How to (NOT) Drain a Patio Using Decomposed Granite
I have an interesting drainage problem on a patio surface made of decomposed granite. I am looking for proposed solutions, but first, let me describe the problem. Someone who follows @tapla (mid-Michigan, USDA z5b-6a) 's work on perched water tables will see here that this science applies to domains other than container gardening.
https://www.houzz.com/discussions/6167237/container-soils-water-movement-and-retention-8-26-21
Someone helping me to design the area around a water fountain insisted I use decomposed granite. I don't like the material because normally in the rain, it develops puddles of water on top, and they can be very slow to drain and dry. So I took on the challenge of how to make a decomposed granite patio drain as well as it can drain, and unfortunately, I only solved half of the problem.
I dug a deep ditch about 12 inches deep where the patio is and lined it with landscape cloth, filled up to 8 inches of that with 4 to 8 inch rock, and then I covered it with another landscape cloth. The point of that was to create a drainage basin under the patio, which is in a part of the yard that has a flooding issue in the rain. That drainage ditch works fantastically well. and it removes all issues with water saturating the subsoil under the patio. If I had put crushed rock on top of this, I would have had a very nice patio that is incredibly well-drained.
Unfortunately, I put decomposed granite 4 inches deep on top of the landscape cloth. The problem is that the decomposed granite sitting on top of the landscape cloth is very similar to a container, rather than a raised bed. That container is forming a perched water table! After I saw what was happening, I researched, and in horror, I discovered that decomposed granite in a container will have a 6-inch perched water table. And the entire depth of my decomposed granite is only 4 inches.
That causes water to flood the patio and stay there for a long time, not draining through the landscape cloth. As in a container soil scenario, the very small dust-like particles of the decomposed granite have huge surface area, and this is causing both adhesion of water to those particles and cohesion of water to itself. This forms a stable perched water table, and the decomposed granite is simply not deep enough to overcome this perched water table.
What are some solutions? I want the patio to drain rapidly.
I tried adding 20% 1/4 inch and 3/8 inch crushed rock into the decomposed granite, but this did not overcome the issue. Decomposed granite is too much like clay, and even a small amount in a container creates substantial perched water.
I could redo this and make the depth of the decomposed granite around 10 inches, which gives 4 inches above the top of the 6-inch perched water table. I'm testing around this idea now in a large nursery container.
Tapla uses wicks to help reduce a perched water table in a container. I guess I could use a garden pot wick every two feet throughout the small patio area to help with the drainage. But I am skeptical that a wick will sufficiently drain the area, and I probably need to combine wicks together with a thicker layer of decomposed granite.
In truth, I probably just need to take out the decomposed granite and go back to the idea of crushed rock.
If anyone sees another solution, please share.
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@tapla (mid-Michigan, USDA z5b-6a) The decomposed granite is the walking surface. I added a simple schematic to the original post, and I am copying that here as well.
The total area is small, let's call it about 100 square feet.
It would be difficult to grade the surface underneath the decomposed granite. I'm not sure that would be less work than other solutions, and my guess is that it won't fix the issue well because the small particle sizes in the DG create a strong capillary action/adhesion/cohesion. In other words, grading the subsoil/surface under the DG helps but does not really "fix" the problem.
As I see it I have a few ways to go on this:
1) I can replace the DG with crushed rock that has particle sizes > 0.1 inch / 2.5 mm, and the problem instantly disappears as there will be no perched water. The person who wanted DG will throw a fit, but that's my problem.
2) I can amend the DG with some other crushed rock, trying to greatly reduce the perched water in the mix. For example, I have a beautiful gold 3/4" crushed rock, and I could try to densely pack that and let the DG fills spaces between those rocks. There would still be a lot of DG. Getting specific, tightly packed 3/4" crushed rock has about 30% to 40% void space between the rock. That's all space that the DG is going to fill. Having my mix be 30% as decomposed granite is probably not going to avoid perched water?
ChatGPT suggested this mix:
55-60% primary 3/4" crushed rock as the base.
25-30% intermediate filler like 3/8" crushed rock (#8).
10-15% fine sand or 1/8" screenings for further void filling.
Less than 5% DG to act as a binder but not create perched water.
How would you change that?
3) I can add wicks throughout the patio, but I think this, at best, speeds up the drainage of the perched water, but doesn't really avoid the fact that the patio will be a muddy mess for many hours.
4) I can add wicks and also increase the depth of the decomposed granite to 10+ inches, to put the perched water far below the surface of the patio. That's a lot of work.
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Just for kicks, I put 10 inches of decomposed granite into a container and filled it. The water is draining about 1/2 inch per hour. After three hours, it's still a lake!
tapla (mid-Michigan, USDA z5b-6a)