Agricultural drains (also known as ag pipes, agi pipes, ag line, french drains (in France?!?), soakage drains and slotted drainage pipes – phew!!) are sometimes used by homeowners and engineers to improve house subsoil drainage.
Sometimes agricultural drains are used to fix slab heave and reduce cracking in houses.
However, if agricultural drains are specified or installed incorrectly, they can easily be the cause of uncontrolled movement and cracking in your house.
What is an Agricultural Drain?
An agricultural drain is a slotted PVC pipe placed into a trench that is backfilled with rocks or sand. The purpose of the agi pipe is to drain away any water that is in the ground.
Groundwater or surface water that seeps into the trench falls through the rocks and is carried away by the slotted PVC pipe.
An agricultural drain is constructed by
- digging a trench
- placing a slotted PVC pipe in the bottom of the trench
- sometimes the agi pipe is protected with a geofabric filter (a sock that keeps the dirt out)
- backfilling the trench either to the surface or just below the surface with 10mm or 20mm gravel
- sometimes the entire trench is wrapped in geofabric filter material for maximum protection from silt
How Agricultural Pipes Wreck Houses
Agricultural pipes are meant to remove water from the ground. Agricultural pipes wreck houses when they become the source of moisture in the ground.
Do not connect your ag pipes to your stormwater pipes. Run separate pipes.
If your agricultural pipes are connected to your roof downpipes, every time it rains water can flood back into the agricultural pipe and flood the trench that is meant to be collecting water!
That water then soaks into the ground and can affect your house footings.
Never connect agricultural pipes to your roof stormwater system!
The Best Way to Install Agricultural Pipes
This is the best way to install agricultural pipes:
- If water is already on the surface, use spoon drains rather than aggi-drains. Surface water should be managed at surface level. Ag pipes are for water that is already in the ground.
- Connect agricultural drains to their own pipework system. Do not connect agi pipes and soakage trenches to the roof stormwater system.
- Ensure the agricultural pipe AND the bottom of the trench are graded in the direction of water flow. Water in and around the pipe should always be able to drain by gravity along the trench.
- Position agricultural drains no closer than 1m from your house footings. Use solid pathways or grade the ground away from your house for the first metre.
- Never run slotted pipe drains UNDER your house. Use only solid pipes under your house. Try not to introduce water under your house.
- Use silt pits where required by the National Construction Code. Don’t forget to keep silt pits clean of silt and debris.
- Use geofabric and geotextiles to keep silt out of the soakage drain and pipe.
- Bring the end of an ag pipe to the surface and cap it so that the pipe can be flushed out from time to time.
- Discharge the low end of the agi pipe downhill of the house you are protecting or into the stormwater system downhill from the house.
More About House Drainage
Keeping water away from your house footings is one of the best ways to improve the performance of your house. This is especially true if your house is built on reactive clay soils and you are trying to avoid slab heave.
The Australian standard for residential footings and slabs (AS2870) stipulates 50mm ground fall away from your home over the first metre.
This is called surface drainage and it is getting harder to achieve on small sites.
- Some buildings are built right to the boundary and you don’t have control of the ground levels or surface drainage next door.
- On sloping blocks that are dug out to create a flat block, stormwater that falls between the house and the retaining wall has nowhere to go unless there is good surface or sub-surface drainage.
In these cases, your engineer and plumber should have a chat.
When Should Agricultural Pipes Be Used?
Agricultural drains should be used as a last choice when management of surface water isn’t enough to improve the performance of your house.
Sometimes water is already in the ground and it needs to be removed. This is the true benefit of soakage trenches and agricultural drains.
Subsoil drainage systems should be used when:
- Water is soaking into the ground uphill from your house and running through the ground towards your house.
- Behind retaining walls.
- Adjacent to basement walls.
- Adjacent to deep footings.
- On the uphill side of cut and fill sites.
If water is on the ground try your hardest to achieve drainage by overland flow rather than letting the water soak into the ground and be collected by an agi pipe.
A Quick Summary about Agi Pipes
Don’t connect agi pipes to the stormwater system
Do make sure soakage trenches slope downhill
Do keep agi pipe trenches at least 500mm from a house
Do use soakage trenches to remove water from the ground
Do discharge water from soakage trenches downhill of the house you are protecting