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Active, At-Rest, Passive Soil Pressure

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It has been only in the last couple of years that I have heard the term, 
"at-rest" pressure.  None of my soils books (Spangler, and Peck, Hanson & 
Thornburn in particular) have the term.

One former local soils engineer had recommended/advocated passive pressure be 
used on all restrained walls, such as basement walls restrained by 
diaphragms.  I have had a difficult time justifying that in my mind as 
passive pressure requires the element to be "pushed" against the soil, which 
a restrained basement wall does not do.

I also have a difficult time visualizing "at-rest" soils pressures.  For soil 
to exert pressure against an object, it must be mobilized, i.e., unless 
the soil particles move, there can be no pressure.  Likewise, the soil 
particles cannot move until the object deflects, and if it deflects, then 
isn't that the criteria for "active pressure?"  And, if the object 
deflects, and the soil particles are not mobilized, isn't the pressure 
released?  If the "at-rest" pressure exists without a failure plane in the 
soil, what affect does the soil bridging between the restrained top of the 
wall and the restrained footing have on the "at-rest" pressure?  (Similar to 
material in a hopper bridging.)

Likewise, compacting behind a retaining wall has also been curious to me as 
it seems that it could cause pressures greater than the active soil 
pressures, yet, I have never seen or heard of a retaining wall failing when 
the soil behind it was compacted using hand equipment.  (I did see a 12-ft to 
15-ft cmu retaining wall that collapsed when the contractor was using a 
sheepsfoot roller and dozer to compact the backfill.  And, of course, all of 
the reinforcing was spliced at the top of the footing.)

I would appreciate hearing others thoughts on this.  And to be able to 
compare apples to apples, let us limit our discussion to non-expansive soils 
above the water table.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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