across our back boundry into other yards. it was like a geisher.

The best most expensive treatment would have been to seek pemission to put a pvc drain through 2 yards (these 2 yards efected from the overflow and it would have saved another 2 from ever being affected) to the lower storm water drain, you can imagine expensive hey (app' 50 meters in length) and this overflow water was never going to affect him, out of sight out of mind.

Anyway this drain scenerio had same/similar issues, the builder could have gotten pemission to install a pvc drain down to the wetland then there would be no problem for any others, I look carefully now at topography before we buy.

So that was a major issue in my neighbours issue, some hidden ones we found the digger guy helped here also was a disused septic tank with the top smashed in so they could fill it with dirt but they didn't punch a hole through the bottom which would have helped, the tanks should have been removed but guess the owner then was not going to fork out the cash. This was the cause of why the water continued until the level dropped in that tank.

The digger bloke pointed out that the old leach field pipes all had a downhill flow back toward the house, a bad plumber I would say, must have been issues with the owners who had it then? so we factored all that in as the job the ag' drain had to control. the old septic system actually had some uphill flow factor in it between the tank and the leach field pipes. Legacy of the house being built without owner supervision (probably a spec' home)there is a lesson to be learnt there, with the home we built we were there each and every day even then they tried short cuts.

We ran the drain and connected it to a lower storm water pipe to evacuate that water away from his house to the gutter, we use pvc in leiu of black flexable as the pvc can be cleaned at anytime using a plumbers eel. You can in fact make your own pvc ag' pipe by buying normal pipe then with a grinder cut slots down 2 sides.

Anyhow the job was a success beyond all expectations.


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