Soils & Foundations: Angle Of Repose

I'd like to talk about the angle of repose for granular soils, which are non-cohesive soils, which are basically sand and gravel. Clay is cohesive, so this is not part of the discussion. But the angle of repose is the angle that a heap of soil, be it gravel or sand, makes with the horizontal. So, in the case of gravel, this would be the angle. And in the case of sand, the angle of repose is a lot shallower. In the case of gravel, it's a lot steeper. And that has implications on the lateral push that a soil exerts on a retaining wall or a basement wall. And that's the topic of this video. So, if we were to put a retaining wall here and lose this gravel, then we need to recognize that most of the weight of this pile of gravel is going downward, because the particles are large, and the push on that retaining wall is going to be small. It's not going to be very large, because most of the weight is going downward. So we have a steep angle of repose, and it's pushing on that retaining wall. but most of the weight is going downward. So the vertical component of the weight is much larger than the horizontal push on the retaining wall, in the case of gravel. Now in the case of sand, if we were to put a retaining wall here and remove this dirt and retain it with that wall, then we need to recognize that the vertical is pretty small. And the push, the lateral push of the soil is much greater. The horizontal component of push on that retaining wall is much greater. There is not much push here, but as you go down, there's increasing pressure. So in the case of sand, the horizontal component is larger than the vertical component. And as we said, that has implications. So looking at this again, the angle of repose here, let's use the same color, sorry. The angle of repose here means that there's a lot of push on that retaining wall and a lot less push. The angle here is steeper. Okay, so looking at this, there's going to be a little bit of a push on this precast concrete barrier from the gravel onto this barrier. And there's going to be a lot more push from sand. And the angle of repose depends on two things. One is the grain size. So with gravel, larger particles or larger grain size means V is greater than H, and the push is relatively small. The second factor that affects the angle of repose is the moisture content. of the soil. So the moisture content is telling us here that if it were to rain, it would drain very nicely in gravel. There's a lot of surface area, so it's a good filter for water. But with sand, we need to know and we need to distinguish dry sand on a question on the ARE, for example. Is it dry sand? Is it moist sand? Is it wet sand? Or is it saturated? Saturated means there's a lot of water, which means the horizontal push might even be greater than that, because the water content is high. So a dry sand might have that much lateral component. A wet sand or a moist sand would have more and then a saturated sand or water, pure water, will push a lot more on a vertical surface than a dry sand. So I have this slide here of Morton salt and salt is granular and we can see what happened here. It pushed through the side of the building. So anything with a certain size grain is going to have a lateral push that needs to be addressed. So sand, gravel, salt, cat litter, dog food, all that stuff will have an angle of repose. So let's put this to work here, and let's understand that in a basement, what we're going to do here is, first of all, we're going to waterproof this surface of the basement. And the next thing we're going to do is we're going to put some kind of French drain down here, which is a drain that is perforated on the top. And then we're going to backfill with gravel. Why gravel? Because it's a very good drainage bed, and the water would go straight down, or else go into the barrier and come down here. So what is happening to that basement wall is the following. There is a certain push from the soil on that basement wall, and the wetter the soil, the greater the push. If it's sand. If it's gravel, it's filtering the water, sending it straight to the French drain. So, I'll repeat the words I had earlier. Dry sand versus it just rained, so it's moist. Or it's wet. Or it rained a lot. Or maybe there's a water table, saturated. Or maybe there's a water table. If there's a water table, then if there's some water here, then the push is even greater. Okay, so let's take this one step further and say we have a certain push from the soil, a lateral push on that basement wall in red. And what is holding that wall is basically this floor and this floor. But clearly, the center of gravity of a triangular load is lower than half point. So it's located at one-third and two-thirds. Two-thirds from the top, one-third from the bottom. So there's more weight, there's more lateral push on the bottom than there is at the top. And therefore, I should recognize that the reaction here is smaller than the reaction down here. So there is this much push from that first slab, well, pushback rather. Well, then there's going to be more on the bottom. And if this is x, then this is going to be 2x. That's the reaction on a basement wall. It has to retain the dirt, the lateral load. And so the first floor gets a certain reaction. The slab on grade, the lowest, the floor of the basement, is going to receive twice as much.