Hi everyone,
I’ve been dealing with ongoing drainage problems in my backyard since purchasing the property. Previously, I laid turf, but the ground stayed constantly soggy, and during heavy rain, water would just pool on the surface and sit there similar to first photo.
To fix this, I’ve removed around 200 mm of the heavy clay, graded the remaining clay base to fall toward a strip drain and agline, and installed two stormwater pits—one at each side of the backyard. I’ve also run ag pipe along the full length of the smaller grey retaining wall and another line alongside the strip drain.
I’m now planning to bring in about 200 mm of quality topsoil and lightly slope the finished surface toward the strip drain for quick runoff.
Is there anything else I should consider or add to make absolutely sure I won’t have any more surface water pooling or soggy patches? Any extra tips from people who’ve tackled similar clay soil drainage issues would be greatly appreciated!
by Kooky-Sheepherder-17
2 Comments
Create the slope toward the strip drain with something inorganic, hard and impermeable like clay and then top off with topsoil or sod, otherwise water will simply soak the topsoil mass and create a bog in that enclosure.
You have excavated a depression into impermeable clay and you are planning to backfill it with permeable topsoil. That topsoil acts like a sponge; water will rush through it, hit the hardpan clay bottom, and get stuck because it can’t infiltrate fast enough. Even with your ag lines (we call them French drains or drain tiles in the US), if that clay sub-base isn’t perfectly pitched to the drains, you will have a marsh floating on top of a clay bowl.
You need to treat the interface between the clay and the new soil. Do not just dump dirt on top of the pipes. You need to create a “drainage sandwich.” The ag pipe needs to be surrounded by 3/4-inch clean crushed gravel (no fines/dust), and that gravel needs to be wrapped in a non-woven geotextile fabric. If you skip the fabric or the gravel, the fines from your new fancy topsoil will migrate down, clog the gravel voids, and choke your ag line within two seasons. This is the subsurface migration syndrome. I usually recommend to lay fabric across the entire clay bottom, put the pipe and gravel down, and then fold the fabric over the top before adding the soil. This keeps your drainage system chemically and physically separate from your growing medium.
That tight 90-degree turn in the PVC is a choke point for debris. Water carries silt, and silt settles in corners. If you can, swap that hard 90 for two 45-degree fittings or a “sweep” elbow to keep the velocity up. Also, ensure that strip drain (channel drain) is set in concrete or a very stable aggregate base. If you set it just on dirt, it will settle over time, creating a lip that actually prevents water from entering the drain.