What is the purpose of a 6" base gravel layer in patio installation?

Question

All standard drawings for patio installation show a base gravel layer. What is the purpose of this layer? Depending on the answer, I am curious what this forum feels about this product as replacement for the base gravel layer: https://www.alliancegator.com/products/gator-base/

My thoughts.

A base gravel layer (with different stone sizes, larger at the bottom) is clearly needed for permeable patios to absorb/hold the water. But the base layer was required before permeability/water runoff became a thing, so I wonder if there is an additional purpose? Or has water absorption to prevent pooling/runoff mitigation always been he reason in which case I don't see what problem his product solves.

Thanks,

Answer 1

It mainly serves two functions:

1. The density of the in-situ soil usually is uneven (due to voids) throughout the site, and difficult, if not impossible, to compact to the desired strength (eliminate/minimize the void spaces) to avoid uneven settlement that would lead to structural cracks. A 6" course of gravel fill solves that problem, as it is more rigid to work with.

2. For cold regions, the groundwater or moisture present in the finer in-situ soil tends to rise to the surface that is prone to freezing and will be subject to the subsequent heave. During freezing, the volume of the water expands against the slab, thereafter, void spaces are created during heave with soil softening as a result. The gravel course provides an escape for the undesirable water and moisture, as it has larger voids to accommodate/minimize the expansion, cut down, if not cut off, the upward migration of water. Note that if the groundwater level is too close to the ground surface, you might need a thicker layer of coarse gravel, or to lower the groundwater table mechanically.

Note, as mentioned in 1 above, good compaction is absolutely essential to ensure a quality and durable slab/foundation. Don't skip it.

Answer 2

I think the purpose is to make money for a vendor. I have layed two patios, one in northern IL ( freezing) and one in east TX ( no freezing but average 50" annual rain). I layed them both on the native soil which were both very sandy. No problems, I admit the sandy soil may have been a help.