Many boulder retaining walls are in a state of failure because the contractor built it the wrong way. We're going to talk about the fundamentals of building a boulder retaining wall and why these simple principles are usually ignored. The two main failure factors are the size of the boulders typically used and how those boulders actually get laid.
In general, boulders make an absolutely phenomenal building material. They never suffer from weathering or erosion, salt degradation has absolutely no impact on them. The weight of the boulders allows them to tolerate massive amounts of earth movement without transmitting that through the wall. So let's start talking about how boulder retaining walls are typically built the wrong way. You cannot build a retaining wall like that, and I'm going to go right down and point at this boulder right here specifically. We've got this nice four footer laid the right way, the depth is maximized into the slope. But beneath it look at this thing. It ends right here, there's no depth. That boulder, if it was to be used where it is, needs to be rotated and laid into the bank. This wall was built to maximize space, not depth.
Now here's a pro tip: contractors will usually buy boulders on a price per pound. But they will usually charge to install the boulders on a price per square foot. What this means is big boulders cost big bucks, but that doesn't always mean you get the most square footage out of those big boulders. Now the first rule with boulders is bigger is better. With this in mind, contractors will usually try to install boulder retaining walls using stones of inadequate size for what they must retain. This means that the stones are simply too small. The strength of a boulder retaining wall and its overall holding capacity comes from the sheer weight of the boulders being used. Small stones simply can't hold back big banks. The second misapplication when building a boulder retaining wall is using a large boulder, but using it in the wrong way. Boulders are by nature very irregular in their shape and size, meaning that they will typically have a small side and have a big side.
Here's another great example. These are phenomenal stones in and of themselves. I can reuse every single boulder here. I'm not going to be reusing it for its base, but I'm going to be reusing it for its depth. This boulder right there, rotate it 90 degrees get the depth into it, not the face surface area, but the depth. This boulder right now probably has six square feet face. By the time I'm done we'll be lucky if we get two and a half feet because it's not a very thick stone this way. It's going to be deep into the bank which is going to resist movement. Oftentimes contractors will try to put the big side on the face of the actual retaining wall. But the strength of the retaining wall lies in putting the big size into the bank. One boulder may have a face that is eight square feet and have a side that is four square feet. Now the contractor is going to try to optimize the face square footage of that stone. So instead of putting the big side into the bank and burying it, they're going to flip it.
So we are at the site of a failing boulder retaining wall probably a 10 footer and with great boulders. But the construction itself, how they put it together is where this wall has its main issues. You can see that they were optimizing the face of these boulders. Instead of having a boulder like this, they use the big face area. That should have been flipped sideways so it went into the hill, because now there's nothing behind that holding it. Wide open there's nothing back here pulling this one to that one, and the soil behind it is pushing it out. Even boulder retaining walls need to be engineered when they're over four feet. Typically you can build a gravity based retaining wall much taller with boulders than you can with a modular block. Boulder retaining walls also will need a certain amount of drainage and base material depending on the subsoil conditions. But if your soils are soft and compressionable, you're going to want to fortify that base by laying something down before you put the boulders in place. We're going to talk about filter fabric right now, we're going to dispel some of the myths. Boulder retaining walls in certain applications will require filter fabric. At other times you will not want to use filter fabric.
If you have very sandy soils with very little clay binder behind it, it is mandatory that you put filter fabric in. Or, the soils will wash through the face of the boulder retaining wall. If you have a high clay content soil with a lot of silt in it, beware that when you put filter fabric behind that boulder retaining wall that fabric is going to plug up. And when that fabric plugs up, it is going to start to bulge outward. A way to get around it if you have a high clay content soil and you're willing to do some work, is to plant the heck out of that retaining wall. It's almost like stitching the boulder retaining wall together. Those plants will stop the soil from weeping, from eroding through the face of that retaining wall, and furthermore. It will knit everything together actually creating a stronger retaining wall than if you build a boulder retaining wall with just fabric. You obviously want to be cautious when applying filter fabric; it is not a universal one-size-fits-all application.
So let's repeat this - high clay content soil, and you don't want to do anything with the wall? You put fabric behind it, knowing that that fabric is going to bulge. Sandy gravelly soil, something that doesn't have a clay binder in it, you put filter fabric in because that filter fabric has to retain that sand from eroding through the base. But if you have a high clay content soil and you're willing to do some work on your own, don't put filter fabric in. Plant the heck out of the joint spacing. Eventually, those joint spaces will grow over and will create a stronger longer-lasting retaining wall. Hey, one last thing - look me up on Instagram. The majority of my projects do not make it to YouTube. I do almost daily updates on my Instagram accounts and a lot of projects get posted there that never see YouTube.