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RE: soils pressure

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> Bill's response matches my understanding.  What originally brought this
> issue to my attention was looking at different widths of granular granular
> backfill specified for retaining walls.  Why were different widths called
> out? Was I designing for the backfill or the material behind the backfill
> (not really a question)?  It seemed (not exactly) that 35 pcf with a
> granular backfill was being specified regardless of the type of soils
> encountered at the site (okay clay-cohesion had an impact) but really:
> does
> the width of granular material matter if it can be placed and if it
> drains.
> Backfill with native material setting the EFP makes sense to me.  An EFP
> of
> 35 with granular backfill regardless of native noncohesive soil type does
> not without further clarification.

To be able to use the pressure for granular backfill, the granular material
has to fill the entire failure wedge for the NATIVE SOIL.  If all of the
native soil is below the failure plane, it will not mobilize, and will not
influence the wall pressure.  Typically, this is specified as a 45 deg. line
from the base of the footing.  I've also seen requirements for extending the
granular fill for a distance and depth equal to the height of the wall, but
this is overkill.

A 1-2' drainage layer immediately behind the wall will not significantly
reduce the pressure (unless the wall is only 2 feet tall).

With MSE walls the soil immediately behind the face is typically granular
fill, with the retained soil being native soil.  BOTH pressures are used in
design.  The pressure from the retained (native) soil is used to design for
global parameters (overturning and sliding).  The pressure from the
reinforced soil is used to design for internal parameters (grid strength,
face stability, etc.).

Jason Kilgore
Leigh & O'Kane, LLC
Kansas City, Missouri

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