How to Build a Retaining Wall and 6 Reasons they Fail
How to Build a Retaining Wall and 6 Reasons they Fail
Today we’re going to cover six big mistakes when you’re building a retaining wall and how to avoid them so let’s get started.
I predicted this wall would fail over a year ago in this exact spot, in this exact way. You can see where the grades behind this wall have dipped down. It’s going to blow that wall out and in fact, we had one catastrophic wall failure on this project last year and we had to repair it. We got another one coming up; it’s not because I have a crystal ball it’s because the writing was on the wall.
It’s just completely shot right there, that’s from drainage, from not maintaining it. You can see where all the grades have dropped down in here and now we’ve got big sinkholes behind this retaining wall. I can literally stick my leg down in here, up to my knees. You can see this distinct angle, it’s all bowed out.
This problem could have easily been avoided had the property owner, the development company listened when I screamed my head off. All you need to do is either A: raise the height of the retaining wall up, just tall enough to direct the water that’s coming down from the other property away from this one spot, and divert it back onto their own property. Or, B: create a berm above the retaining wall which keeps the water from funneling into this location, two simple fixes not done in time.
Retaining walls come in all shapes and sizes. Some of the absolute strongest retaining walls made are actually poured retaining wall. To remove a retaining wall like this, we’ll be putting a jack hammer on the front of this skid loader. We will pound in the front just to soften up the concrete. Why is this wall so strong? Well, it’s how it’s designed, it’s one continuous element.
This has a lateral footing, down at the base of the soil keeping the retaining wall from tipping over. It’s all about the base, and they knew how to build them a hundred years ago. Now if you’re building a retaining wall today, make sure that you design the retaining wall with the exact type of material that is going to be on the face. That means, if it’s a modular rock retaining wall you’ve got to have the right drainage, you’ve got to have the right Geogrid. If you’re using a boulder retaining wall, you’ve got to have the right size stones. It doesn’t matter what material you build the retaining wall out of, you’ve got to design it to withstand the elements. This is a great example.
Here’s another typical problem with retaining walls; people rely on the drain tile to actually function. The wall behind me looks OK, but it’s actually in a state of failure. The first sign that it’s in a state of failure, if you look at the base of the wall you can actually see where the drain tile opening has water percolating through. That means that the water in this entire retaining wall is concentrated into that drain tile system. That’s just a four inch perforated drain tile, it’s not meant to handle large amounts of water. It’s meant to only act as an emergency overflow at best.
The entire retaining wall should allow water to pass through evenly; meaning, the water behind the retaining wall should never get down to the drain tile system. If you see water pumping through a drain tile opening, that means that entire wall is in a state of failure. All of the drainage aggregates are bound up and it’s not allowing the water to pass through it, eventually this wall, “boom”, is going to pop like a balloon.
Behind me is a small portion of a 30,000+ square foot retaining wall we’ve built over a decade ago. Now they have failed at one important component. That’s maintaining their retaining walls. The most important that you can do for a retaining wall is to make sure that the grades stay flush behind the back of the block.
This is a great example of what happen when you allow the grade to fall behind the wall. You’re going to find cavities. I can reach in and put my whole hand inside this cavity behind this retaining wall. You can see other cavities, and that’s going to create premature failure in this retaining wall.
Often times I see retaining walls, even massive ones like this, get neglected and when that the neglect sets in, failure will rapidly occur. So what you can see is they’ve actually allowed the vegetation to grow right through the face of the retaining wall and that’s how far the neglect has gotten, on this site. What happens, is that when that vegetation grows through, it creates a gap between the blocks.
So as the plants grow, the roots expand and they create spacing in the blocks. That spacing is a cavity for the aggregate behind it to flow right through the face of the retaining wall. That also creates funnel points. Where the water is going to want to migrate instead of dispersing evenly behind the retaining wall. It’s now going to find a spot and it’s going to puncture through that retaining wall. This is creating potential issues during heavy rainstorms.
Plants on Walls
Here’s another great rule of thumb that you can use on almost any retaining wall if you want to plant a major tree bush or shrub on it, like the pine tree you see behind me. Take the height of the retaining wall (in this case, it’s just a five footer) and multiply it by two. So 5 feet x 2 means you go ten feet behind the retaining wall before you would plant this tree.
If you plant it too close to the wall it creates undue stress (called a surcharge) as it grows. So as the tree is growing up, the roots are growing down and it’s going to push that wall out. That should put you safely outside of the surcharge zone of most structural retaining walls.
Here’s another myth for you; pea rock or sand makes great backfill material. Nothing could actually be further from the truth especially in the drainage zone. You want a dedicated material that is angular in nature, that can lock together and doesn’t have a lot of fines in it. The stone of choice that I use is three-quarter inch of clear rock. The reason that’s important is, that as water flows through it and then through the face of the wall, it’s not going to move. It can resist all types of ground movement behind the wall and not transfer that into the actual face of the structure.
Today we covered six issues that typically make retaining walls fall apart. Sadly there are a lot more reasons why they fail. Let me know in the comments if there are any specific videos that you would like to see.
God bless and go get them.