Is it OK to use washed, smooth gravel under a pier block footing?

Question

I am preparing footings for an outbuilding, using pier blocks. Our frost depth is only 12". There is a significant amount of clay in the soil, so I am putting gravel under the pier blocks.

The plan was: excavate holes to 12". Fill with 4" of 1 1/4" rock, compacted. Then 4" of 3/4" rock, compacted and leveled. This leaves the pier block 4" below grade. Fill around the block with soil, to resist sideways movement.

I have excavated to frost depth and bought a load of gravel. It's not exactly what I expected: it's smooth, round drain rock, and I was expecting coarse crushed rock. I assumed that coarse rock was important to make a stable base for the footings.

Is it OK to use round, smooth rock in this application, or would that doom my building project?

Answer 1

Really Jay, that is the wrong stuff. I wouldn't use it alone. It will not pack properly and will have a tendency to shift and let water rush through and undercut the surrounding soils. You really need a good sandy gravel that will pack hard. I'm absolutely sure you will regret using washed round stone without a binder. You could add Sacrete mix to it to firm it up. That would help. Mix the sacrete with the stones dry and just wet it down with a hose or a couple of gals of water, let it set up and continue. The stones will become the aggregate in the concrete mix and make an excellent base for you.