Subbases and Subgrades for Concrete Foundations
A concrete surface is only as stable and level as its subgrade and subbase. Weak subgrade and subbase layers can settle differently across the surface and cause cracks in the concrete. Any concrete project needs at least a compacted soil subgrade layer. A thick gravel subbase and base may also be required, and are usually worth the effort.
How to Prepare the Subgrade Level
A sidewalk or other concrete surface that isn’t taking heavy loads can stand to have only a subgrade layer under the concrete. However, even for such simple projects, it may be advisable to add a gravel subbase to improve drainage and help level the surface. A subbase also will make the concrete work easier because a soil subgrade can get muddy and make a mess during the concrete pour.
Either way, the subgrade soil needs to be flat and compacted. If you are adding a subbase, dig down least four inches deeper than the thickness of your concrete, and compact the soil. Also dig out the soil at least six inches laterally beyond the project outline. You may need to add or remove soil to level the project area. Level the soil before adding a subbase so the subbase and concrete will have a uniform thickness.
The compaction process varies according to soil type. Sandy, granular soils are the easiest to compact and cover. Use a vibratory plate compactor to push the spaces out between particles of granular soils. These compactors look like large gas-powered lawnmowers and can be rented at some home improvement stores. If the soil is predominantly organic, like a rich gardening topsoil, several inches of soil will need to be excavated and replaced with a more granular fill soil.
Clay-heavy soil poses a greater challenge. Laying concrete slabs or even a driveway on expansive soil without careful engineering and adequate drainage can cause major headaches. For small projects, rammers are best for compacting clay soils.
How to Make the Subbase
To make the subbase, pour gravel about four inches thick, flatten with a screed rail and check for level. Then tamp down so it is about four inches thick. Flatten and level again. Particularly for projects adjoining or close to the house, ensure that the subbase slopes away from the house about 1/8th inch per foot.
An additional option is to replace the top inch with a finer gravel or sand base. (You can also add this type of base on top of a full four-inch subbase.) The finer base will make it easier to keep the concrete level, and require less concrete, which can be expensive.
Once the base is level and thoroughly compacted, check your work by rolling a heavy truck over it. The truck tires should sink uniformly, and not more than 1 inch into the soil.
Proper preparation of the soil subgrade and concrete subbase are crucial for a successful concrete project. For expert help, hire a reliable concrete contractor.
Updated February 13, 2018.