One of the biggest problems folks tend to have around here is that the topsoil is so thin that rocks from below ground will easily work their way up to the surface. I'm having the reverse problem. I've got a couple little garden plots in the yard and the rocks are all sinking into the ground. I'd kind of like to prevent this, but I've no idea how to do it. I know I can dig the rocks out and put more dirt under them, but that will only work for a short period of time, until the dirt compacts some more. Is there some way I can put something under the rocks so that I don't have to worry about them sinking for a long (say 10+ years) or so? Or am I just SOL?

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Are these stepping-stone sized rocks or larger (not flat)?

I'm not entirely sure of this, but I thought that one contributing factor for sinking stepping-stones is the activity of earthworms. By churning and reworking the soil under and around the stones, it causes them to sink . Maybe you have a lot of worms in the soil.

They're slightly larger than fist size and all different shapes.

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That's interesting. In areas that get deep frosts, rocks of that size often seem to "grow" out of the soil, like you've already described.

My guess is that you need something that will resist the slow and steady compaction from people stepping on them as well as undermining by soil organisms, in other words, a foundation. Where I live, we also have the problem of sand that is placed on icy streets and sidewalks in the winter, some of which finds its way onto lawns, causing them to rise over the years.

I've done brick walkways in front of a couple of homes I've owned. In each case I prepared the base by digging out the extant soil (clay and silt) to a depth of several feet, replacing it with coarse gravel that I machine-packed, and capping with 1/4"- sand that I could screed to a uniform surface. Neither of these walkways have shown substantial movement in many years, and this is an environment that gets deep frosts that often cause heaving in wet, heavy soils. I think that the environment I have created under those bricks is well drained and inhospitable to most creatures except ants. It was lot of work, but I think it was worth it.

There's nobody save some cats walking on the rocks and certainly no deep frosts. And while I appreciate your suggestion, that's a bit beyond my means at the moment.

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Easy (with no extra digging).

1. Pull the rocks and dirt.

2. Place large, 1/2" to 1" thick closed-cell foam (polyurethane) sheet on the bottom of the pebble bed (cut to countoured shape), wrapped in black plastic of the garbage sack variety (available in wide rolls). This keeps the white foam from showing through. Ensure you cut a piece larger than the foam, then tuck the edges beneath the foam.

3. Replace dirt.

4. Replace rocks.

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According to Darwin (The Formation of Vegetable Mould Through the Action of Worms) earthworms wriggling around, doing their thing, causes stones to sink.

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it's beyond your means to dig a few holes and pack some gravel in there to stabilize things? unless we are talking about a huge area here, the monetary outlay will be minimal- whatever a pickup truck full of gravel costs- and who doesn't like playing with shovels and rakes?

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Tuckerfan, I think you need to put something under the rocks to make them “float” on top of the ground. It could be that when it rains, the soil becomes soft and gravity is pulling the rocks downward into the mud. It would be like the difference between putting a rock on top of dry ground, and then putting the same rock on top of mud. It will sink into the mud. I think mugaliens idea should work, with some pieces of thick foam placed under the rocks. I think the foam should be slightly larger than the diameter of the rocks. You can get a sheet of this kind of foam at a large hardware store, and then cut it or break it up into small pieces.

Did you miss the part about "depth of several feet" and "machine-packed"? Digging several feet with a shovel is not exactly easy (I know, I've done it more times than I care to think about) and seeing as how I don't have a pick up truck, I'd have to pay to have gravel hauled here (can't fit a load of gravel into the trunk of a Honda Prelude very well), which is expensive with fuel costs being what they are. Then there's the "machine packed" part. Again, renting said machine, getting it out here, and using it, is beyond my means at the moment.

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Tuckers! Science! Automotive Oddities! Boycott Trek XI! Building my hot rod with the help of the intarwebs Those who would delay scientific progress for a little temporary prosperity shall have neither.

I think you should find out for sure what the real cause of the sinking is
before trying to combat it. Some garden stores do a lot of business in
decorative garden stones and paving stones. Ask them. If they aren't
sure, try another one.

I'm sure that Better Homes and Gardens has put out books explaining the
problem and how to deal with it. A quick look through their titles should
turn up the right books. The library will have copies.

Mugaliens' idea sounds weird. Torsten's suggestion is appropriate for a
walkway in the freezing zone, not for mere decorative, softball-size rocks.

-- Jeff, in Minneapolis

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Hmmm... A lot easier and cheaper to do this.

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I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that.

I am human. Fully human.

My idea isn't my idea. It's the idea of a 6-figure a year landscape architect who's designs grace many corporate and government office buildings and city parks.

She's been doing it for 26 years, lives up the street from my parents, and their family and mine are friends.

Regardless of how weird it may sound to you, Jeff, the proof is in the pudding.

It simply works.

PS: It doubles as an excellent barrier to weeds with deep tap roots, as well as most flora.

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I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that.

I am human. Fully human.

Ground cover - available in large rolls from most home/garden centers. You have to remove all the rocks that you want to keep - lay down the ground cover, and vola' no sinking rocks AND very few weeds...at least for a few years. We zeroscaped ~4 years ago with ground cover and it should last a few more. But as dust blows in, the rocks will eventually be covered. It doesn't sound like you are in a dusty area, so the ground cover should last for a while.

Oh, and if you want something to grow, cut a hole where you want it.

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Yeah, I probably should have said "This is probably overkill for your particular problem", but I just couldn't resist writing about that particular success.

Regarding Mugs' idea, perhaps the low density of the foam platform contributes to its success, in effect, helping the rocks to float relative to the other material. I wonder how well it works where there are lots of worms in the soil.

Unrelated to this, on a trip to Finland I once saw foam being installed in a roadway where they had heaving problems. It was in an area of wet soils. I think the idea was to insulate the warm earth below from the cold, and prevent its expanding in the winter. If you've never experienced the kind of heaving that happens to a poorly built subgrade in a cold winter it's hard to appreciate the frustration it causes motorists. The heaves can be bad enough to launch the vehicle and have it bottom out on landing. Terrible phenomenon.

Provided the rocks are forming a pebble bed (usually white, rounded river rocks are used for this), it works great, as the foam + rocks are less dense than the loam that's highly aerated by Earthworms. Boulders would tend to overload it *a bit,* though...[/quote]

Good comments, Jerry.

Plain ground cover should work just fine for sandy or other dense soils. For lighter soils, you'd want to use something to lessen the density of your system (aka, the foam).

As for dust, at my folk's, we hit it with the blower a couple times a year, and about 1 year in three, we remove the rocks, give them a nice bath in a washtub with a very mild (small) addition of soap and the naked end of a broomstick, and sweep and blow the settled dust/dirt/debris before replacing the rocks.

__________________
I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that.

I am human. Fully human.