Retaining Walls: 2c. Retaining Walls Forces

Let's study the forces that are exerted on a retaining wall or a basement wall. There's gravity loads, which is basically the dead weight of the wall and the soil resistance. Now bedrock can handle dead loads very easily. Clay, the dead load might sink a little bit or settle. So gravity is in the up-down direction versus in the horizontal direction or the lateral direction. We have the active lateral soil pressure and we have friction between the retaining wall and the soil. Now that soil underneath that retaining wall could be a very slippery clay with very little friction, or it could be sand mixed in with gravel with a lot of friction. So that is a factor that we need to look at. So in the lateral direction, there's the push on the wall, on the stem mostly, and then there is friction between the ground and the heel and toe of the retaining wall. So here we are looking at the gravity loads. It's basically these two. Oops, let's get a fatter pen, a highlighter. These two are the gravity ones. And then in the lateral direction, I have the active lateral soil pressure, P, and the resistance with the soil, the friction, or V. So looking at this diagram and trying to understand just qualitatively what happens if the dead load, W, is greater than what the soil can handle. So the bearing pressure, if that dead load is greater, then you get settlement. The wall sinks, settlement or sinking. as long as that FP or the soil bearing pressure can hold the weight of the wall we're in good shape so in the horizontal direction in the lateral direction if this active lateral soil pressure is much greater than the friction between the soil and the bottom of the retaining wall a slippery clay, then you get sliding. The wall will slide. And if P, the active lateral soil pressure, keeps pushing, but then if there is some friction with the soil, then that starts resisting. And now all of a sudden we get overturning. So we have three basic phenomena happening on any of the retaining walls, whether it's a cantilever, a counterforce, a buttress, or a gravity retaining wall. They're all subjected to these phenomena. Vertically settling or sinking, horizontally sliding, and if there's a lot of friction, I'm worried about overturning. So let's study each one of these very carefully and try to understand. There are no numbers here. I'm sorry, there's a lot of arrows. Can't help it. Got to do that in structures. Sorry for you. Okay. So, I have a dead load. And it is, the retaining wall is clearly made of concrete, reinforced concrete, that weighs, or sorry, that has a density of 150 pounds per cubic foot. And I can figure out the volume of this wall and figure out how much dead load I have. And from soil boring tests, I can figure out the bearing capacity, FP, based on the soil type. They'll do borings. They'll figure out how much the soil can handle in a certain location. They'll compare it to what is expected of the dead load of the wall. And clearly bedrock can bear much more load than gravel, which can bear more load than sand, sand more than silt, and silt more than clay. And we must avoid organic matter in our soils. Very good. So increasing bearing pressure, bedrock, of course, is the greatest. Excellent. So that's in the gravity direction, and we're worried again about settlement or sinking. So the wall will travel downward if the soil cannot handle it. Very good. Let's look at some pictures here. It looks like on this side, the wall is sinking. On this side, it looks like it's in place, and I don't know if it's a different kind of soil or what have you. But you can notice that the failure is a stepped pattern that follows the mortar. And it looks like, unfortunately, this is unreinforced masonry. Had it been reinforced, this would not have happened if I had rebar going in here and in the bed joints. Then it would have been fine. Okay. This other case also, I have a wall. It looks like it's sinking. And I'm not sure if there's mortar here. There's no mortar. It's just the weight of the stone is going down. And here it looks like it's stuck to the soil, to the clay maybe. It's not going with it. Again, there's a battered wall, so the contact is more between the stone and the dirt than it is between the stone and the stone below it. That's when the wall is more vertical. Okay, so settlement. That's the outcome of not enough bearing pressure and too much weight. Let's look at sliding. Sliding is when that active lateral soil pressure is way more than the friction. So two measures that are usually done. One is to cast in a keyway. So this retaining wall here is going to have a keyway. I don't know if this picture shows it well. Okay, so here's the wall, and it's going to have a stem and a heel. The dirt's going to be on this side. So what they do is they go in here and they pour a little extra concrete, which is this hole here. So that is the key way. We can see it. This is where the keyway, this additional concrete is where the keyway is. We can see it a little bit better in this picture. It's the profile looks like this and then down and then up and then the wall goes up here. So keyway, that's one way. Another way is to have a straight pile driven in the ground or else drive it at an angle and call it a battered pile and then cast it in the concrete of that gravity retaining wall. So in this image here, I can see I have a wide flange or an HP pile that is driven in the ground at an angle. And this one here is a straight pile versus a battered pile. And we can see in this case, the battered pile is sticking its head up in the pour of that retaining wall. Once they pour the concrete, that's it. This guy cannot slide anymore. So here it is. It's reinforced. This guy is the pile. It's going 30, 40 feet in the ground at an angle in this case. Well then, if you want to push on me laterally, we're not going to take it. So we're going to fight that and we're not going to let any sliding occur. Same with the key way. If you push on me, I'm going to push back so that there is resistance to sliding. So two ways of hindering sliding. One is with piles, straight or battered. The other one is with casting a keyway. Okay, let's look at this one here. Another method is something called soiled nails, also called tiebacks, also called deadmen. We can see what's going on here. There is a coring bit here that is angled on the truck and it's driven in at an angle and then they tie these HP piles. They tie them and they're retaining that dirt over there. So soil nails are a way of stabilizing the walls of a construction site so that it doesn't cave in. They're also called tiebacks. Deadman we saw in a previous video is the big old block. If it's buried in the ground, then it's a dead man. It's not coming out ever. Okay. Let's understand the last one, which is very important. It is overturning. So we said up, down, we're worried about settlement and sinking. Left, right, we said there is friction. If the friction is not adequate and the lateral soil pressure is greater, the wall will slide. we prevent it with a keyway or we drive in some piles and pour them in the bottom of the retaining wall. So if those two are taken care of, we're still worried about one more thing, which is overturning, which is pretty common in retaining walls. I saw you, I showed you most of those damaged retaining walls was due to overturning, not sliding and not sinking, but just overturning. So, if there is a lot of push laterally, and there's a lot of resistance, now those two are roughly equal, well, then there's no sliding. But if this is a little bit larger than this one, and this one is fighting back, but this one beats it, then the wall will overturn about the toe. The toe is over here, and it looks like U1. So it overturns, in this case, counterclockwise. But the idea is the active lateral pressure was resisted by a lot of friction. And then you have two equal but opposite forces. They make something called a couple. And the overturning moment is one of them times that distance. We're not going to do the math. Very good. So we talked about stabilizing. We can stabilize this wall and anchor it with soil anchors. then it will not overturn. So these help me with sliding, they help me with overturning. Now the next video will address this in much more detail but right now I would like to say that if the soil pressure, oops, if the soil pressure is over here and it's pushing and can we copy and paste? Let's see. Oops, undo. Sorry. This one is select. Where is select? Here it is. Copy and paste. I don't know how to do this yet. Okay, forget it. Paste. There it is. And one more time. Oops. Paste. Paste. Paste. Good. So there's active lateral soil pressure pushing on the stem of the wall. What does it do? Regardless of what's going on, the stem is going to do that. It's going to bend a little bit at the top. It's going to stretch on this side. It's going to stretch on this side. And therefore, wherever you have blue, you're going to need larger diameter reinforcing or closer spacing or both. So that's one thing. But then there is also, where is that drawing? It's in the beginning. Sorry, I'm thinking to myself, oh, it's in the previous video. Forget it. So we're here. And this dirt is also sitting on the heel and pushing down on the heel. So it does that to the heel. And it makes it do that. Don't worry if you're not following on this one. There's another total video that explains it much better. but basically I wanted to ask which is most likely to overturn of these three configurations configuration a is no toe or very small toe and c is very small heel and b is has both toe and heel So which one is most likely to overturn? Let's remember, this is the toe in all cases. We're going to overturn about that point if the dirt is on the right-hand side of the wall. So I think the answer is as follows. Not I think, I know. There's a mass of dirt here. The overturning is going to happen about this point. So the overturning due to this active lateral soil pressure, it wants to overturn the retaining wall about the toe. It's going to do one third, two thirds. There's the resultant. It's going to do that. But then I am a counterweight. All this dirt is sitting on this side and it has a resultant. And this counterweight is doing the opposite rotation. So I have stability on the back. The backfill is helping me. To overturn the wall, you have to lift all that backfill. That's not happening. So over here, I have a dead load of dirt. And you are trying to overturn me. And I'm not going to let you because I am clockwise. You are counterclockwise. To overturn the wall, you must get rid of all that dirt. Versus in option C, that dirt is sitting on itself. I didn't get the benefit of that counterweight on the back. It's a lot easier to overturn wall number C. And in fact, wall C wants a longer toe to prevent overturning because it doesn't have the counterweight on its backside. Okay, I think that's good for this one. Please listen to the next one because that's where things are going to be explained very clearly with demonstrations.