but I am not sure. the lateral force from soil, surcharge... will give the headwall a clockwise moment to overturn. how the soil pressure prism to balance it?
The dead load of the headwall structure will resist the overturning, and this, coupled with the overturning moment, will impart a prismatic soil pressure diagram to the base, with the maximum pressure most likely at the toe of the structure.
Additionally, depending on the depth of the slab, you may have some passive resisting pressure at the toe of the structure that may help resist overturning.
P/A +- M/S (simplistically) will give you the max and min soil pressures.
You have a shear key at the toe or outflow which also serves a erosion protection there, and that’s good. You could run the floor slab out to the left and out on the sides near the headwall, a few feet, to pick up soil weight as part of the righting moment.
Mike- I'm a little confused here...by no means am I a retaining wall guru, but why would the overturning not be determined at point 'A' ?
Isn't the overturning for this the same as any retaining wall or foundation?
Righting moment is just from the dead weight of the structure and the overturning from the lateral soil pressure, no?
If need be, depending on the construction of the sewer pipe and its attachment, you could also count on it + some % of the soil above the pipe in calculating the righting moment (probably not a good idea though).
I suppose there may be other ways of coming up with the same answer, but the factor of safety against overturning is the ratio of the resisting moment to the overturning moment, both taken about the toe of the wall.
Mike, why would you select the heel as the point to sum moment? The footing doesn't rotate about the heel.
Hi, guys, still confused, if the check the overturning at the toe (point "A"), how the prismatic soil pressure (P/A +- M/S ) at the base to resist this moment?
the prismatic soil pressure also gives a clockwise moment if rotate at "A".
Not to distract from the OP but the more interesting question is the design of the sidewalls. Where you have a U-shaped structure with soil on the outside. I image there a couple different ways to do this however I'd be interested to see what is typical. Do you use half the footing for each wall or assume a resultant reaction at midspan of the footing? I will think about this and if I get some time I may start a new thread and clarify.
This discussion got me looking at the computer program I wrote 25 years ago for retaining walls, and you are right. I also used the toe when I wrote it. That being said though, the point should be able to be taken anywhere and get the same result. The toe is just the preferred point and easier to analyze.
In looking at the OP's problem though, if the overturning moments from the soil are greater than the resisting moments of the dead load of the structure, the heel should be extended under the soil to the rear of the outfall as needed to pick up the overturning resistance of the dead weight of that soil. It could be extended beyond the sidewalls too, but not with quite as effective of a result.
And really, the soil prism is just an overall summary of the total effect of the gravity and overturning forces on the base of the structure. It is best to keep the resultant of this prism within the kern of the structure, but it can vary from there if stability and soil bearing parameters are satisfied.
I am most likely wrong, but in analyzing OT in some foundations over the years, I found that the point you pick does matter.
If you don't use the point about which the footing is assumed to rotate as it turns over, then I believe you will have to determine bearing pressures and use those in your OT calc...more work than is necessary.
I'm not going to think about this too much for fear of confusing myself, but checking overturning about the point of rotation is just logical. Of course, the point of rotation is debateable. In rock, most would agree that it is the toe, but in soft soils, a footing can rotate about a point behind the toe. Some use an arbitrary distance behind the toe for this calculation.
hokie-
I agree, but the only way to logically eliminate bearing pressure on the bottom of the footing is to assume rotation about the extreme end of the 'toe', right?
Besides, I'm inclined to believe that the conservative FOS used in checking overturning takes into account the point about which the calculation is done. I doubt any footing would actually rotate about the extreme edge unless on rock as you have noted.
