Building Dry Stone Retaining Walls (2002-06)

Building Dry Stone Retaining Walls (2002-06)

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Summary

This video shows how to build small drystone retaining walls from beginning to end; laying out the shape, digging the foundation, determining the wall angle, building the face, packing the back, and leveling the top. The techniques are suitable for all rock types, whether glacially rounded, angular, or flat-bedded.

A series of graphic drawing illustrates the principles of retaining wall construction, followed by two case studies showing on-site training classes. The first project is a low, two-foot wall suitable for a garden, with a Master Craftsman instructor who discusses the plans as well as the problems the class members encounter and solve. The second project is a four-foot wall that supports and equestrian trail and traverses a wet-weather water course in a Louisville park.

From small retaining walls in home gardens to road and trail walls in state and national parks, drystone is increasingly the material of choice. The flexibility of mortarless masonry is ideal for terraces and stream banks; it is functional, beautiful, and natural. Most projects do not require poer tools or machinery, and there is no need for harsh chemicals common in most construction.

The projects in the video were organized and managed by the Dry-Stone Conservancy. For instructions in print, you may want to consult the section on retaining walls in the DSC handbook, “Building and Repairing Drystone Fences and Retaining Walls.”

Video Transcript

Batter - Inward slope of the front face of the wall.

Batter – Inward slope of the front face of the wall.

NARRATOR: The craft of the dry stone mason combines functionality with beauty. Dry stone retaining walls hold back time as well as earth. Durability, versatility, even in water. Strong enough for trucks, strong enough for wide bridges, yet always blending with nature. Whatever the shape, whatever the size, there are important principles which can ensure dry stone retaining walls are built to last.

PAUL WEBLEY: Now the differences we have between a free standing rock fence and a retaining wall is that on a free standing rock fence, the majority of the pressure is downwards and the problem…

NARRATOR: A group of beginners are learning how to build a simple retaining wall. None of them are experienced masons. Their instructor is Paul Webley, one of the dry stone conservancy’s colleagues from Great Britain.

PAUL WEBLEY: When we talk about a retaining wall, we’ve still got some of that downward pressure, but we’ve also got pressure from the back. We’ve got the soil, or the material we’re facing to, that may actually cause bulges to appear in the wall.

Most small retaining walls have a batter of one to six.

Most small retaining walls have a batter of one to six.

PAUL WEBLEY: If you’re building a small, low retaining wall, by low retaining wall I mean something not much higher than my knees, we’re only going up about twenty-one inches. In that case, all you need to do is to build a good face and to actually pack the back with good, solid, stony material. You don’t want to pack any soil into that area, you want it to be big, chunky, solid lumps of stone and that will actually take a lot of the pressure that comes with the soil pushing forward. Any questions?

NARRATOR: Retaining walls convert sloping ground into usable level spaces. If the ground is level above and the footing is stable below, the simplest form of retaining wall is appropriate. Dry stone walls are almost always built with a batter, which is the inward slope of the front face of the wall. The batter adds strength, giving the wall resistance to the weight behind it. Simple walls have a horizontal foundation, and rock laid in horizontal courses that run from the wall face to the back. When the wall is located in front of the slope, a freestanding wall can be built, which is then back-filled to create the level area behind the wall. This reduces the amount of rock required to build the wall, compared to coursing rock all the way to the top.

NARRATOR: All principles are the same for any rock type, even rounded boulder walls. The simplest retaining walls are those up to two and a half feet tall. In short retaining walls, it is easiest to course the wall to the back. Lay the footing and all courses horizontally. As for all retaining walls, the foundation course projects out from the rest of the retaining wall by two to four inches. This spreads the weight of the wall and prevents rain water from undermining the bottom of the wall. The width of the foundation for a wall of this size is 22-24 inches, including the front projection. Gaps under the foundation rocks are carefully packed to evenly distribute the weight of the wall and to minimize any future settlement. Once the foundation course is finished, the wall is built up with a battered front face. The angle of this batter is typically one to six. In other words, the wall steps in one inch for every six inches in height.

The space behind the wall is filled in with earth.

The space behind the wall is filled in with earth.

NARRATOR: The front stones of each course are called face stones, and ideally must be eight inches deep so that they’ll be securely anchored inside the wall. The space behind the face stones must be carefully packed with what is called harding, or packing, which may be sized a little smaller than the face rock. Gravel is not acceptable. These rocks form the core of the wall, and their careful packing plays an important part in preventing any slumping of the wall interior.

NARRATOR: The wall is built up in four to five inch layers. To provide weight that helps tie the face to the core, the heaviest available rocks are used as cap stones for the final course. The cap stones are pinned to ensure they do not move, and the space behind the wall is filled in with earth.

NARRATOR: While most small retaining walls have a batter of one to six, it is acceptable for some small walls to have a vertical front face, and no batter.

Each tie rock is placed thirty-six inches apart.

Each tie rock is placed thirty-six inches apart.

NARRATOR: Simple walls can be up to four to five feet high. Because the wall is higher, the foundation forward not projects four inches out more than the rest of the wall to further spread the load. The width of the foundation from front to back is increased to thirty inches, including the four inch front projection. There is a greater angle to the batter, now it’s one to five. The wall is built up course by course, with the lower courses running all the way to the back, where possible. The face stones for a wall this high should now be eight to ten inches deep. To tie the face to the core, long tie rocks are built into the wall, eighteen inches above the foundation. Each tie rock is placed thirty-six inches apart.

NARRATOR: The front should project two inches from the base to provide support for future wall settlement. If possible, the back should tie into the earth bank. For walls that are built in front of the bank, a freestanding upper section can be built to conserve rock. The back face can be nearly vertical, or with a slight one to twelve batter. Lay a second row of tie rocks eighteen inches above the right row, and alternating between them. If the wall is less than four feet high, build only one row of ties halfway up the wall. Use the heaviest rocks available for the top cap stones. Waste rock can be discarded and packed behind the wall, and the remaining space filled with earth.

Strings are first set to act as a guide line.

Strings are first set to act as a guide line.

NARRATOR: The beginners’ class will build a demonstration section of retaining wall in a gap of old wall. Strings are first set to act as a guideline for the edge of the foundation course. The area for the foundation has already been cleared, and all loose earth has been removed so there is a solid base on which to build.

PAUL WEBLEY: What I’ve got here is a mini-spirit level. When you think you’re right, hook it onto the line, bring the line down until you get it as level as possible, and then we will see whether we can get a level foundation course or not.

NARRATOR: The foundation course of rocks for the wall should be laid horizontally. Choose large rocks for the face of the foundation. Lay them so that the length runs into the wall, and so that the face of each rock is in line with the string. Pack smaller stones under the rocks so that there are as few gaps as possible. The rocks should not move at all. If there are gaps, the pressure of rocks put on top might crack these stones, so thorough packing is necessary, and takes time.

PAUL WEBLEY: So this is the slow part. We don’t want that stone to wobble in any way at all. I will put my big, heavy boot on it downwards and make sure it’s embedded right solidly into the ground.

PAUL WEBLEY: If you’ve got foundation stones which are curved on any of the faces, that’s the face to use downwards. You can put that into a dip in the soil. Then you can give yourself a nice flat surface to work off of next time. We’re trying to always create as flat as a surface as we can. It looks good. The only thing I would say to you is ideally, you need your length into the wall. That is beginning to look a lot better.

NARRATOR: Every time you lay a rock, it is important to consider the space you are leaving for the rocks that follow.

PAUL WEBLEY: That’s going to be difficult to meet up with. So you’re going to either have to address that or lay the stone in a different way so that doesn’t come into the equation. Close, but one more turn of the stone would probably solve it completely. That is beautiful. That’s perfect.

Pack smaller stones under the rocks so that there are as few gaps as possible.

Pack smaller stones under the rocks so that there are as few gaps as possible.

NARRATOR: Lay large rocks behind the face stones, and pack any voids with smaller rocks.

PAUL WEBLEY: What you can actually do with a retaining wall is you can actually use rough stone tied to the bank. As you are aware, people have fun because you don’t have to worry about meeting up perfectly on the front edge. Here you can push your stones back to the bank, and get them so that they do bend down firmly, because they’re going to take weight, and so they do need to bend down firmly, and make sure you get- it doesn’t matter about an absolutely perfect back line, you can work like that, you know, to the contour of the bank, right down there. Still pin and wedge exactly in the same way.

PAUL WEBLEY: This isn’t how we’re going to layer the next course, it’s simply to hold the string.

NARRATOR: Once the foundation course is level and packed, raise the string to guide the shape of the face of the batter. The edge of this course is set back from the foundation. When the wall is finished, the foundation will project. The face stones are again laid in line with the string because the face of the wall must slope inwards to give the wall its batter. Try and choose stones that have a natural slope.

NARRATOR: It would cry out to be used that way up, because it’s got what I call natural batter on it. You see where my thumb is, it’s sloping like that. You turn it that way up, suddenly your wall goes up to there, comes out, and then back in again.

Consider the space you are leaving for the rocks that follow.

Consider the space you are leaving for the rocks that follow.

NARRATOR: It is particularly important to avoid what are called running joints. An example of this can be seen on a neighboring wall.

PAUL WEBLEY: What you see here is actually a line of weakness up the wall. We actually call it a running joint. This running joint has been caused by- the person who did this actually, first working up all the way through, two stones brushing up against that one, but at that point, he actually should have tried to produce this t-joint that we talk about, he should have covered the joint or broken the joint by actually putting a stone right across the wall at this position in order to prevent the straight line, the weakness, continuing on up the wall. The wall would open at that particular point. So he made a bad choice of stone. At this particular point he needed not to have put these stones in here where they’re not butting up with the same position, he should have taken a stone like this one and he should have stretched it across that joint there, and ensured the run of weakness that went up the wall was actually stopped.

PAUL WEBLEY: Is it wobbling on something?

MASON: No.

PAUL WEBLEY: No? That’s pretty close.

The wall is almost finished.

The wall is almost finished.

MASON: It’s pretty close.

PAUL WEBLEY: Right, pin it, pin it.

PAUL WEBLEY: If I put something in, and I’m using this in a totally inappropriate way, if I put something like that in there, what have I done? I’ve created another double joint. Where are you going to seat your next stone to in order to ensure that you cover the joint? Up to here. Now what you’re going to do is measure that space with your eye, and then you go away and find a stone that will do that. You build a wall with your brains.

NARRATOR: The wall is almost finished.

PAUL WEBLEY: The next stage in the job here is actually to level this stone and move right along, and try to get the front absolutely level, the back absolutely level, and then we’re going to fill in and place large stones on top, and this will create the seat.

PAUL WEBLEY: Still do exactly what we’ve done everywhere on the wall with this. Keep filling in. The problem is the stones that you’re actually going to put on are not going to be absolutely level. So we’re still doing to fiddle around in trying to get these in.

PAUL WEBLEY: You push the front stone out when you do that.

Cover stones help bind the wall together.

Cover stones help bind the wall together.

NARRATOR: Now, the final layer of cover stones can go on. These are large rocks that go across the top of the wall, and help bind the wall together.

PAUL WEBLEY: Right, as carefully as you can, right across here. Now, on this wall, because we’ve got such a long length to the back, it’s possible that we won’t get this all the way across, so you want that stone to sit across the joint there, a very slight overhang at the front, like that, OK? What we want to try to do now is find two or three more of these. These are for me to rest on in a minute or two.

PAUL WEBLEY: Ideally we want single ones if we can. We’ll just do a quick test look… Here we are, I’m settled here for the rest of the afternoon.

MASONS: (laughing)

MASON: And he’s sitting on our section of the wall, which means we can’t build it, or we get to sit, too.

Michael L. Smiley

Michael L. Smiley

PAUL WEBLEY: Right so, that’s it. Just position your legs so that you cover up all the faults, look.

MASON: And the gin and tonic is where?

NARRATOR: In this next example, a group of experienced masons are constructing a longer dry stone retaining wall in one of Louisville’s homestead parks.

MICHAEL SMILEY: We’re building a bridal trail here, in this part of Iroquois Park. Rebuilding the bridal trail from down along the busy road to the edge of the woods to give the horse enthusiasts sort of a better experience with riding through the woods.

NARRATOR: The wall will curve around a hillside, and will be seventy-five feet long. Where the wall spans a water drainage point, a culvert spans under the path. This requires a small retaining wall along the uphill side of the bridal path, and an arch in the main retaining wall to provide an outlet for the pipe.

The wall will curve around a hillside, and will be seventy-five feet long.

The wall will curve around a hillside, and will be 75 feet long.

MICHAEL SMILEY: The wall will be at one downhill edge of the bridal path, so as you can see the foundation line here, we’re going to raise that up about two feet, and the bridal path will be on this side, on the uphill side of the wall, so there can be a drop-off at the edge of the trial. And then the trail will be about eight feet wide on the uphill side of the slope, just curving around very nicely through there.

NARRATOR: On a more complex site such as this, many factors affect the design. Wall height, stone type, soil type, load, water drainage, and ground slope. The section at each end will be a small retaining wall. The taller middle section will also have a freestanding section, with a rough backface that will be backfilled later.

NARRATOR: Before any construction begins, the masons sort through the rock supply to separate out enough tie rocks and capstones for later use. Otherwise, they may inadvertently be broken and used as face stones. The footing is cleared of loose earth, and firmly tamped down. Since drystone walls are designed to cope with settlement, they do not need a concrete foundation.

Sort through the rock supply to separate tie rocks and capstones.

Sort through the rock supply to separate tie rocks and capstones.

NARRATOR: The foundation course is built in a series of steps so that the courses still run in rows, but will step down the hillside. A series of wooden batter frames are used to attach string lines. These guide the masons in building the face at the correct angle. They check each frame to make sure it is correctly aligned.

NARRATOR: Each end of the frame is anchored with large rocks. In addition to making sure that each frame is correctly set, they are aligned precisely to each other. This positioning determines the face of the wall.

NARRATOR: A string is tied at the average height of the larger stones to be put in the foundation. The distance from the bank to the string sets the width of the foundation. For the middle, taller sections, the foundation will be thirty inches wide. At each end, it will be twenty-four inches wide.

NARRATOR: The front edge of the foundation course is laid close to the string. The foundation is carefully packed to make sure that there are no gaps.

MASON: Just trying to pack underneath as much of the rock as I can. Less movement means the better off we’ll be.

he distance from the bank to the string sets the width of the foundation.

The distance from the bank to the string sets the width of the foundation.

JAMES MILES: Realistically, you just want to get it packed in as tight as you can. Start out with the bigger stones as this one and that one and then work in all these smaller stones.

NARRATOR: Every packing stone is just as important as the face stones, and must have maximum contact with adjacent stones. Fill the spaces with diminishing sizes and pin each carefully.

RICHARD TUFNELL: If you’re down in the lower courses, then we try to have eight or ten or twelve inches extend from the front face to the back of the wall. This end is running along the face of the wall, but these stones are much fatter, much heavier, and extend back into the wall more than a foot. Stones like this, if I can just put that there for an example for a moment. That’s running along the wall, but it has no depth. Alright, and so it’s seven inches, so that’s not enough to be stable.

NARRATOR: Once the foundation course is finished, the masons raise the batter frames and set them on top of the foundation course. This ensures that the face of the wall will be set back, leaving the foundation course projecting by the right amount.

JAMES MILES: More, just a little more. Oh, that’s it.

masons raise the batter frames and set them on top of the foundation course

Masons raise the batter frames and set them on top of the foundation course

NARRATOR: Once the frames have been raised, their alignment must be checked again. The batter frames also mark points in the curve of the wall, and are positioned as close together as possible while still allowing enough space to work. The masons adjust the face stones by eye to produce a smooth arch between the frames and curve.

RICHARD TUFNELL: You push the batter frames as close together as you can to make for easy working environment, and then you have to push string in while you’re building. So you get the exact curve. And also, when you finish laying in a particular course, you stand back and do what is known as sweetening the curve, by pushing the stones gradually in a little bit more until you come to a center point, and then a little bit less as you approach the next batter frame.

NARRATOR: Because the bridal path spans a drainage ravine, the masons are building a culvert with a pipe under the path. On the front wall face, an arch across the pipe gives a more attractive finish to the wall. A semicircle of carefully trimmed stones are set like spokes in a bicycle wheel. A string set at the center of the pipe is used to check that each rock is running into the center, and that each edge forms a neat semicircle. Wedge-shaped ends align and secure the stones carefully around the pipe.

NARRATOR: On the uphill side, masons are building a second small retaining wall around the pipe. There will be stone retaining walls both upside and downside the finished trail. Because this second wall is low, it does not need a batter, and has a vertical face. It still needs a projecting foundation. This time, the pipe is bridged with a stone lentil.

A rock must be sufficiently pinned underneath.

A rock must be sufficiently pinned underneath.

NARRATOR: One of the problems that inexperienced masons is being able to tell when a rock is sufficiently pinned underneath.

JAMES MILES: Any place that you can find a hole, you want to find a wedge that will fit in there just as tight as you can possibly get it. When you get done, it shouldn’t move, shouldn’t rock around as you’re trying to move. And once you’ve done that you’ve got a stone that’s set. You should be seeing some daylight under there now. So you try to go in with just as thin a piece as you possibly can, until it won’t take any more, and then you work your way out to the edges with thicker stuff in there. People kind of cheat sometimes, and just pack at the edges instead of filling in the center, but if you don’t pack all the way to the center then the compression of the other rocks on this could conceivably break the rock in the middle. So it needs to be packed in just as tight as you can possibly get it. A lot less light in there now than what you could originally see, that’s just what were basically trying to do. You just want to get as much in there as you possibly can. It just helps prevent the settling.

NARRATOR: Remember to break the joint at all times.

Remember to break the joint at all times.

Remember to break the joint at all times.

JAMES MILES: Whenever possible you want two rocks on top of one, and…. this little bit is essential as far as knitting the wall together. In places like this you have no option, where you have two rocks that come up to equal the height of one rock, but you know, that’s fine because this one goes on top of that one, this one breaks those two.

NARRATOR: It takes experience to judge how much the rocks overlap.

MASON: I like to half the stone, but it doesn’t always work out to where you can get half the stone. Like right here we’ve got four inches, three and a half to four inches. And that’s very acceptable.

NARRATOR: Since the middle section of the wall is more than four feet tall, it needs two rows of tie rocks.The first row is eighteen inches above the foundation. The ties run right through the wall, tying the two faces together and giving the wall additional strength.

RICHARD TUFNELL: You’ll need to position yourself well because it’s quite heavy.

NARRATOR: The tie rocks need to lie flat.

RICHARD TUFNELL: Can you jump on that, see what happens? Compress it a bit. Ok, I think that’s it.

The tie rocks are thirty-six inches apart.

The tie rocks are thirty-six inches apart.

NARRATOR: The tie rocks are thirty-six inches apart, and each one is placed so that it covers a joint in the course below. Masons check the front of the tie rock to make sure it projects two inches out from the face.

NARRATOR: Now the tie rock can be packed underneath to make sure there are no void spaces, and that it does not move. Projecting tie rocks continue to support the wall, even if the wall levels and spreads.

NARRATOR: The courses are built up to level off the wall between the tie rocks. At the top of the wall, the pressure from the bank is much less. A freestanding wall section with a rough back base can be built instead of coursing all the way to the bank. This helps to conserve stone. Although it can be rough in appearance, this back face must be as carefully built as the front face.

RICHARD TUFNELL: That’s exactly what we’re after. Yes, that’s right. So it’s strong, but we really don’t mind if there are these dents and fissures, and not-looking-so-good. So as long as it’s doing its job structurally that’s all we’re worried about because it’s going to be buried by the field in the back and this wall is more than wide enough to be amply strong for the job it’s got to do.

The second row of tie rocks is eighteen inches above the first.

The second row of tie rocks is eighteen inches above the first.

NARRATOR: The second row of tie rocks is eighteen inches above the first. Masons set these tire rocks midway between those in the first row.

RICHARD TUFNELL: We need to center it up. The two other tie rocks are here and here. So, yep, that’s very close. That’s fine, we’re breaking the joint. That’s fine. We’ll pack in a minute.

RICHARD TUFNELL: Ok, if we can break this joint.

RICHARD TUFNELL: Next one.

NARRATOR: The spacing between the ties is the same as in the first row. The center of each tie rock is thirty-six inches apart.

NARRATOR: Once the tie rocks are in position, the masons build up the courses so that the two layers of rocks are interwoven in the wall.

NARRATOR: A wall head finishes each end of the wall. Use heavier rocks for the wall head. They should alternate between spanning the full wall and ties, and running into the wall in pairs.

NARRATOR: Continue to carefully pack under and around every rock.

The wall is almost finished and ready for the final row: the capstones.

The wall is almost finished and ready for the final row: the capstones.

NARRATOR: The wall is almost finished and ready for the final row: the capstones. These are heavily rocks, measured precisely so that the top of the wall is level with the final string line.

NARRATOR: Once a rock of the correct thickness has been selected, it may be necessary to trim its face to give the wall a neater finish.

NARRATOR: Pin the capstones so that there is no movement whatsoever.

NARRATOR: The cap must be pinned so that it cannot move.

RICHARD TUFNELL: Just hop and there a minute Trey and just wobble it. There’s very little now. So keep the weight down on this side and then we need a really good triangle. That’s it. Ok,now, let’s try that. Ok, at last.

NARRATOR: The wall is now finished. The space behind the wall is backfilled. First with waste rock, then with earth. And the masons clear the work site.

The cap must be pinned so that it cannot move.

The cap must be pinned so that it cannot move.

END

This video was produced in 2001 by the Dry Stone Conservancy through the National Center for Preservation Technology and Training of the National Park Service.

Credits

Special thanks to:

Jim Barton, Narrator

Ivan D. Symes, Music

Brandon Codispoti, Graphics

Chris McElhone, Editor

Graham Maughan, Direction and Filming

This video was made possible through Grant MT-2210-0-NC-15 from the National Center for Preservation Technology and Training (NCPTT).

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4 Responses to Building Dry Stone Retaining Walls (2002-06)

  1. kim says:

    big job do that by hand i have been there doing it too. just finished one up handled 550 thousand pounds go to my site

  2. Erik says:

    The walls are is pretty, but beyond that I have some criticisms and genuine concerns. Why there is only face batter and flat/horizontal courses rather than a battered course seems like a risky proposition at any height, especially for a crib wall…and 6:1? Why not 2:1 to 4:1? At 6:1 it seems more like a free standing wall with fill behind it than a crib or retaining wall, and more likely to fail. To me, off setting with face batter alone is a “false batter.” Why rely solely on static friction with flat courses, not friction and an insloped batter with Force deflected toward the containment? While the Normal Force is reduced with an inslope, as opposed to a flat batter, the Force of retainment is amplified by more than the reduction with an insloped batter…which should nearly match the face batter. In other words, we have a “true” face batter that reflects the insloped course batter, as the angles are congruent. With true batter the reduction in Normal Force is small, and the gain of force towards the retained material is now a positive integer, not zero as in the case of a horizontal course relying solely on static friction and Normal Force. The Force directed in towards the retaining material is mg x sin(of the inslope batter angle). Lastly, did I miss it or was the width of the wall mentioned? Should retaining and crib wall widths not be at minimum 1/2 their height? One of the diagrams shows the wall becoming narrower with height…it looks like a a free standing wall on top of a thicker wall…it looks like failure to me.

  3. Erik says:

    The walls are pretty, but beyond that I have some criticisms and genuine concerns. First, why 6:1 and 5:1, not 2:1, 3:1, or 4:1? At 5 and 6:1 it seems more like a free standing wall with fill behind it rather than a crib or retaining wall, and more likely to fail. Second, why only face batter and flat/horizontal courses rather than battered courses? Flat courses seem like a risky proposition at any height, especially for a crib wall. To me, off-setting with face batter alone is a “false batter,” but “true” face batter is a result of, or reflects, an insloped course batter. Insloped courses will cause the face to have a batter naturally, as the face will reflect the inslope as the course and face angles are congruent. There are benefits to true batter over face batter alone. The gain of force towards the retained material is now a positive integer, not zero as in the case of a horizontal course relying solely on static friction and the Normal Force. Why rely solely on static friction with flat courses? Is it not better to rely on friction and an insloped batter with Force deflected toward the containment? While the Normal Force is reduced with an inslope, as opposed to a flat batter, the Force of ‘retainment’ is amplified by more than the normal Force reduction with an insloped batter. On a somewhat positive note, one good point that could have been clearer, or said outright, is that retaining and crib wall widths should be at minimum 1/2 their height. However, what’s confusing, is the diagram showing the 5 foot high wall becoming narrower with height…it looks like a free standing wall on top of a thicker wall…it looks like failure to me depending on how much of what is being contained. Other than those concerns, thank you. Image to explain some of my comments: http://www.trailove.com/trail-science/crib-walls-and-retaining-walls/

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