Return to index: [Subject] [Thread] [Date] [Author]

Re: Active, At-Rest, Passive Soil Pressure

[Subject Prev][Subject Next][Thread Prev][Thread Next]
To me at-rest pressures make some sense.  Think about a cantilevered retaining
wall.  One would design the wall for active pressures, based on an expected
movement / rotation of approximately 1" in 20' in granular material.  Before the
wall moves, by your thinking, then the soil would exert no pressure on the wall.
If that were the case, then what force makes the wall move?  There has to be soil
pressure.
When I was in school, we talked about there being significant additional
pressures in walls compacted with heavy equipment.  There are methods to account
for the added pressures but I've never used them in practice.  I doubt that many
walls built and designed properly (without considering compaction method) have
failed.  I think more failures are related to poor backfill selection or
foundation material (clays).

Eric Ober, P.E.
Rockville, MD

Roger Turk wrote:

> 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
>
> *
> *   This email was sent to you via Structural Engineers
> *   Association of Southern California (SEAOSC) server. To
> *   subscribe (no fee) or UnSubscribe, please go to:
> *
> *   http://www.seaint.org/sealist1.asp
> *
> *   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
> *   send to the list is public domain and may be re-posted
> *   without your permission. Make sure you visit our web
> *   site at: http://www.seaint.org


* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org