"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?

 

[JedClampett]

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!

 

[DRC1]

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

 

[trilinga]

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.

 

[pba]

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!