mortarless fieldstone retaining wall

[drystacker]

There seems to be a fair amount of discrepancies when it comes to designing dry stacked natural stone retaining walls. While I have read the federal rockery manual concerning dry stacking with large boulders (250 pounds and up) and it seems there is consensus for drainage practices (drain rock, filter fabric, perf pipe at base) with these walls that are 10-15 feet tall. I am interested in thoughts on walls between 4-8 feet tall using one and two man sized rocks (and some about the size of a brick) with relatively angular sides. One trade authority (The Dry Stone Conservancy) appears to endorse no backfill other than local soil with no fabric and no drain pipe and no special footing other than trenching down to subsoil. (judging from their project pictures and handbook I read) (

http://www.drystone.org)

Some contractors I have worked for insist on jumping jack compaction of a crusher run base and drain rock behind the wall face with a perforated pipe at the bottom daylighting on one end of the wall and occassionaly through the face. Most everyone seems to agree with a base course width of 40-50% of total height and a batter of 1 to 2 inches per foot. For the most part I am talking walls retaining slopes that have not shown obvious signs of instability.

So questions are:

1. Drain rock, filter fabric, perf pipe: necessary or overkill?

2. Most of you will probably give me 4-5 feet without concern for design (beyond good, common sense placement of stone) how about 6-8 feet with such small rocks?

3. I have read on a previous post a comment suggesting even though hydrostatic pressure is nil being that the face is open draining that there are still hydrodynamic concerns during large storm events. Can anyone explain this?

4. How much do clay soils influence such design suggestions?

Thanks in advance to anyone willing to give some of their time and expertise.

 

[dirtydude]

It is always good practice to backfill with open graded material, filter fabric and a perf pipe. Hydrostatic pressure will eventually be placed on the wall as the open joints silt up, so the hydrostatic forces must be accounted for if no drainage system is put in place.

If clay is used as material to backfill, a drain board should be used, instead of just filter fabric.

 

[fattdad]

Whether you need a drainage media behind the wall has more to do with the on-site soils and the position of the water table. If you are dealing with a water table that's well below the toe elevation of the retaining wall, I can't quite think of what would make a drop of water make a left hand turn as it's percolating just to puddle up against the retaining wall.

Folk use drainage media as a default 'cause it can't hurt, but that doesn't mean it's needed.

Nice project!

f-d

¡papá gordo ain’t no madre flaca!

 

[ishvaaag]

I see valid your concern relating the size of the stones and the total height. By whatever the reasons, but surely practical, most of the ancient works of this type use to be less or around 1 man's height. This makes a human scale, sight can attain over the wall, step is smaller between levels, when failing can't crush the head, and have proven to be stable for centuries for the works' effort.

 

[Geostructsparks]

The drystack site is providing more traditional wall construction, perhaps why they do not advocate drain materials. Clearly most drystack walls that survive to the current day have had adequate drainage (and the ones that did not fell down long ago).

In Nevada, there is considerable use of rockery walls of the type in the Fed manual. Most designs use the 2 - 3 foot wide 3"- rock zone behind the stacked boulders with a drain pipe at the base, some designers omit. Unlike drystack, the spaces in the wall range from 3 - 4 inches max (good walls) to 1 - 2 feet (unsightly but stands). One use of the drain backfill is that the back face of all those boulders is uneven, it fills the gap. Even in our dry state, most users consider the drain rock and pipe are considered appropriate to handle the expected occasional extreme weather event or snowmelt. Its usually next to a vertical unsupported soil face that exceeds OSHA, so there is no compaction, and proscribed in the design report that no utilities, pavements, buildings be located over the drain fill. The geotextile between native and drain zone is appropriate in more erodable soils, so that there is not long-term loss of soil into the filter (clogging filter/ground loss in soil behind, however in some soils it may be omitted at the designer's discretion. I would think that drain materials and filter would be more critical in a climate with greater moisture.

 

[drystacker]

Well looks like it's two for drain rock and perf pipe and one for overkill. Thanks for everybody's input. Anybody else want to give their two cents?

What are everybody's thoughts on frost heave concerns for a wall of this type. Any good mitigating measure there or just let it rise and fall as it pleases since there's no mortar? Would a blow out from an extreme rain event be much more likely than frost heave causing the wall to tumble over?

This would be mountainous Virginia climate we're talking. Forty inches of rain and two feet of snow a year. (though make that five feet and counting this winter)

Thanks!

 

[fattdad]

Regarding frost depth, wouldn't you install the lowest rocks on some aggregate, "leveling pad?" For that case I'd just have the base of the leveling pad at the frost depth. You'd probably start the wall below finish toe grade anyhow, eh?

f-d

¡papá gordo ain’t no madre flaca!

 

[drystacker]

f-d,

I'm sure there are some contractors out there putting a leveling pad down to frost depth, but I think it's the exception more than the rule. Most are content just trenching 6-12" to get rid of the topsoil, burying half or more of the base course of rocks. But like I say there is a lot of variation on these smaller dry stack walls of which design features are more the domain of a landscape installation contractor than an engineer.

Earlier you said, "Whether you need a drainage media behind the wall has more to do with the on-site soils and the position of the water table." What I take from that is well draining sandy to loamy soils without the presence of groundwater nearby would probably not need a drainage media while clayey soils with their smaller pore spaces probably would. Is that general statement what you are implying or did I take that wrong?

Thanks.

 

[oldestguy]

A good rule of thumb for gravity walls is the width, front to back at 1/3 the height. Some narrowing of width as you go up can be gotten away with.

It seems most of these walls do not have an absolutely level backfill area. Thus, surface drainage goes towards the wall and that's part of the reason we some of them fail

I've seen few fail because of erosion of soil thru the walls, most being due to too heavy a backfill pressure, now saturated and weakened. So this says more about keeping water from getting in behind the wall than draining the backfill. Diverting the surface water or sealing the top of the backfill helps a lot.

 

[fattdad]

without getting too specific, whether it's clay or sand affects the horizontal earth pressures, irrespective of the position of the water table. For clay, you have to consider that the affect of cohesion will attenuate with time and design the earth pressures using just the contribution of friction angle.

I'm sure I wouldn't have to look too far to find a geotechnical engineer (and a good one) that would say the porous nature of the dry stacked stones function as a drainage media and installing even more drainage media behind the wall is overkill. If I had more case-history I'd likely be in that camp, but alas, I have to look at this somewhat more conservatively.

Let's say you are building a retaining wall in lowground and the water table is just below the ground surface. Let's say you are going to pile up the stones 10 ft and then backfill the wall to create flat land. Let's say, you're then going to irrigate for some landscaping on the newly-constructed flat ground. Then during the winter you get falling weather and more of it infiltrates (after all it is flatter). There's a chance that you will create a ground water mound in response to the regrading and in response to the increased infiltration. If the field stones are porous enough, you'll never get hydrostatic pressures, but if it's not, you will and that could be bad. So, I'd consider a drainage media and give it some gravity discharge (i.e., perorated pipe, weep holes or such).

Let's consider another scenario. You are in the Piedmont about 100 ft higher than the base flow from a nearby stream and the geology is colluvium over fractured bedrock. The water table is in the bedrock and it's 40 ft below the ground. Maybe drainage considerations are not that relavent.

Here's a good web page:

http://www.fatt-dad.com

There is contact information and I'm in Richmond. Bear in mind, I have no experience in dry stack walls. But they look cool!

f-d

¡papá gordo ain’t no madre flaca!

 

[drystacker]

Thanks for the details f-d! (I'm in Rockbridge county)

I'm reporting directly to homeowners now as I attempt to start my own retaining wall and landscaping business rather than just working on a crew building according to the boss's standards. Just figuring out how to explain why it is or isn't worth it to add 10-15% to total cost when clearly dry stack walls are meant to be able to handle some or most drainage through the face. I'm beginning to think a big reason all the contractors I've worked for use the additional drainage is just to take the most conservative approach possible. Almost a train of thought that says hey it's easier to just throw it back there than gain or pay for the expertise needed to thoroughly investigate the situation. Plus if during an initial walk through with a prospective client you don't mention the need for drainage and the next contractor does you might be left looking like the short cutter. Just my thoughts at least.