at rest vs active earth pressure

huangyhg

"at rest" vs "active" earth pressure
it's been a spell since i've designed a retaining wall and previously i always designed in based on active earth pressure forces.
in browsing through my literature for a refresher, i happened upon a section that suggested designing the retaining wall based on the "at rest" earth pressure. the idea being that inherent conservacy in wall design generally made the wall too stiff to adaquately rotate and allow earth shear forces to develop and lessen the pressure to the active level.
i ran a quick check on a retaining wall deflection and sure enough the wall did not deflect as much as the stated requirement for active earth pressures. i haven't previously had any problems arise and don't relish the idea of designing with the increased loading of "at rest" earth pressures. has anybody else encountered this and what's your take on it?
at rest pressure requires the wall to move (rotate) slightly (about .2 to .5% of wall height). if the wall can accomodate this movement, it can be designed for active pressure. if not, it should be designed for at rest pressure. typically the front face of a retaining wall is battered back slightly so if any movement does occur it is not noticable. if the wall were plumb, the top of the wall would overhang the base and not look good. building walls are typically designed on the basis of at rest pressures. one final note is that soil proerties are approximate values, not exact values, always use a little margin with them.
good luck!
nearly all retaining walls and wingwalls i design are based on active pressure unless there's some compelling reason to use "at rest."
drc1, i assume you meant active pressure requires the wall to move slightly. you typed "at rest". if that's the case, i guess my suspicions were correct. just wasn't looking forward to the +50% increase in loads.
thanks!
no! at rest(passive) preassures require the wall to move to engage the soil. think of it as the ability of the soil to resist something pushing against it. in order for the soil to "compress" slightly to develop strength the wall must move to push on the soil so the soil can push back.
don't know if this helps
i thought passive pressure occured when compression was applied to the soil. the wall needed to rotate "into" the soil to create passive pressure.if i build a retaining wall, and put in the backfill, unless the wall rotates backwards, you don't get passive pressure. at rest implies none of the earths shear strength is utilized to resist the spread of the soil, only the wall resists it.
active and passive both utilize the earths shear strength, just in opposite directions.
am i misunderstanding something here?
grizzman
you are correct. i meant to say active pressure requires rotation of the wall.
hi
i read some of the post and might be able to increase(?) the confusion by saying there are typically three types of pressure.
active: the wall moves away from the soil. pressurecoefficients about 0.2 - 0.3. this might be suitable for a flexible wall.
"at rest" or something similar: the wall doesn't move. pressurecoefficients about 0.3 - 0.5. ths is usually suitable for a wall.
passive: the wall moves against the soil. pressurecoefficients about 3.5 - 6.0. this is probably not "normal" for a wall.
in my terminology active and passive requires movement while "at rest" doesn't.
this i by no means my area of expertice (if i have any) but it's the terminology i would use. i would probably use "at rest" for a underground wall in a building.
as for the exact value of the pressurecoefficients they are a function of the soils internal friction angle.
hope i wasn't to confusing.
thomas
i thought if the wall moved into the soil you would have active pressure in the soil resisting the movement of the wall (which is much higher than at rest or passive pressure) and passive pressure on the other side of the wall (which is less that at-rest pressure) because the soil on this side of the wall relieves some of its stress due to movement of the wall away from the soil.
at rest is based on the wall not moving at all?
oops, was wrong....i have it backwards!!
there seems to be some confusion here.  for those that need a quick refresher, try this link.