Retaining Walls: 2a. Retaining Walls Overview Images

So let's take a look at the sequence of videos I've prepared for retaining walls. We're going to start with an overview. The first video is about the purpose of retaining walls, construction materials, what they're made of typically, and the concerns, structural concerns with retaining walls. Then the next video will get into terminology and types of retaining walls. Then there will be a video about the forces on retaining walls, and don't panic. There is absolutely no math in that. Yes, there's arrows. I'm sorry, I can't help it, but focus on what is going on behind the retaining wall. Then the final video will be about the reinforcing of cast-in-place cantilever retaining walls versus basement walls. That's a very important topic in practice and on the architect registration exam. Okay, let's get started. So a retaining wall is basically the primary function of a retaining wall is to negotiate a level change in the landscape. So we have dirt here. We want to put a sidewalk and the dirt is up here. So we put a little retaining, a gravity retaining wall over here. We cut back the dirt. Otherwise, if the dirt were up to here, we'd have to put the retaining wall all the way up to there. So that's the basic function of a retaining wall to negotiate grade or grade changes. So construction materials, I've categorized them into basically two. One is a retaining wall made of individual cellular pieces. and the other one is monolithic or cast-in-place concrete. So under the individual units, we have basically stone retaining walls. No matter their height, they're made of individual stones and they must have gaps in there for any water that accumulates behind the wall to be able to escape. So it could be CMU, concrete masonry units, or split face, or concrete masonry units. It could be, come on, it could be brick. It could be timber. So wood cribbing is an example where they drive in some HP steel shapes. and then they run these timber pieces between them and we're retaining the dirt behind the wall and there's some gaps for water that accumulates to escape. Same thing with railroad ties or wood logs. So sheet piling is another example of a retaining wall. Usually this is used in construction temporary and it's got this profile, corrugated profile. It's got the section modulus. It's got the moment of inertia and it is capable of retaining dirt in this direction. It has that much depth in the direction of load and they drive it into the ground quite a bit of distance so it can act as a cantilever, maybe twice its projected length, twice that amount into the ground. Then we have precast concrete units, for example, at a precasting yard. But this might be a good time to discuss the active lateral soil pressure of different soil materials. So in the case of gravel, the size of the gravel molecule is pretty large. So it will push against that retaining wall, but relatively mildly, because most of it is stacking and the vertical is much more than the horizontal push. it's applying most of its load is going down on the ground very little is pushing on that retaining wall versus sand. Sand has a very fine molecule size so the smaller the grain size the greater the lateral push so this one is not gonna stack much it's vertical is small but it's horizontal push on that retaining wall is very large and if it rains it's even larger so the angle of repose or the active lateral soil pressure depends on the grain size and the water content the smaller the grain size the greater the lateral push the greater the grain size the more the vertical stacking and most of the weight is going down instead of pushing on the wall. So active lateral soil pressure depends on the soil and the water content because a saturated sand is quite different than a dry sand. Okay, so there's geofabric walls, masonry, and retaining walls. This is a fascinating system where they have these pegs that go into the units and there's geo fabric here and they put gravel and then they backfill and then they do the same on the next layer and they can go pretty tall on these walls and they retain the dirt and they can resist lateral loads. So also of course cast in place concrete monolithic walls is the premium for taller spaces. That's the best way to go. So construction concerns of course involve draining the water from behind the retaining wall. So the more you can get rid of the water the steeper the angle being pushed on the retaining wall. The more the water, the more horizontal the push. So draining the water is a critical thing from behind the retaining wall. So we can see here clearly there's a serious attempt to have drains in the wall. This one is not working very well because the drain is not behind the wall. It was added afterwards. And it's to drain this soil instead of the soil that's behind the wall. So I'm saying this because this wall eventually failed. It's in my neighborhood. I kept tabs on it. And then I'll show it to you in a little while. So a battered wall is not vertical. It makes an angle with the vertical. And it's based on the type of soil behind it. So they work with the angle of repose of the soil. and then they lay the stones based on that angle, knowing that the soil is at rest and it's not going to push the retaining wall, push it less than if it were vertical, versus this retaining wall is totally vertical, and it's pretty easy to overturn this wall, and it's a lot harder to overturn a battered wall because it's at an angle, and if this one were battered like this, then it would be the center of gravity moved to the right. It would be much harder to overturn it than if it were vertical. Okay, so here's one of those walls in the neighborhood, and they've added some dead men. They have anchors here, soil nails in the ground, and they've done the drainage. The problem is this piece of metal plate is just retaining that many, is keeping together that many stones, but the others are free to play as they wish. This one can probably keep that many in place. So eventually this wall collapsed. I took my picture. This wall, the brick wall was being pushed a little bit more on that side compared to this side. And it cracked. It failed. Again, I don't know what kind of soil is behind the wall, but for sure there is water accumulation and it pushed more on this side than it did on this side and it cracked and failed in shear. So no matter what the height of the wall, failure is possible. I have students all around the country and they send me pictures of failure. I love them and I show them in my presentations. So these are my own pictures. We can see here the wall is starting to separate They're putting a French drain there, a perforated tube to get the water out from behind the wall. And then eventually it just failed. So they replaced it with railroad ties. It took all of three days. And these things pulled out of the dirt and were in the ground. And I took my picture. And then finally they put in proper drainage. And they went in with split-face CMU. and you can see it doesn't look pretty, but at least it's together on the right-hand side. Okay.