"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?
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
Good Luck!
RE: "at rest" vs "active" earth pressure
RE: "at rest" vs "active" earth pressure
Thanks!
RE: "at rest" vs "active" earth pressure
don't know if this helps
RE: "at rest" vs "active" earth pressure
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
You are correct. I meant to say active pressure requires rotation of the wall.
RE: "at rest" vs "active" earth pressure
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
At rest is based on the wall not moving at all?
RE: "at rest" vs "active" earth pressure
RE: "at rest" vs "active" earth pressure
ht
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
RE: "at rest" vs "active" earth pressure
RE: "at rest" vs "active" earth pressure
RE: "at rest" vs "active" earth pressure
live and learn
RE: "at rest" vs "active" earth pressure
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
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
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
RE: "at rest" vs "active" earth pressure
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
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
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
'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
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!