"at rest" vs "active" earth pressure 3

[Grizzman]

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?

 

[DRC1]

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!

 

[crossframe]

Nearly all retaining walls and wingwalls I design are based on active pressure unless there's some compelling reason to use "at rest."

 

[Grizzman]

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!

 

[TFL]

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

 

[Grizzman]

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?

 

[DRC1]

Grizzman

You are correct. I meant to say active pressure requires rotation of the wall.

 

[ThomasH]

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

 

[KarlT]

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?

 

[KarlT]

Oops, was wrong....I have it backwards!!

 

[nwm94]

There seems to be some confusion here. For those that need a quick refresher, try this link.

http://www.pdhengineer.com/Course F...h Pressure for Non-Geotechnical Engineers.pdf

Good basic knowledge for free.

To the original question, if the wall won't deflect enough to develop active pressure, it makes sense to me that you should use at-rest pressures. However, minimum recommended soil lateral loads are documented in the building code for the specific application of retaining walls. Based on the values printed (i.e. Table 1610.1 in 2000 IBC)it appears that the codes allow you to use the active pressures. Please correct me if I interpret this incorrectly.

 

[PEinc]

This discussion is SCARY!!!!! Get the books out!

 

[BFPartners]

Hopefully I can help clear the air on this subject. Passive pressures result when the wall rotates into the retaining soil. Active pressures result when the wall rotates away from the soil mass. Both conditions require shear planes to form within the soil mass (ie the soil must plasticize). Normally the number in the design literature is H/1000. (H is the height of the wall. 1/1000 = 0.1%) The at-rest condition is when the soil is "at-rest" or defined as the ratio of the horizontal stress to the vertical stress in the soil. Normally, wing walls, concrete gravity structures in dams, basement foundations, etc.. do not rotate enough to plasticize the soil mass behind the wall thus no passive or active pressures develope. Most examples in classic text books show examples of thin sheet piling walls whereby the wall can deflect H/1000 and thus active or passive pressures can develope. (Not the case for all textbooks and the author should be questioned on "their" choice of wall. Basement walls should be designed utilizing the at-rest pressure coefficients. (Not to mention the potential for hydrostatic head behind the wall... we won't even begin to talk about weeping tiles and geosocks... Also remember most engineers neglect soil cohesion (even small amounts in gravel) which helps the soil remain "at-rest".

 

[Ibeam]

Grizzman, I consistently have a run-in with the same quandary when doing retaining walls. Here's the way I see it - reinf. concrete of the stem wall more than likely experiences the at-rest pressure, so design the reinf with that in mind. But for stability, it seems that active and passive pressures are the logical choices, as they go hand-in-hand, and at-rest pressure would be an overly conservative design for stability.

 

[TFL]

i'm sorry apparently i was wrong. i thought at rest preassure was another term for passive preasuure i guess i was wrong.
live and learn

 

[Grizzman]

nwm94,
Your link is one of the sources I was looking at. Also, It does appear the code addresses the issue to some extent. They simply say "for a relatively rigid wall" to double lateral forces. This is a touch conservative, and leaves a lot open for interpretation, but clears the air well enough.
Ibeam,
Re: your suggestion of active pressure for stability, it makes some sense in that if the wall rotates slightly due to the "at rest" pressures, the soil will eventually plasticize with enough rotation and active soils will control. I guess that's where the idea of a front batter comes in to make the wall still appear straight.
Thanks everyone for their thoughts.

 

[BigH]

BFPartners - nice post - and of course, we won't go into the "wall friction" which most can't figure out anyway or the variability of the unit weights - such as you design for a sand at 125pcf but the contractor uses a good sand and gravel at 135pcf and "exceeds" the spec of 95%MDD(Std) by mistakingly using 95%MDD (Mod).
One of the funniest, in my view, notes on many structural drawings of retaining walls is that the designer says to use soil with 120pcf unit weight, 20deg wall friction and 32deg friction angle. Then the specs say to use a specific soil and compact it to 95%MDD(mod) - but the soil that is specified when compacted to 95% can't weigh anything less than 135pcf and, if done to 100% is 142pcf; and at 95%MDD would have a friction angle of 38deg or more! Fiction in design.

 

[Grizzman]

BigH
Of course by increasing the friction angle, you're decreasing the horizontal thrust. Maybe that offsets the additional density.

 

[BigH]

Grizzman - that's true, but remember, what you use as 'design' has to be built so the design should be consistant with the specs and construction techniques used (the designer, as I put forth, needs some realism).

 

[bridgebuster]

A few years ago, we had this discussion in the office. Someone designed a U-Shaped abutment using active pressure, as allowed by AASHTO-LRFD. The design was checked but when the QA reviewer looked at it he insisted that the design be done over - using at-rest pressure mear the corners and active everywhere else.

I thought it was overkill. In reality we have 3-dimensional structure with the wingwalls and abutment sharing the same soil wedge. Anyway, it was redesigned because the group manager said the QA reviewer had more experience than any of us. Meanwhile, at the time some of us were designing retaining walls for 20+ years and hadn't lost one yet.