How deep do rocks migrate

[FJ and G]

I'm beginning to think that rocks are alive and have bird-like migratory tendancies. When I removed my outdoor train shed and brought it inside for repairs (another post describes the repairs), I found that the 1:1 sized ballast rocks had slid down into the clay and when I stepped on them, they went down even further.

The structure weighs about 200 pounds when filled w/trains and now I'm wondering if the structure might disappear into the clay? Actually, I don't think this will happen (tongue in cheek) but at the least, it could cause the structure to sag.

What's the solution to the migratory problem?

Should I use cement-sized blocks as a foundation or will they too disappear?

I was out walking the beagle this morning (actually she was walking me) and I went thru a construction site and noticed that sewer pipes were put about 4 feet into the ground, laying on ballast rocks, topped with crusher fines (just like I'm using for my roadbed).

Perhaps they too are trying to keep the pipes from disappearing.

Your solutions to this sinking problem would be welcome.

[Capt Bob Johnson]

Foundation is the key!

Perhaps some of the 2" thick 8" X 16" cement blocks on a bed of crushed stone or gravel and all leveled up with each other would provide sufficient footprint to keep your structure from sinking or sagging!

Another way to do it would be to excavate the area about 6" deep and fill it with crusher run, level it off, then place structure.

Third idea would be to put about a 1" layer of crushed stone, use 1/2" thick cement board ( I like hardieboard) as a structure base, then place structure.

[tangerine-jack]

I know the feeling Dave. The soil in VA is very soft, especialy after rain. You need to dig down about 12-18", fill with 3-4" of gravel/sand, then put down some brick or cinder-block. You've got to get beneath the clay into the hardpack or your stuff will disappear forever. I've had to do this countless times on everything from my walkway flagstones to my flower bed trim. Yes, it's a little work, but the alternative is to restone your foundations every few weeks.

[grandpopswalt]

In my area (Northeast) the opposite is true. We have frost heave problems. I've seen three foot diameter boulders migrate up out of the ground 2" to 3" over the course of several years. Any structure whose foundation is not below the frost line (42" in these parts) will, over time, be pushed upward.

Walt

[FJ and G]

Jack,

Ok. Could I reverse what you did and have the flagstones on the bottom layer, followed by ballast on top?

[ttrigg]

Dave:

On the bottom layer you will need to use stone, to achieve proper drainage. Use at least 3-4 inch of small (3/4 inch minus) then use cinder block or brick. The flagstone would be too expensive (at least out here on the west coast) to use as foundation filler.

[tangerine-jack]

QUOTE: Originally posted by FJ and G

Jack,

Ok. Could I reverse what you did and have the flagstones on the bottom layer, followed by ballast on top?

I would recommend against doing it that way. The stone acts as drainage like ttrig said, then the base blocks rest on top nice and firm. If you like you can cover over the base blocks with another layer of stone for visual effect; the base can be slightly below ground level if needed.

Depending on your yard, you may not have to go down as far as 12", you'll see the top layer that is all spongy, then there will be a clearly defined layer of packed dry clay. It's the dry level that you want to get to. I've done from 4" to 18" on different things, which is why I said 12-18" in my first post, with 12' being the average and mentaly prepping you for up to an 18" dig.

[FJ and G]

Thanks, all. About 18 " of clay here. I'm talking CLAY. You can actually model with the stuff!

Below 18" is a layer of dark shale. I suppose that is "bedrock". Below that must be the Earth's molten core.