Broken Concrete for retaining wall

Grinche(7b WA)December 17, 2004

I have a plan, to use broken concrete in several terracing retaining walls. This is located on a slope of aprox 50 ft in elevation, and 100+ ft in length. Wall height will be 4 ft or less. This of course will lead to many levels. My question is, can I get away with stacking the broken concrete, and using sand/soil to level each piece, or do I need to use mortar. I plan on river rock for drainage behind the wall, and sloping the walls to the end of each run.


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treebeard(z5 MA)

Are you saying that this slope is 100 feet long and has an elevation difference (top to bottom) of 50 feet?

    Bookmark   December 28, 2004 at 11:14AM
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matteow(z8 WA)

Sounds like a lot of concrete! You'll use more than you think. Make sure you know the building codes in your area, all it take is some inspector driving by to stick his nose into your business.
As far as the leveling of each stone goes use a crushed gravel, sand or soil will just wash out over time.
Good luck,

    Bookmark   January 3, 2005 at 5:47PM
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lynnt(Z7 MD)

I am working on a project that sounds similar to what you have in mind. Sorry, I have no camera, so words will have to do. My yard is a long narrow valley, 300' X 80 wide, whose sides are three to six feet above the ground level in the center. There are high wooden fences along the crest of the hill on each long side. Last summer I arranged to get broken-concrete chunks from a paving company working in the area (I paid about $50/truckload to cover the labor of loading and unloading, even though I saved him the cost and gas of making dump runs) and used the three truckloads I thus obtained to make the first/outermost level of three terraced rows of beds I plan for the two long edges of my backyard.

This first level consists of about 75 feet of walls about a foot thick, two to four feet in height, in long looping curves punctuated by a couple half-rounds that look like ruined castle towers, to bring the outermost beds level with the surrounding yards. These upper beds vary from four to twelve feet in width. I laid out hose to determine the curves (wider is always better!), used a Mantis tiller to dig foot-deep foundations, which I shoveled out and lined with landscape cloth and 6" of gravel, before I started dry-stacking the chunks. I was careful to slant the walls back into the hillside a bit, maybe six inches for every two feet of height. I used no mortar or other leveling gravel; instead it was important to me not to move on to the next stone until the one I was working on was well-balanced and unwilling to shift. If it wouldn't fit, I used a different chunk, or tried that chunk elsewhere. I was also careful to look to establish smooth linkages between stones on the face of the wall folks would see, even if there were jagged backsides. Whenever there was an available crack in the face of a row I was capping I tucked in an egg-shaped local stone or two, mostly stained orange by the local clay, for contrast with the rough rectangles of the grey concrete. I also built a wide flight of broken-concrete steps to climb the slope from the back yard to the side of the house, with a paved slope adjacent to allow wheelbarrow access to the side-yard beds.

Once the beds were built, I filled them lasagna style: layers of leaf mulch, well-rotted horse manure, local clay tilled up with sand and leafmulch, and so on, tamping them down a bit every couple of feet of depth and making sure to fill the beds a foot or so above the walls to allow for further settling. It took an amazing amount of stuff to fill those beds, and about 2/3 of it was free for the hauling!

I have planted the top row already: several flowering trees I got in the fall sales (purple plum, crepe myrtle, tree lilac, a Briotia horse chestnut) and tall shrubs like lilac, eleaginus, kerria and the like, shorter shrubs like goldmound spirea and dwarf blueberry in front, and all underplanted with lots of shade-lovers like hellebores, gentians, ferns, hostas. And bulbs, especially along the walls themselves. There are clumps of various daylily planted hard against the walls too.

This summer's project is the next row -- 18"-high walls in counter-curves to create three garden rooms on the right-hand edge of the property, and incidentally refresh the iris beds along that side. I have to be done by May 22, when my local garden club is holding a planning meeting at my house...

Oh yes. To cut the recurrent labor of edging/weedwhacking lawn along all those beds, I plan to plant a foot or so swathe of ajuga against the lawn-edge of the bottom-most walls.

What d'you think? Does this help any?


    Bookmark   February 4, 2005 at 5:45PM
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Grinche(7b WA)

Wow Lynn, would love a picture. Thanks for all the info.

Treebeard, yes it is as you describe.

Matt, thanks for the reply.

I will post a couple of pictures in the future provided I figure out how to do that.


    Bookmark   February 5, 2005 at 12:42AM
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lynnt(Z7 MD)

Grinche, are you really talking about a 50' difference in height from the bottom to the top of that slope? Then you're working on a different order of magnitude than my project,where the max height wall I drystacked was just over four feet, and I do believe you need professional advice from both a soil engineer and an experienced landscape construction expert before you start anything. If you really mean four-foot-high walls, it'd take nearly a dozen of them to span the space! How much sideways area do you have to stretch these terraces out over? Even at first thought, some significant issues come to mind:

  1. Walls at the bottom of a cliff like that get sideways pressure from indented walls above them, so there are rules for thickness of lower walls/footings and for the setback of upper walls to distribute these significant forces. I do not believe drystacked walls can necessarily handle those kind of stresses. A pro can advise you better.

  2. In a slope of that height, how do you plan to deal with rainwater runoff, or the natural springs that may push water within the slope? Your footings will be different and you'll need a series of water-pipes and weepholes to prevent water pressure from behind from collapsing your whole setup. Think Las Conchitas, eh? But the nature of your soil and climate will impact what you need to do to address this -- another thing a pro can advise you on.

  3. How do you plan to get up and down this slope, or maneuver materials like fill, mulch and large plants around? I built both steps and wheelbarrow ramps into my plan, so I can get bulky materials to each of the upper beds without having to dissassemble walls.



    Bookmark   February 6, 2005 at 8:54AM
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Broc(Omaha, 4b)

I don't wish to pretent I'm an expert wall-builder -- am not!

But here on the Eastern bank of the Muddy Mo, we have a "mushy" soil called "Loess." This soil extends for about 40 miles N - S along the river, and exists in only one other place in the world -- Mongolia.

The soil is... well... the weirdest stuff I've ever seen! It is not very fertile -- but we have "mounds" several hundred feet high, upon which are housing developments, roads with sewers, etc.

Our wash-outs are slow developing, but every summer, someone here loses their road -- or part of it -- and the city is always rebuilding roads that were "properly" installed twenty years earlier.

So, we have to protect our homes. He who does not learn how to build a wall [and who won't pay someone else to do so], will lose his home! Or, more likely, his yard will end up in his neighbor's basement.

It is common for basement walls to cave inward, etc.

In addition to RR ties [those that "ties" back into the hillside aren't too bad], and poured concrete [terrible walls!], and concrete blocks [waste of money!] is broken concrete -- only a mediocre solution:

1] Foundations -- it is imperative to establish a below grade foundation. Firm pack at least 12" below grade, and fill and pack 4" - 6" of pea gravel &/sand.
2] Establish base line of heavy concrete block, perfectly level from side-to-side and tipping back into the hillside approx 3 degrees.
3] Overlap blocks irregularly from course-to-course, so there are no vertical seams [of course!]
4] Make sure the wall "leans" into the hillside, yet --
5] Backfill the wall with sand & pea gravel. The idea is for water to weep through the cement blocks, so it does not freeze and push the wall over. Most people here do not use drainpipe at the base of their walls, because --
6] All walls will fail in time, due to uncomtrollable pockets of collapse within the loess soil and/or repeated freeze/thaw which causes the wall to buckle outwards **above** the foundation courses of the wall.

Of courses[!], when this happens, you mutter -- maybe offer some special words of blessing -- tear your wall down and rebuild it wherever appropriate.

Here, in "the Bluffs," we praise the "inventor" of the modern preformed concrete block, as expensive as they are. Those things are ten times more stable than randomly broken concrete... for two reasons -- they're flat on the top and the bottom and those wonderful little "tail bones" interlock from back to front.

I hope whoever invented these things is getting rich on royalties and residuals. That I haven't had to make the first repair to any retaining wall I've built in the last 20 years is worth whatever I have to pay for the damn things!

    Bookmark   February 7, 2005 at 6:13PM
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You'd be much better off with a segmental retaining wall, made from attractive concrete blocks. They'll last forever and ar almost maintenance-free. Don't waste your time on hollow-block systems--not strong enough and labor-intensive to build. Check out solid-block systems with a mechanical connection, like pins. Versa-Lok makes some really nice looking retaining walls, particularly with "weathered" textures that look old. They can be built to unlimited heights with proper soil reinforcement. Check with a licensed engineer to spec your wall, or better yet, the manufacturer of your retaining wall system.

    Bookmark   March 8, 2005 at 11:24AM
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