stonewalling

October 17, 2005 4:50 PM

Building a stone wall: crushed stone foundation necessary?

I'm having a couple of retaining walls about 1.5-2 feet high (about 30 ft. long) built, to create terraces where there is currently just a slope. The wall will be made of flat irregular slate stones, dry-fitted (no mortar). Everywhere I read I see that it's best to have a foundation of 4-6" of compacted gravel underneath the wall for drainage purposes. My landscaper says there's no need, as the dry-fit wall will allow drainage through it, and besides the ground underneath is already packed hard like concrete. Crushed stone foundation: necessary or not? The polls are open.

posted by stupidsexyFlanders to home & garden (12 comments total)

I haven't built any walls myself but I researched it on and off for two years. As far as I could tell, the gravel base was used to make sure there would be no settling of the wall material. If you have a 4-6" bed of compacted gravel, then you have a stable surface that won't shift or fall or rise no matter how much water might hit it. Depending on your soil type, it's possible you could have this beautiful long low wall start to fall apart if in a year or two a torrential downpour causes the soil underneath to shift.
posted by mathowie at 5:21 PM on October 17, 2005

I use gravel + sand. Or, alternatively just sand. Compact it as much as you can, and you will have a very nice, stable base.

As to my sucess ratio? 50/50 - one wall is perfect, 5+ years later. Another had to be demolished due to it's leaning tendencies - however, in my defense the second was built on a slope, and I did not use proper levelling devices.
posted by jkaczor at 5:35 PM on October 17, 2005

My boss mentioned that his wall in his back yard had collapsed the other day, so he'll be out on Wednesday to fix it. His was also slate, but was about four feet high and about 20' long. He didn't put a bed of gravel down underneath it, and is going to this time.
I'm not sure how much the lack of gravel was to blame, but he's doing it. Just came to mind when I saw your question.
posted by klangklangston at 7:46 PM on October 17, 2005

If you live in an area subject to hard freezing, I would recommend a gravel base. Frost in compact soils tends to expand and heave, pushing upward unevenly. This action can ruin a nice wall quickly.
posted by Hobgoblin at 8:29 PM on October 17, 2005

Compacted gravel base and backfill the area immediately behind the wall with gravel to allow drainage.
posted by electroboy at 8:53 PM on October 17, 2005

As electroboy implies, we've been discussing two issues, not one: minimizing settlement of the wall's foundation and preventing build-up of water behind the wall.

You don't want the latter because it can push your wall over, particularly because it's dry-stack. Pea-gravel (the rounded stuff, usually sorted to be all one size) wrapped with filter fabric (a mesh kind of sheet to prevent soil from plugging up the spaces between your gravel) will distribute any accumulated water along the length of the wall and stop any local accumulation of water. A belt added to these suspenders would be a perforated drain pipe buried in the gravel at the base of the wall to carry any water around your wall, although this would be overkill for the wall you describe unless it rains like crazy where you live and sub-surface runoff is funneled through your yard.

Given that the ground currently slopes rather than steps, it's unlikely that your new wall is going to be resting on undisturbed hard-as-concrete soil after the dirt is redistributed to your satisfaction. The crushed rock (a different animal than the pea-gravel - this stuff is jagged and has a variety of sizes and is designed to pack together with the interstitial spaces filling up so it doesn't shift around) fills the space between the undisturbed hard-as-concrete soil (which probably isn't at the elevation you want the base of your wall to be) and the bottom of the wall.

The real question here is why your landscaper is recommending against it. Did you agree to a fixed price to do the work? If so, it's in the landscaper's best interests (at least from a short-term financial standpoint) to do as little as possible and this is a way of increasing the profit. They should do it right and not charge you more, unless they specifically excluded it from the job. Or did you go for time & materials (where the more they work, the more you pay) and this is helpfully trying to save you money? If so, you're on the hotseat.

Feeling devil-may-care? Leave out the crushed base. Your walls aren't very tall and you might get lucky. Want to do it the way it's supposed to be done? Then listen to the accumulated wisdom of AskMeFi and go with the crushed rock as a foundation and backfill (i.e. put stuff right behind the full height of the wall) with pea gravel. Hopefully any extra cost won't push things beyond what you can afford. When the thing gets built, you want to be able to look at it and think "Damn, that's a handsome wall" not "Oh, s***, hope it doesn't fall over".

On preview, perhaps I should change my name to "verbose" or perhaps "parentheses".
posted by skyscraper at 10:08 PM on October 17, 2005

If you know how to build a stonewall you don't need a base. But it is definetely a skill so if you have never done it before or live in an area where it might freeze then it's probably a good idea.

The key is not to have any stones than slope inwards. Tough to do but it will stand for ever that way.
posted by fshgrl at 11:14 PM on October 17, 2005

It depends on the site conditions and the weather in your area. Are you in the dry and arid southwest, the wet and warm southeast, or the midwest? The needs differ based on the area and of course other onditions on site.

Generally if a 2 foot high wall is built with a foot below grade you won't have any problems. With the expense of slate you probably don't want to put any below grade but you should have something more stable than topsoil. Clay would be somewhat better but that is generally deeper depending on the region that you live. If he is going to build it starting at the grade of the ground on topsoil then I would start to worry. But that height of a wall will not be a problem unless there is lots of water around as well as lots of frost. No wall should be built on topsoil, period. So make sure he removes it if necessary and replace it with something like crushed stone.

As always, you should ask for references and see for yourself other similar work that the landscaper has done before you agree to anything. If he is a hack, then you will find out quickly from the quality of his work elsewhere. At least ask friends or neighbors or co-workers.
posted by JJ86 at 1:35 AM on October 18, 2005

Thanks, great answers, confirming my inclination. I'm going with the crushed stone base but taking my chances re: the gravel backfill. I'll post pictures to MeTa in thirty years and let you know how it turned out.
posted by stupidsexyFlanders at 4:08 AM on October 18, 2005

My husband has built many such walls, and always swears by "battering" the wall (see A stone wall higher than about 30 inches looks better and lasts longer if you build it with "battered" walls—thick at the base and narrow at the top)
. He never used a crushed stone base, and the two longest are still perfect more than ten years later.
posted by dbmcd at 9:58 AM on October 18, 2005

Belatedly, I see that, as these walls will be creating terraces, they will be backfilled. I now think that your biggest problem is going to be the pressure on the wall from the soil behind it. I would not expect a dry fit wall to withstand the pressure - even such a minimal wall as this.
ymmv.
posted by Hobgoblin at 4:03 AM on October 19, 2005

I've built several walls in my yard without any crushed stone base. All of them are mortarless field stone retaining walls. Never had an issue with heaves or water building up behind them. I think it depends more on your soil condition. The yard I've done my walls in, is filled with boulders and clay once you get down below the topsoil.

The oldest walls I've built are now 8 years old, probably around 200 feet of wall by hand. I built them like the ones my father built in the same yard 20+ years earlier and those walls are still in place with no problems. Drive through New England and you'll see stone walls in the woods that are all over 200 years old. If you think the farmer building those walls laid a stone base first, guess again.

Just start with larger rocks at the bottom, and lean it into backwards into to the soil. Any pressue from behind will lock the rocks into place over time, provided that the rocks won't roll forward as you're putting them in place.
posted by inthe80s at 9:19 AM on October 19, 2005