View Full Version : Clear stone behind walls - exceptions

04-16-2002, 11:16 AM
Are there any materials that you build walls with that you do not use clear, washed stone behind them for drainage?

For example, with segmental retaining wall units, snapped-face limestone, outcroppings, fieldstone, timbers, sandstone, etc, are there certain constructions where you don't use clear stone behind them?

The reason I ask is I don't use it in fieldstone retaining wall construction, because there are sufficient opportunities within the nooks of the wall for water to escape.

What do the rest of you do?

04-16-2002, 12:06 PM
Even with field stone type walls we still put gravel behind it for drainage to keep the heaving from occuring with the freeze/thaw cycle. We just don't install drain pipe since the water can seep through the wall. When we build it, we lay landscape fabric on the slope behind the wall, back fill with gravel as we build and then wrap the fabric over the top of the gravel about 8" from the surface - and back fill the rest of the way with the soil from the bed for planting. We try to slow the soil from migrating into the gravel just as you would with a block wall.

When you say clear gravel - do you mean a "washed" gravel, like #57 wash?

04-16-2002, 06:03 PM
Okay, here's an add-on question regarding fieldstone walls...

They don't lend themselves to using grid for walls in excess of 4'. But they are more dense than segmental retaining wall units, making grid less necessary, at least at 4'. But I'm wondering, in order to not involve grid or other methods of tieback in the wall, what size stones do you need to use to build larger walls? Say, 6-10'? Is there a min size you'll work with when building MAN-SIZED walls like that?

04-16-2002, 06:09 PM
I don't know if I'm qualified to answer this... but I'm sure to some degree it depends on what you're retaining and the definition of "man sized" walls. If you're looking at a 5' tall wall, I think the rake and drainage are going to be most critical. I've seen some barnstone walls that tall - but do you need to build it with 2,000 lb pieces? No - I think the drainage and the rake are the most important factors.

Largest stone wall we've laid out of natural material was about 3' tall, built into a hill. We built an ashlar blend pattern from sandstone, with 12" deep block, up to 8" thick and 3' long. Those being the largest pieces, many much smaller. Over 3' - I can't say.

Stonehenge - what do you mean by "clear" stone?

04-16-2002, 07:15 PM
clear = washed

04-16-2002, 07:42 PM
And LL, I'm sure that batter and drainage continue to play a large role, but just like segmental 'handy wall' units are only rated for walls of 2' or less, there must be some formula for fieldstone boulder size relative to wall height.

04-16-2002, 09:03 PM
Some of my own thoughts and ideas...lol.

I can't ever say I read or heard anything regarding the size of stone used compared to the height of wall.....I think it is more or less a common sense idea.

I've seen tall walls (4+) built from smaller stacked stone, but 95% of the time they are mortared walls, not stacked. I do see taller walls on occasion that are dry stacked, ans will say that I remember many of them not looking too good as they were beginning to fall.

One question I ask is how thick do you build your walls? are you just building the face and backfilling to that, do you build the face and add stone behind it, or do you build the face and build 'another wall' almost behind that one.

I've always build stone walls very thick. For example, a 2 foot dry laid may be 2 feet wide at the bottom and taper up to 1 foot at the top. When using palletized, I find this easy to do as you end up not needing half the crap pieces in the pallet and can use them behind the wall.

I think the laws of common sense say not to build a 10 ft wall out of 1" thick slates dry stacked though if thats what you are asking.

I'm still not liking the idea of not having any gravel behind your walls though. If it works for you though, I can't say a thing. However, I just like the comfort of know that water is going to flow from behind the wall into a nice pipe and not heave my wall out.


04-16-2002, 09:48 PM
Flat stone or limestone

On larger walls we've built, some over 10' tall we always back fill with clear gravel and fabric behind that. Most of these walls have been built with outcropping stone, this stone is 6" to 8" thick and sizes range 3'x3' to 6'x3'. One of the most important factors in these walls are batter or back pitch. With out having a 1/4" to 1" batter the walls will slide.


Boulder walls are back pitched naturaly and we don't back fill them with gravel unless we are working in rivers. Again with these type of walls well use larger stones at the bottom and smaller ones on top. Good stone sizes for tall boulder walls are 4' dia to 5', scale has a lot to do with these walls, most home owners don't have the room for this type of wall.

04-17-2002, 12:35 AM
We generally backfill all walls with 3/4 clean and fabric, but as Paul had mentioned, when we do boulder walls 3'-5' we rarely ever fill behind them

04-17-2002, 12:52 AM
When we build SRW planters 3 units high that get filled with nice fluffy soil we only put the cut coping and some scraps of bricks behind the first row.

04-17-2002, 06:47 AM
Don't overlook geogrid. The stabilization strength of it is not the effect of tieing the wall face to the slope so much as tieing the material behind the wall together. I'm not sure I will describe this well, but here goes. ...
Geogrid is used to increase the effectiveness of gravity walls. A gravity wall is one that uses the mass of the material that it is built of to counter act the force that wants to fail the slope. The primary purpose of geogrid is to "glue" the fill material together so that it acts as one unit much like a dam. The surcharge of the slope has to be greater than that mass to get it to move. Tieing it into a modular block wall just adds the mass of the wall to the "dam" and is so easy it would be foolish not to.
If you take apart a wall that has geogrid, look and see if the geogrid has tension between it and the face of the wall. What you will see might surprise you.

04-17-2002, 11:06 PM
Originally posted by AGLA
If you take apart a wall that has geogrid, look and see if the geogrid has tension between it and the face of the wall. What you will see might surprise you.

Please expand on that, I've never done anything that required Geogrid so I'm open to any information I can get.

04-18-2002, 12:04 AM
Alan, what he's saying is the geogrid makes the backfill a single unit. It locks the soils in by spreading shear forces over a larger area. Imaging a sheet paper in a book, you can pull it out but you feel resistance, now press down on the book and try to pull out the paper, it's much harder. For stone walls you can use a geotextile fabric, there are some installers that use these all the time on retaining walls.

This works to a point, but understand the soils have to be compacted into the grid AND the soils have to be the right type. Mixed soils or water absorbing soils will still fail. Knowing your soil types and making sure you've compacted them properly.

One thing we've not talked about is load factors on the wall. This takes in to loads above the wall and behind them. Drives, walkways, patios, parking areas and other walls.

04-18-2002, 10:32 AM

Have you used grid on fieldstone (boulder) retaining walls? I'd be interested to hear how the installation differs from a flat stone or segmental retaining wall block.

04-18-2002, 11:43 PM
I have not used geogrid on anything, but segmental walls. But, you would get effective use out of it by using it in backfill much like you would with blocks - except you would not go between the courses of stone. Just use it in the back fill.
I have used heavy weed cloth to reinforce dry stacked shale by unrolling a few feet flat on the ground where the back fill is going. adding and compacting a 9" lift of fill, wrapping the fabric back over that, and repeating as I back filled. King of like a dike of sand bags, but only one continuous piece of fabric back and forth over itself with backfill in between. This made a pretty solid unit of fill...as long as you did not have to plant deep behind it. It stood by itself, the wall went up in front of that with a little drain rock in between.
This was not an original idea of mine.