backfilling a french drain trench

Offering reliable landscaping services to homeowners in British Columbia since 2013

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Natasha Maerz

Owner/Office Manager

 If you have drainage issues, and your yard is always wet, it is most likely because you have clay soil in it. If your yard was sandy, you wouldn’t have a water problem, because water can percolate through sand no problem, and not allow surface water to pool.

Clay soil does not allow water to go through it, and therefore the water will have nowhere to go.

French Drains can be backfilled by either using the excavated clay soil from your trench, or by using 1 ½’ round rock.

If you use the same material that you already have in your yard as backfill, you will have the same drainage issues shortly after your system is installed. Yes, a drainage pipe will be put in your French drain trench which should carry your water problems away; however, water has to be able to get into the pipe that was placed in the trench.

What ends up happening is the French drain system works initially because the clay soil is uncompacted, and allows water to pass through it into the drainage pipe.

Shortly afterwards though (2-24 months) this clay soil compacts just like the rest of your yard. Once it compacts again water will sit on the surface of your lawn.

 

Clay Soil Disadvantages

  • When there is no void space in the trench the surface of the yard dries slowly, and the subsurface never dries out completely. As we said before, this is because water cannot percolate through the clay soil.
  • Using soil gives your system a short lifespan, as clay soil will compact and not be able to provide the drainage you are looking for.
  • System is Expensive for how long it lasts (2-24 months).
  • Grass roots will be waterlogged due to standing surface water. This will cause a yellowish tinge to your lawn as its oxygen supply is cut off

 

Clay soil effects a French drain in two main ways.

1: The first is it creates standing water on the surface of a lawn. If you have ever made a clay bowl as a child you will remember that water will stay in this bowl almost indefinitely. Light clay soils will eventually pass water, but heavy clays will almost never allow water through it on its own.

Lawns with healthy grass roots will help to penetrate this clay soil, and provide microscopic paths for the water to get through. It generally takes a year for the grass to establish healthy roots, however if this is in a shady area, or an unmaintained area the grass may never provide adequate roots to penetrate the clay soil.

2: The second reason clay soil is a problem for French Drains is once the water does eventually make it into the ground, it moves laterally very slowly. This slow process causes water to back up on the surface and if not addressed will prevent the system from ever draining the water.

 

1 ½’ round rock can be used in place of clay soil to backfill the trench. When backfilling with 1 ½’ round rock, your system will have an ample amount of void space due to the size of the rock that is placed in the trench. Void space in a trench is important, because the greater amount of void space that is in the trench, the quicker your yard will dry. Void space allows both the surface and subsurface of your yard to dry out quickly by allowing air movement in the trench, creating an underground drying effect. The water that is on top of your yard is able to go through the round rock, and into the pipe. Once that water gets into the pipe, the pipe can take it to its discharge station, and your yard will be dry.

Best practises to overcome clay soil:

  • Install a French drain system with void spaces underground (large round rock). The void spaces will dry out the clay between rain events, and instead of starting a rain event with saturated soil, you will go into a rain event with dry soil that is capable of absorbing much more water.
  • Replace the top 2-4” of soil in the yard with free draining sandy soil. This will allow the water to get below the grass and move laterally to the nearest French Drain trench.
  • Increase the size of a French Drain system. The closer together the Drain trenches are excavated the shorter the distance the water has to travel to be removed. This will increase the speed in which your lawn dries up.
  • Grade the area to slope to a low spot and instal the French drain system there. This way the water will run on top of the grass and move quickly to a centralized location to be collected from.

What Material Should Be Used To Backfill A French Drain System instead of clay soil?

An ideal backfill for a French Drain system is 1 ½’ round rock. When backfilling with 1 ½’ round rock, your system will have an ample amount of void space due to the size of the rock that is placed in the trench. Void space in a trench is important, because the greater amount of void space that is in the trench, the quicker your yard will dry. Void space allows both the surface and subsurface of your yard to dry out quickly by allowing air movement in the trench, creating an underground drying effect.

The water that is on top of your yard is then able to go through the round rock, and into the pipe. Once that water gets into the pipe, the pipe can take it to its discharge station.