Retaining Walls: 2b. Retaining Walls Terminology & Types

So continuing on with retaining walls, I'd like to define some terminology and look at the types of retaining walls in this video and then go on to a little bit more qualitative structure. No numbers here. So let's understand some of the terminology. If the dirt is pushing on this side, then it wants to overturn the wall. Oops. And that would be the toe. So the side opposite, bottom most side the bottom most point opposite of the retaining wall is called the toe so I have a toe here and I didn't do that not sure why this is happening undo okay so this is the toe right here oh geez okay I'm sorry I'm getting used to things here so let's get a little more weight okay good so this is the toe. Any dirt above the top of the wall is called surcharge. That could include a roadway, a guardrail, a car, or whatever it is, load above that line is called surcharge. So the toe is over here. Well then this is the heel because it's going to rotate counterclockwise on the toe. So the heel is the opposite side and this one is the stem. Active lateral soil pressure. This one is an important term. It's behind the wall. There is no pressure at this point, but then as you go down one foot, then the pressure starts increasing. You go down another foot, the pressure increases. For every foot you go down, that pressure increases. So by the time you reach the bottom, it's that much. So active. Active because it could drain, and then we have more water, and then it's pushing more against the stem of the retaining wall. So it's active, it's changing, it's not constant. So to remedy that, they will put a French drain, which is a perforated pipe. Usually it's perforated all around, or else it could be just pieces of clay with gaps in between them. So something like that, where the water can go in and drain away from the retaining wall. A French drain. This guy is sloped to get the water out from behind the retaining wall. Passive lateral soil pressure is on this side of the wall. It's opposite the active lateral soil pressure. Usually, the passive lateral soil pressure is ignored in calculations. So that way, it's like a factor of safety. We're not going to count on that one. And then we have the bearing pressure. We have a backfill here that has some weight. This soil here weighs, depending on the density of the soil and its water content, it has a certain density. And based on the volume, it's going to be resting on the heel of this cantilever retaining wall. Okay, so you have all this soil pressure and you have a response or a bearing pressure resisting all this weight. And, okay, that's good enough. Let's go to the next slide. So angle of repose of a granular soil. Granular soil means gravel or sand, not silt or clay. Those are cohesive soils. Their particles are pretty small and they stick to each other, unlike sand. And a greater particle, which is gravel versus bedrock. Now, clearly, the larger the grain size, the more the bearing capacity of the soil. So essentially bedrock can handle more load than gravel, which can handle more load than sand, more load than silt, which is also more capable in supporting load than clay. And organic is to be avoided at all costs. So yeah, soil bearing capacity depends on grain size and moisture content. So the angle of repose is basically the steeper the angle, the less the soil pushes on a vertical surface, a retaining wall, a basement wall, things like that. So gravel has a larger grain size, and so it stacks nicely. And most of the weight of this pile of gravel is going straight down to the ground. So most of it is vertical and very little of it is pushing horizontally. So the vertical component of that weight is much greater than the horizontal component. Versus with sand, it is a much smaller grain size with sand. And so it doesn't stack as much, but instead it pushes horizontally a lot more. So let's look at this diagram here. It says the shallower the angle of repose, because of decreasing grain size, then it affects the stem of the, for example, cantilever or any other retaining wall. So a steep angle because the grain size is large, but as that grain size becomes smaller, then the push on the vertical surface, sorry, the horizontal push becomes much larger. it's important to note the description of the sand. So, for example, a dry sand versus a moist sand versus a saturated sand, we're describing more the content of water, how much water is in the soil, did it just rain? So water is the biggest culprit, and it has 62.4 pounds per cubic foot. So don't worry about the number. All I'm saying is one foot down on the wall of a swimming pool, you have 62.4 pounds. Two feet later, you have two times 62.4, three times 62.4, four times 62.4. Ten feet later, ten feet deeper, you have 624 pounds per every square foot of elevation of that pool wall. Water pushes a lot horizontally. That's why your divers have to take precaution. As they go down, the pressure is much greater. Very good. So smaller grain size causes greater horizontal push. Wetter soils cause greater horizontal push as well. Very good. Let's look at retaining wall types. The simplest one that can really not go, that's really pushing it, it cannot go very high, is a garden wall or something low, up to 12, 14, 15 feet, somewhere in that neighborhood. a gravity retaining wall counts on its dead load to keep everything in position. And in this case, this would be the toe because the pressure is on this side and it's increasing with depth. And we will see with another, the coming video, what are the forces and what they do. But right now, this active lateral soil pressure wants to rotate that wall in this direction, and that becomes a problem. Okay. In this case, for example, the gravel is pushing against that precast retaining wall, and the dirt, sorry, the sand is pushing at a greater pressure just because of angle of repose and grain size. Garden variety, retaining wall, gravity retaining wall versus a cantilever retaining wall. Okay, in a cantilever retaining wall, let's look at condition B, which is basically a stem. and the dirt is on this side. Let's put a French drain. And there is a pressure increasing with depth on this side, wherever the dirt is. And so that makes this one the toe and this one the heel. Very good. It could be a cantilever wall that has a small toe, or it could be a cantilever wall that has a small heel. So these are different configurations, and we'll discuss in the next video what the ramifications on what happens to the retaining wall. So let's look at some pictures. Here is, let me see if I can draw here. The wall will come in here. It has a footing over here. And it's going to be cast. And it has reinforcing in two directions. I see dowels coming out of the ground and then they're up to here. I see them terminating here. And then they go up with another layer of reinforcing. And there is dead men here, which is these blocks that they have positioned with a crane. So that when the wet concrete goes in the form, it wants to push outward. This guy goes into compression and keeps it in place. Instead of, they didn't have concrete here, so they put the dead men and anchored these tiebacks or diagonal braces. These are all temporary. Once the formwork is done, all of that goes away. So we can see here, again, the reinforcing. Hard to see. Okay. And there is the footing for that retaining wall, and it goes to the other side. And the wall is up here. I'm not sure if it has a stem, sorry, if it has a toe or a heel or both. I don't remember. So a cantilever retaining wall. I mirrored the image here just to make sure we understand that now the toe is over here. Oops. The toe is over here and the heel is over here. So this is discussed extensively in the next video, but I just like to make sure we understand what is happening to the stem. The stem exaggerated is going to be doing that. So the dirt side looks like it's stretching, and the side opposite the dirt of the stem looks like it's in compression. So likewise, in this mirror image, that's what's happening to the stem. The dirt side is being pushed by this active lateral soil pressure, and it's making it stretch. So this side is in tension, and that's really what I need to know, because the reinforcing does not need to be the same on both sides. Typically will not be the same on both sides of the retaining wall. It will be greater on the side that is in tension than the side that is in compression. Very good. Okay, so another variety, sorry, the cantilever wall is good between the range of 15 to 25 feet, so the gravity retaining wall is for low amount of dirt, and if the dirt is between 10, 12, 15 to 25 feet, then you can use a cantilever retaining wall. Now, if there is more dirt to retain or the height is greater than what the cantilever retaining wall can handle, so if we're more than 25 feet and probably up to 50 feet, now we're going to take the cantilever wall, but then we're going to add a counter four. A counter four is basically a wedge, a triangular wedge that ties the stem to the heel or to the toe. So in this case, when I look at this, I see the dirt pushing over here. And that triangular piece is in compression. Versus if the dirt is on this side and pushing, then this triangular piece is in tension. It's being pulled. So this one, when the fin is in tension, when that fin, I use the word fin, but actually it's called a counter four. Regardless, whether it is in a counter four wall or a buttress wall, that triangular reinforcing that ties the stem to the heel is called a counterfor. So in a counterfor retaining wall, the fin is in tension. In a buttress retaining wall, the counterfor, or the triangular fin, is in compression. It's being pushed by the dirt versus it's being pulled by the dirt in a counterfor. So we can see here in this image, if the dirt, sorry, this is a pretty tall wall, obviously. I think it was 30 feet. This was a prison. So they have reinforcing dowels sticking out of that wall, and then they're going to pour the counterforce separately. And then they will backfill, and it looks like this fin or counterforce is going to tension. This is a counter four retaining wall. Had the dirt been on this side and pushing, then this guy would have been in compression. And we would have called it a buttress retaining wall. They're exactly identical, except which side has the dirt. Then they will go in and put some water and damp proofing. And then they will backfill. And now these guys are in tension. So they will usually run a French train somewhere here to get rid of all that water that may accumulate in the dirt, get it out and have less push on that stem of the counter floor or buttress wall. Okay, that's good enough. Next, we're going to talk about the forces that are being exerted on this type, on a cantilever retaining wall, and where the reinforcing needs to be. Very important. .