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"at rest" vs "active" earth pressure
3

"at rest" vs "active" earth pressure

"at rest" vs "active" earth pressure

(OP)
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?

RE: "at rest" vs "active" earth pressure

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!

RE: "at rest" vs "active" earth pressure

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

RE: "at rest" vs "active" earth pressure

(OP)
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!

RE: "at rest" vs "active" earth pressure

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

RE: "at rest" vs "active" earth pressure

(OP)
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?

RE: "at rest" vs "active" earth pressure

Grizzman

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

RE: "at rest" vs "active" earth pressure

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

RE: "at rest" vs "active" earth pressure

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?

RE: "at rest" vs "active" earth pressure

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

RE: "at rest" vs "active" earth pressure

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

http://www.pdhengineer.com/Course%20Files/Completed%20Course%20PDF%20Files/Lateral%20Earth%20Pressure%20for%20Non-Geotechnical%20Engineers.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.

RE: "at rest" vs "active" earth pressure

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

RE: "at rest" vs "active" earth pressure

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".

RE: "at rest" vs "active" earth pressure

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.

RE: "at rest" vs "active" earth pressure

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

RE: "at rest" vs "active" earth pressure

(OP)
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.

RE: "at rest" vs "active" earth pressure

     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.

RE: "at rest" vs "active" earth pressure

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

RE: "at rest" vs "active" earth pressure

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).

RE: "at rest" vs "active" earth pressure

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.

RE: "at rest" vs "active" earth pressure

Looking at this thread and remembering some discussions in my office, I'd like to add that this subject is very poorly covered in college coursework.  I had to learn it on the job and I suspect so does most everyone else.
I think the problem is that Soil Mechanics at the college level is taught at a very theoretical level oriented toward future Geotechnical Engineers, not the end users, structural engineers.
Another observation is that there has been significant inflation in all soil pressures in the last twenty or thirty years.  Old soil reports used values of 30 pcf and 45 pcf for active and at-rest pressures respectively.  Now the values approach 45 pcf and 60 pcf for the smae area.  I guess soil is heavier now!

RE: "at rest" vs "active" earth pressure

A couple of things
1.) A structure will be subjected to at rest and may rotate to a postion where it has active pressure. Active and passive are limiting pressures. Actual pressures may not reach these values.
2.) Structures should be designed for active or at rest pressures. It does not make sense to design part of the structure for one pressure and the rest for another.
3.) Sheet pile walls are ussually designed on active pressure or some varriation of active pressure. Rigid concrete structures should be designed for at rest pressure, which brings us to:
4.) The biggest reason to use at rest pressure is compaction. Rollers and even large tampers generate considerable amonts of compactive effort, which locally can approach passive pressure, even if you hand tamp close to the wall. Although values can be high locally, it usually averages out across the face of the wall to value between active and at rest. Thus for permenant structures, usually at rest pressure should be used

RE: "at rest" vs "active" earth pressure

One specific point needs to be highlighted here.

'At rest' and active pressures are like applied loads on the wall and the 'at rest' and active pressure coefficients can be directly used to calculate the loading on the wall.

In the case of passive earth pressure, it is the resistence offered by soil for the inward movement of the wall.It is not constant always but is mobilised to the extent required to equalise the effect of wall movement, similar to static frictional resistance. Hence,the passive pressure coefficient represents the limiting value of the resistance and can not be used directly to obtain the passive pressure on the wall.

RE: "at rest" vs "active" earth pressure

Ok - This is my experience:

Walls designed for Active/Passive pressure are free- standing walls. These walls generally have high factors of safety (usually 2 to 3 for stability conditions).

Walls which form part of a structure are designed for 'At Rest' pressure as the potential deflections are usually to large to tolerate. Passive pressure generated by the wall pushing into the ground in front cannot be generated (as the wall doesn't move). I usually consider the earth force for these walls to be the 'worst case' and use a lower overall factor of safety (usualy 1.5 to 2).

BigH mentioned the specifiaction of backfilling. Ingold (someone else please supply further reference if possible - I can't find the paper) did some research into the horizontal pressures imposed on walls by the process of backfilling. Often these forces are DOUBLE the 'At Rest' forces if serious machines are used for compaction.

Having discovered Ingolds work I now take particular care in specifying compaction forces!

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